The active component of the marijuana plant Cannabis sativa, Δ⁹-tetrahydrocannabinol (THC), produces numerous beneficial effects, including analgesia, appetite stimulation and nausea reduction, in addition to its psychotropic effects. THC mimics the action of endogenous fatty acid derivatives, referred to as endocannabinoids. The effects of THC and the endocannabinoids are mediated largely by metabotropic receptors that are distributed throughout the nervous and peripheral organ systems. There is great interest in endocannabinoids for their role in neuroplasticity as well as for therapeutic use in numerous conditions, including pain, stroke, cancer, obesity, osteoporosis, fertility, neurodegenerative diseases, multiple sclerosis, glaucoma and inflammatory diseases, among others. However, there has been relatively far less research on this topic in the eye and retina compared with the brain and other organ systems. The purpose of this review is to introduce the “cannabinergic” field to the retinal community. All of the fundamental work on cannabinoids has been performed in non-retinal preparations, necessitating extensive dependence on this literature for background. Happily, the retinal cannabinoid system has much in common with other regions of the central nervous system. For example, there is general agreement that cannabinoids suppress dopamine release and presynaptically reduce transmitter release from cones and bipolar cells. How these effects relate to light and dark adaptation, receptive field formation, temporal properties of ganglion cells or visual perception are unknown. The presence of multiple endocannabinoids, degradative enzymes with their bioactive metabolites, and receptors provides a broad spectrum of opportunities for basic research and to identify targets for therapeutic application to retinal diseases.

2.1. Synthesis and Release

Unlike water soluble transmitters (e.g., amino acids and biogenic amines), AEA and 2-AG are highly lipophilic and not stored in synaptic vesicles. Rather, membrane phospholipids are metabolized ‘on demand’ to liberate AEA and 2-AG by calcium-dependent phospholipases. There are numerous reviews that illustrate the details of these pathways (i.e., Piomelli, 2003;Bisogno et al 2005; Hohmann and Suplita, 2006). Briefly, the precursor of AEA is the membrane phospholipid, N-arachidonyolphosphatidyl ethanolamine (NAPE) that is formed by the transfer of arachidonic acid from the sn-1 position of 1,2-sn-diarachidonoylphosphatidylcholine (diAPC) to phosphatidylethanolamine (PE), a process that is catalyzed by a calcium-dependent N-acyltransferase (NAT). In a one-step pathway, NAPE is hydrolyzed by a phospholipase D to release AEA and phosphatidic acid (Di Marzo et al 1994). In a two-step pathway, NAPE is hydrolyzed to N-Acyl-lyso-PE by phospholipase A₁/A₂, then, AEA is released from N-Acyl-lyso-PE by lysophospholipase D (Sun et al., 2004). A third pathway has been described in which phospholipase C (PLC) cleaves NAPE to generate phosphoanandamide, which is dephosphorylated by phosphotases to liberate AEA (Liu et al., 2006). It has been suggested that this latter “PLC” pathway is involved in the ‘on demand’ synthesis of AEA rather than in maintaining basal tissue levels of AEA (Liu et al 2008). The primary pathway for 2-AG synthesis appears to involve hydrolysis of diacylglycerols (DAG) by two DAG lipase isozymes, DAGLα and DAGLβ (Stella et al., 1997; Bisogno et al., 2003;2005). DAGs may be produced either by the phospholipase C β catalyzed hydrolysis of phophotidylinositol or the hydrolysis of phosphatidic acid by a phosphohydrolase (Bisogno et al., 2005, for review). In addition to arachidonic acid, these same enzymes incorporate other fatty acids into membrane phospholipids which then serve as precursors for numerous other endocannabinoids, (i.e., palmitic acid and oleic acid into N-palmitoyl ethanolamine (PEA) and oleoylethanolamide (OEA), respectively).

As indicated in the previous paragraph, eCBs are stored as fatty acid components of membrane phospholipids. The liberation of eCBs from the phospholipids is due to an increase in intracellular calcium that leads to phosphorylation by calcium-dependent phospholipases. The increase in intracellular calcium may be achieved by depolarization and resultant Ca²⁺influx through N-type calcium channels (Kreitzer and Regehr, 2001; Ohno-Shosaku et al., 2001; Wilson and Nicoll, 2001), activation of postsynaptic G_q/11-coupled receptors such as mAChR or group I mGluRs (Maejima et al., 2001), or a combination of the two (Ohno-Shosaku et al., 2002; Maejima et al., 2005; Ohno-Shosaku et al., 2005).

As AEA and 2-AG are components of plasma membrane phospholipids, direct histological localization of endocannabinoids has not been possible. Their presence and localization have been inferred from the distribution of synthesizing and inactivating enzymes as well as physiological effects on identified cells. Even though AEA and 2-AG are released from membrane phospholipids by calcium-dependent phospholipases, the problem is how and if the eCBs leave the plasma membrane to affect target receptors. Unlike water-soluble transmitters (i.e., amino acids, acetylcholine, peptides), eCBs are highly lipophilic with a tendency to remain in the membrane. The eCBs freely diffuse within the membrane where they can interact with the active sites of degradative enzymes and receptors (see below). However, the phenomenon of retrograde transmission clearly demonstrates that eCBs can leave the membrane, traverse the aqueous medium of the intercellular space to act on presynaptic cannabinoid receptors as far as 20 μm from a release site (Wilson and Nicoll, 2001). It is not clear how eCBs leave the lipid phase of the membrane once they are released from the phospholipids or how they are able to travel in the aqueous phase of the tissues. AEA binds reversibly to serum albumin and it is likely that such binding is critical for the movement of AEA, 2-AG, etc in blood, the extracellular matrix and the cytoplasm (Bojensen et al., 2003).

2.2. Inactivation

AEA and 2-AG are inactivated following intracellular accumulation by several enzymes including: fatty acid amide hydrolase (FAAH), monoacylglycerol lipase (MGL), cyclooxygenase-2 (COX-2) and lipoxygenase (LOX). FAAH is an integral membrane protein with its hydrolytic site facing the lipid bilayer (McKinney and Cravatt, 2005). COX-2 is structurally similar to FAAH in that it is a monotopic membrane protein that is present in the lumen of the endoplasmic reticulum. MGL, in contrast, is a cytosolic enzyme. An issue of continuing contention is the means by which the highly lipophilic AEA and 2-AG cross the plasma membrane prior to their enzymatic inactivation. Although eCB movement across the plasma membrane is a sodium- and ATP-independent process that is driven by a concentration gradient, there are three competing ideas regarding the mechanism of uptake: a high affinity membrane transporter, inward transport by lipid rafts, and passive diffusion through the plasma membrane. A high-affinity membrane transporter for AEA has not yet been identified or cloned. The argument seems to center on the observations that most drugs that are used to block uptake of AEA, also block AEA hydrolysis by FAAH. Since FAAH is an integral membrane protein of the endoplasmic reticulum, one cannot differentiate the action of these drugs (i.e., AM404, MAFP) on a transporter or the enzyme. Detailed arguments for the relative merits of these mechanisms are beyond the scope of this review and may be found in older and more recent reviews (Hillard and Jarrahian, 2003; Glaser et al., 2005b;Hermann et al., 2006; Dainese et al., 2007).

AEA and 2-AG are hydrolyzed by FAAH into arachidonic acid and ethanolamine or glycerol, respectively (Deutsch & Chin 1993; Yu et al., 1997;Goparaju et al., 1998). In a similar manner 2-AG, but not AEA, is hydrolyzed by MGL (Goparaju et al., 1999; Dinh et al., 2002). Following hydrolysis of AEA or 2-AG, arachidonic acid is promptly incorporated into membrane phospholipids. In vivo, AEA is hydrolyzed principally by FAAH (Deutsch and Chin, 1993; Cravatt et al., 2001), while MGL appears to be the predominant 2-AG hydrolyzing enzyme (Goparaju et al., 1999; Saario et al., 2004). COX-2 oxidizes arachidonic acid, AEA and 2-AG to prostamides or prostaglandin glyceryl esters, leading to prostaglandins that also are biologically active (Yu et al., 1997; Kozak et al., 2000). In addition, oxidation of arachidonic acid by lipoxygenase produces 12-(S)-hydroperoxyeicosatetraenoyl acid (15-(S)-HPETE) 5-(S)-HETE and leukotriene B₄, all of which are agonists of TRPV1 receptors (Hwang et al., 2000). It is important to note that all these enzymes have other fatty acid derivatives as substrates. The above pathways are not all present in any given tissue. Rather, the effects of AEA and 2-AG may be modulated by the balance of metabolic enzymes that is specific to each cell type. A summary schematic of these pathways is shown in Figure 2.

Fig. 2

This schematic illustrates some of the metabolic pathways of the degradation of AEA and 2-AG. In the dominant pathways (bold arrows), AEA and 2-AG are hydrolyzed to arachidonic acid (AA) and then rapidly incorporated into membrane phospholipids via N-acyltransferase …

2.3. Receptors

Effects of cannabinoids in the brain are mediated by metabotropic and ionotropic receptors (Diaz-Laviada & Lorez-Llorente, 2005, for review) (Fig 2). Cannabinoid 1 receptors (CB1R) are the most numerous G protein coupled receptor in the brain, accounting for much of the physiological and psychotropic actions of marijuana. In general, activation of CB1R, via Gi/o, modulates voltage-gated K⁺ and Ca²⁺ conductances, resulting in a reduction of neurotransmitter release, particularly GABA and glutamate (McAllister and Glass, 2002; Pertwee and Ross, 2002; Straiker and Mackie, 2005). CB2 receptors also signal through Gi/o (Munro et al., 1993). CB2 receptors, though mainly expressed in cells of the immune system, also are found in the CNS, particulalry in astrocytes (Pazos et al., 2005; Martínez-Orgado et al., 2007; Cabral et al., 2008). There also is mounting evidence based on the persistence of cannabinoid-mediated effects in CB1R knockout mice for additional unidentified targets, or ‘CB3R’ (Zimmer et al., 1999; Begg et al., 2005). There is evidence that G-protein-coupled receptor 55 (GPR55), is the novel cannabinoid receptor (Baker et al., 2006). Some endocannabinoids (i.e., AEA but not 2-AG) activate the ionotropic transient receptor potential type vanilloid 1 receptor (TRPV1), previously referred to as vanilloid receptor 1 (VR1) (Ross, 2003; Toth et al., 2005). CB1R and TRPV1 are widely distributed in the CNS (Tsou et al., 1998; Toth et al., 2005), and along with CB2R, are expressed in the periphery, and in non-neural tissues (Szallasi et al., 1993; Galiegue et al., 1995; Funakoshi et al., 2005; Watanabe et al., 2005). Prostamides and prostaglandin glycerol esters, produced by endocannabinoid oxidation by COX-2 bind to a variety of prostaglandin receptors. Recently, a presynaptic PGE2 EP2 receptor has been implicated in synaptic transmission in the hippocampus, indicating a role for COX-2 and perhaps AEA in this mechanism (Zhu et al., 2005; Sang et al., 2005). Endocannabinoids also have been identified as ligands for peroxisome proliferator-activated receptors (PPAR), members of the nuclear receptor superfamily that are involved in lipid metabolism, insulin sensitivity, regulation of inflammation and cell proliferation (Bernstein, 2005). Independent of receptors, endocannabinoids exert additional effects upon signal transduction pathways and lipid rafts (Sarker et al., 2003; Barnett-Norris et al., 2005)

2.4. Distribution

The cellular localization of CB1 receptors in the mammalian central nervous system (CNS) was first described by in vitro autoradiography (ARG) of receptor binding and in situ hybridization of CB1 mRNA. These studies show enrichment of CB1 receptors in the hippocampus, basal ganglia, cerebellum, pyriform and cerebral cortices (Herkenham et al., 1991; Mailleux and Vanderhaeghen, 1992; Westlake et al., 1994), consistent with regions associated with the psychomotor effects of cannabis. Higher resolution was obtained with the independent introduction of three antibodies against CB1 receptors at about the same time, 1997 and 1998. These were rabbit polyclonal antibodies against the N-terminus, aa 1−77 (Twitchell et al., 1997; Tsou et al., 1998a), aa 83−98 (Pettit et al., 1998) and C-terminus 13 amino acids (Egertová et al 2000). A fourth rabbit polyclonal antibody against the N-terminus, aa 1−14 (McIntosh et al 1998) was not used in the mammalian CNS but was used in the retinas of rat, goldfish and zebrafish (Yazulla et al., 1999, 2000; Yazulla and Studholme, 2001) and the developing chick retinotectal pathway (Leonelli et al., 2005). Fukodome et al. (2004)produced a guinea pig polyclonal antibody against the C-terminus aa 443−473; this antibody was used to study the comparative distributions of CB1 receptors and diacylglycerol lipase α (DGL-α) in mouse hippocampus (Katona et al., 2006). In general, and quite happily, the immunocytochemistry supported the ARG localization of CB1 receptors throughout the rat brain. However, one difference was noted. The N-terminus antibodies labeled the dendrites, axon and cell body (Twitchell et al., 1997; Tsou et al., 1998; Pettit et al., 1998) whereas the C-terminus antibody did not label the cell body (Egertová et al., 1998). All of the aforementioned antibodies appeared to be well characterized. This issue of differential labeling will come up again in the discussion on CB1R-IR in the retina.

The role of each endocannabinoid metabolizing enzyme is dictated, in part, by its expression pattern. Of the three endocannabinoid inactivating enzymes, FAAH expression was the first to be described (Thomas et al., 1997) and is the most widely studied. Comparative distributions of FAAH-IR and CB1R-IR show complimentary, overlapping and unrelated distributions. In the complimentary pattern, FAAH is very prominent in the cerebellum, hippocampus, neocortex, olfactory bulb and amygdala in which it is present in large caliber processes of major output neurons (i.e., pyramidal cells) that are postsynaptic to processes containing CB1R. This pattern suggested presynaptic retrograde regulation of transmitter release by endocannabinoids (Thomas et al ., 1997; Egertová et al., 1998; Tsou et al., 1998b; Egertová et al., 2003). In the overlapping pattern, CB1 mRNA has been detected in cell types known to contain FAAH-IR, that is, some pyramidal cells in the mouse hippocampus, amygdala and entorhinal cortex (Marsicano and Lutz, 1999) and in pyramidal neurons of rat cortex (Hill et al., 2007). Even in the absence of double-labeling, the inference is that FAAH and CB1 receptors would co-localize in these pyramidal cells. In the unrelated pattern, there are FAAH-immunoreactive neurons in the thalamus, mid-brain and hind brain that are not associated with CB1 receptors (Egertová et al., 2003). As mentioned, FAAH has substrates in addition to AEA and 2-AG that are not ligands for CB1R, for example oleamide that may interact with serotonin receptors in the thalamus (Thomas et al., 1997). GABAergic output terminals from the striatum to the globus pallidus, entopenduncular nucleus and substantia nigra are rich in CB1 receptors, but FAAH is markedly reduced or absent. One possibility it that FAAH has a different role in regulating retrograde transmission in GABAergic synapses compared with glutamatergic synapses.

MGL, though it has a somewhat similar distribution as FAAH, is localized presynaptically rather than postsynaptically, with a near complimentary distribution of MGL and FAAH immunoreactivities in rat hippocampus, cerebellum and amygdala; with MGL in presynaptic and FAAH in postsynaptic processes (Dinh et al., 2002; Gulyas et al., 2004). An ultrastructural study in the hippocampus showed that diacylglycerol lipase α (DGL-α) was present in postsynaptic dendritic spines (Katona et al., 2006). The presynaptic contacts contained CB1R-IR. These data were consistent with the presence of MGL presynaptically as well as the identity of 2-AG as a retrograde transmitter at excitatory synapses as described below. COX-2, as with FAAH is present in postsynaptic dendrites, particularly spines, of large output neurons in the rat cortex, hippocampus CA3, and amygdala (Kaufmann et al., 1996). Although FAAH, COX-2 and MGL overlap in CNS distribution, they are not identical, and together may complement the overall distribution of CB1, TRPV1 and EP2 receptors in the brain. The unique cellular and subcellular distribution of endocannabinoid metabolizing enzymes provides multiple opportunities of local regulation of endocannabinoid tone. In addition to the spatial regulation, endocannabinoid activity is also temporally regulated, with in vivo AEA and 2-AG levels varying with a circadian pattern in the nucleus accumbens, prefrontal cortex, striatum, and hippocampus. Specifically, AEA levels are at a maximal in the dark-phase, and 2-AG at a maximal in the light phase. This corresponded to observed reductions of FAAH activity of these regions in the dark phase. MGL activity, however, only significantly differed in the striatum, suggesting additional mechanism(s) regulating 2-AG levels (Egertová et al., 2000; Valenti et al., 2004).

2.5. Functions

AEA and 2-AG have short and long term effects. In general, short term mechanisms involve presynaptic modulation of ion channels that result in inhibiting glutamate or GABA release; while in the long term, endocannabinoids modulate protein kinases and gene transcription (van der Stelt and Di Marzo, 2005, for review). The effects on synaptic plasticity and neuroprotection appear to depend on retrograde transmission in which postsynaptic dendrites release an eCB that binds to presynaptic CB1 receptors to reduce transmitter release (Wilson and Nicoll, 2001). Retrograde transmission of eCBs can be evoked by two mechanisms: 1) voltage-dependent and 2) activation of G_q/11 coupled metabotropic receptors. In the voltage dependent mechanism, depolarization of postsynaptic dendrites opens calcium channels, increases intracellular Ca²⁺, activates Ca-dependent PLD to release an eCB that diffuses to the presynaptic cell to reduce transmitter release. In the brain, strong depolarization of postsynaptic cells elicits a transient suppression of an inhibitory input. This was termed DSI (depolarization-induced suppression of inhibition) and was shown by Wilson and Nicoll (2001) to be due to activation of presynaptic cannabinoid receptors that reduced GABAergic input, thereby relieving the postsynaptic cell from inhibition. Analogous to this in excitatory synapses, retrograde inhibition of glutamate release was the basis of depolarization-induced suppression of excitation (DSE) (Kreitzer and Regehr, 2001; Ohno-Shosaku et al., 2001). A complication of DSI is that the GABAergic neurons in the hippocampus that contain CB1R also contain CCK, which is excitatory. Thus the retrograde suppression of these hippocampal neurons reduces the output of excitatory and inhibitory substances, the combined effect of which is still not clear (Beinfeld et al 2001). The second mechanism involves activation of G_q/11 coupled metabotropic receptors, usually Group I mGluR (mGluR1 and mGluR5) and muscarinic (M1 and M3) receptors. The events, metabotropic-induced suppression of excitation and inhibition, may be abbreviated as MSE and MSI, respectively. DSE, DSI, MSE and MSI are not potentiated by FAAH inhibitors but are blocked by inhibitors of DAG-lipase, implicating 2-AG rather than AEA as a retrograde transmitter (Kim and Alger, 2004; Slanina and Schweitzer, 2005; Fan and Yazulla, 2007).

In summary, the eCB-induced reduction of glutamate and GABA release contributes to short-term synaptic plasticity, while the reduction of glutamate release inhibits excitotoxicity following ischemia (Diana and Marty, 2004; Pertwee, 2005a, for reviews). Evidence implicates 2-AG rather than AEA in plasticity (Kim and Alger, 2004; Slanina and Schweitzer, 2005;Fan and Yazulla, 2007), while both AEA and 2-AG are involved in neuroprotection, in that they are released in response to brain trauma and confer protection against inflammation and excitotoxicity. Antioxidant properties of endocannabinoids also contribute to neuroprotection (Pertwee, 2005b). Whether both AEA and 2-AG serve as substrates for COX-2 to modulate synaptic transmission via EP2 receptors as demonstrated for AEA in the hippocampus (Sang et al., 2005) remains to be determined.

5.1. Biochemical assay

The earliest evidence for direct action of a cannabinoid agonist on the retina was provided by Gawienowski et al. (1982) who showed that THC increased MAO activity in a sampling of ocular tissues. The effect of THC was biphasic with the largest increase (~66%) in MAO activity occurring with 10⁻¹²M THC in the retina, followed by the trabecular meshwork (~44%), ciliary process (~26%), iris (~17%) and choroid (~12%). There was no further work on the retina until the seminal studies on the identification and characterization of endocannabinoids, their receptors and biosynthetic pathways in other tissues that occurred between 1988 and 1992. The ability of the retina to hydrolyze³H-AEA was shown first in homogenates of porcine retina (Matsuda et al., 1997), followed by bovine retina (Bisogno et al 1999) and goldfish retina (Glaser et al., 2005a). Hydrolysis of AEA in these studies was inhibited by a variety of FAAH inhibitors including MAFP, an irreversible blocker of FAAH (Deutsch et al., 1997), strongly indicating that the hydrolysis of AEA was accomplished by FAAH. FAAH hydrolysis of AEA was comparable in the retina and brain of rat and goldfish. However, the measured FAAH activity was much higher in porcine retina compared to bovine and goldfish retinas probably because a saturating concentration of the substrate (AEA) was used in porcine retina. Matsuda et al. (1997) also showed that AEA could be synthesized in porcine retina by reversal of FAAH activity in the presence of a high concentration (250 mM) of ethanolamine. It is unlikely that this ‘condensation’ pathway operates in vivo to synthesize AEA in membrane phospholipids because of the very high concentration of ethanolamine required to drive FAAH activity in reverse.

The first report of endocannabinoids in the retina was the detection of AEA and 2-AG by gas chromatography in bovine retina (Bisogno et al., 1999). Straiker et al. (1999a) detected 2-AG but not AEA in rat retina, while AEA, 2-AG and PEA were detected later in human retina (Chen et al., 2005; Matias et al., 2005). An important observation was that AEA could be released from bovine retinal extracts by incubation in a physiological buffer a time-dependent fashion (Bisogno et al., 1999). This finding implied that the retinal extracts contained both the precursor N-acylphosphotidylethanolamine and phospholipase D that releases AEA from the membrane phospholipids. The presence of endocannabinoids along with their synthetic and hydrolytic enzymes was demonstrated in the retina; the localization of eCBs to specific cell types has been accomplished by in situ hybridization, autoradiography and immunohistochemistry.

5.2. Localization — cannabinoid receptors

The first cellular localization of CB1 receptors in the retina was by in situhybridization in rat retina (Buckley et al., 1998). This study was part of a developmental study of CB1 mRNA in the entire rat embryo. CB1 mRNA appeared in retinal ganglion cells by E15 and “another cell layer within the retina” by E20. Given the low cellular resolution of the images, it is likely that the additional CB1 labeled cells at E20 were in the inner nuclear layer.Begbie et al. (2004) reported CB1 receptor expression by in situ hybridization in ganglion cells at E18 in chick embryo. CB2 mRNA was not detected in embryonic rat retina at E20 by in situ hybridization (Buckley et al., 1998). However, CB2 mRNA was detected by RT-PCR in adult mouse retina and in all cellular layers of adult rat retina by in situ hybridization (Lu et al., 2000). From the data presented by Lu et al. (2000), it was not possible to determine if the CB2 mRNA label was in neurons, Müller’s cells (INL) or astrocytes associated with the vascular system or optic fiber layer.

Straiker et al. (1999a,b) described the localization of CB1R-immunoreactivity (CB1-IR) in the retinas of human, monkey, mouse, rat, chick, salamander, and goldfish. All species showed prominent label in the OPL and IPL. Labeling in the OPL appeared to be derived from the synaptic terminals of cones and perhaps rods. The intense labeling in the IPL could not be ascribed to a cell type as labeling of cell bodies in the INL appeared sporadic. It also was reported that ganglion cells of all the species except goldfish were labeled by CB1R-IR. It was mentioned that the GCL and optic fiber layer were labeled intensely in the human, chick and salamander retina. However, from the images shown in Straiker et al. (1999a) in chick and salamander, it is difficult to see labeled ganglion cell bodies. In any event, the cells in the GCL were labeled far less intensely than the IPL and even appear surrounded by label in the GCL. Leonelli et al. (2005), in a developmental study of chick retina, showed a pattern at E18 that was similar to that shown by Straiker et al. (1999a) in chick, with intense label in the IPL and optic fiber layer. Although numerous amacrine cells were labeled, only few ganglion cells were labeled at E18. Yazulla et al. (1999) showed that CB1R-IR in rat was intense in the OPL and IPL, as well as in rod bipolar cells and PKC-immunoreactive GABAergic amacrine cells, but not in ganglion cells. The difference in the reported patterns of CB1R-IR in rat retina between the two studies (Straiker et al., 1999a; Yazulla et al., 1999) is disconcerting. Straiker et al. (1999a) used the Twichell et al. (1998) antibody against the N-terminus aa 1−77, while Yazulla et al. (1999) used the McIntosh et al. (1998) antibody against N-terminus aa 1−14. Both studies showed preadsorbed controls in rat retina and Yazulla et al. (1999) showed appropriate labeling in immunoblots of rat retina. Considering that CB1 receptor-mediated activity was demonstrated in rat retinal ganglion cells by [³⁵S]GTPγS autoradiography (Savinainen and Laitinen, 2004) and by RT-PCR and immunocytochemistry in a purified culture of rat ganglion cells (Lalonde et al., 2006), it is likely that the punctate label in the rat IPL described by both studies was derived from ganglion cell dendrites, while somatic labeling of ganglion cells in rat was not detected by the antibody used by Yazulla et al. (1999). However, we found that the McIntosh antibody labels ganglion cells in mouse (Fig. 3) as reported, but not illustrated by Straiker et al (1999). As in rat, we verified that the vertical streaks of CB1R-IR in the INL and IPL co-localized with PKC-IR and thus were rod bipolar cells. The labeled ganglion cells in mouse are not nearly as frequent as is observed with FAAH or MGL (to be described). As will be discussed in more detail below, the presence of CB1R-IR in bipolar cells is consistent with the presence of CB1 receptors on glutamatergic neurons in other regions of the mammalian CNS.

Fig. 3

Localization of CB1R-IR in mouse retina (unpublished observation)

The distribution of CB1R-IR in goldfish retina reported by Yazulla et al. (2000) differs from Straiker et al. (1999a) who reported intense label in the OPL, associated with rod and cone terminals, and IPL. In addition to the CB1R-IR in the OPL and IPL, Yazulla et al. (2000) showed intense label over Müller’s cells and lighter label over bipolar cell bodies and synaptic terminals (Fig. 4). At the ultrastructural level, CB1R-IR was preferentially located on the presynaptic membrane of cone pedicles and bipolar cell terminals. Rod spherules were not labeled. Yazulla et al. (2000) used the McIntosh CB1−14 antibody and showed a single band at ~70 kDa in immunoblots of goldfish retina as well as preadsorbed controls in immunoblots and immunocytochemistry. Of great interest in this regard is that Valenti et al. (2005) studied the goldfish brain with the same Twitchell antibody as Straiker et al. (1999a). In immunoblots of goldfish brain, that antibody labeled a single band at about 53 kDa and was blocked by preadsorption. Given that the appropriate controls appear to have been done it is difficult to determine which labeling pattern in the goldfish retina is more accurate. For that decision, corroborative data from other techniques are required. Electrophysiological studies to be described demonstrate the presence of CB1 receptors on goldfish cones and ON mixed rod/cone bipolar cells (Fan and Yazulla, 2003, 2004, 2005, 2007) supporting the localization data of Yazulla et al (2000).

Fig. 4

Distribution of CB1R-IR in goldfish retina (from Yazulla et al., 2000)

CB1R-IR is present on cone terminals of all species. Ultrastructural analysis showed that the CB1R-IR in goldfish cone pedicles was located on plasma membrane at the perimeter of the pedicle as well as within the invagination. CB1R-IR was not immediately apposed to the synaptic ribbon, but was at some distance from it. In this regard, the absence of CB1R-IR near the arciform density within the invagination is similar to that reported for GABA_A receptors (Yazulla et al., 1989; Yazulla and Studholme, 1997), perhaps consistent with this region being the site of active vesicular release and membrane fusion due to exocytosis. Studies also agree that CB1R-IR is found on a small population of amacrine cells. However, only in the rat has a population been identified, namely a relatively rare type that is immunoreactive for both PKC and GABA (Yazulla et al., 1999).

In the goldfish, CB1R-IR was located presynaptically in amacrine cell synaptic boutons. These boutons were found throughout the full depth of the IPL and were presynaptic to either bipolar cell terminals or small processes that likely were derived from ganglion cells. Given the relative rarity of these CB1R-immunoreactive processes it seems likely that they were derived from a single type of amacrine cell that ramifies throughout the IPL. There is about a 75% chance that an amacrine cell type is GABAergic (Marc et al., 1995). Cannabinoids presynaptically inhibit GABA release from neurons in the rat substantia nigra, corpus striatum and hippocampus (Szabo et al., 1998,2006; Katona et al., 1999), and it is likely that a similar function would be found in the retina.

A striking finding in rat and goldfish was the localization of CB1R-IR in bipolar cells. In rat, doubling labeling of antisera against CB1R and PKC showed that every PKC-immunoreactive cell was double labeled for CB1R-IR, including all rod bipolar cells and large PKC-immunoreactive amacrine cells. CB1R-immunoreactive cell bodies in the distal INL were identified as horizontal cells by double labeling with calbindin-IR. Curiously, we found that CB1R-IR in dendrites in the OPL was due exclusively to dendrites of the rod bipolar cells and not horizontal cells (Yazulla et al., 1999). In goldfish, CB1R-IR was present on the plasma membrane of bipolar cell synaptic terminals (Yazulla et al., 2000). Furthermore, there was a significant difference in the ON and OFF sublayers of the IPL with respect to frequency of membrane-bound CB1R-IR. All Mb bipolar cell terminals were CB1R-IR as were the vast majority (86%) of Cb bipolar cell terminals. This is in contrast to far lower frequency of labeling on the Ma (38%) and Ca (27%) terminals that occupy the OFF sublayer. Thus, there is a marked preference for ON bipolar cells in the membrane-associated labeling of CB1R-IR. This labeling of the membrane, as with the cone pedicles, was not adjacent to the synaptic ribbons; rather the CB1R-IR was always some distance removed from the ribbon. This is not surprising because presynaptic modulation of neural function by receptor-activated second messengers does not require spatial contiguity of the receptors with the presynaptic zone. The functional implications of these findings of CB1R-IR on cones and bipolar cells were investigated with whole cell recordings and will be discussed in the sections on physiology.

A relatively new and unexplored area of research is the presence of CB1 receptors on neuroglia. CB1R-IR was reported on Müller’s cells in goldfish retina (Yazulla et al., 2000) but not in any other retinal preparation. There are also inconsistent reports of CB1R-IR in mammalian astrocytes, microglia and oligodendrocytes (Pazos et al 2005, for review). Recent studies show that activation of CB1 receptors can inhibit high affinity excitatory amino acid transport in rat cortical astrocytes (Shivachar, 2007) and induce release of glutamate from hippocampal astrocytes (Navarrete and Arague, 2008). This modulation of extracellular glutamate could be critical in controlling neuronal excitability in normal and pathological states. There is evidence that CB1 receptors and, particularly CB2 receptors are involved in gliotic responses to injury and are targets of investigation for pathological conditions, for example multiple sclerosis (Martínez-Orgado et al., 2007;Cabral et al., 2008). As far as I know the interaction of endocannabinoids and glia has not been investigated in the retina. It is possible that the CB2 mRNA described in all cellular layers of rat retina (Lu et al., 2000) could include glial labeling, particularly the Müller’s cells in the proximal INL.

5.3. Localization — metabolizing enzymes

FAAH – The existence of FAAH in the retina was inferred by studies demonstrating that retinal tissue was able to hydrolyze AEA (Matsuda et al., 1997; Bisogno et al., 1999; Glaser et al., 2005a). There are only four studies that have localized FAAH-IR in the retina. In brain, FAAH-IR often is present in large caliber processes of major output neurons (i.e., Pyramidal cells, Purkinje cells) that are postsynaptic to processes containing CB1R (Thomas et al., 1997; Egertová et al.,1998; Tsou et al., 1998b; Egertová et al., 2003). This is the case in rat and mouse retina but not in goldfish or zebrafish. In rat retina, FAAH-IR was most prominent in medium and large ganglion cells, whose dendrites projected to narrow bands in the IPL. Weaker FAAH-IR was observed in the soma of horizontal cells (identified by calbindin-IR), the soma of large, but not small, dopaminergic amacrine cells (identified by tyrosine hydroxylase-IR), dendrites of orthotopic- and displaced-starburst amacrine cells (identified by choline acetyltransferase-IR), but in less than 50% of the starburst amacrine cell somata (Yazulla et al., 1999). Labeling at the inner limiting membrane, surrounding cell bodies in the ONL, the inner segments and occasionally vertical streaks in the IPL suggested that Müller’s cells were FAAH-immunoreactive. This finding is consistent with the report of CB2 mRNA in rat retina (Lu et al., 2000) and the role of endocannabinoids in glia function (Martínez-Orgado et al., 2007;Cabral et al., 2008). The pattern of FAAH-IR in mouse retina (Fig. 5A) is very similar to what was found in rat, with the most prominent label in ganglion cells and around rods. The major difference in the mouse IPL was the presence of a single rather than the double lamination of starburst amacrine cell dendrites. As in rat (Yazulla et al., 1999), we found that FAAH-immunoreactive bipolar cells in mouse are not PKC-immunoreactive and thus, are cone bipolar cells rather than rod bipolar cells. This is in contrast to CB1R-IR, which is exclusively in rod bipolar cells. One possibility is that CB1 receptors in rod bipolar cells respond to retrogradely released 2-AG, which is not an in vivo substrate for FAAH, whereas FAAH in cone bipolar cells may be involved in hydrolysis of AEA with targets other than rod bipolar cell CB1 receptors. In contrast to the exclusive distribution of FAAH and CB1 receptors in bipolar cells, FAAH and CB1 receptors colocalize in horizontal cells and some ganglion cells. The possibility of autoregulation of amacrine cell and ganglion cell activity by endocannabinoids has been raised in studies on cultured rat ganglion cells and chick amacrine cells (Lalonde et al., 2006;Warrier and Wilson, 2007). These will be discussed in a following section.

Fig. 5

Distribution of endocannabinoid metabolizing enzymes in mouse retina. (unpublished observations)

The distribution of FAAH-IR and the activity of FAAH have been studied in goldfish retina (Glaser et al., 2005a; Zimov and Yazulla, 2007). Only one of the four commercially available antibodies against FAAH that we tried was appropriate for goldfish. This was a rabbit polyclonal against the C-terminus (aa 651−571) of rat FAAH that was obtained from ALEXIS Biochemical. This antibody labeled a single band in immunoblots at ~61 kDa in homogenates of goldfish retina and brain and rat brain (Glaser et al., 2005a). Labeling in immunoblots and tissue was blocked by preadsorption with the immunizing peptide antigen. FAAH-IR was most prominent over Müller’s cells, cone inner segments and some large amacrine cells (Fig. 6). FAAH-activity assays showed that goldfish-retinal and brain homogenates hydrolyzed anandamide (AEA) at rates comparable to rat brain homogenate, and the hydrolysis was inhibited by methyl arachidonyl fluorophosphonate (MAFP), an irreversible inhibitor of FAAH (Deutsch et al., 1997) with an IC₅₀ of 21 nM. The first demonstration of cellular accumulation of an endocannabinoid in an intact neural system was performed in goldfish retina by Glaser et al. (2005a). ³H-AEA uptake was determined by in vitro autoradiography. As illustrated in Fig. 2, following uptake, ³H-AEA is hydrolyzed by FAAH with³H-arachidonic acid rapidly incorporated into membrane phospholipids. Silver-grain deposition then represents the trapping of ³H-arachidonic acid in plasma membrane following glutaraldehyde and osmium fixation. Silver-grain density was most prominent over cone photoreceptors and Müller’s cells, with some accumulation over amacrine cell bodies (Fig. 7). Uptake was significantly reduced when retinas were preincubated with 100 nM MAFP. The co-distribution of FAAH-IR and ³H-AEA uptake in cones, Müller’s cells and amacrine cells indicated that the bulk clearance of AEA from the extracellular space in the retina occurs as a consequence of a concentration gradient across the plasma membrane created by FAAH activity. These experiments did not address the issue of a membrane transporter for AEA but demonstrated the importance of FAAH activity in selective AEA accumulation.

Fig. 6

FAAH-IR in the goldfish retina (from Glaser et al., 2005a)

Fig. 7

In vitro autoradiographs of ³H-AEA uptake in intact goldfish retina, incubated at 20°C at two exposures. The silver grain pattern represents the deposition of ³H-AEA and its metabolites (from Glaser et al., 2005a)

In goldfish retina, CB1R-IR is localized all over Müller’s cells and presynaptically on the membrane of photoreceptor- and bipolar cell-synaptic terminals (Yazulla et al., 2000). FAAHIR as well as ³H-AEA uptake are predominately found in Müller’s cells, cone-inner segments and occasionally in amacrine cells. Thus, although CB1R-IR and FAAH-IR colocalize to cones and Müller’s cells in goldfish retina, there is a difference. In Müller’s cells, CB1R-IR and FAAH-IR are distributed over the entire cell with the same overall distribution within the cells. However, in cones, CB1R-IR is restricted to the synaptic terminal, whereas FAAH-IR is concentrated at the inner segment. The FAAH-IR in the OPL is rather diffuse and it is not clear what structures in the OPL are labeled. As will be discussed in a later section, the CB1 receptors on the cone terminals are involved in retrograde transmission (Fan and Yazulla, 2007). The retrograde endocannabinoid is 2-AG, not AEA; the hydrolyzing enzyme is MGL, not FAAH. Although there is no evidence yet for a function of AEA and FAAH in cones and Müller’s cells, a role in protection against excitotoxicity as reported in rat hippocampus (Karanian et al., 2005) and most recently in rat retina (Nucci et al., 2007) could prove to be a fruitful line of research.

AEA also is an endogenous ligand for the ionotropic TRPV1 (formerly VR1) receptor (Zygmunt et al., 1999; van der Stelt and Di Marzo, 2004). Activation of TRPV1 increases intracellular calcium either by entry through the plasma membrane or by calcium release from intracellular stores (Olah et al., 2001;Liu et al., 2003; Karai et al., 2004). There is evidence that the binding site of TRPV1 receptors for AEA is on an intracellular domain (Jung et al., 1999,2002; De Petrocellis et al., 2001). As FAAH and TRPV1 are integral membrane proteins of the endoplasmic reticulum and plasma membrane, respectively, it is likely that FAAH activity regulates the levels of AEA for TRPV1 activation (Ross, 2003; van der Stelt & Di Marzo, 2004). Such an effect has been demonstrated for increases in intracellular calcium (i.e., De Petrocellis et al., 2001; Millns et al., 2006). In addition, following hydrolysis of AEA by FAAH, lipoxygenase metabolites of arachidonic acid could activate TRPV1 (Hwang et al., 2000). Histological support for this idea in intact tissue was first reported in retina by Zimov and Yazulla (2007) who showed colocalization of TRPV1-IR with FAAH-IR in three anatomically distinct types of amacrine cell of goldfish. The cells are rare and include fusiform cells that ramify in sublaminae a and b, ovoid cells that ramify in sublaminaa and pyriform cells that ramify in sublamina b. In addition, the ovoid and pyriform cell types labeled for GAD67-IR and are subsets of GABAergic amacrine cells, whereas the fusiform cells are not. Based on the lamination of dendrites in sublaminae a, a/b and b, TRPV1/FAAH amacrine cells function in the OFF, ON/OFF and ON pathways of the retina. The effect of TRPV1 activation would depend on the downstream cascade triggered by the increase in cytosolic calcium concentration, and presumably would differ for the GABAergic and non-GABAergic amacrine cells. A recent report (Cristino et al., 2008) showed co-localization of TRPV1 and FAAH in (CA3) hippocampal pyramidal neurons and in some Purkinje cells in mouse brain. These data in fish retina and mouse brain support the idea that AEA can act as an intracellular mediator by being produced from and/or degraded by the same neurons that express TRPV1 receptors. These data also are consistent with Glaser et al. (2005a) who showed ³H-AEA uptake in a rare population of amacrine cells in goldfish retina, although it has not been demonstrated directly that these are the same cells that contain FAAH-IR and TRPV1-IR.

MGL and COX-2 – There are no published studies on the distribution of MGL-IR in the retina. Our preliminary data in mouse retina (Fig. 5B) show that MGL-IR is most prominent over most cells in the GCL (presumably including ganglion cells), proximal and distal INL. Double label with PKC-IR showed co-localization in all rod bipolar cells. MGL-IR was far less intense on rod bipolar cell axons and dendrites of amacrine and ganglion cells. There were numerous MGL-immunoreactive boutons throughout the IPL that could not be assigned to any particular cell type. Immunoreactivity was absent after preadsorption of the MGL antiserum with the immunizing peptide antigen.

Ju et al. (2003) reported that in rat retina, COX-2-IR was constitutive in horizontal cells, amacrine cells and ganglion cells. Following transient ischemia, COX-2 was upregulated in these same cell types and was induced in Müller’s cells. This pattern in rat differs from our preliminary data in mouse in which constitutive COX-2 is largely restricted to several layers of bipolar cell bodies and their axons in the IPL (Fig. 5C). Double labeling with PKC showed that 65% of COX-2 bipolar cells were rod bipolar cells while the remaining 35% presumably were cone bipolar cells. Unexpectedly, only 68% of rod bipolar cells were COX-2-immunoreactive, indicating that rod bipolar cells were of two types, those with and those without COX-2-IR. The same rabbit anti-COX-2 from Cayman Chemical was used in our study and by Ju et al. (2003). We have not as yet tried to replicate the COX-2 pattern in rat (Ju et al., 2003).

The cannabinergic system in vertebrate retinas, as indicated by CB1R-IR, FAAH-IR, COX-2-IR and MGL-IR, is concentrated in the ‘through pathway’, that is, in photoreceptors, bipolar cells and ganglion cells. These are the same cells that for the most part, use L-glutamate as their neurotransmitter. Cells of the inhibitory lateral pathways, horizontal cells and amacrine cells, do not feature as prominently. Exceptions seem to be the horizontal cells, dopaminergic amacrine cells and some cholinergic starburst amacrine cells that label weakly for FAAH-IR. Bipolar cells appear to be differentiated depending on whether they receive rod input. CB1RIR and MGL-IR are restricted to rod bipolar cells, FAAH-IR is restricted to cone bipolar cells and COX-2-IR is found in subtypes of rod and cone bipolar cells. As of now there are no electrophysiological data in mammalian retina to do more than speculate on the function of the differential distribution of CB1 receptors and related enzymes among the retinal cell types. Electrophysiological studies in salamander and goldfish and transmitter release studies in mammalian retina have provided tantalizing glimpses at the richness of cannabinoid function at all stages of retinal processing.

9.1 Voltage gated currents

Effects of the CB1 agonist WIN 55,212−2 on membrane currents of photoreceptors were reported in goldfish cones and tiger salamander rods and cones (Fan and Yazulla, 2003; Straiker and Sullivan, 2003). There is considerable agreement in the data from these two studies: 1. all effects of WIN 55,212−2 were blocked by SR141716A that had no effect of its own, and 2. all effects were mediated by PKA. Salamander rods and cones responded differently to 1 μM WIN 55,212−2. I_K was suppressed in single cones and rods whereas I_Ca was suppressed in cones but enhanced in rods. It was suggested that the differential effect on I_Ca and I_K in rods would increase transmitter release, the result of which would be to reduce sensitivity. This positive feedback mechanism appears to be counter-adaptive and at odds with virtually all other data that show a CB1-mediated inhibition of calcium currents, including salamander cones. This issue will be addressed again in the discussion on the biphasic effect of WIN 55,212−2 and retrograde inhibition of goldfish cones.

Fan and Yazulla (2003) recorded from goldfish cones and reported a biphasic response to WIN 55,212−2. At concentrations <1 μM there was an enhancement of I_K, I_Cl and I_Ca, while at concentrations >1 μM, I_K, I_Cl andI_Ca were suppressed. The voltage-activation ranges of these currents were not affected (Fig. 9). All effects of WIN 55,212−2 were blocked by the SR 141716A as well as the PKA inhibitor, Wiptide. The enhancement was blocked by 0.5 nM cholera toxin and the suppressive effect was blocked by pertussis toxin. The conclusion drawn from these data was that activation of CB1 receptors enhanced I_K, I_Cl and I_Ca by G protein G_s and suppressed these currents by G protein G_i/o. This biphasic regulation of cone currents seemed consistent with reports of CB1 receptor coupling to G_s and G_i/o G-proteins in other preparations (Glass and Felder, 1997; Maneuf and Brotchie, 1997;Sulcova et al., 1998; Calandra, 1999). As both D2 receptors and CB1 receptors are located on goldfish cones (Wagner and Behrens, 1993; Yazulla and Lin, 1995; Yazulla et al., 2000), the next study was to investigate potential interactions between the cannabinoid and dopamine systems.

Fig. 9

Reversible biphasic effects of WIN 55,212−2 on voltage-dependent currents in goldfish cones (from Fan and Yazulla, 2003)

The idea was that dopamine is a light-adaptive signal that desensitizes the retina, while endocannabinoids increase photosensitivity. At first, an opposing action of CB1 and D2 receptors on cone function seemed unlikely because both receptor types ordinarily act by decreasing cAMP via G_i/o. However, the enhancement of cone currents by low concentrations of Win 55,212−2 by G_s seemed to solve this potential problem. Indeed a D2 agonist, quinpirole (<10 μM), completely blocked the enhancement of I_K and I_Caseen with 0.7μM WIN 55,212−2. The effect of quinpirole was blocked by sulpiride and pertussis toxin, indicating action via a D2 receptor-G_i/ocoupled mechanism. The suppressive action of 50 μM quinpirole was not additive with the suppressive effect of high concentrations (3 μM) of WIN 55,212−2 suggesting that both agonists decreased membrane currents via the same transduction pathway, G_i/o PKA. The opposite effects of CB1 and D2 receptor activation on goldfish cone membrane currents occurred only with low concentrations of WIN 55,212−2. Assuming that the effects of WIN 55,212−2 could be generalized to endocannabinoids, we suggested that the physiological condition in fish involves very low extracellular concentrations of endocannabinoids in the OPL. It was only in this condition that a push-pull effect of CB1 receptor with D2 receptor activation on membrane currents could be observed. However, subsequent experiments proved that the best laid plans of fish and men are often gone astray (with apologies to John Steinbeck).

9.2 Retrograde endocannabinoid transmission onto cones

Despite evidence for the presence and actions of endocannabinoids in the retina, a functional role for them had not been determined. In other regions of the CNS it was clear that endocannabinoids, particularly 2-AG, serve as a retrograde transmitter and are responsible for the properties of short term plasticity, including DSI and DSE (Diana and Marty, 2004; Ohno-Shosaku et al., 2005). We attempted to demonstrate and characterize endocannabinoid-mediated retrograde suppression of membrane currents of goldfish cones in a retinal slice (Fan and Yazulla, 2007). Stimulation of retrograde release was achieved by applying a single 50 msec puff of saline with 70 mM KCl or an mGluR1 agonist DHPG through a pipette within a few μm of an Mb bipolar cell body while recording I_K(V) from cone inner segments under whole-cell voltage clamp (Fig. 10A). I_K(V) rather than I_Cawas used to monitor effects on the cone because I_K(V) was more robust and stable and showed the same effects and pharmacology to application of cannabinoid ligands as I_Ca (Fan and Yazulla, 2003). Retrograde inhibition of I_K(V) was reversible and stable over several hours (Fig. 10B,C). It had a latency of about 200 msec after a K⁺ puff, was reduced by an average of 25% and a half time to recovery of 3.4 minutes (Fig. 10D). Puff durations less than 30 msec had no effect on I_K(V) and puffs longer than 50 msec had little additional suppressive effect on I_K(V) (Fig. 10E). Retrograde suppression ofI_K(V) was unaffected by a combination of picrotoxin and CNQX but blocked completely by SR141716A. Experiments with the FAAH inhibitor URB597, Nimesulide (a COX-2 inhibitor), URB754 (a MGL inhibitor), and Orlistat (a blocker of 2-AG synthesis), indicated that 2-AG rather than AEA was the retrograde endocannabinoid. Data also showed that calcium influx or release from intracellular stores could mediate retrograde 2-AG release. The conclusions of this study were that retrograde suppression of I_K(V) is mediated by Ca²⁺-dependent release of 2-AG from Mb bipolar cell dendrites by separate mechanisms (Fig. 11): 1. a voltage-dependent mechanism, mimicked by the K⁺ puff, that would be activated by the depolarizing ON response to light, and 2. a voltage-independent mechanism, occurring under ambient illumination, mediated by tonic activation of mGluR1 that are present on Mb bipolar cells (Klooster et al., 2001).

Fig. 10

Properties of the retrograde responses of cones (from Fan and Yazulla, 2007)

Fig. 11

Schematic illustrations of proposed modulation of retrograde release of 2-AG from Mb bipolar cell dendrites (from Fan and Yazulla, 2007)

Based on our observation that low concentrations (<1 μM) of WIN 55,212−2 enhanced I_K(V) and I_Ca by a CB1/PKA/cholera toxin sensitive (G_s) mechanism (Fan and Yazulla, 2003), it was expected that the concentration of the retrograde endocannabinoid would be very low and therefore enhance cone currents when released. It was thus surprising that the retrograde effect always suppressed the currents. There was no initial enhancement followed by suppression as if there were a buildup of endocannabinoid concentration. The likely explanation appeared to be in ‘agonist-specific’ trafficking in which binding of different agonists to CB1 receptors favors coupling to different G proteins (Bonhaus et al., 1998). For example, WIN 55,212−2 increases intracellular calcium by G_q/11 coupling in hippocampal neurons and human trabecular meshwork cells, while other CB1 agonists including THC, 2-AG, CP55940 and methanandamide may couple to G_i/o but not to G_q/11 (Lauckner et al., 2005; McIntosh et al., 2007). It seemed likely that the reciprocal interaction between CB1/G_s– and dopamine D2/G_i/o-mediated processes that was proposed, based on data obtained with WIN 55,212−2 (Fan and Yazulla, 2004), may have been peculiar to WIN 55,212−2 and did not apply to the endocannabinoid, 2-AG. In the absence of the tidy ‘push-pull’ idea, what could be the function of the retrograde release that is mediated by the same G_i/o mechanism as activated by D2 receptors?

The unwarranted assumption is that activation of the D2 and CB1 receptors on the cones occurs under the same lighting conditions. Two conditions evoke 2-AG release from Mb bipolar cells, strong depolarization and activation of mGluR1, corresponding to voltage-dependent and voltage-independent mechanisms (Fan and Yazulla, 2007). Paradoxically, these two conditions occur in opposite lighting situations. It is presumed that rods and cones release L-glutamate at a steady rate under any ambient illumination; this rate is increased by decrements of light intensity and decreased by increases in light intensity. Also, it was presumed that the data on the retrograde suppression of cone I_K(V) could be generalized to I_Ca with implications for transmitter release from the cones.

The voltage-dependent release of 2-AG from Mb bipolar cells would provide a positive feedback that would amplify the reduction in cone transmitter release caused by increasing light intensity (Fig. 11B). This situation would occur because large increments in light would decrease L-glutamate release from cones, depolarize the Mb bipolar cell, trigger retrograde release of 2-AG that would subsequently reduce L-glutamate release from cones. It is not known how physiologically relevant this sequence would be because the half-time to recovery from a 50 msec pulse of retrograde stimulation is several minutes. This prolonged effect would seem to mitigate any response to rapidly changing intensities of light.

It was suggested that the voltage-independent mechanism was more likely to operate under normal conditions in that it would provide a negative feedback control of the tonic release of glutamate from cones (Fig. 11A). This is because as L-glutamate is released during ambient illumination, mGluR1α on Mb bipolar cells would be activated tonically and 2-AG would be released via a calcium-dependent G_q/11 mechanism. The retrograde inhibition of transmitter release from photoreceptors would provide negative feedback that would dampen the steady transmitter release under ambient illumination; the dimmer the background, the stronger the negative feedback. As background was increased, the feedback would be reduced. In this manner ambient illumination would produce a resting endocannabinoid tone that maintained a steady release of transmitter within relatively narrow limits, regardless of background. In this case, retrograde transmission would occur even though Mb bipolar cells were hyperpolarized by L-glutamate acting on either the EAAT or mGluR6 (Grant and Dowling, 1996; Wong et al., 2004). Also, the long half life of the retrograde effect would make this system insensitive to rapid changes around the background.

Endocannabinoids are only the latest of the neuromodulators that affect membrane currents in photoreceptors. Others include adenosine (Stella et al., 2002, 2003), dopamine (Fan and Yazulla, 2004), GABA (Tachibana and Kaneko, 1984; Picaud et al., 1998), glycine (Balse et al., 2006), nitric oxide (Kurreny et al., 1994; Kourennyi et al., 2004), polyunsaturated fatty acids (Vellani et al., 2000), protons (Barnes and Bui, 1991; Harsanyi and Mangel, 1993), somatostatin (Akopian et al., 2000), as well as modulation of hemichannels (Kamermans et al., 2001). It seems unlikely that all these operate on transmitter release at the same time. Even if paired off as opposing modulators, this makes for excessive control if occurring at the same time. Perhaps reciprocal modulation of photoreceptor activity occurs under multiple conditions of ambient lighting (scotopic, mesopic, photopic), circadian shifts in sensitivity, or even large changes in intensity for a given ambient background condition. The area of influence also may be a factor. Endocannabinoids, nitric oxide, protons and hemichannels could have a very local effect, down to a single synapse, whereas adenosine, dopamine and somatostatin could have global effects on the retina as they can function as paracrine modulators or volume transmitters.

9.3. WIN 55,212−2 affects the cone light response

Goldfish cone photoreceptors contain CB1 receptors at the synaptic terminal, selectively accumulate ³H-anandamide, contain fatty acid amide hydrolase-immunoreactivity, and possess voltage-gated calcium and potassium currents that are modulated by CB1 ligands (Yazulla et al 2000, Fan and Yazulla, 2003, Glaser et al., 2005a). The question was whether effects were restricted to the synaptic output or was there any effect on cone light responses. The previous studies were performed on retinal slices whereas these experiments were done in the isolated retina. Cones were stimulated with a spot of light of variable wavelength and intensity in combination with voltage- and current-clamp protocols (Fig. 12). Bath application of WIN 55,212−2 (10 μM) had no effect on the absolute sensitivity of the cones or on the kinetics of the onset response. However, the light offset response became faster and the depolarizing overshoot was enhanced. This effect was seen at all but dim intensities (Fig. 12A) and was independent of holding potential (Fig. 12B). This was found under current-clamp as well as under voltage-clamp conditions, indicating that the effect of WIN 55,212−2 was mediated by modulation of the cGMP-gated channels in the outer segment of the cones rather than by voltage-dependent currents. The effects of WIN 55,212−2 were not blocked by SR141716A, indicating that the effect was not mediated by CB1 receptors. With a train of ‘dark’ flashes from a steady background, the photocurrent recovered toward baseline more quickly with WIN 55,212−2 than in Control, with the result that the peak-to-peak response to succeeding flashes was greater. It was concluded that WIN 55,212−2 sped up the dynamics of the phototransduction deactivation cascade in the cone outer segments. The functional consequence of this effect was to shorten the recovery time to the offset of bright flashes, perhaps resulting in an increase in contrast sensitivity. A prediction from these experiments is an increase in the flicker fusion frequency at photopic intensities. However, Adams et al. (1978) reported that 15 mg of THC introduced a modest 8% delay in the recovery from a bright flash under photopic conditions. This result is counter to what would be predicted from our finding that during the light offset response, the membrane potential/current returned to baseline more quickly with WIN 55,212−2. Our data indicate that cones would actually recover more quickly at the offset of a flash under a wide range of backgrounds. This effect, if preserved through the visual pathway, could result in increases in contrast detection or critical flicker frequency.

Fig. 12

Effect of WIN 55,212−2 on the responses of goldfish cones in an isolated retinal preparation to flashes of light (from Struik et al., 2006)

These experiments raised several issues. The first has to do with the application of drugs. Concentrations of WIN 55,212−2 (0.2 − 4 μM) that modulated I_K(V) of cones in a retinal slice (Fan and Yazulla, 2003) did not affect the cone-light response when applied to the outer surface of the isolated retina; and 10 μM WIN 55,212−2, when applied to the retinal surface had inconsistent effects on membrane currents. CB1 receptors are concentrated at the cone synaptic terminal (Yazulla et al., 2000). In the slice preparation, access to these receptors is immediate on the exposed surface of the cone terminal. However, in the isolated retina, the cone synaptic terminal is about 100 μm from the outer segments on the surface of the retina. The voltage-independence and insensitivity of WIN 55,212−2 to inhibition by SR141716A in the isolated retina indicates that the ligand did not reach CB1 receptors on the cone terminal. A resolution to this issue could come from recording light responses from cones in a slice preparation. The second issue is the mechanism by which WIN 55,212−2 accelerated the light OFF response in a CB1-independent manner. It was suggested that WIN 55,212−2 stimulated a component involved in the deactivation of G_Tα/PDE, leading to a quicker response to light offset without affecting the kinetics of the onset of the light response (Struik et al., 2006). This idea remains to be tested. A third issue is whether the effect of WIN 55,212−2 on the photoresponse would be observed with other CB1 agonists and the endocannabinoids, AEA and 2-AG. Recall that the enhancing effect of low concentrations of WIN 55,212−2 on cone membrane currents was not observed with retrograde endocannabinoid release (Fan and Yazulla 2003,2007). A combination of agonist-specific trafficking and unidentified CB receptors on cone inner or outer segments could be factors in these findings. There is no evidence that endocannabinoids directly modify the cone light response, and the effect of WIN 55,212−2 may turn out to be agonist specific. For example, a prediction from our experiments is that WIN 55,212−2 would cause an increase in the flicker-fusion frequency at photopic intensities. However, Adams et al, (1978) reported that THC prolonged the recovery time from a bright flash under photopic conditions. Direct comparison of these studies is difficult because of the many potential sites of action of inhaled THC versus the localized sites of action of WIN 55,212−2 in the goldfish isolated retina.

Electrophysiological studies show that endocannabinoids presynaptically suppress the output of the ‘through pathway’ of the retina, photoreceptors and bipolar cells. The effect is subtle and modest as might be expected since smoking marijuana does not render the smoker blind. Evidence for effects of eCBs on amacrine cells is strongest for their suppression of dopamine release. Dopamine, a signal for light adaptation in the retina, also antagonizes the action of cannabinoids on bipolar cells. Historically, psychophysical studies of the effects of cannabinoids on vision were often done in concert with alcohol in the 1970s to determine effects on visual motor behaviors as they may relate to driving (Dawson et al., 1977, for review). Endocannabinoids constitute a major transmitter system in the central and peripheral nervous systems where they are critically involved in neuronal plasticity. Light and dark adaptation also involve processes of neuronal plasticity that are confined to the retina. Given the action of cannabinoids on photoreceptors and bipolar cells and their interaction with dopamine, perhaps it is worth revisiting the issue of the visual consequences of cannabinoids in model animal systems as a prelude to further work in human.

I would like to acknowledge the many colleagues with whom I have collaborated and who have participated in the studies from my laboratory cited in this review. Dr. Dale Deutsch first introduced me to the field of cannabinoids over ten years ago and has been instrumental with his guidance and encouragement. Special thanks go to Mr. Keith Studholme and Dr. Shihfang Fan, long-time members of my lab, who generated so much of the excellent histological and electrophysiological data over last ten years. I also thank Dr. Maarten Kamermans for the opportunity to work in his laboratory twice, to interact with him and his great lab personnel, and to work with Dr. Mieke Struik on the light-dependent effects on cones. I also thank Dr. Helen McIntosh, an early collaborator on the localization and characterization of CB1 receptors in the rat and goldfish retinas, and Dr. Benjamin Cravatt, who supplied us with the FAAH antisera that has been so important in the progress of our work. Also, thanks to Dr. Sarah Zimov, my most recent Doctoral student, for her tireless efforts on the goldfish projects and any other task that needed help. I also thank Dr. Sherrye Glaser who brought cannabinoid biochemistry and pharmacology to my lab and continues to do so. I am also grateful to Sherrye Glaser and Martin for daily interactions over lunch and critical feedback on the progress of this review. Finally, thanks to Dr. Colleen Shields for applying her grammatical and retinal expertise to the content of this review.

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• Adams AJ, Brown B. Alcohol prolongs time course of glare recovery.Nature. 1975;257:481–483. [PubMed]
• Adams AJ, Brown B, Flom MC, Jampolsky A. Alcohol and marijuana effects on static visual acuity. Am. J. Opt. Physiol. Optics.1975;52:729–735. [PubMed]
• Adams AJ, Brown B, Haegerstrom-Portnoy G, Flom MC, Jones RT. Marijuana, alcohol, and combined drug effects on the time course of glare recovery. Psychopharmacology (Berl) 1978;56:81–86.[PubMed]
• Aguado T, Romero E, Monory K, Palazuelos J, Sendtner M, Marsicano G, Lutz B, Guzmán M, Galve-Roperh I. The CB1 cannabinoid receptor mediates excitotoxicity-induced neural progenitor proliferation and neurogenesis. J. Biol. Chem.2007;282:23892–23898. [PubMed]
• Ahmad AS, Zhuang H, Echeverria V, Doré S. Stimulation of prostaglandin EP2 receptors prevents NMDA-induced excitotoxicity.J. Neurotrauma. 2006;23:1895–1903. [PubMed]
• Akopian A, Johnson J, Gabriel R, Brecha N, Witkovsky P. Somatostatin modulates voltage-gated K⁺ and Ca²⁺ currents in rod and cone photoreceptors of the salamander retina. J. Neurosci.2000;20:929–936. [PMC free article] [PubMed]
• Baker D, Pryce G, Davies WL, Hiley CR. In silico patent searching reveals a new cannabinoid receptor. Trends Pharmacol. Sci.2006;27:1–4. [PubMed]
• Balse E, Tessier LH, Forster V, Roux M, Sahel JA, Picaud S. Glycine receptors in a population of adult mammalian cones. J Physiol.2006;571:391–403. [PMC free article] [PubMed]
• Barnes S, Bui Q. Modulation of calcium-activated chloride current via pH-induced changes of calcium channel properties in cone photoreceptors. J. Neurosci. 1991;11:4015–4023. [PubMed]
• Barnett-Norris J, Lynch D, Reggio PH. Lipids, lipid rafts and caveolae: Their importance for GPCR signaling and their centrality to the endocannabinoid system. Life Sci. 2005;77:1629–1639.[PubMed]
• Begbie J, Doherty P, Graham A. Cannabinoid receptor,CB1, expression follows neuronal differentiation in the early chick embryo. J. Anat. 2004;205:213–218. [PMC free article] [PubMed]
• Begg M, Pacher P, Batkai S, Osei-Hyiaman D, Offertaler L, Mo FM, Liu J, Kunos G. Evidence for novel cannabinoid receptors.Pharmacol. Ther. 2005;106:133–145. [PubMed]
• Beinfeld MC, Connolly K. Activation of CB1 cannabinoid receptors in rat hippocampal slices inhibits potassium-evoked cholecystokinin release, a possible mechanism contributing to spatial memory defects produced by cannabinoids. Neurosci. Lett. 2001;301:69–71.[PubMed]
• Bisogno T, Delton-Vandenbroucke I, Milone A, Lagarde M, Di Marzo V. Biosynthesis and inactivation of N-arachidonoylethanolamine (anandamide) and N-docosahexaenoylethanolamine in bovine retina. Arch. Biochem. Biophysics. 1999;370:300–307. [PubMed]
• Bisogno T, Ligresti A, Di Marzo V. The endocannabinoid signalling system: Biochemical aspects. Pharmacol. Biochem. Behav.2005;81:224–238. [PubMed]
• Bisogno T, Melck D, Bobrov MY, Gretskaya NM, Bezuglov VV, De Petrocellis L, Di Marzo V. N-acyl-dopamines: novel synthetic CB(1) cannabinoid-receptor ligands and inhibitors of anandamide inactivation with cannabimimetic activity in vitro and in vivo.Biochem. J. 2000;351:817–824. [PMC free article] [PubMed]
• Biswas S, Bhattacherjee P, Paterson CA. Prostaglandin E2 receptor subtypes, EP1, EP2, EP3 and EP4 in human and mouse ocular tissues–a comparative immunohistochemical study. Prostaglandins Leukot. Essent. Fatty Acids. 2004;71:277–288. [PubMed]
• Bonhaus DW, Chang LK, Kwan J, Martin GR. Dual activation and inhibition of adenylyl cyclase by cannabinoid receptor agonists: Evidence for agonist-specific trafficking of intracellular responses. J. Pharmacol. Exp. Ther. 1998;287:884–888. [PubMed]
• Bortolato M, Mangieri RA, Fu J, Kim JH, Arguello O, Duranti A, Tontini A, Mor M, Tarzia G, Piomelli D. Antidepressant-like Activity of the Fatty Acid Amide Hydrolase Inhibitor URB597 in a Rat Model of Chronic Mild Stress. Biol Psychiatry. 2007;62:1103–1110.[PubMed]
• Bradshaw HB, Walker JM. The expanding field of cannabimimetic and related lipid mediators. Br. J. Pharmacol. 2005;144:459–465.[PMC free article] [PubMed]
• Brandon C, Lam DMK. L-glutamic acid: A neurotransmitter candidate for cone photoreceptors in human and rat retinas. Proc. Nat. Acad. Sci. USA. 1983;80:5117–5121. [PMC free article][PubMed]
• Breivogel CS, Griffin G, DiMarzo V, Martin BR. Evidence for a new G protein-coupled cannabinoid receptor in mouse brain. Mol. Pharmacol. 2001;60:155–163. [PubMed]
• Buckley NE, Hansson S, Harta G, Mezey É. Expression of the CB1 and CB2 receptor messenger RNAs during embryonic development in the rat. Neuroscience. 1998;82:1131–1149. [PubMed]
• Burnstein S. PPAR-γ: A nuclear receptor with affinity for cannabinoids. Life Sci. 2005;77:1674–1684. [PubMed]
• Cabral GA, Raborn ES, Griffin L, Dennis J, Marciano-Cabral F. CB2 receptors in the brain: role in central immune function. Br. J. Pharmacol. 2008;153:240–251. [PMC free article] [PubMed]
• Calandra B, Porta G, Kerim S, Delpech M, Carillon C, Lazennec C, Ferrara P, Shiono H. Dual intracellular signaling pathways mediated by the human cannabinoid CB₁ receptor. Eur. J. Pharmacol.1999;374:445–455. [PubMed]
• Carlin AS, Bakker CB, Halpern L, Post RD. Social facilitation of marijuana intoxication: impact of social set and pharmacological activity. J. of Abnormal Psych. 1972;80:132–140. [PubMed]
• Chaperon F, Thiébot MH. Behavioral effects of cannabinoid agents in animals. Crit. Rev. in Neurobiol. 1999;13:243–281. [PubMed]
• Chen J, Matias I, Dinh T, Lu T, Venezia S, Nieves A, Woodward DF, Di, Marzo V. Finding of endocannabinoids in human eye tissues: implications for glaucoma. Biochem. Biophys. Res Commun.2005;330:1062–1067. [PubMed]
• Chien FY, Wang RF, Mittag TW, Podos SM. Effect of WIN 55212-2, a cannabinoid receptor agonist, on aqueous humor dynamics in monkeys. Arch. of Ophthalmol. 2003;121:87–90. [PubMed]
• Chin MS, Nagineni CN, Hooper LC, Detrick B, Hooks JJ. Cyclooxygenase-2 gene expression and regulation in human retinal pigment epithelial cells. Invest. Ophthalmol. Vis. Sci. 2001;42:2338–2346. [PubMed]
• Consroe P, Musty R, Rein J, Tillery W, Pertwee RG. The perceived effects of smoked cannabis on patients with multiple sclerosis. Eur. Neurol. 1997;38:44–48. [PubMed]
• Crandall J, Matragoon S, Khalifa YM, Borlongan C, Tsai NT, Caldwell RB, Liou GI. Neuroprotective and intraocular pressure-lowering effects of (−)Delta9-tetrahydrocannabinol in a rat model of glaucoma. Ophthalmic Res. 2007;39:69–75. [PubMed]
• Cravatt BF, Demarest K, Patricelli MP, Bracey MH, Giang DK, Martin BR, Lichtman AH. Supersensitivity to anandamide and enhanced cannabinoid signaling in mice lacking fatty acid amide hydrolase. Proc. Nat. Acad. Sci. USA. 2001;98:9371–9376.[PMC free article] [PubMed]
• Cravatt BF, Saghatelian A, Hawkins EG, Clement AB, Bracey MH, Lichtman AH. Functional dissociation of the central and peripheral fatty acid amide signaling systems. Proc. Nat. Acad. Sci. USA.2004;101:10821–10826. [PMC free article] [PubMed]
• Cristino L, Starowicz K, De Petrocellis L, Morishita J, Ueda N, Guglielmotti V, Di Marzo V. Immunohistochemical localization of anabolic and catabolic enzymes for anandamide and other putative endovanilloids in the hippocampus and cerebellar cortex of the mouse brain. Neuroscience. 2008;151:955–968. [PubMed]
• Dacey DM. Wide-spreading terminal axons in the inner plexiform layer of the cat’s retina: evidence for intrinsic axon collaterals of ganglion cells. J. Comp. Neurol. 1985;242:247–262. [PubMed]
• Dainese E, Oddi S, Bari M, Maccarrone M. Modulation of the endocannabinoid system by lipid rafts. Curr. Med. Chem.2007;14:2702–2715. [PubMed]
• Dawson WW, Jimenez-Antillon CF, Perez JM, Zeskind JA. Marijuana and vision – after ten years’ use in Costa Rica. Invest. Ophthalmol. Vis. Sci. 1977;16:689–699. [PubMed]
• de Lago E, Fernández-Ruiz J. Cannabinoids and neuroprotection in motor-related disorders. CNS Neurol. Disorders and Drug Targets.2007;6:377–387. [PubMed]
• De Petrocellis L, Bisogno T, Maccarrone M, Davis JB, Finazzi-Agro A, DiMarzo V. The activity of anandamide at vanilloid VR1 receptors requires facilitated transport across the cell membrane and is limited by intracellular metabolism. J. Biol. Chem. 2001;276:12856–12863.[PubMed]
• Dearry A, Burnside B. Dopaminergic regulation of cone retinomotor movement in isolated teleost retinas: I. Induction of cone contraction is mediated by D2 receptors. J. Neurochem.1986;46:1006–1021. [PubMed]
• Deutsch DG, Chin SA. Enzymatic synthesis and degradation of anandamide, a cannabinoid receptor agonist. Biochem. Pharmac.1993;46:791–796. [PubMed]
• Deutsch DG, Omeir R, Arreaza G, Salehani D, Prestwich GD, Huang Z, Howlett AC. Methyl arachidonyl fluorophosphonate: a potent irreversibe inhibitor of anandamide amidase. Biochem. Pharmac.1997;53:255–260. [PubMed]
• Devane WA, Dysarz FAI, Johnson MR, Melvin LS, Howlett AC. Determination and characterization of a cannabinoid receptor in rat brain. Mol. Pharmacol. 1988;34:605–613. [PubMed]
• Devane WA, Hanus L, Breuer A, Pertwee RG, Stevenson LS, Griffin G, Gibson D, Mandelbaum A, Etinger A, Mechoulam R. Isolation and structure of a brain constituent that binds to the cannabinoid receptor. Science. 1992;258:1946–1949. [PubMed]
• Diana MA, Marty A. Endocannabinoid-mediated short-term synaptic plasticity: depolarization-induced suppression of inhibition (DSI) and depolarization-induced suppression of excitation (DSE).Br. J. Pharmacol. 2004;142:9–19. [PMC free article] [PubMed]
• Diaz-Laviada I, Ruiz-Llorente L. Signal transduction activated by cannabinoid receptors. Mini Rev. Med. Chem. 2005;5:619–630.[PubMed]
• Egertova M, Cravatt BF, Elphick MR. Comparative analysis of fatty acid amide hydrolase and cb(1) cannabinoid receptor expression in the mouse brain: evidence of a widespread role for fatty acid amide hydrolase in regulation of endocannabinoid signaling. Neuroscience.2003;119:481–496. [PubMed]
• Egertová M, Elphick MR. Localisation of cannabinoid receptors in the rat brain using antibodies to the intracellular C-terminal tail of CB1. J. Comp. Neurol. 2000;422:159–171. [PubMed]
• Egertová M, Giang DK, Cravatt BF, Elphick MR. A new perspective on cannabinoid signalling: complementary localization of fatty acid amide hydrolase and the CB1 receptor in rat brain. Proc. Roy. Soc. Lond. B:Biol Sci. 1998;265:2081–2085. [PMC free article] [PubMed]
• Ehinger B. [³H]-D-aspartate accumulation in the retina of pigeon, guinea pig and rabbit. Exp. Eye Res. 1981;33:381–391. [PubMed]
• El Remessy AB, Khalil IE, Matragoon S, Abou-Mohamed G, Tsai NJ, Roon P, Caldwell RB, Caldwell RW, Green K, Liou GI. Neuroprotective effect of delta(9)-tetrahydrocannabinol and cannabidiol in N-methyl-d-aspartate-induced retinal neurotoxicity: involvement of peroxynitrite. Am. J Pathol. 2003;163:1997–2008.[PMC free article] [PubMed]
• Emrich HM, Weber MM, Wendl A, Zihl J, Von Meyer L, Hanish W. Reduced binocular depth inversion as an indicator of cannabis-induced censorship impairment. Pharm. Biochem. & Behav.1991;40:689–690. [PubMed]
• Fan SF, Yazulla S. Modulation of voltage-gated K⁺ currents (I_K(v)) in retinal bipolar cells by ascorbate is mediated by dopamine D1 receptors. Visual Neurosci. 1999;16:928–931. [PubMed]
• Fan SF, Yazulla S. Biphasic modulation of voltage-dependent currents of retinal cones by cannabinoid CB1 agonist, WIN 55212−2.Visual Neurosci. 2003;20:177–188. [PubMed]
• Fan SF, Yazulla S. Inhibitory interaction of cannabinoid CB1 receptor and dopamine D2 receptor agonists on voltage-gated currents of goldfish cones. Visual Neurosci. 2004;21:69–79.[PubMed]
• Fan SF, Yazulla S. Reciprocal inhibition of voltage-gated potassium currents (I K(V)) by activation of cannabinoid CB1 and dopamine D1 receptors in ON bipolar cells of goldfish retina. Visual Neurosci.2005;22:55–63. [PubMed]
• Fan SF, Yazulla S. Retrograde endocannabinoid inhibition of goldfish retinal cones in mediated by 2-arachidonoyl glycerol. Visual Neurosci. 2007;24:257–267. [PubMed]
• Felder CC, Glass M. Cannabinoid receptors and their endogenous agonists. Annu. Rev. Pharmacol. Toxicol. 1998;38:179–200.[PubMed]
• Fernández-Ruiz J, Gómez M, Hernández M, de Miguel R, Ramos JA. Cannabinoids and gene expression during brain development.Neurotox. Res. 2004;6:389–401. [PubMed]
• Fernández-Ruiz JJ, Berrendero F, Hernández ML, Romero J, Ramos JA. Role of endocannabinoids in brain development. Life Sci.1999;65:725–736. [PubMed]
• Flom MC, Brown B, Adams AJ, Jones RT. Alcohol and marijuana effects on ocular tracking. Amercian J. Optom. Physiol. Opt.2000;53:764–773. [PubMed]
• Fowler CJ. The contribution of cyclooxgenase-2 to endocannabinoid metabolism and action. Br. J. Pharmacol. 2007;152:594–607.[PMC free article] [PubMed]
• Fowler CJ, Holt S, Nilsson O, Jonsson KO, Tiger G, Jacobsson SO. The endocannabinoid signaling system: Pharmacological and therapeutic aspects. Pharmacol. Biochem. Behav. 2005;81:248–262.[PubMed]
• Fride E. Multiple roles for the endocannabinoid system during the earliest stages of life: pre- and postnatal development. J. Neuroendocrin. 2008;20(Suppl 1):75–81. [PubMed]
• Fried PA. The Ottawa Prenatal Prospective Study (OPPS): methodological issues and findings – It’s easy to throw the baby out with the bath. Life Sci. 1995;56:2159–2168. [PubMed]
• Fukudome Y, Ohno-Shosaku T, Matsui M, Omori Y, Fukaya M, Tsubokawa H, Taketo MM, Watanabe M, Manabe T, Kano M. Two distinct classes of muscarinic action on hippocampal inhibitory synapses: M₂-mediated direct suppression and M₁M₃-mediated indirect suppression through endocannabinoid signaling. Eur. J. Neurosci. 2004;19:2682–2692. [PubMed]
• Funakoshi K, Nakano M, Atobe Y, goris RC, Kadota T, Yazama F. Differential development of TRPV1-expressing sensory nerves in peripheral organs. Cell Tiss. Res. 2005:1–15. [PubMed]
• Galiegue S, Mary S, Marchand J, Dussossoy D, Carriere D, Carayon P, Bouaboula M, Shire D, Le Fur G, Casellas P. Expression of central and peripheral cannabinoid receptors in human immune tissues and leukocyte subpopulations. Eur. J. Biochem. 1995;232:54–61.[PubMed]
• Gallego A, Cruz J. Mammalian retina: Associational nerve cells in ganglion cell layer. Science. 1965;150:1313–1314. [PubMed]
• Galve-Roperh I, Aguado T, Rueda D, Velasco G, Guzmán M. Endocannabinoids: a new family of lipid mediators involved in the regulation of neural cell development. Curr. Pharm. Des.2006;12:2319–2325. [PubMed]
• Gaoni Y, Mechoulam R. Isolation, structure and partial synthesis of an active constituent of hashish. J. Am. Chem. Soc. 1964;86:1646–1647.
• Gawienowski AM, Chatterjee D, Anderson PJ, Epstein DL, Grant WM. Effect of delta 9-tetrahydrocannabinol on monoamine oxidase activity in bovine eye tissues, in vitro. Invest Ophthalmol. Vis. Sci.1982;22:482–485. [PubMed]
• Gilbert GL, Kim HJ, Waataja JJ, Thayer SA. Delta9-tetrahydrocannabinol protects hippocampal neurons from excitotoxicity. Brain Res. 2007;1128:61–69. [PubMed]
• Glaser ST, Deutsch DD, Studholme KM, Zimov S, Yazulla S. Endocannabinoids in the intact retina: ³H-anandamide uptake, fatty acid amide hydrolase immunoreactivity and hydrolysis of anandamide. Visual Neurosci. 2005;22:693–705. [PubMed]
• Glaser ST, Kaczocha M, Deutsch DG. Anadamide transport: a critical review. Life Sci. 2005;77:1584–1604. [PubMed]
• Glass M, Felder CC. Concurrent stimulation of cannabinoid CB1 and dopamine D2 receptors augments cAMP accumulation in striatal neurons: Evidence for a G_s linkage to the CB1 receptor. J. Neurosci.1997;17:5327–5333. [PubMed]
• Goparaju SK, Ueda N, Taniguchi K, Yamamoto S. Enzymes of porcine brain hydrolyzing 2-arachidonoylglycerol, an endogenous ligand of cannabinoid receptors. Biochem. Pharmac. 1999;57:417–423. [PubMed]
• Goparaju SK, Ueda N, Yamaguchi H, Yamamoto S. Anandamide amidohydrolase reacting with 2-arachidonoylglycerol, another cannabinoid receptor ligand. FEBS Lett. 1998;422:69–73. [PubMed]
• Grant GB, Dowling JE. On bipolar cell responses in the teleost retina are generated by two distinct mechanisms. J. Neurophysiol.1996;76:3842–3849. [PubMed]
• Green K. The ocular effects of cannabinoids. Curr. Top. Eye Res.1979;1:175–215. [PubMed]
• Gulyas AI, Cravatt BF, Bracey MH, Dinh TP, Piomelli D, Boscia F, Freund TF. Segregation of two endocannabinoid-hydrolyzing enzymes into pre- and postsynaptic compartments in the rat hippocampus, cerebellum and amygdala. Eur. J. Neurosci.2004;20:441–458. [PubMed]
• Harsanyi K, Mangel SC. Modulation of cone to horizontal cell transmission by calcium and pH in the fish retina. Visual Neurosci.1993;10:81–91. [PubMed]
• Hartwick AT, Lalonde MR, Barnes S, Baldridge WH. Adenosine A1-receptor modulation of glutamate-induced calcium influx in rat retinal ganglion cells. Invest. Ophthalmol. Vis. Sci. 2004;45:3740–3748. [PubMed]
• Heidelberger R, Matthews G. Dopamine enhances Ca²⁺ responses in synaptic terminals of retinal bipolar neurons. Neuroreport.1994;5:729–732. [PubMed]
• Herkenham M, Lynn AB, Ross Johnson MR, Melvin LS, de Costa BR, Rice KC. Characterization and localization of cannabinoid receptors in rat brain: A quantitative in vitro autoradiographic study. J. Neurosci. 1991;11:563–583. [PubMed]
• Hermann A, Kaczocha M, Deutsch DG. 2-Arachidonoylglycerol (2-AG) membrane transport: history and outlook. AAPS. J.2006;8:E409–E412. [PMC free article] [PubMed]
• Hewett SJ, Uliasz TF, Vidwans AS, Hewett JA. Cyclooxygenase-2 contributes to N-methy-D-aspartate-mediated neuronal cell death in primary cortical cell culture. J. Pharm. & Exp. Therap.2000;293:417–425. [PubMed]
• Hill EI, Gallopin T, Férézou I, Cauli B, Rossier J, Schweitzer P, Lambolez B. Functional CB1 receptors are broadly expressed in neocortical GABAergic and glutamatergic neurons. J. Neurophysiol.2007;97:2580–2589. [PubMed]
• Hillard CJ, Jarrahian A. Cellular accumulation of anandamide: consensus and controversy. Br. J. Pharmacol. 2003;140:802–808.[PMC free article] [PubMed]
• Hohmann AG, Suplita RL. Endocannabinoid mechanisms of pain modulation. AAPS Journal. 2006;8:E693–E708. [PMC free article][PubMed]
• Hollister LE. Marihuana in man: Three years later. Science.1971;172:21–29. [PubMed]
• Howlett AC, Breivogel CS, Childers SR, Deadwyler SA, Hampson RE, Porrino LJ. Cannabinoid physiology and pharmacology: 30 years of progress. Neuropharm. 2004;47(Suppl 1):345–358. [PubMed]
• Hwang SW, Cho H, Kwak J, Lee SY, Kang CJ, Jung J, Cho S, Min KH, Suh YG, Kim D, Oh U. Direct activation of capsaicin receptors by products of lipoxygenases: endogenous capsaicin-like substances.Proc. Nat. Acad. Sci. USA. 2000;97:6155–6160. [PMC free article][PubMed]
• Iribane M, Tordidoni V, Julian K, Prestifilippo JP, Sinha D, Rettori V, Berra A, Suburo AM. Cannabinoid receptors in conjunctival epithelium: identification and functional properties. Invest. Ophthalmol. Vis. Sci. 2008 June 19; [Epub ahead of print]
• Iversen L. Cannabis and the brain. Brain. 2003;126:1252–1270.[PubMed]
• Iversen LL. The Science of Marijuana. Oxford University Press; New York: 2000.
• Järvinen T, Pate DW, Laine K. Cannabinoids in the treatment of glaucoma.. Pharm. & Therapeutics. 2002;95:203–220. [PubMed]
• Jayamanne A, Greenwood R, Mitchell VA, Aslan S, Piomelli D, Vaughan CW. Actions of the FAAH inhibitor URB597 in neuropathic and inflammatory chronic pain models. Br. J Pharmacol. 2006;147:281–288. [PMC free article] [PubMed]
• Ju WK, Kim KY, Neufeld AH. Increased activity of cyclooxygenase-2 signals early neurodegenerative events in the rat retina following transient ischemia. Exp. Eye Res. 2003;77:137–145. [PubMed]
• Jung J, Hwang SW, Kwak J, Lee SY, Kang CJ, Kim WB, Kim D, Oh U. Capsaicin binds to the intracellular domain of the capsaicin-activated ion channel. J. Neurosci. 1999;19:529–538. [PubMed]
• Jung J, Lee SY, Hwang SW, Cho H, Shin J, Kang YS, Kim S, Oh U. Agonist recognition sites in the cytosolic tails of vanilloid receptor 1.J. Biochem. (Tokyo) 2002;46:44448–44454. [PubMed]
• Kamermans M, Fahrenfort I, Schultz K, Janssen-Bienhold U, Sjoerdsma T, Weiler R. Hemichannel-mediated inhibition in the outer retina. Science. 2001;292:1178–1180. [PubMed]
• Kaplan-Messas A, Naveh N, Avni I, Marshall J. Ocular hypotensive effects of cholinergic and adrenergic drugs may be influenced by prostaglandins E2 in the human and rabbit eye. Eur. J Ophthalmol.2003;13:18–23. [PubMed]
• Kárai L, Russell JT, Iadarola MJ, Oláh Z. Vanilloid receptor 1 regulates multiple calcium compartments and contributes to Ca^2+-induced Ca²⁺-release in sensory neurons. J. Biol. Chem.2004;279:16377–16387. [PubMed]
• Karanian DA, Brown QB, Makriyannis A, Kosten TA, Bahr BA. Dual modulation of endocannabinoid transport and fatty acid amide hydrolase protects against excitotoxicity. J Neurosci. 2005;25:7813–7820. [PubMed]
• Katona I, Urban GM, Wallace M, Ledent C, Jung KM, Piomelli D, Mackie K, Freund TF. Molecular composition of the endocannabinoid system at glutamatergic synapses. J Neurosci.2006;26:5628–5637. [PMC free article] [PubMed]
• Kaufmann WE, Worley PF, Pegg J, Bremer M, Isakson P. COX-2, a synaptically induced enzyme, is expressed by excitatory neurons at postsynaptic sites in rat cerebral cortex. Proc. Nat. Acad. Sci. USA.1996;93:2317–2321. [PMC free article] [PubMed]
• Kim J, Alger BE. Inhibition of cyclooxygenase-2 potentiates retrograde endocannabinoid effects in hippocampus. Nat. Neurosci.2004;7:697–698. [PubMed]
• Kiplinger GF, Manno JE, Rodda BE, Forney RB. Dose-response analysis of the effects of tetrahydrocannabinol in man. Clin. Pharm. & Therapeutics. 1971;12:650–657. [PubMed]
• Klooster J, Studholme KM, Yazulla S. Localization of the AMPA subunit GluR2 in the outer plexiform layer of goldfish retina. J. Comp. Neurol. 2001;441:155–167. [PubMed]
• Kogan NM, Mechoulam R. Cannabinoids in health and disease.Dialog. Clin. Neurosci. 2007;9:413–430. [PMC free article][PubMed]
• Kourennyi DE, Liu X, Hart J, Mahmud F, Baldridge WH, Barnes S. Reciprocal modulation of calcium dynamics at rod and cone photoreceptor synapses by nitric oxide. J. Neurophysiol.2004;92:477–483. [PubMed]
• Kozak KR, Rowlinson SW, Marnett LJ. Oxygenation of the endocannabinoid, 2-Arachidonylglycerol, to glyceryl prostaglandins by cyclooxygenase-2. J. Biol. Chem. 2000;275:33744–33749.[PubMed]
• Kreitzer AC, Regehr WG. Retrograde inhibition of presynaptic calcium influx by endogenous cannabinoids at excitatory synapses onto Purkinje cells. Neuron. 2001;29:717–727. [PubMed]
• Kurreny DE, Moroz LL, Turner RW, Sharkey KA, Barnes S. Modulation of ion channels in rod photoreceptors by nitric oxide.Neuron. 1994;13:315–324. [PubMed]
• Ladanyi M, Beaudet A. In-vivo labeling of (3H)D-aspartate uptake sites in monkey retina. Cell Tissue Res. 1986;243:59–63. [PubMed]
• Lambert DM, Fowler CJ. The endocannabinoid system: drug targets, lead compunds, and potential therapeutic applications. J. Med. Chem. 2005;48:5059–5087. [PubMed]
• Lanonde MR, Jollimore CA, Stevens K, Barnes S, Kelly ME. Cannabinoid receptor-mediated inhibition of calcium signallin in rat retinal ganglion cells. Mol. Vis. 2006;12:1160–1166. [PubMed]
• Lauckner JE, Hille B, Mackie K. The cannabinoid agonist WIN55,212−2 increases intracellular calcium via CB1 receptor coupling to Gq/11 G proteins. Proc. Nat. Acad. Sci. USA.2005;102:19149. [PMC free article] [PubMed]
• LaVail MM, Sidman RL. C57BL/6J mice with inherited retinal degeneration. Arch. of Ophthalmol. 1974;91:394–400. [PubMed]
• Ledent C, Valverde O, Cossu G, Petitet F, Aubert JF, Beslot F, Bohme GA, Imperato A, Pedrazzini T, Roques BP, Vassart G, Fratta W, Parmentier M. Unresponsiveness to cannabinoids and reduced addictive effects of opiates in CB1 receptor knockout mice. Science.1999;283:401–404. [PubMed]
• Leonelli M, Britto LRG, Chaves GP, Torrã AS. Developmental expression of cannabinoid receptors in the chick retinotectal system.Dev. Brain Res. 2005;156:176–182. [PubMed]
• Leweke FM, Schneider U, Thies M, Emrich HM. Effects of synthetic 9-tetrahydrocannbinoi on binocular depth inversion in man.Psychopharmacology (Berl) 1999;142:230–235. [PubMed]
• Liang Y, Li C, Guzman VM, Chang WW, Evinger AJ, Pablo JV, Woodward DF. Upregulation of orphan nuclear receptor Nur77 following PGF(2alpha), Bimatoprost, and Butaprost treatments. Essential role of a protein kinase C pathway involved in EP(2) receptor activated Nur77 gene transcription. Br. J Pharmacol.2004;142:737–748. [PMC free article] [PubMed]
• Liang Y, Li C, Guzman VM, Evinger AJ, III, Protzman CE, Krauss AH, Woodward DF. Comparison of prostaglandin F2alpha, bimatoprost (prostamide), and butaprost (EP2 agonist) on Cyr61 and connective tissue growth factor gene expression. J Biol Chem.2003;278:27267–27277. [PubMed]
• Lichtman AH, Shelton CC, Advani T, Cravatt BF. Mice lacking fatty acid amide hydrolase exhibit a cannabinoid receptor-mediated phenotypic hypoalgesia. Pain. 2004;109:319–327. [PubMed]
• Liu D, Wu L, Breyer R, Mattson MP, Andreasson K. Neuroprotection by the PGE2 EP2 receptor in permanent focal cerebral ischemia.Annals Neurol. 2005;57:748–761. [PubMed]
• Liu J, Wang L, Harvey-White J, Huand BX, Kim H-Y, Luquet S, Palmiter RD, Krystal G, Rai R, Mahadevan A, Razdan RK, Kunos G. Multiple pathways involved in the biosynthesis of anadamide.Neuropharm. 2008;54:1–7. [PMC free article] [PubMed]
• Liu M, Liu MC, Magoulas C, Priestley JV, Willmott NJ. Versatile regulation of cytosolic Ca²⁺ by vanilloid receptor I in rat dorsal root ganglion neurons. J. Cell Biol. 2003;278:5462–5472. [PubMed]
• Lograno MD, Romano MR. Cannabinoid agonists induce contractile responses through Gi/o-dependent activation of phospholipase C in the bovine ciliary muscle. Eur. J. Pharmacol. 2004;494:55–62.[PubMed]
• Lu QJ, Straiker A, Lu QX, Maguire G. Expression of CB2 cannabinoid receptor mRNA in adult rat retina. Visual Neurosci.2000;17:91–95. [PubMed]
• Mackie K. Cannabinoid receptors as therapeutic targets. Annu. Rev. Pharmacol. Toxicol. 2006;46:101–122. [PubMed]
• Maejima T, Hashimoto K, Yoshida T, Aiba A, Kano M. Presynaptic inhibition caused by retrograde signal from metabotropic glutamate to cannabinoid receptors. Neuron. 2001;31:463–475. [PubMed]
• Maejima T, Oka S, Hashimotodani Y, Ohno-Shosaku T, Aiba A, Wu D, Waku K, Sugiura T, Kano M. Synaptically driven endocannabinoid release requires Ca2+-assisted metabotropic glutamate receptor subtype 1 to phospholipase Cbeta4 signaling cascade in the cerebellum. J. Neurosci. 2005;25:6826–6835.[PubMed]
• Maihofner C, Schlotzer-Schrehardt U, Guhring H, Zeilhofer HU, Naumann GO, Pahl A, Mardin C, Tamm ER, Brune K. Expression of cyclooxygenase-1 and -2 in normal and glaucomatous human eyes.Invest Ophthalmol. Vis. Sci. 2001;42:2616–2624. [PubMed]
• Mailleux P, Vanderhaeghen JJ. Distribution of neuronal cannabinoid receptor in the adult rat brain: A comparative receptor binding radioautography and in situ hybridization histochemistry.Neuroscience. 1992;48:655–668. [PubMed]
• Maneuf YP, Brotchie JM. Paradoxical action of the cannabinoid WIN 55,212−2 in stimulated and basal cyclic AMP accumulation in rat globus pallidus slices. Br. J. Pharmacol. 1997;120:1397–1398.[PMC free article] [PubMed]
• Marc RE, Lam DMK. Uptake of aspartic and glutamic acid by photoreceptors in goldfish retina. Proc. Nat. Acad. Sci. USA.1981;78:7185–7189. [PMC free article] [PubMed]
• Marc RE, Murry RF, Basinger SF. Pattern recognition of amino acid signatures in retinal neurons. J. Neurosci. 1995;15:5106–5129.[PubMed]
• Marsicano G, Lutz B. Expression of the cannabinoid receptor CB1 in distinct neuronal subpopulations in the adult mouse forebrain. Eur. J. Neurosci. 1999;11:4213–4225. [PubMed]
• Marsicano G, Moosmann B, Hermann H, Lutz B, Behl C. Neuroprotective properties of cannabinoids against oxidative stress: role of the cannabinoid receptor CB1. J. Neurochem. 2002;80:448–456. [PubMed]
• Martínez-Orgado J, Fernádez-López D, Lizasoain I, Romero J. The seek of neuroprotection: introducing cannabinoids. Rec. Patents CNS Drug Discov. 2007;2:131–139. [PubMed]
• Matias I, Wang JW, Moriello AS, Nieves A, Woodward DF, Di,Marzo V. Changes in endocannabinoid and palmitoylethanolamide levels in eye tissues of patients with diabetic retinopathy and age-related macular degeneration. Prostaglandins Leukot. Essent. Fatty Acids.2006;75:413–418. [PubMed]
• Matsuda LA, Lolait SJ, Brownstein MJ, Young AC, Bolnier TI. Structure of a cannabinoid receptor and functional expression of the cloned cDNA. Nature. 1990;346:561–564. [PubMed]
• Matsuda S, Kanemitsu N, Nakamura A, Mimura Y, Ueda N, Kurahashi Y, Yamamoto S. Metabolism of anandamide, an endogenous cannabinoid receptor ligand, in porcine ocular tissues.Exp. Eye Res. 1997;64:707–711. [PubMed]
• McAllister SD, Glass M. CB1 and CB2 receptor-mediated signalling: a focus on endocannabinoids. Prostaglandins Leukot. Essent. Fatty Acids. 2002;66:161–171. [PubMed]
• McCullough L, Wu L, Haughey N, Liang X, Hand T, Wang Q, Breyer RM, Andreasson K. Neuroprotective function of the PGE2 EP2 receptor in cerebral ischemia. J. Neurosci. 2004;24:257–268.[PubMed]
• McIntosh BT, Hudson B, Yegorova S, Jollimore CA, Kelly ME. Agonist-dependent cannabinoid signalling in human trabecular meshwork cells. Br. J. Pharmacol. 2007;152:1111–1120.[PMC free article] [PubMed]
• McIntosh HH, Song C, Howlett AC. CB₁ cannabinoid receptor: cellular regulation and distribution in N18TG2 neuroblastoma cells.Mol. Brain Res. 1998;53:163–173. [PubMed]
• McKinney MK, Cravatt BF. Structure and function of Fatty Acid amide hydrolase. Annu. Rev. Biochem. 2005;74:411–432. [PubMed]
• Mechoulam R. Discovery of endocannabinoids and some random thoughts on their possible roles in neuroprotection and aggression.Prostaglandins Leukot. Essent. Fatty Acids. 2002;66:93–99.[PubMed]
• Mechoulam R, Ben-Shabat S, Hanus L, Ligumdky M, Kaminski NE, Shatz AR, Gopher A, Almog S, Martin BR, Compton DR. Identification of an endogenous 2-monoglyceride, present in canine gut, that binds to cannabinoid receptors. Biochem. Pharmac.1995;50:83–90. [PubMed]
• Mechoulam R, Gaoni Y. The absolute configuration of D¹-tetrahydrocannabinol, the major active constituent of hashish.Tetrahedron Lett. 1967:1109–1111. [PubMed]
• Mechoulam R, Shohami E. Endocannabinoids and traumatic brain injury. Mol. Neurobiol. 2007;36:68–74. [PubMed]
• Micale V, Massola C, Drago F. Endocannabinoids and neurodegenerative diseases. Pharmacol. Res. 2007;56:382–392.[PubMed]
• Millns PJ, Chimenti M, Ali N, Ryland E, de Lago E, Fernandez-Ruiz J, Chapman V, Kendall DA. Effects of inhibition of fatty acid amide hydrolase vs. the anandamide membrane transporter on TRPV1-mediated calcium responses in adult DRG neurons; the role of CB₁receptors. Eur. J. Neurosci. 2006;24:3489–3495. [PubMed]
• Mora-Ferrer C, Yazulla S, Studholme KM, Haak-Frendscho M. Dopamine D1-receptor immunolocalization in goldfish retina. J. Comp. Neurol. 1999;411:704–714. [PubMed]
• Munro S, Thomas KL, Abu-Shaar M. Molecular characterization of a peripheral receptor for cannabinoids. Nature. 1993;365:61–65.[PubMed]
• Nagayama T, Sinor AD, Simon RP, Chen J, Graham SH, Jin KL, Greenberg DA. Cannabinoids and neuroprotection in global and focal cerebral ischemia and in neuronal cultures. J. Neurosci.1999;19:2987–2995. [PubMed]
• Nakamichi N, Chidlow G, Osborne NN. Effects of intraocular injection of a low concentration of zinc on the rat retina.Neuropharm. 2003;45:637–648. [PubMed]
• Navarrete M, Araque A. Endocannabinoids mediate neuron-astrocyte communication. Neuron. 2008;57:883–893. [PubMed]
• Neal MJ, Cunningham JR, Matthews KL. Activation of nicotinic receptors on GABAergic amacrine cells in the rabbit retina indirectly stimulates dopamine release. Visual Neurosci. 2001;18:55–64.[PubMed]
• Nogawa S, Zhang F, Ross ME, Iadecola C. Cyclo-oxygenase-2 gene expression in neurons contributes to ischemic brain damage. J. Neurosci. 1997;17:2746–2755. [PubMed]
• Noyes R, Jr., Brunk SF, Avery DAH, Canter AC. The analgesic properties of delta-9-tetrahydrocannabinol and codeine. Clin. Pharm. & Therap. 1975;18:84–89. [PubMed]
• Nucci C, Gasperi V, Tartaglione R, Cerulli A, Terrinoni A, Bari M, De Simone C, Agrò AF, Morrone LA, Corasaniti MT, Bagetta G, Maccarrone M. Involvement of the endocannabinoid system in retinal damage after high intracellular pressure-induced ischemia in rats. Invest. Ophthalmol. Vis. Sci. 2007;48:2997–3004. [PubMed]
• Ohno-Shosaku T, Hashimotodani Y, Maejima T, Kano M. Calcium signaling and synaptic modulation: Regulation of endocannabinoid-mediated synaptic modulation by calcium. Cell Calcium.2005;38:369–374. [PubMed]
• Ohno-Shosaku T, Maejima T, Kano M. Endogenous cannabinoids mediate retrograde signals from depolarized postsynaptic neurons to presynaptic terminals. Neuron. 2001;29:729–738. [PubMed]
• Ohno-Shosaku T, Tsubokawa H, Mizushima I, Yoneda N, Zimmer A, Kano M. Presynaptic cannabinoid sensitivity is a major determinant of depolarization-induced retrograde suppression at hippocampal synapses. J. Neurosci. 2002;22:3864–3872. [PubMed]
• Oku H, Goto W, Kobayashi T, Okuno T, Hirao M, Sugiyama Y, Yoneda S, Hara H, Ikeda T. Adenosine protects cultured retinal neurons against NMDA-induced cell death through A1 receptors.Curr. Eye Res. 2004;29:449–455. [PubMed]
• Olah Z, Szabo T, Karai L, Hough C, Fields RD, Caudle RM, Blumberg PM, Iadarola MJ. Ligand-induced dynamic membrane changes and cell deletion conferred by vanilloid receptor 1. J. Biol. Chem. 2001;276:11021–11030. [PubMed]
• Onaivi ES, Sugiura T, Di Marzo V, editors. Endocannabinoids: The Brain and Body’s Marijuana and Beyond. CRC Press; Boca Raton: 2006.
• Opere CA, Zheng WD, Zhao M, Lee JS, Kulkarni KH, Ohia SE. Inhibition of Potassium- and Ischemia-Evoked [3H] D-Aspartate Release from Isolated Bovine Retina by Cannabinoids. Curr. Eye Res. 2006;31:645–653. [PubMed]
• Osborne NN, Casson RJ, Wood JP, Chidlow G, Graham M, Melana J. Retinal ischemia: mechanisms of damage and potential therapeutic strategies. Prog. Ret. Eye Res. 2004;23:91–147.[PubMed]
• Patrick G, Straumanis JJ, Struve FA, Nixon F, Fitz-Gerald MJ, Manno JE, Soucair M. Auditory and visual P300 event related potentials are not altered in medically and psychiatrically normal chronic marihuana users. Life Sci. 1995;56:2135–2140. [PubMed]
• Pazos MR, Núñez E, Benito C, Tólon RM, Romero J. Functional neuroanatomy of the endocannabinoid system. Pharmacol. Biochem. Behav. 2005;81:239–247. [PubMed]
• Pertwee RG. The therapeutic potential of drugs that target cannabinoid receptors or modulate the tissue levels or actions of endocannabinoids. AAPS Journal. 2005;7:E625–E654.[PMC free article] [PubMed]
• Pertwee RG. Cannabinoid pharmacology: the first 66 years. Br. J. Pharmacol. 2006;147(Suppl 1):S163–S171. [PMC free article][PubMed]
• Pertwee RG, Ross RA. Cannabinoid receptors and their ligands.Prostaglandins. Leukot. Essent. Fatty Acids. 2002;66:101–121.[PubMed]
• Pettit DA, Harrison MP, Olson JM, Spencer RF, Cabral GA. Immunohistochemical localization of the neural cannabinoid receptor in rat brain. J. Neurosci. Res. 1998;51:391–402. [PubMed]
• Picaud S, Pattnaik B, Hicks D, Forster V, Fontaine V, Sahel J, Dreyfus H. GABA_A and GABA_C receptors in adult porcine cones: evidence from a photoreceptor-glia co-culture model. J. Physiol.1998;513:33–42. [PMC free article] [PubMed]
• Piomelli D. The molecular logic of endocannabinoid signalling. Nat Rev Neurosci. 2003;4:873–884. [PubMed]
• Porcella A, Casellas P, Gessa GL, Pani L. Cannabinoid receptor CB₁mRNA is highly expressed in the rat ciliary body: implications for the antiglaucoma properties of marihuana. Mol. Brain Res.1998;58:240–245. [PubMed]
• Porcella A, Maxia C, Gessa GL, Pani L. The human eye expresses high levels of CB1 cannabinoid receptor mRNA and protein. Eur. J. Neurosci. 2000;12:1123–1127. [PubMed]
• Porcella A, Maxia C, Gessa GL, Pani L. The synthetic cannabinoid WIN55212−2 decreases the intraocular pressure in human glaucoma resistant to conventional therapies. Eur. J. Neurosci.2001;13:409–412. [PubMed]
• Quigley HA. Neuronal death in glaucoma. Prog. Ret. Eye Res.1999;18:39–57. [PubMed]
• Ramos JA, Gómez M, de Miguel R. Effects on development.Handbook of Exp. Pharmacol. 2005;168:643–656. [PubMed]
• Reese KM. Cannabis seems to improve night vision of fisherman.Chem. Engin. News. 1991;69:44.
• Rosch S, Ramer R, Brune K, Hinz B. Prostaglandin E(2) induces cyclooxygenase-2 expression in human non-pigmented ciliary epithelial cells through activation of p38 and p42/44 mitogen-activated pritein kinases. Biochem. Biophys. Res. Comm.2005;338:1171–1178. [PubMed]
• Rosch S, Ramer R, Brune K, Hinz B. R(+)-methanandamide and other cannabinoids induce the expression of cyclooxygenase-2 and matrix metalloproteinases in human non-pigmented ciliary epithelial cells. J. Pharmacol. Exp. Ther. 2006;316:1219–1228.[PubMed]
• Ross RA. Anandamide and vanilloid TRPV1 receptors. Br. J. Pharmacol. 2003;140:790–801. [PMC free article] [PubMed]
• Russo EB. Clinical Endocannabinoid Deficiency (CECD): Can this Concept Explain Therapeutic Benefits of Cannabis in Migraine, Fibromyalgia, Irritable Bowel Syndrome and other Treatment-Resistant Conditions? Neuroendocrinol. Lett. 2004;25:31–39.[PubMed]
• Russo EB, Merzouki A, Mesa JM, Frey KA, Bach PJ. Cannabis improves night vision: a case study of dark adaptometry and scotopic sensitivity in kif smokers of the Rif mountains of northern Morocco. J. Ethnopharmacol. 2004;93:99–104. [PubMed]
• Saario SM, Savinainen JR, Laitinen JT, Niemi R. Monoglyceride lipase-like enzymatic activity is responsible for hydrolysis of 2-arachidonoylglycerol in rat cerebellar membranes. Biochem. Pharmac. 2004;67:1381–1387. [PubMed]
• Sagar SM, Marshall PE. Somatostatin-like immunoreactive material in associational ganglion cells of human retina. Neuroscience.1988;27:507–516. [PubMed]
• Sang N, Zhang J, Chen C. COX-2 oxidative metabolite of endocannabinoid 2-AG enhances excitatory glutamatergic synaptic transmission and induces neurotoxicity. J. Neurochem.2007;102:1966–1977. [PubMed]
• Sang N, Zhang J, Marcheselli V, Bazan NG, Chen C. Postsynaptically synthesized prostaglandin E2 (PGE2) modulates hippocampal synaptic transmission via a presynaptic PGE2 EP2 receptor. J. Neurosci. 2005;25:9858–9870. [PubMed]
• Sarker KP, Maruyama I. Anandamide induces cell death independently of cannabinoid receptors or vanilloid receptor 1: possible involvement of lipid rafts. Cell. Mol. Life Sci. 2003;60:1200–1208. [PubMed]
• Savinainen JR, Laitinen JT. Detection of cannabinoid CB1, adenosine A1, muscarinic acetylcholine, and GABA(B) receptor-dependent G protein activity in transducin-deactivated membranes and autoradiography sections of rat retina. Cell Mol. Neurobiol.2004;24:243–256. [PubMed]
• Schlicker E, Timm J, Göthert M. Cannabinoid receptor-mediated inhibition of dopamine release in the retina. Naunyn-Schmiedebergs Arch. of Pharmacol. 1996;354:791–795. [PubMed]
• Schlotzer-Schrehardt U, Zenkel M, Nusing RM. Expression and localization of FP and EP prostanoid receptor subtypes in human ocular tissues. Invest Ophthalmol. Vis. Sci. 2002;43:1475–1487.[PubMed]
• Schultheiss T, Flau K, Kathmann M, Gothert M, Schlicker E. Cannabinoid CB(1) receptor-mediated inhibition of noradrenaline release in guinea-pig vessels, but not in rat and mouse aorta.Naunyn Schmiedebergs Arch. Pharmacol. 2005;372:139–146.[PubMed]
• Semple DM, Ramsden F, McIntosh AM. Reduced binocular depth inversion in regular cannabis users. Pharmacol. Biochem. Behav.2003;75:789–793. [PubMed]
• Sharif NA, Williams GW, Crider JY, Xu SX, Davis TL. Molecular pharmacology of the DP/EP2 class prostaglandin AL-6598 and quantitative autoradiographic visualization of DP and EP2 receptor sites in human eyes. J Ocul. Pharmacol. Ther. 2004;20:489–508.[PubMed]
• Shivachar AC. Cannabinoids inhibit sodium-dependent, high affinity excitatory amino acid transport in cultured rat cortical astrocytes.Biochem. Pharmac. 2007;73:2004–2011. [PubMed]
• Skosnik PD, Krishnan GP, Vohs JL, O’donnell BF. The effect of cannabis use and gender on the visual steady state evoked potential.Clin. Neurophysiol. 2006;117:144–156. [PubMed]
• Slanina KA, Schweitzer P. Inhibition of cyclooxygenase-2 elicits a CB1-mediated decrease of excitatory transmission in rat CA1 hippocampus. Neuropharm. 2005;49:653–659. [PubMed]
• Sommer C, Schomacher M, Berger C, Kuhner K, Müller HD, Schwab S, Schäbitz WR. Neuroprotective cannabinoid receptor antagonist SR141716A prevents downregulation of excitotoxic NMDA receptors in the ischemic penumbra. Acta Neuropatholgica. 2006;112:277–286. [PubMed]
• Song ZH, Slowey CA. Involvement of cannabinoid receptors in the intraocular pressure-lowering effects of WIN55212−2. J. Pharmacol. Exp. Ther. 2000;292:136–139. [PubMed]
• Stamer WD, Golightly SF, Hosohata Y, Ryan EP, Porter AC, Varga E, Noecker RJ, Felder CC, Yamamura HI. Cannabinoid CB(1) receptor expression, activation and detection of endogenous ligand in trabecular meshwork and ciliary process tissues. Eur. J. Pharmacol.2001;431:277–286. [PubMed]
• Stella N, Schweitzer P, Piomelli D. A second endogenous cannabinoid that modulates long-term potentiation. Nature.1997;388:773–778. [PubMed]
• Stella SL, Bryson EJ, Cadetti L, Thoreson WB. Endogenous adenosine reduces glutamatergic output from rods through activation of A(2)-like adenosine receptors. J. Neurophysiol.2003;90:165–174. [PubMed]
• Stella SL, Bryson EJ, Thoreson WB. A(2) adenosine receptors inhibit calcium influx through L-type calcium channels in rod photoreceptors of the salamander retina. J. Neurophysiol.2002;87:351–360. [PubMed]
• Straiker A, Mackie K. Cannabinoids, electrophysiology, and retrograde messengers: challenges for the next 5 years. AAPS Journal. 2006;8:E272–E276. [PMC free article] [PubMed]
• Straiker A, Sullivan JM. Cannabinoid receptor activation differentially modulates ion channels in photoreceptors of the tiger salamander. J. Neurophysiol. 2003;89:2647–2654. [PubMed]
• Straiker AJ, Maguire G, Mackie K, Lindsey J. Localization of cannabinoid CB1 receptors in the human anterior eye and retina.Invest. Ophthalmol. Vis. Sci. 1999;40:2442–2448. [PubMed]
• Struik M, Yazulla S, Kamermans M. Cannabinoid agonist WIN 55212−2 speeds up the cone light offset response in goldfish. Visual Neurosci. 2006;23:285–293. [PubMed]
• Stumpff F, Boxberger M, Krauss A, Rosenthal R, Meissner S, Choritz L, Wiederholt M, Thieme H. Stimulation of cannabinoid (CB1) and prostanoid (EP2) receptors opens BKCa channels and relaxes ocular trabecular meshwork. Exp. Eye Res. 2005;80:697–708. [PubMed]
• Sugiura T, Kondo S, Sukagawa A, Nakane S, Shinoda A, Itoh K, Yamashita A, Waku K. 2-Arachidonoylglycerol: a possible endogenous cannabinoid receptor ligand in brain. Biochem. Biophys. Res. Comm. 1995;215:89–97. [PubMed]
• Sulcova E, Mechoulam R, Fride E. Biphasic effects of anandamide.Pharmacol. Biochem. Behav. 1998;59:347–353. [PubMed]
• Sun YX, Tsuboi K, Okamoto Y, Tonai T, Murakami M, Kudo I, Ueda N. Biosynthesis of anandamide and N-palmitoylethanolamne by sequential actions of phospolipase A2 and phospholipase D.Biochem. J. 2004;380:749–756. [PMC free article] [PubMed]
• Szabo B, Dörner L, Pfreundtner C, Nörenberg W, Starke K. Inhibition of gabaergic inhibitory postsynaptic currents by cannabinoids in rat corpus striatum. Neuroscience. 1998;85:395–403. [PubMed]
• Szabo B, Urbanski MJ, Bisogno T, Di M, V, Mendiguren A, Bar W, Freiman I. Depolarization-induced retrograde synaptic inhibition in the cerebellar cortex is mediated by 2-arachidonoylglycerol. J. Physiol. 2006;577:262–280. [PMC free article] [PubMed]
• Szallasi A, Conte B, Goso C, Blumber PM, Manzini S. Vanilloid receptors in the urinary bladder: regional distribution, localization on sensory nerves, and species-related differences. Naunyn-Schmiedebergs Arch. Pharmacol. 1993;347:624–629. [PubMed]
• Tachibana M, Kaneko A. g-Aminobutyric acid acts at axon terminals of turtle photoreceptors: Difference in sensitivity among cell types.Proc. Nat. Acad. Sci. USA. 1984;81:7961–7964. [PMC free article][PubMed]
• Takadera T, Ohyashiki T. Prostaglandin E2 deteriorates N-methyl-D-aspartate receptor-mediated cytotoxicity possible by activating EP2 receptors in cultured cortical neurons. Life Sci. 2006;78:1878–1883. [PubMed]
• Takadera T, Yumoto H, Tozuka Y, Ohyashiki T. Prostaglandin E(2) induces caspase-dependent apoptosis in rat cortical cells. Neurosci. Lett. 2002;317:61–64. [PubMed]
• Thomas EA, Cravatt BF, Danielson PE, Gilula NB, Sutcliffe JG. Fatty acid amide hydrolase, the degradative enzyme for anandamide and oleamide, has selective distribution in neurons within the rat central nervous system. J. Neurosci. Res. 1997;50:1047–1052. [PubMed]
• Tomida I, Pertwee RG, Azuara-Blanco A. Cannabinoids and glaucoma. Br. J Ophthalmol. 2004;88:708–713. [PMC free article][PubMed]
• Toth A, Boczan J, Kedei N, Lizanecz E, Bagi Z, Papp Z, Edes I, Csiba L, Blumberg PM. Expression and distribution of vanilloid receptor 1 (TRPV1) in the adult rat brain. Brain Res. Mol. Brain Res.2005;135:162–168. [PubMed]
• Tsou K, Brown S, Sañudo-Peña MC, Mackie K, Walker JM. Immunohistochemical distribution of cannabinoid CB1 receptors in the rat central nervous system. Neuroscience. 1998;83:393–411.[PubMed]
• Tsou K, Noguerón I, Muthian S, Sañudo-Peña MC, Hillard CJ, Deutsch DG, Walker JM. Fatty acid amide hydrolase is located preferentially in large neurons in the rat central nervous system as revealed by immunohistochemistry. Neurosci. Lett. 1998;254:1–4.[PubMed]
• Valenti M, Cottone E, Martinez R, De Pedro N, Rubio M, Viveros MP, Franzoni MF, Delgado MJ, Di M, V The endocannabinoid system in the brain of Carassius auratus and its possible role in the control of food intake. J. Neurochem. 2005;95:662–672. [PubMed]
• Valenti M, Vigano D, Casico MG, Rubino T, Steardo L, Parolaro D, Di M, V Differential diurnal variations of anandamide and 2-arachidonoyl-glycerol levels in rat brain. Cell. Mol. Life Sci.2004;61:945–950. [PubMed]
• van der Stelt M, Di Marzo V. Endovanilloids. Eur. J. Biochem.2004;271:1827–1834. [PubMed]
• Van Der SM, Di M, V Cannabinoid receptors and their role in neuroprotection. Neuromol. Med. 2005;7:37–50. [PubMed]
• Varvel SA, Cravatt BF, Engram AE, Lichtman AH. Fatty acid amide hydrolase (−/−) mice exhibit an increased sensitivity to the disruptive effects of anandamide or oleamide in a working memory water maze task. J. Pharmacol. Exp. Ther. 2006;317:251–257.[PubMed]
• Veldhuis WB, van der Stelt M, Wadman MW, van Zadelhoff G, Maccarrone M, Fezza F, Veldink GA, Vliegenthart JFG, Bär PR, Nicolay K, Di Marzo V. Neuroprotection by the endogenous cannabinoid anandamide and arvanil against in vivo excitotoxicity in the rat: role of vanilloid receptors and lipoxygenases. J. Neurosci.2003;23:4127–4133. [PubMed]
• Vellani V, Reynolds AM, McNaughton PA. Modulation of the synaptic Ca2+ current in salamander photoreceptors by polyunsaturated fatty acids and retinoids. J. Physiol. (Lond.)2000;529:333–344. [PMC free article] [PubMed]
• Wagner H-J, Behrens UD. Microanatomy of the dopaminergic system in the rainbow trout retina. Vision Res. 1993;33:1345–1358.[PubMed]
• Warrier A, Wilson M. Endocannabinoid signaling regulates spontaneous transmitter release from embryonic retinal amacrine cells. Visual Neurosci. 2007;24:25–35. [PubMed]
• Watanabe N, Horie S, Michael GJ, Spina D, Page CP, Priestley JV. Immunohistochemical localization of vanilloid receptor subtype 1 (TRPV1) in the guinea pig respiratory system. Pulmonary Pharm. Ther. 2005;18:187–197. [PubMed]
• Weber B, Schlicker E. Modulation of dopamine release in the guinea-pig retina by G(i)- but not by G(s)- or G(q)-protein-coupled receptors. Fund. & Clin. Pharmacol. 2001;15:393–400. [PubMed]
• Weil AT, Zinberg NE, Nelsen JM. Clinical and psychological effects of marihuana in man. Science. 1968;162:1234–1242. [PubMed]
• West ME. Cannabis and night vision. Nature. 1991;351:703–704.[PubMed]
• Westlake TM, Howlett AC, Bonner TI, Matsuda LA, Herkenham M. Cannabinoid receptor binding and messenger RNA expression in human brain: an in vitro receptor autoradiography and in situhybridization histochemistry study of normal aged and Alzheimer’s brains. Neuroscience. 1994;63:637–652. [PubMed]
• Wilson RI, Nicoll RA. Endogenous cannabinoids mediate retrograde signaling at hippocampal synapses. Nature. 2001;410:588–592.[PubMed]
• Wise LE, Shelton CC, Cravatt BF, Martin BR, Lichtman AH. Assessment of anandamide’s pharmacological effects in mice deficient of both fatty acid amide hydrolase and cannabinoid CB1 receptors. Eur. J Pharmacol. 2007;557:44–48. [PubMed]
• Wong KY, Cohen ED, Dowling JE. Retinal Bipolar Cell Input Mechanisms in Giant Danio: II. Patch-Clamp Analysis of ON Bipolar Cells. J. Neurophysiol. 2004;93:94–107. [PubMed]
• Yazulla S, Lin Z-S. Differential effects of dopamine depletion on the distribution of ³H-SCH23390 and ³H-spiperone binding sites in the goldfish retina. Vision Res. 1995;35:2509–2514. [PubMed]
• Yazulla S, Studholme KM. Light adaptation affects synaptic vesicle density but not the distribution of GABA_A receptors in goldfish photoreceptor terminals. Microsc. Res. Tech. 1997;36:43–56.[PubMed]
• Yazulla S, Studholme KM. Vanilloid receptor like 1 (VRL1) immunoreactivity in mammalian retina: Colocalization with somatostatin and purinergic P2X1 receptors. J. Comp Neurol.2004;474:407–418. [PubMed]
• Yazulla S, Studholme KM, McIntosh HH, Deutsch DG. Immunocytochemical localization of cannabinoid CB1 receptor and fatty acid amide hydrolase in rat retina. J. Comp. Neurol.1999;415:80–90. [PubMed]
• Yazulla S, Studholme KM, McIntosh HH, Fan SF. Cannabinoid receptors on goldfish retinal bipolar cells: Electron-microscope immunocytochemistry and whole-cell recordings. Visual Neurosci.2000;17:391–401. [PubMed]
• Yu M, Ives D, Ramesha CS. Synthesis of prostaglandin E₂ethanolamide from anandamide by cyclooxygenase-2. J. Biol. Chem.1997;272:21181–21186. [PubMed]
• Zheng L, Gong B, Hatala DA, Kern TS. Retinal ischemia and reperfusion causes capillary degeneration: similarities to diabetes.Invest. Ophthalmol. Vis. Sci. 2007;48:361–367. [PubMed]
• Zhu P, Genc A, Zhang X, Zhang J, Bazan NG, Chen C. Heterogeneous expression and regulation of PGE2 receptors in the hippocampus. J. Neurosci. Res. 2005;81:817–826. [PubMed]
• Zimmer A, Zimmer AM, Hohmann AG, Herkenham M, Bonner TI. Increased mortality, hypoactivity, and hypoalgesia in cannabinoid CB1 receptor knockout mice. Proc. Natl. Acad. Sci. USA.1999;96:5780–5785. [PMC free article] [PubMed]
• Zimov S, Yazulla S. Vanilloid receptor 1 (TRPV1/VR1) co-localizes with fatty acid amide hydrolase (FAAH) in retinal amacrine cells.Visual Neurosci. 2007;24:581–591. [PubMed]
• Zygmunt PM, Petersson J, Andersson DA, Chuang H, Sorgard M, DiMarzo V, Julius D, Hogestatt ED. Vanilloid receptors on sensory nerves mediate the vasodilator action of anandamide. Nature.1999;400:452–457. [PubMed]