Glutamate and glutamate receptors in the vertebrate retina

[General overview] [Histological techniques] [Glutamate receptors] [Ionotropic glutamate receptors] [Metabotropic receptors] [Glutamate transporters] [Localization of glutamate receptors] [Retinal neurons expressing ionotropic] [Retinal neurons expressing metabotropic] [Retinal neurons expressing transporters] [Summary and conclusions] [References] [Author]

1. General overview of synaptic transmission.

Cells communicate with each other electrically, through gap junctions, and chemically, using neurotransmitters. Chemical synaptic transmission allows nerve signals to be exchanged between cells which are electrically isolated from each other. The chemical messenger, or neurotransmitter, provides a way to send the signal across the extracellular space, from the presynaptic neuron to the postsynaptic cell. The space is called a cleft and is typically more than 10 nanometers across. Neurotransmitters are synthesized in the presynaptic cell and stored in vesicles in presynaptic processes, such as the axon terminal. When the presynaptic neuron is stimulated, calcium channels open and the influx of calcium ions into the axon terminal triggers a cascade of events leading to the release of neurotransmitter. Once released, the neurotransmitter diffuses across the cleft and binds to receptors on the postsynaptic cell, allowing the signal to propagate. Neurotransmitter molecules can also bind onto presynaptic autoreceptors and transporters, regulating subsequent release and clearing excess neurotransmitter from the cleft. Compounds classified as neurotransmitters have several characteristics in common (reviewed in Massey, 1990, Erulkar, 1994). Briefly, (1) the neurotransmitter is synthesized, stored, and released from the presynaptic terminal. (2) Specific neurotransmitter receptors are localized on the postsynaptic cells, and (3) there exists a mechanism to stop neurotransmitter release and clear molecules from the cleft. Common neurotransmitters in the retina are glutamate, GABA, glycine, dopamine, and acetylcholine. Neurotransmitter compounds can be small molecules, such as glutamate and glycine, or large peptides, such as vasoactive intestinal peptide (VIP). Some neuroactive compounds are amino acids, which also have metabolic functions in the presynaptic cell.

Fig. 1. Structure of the glutamate molecule.).

Glutamate (Fig. 1) is believed to be the major excitatory neurotransmitter in the retina. In general, glutamate is synthesized from ammonium and alpha-ketoglutarate (a component of the Krebs Cycle) and is used in the synthesis of proteins, other amino acids, and even other neurotransmitters (such as GABA; Stryer, 1988). Though glutamate is present in all neurons, only a few are glutamatergic, releasing glutamate as their neurotransmitter. Neuroactive glutamate is stored in synaptic vesicles in presynaptic axon terminals (Fykse and Fonnum, 1996). Glutamate is incorporated into the vesicles by a glutamate transporter located in the vesicular membrane. This transporter selectively accumulates glutamate through a sodium-independent, ATP-dependent process (Naito and Ueda, 1983, Tabb and Ueda, 1991, Fykse and Fonnum, 1996), resulting in a high concentration of glutamate in each vesicle. Neuroactive glutamate is classified as an excitatory amino acid (EAA) because glutamate binding onto postsynaptic receptors typically stimulates, or depolarizes, the postsynaptic cells.

2. Histological techniques identify glutamatergic neurons.

Fig. 2. Glutamate immunoreactivity.

Using immunocytochemical techniques, neurons containing glutamate are identified and labeled with a glutamate antibody. In the retina, photoreceptors, bipolar cells, and ganglion cells are glutamate immunoreactive (Ehinger et al, 1988, Marc et al., 1990, Van Haesendonck and Missotten, 1990, Kalloniatis and Fletcher, 1993, Yang and Yazulla, 1994, Jojich and Pourcho, 1996) (Fig. 2). Some horizontal and/or amacrine cells can also display weak labeling with glutamate antibodies (Ehinger et al., 1988, Marc et al., 1990, Jojich and Pourcho, 1996; Yang, 1996). These neurons are believed to release GABA, not glutamate, as their neurotransmitter (Yazulla, 1986), suggesting the weak glutamate labeling reflects the pool of metabolic glutamate used in the synthesis of GABA. This has been supported by the results from double-labeling studies using antibodies to both GABA and glutamate: glutamate-positive amacrine cells also label with the GABA antibodies (Jojich and Pourcho, 1996, Yang, 1996).

Fig. 3. Autoradiogram of glutamate uptake through glutamate transporters.

Photoreceptors, which contain glutamate, actively take up radiolabeled glutamate from the extracellular space, as do Muller cells (Fig. 3) (Marc and Lam, 1981; Yang and Wu, 1997). Glutamate is incorporated into these cell types through a high affinity glutamate transporter located in the plasma membrane. Glutamate transporters maintain the concentration of glutamate within the synaptic cleft at low levels, preventing glutamate-induced cell death (Kanai et al., 1994). Though Muller cells take up glutamate, they do not label with glutamate antibodies (Jojich and Pourcho, 1996). Glutamate incorporated into Muller cells is rapidly broken down into glutamine, which is then exported from glial cells and incorporated into surrounding neurons (Pow and Crook, 1996). Neurons can then synthesize glutamate from glutamine (Hertz, 1979, Pow and Crook, 1996).

Thus, histological techniques are used to identify potential glutamatergic neurons by labeling neurons containing glutamate (through immunocytochemistry, Fig. 2) and neurons that take up glutamate (through autoradiography, Fig. 3). To determine if these cell types actually release glutamate as their neurotransmitter, however, the receptors on postsynaptic cells have to be examined.

3. Glutamate receptors.

Once released from the presynaptic terminal, glutamate diffuses across the cleft and binds onto receptors located on the dendrites of the postsynaptic cell(s). Multiple glutamate receptor types have been identified. Though glutamate will bind onto all glutamate receptors, each receptor is characterized by its sensitivity to specific glutamate analogues and by the features of the glutamate-elicited current. Glutamate receptor agonists and antagonists are structurally similar to glutamate (Fig. 4), which allows them to bind onto glutamate receptors. These compounds are highly specific and, even in intact tissue, can be used in very low concentrations because they are poor substrates for glutamate uptake systems (Tachibana and Kaneko, 1988, Schwartz and Tachibana, 1990).

Fig. 4. Glutamate receptor agonists and antagonists.

Two classes of glutamate receptors (Fig. 5) have been identified: (1) ionotropic glutamate receptors, which directly gate ion channels, and (2) metabotropic glutamate receptors, which may be coupled to an ion channel or other cellular functions via an intracellular second messenger cascade. These receptor types are similar in that they both bind glutamate and glutamate binding can influence the permeability of ion channels. However, there are several differences between the two classes.

Fig. 5. Ionotropic and metabotropic glutamate receptors and channels.

4. Ionotropic glutamate receptors.

Glutamate binding onto an ionotropic receptor directly influences ion channel activity because the receptor and the ion channel form one complex (Fig. 5a). These receptors mediate fast synaptic transmission between neurons. Each ionotropic glutamate receptor, or iGluR, is formed from the co-assembly of individual subunits. The assembled subunits may or may not be homologous, with the different combinations of subunits resulting in channels with different characteristics (Keinanen et al., 1990, Verdoorn et al., 1991, Moyner et al., 1992; Nakanishi, 1992, Ozawa and Rossier, 1996).

Fig. 6. Comparison between NMDA and non-NMDA receptors.

Two iGluR types (see Fig. 6) have been identified: (1) NMDA receptors, which bind glutamate and the glutamate analogue N-Methyl-D-Aspartate (NMDA) and (2) non-NMDA receptors, which are selectively agonized by kainate, AMPA, and quisqualate, but not NMDA.

Non-NMDA receptors. Glutamate binding onto a non-NMDA receptor opens non-selective cation channels more permeable to sodium (Na+) and potassium (K+) ions than calcium (Ca+2) (Mayer and Westbrook, 1987). Glutamate binding elicits a rapidly activating inward current at membrane potentials negative to 0 mV, and an outward current at potentials positive to 0 mV. Kainate, quisqualate, and AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) are the specific agonists at these receptors; CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), NBQX (1,2,3,4-tetrahydro-6-nitro-2,3-dione-benzo[f]quinoxaline-7-sulfonamide), and DNQX (6,7-dinitroquinoxaline-2,3-dione) are the antagonists.

Fig. 7. Whole-cell patch clamp to show quisqualate and kainate gated currents.

In retina, non-NMDA receptors have been identified on horizontal cells, OFF-bipolar cells, amacrine cells, and ganglion cells (see below). Patch clamp recordings (Gilbertson et al., 1991, Zhou et al., 1993, Boos et al., 1993, Cohen and Miller, 1994, Yu and Miller, 1995) indicate that AMPA, quisqualate, and/or kainate application can evoke currents in these cells. However, the kinetics of the ligand-gated currents differ. AMPA - and quisqualate-elicited currents rapidly desensitize; whereas, kainate-gated currents do not (Fig. 7a). The desensitization at AMPA/quisqualate receptors can be reduced (Fig. 7b) by adding cyclothiazide (Yamada and Tang, 1993), which stabilizes the receptor in an active (or non-desensitized) state (Yamada and Tang, 1993, Kessler et al., 1996).

Each non-NMDA receptor is formed from the co-assembly of several subunits (Boulter et al., 1990, Nakanishi et al., 1990, Nakanishi, 1992). To date, seven subunits (named GluR1 through GluR7) have been cloned (Hollmann et al., 1989, Boulter et al., 1990, Keinanen et al., 1990, Nakanishi et al., 1990, Bettler et al., 1990, 1992, Egebjerg et al., 1991). Expression of subunit clones in Xenopus oocytes revealed that GluR5, GluR6, and GluR7 (along with subunits KA1 and KA2) co-assemble to form kainate(-preferring) receptors; whereas, GluR1, GluR2, GluR3, and GluR4 are assembled into AMPA(-preferring) receptors (Nakanishi, 1992).

NMDA receptors. Glutamate binding onto an NMDA receptor also opens non-selective cation channels, resulting in a conductance increase. However, the high conductance channel associated with these receptors is more permeable to Ca+2 than Na+ ions (Mayer and Westbrook, 1987) and NMDA-gated currents typically have slower kinetics than kainate- and AMPA-gated channels. As the name suggests, NMDA is the selective agonist at these receptors. The compounds MK-801, AP-5 (2-amino-5-phosphonopentanoic acid), and AP-7 (2-amino-7-phosphoheptanoic acid) are NMDA receptor antagonists.

NMDA receptors are structurally complex, with separate binding sites for glutamate, glycine, magnesium ions (Mg+2), zinc ions (Zn+2), and a polyamine recognition site (Fig. 6b). There is also an antagonist binding site for PCP and MK-801 (Lodge, 1997). The glutamate, glycine, and magnesium binding sites are important for receptor activation and gating of the ion channel. In contrast, the zinc and polyamine sites are not needed for receptor activation, but affect the efficacy of the channel. Zinc blocks the channel in a voltage-independent manner (Westbrook and Mayer, 1987). The polyamine site (Ransom and Stec, 1988, Williams et al., 1994) binds compounds such as spermine or spermidine, either potentiating (Ranson and Stec, 1988; Williams et al., 1994) or inhibiting (Williams et al., 1994) the activity of the receptor, depending on the combination of subunits forming each NMDA receptor (Williams et al., 1994).

To date, five subunits (NR1, NR2a, N2b, N2c, and N2d) of NMDA receptors have been cloned (Moriyoshi et al., 1991, Ikeda et al., 1992, Katsuwada et al., 1992, Meguro et al., 1992, Ishii et al., 1993). As with non-NMDA receptors, NMDA receptor subunits can co-assemble as homomers (i.e., five NR1 subunits; Moyner et al., 1992, Moriyoshi et al., 1992) or heteromers (one NR1 + four NR2 subunits; Meguro et al., 1992, Katsuwada et al., 1992, Moyner et al., 1992, Ishii et al., 1993). However, all functional NMDA receptors express the NR1 subunit (Moyner et al., 1992, Nakanishi, 1992, Ishii et al., 1993).

Fig. 8. NMDA receptor activation

The glutamate, glycine, and Mg+2 binding sites confer both ligand-gated and voltage-gated properties onto NMDA receptors. NMDA receptors are ligand-gated because the binding of glutamate (ligand) is required to activate the channel. In addition, micromolar concentrations of glycine must also be present (Fig. 8) (Johnson and Ascher, 1987, Kleckner and Dingledine, 1988). The requirement for both glutamate and glycine makes them co-agonists (Kleckner and Dingledine, 1988) at NMDA receptors.

Mg+2 ions provide a voltage-dependent block of NMDA-gated channels (Nowak et al., 1984). This can be seen in the current-voltage (I-V) relationship presented in Fig. 9 (from Nowak et al., 1984).

Fig. 9. Mg+2 ions block NMDA receptor channels.

I-V curves plotted from currents recorded in the presence of Mg+2 have a characteristic J-shape (dotted line); whereas, a linear relationship is calculated in Mg+2-free solutions (solid line). At negative membrane potentials, Mg+2 ions occupy the binding site causing less current to flow through the channel. As the membrane depolarizes, the Mg+2 block is removed (Nowak et al., 1984).

Retinal ganglion cells and some amacrine cell types express functional NMDA receptors in addition to non-NMDA receptors (i.e., Massey and Miller, 1988, 1990, Mittman et al., 1990, Dixon and Copenhagen 1992, Diamond and Copenhagen, 1993, Cohen and Miller, 1994). The currents elicited through these different iGluR types can be distinguished pharmacologically. Non-NMDA receptor antagonists block a transient component of the ganglion cell light response; whereas, NMDA receptor antagonists block a more sustained component (Mittman et al., 1990, Diamond and Copenhagen, 1993, Hensley et al., 1993, Cohen and Miller, 1994). These findings suggest the currents elicited through co-localized NMDA and non-NMDA receptors mediate differential contributions to the ON- and OFF-light responses observed in ganglion cells (i.e., Diamond and Copenhagen, 1993).

5. Metabotropic glutamate receptors.

Unlike ionotropic receptors, which are directly linked to an ion channel, metabotropic receptors are coupled to their associated ion channel through a second messenger pathway. Ligand (glutamate) binding activates a G-protein and initiates an intracellular cascade (Nestler and Duman, 1994). Metabotropic glutamate receptors (mGluRs) are not co-assembled from multiple subunits, but are one polypeptide (Fig. 5b). To date, eight mGluRs (mGluR1-mGluR8) have been cloned (Houamed et al., 1991, Masu et al., 1991, Abe et al., 1992, Tanabe et al., 1992, Nakajima et al., 1993, Saugstad et al., 1994, Duvoisin et al., 1995). These receptors are classified into three groups (I, II, and III) based on structural homology, agonist selectivity, and their associated second messenger cascade (Table 1, end of chapter) (reviewed in Nakanishi, 1994, Knopfel et al., 1995, Pin and Duvoisin, 1995, Pin and Bockaert, 1995).

In brief, Group I mGluRs (mGluR1 and mGluR5) are coupled to the hydrolysis of fatty acids and the release of calcium from internal stores. Quisqualate and trans-ACPD are Group I agonists. Group II (mGluR2 and mGluR3) and Group III (mGluRs 4, 6, 7, and 8) receptors are considered inhibitory because they are coupled to the downregulation of cyclic nucleotide synthesis (Pin and Duvoisin, 1995). L-CCG-1 and trans-ACPD agonize Group II receptors; L-AP4 (also called APB) selectively agonizes Group III receptors. In situ hybridization studies have revealed that the mRNAs encoding Group I, II, and III mGluRs are present in retina (see below); however, with the exception of the APB receptor, the function of all these receptor types in retina has not been characterized.

APB receptor. In contrast to non-NMDA and NMDA receptors, glutamate binding onto an APB receptor elicits a conductance decrease (Slaughter and Miller, 1981, Nawy and Copenhagen, 1987, 1990) due to the closure of cGMP-gated non-selective cation channels (Nawy and Jahr, 1990) (Fig. 10).

Fig. 10.Whole-cell current traces to show kinetics of APB receptor gated currents.

APB application selectively blocks the ON-pathway in the retina (Fig. 11) (Slaughter and Miller, 1981), i.e., ON-bipolar cell responses and the ON-responses in amacrine cells (Taylor and Wassle, 1995) and ganglion cells (Cohen and Miller, 1994, Kittila and Massey, 1995, Jin and Brunken, 1996) are eliminated by APB. Experimental evidence (Slaughter and Miller 1981, Massey et al., 1983) suggests the APB receptor is localized to ON-bipolar cell dendrites. Inhibition of amacrine and ganglion cell light responses, therefore, is due to a decrease in the input from ON-bipolar cells, not a direct effect on postsynaptic receptors.

Fig. 11. Intracellular recordings to show that APB selectively antagonizes the ON-pathways.

APB (2-amino-4-phosphobutyric acid, also called L-AP4) is the selective agonist for all Group III mGluRs (mGluR4, 6, 7, and 8). So, which is the APB receptor located on ON-bipolar cell dendrites? MGluR4, 7, and 8 expression has been observed in both the inner nuclear layer and the ganglion cell layer (Duvoisin et al., 1995, Hartveit et al., 1995) suggesting these mGluRs are associated with more than one cell type. In contrast, mGluR6 expression has been localized to the INL (Nakajima et al., 1993, Hartveit et al., 1995) and the OPL (Nomura et al., 1994) where bipolar cell somata and dendrites are located. Furthermore, ON-responses are abolished in mice lacking mGluR6 expression (Masu et al., 1995). These mutants also display abnormal ERG b-waves, suggesting an inhibition of the ON-retinal pathway at the level of bipolar cells (Masu et al., 1995). Taken together, these findings suggest the APB receptor on ON-bipolar cells is mGluR6.

6. Glutamate transporters and transporter-like receptors.

Glutamate transporters have been identified on photoreceptors (Marc and Lam, 1981, Tachibana and Kaneko, 1988, Eliasof and Werblin, 1993) and Muller cells (Marc and Lam, 1981, Yang and Wu, 1997). From glutamate labeling studies, the average concentration of glutamate in photoreceptors, bipolar cells, and ganglion cells is 5mM (Marc et al. 1990). Physiological studies using isolated cells indicate that only µM levels of glutamate are required to activate glutamate receptors (i.e., Aizenman et al., 1988, Zhou et al., 1993, Sasaki and Kaneko, 1996). Thus, the amount of glutamate released into the synaptic cleft is several orders of magnitude higher than the concentration required to activate most postsynaptic receptors. High affinity glutamate transporters located on adjacent neurons and surrounding glial cells rapidly remove glutamate from the synaptic cleft to prevent cell death (Kanai et al., 1994). Five glutamate transporters, EAAT-1 (or GLAST), EAAT-2 (or GLT-1), EAAT-3 (or EAAC-1), EAAT-4, and EAAT-5, have been cloned (Kanai and Hediger, 1992, Pines et al., 1992, Fairman et al., 1995, Schultz and Stell, 1996, Arriza et al., 1997, Kanai et al., 1997).

Glutamate transporters are pharmacologically distinct from both iGluRs and mGluRs. L-glutamate, L-aspartate, and D-aspartate are substrates for the transporters (Brew and Attwell, 1987, Tachibana and Kaneko, 1988, Eliasof and Werblin, 1993); glutamate receptor agonists (Brew and Attwell, 1987, Tachibana and Kaneko, 1988, Schwartz and Tachibana, 1990, Eliasof and Werblin, 1993) and antagonists (Barbour et al., 1991, Eliasof and Werblin, 1993) are not. Glutamate uptake can be blocked by the transporter blockers dihydrokainate (DHKA) and DL-threo-beta-hydroxyaspartate (HA) (Barbour et al., 1991, Eliasof and Werblin 1993).

Fig. 12 Glutamate transporters in Muller cells are electrogenic.

Glutamate transporters incorporate glutamate into Muller cells along with the co-transport of three Na+ ions (Brew and Attwell, 1987, Barbour et al., 1988) and the antiport of one K+ ion (Barbour et al., 1988, Bouvier et al., 1992) and either one OH- or one HCO3- ion (Bouvier et al., 1992) (Fig. 12). The excess sodium ions generate a net positive inward current which drives the transporter (Brew and Attwell, 1987, Barbour et al., 1988). More recent findings indicate a glutamate-elicited chloride current is also associated with some transporters (Eliasof and Jahr, 1996, Arriza et al., 1997).

It should be noted that the glutamate transporters located in the plasma membrane of neuronal and glial cells (discussed in this section) are different from the glutamate transporters located on synaptic vesicles within presynaptic terminals (see section 1). The transporters in the plasma membrane transport glutamate in a Na+- and voltage-dependent manner independent of chloride (Brew and Attwell, 1987, Barbour et al., 1988, Kanai et al., 1994). L-glutamate, L-aspartate, and D-aspartate are substrates for these transporters (i.e., Brew and Attwell, 1987). In contrast, the vesicular transporter selectively concentrates glutamate into synaptic vesicles in a Na+-independent, ATP-dependent manner (Naito and Ueda, 1983, Tabb and Ueda, 1991, Fykse and Fonnum, 1996) that requires chloride (Tabb and Ueda, 1991, Fykse and Fonnum, 1996).

Glutamate receptors with transporter-like pharmacology have been described in photoreceptors (Picaud et al., 1995a, b, Grant and Werblin, 1996) and ON-bipolar cells (Grant and Dowling 1995, 1996). These receptors are coupled to a chloride current. The pharmacology of these receptors is similar to that described for glutamate transporters, as the glutamate-elicited current is (1) dependent upon external Na+, (2) reduced by transporter blockers, and (3) insensitive to glutamate agonists and antagonists. However, altering internal Na+ concentration does not change the reversal potential (Picaud et al., 1995b) or the amplitude (Grant and Werblin, 1995, Grant and Dowling, 1996) of the glutamate-elicited current, suggesting the receptor is distinct from glutamate transporters. At the photoreceptor terminals, the glutamate-elicited chloride current may regulate membrane potential and subsequent voltage-gated channel activity (i.e., Picaud et al., 1995a). Postsynaptically, this receptor is believed to mediate conductance changes underlying photoreceptor input to ON- cone bipolar cells (Grant and Dowling, 1995).

7. Localization of glutamate receptor types in the retina.

Fig. 13. The types of neurons in the vertebrate retina.

Photoreceptor, bipolar, ganglion cells comprise the vertical transduction pathway in the retina. This pathway is modulated by lateral inputs from horizontal cells in the distal retina and amacrine cells in the proximal retina (Fig. 13). As described in the previous sections, photoreceptor, bipolar, and ganglion cells show glutamate immunoreactivity. Glutamate responses have been electrically characterized in horizontal and bipolar cells, which are postsynaptic to photoreceptors, and in amacrine and ganglion cells, which are postsynaptic to bipolar cells. Taken together, these results suggest glutamate is the neurotransmitter released by neurons in the vertical pathway. Recent in situ hybridization and immunocytochemical studies have localized the expression of iGluR subunits, mGluRs, and glutamate transporter proteins in the retina. These findings are summarized below.

8. Retinal neurons expressing ionotropic glutamate receptors.

Fig.14. Whole-cell currents in OFF bipolar cells.

Fig. 15. Whole-cell currents in horizontal cells.

    In both higher and lower vertebrates, electrophysiological recording techniques have identified ionotropic glutamate receptors on the neurons comprising the OFF-pathway (Table 2, end of chapter). In the distal retina, OFF-bipolar cells (Fig. 14) (Euler et al., 1996, Sasaki and Kaneko, 1996, Hartveit, 1997) and horizontal cells (Fig. 15) (Yang and Wu, 1991, Zhou et al., 1993, Kriaj et al., 1994) respond to kainate, AMPA, and quisqualate application, but not NMDA nor APB. (However, NMDA receptors have been identified on catfish horizontal cells (OÍDell and Christensen, 1989, Eliasof and Jahr, 1997) and APB-induced hyperpolarizations have been reported in some fish horizontal cells (Nawy et al., 1989, Takahashi and Copenhagen, 1992, Furukawa et al., 1997)).

    Non-NMDA agonists also stimulate both amacrine cells (Fig. 16a) (Massey and Miller, 1988, Dixon and Copenhagen, 1992, Boos et al., 1993) and ganglion cells (Fig. 16b) (Mittman et al., 1990, Diamond and Copenhagen, 1993, Hensley et al., 1993, Cohen and Miller, 1994, Yu and Miller, 1995). Ganglion cells responses to NMDA have been observed (Massey and Miller, 1988, 1990, Mittman et al., 1990, Diamond and Copenhagen, 1993, Cohen and Miller, 1994); whereas, NMDA responses have been recorded in only some types of amacrine cells (Massey and Miller, 1988, Dixon and Copenhagen, 1992, Boos et al., 1993, but see Hartveit and Veruki, 1997).

    Fig. 16. Glutamate receptors on amacrine and ganglion cells.

    Consistent with this physiological data, antibodies to the different non-NMDA receptor subunits differentially label all retinal layers (Table 3, end of chapter; Hartveit et al., 1994, Peng et al., 1995, Hughes, 1997, Pourcho et al., 1997) and mRNAs encoding the different non-NMDA iGluR subunits are similarly expressed (Hughes et al., 1992, Hamassaki-Britto et al., 1993, Brandstatter et al., 1994). In contrast, mRNAs encoding NMDA subunits are expressed predominantly in the proximal retina, where amacrine and ganglion cells are located (INL, IPL, GCL; Table 3) (Brandstatter et al., 1994, Hartveit et al., 1994), though mRNA encoding the NR2a subunit (Hartveit et al., 1994) has been observed in the OPL and antibodies to the NR2d (Wenzel et al., 1997) and the NR1 subunits (Hughes, 1997) label rod bipolar cells.

    9. Retinal neurons expressing metabotropic glutamate receptors.

    All metabotropic glutamate receptors, except mGluR3, have been identified in retina either through antibody staining (Peng et al., 1995, Brandstatter et al., 1996, Koulen et al., 1997, Pourcho et al., 1997) or in situ hybridization (Nakajima et al., 1993, Duvoisin et al., 1995, Hartveit et al., 1995). MGluRs are differentially expressed throughout the retina, specifically in the outer plexiform layer, inner nuclear layer, inner plexiform layer, and the ganglion cell layer (Table 4, end of chapter). Though different patterns of mGluR expression have been observed in the retina, only the APB receptor on ON-bipolar cells has been physiologically examined.

    10. Retinal neurons expressing glutamate transporters.

    The glutamate transporters GLAST, EAAC1, and GLT-1have been identified in retina (Table 5, end of chapter). GLAST (L-glutamate/L-aspartate transporter) immunoreactivity is found in all retinal layers (Otori et al. 1994), but not in neuronal tissue. GLAST is localized to Muller cell membranes (Otori et al. 1994, Derouiche and Rauen, 1995, Rauen et al., 1996, Lehre et al., 1997). In contrast, EAAC-1 (excitatory amino acid carrier-1) antibodies do not label Muller cells or photoreceptors. EAAC-1 immunoreactivity is observed in ganglion and amacrine cells in chicken, rat, goldfish, and turtle retinas. In addition, bipolar cells positive labeled with EAAC-1 antibody in lower vertebrates and immunopositive horizontal cells were observed in rat (Schultz and Stell, 1996). GLT-1 (glutamate transporter-1) proteins have been identified in monkey (Grunert et al., 1994), rat (Rauen et al., 1996), and rabbit (Massey et al., 1997) bipolar cells. In addition, a few amacrine cells were weakly labeled with the GLT-1 antibody in rat (Rauen et al., 1996), as were photoreceptor terminals in rabbit (Massey et al., 1997).

    11. Summary and conclusions.

    Fig. 17. The ribbon glutamatergic synapse in the retina.

    Histological analyses of presynaptic neurons and physiological recordings from postsynaptic cells suggest photoreceptor, bipolar, and ganglion cells release glutamate as their neurotransmitter. Multiple glutamate receptor types are present in the retina. These receptors are pharmacologically distinct and differentially distributed. IGluRs directly gate ion channels and mediate rapid synaptic transmission through either kainate/AMPA or NMDA receptors. Glutamate binding onto iGluRs opens cation channels, depolarizing the postsynaptic cell membrane. Neurons within the OFF-pathway (horizontal cells, OFF-bipolar cells, amacrine cells, and ganglion cells) express functional iGluRs. MGluRs are coupled to G-proteins. Glutamate binding onto mGluRs can have a variety of effects depending on the second messenger cascade to which the receptor is coupled. The APB receptor, found on ON-bipolar cell dendrites, is coupled to the synthesis of cGMP. At these receptors, glutamate decreases cGMP formation leading to the closure of ion channels. Glutamate transporters, found on glial and photoreceptor cells, are also present at glutamatergic synapses (Fig. 17). Transporters remove excess glutamate from the synaptic cleft to prevent neurotoxicity. Thus, postsynaptic responses to glutamate are determined by the distribution of receptors and transporters at a glutamatergic synapses which, in retina, determine the conductance mechanisms underlying visual information processing within the ON- and OFF-pathways.

    12. References.

    Abe, T., Sugihara, H., Nawa, H., Shigemoto, R., Mizuno, N., and Nakanishi, S. (1992) Molecular characterization of a novel metabotropic glutamate receptor mGluR5 coupled to inositol phosphate/Ca+2 signal transduction. J. Biol. Chem., 267, 13361-13368.

    Aizenman, E., Frosch, M.P., and Lipton, S.A. (1988) Responses mediated by excitatory amino acid receptors in solitary retinal ganglion cells from rat. J. Physiol., 396, 75-91.

    Arriza, J.L., Eliasof, S., Kavanaugh, M.P., and Amara, S.G. (1997) Excitatory amino acid transporter 5, a retinal glutamate transporter coupled to a chloride conductance. Proc. Natl. Acad. Sci., 94, 4155-4160.

    Barbour, B., Brew, H., and Attwell, D. (1988) Electrogenic glutamate uptake in glial cells is activated by intracellular potassium. Nature, 335, 433-435.

    Barbour, B., Brew, H., and Attwell, D. (1991) Electrogenic uptake of glutamate and aspartate into glial cells isolated from the salamander (Ambystoma) retina. J. Physiol., 436, 169-193.

    Bettler, B., Boulter, J., Hermans-Borgmeyer, I., OÍShea-Greenfield, A., Deneris, E.S., Moll, C., Borgmeyer, U., Hollmann, M., and Heinemann, S. (1990) Cloning of a novel glutamate receptor subunit, GluR5: expression in the nervous system during development. Neuron, 5, 583-595.

    Bettler, B., Egebjerg, J., Sharma, G., Pecht, G., Hermans-Borgmeyer, I., Moll, C., Stevens, C.F., and Heinemann, S. (1992) Cloning of a putative glutamate receptor: a low affinity kainate-binding subunit. Neuron, 8, 257-265.

    Boos, R., Schneider, H., and Wassle, H. (1993) Voltage- and transmitter-gated currents of AII amacrine cells in a slice preparation of the rat retina. J. Neurosci., 13, 2874-2888.

    Boulter, J., Hollmann, M., OÍShea-Greenfield, A., Hartley, M., Deneris, E., Maron, C., and Heinemann, S. (1990) Molecular cloning and functional expression of glutamate receptor subunit genes. Science, 249, 1033-1037.

    Bouvier, M., Szatkowski, M., Amato, A., and Attwell, D. (1992) The glial cell glutamate uptake carrier countertransports pH-changing ions. Nature, 360, 471-474.

    Brandstatter, J.H., Hartveit, E., Sasso-Pognetto, M., and Wassle, H. (1994) Expression of NMDA and high-affinity kainate receptor subunit mRNAs in the adult rat retina. Eur. J. Neurosci., 6, 1100-1112.

    Brandstatter, J.H., Koulen, P., Kuhn, R., van der Putten, H., and Wassle, H. (1996) Compartmental localization of a metabotropic glutamate receptor (mGluR7): two different active sites at a retinal synapse. J. Neurosci., 16, 4749-4756.

    Brew, H. and Attwell, D. (1987) Electrogenic glutamate uptake is a major current carrier in the membrane of axolotl retinal glial cells. Nature, 327, 707-709.

    Cohen, E.D. and Miller, R.F. (1994) The role of NMDA and non-NMDA excitatory amino acid receptors in the functional organization of primate retinal ganglion cells. Vis. Neurosci., 11, 317-332.

    Derouiche, A. and Rauen, T. (1995) Coincidence of L-glutamate/L-aspartate transporter (GLAST) and glutamine synthetase (GS) immunoreactions in retinal glia: evidence for coupling of GLAST and GS in transmitter clearance. J. Neurosci. Res., 42, 131-143.

    Diamond, J.A. and Copenhagen, D.R. (1993) The contribution of NMDA and non-NMDA receptors to the light-evoked input-output characteristics of retinal ganglion cells. Neuron, 11, 725-738.

    Dixon, D.B. and Copenhagen, D.R. (1992) Two types of glutamate receptors differentially excite amacrine cells in the tiger salamander retina. J. Physiol., 449, 589-606.

    Duvoisin, R.M., Zhang, C., and Ramonell, K. (1995). A novel metabotropic glutamate receptor expressed in the retina and olfactory bulb. J. Neurosci., 15, 3075-3083.

    Egebjerg, J., Bettler, B., Hermans-Borgmeyer, I., and Heinemann, S. (1991) Cloning of a cDNA for a glutamate receptor subunit activated by kainate but not AMPA. Nature, 351, 745-748.

    Ehinger, B., Ottersen, O.P., Storm-Mathisen, J., and Dowling, J.E. (1988) Bipolar cells in the turtle retina are strongly immunoreactive for glutamate. Proc. Natl. Acad. Sci., 85, 8321-8325.

    Eliasof, S. and Jahr, C.E. (1996) Retinal glial cell glutamate transporter is coupled to an anionic conductance. Proc. Natl. Acad. Sci., 93, 4153-4158.

    Eliasof, S. and Jahr, C.E. (1997) Rapid AMPA receptor desensitization in catfish cone horizontal cells. Vis. Neurosci., 14, 13-18.

    Eliasof, S. and Werblin, F. (1993) Characterization of the glutamate transporter in retinal cones of the tiger salamander. J. Neurosci., 13, 402-411.

    Erulkar, S.D. (1994) Chemically mediated synaptic transmission: an overview, pp. 181-208 In Basic Neurochemistry, 5th ed. Siegel, G.J., Agranoff, B.J., Albers, R.W., and Molinoff, P.B. (eds). Raven Press, New York.

    Euler, T., Schneider, H., and Wassle, H. (1996) Glutamate responses of bipolar cells in a slice preparation of the rat retina. J. Neurophysiol., 16, 2934-2994.

    Fairman, W.A., Vandengerg, R.J., Arriza, J.L., Kavanaugh, M.P., and Amara, S.G. (1995) An excitatory amino-acid transporter with properties of a ligand-gated chloride channel. Nature, 375, 599-603.

    Furukawa, T., Yamada, K.M., Petruv, R., Djamgoz, M.B.A., and Yasui, S. (1997) Nitric oxide, 2-amino-4-phosphobutyric acid and light/dark adaptation modulate short-wavelength-sensitive synaptic transmission to retinal horizontal cells. Neurosci. Res., 27, 65-74.

    Fykse, E.M. and Fonnum, F. (1996) Amino acid neurotransmission: dynamics of vesicular uptake. Neurochem. Res., 21, 1053-1060.

    Gilbertson, T.A., Scobey, R., and Wilson, M. (1991) Permeation of calcium ions through non-NMDA glutamate channels in retinal bipolar cells. Science, 251, 1613-1615.

    Grant, G.B. and Dowling, J.E. (1995) A glutamate-activated chloride current in cone-driven ON bipolar cells of the white perch retina. J. Neurosci., 15, 3852-3862.

    Grant, G.B. and Dowling, J.E. (1996) ON bipolar cell responses in the teleost retina are generated by two distinct mechanisms. J. Neurophysiol., 76, 3842-3849.

    Grant, G.B. and Werblin, F.S. (1996) A glutamate-elicited chloride current with transporter-like properties in rod photoreceptors of the tiger salamander. Vis. Neurosci., 13, 135-144.

    Grunert, U., Martin, P.R., and Wassle, H. (1994) Immunocytochemical analysis of bipolar cells in the Macaque monkey retina. J. Comp. Neurol., 348, 607-627.

    Hamassaki-Britto, D.E., Hermans-Borgmeyer, I., Heinemann, S., and Hughes, T.E. (1993) Expression of glutamate receptor genes in the mammalian retina: the localization of GluR1 through GluR7 mRNAs. J. Neurosci., 13, 1888-1898.

    Hartveit, E. (1997) Functional organization of cone bipolar cells in the rat retina. J. Neurophysiol., 77, 1716-1730.

    Hartveit, E., Brandstatter, J.H., Sasso-Pognetto, M., Laurie, D.J., Seeburgh, P.H., and Wassle, H. (1994) Localization and developmental expression of the NMDA receptor subunit NR2A in the mammalian retina. J. Comp. Neurol., 348, 570-582.

    Hartveit, E., Brandsttter, J.H., Enz, R., and Wassle, H. (1995) Expression of the mRNA of seven metabotropic glutamate receptors (mGluR1 to 7) in the rat retina. An in situ hybridization study on tissue section and isolated cells. Eur. J. Neurosci., 7, 1472-1483.

    Hartveit, E. and Veruki, M.L. (1997) AII amacrine cells express functional NMDA receptors. Neuroreport,8, 1219-1223.

    Hensley, S.H., Yang, X.-L., and Wu, S.M. (1993) Identification of glutamate receptor subtypes mediating inputs to bipolar cells and ganglion cells in the tiger salamander retina. J. Neurophysiol., 69, 2099-2107.

    Hertz, L. (1979) Functional interactions between neurons and astrocytes I. Turnover and metabolism of putative amino acid transmitters. Progr. Neurobiol., 13, 277-323.

    Hirano, A.A. and MacLeish, P.R. (1991) Glutamate and 2-amino-4-phosphobutyric acid evoke an increase in potassium conductance in retinal bipolar cells. Proc. Natl. Acad. Sci., 88, 805-809.

    Hollmann, M., OÍShea-Greenfield, A., Rogers, S.W., and Heinemann, S. (1989) Cloning by functional expression of a member of the glutamate receptor family. Nature, 342, 643-648.

    Houamed, K.M., Kuijper, J.L., Gilbert, T.L., Haldeman, B.A., OÍHara, P.J., Mulvihill, E.R., Almers, W., and Hagen, F.S. (1991) Cloning, expression, and gene structure of a G protein-coupled glutamate receptor from rat brain. Science, 252, 1318-1321.

    Hughes, T.E. (1997) Are there ionotropic glutamate receptors on the rod bipolar cell of the mouse retina? Vis. Neurosci., 14, 103-109.

    Hughes, T.E., Hermans-Borgmeyer, I., and Heinemann, S. (1992) Differential expression of glutamate receptor genes (GluR1-5) in the rat retina. Vis. Neurosci., 8, 49-55.

    Ikeda, K., Nagasawa, M., Mori, H., Araki, K., Sakimura, K., Watanabe, M., Inoue, Y., and Mishina, M. (1992) Cloning and expression of the ,4 subunit of the NMDA receptor channel. FEBS Lett., 313, 34-38.

    Ishii, T., Moriyoshi, K., Sugihara, H., Sakurada, K., Kadotani, H., Yokoi, M., Akazawa, C., Shigemoto, R., Mizuno, N., Masu, M., and Nakanishi, S. (1993) Molecular characterization of the family of N-methyl-D-aspartate receptor subunits. J. Biol. Chem., 268, 2836-2843.

    Jahr, C.E. and Lester, R.A. (1992) Synaptic excitation mediated by glutamate-gated ion channels. Curr. Opin. Neurobiol., 2, 270-274.

    Jin, X.T. and Brunken, W.J. (1996) A differential effect of APB on ON- and OFF-center ganglion cells in the dark adapted rabbit retina. Brain Res., 708, 191-196.

    Johnson, J.W. and Ascher, P. (1987) Glycine potentiates the NMDA response in cultured mouse brain neurons. Nature, 325, 529-531.

    Jojich, L. and Pourcho, R.G. (1996) Glutamate immunoreactivity in the cat retina: a Quantitative study. Vis. Neurosci., 13, 117-133.

    Kalloniatis, M. and Fletcher, E.L. (1993) Immunocytochemical localization of the amino acid neurotransmitters in the chicken retina. J. Comp. Neurol., 336, 174-193.

    Kanai, Y. and Hediger, M.A. (1992) Primary structure and functional characterization of a high-affinity glutamate transporter. Nature, 360, 467-471.

    Kanai, Y., Smith, C.P., and Hediger, M.A. (1994) A new family of neurotransmitter transporters: the high-affinity glutamate transporters. FASEB J., 8, 1450-1459.

    Kanai, Y., Trotti, D., Nussberger, S., and Hediger, M.A. 1997. The high-affinity glutamate transporter family, structure, function, and physiological relevance, pp. 171-213. In M.E.A. Reith (ed) Neurotransmitter transporters: structure, function, and regulation, Humana Press, Totowa, NJ.

    Kandel, E.R., Schwartz, J.H., and Jessell, T.M. (1991) Principles of neuroscience, 3rd ed. Elsevier Publishing Co, New York.

    Katsuwada, T., Kashiwabuchi, N., Mori, H., Sakimura, K., Kushiya, E., Araki, K., Megure, H., Masaki, H., Kumanishi, T., Arakawa, M., and Mishina, M. (1992) Molecular diversity of the NMDA receptor channel. Nature, 358, 36-41.

    Keinanen, K., Wisden, W., Sommer, B., Werner, P., Herb, A., Versoorn, T.A., Sakmann, B., and Seeburg, P.H. (1990) A family of AMPA-selective glutamate receptors. Science, 249, 556-560.

    Kessler, M., Arai, A., Quan, A., and Lynch, G. (1996) Effect of cyclothiazide on binding properties of AMPA-type glutamate receptors: lack of competition between cyclothiazide and GYKI 52466. Molecular Pharmacology, 49, 123-131.

    Kittila, C.A. and Massey, S.C. (1995) Effect of ON pathway blockade on directional selectively in the rabbit retina. J. Neurophysiol., 73, 703-712.

    Kleckner, N.W. and Dingledine, R. (1988) Requirement for glycine activation of NMDA-receptors expressed in Xenopus oocytes. Science, 241, 835-837.

    Knopfel,T., Kuhn, R., and Allgeier, H. (1995) Metabotropic glutamate receptors: novel targets for drug development. J. Med Chem., 38, 1417-1426.

    Koulen, P., Kuhn, R., Wassle, H., and Brandstatter, J.H. (1997) Group I metabotropic glutamate receptors mGluR1" and mGluR5a: localization in both synaptic layers of the rat retina. J. Neurosci., 17, 2200-2211.

    Kriaj, D., Akopian, A., and Witkovsky, P. (1994) The effects of L-glutamate, AMPA, quisqualate, and kainate on retinal horizontal cells depend on adaptational state: implications for rod-cone interactions. J. Neurosci., 14, 5661-5671.

    Lehre, K.P., Davanger, S., and Danbolt, N.C. (1997) Localization of the glutamate transporter protein GLAST in rat retina. Brain Res., 744, 129-137.

    Lodge, D. (1997) Subtypes of glutamate receptors. Historical perspectives on their pharmacological differentiation, pp. 1-38 In The ionotropic glutamate receptors, Monaghan, D.T. and Weinhold, R.J. (eds), Humana Press, New Jersey.

    Marc, R.E. and Lam, D.M.K. (1981) Uptake of aspartic and glutamic acid by photoreceptors in goldfish retina. Proc. Natl. Acad. Sci., 78, 7185-7189.

    Marc, R.E, Liu, W.-L.S., Kalloniatis, M., Raiguel, S.F., and Van Haesendonck, E. (1990) Patterns of glutamate immunoreactivity in the goldfish retina. J. Neurosci., 10, 4006-4034.

    Massey, S.C. (1990) Cell types using glutamate as a neurotransmitter in the vertebrate retina. Progr. Retinal Res., 9, 399-425.

    Massey, S.C., Koomen, J.M., Liu, S., Lehre, K.P., and Danbolt, N.C. (1997) Distribution of the glutamate transporter GLT-1 in the rabbit retina. Invest. Ophthal. Vis. Sci., 38, S689.

    Massey, S.C. and Miller, R.F. (1988) Glutamate receptors of ganglion cells in the rabbit retina: evidence for glutamate as a bipolar cell transmitter. J. Physiol., 405, 635-655.

    Massey, S.C. and Miller, R.F. (1990) N-Methyl-D-Aspartate receptors of ganglion cells in rabbit retina. J. Neurophysiol., 63, 16-30.

    Massey, S.C., Redburn, D.A., and Crawford, M.L.J. (1983) The effects of 2-amino-4-phosphobutyric acid (APB) on the ERG and ganglion cell discharge of rabbit retina. Vision Res., 23, 1607-1613.

    Masu, M., Tanabe, Y., Tsuchida, K., Shigemoto, R., and Nakanishi, S. (1991) Sequence and expression of a metabotropic glutamate receptor. Nature, 349, 760-765.

    Masu, M., Iwakabe, H., Tagawa, Y., Miyoshi, T., Yamashita, M., Fukuda, Y., Sasaki, H., Hiroi, K., Nakamura, Y., Shigemoto, R., Takada, M., Nakamura, K., Makao, K., Katsuki, M., and Nakanishi, S. (1995) Specific deficit of the ON response in visual transmission by targeted disruption of the mGluR6 gene. Cell, 80, 757-765.

    Mayer, M.L. and Westbrook, G.L. (1987) Permeation and block of N-Methyl-D-Aspartic acid receptor channels by divalent cations in mouse cultured central neurones. J. Physiol., 394, 501-527.

    Meguro, H., Mori, H., Araki, K., Kushiya, E., Kutsuwada, T., Yamazaki, M., Kumanishi, T., Arakawa, M., Sakimura, K., and Mishina, M. (1992) Functional characterization of a heteromeric NMDA receptor channel expressed from cloned cDNAs. Nature, 357, 70-74.

    Mittman, S., Taylor, W.R., and Copenhagen, D.R. (1990) Concomitant activation of two types of glutamate receptor mediates excitation of salamander retinal ganglion cells. J. Physiol., 428, 175-197.

    Moyner, H., Sprengel, R., Schoepfer, R., Herb, A., Higuchi, M., Lorneli, H., Burnashev, N., Sakmann, B., and Seeburg, P.H. (1992) Heteromeric NMDA receptors: molecular and functional distinction of subtypes. Science, 256, 1217-1221.

    Moriyoshi, K., Masu, M., Ishii, T., Shigemoto, R., Mizuno, N., and Nakanishi, S. (1991) Molecular cloning and characterization of the rat NMDA receptor. Nature, 354, 31-37.

    Naito, S. and Ueda, T. (1983) Adenosine triphosphate-dependent uptake of glutamate into Protein I-associated synaptic vesicles. J. Biol. Chem., 258, 696-699.

    Nakajima, Y., Iwakabe, H., Akazawa, C., Nawa, H., Shigemoto, R., Mizuno, N., and Nakanishi, S. (1993) Molecular characterization of a novel retinal metabotropic glutamate receptor mGluR6 with a high agonist selectively for L-2-amino-4-phosphobutyrate. J. Biol. Chem., 268, 11868-11873.

    Nakanishi, S. (1992) Molecular diversity of glutamate receptors and implications for brain function. Science, 258, 597-603.

    Nakanishi, S. (1994) Metabotropic glutamate receptors: synaptic transmission, modulation, and plasticity. Neuron, 13, 1031-1037.

    Nakanishi, N., Schneider, N.A., and Axel, R. (1990) A family of glutamate receptor genes: evidence for the formation of heteromultimeric receptors with distinct channel properties. Neuron, 5, 569-581.

    Nawy, S. and Copenhagen, D.R. (1987) Multiple classes of glutamate receptor on depolarizing bipolar cells in retina. Nature, 325, 56-58.

    Nawy, S. and Copenhagen, D.R. (1990) Intracellular cesium separates two glutamate conductances in retinal bipolar cells of goldfish. Vision Res., 30, 967-972.

    Nawy, S. and Jahr, C.E. (1990) Suppression by glutamate of cGMP-activated conductance in retinal bipolar cells. Nature, 346, 269-271.

    Nawy, S.E., Sie, A., and Copenhagen, D.R. (1989) The glutamate analog 2-amino-4-phosphobutyrate antagonizes synaptic transmission from cones to horizontal cells in the goldfish retina. Proc. Natl. Acad. Sci., 86, 1726-1730.

    Nestler, E.J. and Duman, E.S. (1994) G proteins and cyclic nucleotides in the nervous system, pp. 429-448 In Basic Neurochemistry, 5th ed. Siegel, G.J., Agranoff, B.W., Albers, R.W., and Molinoff, P.B. (eds). Raven Press, New York.

    Nomura, A., Shigemoto, R., Nakamura, Y., Okamoto, N., Mizuno, N., and Nakanishi, S. (1994) Developmentally regulated postsynaptic localization of a metabotropic glutamate receptor in rat bipolar cells. Cell, 77, 361-369.

    Nowak, L., Bregestovski, P., Ascher, P., Herbet, A., and Prochiantz, A. (1984) Magnesium gates glutamate-activated channels in mouse central neurones. Nature, 307, 462-465.

    OÍDell, T.J. and Christensen, B.N. (1989) Horizontal cells isolated from catfish retina contain two types of excitatory amino acid receptors. J. Neurophysiol., 61, 1097-1109.

    Otori, Y., Shimada, S., Tanaka, T., Ishimoto, I., Tana, Y. and Tohyama, M. (1994) Marked increase in glutamate-aspartate transporter (GLAST/GluT-1) mRNA following transient retinal ischemia. Mol. Brain Res., 27, 310-314.

    Ozawa, S. and Rossier, J. (1996) Molecular basis for functional differences of AMPA-subtype glutamate receptors. News Physiol Soc., 11, 77-82.

    Peng, Y.W., Blackstone, C.D., Huganir, R.L., and Yau, K.W. (1995) Distribution of glutamate receptor subtypes in the vertebrate retina. Neurosci., 66, 483-497.

    Picaud, S., Larsson, H.P., Wellis, D.P., Lecar, H., and Werblin, F. (1995a). Cone photoreceptors respond to their own glutamate release in the tiger salamander. Proc. Natl. Acad. Sci., 92, 9417-9421.

    Picaud, S.A., Larsson, H.P., Grant, G.B., Lecar, H., and Werblin, F.S. (1995b) Glutamate-gated chloride channel with glutamate-transporter-like properties in cone photoreceptors of the tiger salamander. J. Neurophysiol., 74, 1760-1771.

    Pin, J.P. and Bockaert, J. (1995) Get receptive to metabotropic glutamate receptors. Curr. Opin. Neurobiol., 5, 342-349.

    Pin, J.P. and Duvoisin, R. (1995) Review: Neurotransmitter receptors I: The metabotropic glutamate receptors: structure and functions. Neuropharmacol., 34, 1-26.

    Pines, G., Danbolt, N.C., Bjoras, M., Zhang, Y., Bendahan, A., Eide, L., Koepsell, H., Storm-Mathisen, J., Seeberg, E., and Kanner, B.I. (1992) Cloning and expression of a rat brain L-glutamate transporter. Nature, 360, 464-467.

    Pourcho, R.G., Cai, W., and Qin, P. (1997) Glutamate receptor subunits in cat retina: light and electron microscopic observations. Invest. Ophthal. Vis. Sci., 38, S46.

    Pow, D.V. and Crook, D.R. (1996) Direct immunocytochemical evidence for the transfer of glutamine from glial cells to neurons: use of specific antibodies directed against the D-stereoisomers of glutamate and glutamine. Neurosci., 70, 295-302.

    Ransom, R.W. and Stec, N.L. (1988) Cooperative modulation of [3H}MK-801 binding to the N-Methyl-D-Aspartate receptor-ion channel complex by L-glutamate, glycine, and polyamines. J. Neurochem., 51, 830-836.

    Rauen, T., Rothstein, J.F., and Wassle, H. (1996) Differential expression of three glutamate transporter subtypes in the rat retina. Cell Tissue Res., 286, 325-336.

    Sasaki, T. and Kaneko, A. (1996) L-glutamate-induced responses in OFF-type bipolar cells of the cat retina. Vision Res., 36, 787-795.

    Saugstad, J.A., Kinzie, J.M., Mulvihill, E.R., Segerson, T.P., and Westbrook, G.L. (1994) Cloning and expression of a new member of the L-2-amino-4-phosphobutyric acid-sensitive class of metabotropic glutamate receptors. Mol. Pharmacol., 45, 367-372.

    Schultz, K. and Stell, W.K. (1996) Immunocytochemical localization of the high-affinity glutamate transporter, EAAC1, in the retina of representative vertebrate species. Neurosci. Lett., 211, 191-194.

    Schwartz, E.A. and Tachibana, M. (1990) Electrophysiology of glutamate and sodium co-transport in a glial cell of the salamander retina. J. Physiol., 426, 43-80.

    Slaughter, M.M. and Miller, R.F. (1981) 2-amino-4-phosphobutyric acid: a new pharmacological tool for retina research. Science, 211, 182-184.

    Slaughter, M.M. and Miller, R.F. (1983) The role of excitatory amino acid transmitters in the mudpuppy retina: an analysis with kainic acid and N-methyl aspartate. J. Neurosci., 3, 1701-1711.

    Stryer, L. (1988) Biochemistry, 3rd edition. W.H. Freeman and Co. New York.

    Tabb, J.S. and Ueda, T. (1991) Phylogenetic studies on the synaptic vesicle glutamate transporter. J. Neurosci., 11, 1822-1828.

    Tachibana, M. and Kaneko, A. (1988) L-glutamate-induced depolarization in solitary photoreceptors: a process that may contribute to the interaction between photoreceptors in situ. Proc. Natl. Acad. Sci., 85, 5315-5319.

    Takahashi, K.-I. and Copenhagen, D.R. (1992) APB suppresses synaptic input to retinal horizontal cells in fish: a direct action on horizontal cells modulated by intracellular pH. J. Neurophysiol., 67, 1633-1642.

    Tanabe, Y., Masu, M., Ishii, T., Shigemoto, R., and Nakanishi, S. (1992) A family of metabotropic glutamate receptors. Neuron, 8, 169-179.

    Taylor, W.R. and Wassle, H. (1995) Receptive field properties of starburst cholinergic amacrine cells in the rabbit retina. Eur. J. Neurosci., 7, 2308-2321.

    Van Haesendonck, E. and Missotten, L. (1990) Glutamate-like immunoreactivity in the retina of a marine teleost, the dragonet. Neurosci. Lett., 111, 281-286.

    Verdoorn, T.A., Burnashev, N., Monyer, H., Seeburg, P.H., and Sakmann, B. (1991) Structural determinants of ion flow through recombinant glutamate receptor channels. Science, 252, 1715-1718.

    Wenzel, A., Benke, D., Mohler, H., and Fritschy, J.-M. (1997) N-methyl-D-aspartate receptors containing the NR2D subunit in the retina are selectively expressed in rod bipolar cells. Neuroscience, 78, 1105-1112.

    Westbrook, G.L. and Mayer, M.L. (1987) Micromolar concentrations of Zn+2 antagonize NMDA and GABA responses of hippocampal neurons. Nature, 328, 640-643.

    Williams, K., Zappia, A.M., Pritchett, D.B., Shen, Y.M., and Molinoff, P.B. (1994) Sensitivity of the N-Methyl-D-Aspartate receptor to polyamines is controlled by NR2 subunits. Mol. Pharmacol., 45, 803-809.

    Yamada, K.A. and Tang, C.-M. (1993) Benzothiadiazides inhibit rapid glutamate receptor desensitization and enhance glutamatergic synaptic currents. J. Neurosci., 13, 3904-3915.

    Yang, C.-Y. (1996) Glutamate immunoreactivity in the tiger salamander retina differentiates between GABA -immunoreactive and glycine-immunoreactive amacrine cells. J. Neurocytol., 25, 391-403.

    Yang, C.-Y. and Yazulla, S. (1994). Glutamate-, GABA-, and GAD-immunoreactivities co-localize in bipolar cells of tiger salamander retina. Vis. Neurosci., 11, 1193-1203.

    Yang, J.H. and Wu, S.M. (1997) Characterization of glutamate transporter function in the tiger salamander retina. Vision Res.,37, 827-838.

    Yang, X.-L. and Wu, S.M. (1991) Coexistence and function of glutamate receptor subtypes in the horizontal cells of the tiger salamander retina. Vis. Neurosci., 7, 377-382.

    Yazulla, S. (1986) GABAergic neurons in the retina. Progr. Retinal Res., 5, 1-52.

    Yu, W. and Miller, R.F. (1995) NBQX, an improved non-NMDA antagonist studied in retinal ganglion cells. Brain Res., 692, 190-194.

    Zhou, Z.J., Fain, G.L., and Dowling, J.E. (1993) The excitatory and inhibitory amino acid receptors on horizontal cells isolated from the white perch retina. J. Neurophysiol., 70, 8-19.


      Table 1

    Metabotropic glutamate receptor groups (from Pin and Duvoisin, 1995).
    group mGluR agonist(s) intracellular pathway
    I mGluR1, mGluR5 quisqualate, ACPD increase phospholipase C activity, increase cAMP levels, increase protein kinase A activity
    II mGluR2, mGluR3 L-CCG-1, ACPD decrease cAMP levels
    III mGluR4, mGluR6. mGluR7, mGluR8 L-AP4 (APB) decrease cAMP or cGMP levels

      Table 2

    Glutamate receptor types on retinal neurons, electrophysiological measurements.

    retinal cell type non-NMDA receptor NMDA receptor mGluR Glutamate receptor with transporter-like pharmacology species references

    ++ (cones) salamander Eliasof & Werblin, 1993; Picaud et al., 1995b

    ++ (rods) salamander Grant & Werblin, 1996

    OFF-bipolar cells ++

    mudpuppy Slaughter & Miller, 1981, 1983

    cat Sasaki & Kaneko, 1996

    salamander Hensley et al., 1993

    rat Euler et al., 1996

    mudpuppy Slaughter & Miller, 1983

    ON-bipolar cells ++
    mudpuppy Slaughter & Miller, 1981, 1983

    ++(APB) ++ white perch Grant & Dowling, 1995, 1996

    salamander Hirano & MacLeish, 1991

    salamander Hensley et al., 1993

    rat Euler et al., 1996

    ++(APB and cGMP)
    salamander Nawy & Jahr, 1990

    ++(APB and cGMP)
    cat de la Villa et al., (1995)

    horizontal cells ++

    white perch Zhou et al., 1993

    mudpuppy Slaughter & Miller, 1983

    Yang & Wu, 1991
    ++ ++

    catfish O'Dell & Christensen, 1989; Eliasof & Jahr, 1997

    amacrine cells ++(AII)

    rat Boos et al., 1993
    ++ ++

    mudpuppy Slaughter & Miller, 1983
    ++ ++

    rabbit Massey & Miller, 1988
    ++ ++

    rat Hartveit & Veruki, 1997
    ++ (transient & sustained AC) ++ (transient AC)

    salamander Dixon & Copenhagen, 1992

    ganglion cells ++ ++

    salamander Diamond & Copenhagen, 1993; Mittman et al., 1990; Hensley et al., 1993
    ++ ++

    primates Cohen & Miller, 1994
    ++ ++

    rat Aizenman et al., 1988
    ++ ++

    mudpuppy Slaughter & Miller, 1983
    ++ ++

    cat Cohen et al., 1994
    ++ ++

    rabbit Massey & Miller, 1988; 1990

      Table 3

    Ionotropic glutamate receptor expression in retinal neurons and retinal layers, immunocytochemistry and in situ hybridization.
    retinal cell type or layer non-NMDA receptor subunits NMDA receptor subunits species references
    photoreceptors GluR6/7 (single cone outer segments)
    goldfish Peng et al., 1995
    GluR1 (cone pedicles)
    cat Pourcho et al., 1997

    OPL GluR2, GluR2/3, GluR6/7
    rat Peng et al., 1995

    NR2A (punctate) cat Hartveit et al., 1994
    GluR2, GluR2/3 (photoreceptors)
    goldfish Peng et al., 1995

    bipolar cells GluR2 (Mb cells)
    goldfish Peng et al., 1995
    GluR2, GluR2/3
    rat Peng et al., 1995

    NR2D (RBC) rat Wenzel et al., 1997
    GluR2 and/or GluR4 NR1 (RBC) rat Hughes, 1997
    GluR2 (RBC)
    rat Hughes et al., 1992

    horizontal cells GluR6/7
    goldfish Peng et al., 1995
    cat Pourcho et al., 1997

    INL GluR2/3, GluR6/7
    rat Peng et al., 1995

    NR2A(inner) rat Hartveit et al., 1994
    GluR1, 2, 5 > GluR4 (outer third), GluR1, 2, 5 (middle third), GluR1-5 (inner third)
    rat Hughes et al. 1992
    rat, cat Hamassaki-Britto et al., 1993
    KA2 (homogenous), GluR6 (inner), GluR7 (inner two thirds) NR1 (homogenous), NR2A-B (inner third, patchy), NR2C (inner two-thirds) rat Brandstatter et al., 1994

    IPL GluR1, GluR2/3, GluR6/7
    rat Peng et al., 1995

    NR2A rat, cat, rabbit, monkey Hartveit et al., 1994

    amacrine cells GluR6 NR2A-C rat Brandstatter et al., 1994
    cat Pourcho et al., 1997
    GluR1, GluR2/3
    rat Peng et al., 1995

    ganglion cells GluR1
    rat Peng et al., 1995

    GCL GluR2/3, GluR6/7
    rat Peng et al., 1995
    rat Hughes et al., 1992
    rat, cat Hamassaki-Britto et al., 1993
    GluR6-7, KA2 NR1, NR2A-C rat Brandstatter et al., 1994

    Muller cells GluR4
    rat Peng et al., 1995

      Table 4

    Metabotropic glutamate receptor expression in retinal neurons and retinal layers, immunocytochemistry and in situ hybridization.
    retinal cell type or layer Group 1 Group II Group III species references
    OPL mGluR1alpha, mGluR5a (RBC dendrites)

    rat Koulen et al., 1997

    mGluR6 (RBC dendrites) rat Nomura et al., 1994


    mGluR8 mouse Duvoisin et al., 1995

    mGluR6 rat Nakajima et al., 1993
    mGluR5 (BC, HC), mGluR1 (AC) mGluR2 (AC) mGluR6 (RBC), mGluR7 (BC), mGluR4,7 (AC) rat Hartveit et al., 1995

    IPL mGluR1alpha

    rat Peng et al., 1995

    mGluR7 (CBC terminals; AC dendrites; few GC dendrites) rat Brandstatter et al., 1996
    mGluR1alpha, mGluR5a (AC dendrites)

    rat Koulen et al., 1997

    amacrine cells mGluR1alpha

    rat Peng et al., 1995
    mGluR1alpha mGluR2/3
    cat Pourcho et al., 1997

    ganglion cells mGluR1alpha

    rat Peng et al., 1995


    mGluR8 mouse Duvoisin et al., 1995
    mGluR1alpha mGluR2/3
    cat Pourcho et al., 1997
    mGluR1 mGluR2 mGluR4, 7 rat Hartveit et al., 1995

      Table 5

    Glutamate transporters in retinal neurons and retinal layers, immunocytochemical localizations.
    retinal cell type EAAC-1 GLAST GLT-1 species reference

    + (cone soma to pedicles) rabbit Massey et al., 1997

    OPL ++

    rat Rauen et al., 1996

    ++ (rod spherules > cone pedicles) rabbit Massey et al., 1997

    horizontal cells ++

    rat Schultz & Stell, 1996; Rauen et al., 1996

    bipolar cells

    ++ (2 types of CBCs)
    rabbit Massey et al., 1997
    ++ (faint)
    ++ rat Rauen et al., 1996

    turtle, salamander Schultz & Stell, 1996

    ++ (DB2, flat midget bipolar cells) monkey Grunert et al., 1994


    ++ (diffuse) rabbit Massey et al., 1997

    ++ rat Rauen et al., 1996

    goldfish, salamander, turtle, chicken, rat Schultz & Stell, 1996

    amacrine cells ++
    ++ rat Rauen et al., 1996

    Schultz & Stell, 1996

    ganglion cells ++

    chicken, rat, goldfish, turtle Schultz & Stell, 1996

    rat Rauen et al., 1996

    Muller cells
    rat Rauen et al., 1996; Lehre et al., 1997; Deroiche & Rauen, 1995

    The author

    Dr. Victoria Connaughton was born in Sellersville Pennsylvania. She received her B.A. from Bucknell University in Biology in 1989 and her Ph. D. in Marine Studies from The University of Delaware in 1994. She is currently an Assistant Professor in the Biology Department at American University, Washington D.C. In her thesis work under Dr. Charles Epifano, she studied the visually guided feeding behavior of larval fish. Dr. Connaughton pursued postdoctoral studies with Dr. Greg Maguire at the University of Houston, and Dr. Ralph Nelson at the National Institutes of Health. Dr. Connaughton's current research interests include electrophysiological examination of zebrafish mutants with visual system defects and the characterization of light responses in zebrafish retinal bipolar cells.

    Updated: July, 2011