Color vision in mammals is based on the expression of at least two cone opsins that are sensitive to different wavelengths of light. Furthermore, retinal pathways conveying color-opponent signals are required for color discrimination. Most of the primates are trichromats, and “color-coded channels” of their retinas are unveiled to a large extent. In contrast, knowledge of cone-selective pathways in nonprimate dichromats is only slowly emerging, although retinas of dichromats like mice or rats are extensively studied as model systems for retinal information processing. Here, we review recent progress of research on color-coded pathways in nonprimate dichromats to identify differences or similarities between di- and trichromatic mammals. In addition, we applied immunohistochemical methods and confocal microscopy to retinas of different species and present data on their neuronal properties, which are expected to contribute to color vision. Basic neuronal features such as the “blue cone bipolar cell” exist in every species investigated so far. Moreover, there is increasing evidence for chromatic OFF channels in dichromats and retinal ganglion cells that relay color-opponent signals to the brain. In conclusion, di- and trichromats share similar retinal pathways for color transmission and processing.

Glutamate is believed to be the primary excitatory neurotransmitter in the vertebrate retina, and its fast postsynaptic effects are elicited by activating NMDA-, kainate-, or AMPA-type glutamate receptors. We have characterized the ionotropic glutamate receptors present on retinal horizontal cells of the skate, which possess a unique all-rod retina simplifying synaptic circuitry within the outer plexiform layer (OPL). Isolated external horizontal cells were examined using whole-cell voltage-clamp techniques. Glutamate and its analogues kainate and AMPA, but not NMDA, elicited dose-dependent currents. The AMPA receptor antagonist GYKI 52466 at 100 μm abolished glutamate-elicited currents. Desensitization of glutamate currents was removed upon coapplication of cyclothiazide, known to potentiate AMPA receptor responses, but not by concanavalin A, which potentiates kainate receptor responses. The dose–response curve to glutamate was significantly broader in the presence of the desensitization inhibitor cyclothiazide. Polyclonal antibodies directed against AMPA receptor subunits revealed prominent labeling of isolated external horizontal cells with the GluR2/3 and GluR4 antibodies. 1-Naphthylacetyl spermine, known to block calcium-permeable AMPA receptors, significantly reduced glutamate-gated currents of horizontal cells. Downregulation of glutamate responses was induced by increasing extracellular ion concentrations of Zn2+ and H+. The present study suggests that Ca2+-permeable AMPA receptors likely play an important role in shaping the synaptic responses of skate horizontal cells and that alterations in extracellular concentrations of calcium, zinc, and hydrogen ions have the potential to regulate the strength of postsynaptic signals mediated by AMPA receptors within the OPL.

Preclinical and clinical studies have shown that N-methyl-d-aspartate (NMDA) receptor antagonists can exert antidepressant effects. Thus, a single intravenous injection of ketamine, a non-competitive NMDA receptor antagonists, has been recently demonstrated to produce a rapid and relatively sustained antidepressant effect in patients. Therefore, the role of NMDA receptors and their signalling pathways for pathophysiology and therapy of depression are under intense research.

Some neurochemical features of the neuronal circuitry regulating dopamine release were examined in the retina of the turtle, Pseudemys scripta elegans. Glutamate antagonists that block hyperpolarizing bipolar cells, such as 2,3 piperidine dicarboxylic acid (PDA), produced dose-dependent dopamine release. In contrast, the glutamate agonist 2-amino-4-phosphonobutyric acid (APB), which blocks depolarizing bipolar cell responses with high specificity, had no effect on the release of dopamine. The γ-aminobutyric acid (GABA) antagonist, bicuculline, also produced potent dose-dependent release of dopamine. The release of dopamine produced by PDA was blocked by exogenous GABA and muscimol, suggesting that the PDA-mediated release process was polysynaptic and involved a GABAergic synapse interposed between the bipolar and dopaminergic amacrine cells. The only other agents that produced dopamine release were chloride-free media and high extracellular K+; in particular, kainic acid and glutamate itself were ineffective. These results suggest that the primary neuronal chain mediating dopamine release in the turtle retina is: cone → hyperpolarizing bipolar cell → GABAergic amacrine cell → dopaminergic amacrine cell.

Effects of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), 2-amino-4-phosphonobutyrate (APB), cis-2,3-piperidine dicarboxylic acid (PDA), and kynurenate (KYN) on the depolarizing actions of glutamate and kainate on horizontal cells (HCs) were studied in the larval tiger salamander retina. APB, PDA, and KYN hyperpolarized the HCs, but they failed to block either the actions of glutamate and kainate, or the HC light responses. APB and PDA did not cause membrane polarizations in either rods or cones, suggesting that the HC hyperpolarizations were not mediated by presynaptic actions of these compounds. CNQX, the newly synthesized non-NMDA (N-Methyl-D-Aspartate) receptor antagonist, blocked the HC light responses and the action of kainate, but not that of glutamate. These results suggest that the synaptic receptors in the tiger salamander HCs are probably non-NMDA although extra-synaptic NMDA receptors may exist in these cells.

The biochemical effects triggered by the action of glutamate, the main excitatory amino acid, on a specialized type of glia cells, Bergmann glial cells of the cerebellum, are a model system with which to study glia–neuronal interactions. Neuron to Bergmann glia signaling is involved in early stages of development, mainly in cell migration and synaptogenesis. Later, in adulthood, these cells have an important role in the maintenance and proper function of the synapses that they surround. Major molecular targets of this cellular interplay are glial glutamate receptors and transporters, both of which sense synaptic activity. Glutamate receptors trigger a complex network of signaling cascades that involve Ca2+ influx and lead to a differential gene-expression pattern. In contrast, Bergmann glia glutamate transporters participate in the removal of the neurotransmitter from the synaptic cleft and act also as signal transducers that regulate, in the short term, their own activity. These exciting findings strengthen the concept of active participation of glial cells in synaptic transmission and the involvement of neuron–glia circuits in the processing of brain information.

We have examined the distribution of metabotropic glutamate receptors (mGluRs) 1 and 5 in the adult chicken retina using preembedding immuno-electronmicroscopy. Immunoreactivity for mGluRs 1 and 5 was found in both the outer plexiform layer (OPL) and the inner plexiform layer (IPL). For mGluR1, OPL labeling was observed at cone pedicles and horizontal and bipolar cell processes. In the IPL, mGluR1 labeling could be found on bipolar cell terminals, as well as postsynaptic processes, including amacrine cell processes. Neither presynaptic nor postsynaptic elements were labeled at rod synapses. For mGluR5, OPL labeling was associated with cone pedicles as well as bipolar and horizontal cell processes. As for mGluR1, rod synapses were unlabeled. In the IPL, labeling for mGluR5 was found on bipolar cell terminals and amacrine cell processes. The presynaptic expression of these receptors in the OPL was confirmed at the light level by double-labeling experiments with SV2. The distributions of mGluRs 1 and 5 indicate that they have the potential to regulate function in both synaptic layers. Furthermore, the similar expression patterns for these two receptors indicate that they might be co-expressed and thus have the potential to interact functionally.

The separation of OFF pathways that signal light decrements from ON pathways that signal light increments occurs at the first retinal synapse. The dendrites of OFF bipolar cells abut the cone pedicle at basal positions distal to the site of glutamate release and express ligand-gated or ionotropic glutamate receptors (GluR). The dendrites of ON bipolar cells penetrate narrow invaginations of the cone pedicle proximal to the site of release and express the G-protein-coupled, metabotropic glutamate receptor, mGluR6. However, recent studies demonstrating the expression of GluR subunits in the rodent rod bipolar cell, known to yield an ON response to light, call this basic segregation of receptors into question. The light-microscopic distribution of many glutamate receptors in the primate retina is now well established. We reexamined their ultrastructural localization in the outer retina of Macaca fascicularis to test systematically whether invaginating dendrites at the cone synapse, presumably from ON bipolar cells, also express one or more ionotropic subunits. Using preembedding immunocytochemistry for electron microscopy, we quantified the distribution of the AMPA-sensitive subunits GluR2/3 and GluR4 and of the kainate-sensitive subunits GluR6/7 across 207 labeled dendrites occupying specific morphological loci at the cone pedicle. We report, in agreement with published investigations, that the majority of labeled processes for GluR2/3 (70%) and GluR4 (67%) either occupy basal positions or arise from horizontal cells. For GluR6/7, we find a significantly lower fraction of labeled processes at these positions (47%). We also find a considerable number of labeled dendrites for GluR2/3 (10%), GluR4 (21%), and GluR6/7 (18%) at invaginating positions. Surprisingly, for each subunit, the remainder of labeled processes corresponds to “fingers” of presynaptic cytoplasm within the cone invagination.

Brain injury following cerebral ischaemia (stroke) involves a complex combination of pathological processes, including excitotoxicity and inflammation leading to necrotic and apoptotic forms of cell death. At the cellular level, excitotoxicity is mediated by glutamate and its cognate receptors, resulting in increased intracellular calcium and free radical production, and eventual cell death. Recent evidence suggests that scaffolding molecules that associate with glutamate receptors at the postsynaptic density allow coupling of receptor activity to specific second messengers capable of mediating excitotoxicity. These findings have important implications in the search for effective neuroprotective therapies in treating stroke.

Immunocytochemical localization was carried out for five isoforms of protein kinase C (PKC) in the cat retina. In common with other mammalian species, PKCα was found in rod bipolar cells. Staining was also seen in a small population of cone bipolar cells with axon terminals ramifying near the middle of the inner plexiform layer (IPL). PKCβI was localized to rod bipolar cells, one class of cone bipolar cell, and numerous amacrine and displaced amacrine cells. Staining for PKCβII was seen in three types of cone bipolar cells as well as in amacrine and ganglion cells. Immunoreactivity for both PKCε and PKCζ was found in rod bipolar cells; PKCε was also seen in a population of cone bipolar cells and a few amacrine and ganglion cells whereas PKCζ was found in all ganglion cells. Double-label immunofluorescence studies showed that dendrites of the two PKCβII-positive OFF-cone bipolar cells exhibit immmunoreactivity for the kainate-selective glutamate receptor GluR5. The third PKCβII cone bipolar is an ON-type cell and did not stain for GluR5. The retinal distribution of these isoforms of PKC is consistent with a role in modulation of various aspects of neurotransmission including synaptic vesicle release and regulation of receptor molecules.

Visual information is encoded at the photoreceptor synapse by modulation of the tonic release of glutamate from one or more electron-dense ribbons. This release is highest in the dark, when photoreceptors are depolarized, and decreases in grades when photoreceptors hyperpolarize with increasing light. Functional diversity between neurons postsynaptic at the synaptic ribbon arises in part from differential expression of both metabotropic (G-protein-gated) and ionotropic (ligand-gated) glutamate receptor. In the brain, different subunits also modulate the presynaptic active zone. In hippocampus, ionotropic kainate receptors localize to the presynaptic membrane of glutamatergic axon terminals and facilitate depolarization of the synapse (e.g. Lauri et al., 2001). Such facilitation may be helpful in the retina, where consistent depolarization of the photoreceptor axon terminal is necessary to maintain glutamate release in the dark. We investigated whether such a mechanism could be present in primate retina by using electron microscopy to examine the localization of the kainate subunits GluR6/7 at the rod axon terminal, where only a single ribbon synapse mediates glutamate release. We scored 54 rod axon terminals whose postsynaptic space contained one or more GluR6/7-labeled processes and traced these processes through serial sections to determine their identity. Of 68 labeled processes, 63% originated from narrow “fingers” of cytoplasm extending from the presynaptic axon terminal into the postsynaptic cleft. Each rod terminal typically inserts 4–6 presynaptic fingers, and we scored several instances where multiple fingers contained label. Such consistency suggests that each presynaptic finger expresses GluR6/7. The physiological properties of kainate receptors and the geometry of the rod axon terminal suggest that presynaptic GluR6/7 could provide a steady inward current to maintain consistent depolarization of the rod synapse in the long intervals between photons in the dark.

To determine how a progressive loss of photoreceptor cells and the concomitant loss of glutamatergic input to second-order neurons can affect inner-retinal signaling, glutamate receptor expression was analyzed in the Royal College of Surgeons (RCS) rat, an animal model of retinitis pigmentosa. Immunohistochemistry was performed on retinal sections of RCS rats and congenic controls between postnatal (P) day 3 and the aged adult (up to P350) using specific antibodies against N-methyl-D-aspartate (NMDA) subunits. All NMDA subunits (NR1, NR2A–2D) were expressed in control and dystrophic retinas at all ages, and distinct patterns of labeling were found in horizontal cells, subpopulations of amacrine cells and ganglion cells, as well as in the outer and inner plexiform layer (IPL). NR1 immunoreactivity in the inner plexiform layer of adult control retinas was concentrated in two distinct bands, indicating a synaptic localization of NMDA receptors in the OFF and ON signal pathways. In the RCS retina, these bands of NR1 immunoreactivity in the IPL were much weaker in animals older than P40. In parallel, NR2B immunoreactivity in the outer plexiform layer (OPL) of RCS rats was always reduced compared to controls and vanished between P40 and P120. The most striking alteration observed in the degenerating retina, however, was a strong expression of NR1 immunoreactivity in Müller cell processes in the inner retina which was not observed in control animals and which was present prior to any visible sign of photoreceptor degeneration. The results suggest functional changes in glutamatergic receptor signaling in the dystrophic retina and a possible involvement of Müller cells in early processes of this disease.

Mechanisms that mediate the calcium influx in mammalian horizontal cells were studied. Horizontal cells (HCs) enzymatically dissociated from the rabbit retina were recorded by the whole-cell configuration of the patch-clamp technique and by calcium image ratioespectrophotometry of Fura-2 loaded cells. AMPA-preferring glutamate receptors were shown to permeate Ca2+ in mammalian HCs by ionic substitution experiments. Furthermore, after blocking the L-type calcium current with nifedipine (100 μM), calcium current through the AMPA-preferring glutamate receptors was measured. Calcium image ratioespectrophotometry was performed on the dissociated HCs in order to determine the changes in the intracellular calcium ([Ca2+]i). Fura-2 microspectrophotometry showed that in HCs, K+-induced cell depolarization promoted an increase in [Ca2+]i, mediated by the L-type calcium channels, since it was abolished in the presence of nifedipine. The increase in [Ca2+]i upon cell depolarization was observed throughout each cell; however, it was maximal at the cell soma. Activation of glutamate receptors in dissociated HCs by glutamate, AMPA or kainate promoted an increase in [Ca2+]i. This increase in [Ca2+]i was abolished in nominally Ca2+-free solution (0 mM Ca2+); in contrast, nifedipine decreased the glutamate-induced influx of calcium in ca. 50%. The present study demonstrates that calcium may permeate through glutamate receptors expressed in HCs of the rabbit retina.

The basal ganglia consist of several interconnected nuclei located in the telecephalon, diencephalon and mesencephalon that are involved in a variety of motor and non-motor behavioural functions. Glutamate receptors play a major role in neurotransmission within the basal ganglia and are present in all nuclei of the basal ganglia. This review focuses on the contribution of the NMDA class of glutamatergic receptors to various movement disorders whose primary pathology lies within the basal ganglia and discusses how pharmacological manipulation of such receptors may be therapeutically useful.

Excitatory amino acid (EAA)-induced production of inositolphosphates (IPs) was studied in primary cultures of chick retinal pigment epithelium (RPE) following in vitro incorporation of [3H] myo-inositol. Glutamic acid (L-glu) significantly increased [3H]-IPs accumulation (215%). L-glu agonists stimulated [3H]IPs accumulation in the following order of efficiency: N-methyl-D-aspartate (NMDA) ≥ L-glu > quisqualate ≥ kainate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD). Stimulation was dependent on external Ca2+. The NMDA-induced response was blocked by (+)-5-methyl-10,11-dihydro-5H-dibenzo-cyclohepten-5,10-imine maleate (MK-801) and 3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP) and was decreased by the L-Ca2+-channel blockers verapamil and nifedipine as well as by dantrolene. The metabotropic glutamate receptor (mGluR) antagonist (+)-α-methyl-4-carboxyphenylglycine (+)MCPG inhibited 3,5-dihydroxyphenylglycine (DHPG) and ACPD-induced stimulation, which demonstrates the presence in RPE of mGluRs 1 and/or 5, as well as NMDA receptors coupled directly, or through the influx of external Ca2+, to phospholipase C activation. L-glu agonists showed no effect either on basal level of intracellular cyclic adenosine monophosphate, nor on forskolin- or carbachol-induced stimulation of adenylyl cyclase. Since L-glu is released from the retina upon illumination, and receptors for this compound are present in RPE, the activation of the inositide pathway could be involved in the regulation of retina-RPE interaction, which is essential for the visual process.