Uptake of aspartic and glutamic acid by photoreceptors in goldfish retina

Comparative electrophysiology of retinal Müller glial cells-A survey on vertebrate species

Müller cells are the dominant macroglial cells in the retina of all vertebrates. They fulfill a variety of functions important for retinal physiology, among them spatial buffering of K⁺ ions and uptake of glutamate and other neurotransmitters. To this end, Müller cells express inwardly rectifying K⁺ channels and electrogenic glutamate transporters. Moreover, a lot of voltage- and ligand-gated ion channels, aquaporin water channels, and electrogenic transporters are expressed in Müller cells, some of them in a species-specific manner. For example, voltage-dependent Na⁺ channels are found exclusively in some but not all mammalian species. Whereas a lot of data exist from amphibians and mammals, the results from other vertebrates are sparse. It is the aim of this review to present a survey on Müller cell electrophysiology covering all classes of vertebrates. The focus is on functional studies, mainly performed using the whole-cell patch-clamp technique. However, data about the expression of membrane channels and transporters from immunohistochemistry are also included. Possible functional roles of membrane channels and transporters are discussed. Obviously, electrophysiological properties involved in the main functions of Müller cells developed early in vertebrate evolution. GLIA 2017;65:533-568.

Sigma receptors [σRs]: biology in normal and diseased states

This review compares the biological and physiological function of Sigma receptors [σRs] and their potential therapeutic roles. Sigma receptors are widespread in the central nervous system and across multiple peripheral tissues. σRs consist of sigma receptor one (σ₁R) and sigma receptor two (σ₂R) and are expressed in numerous regions of the brain. The sigma receptor was originally proposed as a subtype of opioid receptors and was suggested to contribute to the delusions and psychoses induced by benzomorphans such as SKF-10047 and pentazocine. Later studies confirmed that σRs are non-opioid receptors (not an µ opioid receptor) and play a more diverse role in intracellular signaling, apoptosis and metabolic regulation. σ₁Rs are intracellular receptors acting as chaperone proteins that modulate Ca²⁺ signaling through the IP₃ receptor. They dynamically translocate inside cells, hence are transmembrane proteins. The σ₁R receptor, at the mitochondrial-associated endoplasmic reticulum membrane, is responsible for mitochondrial metabolic regulation and promotes mitochondrial energy depletion and apoptosis. Studies have demonstrated that they play a role as a modulator of ion channels (K⁺ channels; N-methyl-d-aspartate receptors [NMDAR]; inositol 1,3,5 triphosphate receptors) and regulate lipid transport and metabolism, neuritogenesis, cellular differentiation and myelination in the brain. σ₁R modulation of Ca²⁺ release, modulation of cardiac myocyte contractility and may have links to G-proteins. It has been proposed that σ₁Rs are intracellular signal transduction amplifiers. This review of the literature examines the mechanism of action of the σRs, their interaction with neurotransmitters, pharmacology, location and adverse effects mediated through them.

Protein surface recognition by calixarenes

The present review summarizes recently developed calixarene derivatives for protein surface recognition which are able to identify, inhibit, and separate specific proteins.

Retinal amino acid neurochemistry of the southern hemisphere lamprey, Geotria australis

Lampreys are one of the two surviving groups of the agnathan (jawless) stages in vertebrate evolution and are thus ideal candidates for elucidating the evolution of visual systems. This study investigated the retinal amino acid neurochemistry of the southern hemisphere lamprey Geotria australis during the downstream migration of the young, recently-metamorphosed juveniles to the sea and during the upstream migration of the fully-grown and sexually-maturing adults to their spawning areas. Glutamate and taurine were distributed throughout the retina, whilst GABA and glycine were confined to neurons of the inner retina matching patterns seen in most other vertebrates. Glutamine and aspartate immunoreactivity was closely matched to Müller cell morphology. Between the migratory phases, few differences were observed in the distribution of major neurotransmitters i.e. glutamate, GABA and glycine, but changes in amino acids associated with retinal metabolism i.e. glutamine and aspartate, were evident. Taurine immunoreactivity was mostly conserved between migrant stages, consistent with its role in primary cell functions such as osmoregulation. Further investigation of glutamate signalling using the probe agmatine (AGB) to map cation channel permeability revealed entry of AGB into photoreceptors and horizontal cells followed by accumulation in inner retinal neurons. Similarities in AGB profiles between upstream and downstream migrant of G. australis confirmed the conservation of glutamate neurotransmission. Finally, calcium binding proteins, calbindin and calretinin were localized to the inner retina whilst recoverin was localized to photoreceptors. Overall, conservation of major amino acid neurotransmitters and calcium-associated proteins in the lamprey retina confirms these elements as essential features of the vertebrate visual system. On the other hand, metabolic elements of the retina such as neurotransmitter precursor amino acids and Müller cells are more sensitive to environmental changes associated with migration.

Glutamate metabolic pathways and retinal function

Glutamate is a major neurotransmitter in the CNS but is also a key metabolite intimately coupled to amino acid production/degradation. We consider the effect of inhibition of two key glutamate metabolic enzymes: glutamine synthetase (GS) and aspartate aminotransferase on retinal function assessed using the electroretinogram to consider photoreceptoral (a-wave) and post-receptoral (b-wave) amplitudes. Quantitative immunocytochemistry was used to assess amino acid levels within photoreceptors, ganglion and Müller cells secondary to GS inhibition. Intravitreal injections of methionine sulfoximine reduced GS immunoreactivity in the rat retina. Additionally, glutamate and its precursor aspartate was reduced in photoreceptors and ganglion cells, but elevated in Müller cells. This reduction in neuronal glutamate was consistent with a deficit in neurotransmission (-75% b-wave reduction). Exogenous glutamine supply completely restored the b-wave, whereas other amino acid substrates (lactate, pyruvate, alpha-ketoglutarate, and succinate) only partially restored the b-wave (16-20%). Inhibition of the aminotranferases using aminooxyacetic acid had no effect on retinal function. However, aminooxyacetic acid application after methionine sulfoximine further reduced the b-wave (from -75% to -92%). The above data suggest that de novo glutamate synthesis involving aspartate aminotransferase can partially sustain neurotransmission when glutamate recycling is impaired. We also show that altered glutamate homeostasis results in a greater change in amino acid distribution in ganglion cells compared with photoreceptors.

Neurotransmitter-specific identification and characterization of neurons in the all-cone retina of Anolis carolinensis II: Glutamate and aspartate

Immunocytochemical and autoradiographic methods were used to identify neurons in the pure cone retina of the lizard (Anolis carolinensis) that are likely to employ glutamate (GLU) or aspartate (ASP) as a neurotransmitter.

GLU immunocytochemistry demonstrated high levels of endogenous GLU in all cone types and numerous bipolar cells. Moderate GLU levels were found in horizontal and ganglion cells. Müller cells and most amacrine cells had very low GLU levels. GLU immunoreactivity (GLU-IR) in the cones was present from the inner segment to the synaptic pedicle. A large spherical cell type with moderate GLU-IR was identified in the proximal inner plexiform layer (IPL). These cells also contain ASP and have been tentatively identified as amacrine cells. Uptake of [3H]-L-GLU labeled all retinal layers. All cone types and Müller cells sequestered [3H]-D-ASP, a substrate specific for the GLU transporter.

Anti-ASP labeling was observed in cones, horizontal cells, amacrine cells, and cells in the ganglion cell layer. ASP immunoreactivity (ASP-IR) in the cones was confined to the inner segment. One ASP-containing pyriform amacrine cell subtype ramifying in IPL sublamina b was identified.

Analysis of GLU-IR, ASP-IR, and GABA-IR on serial sections indicated that there were two distinct populations of horizontal cells in the Anolis retina: one containing GABA-IR, GLU-IR, and ASP-IR; and another type containing only GLU-IR and ASP-IR. Light GLU-IR was frequently found in GABA-containing amacrine cells but ASP-IR was not.

The distinct distributions of GLU and ASP may indicate distinctly different roles for these amino acids. GLU, not ASP, is probably the major neurotransmitter in the cone-biploar-ganglion cell pathway of the Anolis retina. Both GLU and ASP are present in horizontal cells and specific subpopulations of amacrine cells, but it is unclear if GLU or ASP have a neurotransmitter role in these cells.

Endocannabinoids in the retina: from marijuana to neuroprotection

The active component of the marijuana plant Cannabis sativa, Delta9-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 works on cannabinoids have 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 adaptations, 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.

Mechanism linking NMDA receptor activation to modulation of voltage-gated sodium current in distal retina

In this study, we investigated the mechanism that links activation of N-methyl-D-aspartate (NMDA) receptors to inhibition of voltage-gated sodium channels in isolated catfish cone horizontal cells. NMDA channels were activated in voltage-clamped cells incubated in low-calcium saline or dialyzed with the calcium chelator BAPTA to determine that calcium influx through NMDA channels is required for sodium channel modulation. To determine whether calcium influx through NMDA channels triggers calcium-induced calcium release (CICR), cells were loaded with the calcium-sensitive dye calcium green 2 and changes in relative fluorescence were measured in response to NMDA. Responses were compared with measurements obtained when caffeine depleted stores. Voltage-clamp studies demonstrated that CICR modulated sodium channels in a manner similar to that of NMDA. Blocking NMDA receptors with AP-7, blocking CICR with ruthenium red, depleting stores with caffeine, or dialyzing cells with calmodulin antagonists W-5 or peptide 290-309 all prevented sodium channel modulation. These results support the hypothesis that NMDA modulation of voltage-gated sodium channels in horizontal cells requires CICR and activation of a calmodulin-dependent signaling pathway.

Retinal neurochemical changes following application of glutamate as a metablolic substrate

BACKGROUND

Retinal neural and glial cells share an intricate relationship that includes uptake and recycling of the amino acid neurotransmitters, glutamate and gamma-amino butyric acid (GABA), as well as metabolic links. The aim of this work was to determine the neurochemical and morphological changes induced by the removal of glucose but with the provision of exogenous glutamate in the isolated retinal preparation incubated under aerobic conditions. The carbon skeleton of glutamate can enter the tricarboxylic acid cycle as alpha-ketogluterate, providing an alternative metabolic substrate in cases of glucose deprivation.

METHODS

Isolated rat retinas were incubated in physiological media with and without glucose, using a range of glutamate concentrations to provide an alternative source of metabolic substrate. We conducted post-embedding immunocytochemistry and quantified the change in glutamate and GABA immunoreactivity within Müller cells under these different incubation conditions.

RESULTS

The provision of glutamate with normal (6 mM) glucose levels resulted in a gradual accumulation of glutamate and GABA in Müller cells, with Müller loading when exogenous glutamate concentrations were above 0.1 mM. However, when these varying levels of glutamate were applied in the absence of glucose, glutamate accumulation in Müller cells was decreased compared to the 6 mM glucose condition and GABA accumulation in Müller cells was at a minimum at moderate (0.5 and 1 mM) glutamate levels. Under hypoglycaemic conditions, exogenous glutamate (0.5 to 1 mM) is rapidly metabolised by Müller cells to the extent that no glial loading is evident, despite the high concentrations.

CONCLUSIONS

Normal neurochemical function appears to be maintained secondary to exogenous glutamate provision of 0.5 to 1 mM when glucose is not in the incubation medium, implying that glutamate can be used as an alternative metabolic substrate. We also show that Müller cells possess more rapid glutamate metabolic capabilities compared to the metabolism of GABA.

Amino acids and their transporters in the retina

This review provides an overview of the distributions, properties and roles of amino acid transport systems in normal and pathological retinal tissues and discusses the roles of specific identified transporters in the mammalian retina. The retina is used in this context as a vehicle for describing neuronal and glial properties, which are in some, but not all cases comparable to those found elsewhere an the brain. Where significant departures are noted, these are discussed in the context of functional specialisations of the retina and its relationship to adjacent supporting tissues such as the retinal pigment epithelium. Specific examples are given where immunocytochemical labelling for amino acid transporters may yield inaccurate results, possibly because of activity-dependent conformation changes of epitopes in these proteins which render the epitopes more or less accessible to antibodies.

Quantitative assessment of localization and colocalization of glutamate, aspartate, glycine, and GABA immunoreactivity in the chick retina

We examined the posthatch chick retina for the frequency of occurrence of localization and colocalization of four amino acid transmitter candidates: glutamate (Glu), aspartate (Asp), gamma aminobutyric acid (GABA), and glycine (Gly) using postembedding methods. We support previous studies of Glu, Asp, GABA, and Gly localization in the direct and indirect functional pathways of the chick retina and extend these studies with new qualitative and quantitative observations. We found that photoreceptors show distinct cellular immunoreactivity for both Glu (Glu+) and Asp+, but not for Gly (Gly-) or GABA. Moreover, there is compartmentalization of Glu and Asp staining within the photoreceptors. All horizontal cells react strongly with Asp and Glu, about three-fourths are GABA+ and three-fourths of these are Gly+. Bipolar cells are uniformly Glu+, heterogeneously Asp+, occasionally Gly+, but GABA-. A majority of amacrine cells stain heterogeneously with all antibodies: 90% are Gly+, slightly more than half colocalize Glu, GABA, and Gly. Furthermore, amacrine cells in the outer two or three rows of cells are more likely to be stained by Gly than Glu, Asp, or GABA. Confirming previous studies, ganglion cells were mostly immunoreactive for Glu and Asp with fewer reactive for GABA and Gly. Strong and distinctly cellular immunoreactivity was found in both central and peripheral retina. Our findings show: 1) there is extensive colocalization of Glu, Asp, GABA, and Gly among most retinal neurons, including some cells that contain all four; 2) cells of the direct functional pathway tend to be labeled by Glu and Asp generally to the exclusion of GABA and Gly, while those of the indirect pathway tend to be labeled by GABA+ and/or Gly+ in addition to Glu+ and Asp+; 3) different cell body layers have distinct patterns of colocalization; and 4) there is no qualitative difference in staining patterns between peripheral and central retina.

Immunocytochemical localization of the glutamate transporter GLT-1 in goldfish (Carassius auratus) retina

Glutamate is the major excitatory neurotransmitter in the retina of vertebrates. Electrophysiological experiments in goldfish and salamander have shown that neuronal glutamate transporters play an important role in the clearance of glutamate from cone synaptic clefts. In this study, the localization of the glutamate transporter GLT-1 has been investigated immunocytochemically at the light and electron microscopical levels in the goldfish retina using a GLT-1-specific antibody. GLT immunoreactivity (IR) was observed at the light microscopical level in Müller cells, bipolar cells, the outer plexiform layer (OPL), and the inner plexiform layer (IPL). At the electron microscopical level, membrane-bound and cytoplasmic GLT-IR in the OPL was located in finger-like protrusions of the cone terminal located near the invaginating postsynaptic processes of bipolar and horizontal cells. GLT-IR was not observed in the vicinity of synaptic ribbons. This location of GLT-1 allows modulation of the glutamate concentration in the synaptic cleft, thereby shaping the dynamics of synaptic transmission between cones and second-order neurons. In the inner IPL, GLT-IR was observed in the cytoplasm and was membrane bound in mixed rod/cone bipolar cell terminals and cone bipolar cell terminals. The membrane-bound GLT-1 was generally observed at some distance from the synaptic ribbon. The morphology of the bipolar cell terminal together with the localization of GLT-1 suggests that at least these glutamate transporters are not primarily involved in rapid uptake of glutamate release by the bipolar cells. The GLT-IR in the cytoplasm of Müller cells was located throughout the entire goldfish retina from the outer limiting membrane to the inner limiting membrane. The location of GLT-1 in Müller cells is consistent with the role of Müller cells in converting glutamate to glutamine.

Regulation of calcium/calmodulin-dependent protein kinase II in the adult rat retina is mediated by ionotropic glutamate receptors

This study is concerned with the transmitter-mediated regulation of the alpha(50 kDa) and beta(60 kDa) subunits of calcium calmodulin dependent protein kinase II (CamKII) in the adult rat retina. The level of antibody binding to the CamKII and the activity of CamKII were found to be increased after intravitreal injection of glutamate. Changes in the levels of the antibody-binding to the subunits of CamKII were observed in different subcellular fractions of the retina with a maximum response observed in crude synaptic membrane fractions. The glutamate mediated increases in CamKII were specific and blocked by 3,5-Dimethyl-1 adamantanamine; 3,5-Dimethylamantadine (Memantine), (+/-) 2-Amino-5-Phosphopentonic (AP-5) and 6-Cyano-7-Nitroquinoxaline-2,3-Dione (CNQX) but not with dl -2-Amino-3-Phosphono-Propionic (AP-3). The results indicate that the retinal neurotransmitter, glutamate, can regulate retinal CamKII activity through ionotropic but not metabotropic glutamate receptors. NMDA-receptors were found to be necessary but insufficient to stimulate CamKII. A model in which cooperative interaction between NMDA and non-NMDA glutamate receptors/ion channels is presented to explain the glutamate stimulated increases in CamKII activity in the retina.

Uptake of precursor and synthesis of transmitter in a histaminergic photoreceptor

As a first step in understanding how the supply of the neurotransmitter histamine is maintained in a photoreceptor, we followed the uptake and metabolism of the immediate precursor of histamine, histidine. [3H]Histidine taken up into photoreceptors and glia was detected using autoradiography, and synthesis of [3H]histamine from [3H]histidine was assayed with thin-layer chromatography. Photoreceptors from barnacles were pulsed (15 min) with [3H]histidine (0.2-200 microM), then maintained in normal saline for up to 24 hr. Autoradiography showed that photoreceptor somata, axons, and presynaptic arbors were labeled, but only weakly, like (nonhistaminergic) ganglion cells. Label instead was concentrated over surrounding glia. Stimulating preparations with light did not increase photoreceptor labeling. Grain counts from photoreceptor axons showed uptake of [3H]histidine into these neurons by a Na+-dependent mechanism with a Km of approximately 50 microM. Over 24 hr only 1% of the [3H]histidine taken up by preparations was converted to [3H]histamine either in the dark or in the light. Injections of [3H]histidine directly into photoreceptors established that synthesis takes place within the photoreceptors and confirmed that stimulation with light did not measurably affect the rate of conversion of [3H]histidine to [3H]histamine. These results suggest that de novo synthesis of transmitter is unlikely to be as important as its reuptake in maintaining neurotransmitter supply in these photoreceptor terminals. In support of this conclusion, photoreceptors accumulated more label when transmitter release was stimulated with high K+ and histamine uptake was antagonized with chlorpromazine.

Uptake of precursor and synthesis of transmitter in a histaminergic photoreceptor

As a first step in understanding how the supply of the neurotransmitter histamine is maintained in a photoreceptor, we followed the uptake and metabolism of the immediate precursor of histamine, histidine. [3H]Histidine taken up into photoreceptors and glia was detected using autoradiography, and synthesis of [3H]histamine from [3H]histidine was assayed with thin-layer chromatography. Photoreceptors from barnacles were pulsed (15 min) with [3H]histidine (0.2-200 microM), then maintained in normal saline for up to 24 hr. Autoradiography showed that photoreceptor somata, axons, and presynaptic arbors were labeled, but only weakly, like (nonhistaminergic) ganglion cells. Label instead was concentrated over surrounding glia. Stimulating preparations with light did not increase photoreceptor labeling. Grain counts from photoreceptor axons showed uptake of [3H]histidine into these neurons by a Na+-dependent mechanism with a Km of approximately 50 microM. Over 24 hr only 1% of the [3H]histidine taken up by preparations was converted to [3H]histamine either in the dark or in the light. Injections of [3H]histidine directly into photoreceptors established that synthesis takes place within the photoreceptors and confirmed that stimulation with light did not measurably affect the rate of conversion of [3H]histidine to [3H]histamine. These results suggest that de novo synthesis of transmitter is unlikely to be as important as its reuptake in maintaining neurotransmitter supply in these photoreceptor terminals. In support of this conclusion, photoreceptors accumulated more label when transmitter release was stimulated with high K+ and histamine uptake was antagonized with chlorpromazine.

Effects of inhibiting glutamine synthetase and blocking glutamate uptake on b-wave generation in the isolated rat retina

The purpose of the present experiments was to evaluate the contribution of the glutamate-glutamine cycle in retinal glial (Müller) cells to photoreceptor cell synaptic transmission. Dark-adapted isolated rat retinas were superfused with oxygenated bicarbonate-buffered media. Recordings were made of the b-wave of the electroretinogram as a measure of light-induced photoreceptor to ON-bipolar neuron transmission. L-methionine sulfoximine (1-10 mM) was added to superfusion media to inhibit glutamine synthetase, a Müller cell specific enzyme, by more than 99% within 5-10 min, thereby disrupting the conversion of glutamate to glutamine in the Müller cells. Threo-hydroxyaspartic acid and D-aspartate were used to block glutamate transporters. The amplitude of the b-wave was well maintained for 1-2 h provided 0.25 mM glutamate or 0.25 mM glutamine was included in the media. Without exogenous glutamate or glutamine the amplitude of the b-wave declined by about 70% within 1 h. Inhibition of glutamate transporters led to a rapid (2-5 min) reversible loss of the b-wave in the presence and absence of the amino acids. In contrast, inhibition of glutamine synthetase did not alter significantly either the amplitude of the b-wave in the presence of glutamate or glutamine or the rate of decline of the b-wave found in the absence of these amino acids. Excellent recovery of the b-wave was found when 0.25 mM glutamate was resupplied to L-methionine sulfoximine-treated retinas. The results suggest that in the isolated rat retina uptake of released glutamate into photoreceptors plays a more important role in transmitter recycling than does uptake of glutamate into Müller cells and its subsequent conversion to glutamine.

Neurochemistry of the mammalian cone 'synaptic complex'

The cone 'synaptic complex' is a unique structure in which a single presynaptic axon secretes glutamate onto processes of bipolar cells (both ON and OFF) and horizontal cells. In turn, the horizontal cell processes antagonize cone and bipolar responses to glutamate (probably by GABA). What still remains largely unknown is the molecular identity of the postsynaptic receptors and their exact locations. We identified several subunits of the glutamate receptor and the GABAA receptor expressed at the cone synaptic complex and localized them ultrastructurally. Glutamate receptors: (i) Invaginating (probably ON) bipolar dendrites in the monkey and rat express the metabotropic glutamate receptor, mGluR6. The stain is intense on the dendritic membrane where it first enters the invagination, and weak at the tip nearest to the ribbon. The cone membrane is electron-dense where it apposes the intense stain for mGluR6. Surprisingly, invaginating bipolar dendrites in the cat also express the AMPA receptor subunits, GluR2/3 and GluR4. (ii) Dendrites forming basal contacts in the cat (probably OFF) express the AMPA subunits GluR2/3, GluR4, and also the kainate subunit, GluR6/7. The stain is especially intense at the dendritic tips in apposition to electron-dense regions of cone membrane. (iii) Horizontal cells in the cat express the AMPA subunits GluR2/3, GluR4 and the kainate subunit, GluR6/7. The stain is strongest in the cytosol of somas and primary dendrites, but is also present in the invaginating terminals where it localizes to the membrane subjacent to the ribbon. GABAA receptors: (i) ON and OFF bipolar dendrites in the monkey express the alpha 1 and beta 2/3 subunits. The stain is localized to the bipolar cell membrane in apposition to horizontal cell processes. (ii) Cones did not express the GABAA subunits tested by immunocytochemistry, but beta 3 mRNA was amplified by RT-PCR from rat photoreceptors.

CONCLUSIONS

(i) mGluR6 receptors concentrate on dendrites at the base of the invagination rather than at the apex. This implies that receptors at both 'invaginating' and 'basal' contacts lie roughly equidistant from the release sites and should therefore receive similar spatiotemporal concentrations of glutamate. (ii) The 'cone' membrane is electron-dense opposite to the receptor sites on both ON and OFF bipolar cells. This suggests a special role for this region in synaptic transmission. Possibly, these densities signify a transporter that would regulate glutamate concentration at sites remote (> 200 nm) from the locus of vesicle release.

Postsynaptic response kinetics are controlled by a glutamate transporter at cone photoreceptors

We evaluated the role of the sodium/glutamate transporter at the synaptic terminals of cone photoreceptors in controlling postsynaptic response kinetics. The strategy was to measure the changes in horizontal cell response rate induced by blocking transporter uptake in cones with dihydrokainate (DHK). DHK was chosen as the uptake blocker because, as we show through autoradiographic uptake measurements, DHK specifically blocked uptake in cones without affecting uptake in Mueller cells. Horizontal cells depolarized from about -70 to -20 mV as the exogenous glutamate concentration was increased from approximately 1 to 40 microM, so horizontal cells can serve as "glutamate electrodes" during the light response. DHK slowed the rate of hyperpolarization of the horizontal cells in a dose-dependent way, but didn't affect the kinetics of the cone responses. At 300 microM DHK, the rate of the horizontal cell hyperpolarization was slowed to only 17 +/- 8.5% (mean +/- SD) of control. Translating this to changes in glutamate concentration using the slice dose response curve as calibration in Fig. 2, DHK reduced the rate of removal of glutamate from approximately 0.12 to 0.031 microM/s. The voltage dependence of uptake rate in the transporter alone was capable of modulating glutamate concentration: we blocked vesicular released glutamate with bathed 20 mM Mg2+ and then added 30 microM glutamate to the bath to reestablish a physiological glutamate concentration level at the synapse and thereby depolarize the horizontal cells. Under these conditions, a light flash elicited a 17-mV hyperpolarization in the horizontal cells. When we substituted kainate, which is not transported, for glutamate, horizontal cells were depolarized but light did not elicit any response, indicating that the transporter alone was responsible for the removal of glutamate under these conditions. This suggests that the transporter was both voltage dependent and robust enough to modulate glutamate concentration. The transporter must be at least as effective as diffusion in removing glutamate from the synapse because there is only a very small light response once the transporter is blocked. The transporter, via its voltage dependence on cone membrane potential, appears to contribute significantly to the control of postsynaptic response kinetics.

Modulation of chromatic difference in receptive field size of H1 horizontal cells in carp retina: dopamine- and APB-sensitive mechanisms

Chromatic aspects of receptive field size in the H1 horizontal cell syncytium of the carp retina were investigated using spectral photostimuli (blue or red) presented in the form of either a pair of a small spot and annulus, or a narrow moving slit. In the light-adapted retina, the receptive field for the blue stimulus was found to be significantly smaller than that for the red, i.e. there was a chromatic difference in the receptive field size. During the course of dark adaptation, the overall receptive field size increased, but the chromatic difference decreased. Immediately after adaptation to bright light, the receptive field sizes were reduced significantly, but the chromatic difference increased, mainly due to a greater reduction in the receptive field for the blue stimulus. Application of dopamine (5 microM) to a dark-adapted retina gradually decreased the receptive field size for both colours, but the chromatic difference became larger, again due to a greater reduction in the receptive field size for the blue stimulus. 2-Amino-4-phosphonobutyrate (APB) applied to light-adapted retinae at a working concentration of 1 mM, greatly expanded the receptive field size and suppressed the chromatic difference due to the effect being greater for the receptive field for the blue stimulus. The effect of APB was slow and cumulative. On the other hand, intracellular injection of cGMP or dibutyryl-cGMP increased the chromatic difference in the receptive field size. It is suggested (i) that the chromatic difference in the receptive field size could be due to a cGMP-coupled, conductance-decreasing receptor mechanism activated by APB; and (ii) that the mechanism is associated with short-wavelength sensitive cone input to the H1 cells and operates in the light-adapted state of the retina.

Localisation of amino acid neurotransmitters during postnatal development of the rat retina

We used postembedding immunocytochemistry to determine the localisation of the amino acid neurotransmitters glutamate, gamma-aminobutyrate (GABA), and glycine, potential neurotransmitter precursors (aspartate and glutamine), and taurine in the rat retina during postnatal development. All amino acids investigated were present at birth; however, only the inhibitory neurotransmitters GABA and glycine displayed neuronal localisation. GABA was localised in a sparse population of amacrine cells, and glycine immunoreactivity was found in cells within the ventricular zone that appeared to migrate through the neuroblastic layer. Glutamate labelling was diffuse across the retina until postnatal day (PND) 8. Localisation of glutamine was evident within Müller's cells by PND 6, in agreement with the known age of onset of glutamine synthetase activity. Based on the findings of uptake of radiolabelled glutamate and GABA by PND 8 and changes in immunoreactivity, we propose that Müller's cells evolve at PND 6-8 from their precursor cells, the radial glial cells. Evidence for differences in glutamate turnover in the infant retina was seen on examination of aspartate and glutamine immunoreactivity. Aspartate labelling was weak until PND 11, when ganglion cells and some amacrine cells were labelled. Unlike the mature retina, a large number of amacrine cells were glutamine immunoreactive in the PND 6 retina. One reason for the observed differences in precursor pooling may be a lack of neuronal neurotransmitter release and overall low metabolic activity. We also investigated the response of the developing retina to ischaemic insult to test the physiological hypoxia model of vascular development. Our findings are consistent with the hypothesis that the developing retina has increased tolerance to ischaemic insult. Our findings suggest that, although the retina is morphologically adult like by PND 8, there are differences in neurotransmitter turnover in the immature rat retina.

Characterization of glutamate transporter function in the tiger salamander retina

Glutamate transporters in the tiger salamander retina were studied by autoradiographic and intracellular recording techniques. When the retina was incubated with 15 microM L-[3H]glutamate, photoreceptors and Muller cells were labeled, indicating that these cells had high-affinity glutamate uptake transporters. A much higher dose of glutamate than kainate was required in the bath to produce the same membrane depolarization in horizontal cells (HCs), and the time course of glutamate-induced depolarization was much slower than that of the kainate-induced depolarization. Since glutamate is a substrate of glutamate transporters whereas kainate is not, we attribute these differences to the buffering of extracellular glutamate by glutamate transporters in the retina. D-aspartate (D-asp) increased the efficacy of bath-applied glutamate. Dihydrokainate (DHKA) exerted little effect on glutamate efficacy when applied alone, but it increased glutamate efficacy in the presence of D-asp. These results are consistent with the notion that glutamate transporters in Muller cells are D-asp sensitive and those in photoreceptors are DHKA and D-asp sensitive. Application of DHKA (1-2 mM) did not affect the dark membrane potential or the light responses in rods and cones, but it depolarized the HC dark membrane potential and reduced the HC peak and tail light responses. Our results suggest that DHKA-sensitive glutamate transporters in photoreceptors regulate glutamate levels in rod and cone synaptic clefts. They modulate dark membrane potential and the relative rod cone inputs in retinal HCs.

Nitric oxide, 2-amino-4-phosphonobutyric acid and light/dark adaptation modulate short-wavelength-sensitive synaptic transmission to retinal horizontal cells

Light-induced changes in the input resistance (Rin) of external, luminosity (i.e. H1) type horizontal cell (HC) perikarya were studied by the bridge-balance method in light-adapted and dark-adapted retinae of carp. Changes in input resistance (delta Rin) induced by short-(460 nm) and long-wavelength (674 nm) flashes, adjusted in intensity to elicit equal-amplitude membrane voltage responses (equal-voltage condition), were measured. In light-adapted retinae, long-wavelength stimuli increased Rin consistently; in contrast, the increase was much less with short-wavelength stimuli. This equal-voltage chromatic delta Rin difference was lost in dark-adapted retinae whereby the delta Rin (an increase) became the same for short- and long-wavelengths. The chromatic delta Rin difference could be recovered by light adaptation or application of sodium nitroprusside to the dark-adapted retinae. Conversely, the equal-voltage chromatic delta Rin difference was eliminated by injection of NG-monomethyl-L-arginine into H1HCs of the light-adapted retinae or by treating the retinae with 2-amino-4-phosphonobutyrate (APB). These results suggest that H1HCs of the carp retina possess distinct postsynaptic mechanisms which mediate short- and long-wavelength signal transmission. Furthermore, it appears that the short-wavelength-sensitive pathway is active only during the light-adapted state of the retina. Taken together, therefore, the short-wavelength transmission to H1HCs probably operates on an APB-sensitive glutamate receptor, with nitric oxide as a light-adaptive messenger.

Iontophoretic study of the action of excitatory amino acids on rod horizontal cells of the dogfish retina

Much interest has been focused on the amino acids, L-glutamate and L-aspartate, as possible neurotransmitters of vertebrate photoreceptors. These amino acids and a number of their analogues were applied iontophoretically to rod horizontal cells on the surface of dark-adapted dogfish retinal slices under visual control. L-glutamate and kainate were found to be of approximately equal potency in depolarizing rod horizontal cells, while L-aspartate was about one tenth as potent. Simultaneous iontophoretic pulses applied to two barrels, each containing either L-glutamate or kainate, produced a larger depolarization than expected for linear summation. Potentiation was most prominent when synaptic transmitter release was reduced by light, demonstrating that these agonists interact with the same postsynaptic receptors as those acted upon by the rod neurotransmitter. Analysis of dose—response curves indicated that at least two molecules of agonist were required to open a cationic channel, presumably the basis for the depolarization. The cells did not become desensitized to long or repeated exposures of the agonists.

Effect of diabetes on levels and uptake of putative amino acid neurotransmitters in rat retina and retinal pigment epithelium

Free amino acid levels and high affinity uptake of glutamate, aspartate, gamma-aminobutyrate, glycine and taurine were studied in retina and retinal pigment epithelium of streptozotocin diabetic rats. Results show that experimental diabetes produces a generalized fall in the content of free amino acids in both retina and retinal pigment epithelium. With regard to the high affinity uptake, in the two tissues of diabetic animals showed decreased aspartate uptake, enhanced taurine and gamma-aminobutyrate uptake, whereas that of glycine and glutamate was unchanged. These results might suggest that diabetes causes alterations of specific amino acid transport systems and/or alterations of some cell populations.

Spectral plasticity of H1 horizontal cells in carp retina: independent modulation by dopamine and light-adaptation

It was shown previously that the spectral sensitivity of luminosity/H1-type horizontal cells (HCs) in carp retinae reflects the absorption spectrum of red-sensitive cones for long wavelengths but can appear highly variable and "truncated' in the short-wavelength region of the spectrum. We have found that light-adaptation sharpened the red-sensitive spectral peak and decreased the blue/red response amplitude ratio (B/R ratio), mainly by decreasing the response to short-wavelength stimuli. The adaptation effect was more pronounced for red background light than for blue. During dark adaptation, the B/R ratio increased steadily. Exogenous dopamine (DA; 5 microM) changed the spectral response profile in a similar way to light-adaptation. However, the effect of light-adaptation in reducing the B/R ratio was still seen in retinae bathed in 5 microM DA. This effect of background adaptation was also recorded in retinae bathed in 37 microM haloperidol, as well as in retinae pretreated with 6-hydroxydopamine (i.e. DA-depleted). The results suggest that (i) short-wavelength-sensitive cones play a dynamic role in determining the spectral response profile of H1 HCs and (ii) spectral response characteristics are modulated independently by exogenous DA and an unknown endogenous neuromodulator which is activated by light-adaptation.

Glutamate responses of bipolar cells in a slice preparation of the rat retina

Whole-cell currents from >70 voltage-clamped bipolar cells were recorded in a slice preparation of the rat retina. The recorded cells were identified and classified by intracellular staining with Lucifer yellow. Glutamate, the specific agonists (+/-)-2-amino-4-phosphonobutyric acid (AP-4) and kainate (KA), and the antagonist 6-cyanoquinoxaline-2,3-dione (CNQX) were applied. The cells could be isolated from presynaptic influences by the co-application of bicuculline, strychnine, and cobalt ions. Responses to AP-4 were elicited only from bipolar cells with axons stratifying in the inner part of the inner plexiform layer (IPL). AP-4 caused an outward current in these cells attributable to the closure of nonspecific cation channels. Responses to kainate representing a direct action of the drug on the recorded cells were observed only in bipolar cells with axons stratifying in the outer part of the IPL. KA caused a CNQX-sensitive inward current in these cells, associated with openings of nonspecific cation channels. The results predict that cone bipolar (CB) cells with axons terminating in the outer IPL are OFF-bipolars, whereas those with axons terminating in the inner IPL are ON-bipolars. Most of the cells expressed GABA-gated Cl- conductances. In rod bipolar and in some CB cells, only part of the GABA-induced currents could be blocked by the application of bicuculline, suggesting the presence of GABAc receptors in addition to GABAA receptors.

Glutamate responses of bipolar cells in a slice preparation of the rat retina

Whole-cell currents from >70 voltage-clamped bipolar cells were recorded in a slice preparation of the rat retina. The recorded cells were identified and classified by intracellular staining with Lucifer yellow. Glutamate, the specific agonists (+/-)-2-amino-4-phosphonobutyric acid (AP-4) and kainate (KA), and the antagonist 6-cyanoquinoxaline-2,3-dione (CNQX) were applied. The cells could be isolated from presynaptic influences by the co-application of bicuculline, strychnine, and cobalt ions. Responses to AP-4 were elicited only from bipolar cells with axons stratifying in the inner part of the inner plexiform layer (IPL). AP-4 caused an outward current in these cells attributable to the closure of nonspecific cation channels. Responses to kainate representing a direct action of the drug on the recorded cells were observed only in bipolar cells with axons stratifying in the outer part of the IPL. KA caused a CNQX-sensitive inward current in these cells, associated with openings of nonspecific cation channels. The results predict that cone bipolar (CB) cells with axons terminating in the outer IPL are OFF-bipolars, whereas those with axons terminating in the inner IPL are ON-bipolars. Most of the cells expressed GABA-gated Cl- conductances. In rod bipolar and in some CB cells, only part of the GABA-induced currents could be blocked by the application of bicuculline, suggesting the presence of GABAc receptors in addition to GABAA receptors.

Glutamate metabolic pathways in displaced ganglion cells of the chicken retina

Glutamate (E) is the putative amino acid neurotransmitter used by ganglion cells, photoreceptors, and bipolar cells. Aspartate (D) and glutamine (Q) are potential precursors of glutamate, and glutamate-utilizing neurons may use one or more of these amino acids to sustain production of glutamate. We used post-embedding immunocytochemistry for several amino acid neurotransmitters to characterize the amino acid signatures for displaced ganglion cells of the avian retina. We found two neurochemical signatures for displaced ganglion cells, EQ and EDQ, in mid-peripheral and far-peripheral retina, respectively. Differences in neurochemical signatures cannot be explained by the existence of two ganglion cell populations, and we propose that the two signature categories for the large-diameter displaced ganglion cells reflect variations in the aspartate precursor pool. The transamination reaction involved in glutamate production, aspartate/oxaloacetate and alpha-ketoglutarate/glutamate, requires an active TCA cycle, since the carbon skeleton of glutamate is derived from alpha-ketoglutarate, a TCA intermediary. We hypothesized that aspartate levels vary in the normal chicken retina because eccentricity-dependent differences in oxygen availability result in changes of alpha-ketoglutarate levels, and hence, alterations in the equilibrium of the transamination reaction. We tested this hypothesis by incubating isolated chicken retinas in anaerobic conditions and found elevated aspartate immunoreactivity in subpopulations of glutamate-utilizing neurons in the central retina. Under aerobic conditions, or in retinas placed directly into fixative, retinal samples from the central edge of the pecten did not show differential cellular staining for aspartate. We have, therefore, identified differences in neurochemical signatures for retinal neurons involving changes in active maintenance of precursor pools.

Noise analysis of the glutamate-activated current in photoreceptors

The glutamate-activated current in photoreceptors has been attributed both to a sodium/glutamate transporter and to a glutamate-activated chloride channel. We have further studied the glutamate-activated current in single, isolated photoreceptors from the tiger salamander using noise analysis on whole-cell patch-clamp recordings. In cones, the current is generated by chloride channels with a single-channel conductance of 0.7 pS and an open lifetime of 2.4 ms. The number of channels per cell is in the range of 10,000-20,000. Activation of the channels requires the presence of both glutamate and sodium. The single-channel conductance and the open lifetime of the channel are independent of the external concentration of glutamate and sodium. External glutamate and sodium affect only the opening rate of the channels. D,L-Threo-3-hydroxyaspartate (THA), a glutamate-transport blocker, is shown to be a partial agonist for the channel. The single-channel conductance is the same regardless of whether glutamate or THA is the ligand, but the open lifetime of the channel is only 0.8 ms with THA as ligand. The glutamate-activated current in rods has a similar single-channel conductance (0.74 pS) and open lifetime (3 ms). We propose a kinetic model, consistent with these results, to explain how a transporter can simultaneously act both as a sodium/glutamate-gated chloride channel and a glutamate/sodium cotransporter.

Glutamate immunoreactivity in the cat retina: a quantitative study

Immunocytochemical methods were used to visualize glutamate immunoreactivity in the cat retina and to compare its localization with that of aspartate, GABA, and glycine. The cellular and subcellular distribution of glutamate was analyzed at the light-microscopic level by optical densitometry and at the electron-microscopic level by immunogold quantification. The findings were consistent with the proposed role for glutamate as the neurotransmitter of photoreceptors and bipolar cells as particularly high concentrations of staining were found in synaptic terminals of these cells. Ganglion cells were also consistently stained. Aspartate was totally colocalized with glutamate in neuronal cell bodies but the synaptic levels of aspartate were much lower than for glutamate. In addition to the staining of photoreceptor, bipolar, and ganglion cells, glutamate immunoreactivity was also observed in approximately 60% of the amacrine cells. These cells exhibited colocalization with either GABA or glycine. The elevated levels of Glu in amacrine cells may reflect its role as a transmitter precursor in GABAergic cells and as an energy source for mitochondria in glycinergic cells.

Cone photoreceptors respond to their own glutamate release in the tiger salamander

Pulse-like currents resembling miniature postsynaptic currents were recorded in patch-clamped isolated cones from the tiger salamander retina. The events were absent in isolated cones without synaptic terminals. The frequency of events was increased by either raising the osmotic pressure or depolarizing the cell. It was decreased by the application of either glutamate or the glutamate-transport blockers dihydrokainate and D,L-threo-3-hydroxyaspartate. The events required external Na+ for which Li+ could not substitute. The reversal potential of these currents followed the equilibrium potential for Cl- when internal Cl- concentration was changed. Thus, these miniature currents appear to represent the presynaptic activation of the glutamate receptor with glutamate transporter-like pharmacology, caused by the photoreceptor's own vesicular glutamate release. Using a noninvasive method to preserve the intracellular Cl- concentration, we showed that glutamate elicits an outward current in isolated cones. Fluorescence of the membrane-permeable form of fura-2 was used to monitor Ca2+ entry at the cone terminal as a measure of membrane depolarization. The increase in intracellular Ca2+ concentration, elicited by puff application of 30 mM KCl, was completely suppressed in the presence of 100 microM glutamate. Puff application of glutamate alone had no measurable depolarizing effect. These results suggest that the equilibrium potential for Cl-, ECl, was more negative than the activation range for Ca2+ channels and that glutamate elicited an outward current, hyperpolarizing the cones.

Glutamate immunoreactivity is enriched over pinealocytes of the gerbil pineal gland

Mammalian pinealocytes have been shown to contain synaptic-like microvesicles with putative secretory functions. As a first step to elucidate the possibility that pinealocyte microvesicles store messenger molecules, such as neuroactive amino acids, we have studied the distributional pattern of glutamate immunoreactivity in the pineal gland of the Mongolian gerbil (Meriones unguiculatus) at both light- and electron-microscopic levels. In semithin sections of plastic-embedded pineals, strong glutamate immunoreactivity could be detected in pinealocytes throughout the pineal gland. The density of glutamate immunolabeling in pinealocytes varied among individual cells and was mostly paralleled by the density of immunostaining for synaptophysin, a major integral membrane protein of synaptic and synaptic-like vesicles. Postembedding immunogold staining of ultrathin pineal sections revealed that gold particles were enriched over pinealocytes. In particular, a high degree of immunoreactivity was associated with accumulations of microvesicles that filled dilated process terminals of pinealocytes. A positive correlation between the number of gold particles and the packing density of microvesicles was found in three out of four process terminals analyzed. However, the level of glutamate immunoreactivity in pinealocyte process endings was lower than in presumed glutamatergic nerve terminals of the cerebellum and posterior pituitary. The present results provide some evidence for a microvesicular compartmentation of glutamate in pinealocytes. Our findings thus lend support to the hypothesis that glutamate serves as an intrapineal signal molecule of physiological relevance to the neuroendocrine functions of the gland.

Taurine, amino acid transmitters, and related molecules in the retina of the Australian lungfish Neoceratodus forsteri: a light-microscopic immunocytochemical and electron-microscopic study

The morphology of the retina of the Australian lungfish Neoceratodus forsteri was investigated by means of light- and electron microscopy, whilst immunocytochemical studies were performed to determine the cellular distributions of the major amino acid neurotransmitters and other amino acids. The distributions of glycine and GABA were similar to those previously described for teleost, amphibian and mammalian retinae. Labelling was abundant in amacrine cells, whilst GABA was also present in one layer of horizontal cells and some bipolar cells. Taurine was present in both rods and cones, but, unlike the mammalian or avian retina, was absent from other cellular structures, including glial elements. Unexpectedly, the photoreceptor terminals lacked an apparent content of the excitatory amino acid transmitter glutamate. The glutamate that was present in the rods and cones occupied a crescentic arc corresponding to the location of glycogen-rich paraboloids. Asparagine was also present in rods, albeit in the modified mitochondria that formed the elipsoids of the rod inner segments. Arginine, the precursor for formation of nitric oxide, was present in glial cells, and in the paraboloids of both rods and cones.

High affinity uptake of glutamate and aspartate in the developing rat retina

Uptake for glutamate and aspartate in both retina and synaptosomes was found to be saturable, temperature sensitive, sodium dependent and reduced by metabolic inhibitors. The P1 and P2 synaptosomal fractions showed high affinity systems for glutamate (3 and 9 microM) and aspartate (6 and 3 microM) respectively. Early after birth, glutamate accumulation was much higher than that of aspartate. It showed a rapid increase reaching the adult values about day 15. Aspartate uptake progressively increases with age up to about day 30. Our findings suggest that glutamate and aspartate may be transmitters at specific cell populations in the rat retina.

Immunocytochemical localization of the amino acid neurotransmitters in the chicken retina

Postembedding immunocytochemistry was used to determine the cellular localization of the amino acid neurotransmitters glutamate, aspartate, gamma-aminobutyric acid (GABA), and glycine in the avian retina. The through retinal pathway was glutamatergic, with all photoreceptors, bipolar cells, and ganglion cells being immunoreactive for glutamate. Bipolar cells displayed the highest level of glutamate immunoreactivity, with the cell bodies terminating just below the middle of the inner nuclear layer. All lateral elements, horizontal cells, amacrine cells, and interplexiform cells were immunoreactive for glycine or GABA. The GABAergic neurons consisted of two classes of horizontal cells and amacrine cells located in the lower part of the inner nuclear layer. GABA was also localized in displaced amacrine cells in the ganglion cell layer, and a population of ganglion cells that co-localize glutamate and GABA. Both the horizontal cells and GABAergic amacrine cells had high levels of glutamate immunoreactivity, which probably reflects a metabolic pool. At least two types of horizontal cells in the avian retina could be discriminated on the basis of the presence of aspartate immunoreactivity in the H2 horizontal cells. Glycine was contained in a subclass of amacrine cells, with their cell bodies located between the bipolar cells and GABAergic amacrine cells, two subclasses of bipolar cells, displaced amacrine cells in the ganglion cell layer, and ganglion cells that colocalize glutamate and glycine. Glycinergic amacrine cells had low levels of glutamate. We have also identified a new class of glycinergic interplexiform cell, with its stellate cell body located in the middle of the inner nuclear layer among the cell bodies of bipolar cells. Neurochemical signatures obtained by analyzing data from serial sections allowed the classification of subclasses of horizontal cells, bipolar cells, amacrine cells, and ganglion cells.

Modulation of cone to horizontal cell transmission by calcium and pH in the fish retina

The effects of small changes in the calcium and sodium concentrations and in the pH of superfusion medium on the membrane potential and light-evoked responses of cone horizontal cells in the goldfish retina were examined. Conventional intracellular recording, a bicarbonate-based superfusion medium, and a specially designed superfusion apparatus that reduced pressure wave disturbances were used. An increase in the extracellular calcium concentration, [Ca2+]o, from control levels (0.1 mM) to 1.0 mM hyperpolarized cone horizontal cells and reduced the magnitude of their light responses at all stimulus intensities. Addition of 20 mM NaCl to the 1.0 mM Ca2+ Ringer's solution reversed the hyperpolarizing effect of the 1.0 mM Ca2+ but addition of 20 mM choline, a monovalent cation that does not pass through cyclic GMP-activated channels, did not. Reduction of the superfusate pH from 7.6 to 7.2 by switching from a Ringer's solution gassed with 3% CO2 to one gassed with 10% CO2 hyperpolarized horizontal cells and reduced the magnitude of their light responses at all stimulus intensities for both 0.1 and 1.0 mM Ca2+ Ringer's solutions. An increase in pH to 8.2 by gassing the superfusate with 1% CO2 slightly depolarized the cells in 0.1 mM Ca2+ Ringer's solution but slightly hyperpolarized the cells in the 1.0 mM Ca2+ Ringer's solution. Following pharmacological isolation of the horizontal cells from synaptic input with high doses of glutamate (4-5 mM) and/or Co2+ (4 mM) treatment, no effect on horizontal cell membrane potential due to changes in pHo or [Ca2+]o was observed. These findings are discussed with respect to the cellular mechanisms and sites of action in the outer retina that are affected by changes in pHo and [Ca2+]o.

Chromaticity of synaptic inputs to H1 horizontal cells in carp retina: analysis by voltage-clamp and spectral adaptation

Cone photoreceptor inputs to H1 horizontal cells (H1 HCs) in carp retina were studied by measuring light-modulated currents (IL) to monochromatic stimuli (460, 533, 688 nm) under a voltage-clamp condition. By using double-barrelled micro-electrodes H1 HCs were voltage-clamped whilst perfusing with dopamine to uncouple the cells. The IL of the H1 HCs driven by each cone input was segregated by selective chromatic adaptation, and differences in the kinetics of the IL of the H1 HCs were revealed. Thus, all together, three types of IL were observed: (1) a 'fast outward' current to the long-wavelength stimulus; (2) a 'slow outward' current to the middle-wavelength stimulus; and (3) a 'delayed inward' current that followed the peak of 'slow outward' current to the short-wavelength stimulus. The reversal potentials of the three currents were estimated to be at least 20 mV more positive than the dark resting potential by extrapolation of the IL-V curve. These observations are consistent with the idea that the H1 HCs receive sign-inverting, conductance decreasing synaptic input(s) from at least one other cone mechanism, in addition to the main conventional EPSP type synaptic input from red-sensitive cones.

Interrelationship between retinal ischaemic damage and turnover and metabolism of putative amino acid neurotransmitters, glutamate and GABA

Conditions causing a reduction of oxygen availability (anoxia), such as stroke or diabetes, result in drastic changes in ion movements, levels of neurotransmitters and metabolites and subsequent neural death. Currently, there is no clinically available treatment for anoxia induced neural cell death resulting in drastic and permanent central nervous system dysfunction. However, there have been some exciting developments in experimentally induced anoxic conditions where several classes of drugs appear to significantly reduce neural cell death. This report aims to provide the foundations for understanding both the basic mechanisms involved in retinal ischaemic damage and experimental treatments used to prevent such damage. We discuss the normal release, actions and uptake of the fast retinal neurotransmitters, glutamate and GABA, in the vertebrate retina. Immunocytochemistry is used to demonstrate that both glutamate and GABA are found in the macaque retina. Following this is a discussion on how ischaemia may enhance neurotransmitter release or disrupt its uptake, thus causing an increase in extracellular concentration of these neurotransmitters and subsequent neuronal damage. The mechanisms involved in glutamate neurotoxicity are reviewed, because excess glutamate is the likely cause of retinal ischaemic damage. Finally, the mechanisms behind four possible modes of treatment of neurotransmitter toxicity and their advantages and disadvantages are discussed. Hopefully, further research in this area will lead to the development of a rational therapy for retinal, as well as cerebral ischaemia.

Ultrastructural and electrophysiological changes associated with K(+)-evoked release of neurotransmitter at the synaptic terminals of skate photoreceptors

Bathing the skate retina in a Ringer solution containing a high concentration (100 mM) of potassium ions depolarized the visual cells, depleted the receptor terminals of synaptic vesicles, and suppressed completely the b-wave of the ERG and the intracellularly recorded response of horizontal cells (the S-potential). The depletion of synaptic vesicles was accompanied by a large increase in the extent of the plasma membrane resulting in distortion of the normal terminal profile, i.e. distension of the basal surface and elaborate infolding of protoplasmic extensions. Morphometric analysis showed that despite the changes in vesicle content and terminal structure, the combined linear extent of the vesicular and plasma membranes was unchanged from control (superfusion with normal Ringer solution); the increase in plasma membrane was equivalent to the observed loss of vesicular membrane. When returned to a normal Ringer solution, the terminals rapidly began to reform, and in about 10 min they were morphologically indistinguishable from receptor terminals seen in control preparations. After 30 min in the normal Ringer solution, the amount of membrane associated with the vesicles and the plasma membrane had reverted to control values, and once again the total membrane estimated morphometrically remained essentially the same. Thus, there is an efficient mechanism at the photoreceptor terminal for the recycling of vesicle membrane following exocytosis. The K(+)-induced depletion of synaptic vesicles was paralleled by a precipitous loss of responsivity in both the b-wave of the ERG and the S-potential of the horizontal cells. However, after 30-min exposure to the high K+ and a return to normal Ringer solution, the recovery of electrophysiological activity followed a much slower time course from that associated with the structural changes; 60 min or longer were required for the potentials to exhibit maximum response amplitudes. It appears that the rate-limiting step in restoring normal synaptic function following massive depletion of vesicular stores is transmitter resynthesis and vesicle loading rather than vesicle recycling.

Localization of aspartate-like immunoreactivity in the retina of the turtle (Pseudemys scripta)

Aspartate has been reported to be a putative excitatory neurotransmitter in the retina, but little detailed information is available concerning its anatomical distribution. We used an antiserum directed against an aspartate-albumin conjugate to analyze the anatomy, dendritic stratification, and regional distribution of cell types with aspartate-like immunoreactivity in the turtle retina. The results showed dramatic differences in immunoreactivity in the peripheral versus the central retina. Strong aspartate-like immunoreactivity was shown in the peripheral retina, with many well-labeled processes in the inner plexiform layer. Many bipolar, horizontal, amacrine, and ganglion cells, some photoreceptors, and some unidentified cells were strongly immunoreactive in the peripheral retina. In contrast, although the central retina showed well-labeled horizontal cells, there was only light labeling in the inner plexiform layer with weakly immunoreactive amacrine and ganglion cells and no labeled bipolar cells. There were several strongly immunoreactive efferent nerve fibers which left the optic nerve head and arborized extensively in the retina. At the electron microscopic level, electron-dense reaction product was associated with synaptic vesicles at bipolar and amacrine cell synapses in the inner plexiform layer. These results suggest that aspartate may be involved in many diverse synaptic interactions in both the outer plexiform layer and the inner plexiform layer of the turtle retina.

Coexistence and function of glutamate receptor subtypes in the horizontal cells of the tiger salamander retina

Effects of the major glutamate receptor agonists, kainate (KA), alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), quisqualate (QA), N-methyl-D-aspartate (NMDA), L-alpha-amino-4-phosphonobutyrate (L-AP4), and trans-1-aminocyclopentane-1,3-dicarboxylic acid (ACPD) on horizontal cells (HCs) were studied in superfused larval tiger salamander retina. 20 microM of KA, AMPA, and QA mimicked the action of 3 mM glutamate in the absence and presence of 1 mM Co2+. 20 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the actions of KA and AMPA, but not those of QA and glutamate, indicative of the existence of CNQX-resistant QA receptors in the tiger salamander HCs. Prolonged application of ACPD hyperpolarized the HCs and enhanced the light responses, probably by shifting the resting HC voltage (Er) to a more hyperpolarized position. It is possible that the KA, AMPA, and CNQX-resistant QA receptors are involved in mediating the postsynaptic light responses in HCs, and ACPD receptors are involved in sensitivity adjustment of the HC responses.