Uptake of aspartic and glutamic acid by photoreceptors in goldfish retina

Characteristics of excitatory amino acid uptake in cultures from neurons and glia from the retina

3H-D-Aspartate uptake was biochemically characterized in cultures from chick retina enriched in glial (Müller) cells or neurons during progressive days in vitro (DIV). In the neuronal cultures a high-affinity, Na(+)-dependent system was found with Km = 8-13 microM and pharmacological characteristics in agreement with those of reuptake systems in other regions of the CNS. The uptake system in glial cells showed a lower affinity, with Km = 100-135 microM. In both cases, uptake was temperature and energy dependent. A sharp increase in the Vmax of uptake was observed in both neuronal and glial cultures at 5 DIV, at which time morphologically mature synapses have been shown to be present in retinal cultures. A parallel increase in the pharmacological specificity of the uptake system in neuronal cultures was observed, with a rise in the efficiency of D-Asp, L-Asp, L-Glu, and DL-asp- beta-hydroxamate for inhibiting 3H-D-Aspartate uptake. Results suggest the possibility of reuptake participating in the regulation of extracellular glutamate concentration during development.

Permeation of calcium ions through non-NMDA glutamate channels in retinal bipolar cells

The conduction of calcium ions through glutamate-gated channels is important in the induction of long-term potentiation and may trigger other cellular changes. In retinal bipolar cells, which lack the N-methyl-D-aspartate (NMDA) type of glutamate-gated channel, calcium permeability through non-NMDA channels was examined. Changes in extracellular calcium concentration unexpectedly affected the reversal potential for glutamate-induced currents in a manner consistent with these channels being highly permeable to calcium. External magnesium ions promote desensitization of these non-NMDA channels in a voltage-independent way. Thus, in addition to non-NMDA channels that conduct only sodium and potassium, there is a class that is also permeable to calcium.

Dopamine and 2-amino-4-phosphonobutyrate differentially modify spectral responses of H1 horizontal cells in carp retina

Cone-driven external H1 horizontal cells (H1 HCs) in the cyprinid fish retina hyperpolarize in response to all visible lights, and their synaptic inputs have been widely believed to be excitatory. Recent experiments indicate, however, that short- and long-lambda (wavelength)-sensitive cone photoreceptors have different types of synaptic mechanisms; a conductance-decreasing, sign-reversing and short-lambda-mediating type, and a more conventional conductance-increasing class of excitatory (sign-conserving) synapse transmitting mainly long-lambda signals to H1 HCs. Here, a new set of evidence is presented for such spectrally segregated synaptic multiplicity, which also supports the notion that H1 HCs are actually color-opponent units where the depolarizing response component due to short-lambda-sensitive cones is normally overshadowed by the dominant hyperpolarizing component ascribed to long-lambda-mediating synaptic inputs. Application of dopamine to the retina preferentially enhanced the H1 HC responses to long-lambda flashes, and also depolarized the resting membrane potential in the dark. The spectral response was also examined after applying APB (2-amino-4-phosphonobutyric acid), in the presence of dopamine included to avoid polysynaptic effects of APB. This treatment enhanced the H1 HC responses to short-lambda stimuli and hyperpolarized the resting potential. These results are consistent with the suggestion that dopamine potentiates the conductance-increasing and long-lambda-mediating excitatory transmitter action, whereas APB acts as an agonist at the receptor involved in the conductance-decreasing and short-lambda-mediating transmitter action.

Effects of CNQX, APB, PDA, and kynurenate on horizontal cells of the tiger salamander retina

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.

H1 horizontal cells of carp retina have different postsynaptic mechanisms to mediate short- versus long-wavelength visual signals

Vertebrate photoreceptors release neurotransmitter substance(s) tonically in the dark and this release is curtailed by light. Recently, we have become increasingly aware of the possibility that short- and long-wavelength visual signals are mediated differently during the synaptic transmission to second-order retinal neurons. The experiment described here advances this notion further by demonstrating a postsynaptic difference. Treatment of the carp retina by dopamine reduced the gap-junctional coupling of horizontal cells, and we made use of this known effect to measure the input resistance (Rin) of H1-type horizontal cells. Flashes of light increased Rin. This increase, however, was found to be smaller with short wavelengths, even though the comparison was made when voltage responses were equal in amplitude. Often, Rin was even found to decrease at the blue end of spectrum. No single postsynaptic mechanism can account for any equal-voltage Rin difference such as this. The synaptic spectral segregation thus revealed is probably subserved by a dual scheme wherein the transmitter from blue-sensitive cone photoreceptors acts to decrease the membrane conductance of H1 cells whereas the synapses made by red- and green-sensitive cones are of a classical excitatory type.

Effects of kynurenate and other excitatory amino acid antagonists as blockers of light- and kainate-induced retinal rod photoreceptor disc shedding

Photoreceptor disc shedding in the retina involves detachment of discs from distal outer segments and phagocytosis of those discs by adjacent pigment epithelial cells. The disc-shedding process balances the continuous renewal of photosensitive membrane. In amphibians, rod disc shedding normally is light-stimulated. However, excitatory amino acids such as kainate stimulate disc shedding independent of a dark-light transition. Excitatory amino acid-induced disc shedding is accompanied by toxic changes within the retina. To evaluate the possible role of an endogenous excitatory amino acid in the regulation of light-evoked disc shedding, we examined the effects of excitatory amino acid antagonists on kainate-induced and light-evoked disc shedding and on retinal toxicity. Using eyecups from Rana pipiens, we found that kynurenate, D-O-phosphoserine, and cis-2,3-piperidine dicarboxylic acid (cis-PDA) all block both the neurotoxic and disc-shedding effects of kainate. Kynurenate and D-O-phosphoserine, but not cis-PDA, also block light-evoked disc shedding. Our analysis suggests that kynurenate blocks the mechanism by which light "triggers" disc shedding rather than directly inhibiting disc detachment and phagocytosis. The observation that cis-PDA antagonizes the effects of kainate, but not light, suggests that the receptor mediating the kainate effect on disc shedding may not be involved in the normal initiation of the response by light. In contrast, our data on kynurenate suggest that it antagonizes an endogenous mechanism involved in the normal control of disc shedding. Thus, analysis of the differences between cis-PDA and kynurenate as antagonists in the retina may yield important insight into the mechanism by which light initiates disc shedding.

L-glutamate-induced depolarization in solitary photoreceptors: a process that may contribute to the interaction between photoreceptors in situ

L-Glutamate is a leading candidate for the vertebrate photoreceptor transmitter. In addition to the signal transmission to second-order neurons, photoreceptors communicate with each other not only electrically but also chemically. In the present study, by using solitary turtle photoreceptors, we examined the possibility that L-glutamate mediates interreceptor communication. L-Glutamate evoked an inward current in all subtypes of photoreceptors voltage-clamped to the resting potential. The highest glutamate sensitivity was located at the axon terminal. Both stereoisomers of aspartate were effective, whereas kainate, quisqualate, N-methyl-D-aspartate, and D-glutamate were ineffective. The presence of Na+ was essential to response generation; even Li+ could not substitute for Na+. The relation between L-glutamate-induced current and the membrane voltage was strongly inward-rectifying. These results favor the hypothesis that the L-glutamate-induced response is generated by an electrogenic uptake carrier. However, L-glutamate-induced current was always accompanied by an increase in current fluctuations, a phenomenon commonly observed in ion channels but not expected for an uptake carrier. Although the underlying mechanism needs further elucidation, it seems likely that L-glutamate is a transmitter for communication between photoreceptors.

Calcium action potential and its use for measurement of reversal potentials of horizontal cell responses in carp retina

1. In the carp retina perfused with a solution containing high-Ca2+, Ba2+ and some K+-channel blockers, the horizontal cell produced a regenerative Ca2+ action potential when the cell was depolarized by bath application of L-glutamate (Glu) or L-aspartate (Asp). The action potential was triggered also by a transretinal electrical stimulation which evoked an e.p.s.p. in the horizontal cell. In this solution, some cells produced the action potential spontaneously. 2. The action potential had an overshoot of about 20 mV which lasted for several seconds or even minutes. It had a threshold and showed refractoriness. In addition, it was insensitive to tetrodotoxin, but was blocked by Co2+. These observations revealed, in horizontal cells in situ, the presence of a voltage-dependent Ca2+ channel similar to that found in dissociated cells. It is supposed that, in a physiological environment, the Ca2+ channel is prevented from becoming regenerative probably because it is counteracted by K+ channel activities. 3. Simultaneous recordings from two separate horizontal cells showed full synchronization of the Ca2+ action potentials whose amplitudes were identical. The potential uniformity thus formed in the S-space (Naka & Rushton, 1967) enabled us to measure reversal potentials of horizontal cell responses irrespective of the electrical coupling between the cells. 4. During an overshoot of the Ca2+ action potential, an electrically evoked e.p.s.p. as well as a light response appeared with polarities reversed to those elicited at the resting state. Their reversal potentials could be estimated within a very narrow range between -5 and -10 mV. At this range, both Glu- and Asp-induced potentials reversed the polarity, too. 5. These observations suggest that the ionic mechanisms are identical in the three kinds of horizontal cell response: light response, e.p.s.p. and amino acid-induced potentials.

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: II. Mechanism of action

The specificity and mechanism of the neurotoxic action of kainic acid (KA) was investigated by histological methods in the isolated retina of toads and goldfish. Particular attention was paid to the earliest and most sensitive response to KA in the outer plexiform layer (OPL). Of 21 compounds tested as potential mimics of KA neurotoxicity in the OPL, only the enantiomers of glutamate and aspartate mimicked KA, inducing a low-level neurotoxic effect at concentrations 5,000-10,000-fold higher than concentrations of KA giving comparable effects. Further, of 22 compounds tested as potential blockers of KA neurotoxicity in the OPL, only D-gamma-glutamylglycine, D,L-alpha-amino pimelic acid, sodium pentobarbital, D,L-alpha-amino adipic acid, L-glutamate, and L-aspartate blocked KA neurotoxicity (IC50 values of 0.1, 0.3, 0.3, 2, 5, and 15 mM, respectively). In ionic substitution experiments, KA-induced vacuolization was found to require sodium and chloride ions but not calcium ions in the extracellular medium. These findings support the hypothesis that KA combines with specific receptors in the membrane of susceptible neurons in the retinal OPL, leading to prolonged opening of membrane channels permeable to sodium and potassium ions. An accompanying equilibrating chloride influx may result in intracellular ion excess, leading to osmotic swelling and vacuolization. The membrane receptors involved in mediating the action of KA in the OPL are likely to be a class of postsynaptic or extrasynaptic glutamate receptor.

Amino acid labeling patterns in the efferent innervation of the cochlea: an electron microscopic autoradiographic study

Light microscopic autoradiography and electron microscopic autoradiography were used to study the distribution of label in the cochlear efferents following in vivo incubation with tritiated amino acids. Two basic patterns of labeling were observed. These patterns correspond closely to the lateral and medial superior olivary complex (SOC) olivocochlear systems identified by Warr and Guinan ('79, Brain Res. 173:152-155). Our electron microscopic observations suggest that, at least in the gerbil, the complete separation of outer hair cell (OHC) versus inner hair cell (IHC) efferent innervation proposed by these investigators based upon light microscopic data does not occur. Rather, our data suggest that while the lateral SOC system supplies endings only to the region under the IHC, the medial SOC system may supply endings beneath both the IHCs and OHCs.

In-vivo labeling of (3H)D-aspartate uptake sites in monkey retina

Following prolonged topical application of (3H)D-aspartate in vivo, selective labeling of three distinct cell classes was observed in light-microscopic radioautographs from squirrel monkey retina. Müller (glial) cell bodies and their processes were intensely and consistently labeled in all preparations. Moderately labeled perikarya were occasionally detected in the area of bipolar cells, within the inner nuclear layer. These were particularly numerous in sections from the central retina where an intense diffuse labeling of the inner plexiform layer was also prominent. Finally, moderate to dense accumulations of label were observed over the cell bodies, internal segments and fiber processes of cone photoreceptors. These results strongly suggest that cones, as well as a sub-population of bipolar cells, use glutamate and/or aspartate as neurotransmitter(s) in monkey retina.

Pharmacological identification of retinal cells releasing taurine by light stimulation

The effect of drugs blocking synaptic activity at different retinal levels was examined in this study, in an attempt to identify the origin of the light-stimulated release of 3H-taurine from the chick retina. It was determined by autoradiography that the chick retina accumulates taurine in photoreceptors, in cells from the inner nuclear layer, and in processes of the inner plexiform layer. All these are possible sites for the release of taurine upon illumination. To discriminate among these possibilities, the effects of aspartate, tetrodotoxin, strychnine, picrotoxin, chlorpromazine, tubocurarine, atropine, glutamate diethyl esther, alpha-amino adipate and 2-amino-4-phosphonobutyrate were studied. Aspartate (10 mM), which is known to eliminate the light response of cells postsynaptic to photoreceptors, induced a marked increase of 150% in the resting efflux of 3H-taurine but did not decrease significantly the light-stimulated release. Tetrodotoxin, which blocks amacrine cell responses, decreased 3H-taurine release stimulated by light by less than 20%. The efflux of taurine was unaffected by strychnine, picrotoxin, tubocurarine, atropine, chlorpromazine, and 2-amino-4-phosphonobutyrate, whereas it was increased by glutamate diethyl esther and alpha-amino adipate. These results, all together, point to photoreceptors as the cells releasing 3H-taurine in response to light.

Stimulation of photoreceptor disc shedding and pigment epithelial phagocytosis by glutamate, aspartate, and other amino acids

It has been reported that aspartate and glutamate selectively impair the structure (Olney, '82) and function (e.g., Furakawa and Hanawa, '55) of second- and third-order retinal neurons while leaving the photoreceptor unaffected. Either amino acid may mimic the endogenous photoreceptor neurotransmitter (Ehinger, '82). We report here that excitatory amino acids also induce massive rod photoreceptor disc shedding in eyecups of Xenopus laevis maintained in vitro. Disc shedding is the process whereby photoreceptors eliminate effete discs. It involves interaction between the distal outer segment and pigment epithelium. Millimolar L-aspartate and L-glutamate, as well as micromolar kainic acid, a glutamate analog, stimulate disc shedding three- to fivefold higher than normal light-evoked shedding levels and result in extensive inner retinal damage. Fifty-millimolar KCl, 1.0 microM ouabain, and replacement of sodium with choline also stimulate disc shedding and alter retinal structure. Extensive neurotoxicity appears unrelated to disc shedding since other amino acids having no significant or marginal effects on retinal structure also stimulate shedding. While the site and mechanism of action of these effectors, and in particular the excitatory amino acids, is now undefined, the data show that amino acids thought to act directly and specifically on inner retinal neurons can also markedly alter photoreceptor and pigment epithelial metabolism.

Aspartate aminotransferase-like immunoreactivity in the guinea pig and monkey retinas

The excitatory amino acids, aspartate and glutamate, have been proposed as retinal neurotransmitters. Aspartate aminotransferase (AAT) is an enzyme which is involved in the routine metabolism of these amino acids and may be involved in the specific synthesis of glutamate and/or aspartate for use as a neurotransmitter. On the basis of the hypothesis that increased levels of aspartate aminotransferase may reflect a transmitter role for aspartate and/or glutamate, we have localized aspartate aminotransferase in the guinea pig and cynamolgus monkey retinas with light and electron microscopic immunohistochemical techniques. AAT-like immunoreactivity is localized to the cones of guinea pig retina and to monkey rods. Both species contain a subpopulation of immunoreactive amacrine cells as well as a subpopulation of immunoreactive cells in the ganglion cell layer. Immunostaining is seen in bipolar cells and terminals in the monkey but not in the guinea pig retina. We have performed quantitative analysis of the immunoreactive staining in the outer plexiform layer and described the synaptic organization of immunoreactive processes in the inner plexiform layer (IPL). Labeled amacrine processes in both species form synaptic contacts predominantly to and from bipolar terminals in the inner third of the IPL and to and from other amacrine and small unidentified processes in the outer portion of the IPL. The majority of labeled bipolar terminals in the monkey retina are seen in the inner third of the IPL where they synapse exclusively onto amacrine processes. Labeled bipolar terminals in the outer third of the IPL occasionally synapse onto ganglion processes.

Dopamine inhibits calcium-independent gamma-[3H]aminobutyric acid release induced by kainate and high K+ in the fish retina

Kainic acid (KA) at micromolar concentrations stimulated the release of gamma-[3H]aminobutyric acid [( 3H]GABA) from a particulate fraction of the carp (Cyprinus carpio) retina. The KA action was dose-dependent but Ca2+-independent. A similar response was elicited by another glutamate receptor agonist, quisqualic acid, and high K+, but not by an aspartate agonist, N-methyl-D-aspartic acid. The stimulatory action of KA on the [3H]GABA release was selectively blocked by the KA blockers gamma-D-glutamylglycine and cis-2,3-piperidine dicarboxylic acid. Dopamine (DA), which is contained in DA interplexiform cells in the carp retina, inhibited the [3H]GABA release induced by KA and high K+ in a dose-dependent manner. 5-Hydroxytryptamine and two well-known GABA antagonists, bicuculline (Bic) and picrotoxin (Pic), also mimicked the DA effect on the GABA release at a comparable concentration. This inhibitory effect of DA as well as Bic and Pic on the [3H]GABA release evoked by KA was clearly antagonized by a DA blocker, haloperidol. The action of these agents (KA, DA, GABA antagonist) belonging to three different receptor categories on the GABAergic neurons (possibly external horizontal cells; H1 cells) is discussed in relation to other electrophysiological studies on the lateral spread of S-potentials between H1 cells.

Permeability changes induced by L-glutamate in solitary retinal horizontal cells isolated from Carassius auratus

Solitary horizontal cells isolated from goldfish retinae are depolarized by L-glutamate (Glu) (Ishida, Kaneko & Tachibana, 1984), a possible candidate for the transmitter of photoreceptors. The underlying mechanisms were analysed under voltage-clamp conditions using 'giga-seal' suction pipettes in the whole-cell recording configuration. Glu induced an inward current at the resting membrane potential (ca. -57 mV). Membrane depolarization decreased the amplitude of Glu-induced current and reversed its polarity to outward beyond approximately -3 mV. Membrane hyperpolarization below the resting potential decreased the amplitude of the Glu-induced inward current. When a K current through the anomalous rectifier, which is activated by membrane hyperpolarization (Tachibana, 1983), was blocked by Cs ions, this phenomenon disappeared and the Glu-induced current increased in amplitude with hyperpolarization. Mg ions had no effect on the reduction of the Glu-induced current at hyperpolarized potentials. It was strongly suggested that Glu produced two types of conductance change; a conductance increase due to an activation of Glu channels and a conductance decrease due to a blockage of the K current through the anomalous rectifier. The latter effect is analysed in detail in the following paper (Kaneko & Tachibana, 1985b). The Glu-activated channel was permeable to cations (Na, K, Ca, Mg, Tris and choline ions) with low selectivity, but not to anions. The least effective dose of Glu was less than 10 microM. The relation between the Glu-induced current and the membrane potential curved upwards near the reversal potential, and this relation was not affected by Mg ions.

The release of gamma-aminobutyric acid from horizontal cells of the goldfish (Carassius auratus) retina

Isolated horizontal cells from goldfish retinas were prepared by enzymatic dissociation using papain and separated from other cells by velocity sedimentation. In the intact retina, H1 horizontal cells possess a high-affinity mechanism for accumulating gamma-aminobutyric acid (GABA). This property is retained in isolated cells, which also release the accumulated GABA in response to depolarization by elevated external K+. L-Glutamic acid and its analogues are highly effective at micromolar concentrations in eliciting the release of preloaded GABA from isolated cells. At saturating concentrations, L-aspartic acid stimulates about one-third as much release as L-glutamic acid. In contrast, the D-isomers of glutamate and aspartate are ineffective. In the intact retina, micromolar concentrations of L-glutamic acid analogues are also capable of eliciting GABA release from H1 horizontal cells. Release of the accumulated GABA from isolated H1 cells is largely independent of external Ca2+ concentrations. In the intact retina, H1 horizontal cells also possess a K+-stimulated GABA release mechanism that is independent of the Ca2+ concentrations in the medium. In addition, there appears to be a small but significant amount of [3H]GABA release that may be Ca2+ dependent. Under our conditions, [3H]GABA release from isolated cells is unaffected by external Na+ concentrations between 20 and 120 mM. However, concentrations of 10 mM or less significantly diminishes this release, with 70% curtailed in Na+-free solutions. Our results, together with morphological observations by a number of other investigators, suggest that there may be two distinct mechanisms for GABA release from goldfish H1 horizontal cells: one being a conventional vesicular mechanism which is Ca2+ dependent, while the other is Na+ driven and Ca2+ independent. H1 horizontal cells in the intact goldfish retina release the accumulated GABA in response to brief incubations in darkness, which is known to be the natural stimulus that depolarizes these neurones.

Responses of solitary retinal horizontal cells from Carassius auratus to L-glutamate and related amino acids

Effects of L-glutamate and its analogues on membrane potentials of solitary horizontal cells were studied by intracellular recording. L-glutamate depolarized these cells at micromolar concentrations (greater than or equal to 10 microM), while D-glutamate and L-alpha-amino adipic acid produced slight depolarizations only at millimolar concentrations. Neither L- nor D-aspartate, even at millimolar doses, produced any change in solitary horizontal-cell resting potential. Solitary horizontal-cell responses to L-glutamate did not desensitize detectably. Responses to pairs of brief, ionophoretic pulses of L-glutamate were nearly equal in amplitude at inter-pulse intervals as short as 50 ms. Responses to maintained applications of low doses of L-glutamate did not decline for as long as 2 min. Depolarizing responses were produced by ionophoretic applications of L-glutamate near cell somata as well as dendrites. The mean sensitivity was 1.4 +/- 1.5 mV/nC with a maximum of 5.1 mV/nC. Depolarizing responses to L-glutamate reversed in polarity at membrane potentials between 0 and -20 mV, were accompanied by a decrease in membrane slope resistance, and were suppressed by replacement of extracellular sodium ions with choline. These results demonstrate that chemosensitivity of retinal horizontal cells to acidic amino acids persists after dissociation protocols, and in several respects resembles that found in horizontal cells in situ. These findings are consistent with the notion that retinal horizontal cells receive a synaptic input involving L-glutamate or a similar substance.

L-glutamic acid: a neurotransmitter candidate for cone photoreceptors in human and rat retinas

We have combined immunocytochemical localization of L-aspartate aminotransferase (L-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1; glutamic-oxaloacetic transaminase) with autoradiographic localization of high-affinity uptake sites for L-glutamate or L-aspartate to identify the neurotransmitters of mammalian photoreceptors. In both human and rat retinas, high aspartate aminotransferase immunoreactivity is found in cones but not in rods; certain putative bipolar and amacrine cells are also heavily stained. In the human retina, and perhaps also in the rat retina, cones possess a high-affinity uptake mechanism for L-glutamate but not L-aspartate, whereas rods and Müller (glial) cells take up both L-glutamate and L-aspartate. Taken together, our results indicate that (i) L-glutamate is much more likely than L-aspartate to be the transmitter for human cones, and possibly for cones of other mammalian species as well, and (ii) major differences exist between mammalian cones and rods in the transport and metabolism or utilization of L-aspartate and L-glutamate.

An excitatory amino acid antagonist blocks cone input to sign-conserving second-order retinal neurons

cis-2,3-Piperidinedicarboxylic acid (PDA), an excitatory amino acid antagonist, reversibly blocked cone input to OFF bipolars and horizontal cells, whereas ON bipolars were relatively unaffected. Kainic acid effects were also blocked, indicating a postsynaptic mechanism of action. The use of PDA helps to characterize one of two classes of excitatory amino acid synaptic receptors that mediate cone influence in the outer retina.

Effects of excitatory amino acids, and of their agonists and antagonists on the release of neurotransmitters from the chick retina

Effects of glutamic, aspartic, and cysteic acid, and of kainic acid and N-methyl aspartate on the release of labeled GABA, glycine, and taurine were examined in isolated, perfused chick retina. Glutamic acid (0.5-2 mM), increased the release of 3H-GABA by more than four times and that of 14C-glycine by about two times. The release of GABA decreased 50% and that of glycine 95% in the presence of the antagonist of glutamic acid receptors, glutamate diethyl ester (300 microM). N-methyl aspartate, used as an agonist of aspartic acid receptors, preferentially increased the release of GABA (seven times) over that of glycine (three times). The stimulatory effect of N-methyl aspartate was antagonized by D-alpha-aminoadipate and by Mg. Kainic acid (10 microM) induced the release of glycine but not that of GABA. Cysteic acid failed to modify the release of any of the amino acids examined. The efflux of labeled taurine was practically unaffected by all the compounds utilized. The release of GABA by the excitatory amino acids and agonists was Ca-independent but Na-dependent, whereas the release of glycine was markedly Ca-dependent. The evidence presented here suggests that experimental conditions activating receptors of excitatory amino acids differently affect the release of inhibitory amino acids.

Carp horizontal cells in culture respond selectively to L-glutamate and its agonists

Horizontal cells were enzymatically isolated from the carp retina and maintained in culture for 2-7 days. Cultured horizontal cells typically had resting membrane potentials of -50 to -70 mV and input resistances of 100-150 m omega. The cells were treated with a number of neurotransmitter agents and their analogues. Significant responses were evoked only by 3,4-dihydroxyphenylalanine (dopamine), L-glutamate, and certain glutamate analogues. The responses to dopamine were inconsistent; most often, the membrane hyperpolarized and input resistances increased. However, highly characteristic responses to L-glutamate and its analogues, quisqualate and kainate, were observed in virtually all of the cells tested. The responses consisted of an initial graded depolarization accompanied by a resistance increase, followed in most cases by a prolonged (1- to 2-min) regenerative depolarization. The regenerative component of the response appears to be Ca2+ dependent, while the underlying graded potential may be due to a decrease in K+ conductance of the membrane.