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

Neurotoxic action of kainic acid in the isolated toad and goldfish retina: I. Description of effects

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). KA caused vacuolization of proximal and distal segments of horizontal cell dendrites in the OPL as well as perikaryal vacuolization and/or chromatin clumping in selected classes of neurons in the inner nuclear layer. Further, KA caused vacuolization and swelling in the inner plexiform layer. These effects were very similar in the retinae of goldfish and toad. The extent of vacuolization in the OPL was graded with KA concentration and with length of incubation. For 15-minute incubations, half-maximal vacuolization was found at 10-20 microM KA. At 25 microM KA, OPL vacuolization was evident within 1-2 minutes of application of KA. In goldfish, but not in toad, rod-connecting dendrites were less sensitive to KA than cone-connecting dendrites.

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.

Light-evoked and kainic-acid-induced disc shedding by rod photoreceptors: differential sensitivity to extracellular calcium

In order to study the light and Ca2+ dependence of disc shedding by rod photoreceptors, we have used eyecups prepared from adult Rana pipiens frogs that had been kept in constant light for 4 days. Disc shedding was initiated by a treatment involving 1 hour of darkness followed by exposure to light or by treatment with kainic acid. Maximal L-evoked disc shedding occurred quickly (within 30-60 minutes) after light onset and could be triggered by brief (15 minutes) exposure to light. L-evoked disc shedding was completely blocked by omission of Ca2+ from culture medium or by treatment with 3mM Co2+ or 12 mM Mg2+ in the presence of Ca2+ (2 mM). The response was also blocked by the organic Ca2+ antagonist nifedipine. Experiments designed to distinguish between Ca2+ dependence of the dark- or light-dependent processes necessary for shedding suggest that voltage-sensitive channels mediate a Ca2+-dependent process involved in light-triggering. Kainic acid caused a dose-dependent stimulation of disc shedding under lighting conditions (continuous culture in light or darkness) that did not normally result in a significant response in the absence of the drug. Disc shedding induced by kainic acid was similar in time course and magnitude to that induced by light. However, kainic-acid-induced disc shedding was not inhibited by medium Ca2+ reduction or by the presence of Co2+. The latter observation suggests that kainic acid activates disc shedding directly, by-passing the Ca2+-dependent process involved in the L-evoked response. The Ca2+-dependent process may involve release of an effector of disc shedding that is mimicked by kainic acid.

Blocking effects of cobalt and related ions on the gamma-aminobutyric acid-induced current in turtle retinal cones

Red-sensitive cone photoreceptors were isolated from the turtle retina, and GABA-induced currents were recorded under voltage clamp. The effect of Co2+, widely used as a blocker of chemical synapses, on the GABA-induced current was studied. Co2+ blocked the GABA-induced current evoked by local application either at the synaptic region (cone pedicle) or at the extra-synaptic region (cell body). 5 microM-Co2+ suppressed the GABA-induced current by 50%, and a few hundred microM-Co2+ blocked it almost completely. Co2+ suppressed the GABA-induced current non-competitively: the saturating response amplitude decreased without a change in the threshold or saturating dose of GABA. The blocking was not voltage dependent in the physiological range of the membrane potential. Ni2+ and Cd2+ also blocked the GABA-induced current non-competitively, and were as effective as Co2+. Tetraethylammonium (25 mM) showed a similar but weaker blocking effect. On the other hand, Mg2+ (20 mM), Mn2+, Sr2+, Ba2+ (10-100 microM each), D-600 (10 microM) or Cs+ (10 mM) did not affect the GABA-induced current. The Ca current in the turtle cones was blocked almost completely by 20 mM-Mg2+ or 4 mM-Co2+, or strongly suppressed by 10 microM-D-600. However, Cd2+ and Ni2+ (10 microM each) blocked the Ca current by ca. 50%, and Co2+ and Mn2+ (10 microM each) suppressed it only partially. The blocking of the GABA-induced current by these agents was, therefore, not directly related to the blocking of the Ca current and/or Ca-mediated currents. These observations present a warning on the use of some divalent cations, such as Co2+, Ni2+ or Cd2+, as a presynaptic blocker at the GABAergic synapse. High concentrations of Mg2+ are recommended as a more appropriate blocker.

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.

Effect of selective kainate lesions on the release of glutamate and aspartate from chick retina

In order to contribute evidence leading to establishing the excitatory pathways in the vertebrate retina, we selectively lesioned chick retinas by intraocular injection of 6, 60, 120, and 200 nmol of kainate, which selectively damages OFF-bipolars, amacrines, horizontals, and ON-bipolars, and measured the K+-stimulated, Ca++-dependent release of L-(3H)-glutamate and L-(3H)-aspartate. We also measured (3H)-GABA release as a marker for horizontal cells and a population of amacrines, as well as (14C)-glycine release as a tracer of a different subpopulation of amacrines. All four amino acids were released from control retinas by a depolarizing K+ concentration in a Ca++-dependent fashion. GABA and glycine, however, showed an additional Ca++-independent component of release. Lesion induced by 6 nmol of kainate decreased by 50% the release of glutamate and by 20% that of aspartate; glycine release was reduced 40% while GABA release was unaffected. Injection of 60 nmol of kainate reduced glutamate release a further 20% and significantly decreased GABA (50%) and glycine (75%) release; aspartate release remained unmodified; 120 nmol of kainate caused a further 30% reduction in aspartate and GABA release. Neither compound was significantly released after treatment with 200 nmol of kainate. These results seem to suggest that while OFF-bipolars could release glutamate as transmitter, aspartate is released from a different cell population which is less sensitive to kainate, probably ON-bipolars.

Synaptic transmission at N-methyl-D-aspartate receptors in the proximal retina of the mudpuppy

The effects of excitatory amino acid analogues and antagonists on retinal ganglion cells were studied using intracellular recording in the superfused mudpuppy eyecup preparation. Aspartate, glutamate, quisqualate (QA), kainate (KA) and N-methylaspartate (NMA) caused depolarization and decreased input resistance in all classes of ganglion cells. The order of sensitivity was QA greater than or equal to KA greater than NMA greater than aspartate greater than or equal to glutamate. All of these agonists were effective when transmitter release was blocked with 4 mM-Co2+ or Mn2+, indicating that they acted at receptor sites on the ganglion cells. At a concentration of 250 microM, 2-amino-5-phosphonovalerate (APV) blocked the responses of all ganglion cells to NMA, but not to QA or KA, indicating that NMA acts at different receptor sites from QA or KA. Responses to bath-applied aspartate and glutamate were reduced slightly or not at all in the presence of APV, indicating that they were acting mainly at non-NMDA (N-methyl-D-aspartate) receptors. In all ganglion cells 250 microM-APV strongly suppressed the sustained responses driven by the 'on'-pathway but not those driven by the 'off'-pathway. In most on-off ganglion cells the transient excitatory responses at 'light on' and 'light off' were not reduced by 500 microM-APV. APV-resistant transient excitatory responses were also present in some on-centre ganglion cells. APV did not block the transient inhibitory responses in any class of ganglion cells. At concentrations which blocked the sustained responses of ganglion cells, APV did not affect the sustained responses of bipolar cells, indicating that it acted at sites which were post-synaptic to bipolar cells. The simplest interpretation of these results is that the transmitter released by depolarizing bipolar cells acts at NMDA receptors on sustained depolarizing amacrine and ganglion cells. It may act at non-NMDA receptors at synapses which produce transient excitatory responses, but this could not be proved. The transmitter released by hyperpolarizing bipolar cells does not appear to act at NMDA receptors on any post-synaptic cells.

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.

Messenger RNA from bovine retina induces kainate and glycine receptors in Xenopus oocytes

The retina contains several types of nerve cells that communicate through chemical synapses. The transmitter and receptor molecules that mediate signal transmission across these synapses need further characterization. For this purpose, poly (A)+ mRNA was isolated from bovine retinas and injected into Xenopus laevis oocytes. Translation of the foreign mRNA induced the oocyte membrane to acquire functional receptors to kainate and, to a lesser extent, also receptors to glycine, gamma-aminobutryic acid (GABA), aspartate and glutamate. Thus, the cells in the retina must contain different messengers coding for these neurotransmitter receptors. Activation of the kainate receptors opens membrane channels, generating an ionic current which has an equilibrium potential close to 0 mv. The current is well maintained during prolonged application of kainate, and hence these receptors may be involved in the neurotoxic effects produced by kainate in the retina.

Dopamine decreases conductance of the electrical junctions between cultured retinal horizontal cells

Horizontal cells from the white perch were isolated by enzymatic treatment and trituration of the retina and were maintained in culture for 1-5 days. Overlapping pairs of horizontal cells were identified, and the two cells were recorded from simultaneously, using whole-cell patch clamp techniques. Electrical coupling between cells was determined by passing current pulses into one cell, the driver cell, while (i) recording voltage changes in the other, follower cell, or (ii) measuring current flow into the follower cell. Most cell pairs of the same morphological type were coupled electrically, with coupling coefficients often greater than 0.9. Junctional resistance was typically found to be between 20 and 60 M omega and junctional conductance was between 150 and 500 nS. After application of 1-microliter pulses of dopamine (200 microM) to coupled pairs of cells, the coupling coefficient fell to approximately equal to 0.1, junctional resistance increased to 300-700 M omega, and junctional conductance decreased to 15-30 nS. Recovery of coupling took, for most cell pairs tested, 8-15 min after dopamine application. The exogenous application of 8-bromo-cyclic AMP (0.5-1 mM) also caused uncoupling of horizontal cell pairs; however, neither isoprenaline nor L-glutamate altered coupling significantly.

Effect of excitatory amino acids on gamma-aminobutyric acid release from frog horizontal cells

The effects of excitatory amino acids, analogues and K on [3H]gamma-aminobutyric acid [3H]GABA) release from horizontal cells of the isolated superfused frog retina were studied. Exposure of the retina to medium containing high concentrations (25-100 mM) of KCl increased the release of [3H]GABA to a maximum which was 40 times the spontaneous resting release. The K-evoked release of [3H]GABA was almost abolished in high-Mg/low-Ca medium. Glutamate, aspartate, kainate and quisqualate also stimulated the release of [3H]GABA from horizontal cells, the maximum evoked release being similar to that produced by KCl. The release of [3H]GABA evoked by glutamate, aspartate, kainate and quisqualate was abolished in high-Mg/low-Ca medium and by Na-free medium. The evoked releases of [3H]GABA were not reduced by tetrodotoxin. N-Methyl-D-aspartate (NMDA) at concentrations up to 10 mM had virtually no effect on [3H]GABA release from horizontal cells. In Mg-free medium, NMDA stimulated [3H]GABA release, but the maximum release was only 10% of that produced by other agonists. Mg-free medium did not significantly affect the evoked release of [3H]GABA by other agonists. NMDA apparently possessed affinity for the kainate receptor, because in normal medium it antagonized the effects of kainate but not glutamate, aspartate or quisqualate. The non-selective antagonist of excitatory amino acids, (+/-)-cis-2,3-piperidine dicarboxylic acid (PDA) antagonized the action of glutamate, aspartate, kainate and quisqualate on horizontal cell [3H]GABA release. D(-)-2-Amino-4-phosphonobutyrate (APB) and D-gamma-glutamylglycine (D-gamma-GG) antagonized the actions of kainate on horizontal cell [3H]GABA release at concentrations which had little affect on quisqualate-evoked responses. Approximate estimates of pA2 values (Schild, 1947) showed that the specificity and potency of the antagonists was low. Nevertheless, the retinal 'non-NMDA' receptors can probably be subdivided into kainate and quisqualate types. Glutamate diethylester (GDEE) did not affect the action of any agonist. We conclude that glutamate (and aspartate) probably stimulate the release of [3H]GABA from frog horizontal cells by activating receptors of the non-NMDA type. This activation may trigger the opening of tetrodotoxin-insensitive Na channels, resulting in the depolarization of the cell membrane and an increase in the conductance of voltage-sensitive Ca-channels. An influx of Ca ions would then trigger the release of [3H]GABA. Our results are not consistent with previous suggestions that GABA release from horizontal cells involves an outwardly directed transport process.

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.

Quisqualate and L-glutamate inhibit retinal horizontal-cell responses to kainate

Currents elicited by L-glutamate and the related agonists quisqualate and kainate were analyzed under voltage clamp in isolated goldfish horizontal cells, using the whole-cell recording configuration of the patch-clamp method [Hamill, O.P., Marty, A., Neher, E., Sakmann, B. & Sigworth, F. J. (1981) Pflügers Arch. 391, 85-100]. These currents resulted from an increase in cationic conductance and were indistinguishable from one another in terms of reversal potential (approximately equal to 0 mV) and apparent elementary conductance (2-3 pS). The power-density spectra of the noise increases produced by each agonist were fit by the sum of two Lorentzian curves having similar cutoff frequencies (tau 1 approximately equal to 5 msec, tau 2 approximately equal to 1 msec), but the relative power of these components were different for quisqualate and glutamate than for kainate. Moreover, the responses to high doses of either quisqualate or glutamate rapidly faded, whereas the responses to kainate did not. Finally, quisqualate and glutamate produced an inhibition of responses to kainate which appeared to be uncompetitive. Kainate, quisqualate, and in our preparation, glutamate appear to activate channels different than those activated by N-methyl-D-aspartate in other preparations. At least some of the effects of quisqualate and glutamate appear to be mediated by receptors bound by kainate.

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.

A voltage-clamp analysis of membrane currents in solitary bipolar cells dissociated from Carassius auratus

Membrane properties of solitary bipolar cells, mechanically dissociated from the enzyme-treated goldfish retina, were studied under current- and voltage-clamp conditions with 'giga-seal' suction pipettes (pipette solution 138 mM-K). The resting potential of solitary bipolar cells was about -30 mV. They responded to depolarizing current pulses with sustained depolarization, and to hyperpolarizing current pulses with an initial hyperpolarizing transient followed by a sag to a less hyperpolarized level. The current-voltage relationship determined under voltage-clamp conditions showed strong outward and inward rectification. The membrane currents consisted of four components; Ca current (ICa), voltage- and Ca-dependent K currents (IK(V) and IK(Ca), respectively), and an inward current activated by membrane hyperpolarization (Ih). ICa was activated by membrane depolarization beyond -40 mV, was maximum at +10 mV and became smaller with further depolarization. No polarity reversal was seen. ICa was enhanced by equimolar replacement of Ca with Ba, and was blocked by 4 mM-Co. IK(Ca) was observed by membrane depolarization beyond -10 mV, was maximum at about +40 mV, and became smaller with further depolarization. This current was suppressed by 4 mM-Co, 1.6 mM-Ba, 35 mM-TEA or 30 microM-quinine. IK(V) was activated by membrane depolarization beyond -60 mV, and had slower kinetics that ICa or IK(Ca). The reversal potential of the tail current was close to the K equilibrium potential (EK), suggesting that this current is carried purely by K ions. IK(V) was inactivated slowly and nearly completely by sustained depolarization. IK(V) was blocked by 35 mM-TEA. Ih was activated by membrane hyperpolarization (less than -60 mV). The current showed a time-dependent increase. It was also dependent on the membrane potential, but not on the driving force of K ions. This current seems to be carried by a mixture of Na and K ions, since (1) in low Na solution, Ih became small in amplitude, and (2) the reversal potential of the tail current was between the Na equilibrium potential (ENa) and EK X Ih was blocked by 10 mM-Cs, but was resistant to 0.2 mM-Ba. The resting potential and voltage responses of solitary bipolar cells are discussed in reference to the characteristics of each membrane conductance isolated in the present study.

Monoclonal antibodies distinguish subtypes of retinal horizontal cells

Sixteen hybridomas have been identified that secrete antibodies specific to horizontal cells in the carp retina. The hybridomas have been classified into three groups based on their antibody staining patterns: group I, staining associated with all horizontal cells; group II, staining associated with the most abundant subtype of horizontal cell (CH1); and group III, staining associated with other subtypes of horizontal cells. Most of the hybridomas fall in group II; some of these antibodies stain the entire horizontal cell, but others are specific only to the cell perikarya and do not stain axonal processes. Our results suggest that there are surface molecules specific (i) to all retinal horizontal cells, (ii) to individual subtypes of horizontal cells, and (iii) to portions of horizontal cells. Furthermore, a group II antibody, which recognizes a 48- to 50-kDa membrane protein, has been found to provide a substrate selective for horizontal cell growth. Horizontal cells plated on coverslips coated with this antibody remain healthy in culture and extend long and elaborate processes for at least 3 weeks.

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.

Kainic acid induces sprouting of retinal neurons

The neurotoxin kainic acid caused dose-dependent morphological changes in horizontal cells of the retinas of adult cats and rabbits. High concentrations of kainic acid killed the cells, but when exposed to sublethal doses they contracted their dendritic fields and sent sprouting processes into the inner retina. It appears that kainic acid can induce neuronal growth as well as degeneration and that the potential for morphological plasticity is still present in neurons of the adult mammalian retina.

Effects of vasoactive intestinal peptide and other peptides on cyclic AMP accumulation in intact pieces and isolated horizontal cells of the teleost retina

The effects of vasoactive intestinal peptide (VIP) and several other peptides have been examined on cyclic AMP accumulation in intact pieces and isolated horizontal cells of the teleost (carp) retina. VIP was the most effective peptide examined, inducing a dose-related response, and an approximately fivefold increase in cyclic AMP production when used at a concentration of 10 microM. Porcine histidine isoleucine-containing peptide and secretin, peptides structurally related to VIP, also stimulated cyclic AMP accumulation, but at concentrations of 10 microM induced responses which were only approximately 40% and 10%, respectively, of the response observed with 10 microM VIP. In contrast, several other peptides, including glucagon, neurotensin, somatostatin, luteinizing hormone-releasing hormone, alpha-melanocyte-stimulating hormone, cholecystokinin octapeptide26-33, gastrin-releasing peptide, thyrotropin-releasing hormone, and VIP10-28 were totally inactive. The response to 10 microM VIP was not antagonized by several dopamine antagonists, indicating the presence of a population of specific VIP receptors coupled to adenylate cyclase, distinct from the population of dopamine receptors coupled to adenylate cyclase also known to be present in this tissue. Finally, experiments involving the use of fractions of isolated horizontal cells indicate that these neurons possess a population of VIP receptors coupled to cyclic AMP production which would appear to share a common pool of adenylate cyclase with a population of similarly coupled dopamine receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Vasoactive intestinal peptide alters membrane potential and cyclic nucleotide levels in retinal horizontal cells

Vasoactive intestinal peptide stimulated the synthesis of adenosine 3',5'-monophosphate in fractions of isolated carp horizontal cells. When applied extracellularly to isolated and cultured horizontal cells, the peptide also induced a slow depolarization (30 to 40 millivolts) accompanied by a decrease in membrane resistance. However, analogs of adenosine 3',5'-monophosphate applied extracellularly or intracellularly, and forscolin applied extracellularly, had no effect on the membrane potential of cultured horizontal cells, indicating that the induced depolarization was not related to the accumulation of adenosine 3',5'-monophosphate in these cells.

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.

Bipolar cells in the mudpuppy retina use an excitatory amino acid neurotransmitter

The bipolar cells of the vertebrate retina are the principal neuronal elements which transmit photoreceptor activity from the outer to the inner retina. An important function of the bipolars is to segregate photoreceptor input into independent ON and OFF channels which are subserved, respectively, by the depolarizing and hyperpolarizing bipolar subtypes. Ultrastructural and physiological observations suggest that chemical neurotransmission is the predominant means of bipolar input to the inner retina. Both ON and OFF bipolars apparently release excitatory transmitters. Histological studies with cytotoxic agents and physiological studies indicate that third-order neurones have excitatory amino acid receptors. In ON-OFF amacrine and ganglion cells, which receive input from both bipolars, ON and OFF excitation have a similar ionic basis, suggesting that the same transmitter may be released by both types of bipolars. We have now found that (+/-)cis-2,3-piperidine dicarboxylic acid (PDA), a new excitatory amino acid antagonist, blocks bipolar input to the inner retina and thus suggests that an excitatory amino acid is a bipolar cell transmitter.

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.

Calcium alters the sensitivity of intact horizontal cells to dopamine antagonists

Horizontal cells of the carp retina possess dopamine receptors linked to adenylate cyclase. Isolated, intact horizontal cells respond to micromolar concentrations of dopamine, whereas nanomolar concentrations of haloperidol, (+)-butaclamol, and flupenthixol block the dopamine response. Preincubation in Ringer's solution containing increased levels of Ca2+ (5-110 mM) decreases the sensitivity of the cells to these antagonists by 1,000-10,000 times. Dopamine sensitivity of the cells is not affected by Ca2+ levels in the preincubation medium. Preincubation of the cells in Ringer's solution containing 500 microM L-glutamate, an agent that increases intracellular Ca2+ levels in intact horizontal cells, also decreases the sensitivity of the cells to haloperidol. These data suggest that antagonist sensitivity of intact horizontal cells may be regulated by intracellular Ca2+.