Pharmacological properties of isolated fish horizontal cells

Characteristics and plasticity of electrical synaptic transmission

Electrical synapses are an omnipresent feature of nervous systems, from the simple nerve nets of cnidarians to complex brains of mammals. Formed by gap junction channels between neurons, electrical synapses allow direct transmission of voltage signals between coupled cells. The relative simplicity of this arrangement belies the sophistication of these synapses. Coupling via electrical synapses can be regulated by a variety of mechanisms on times scales ranging from milliseconds to days, and active properties of the coupled neurons can impart emergent properties such as signal amplification, phase shifts and frequency-selective transmission. This article reviews the biophysical characteristics of electrical synapses and some of the core mechanisms that control their plasticity in the vertebrate central nervous system.

Mesopic state: cellular mechanisms involved in pre- and post-synaptic mixing of rod and cone signals

At least twice daily our retinas move between a light adapted, cone-dominated (photopic) state and a dark-adapted, color-blind and highly light-sensitive rod-dominated (scotopic) state. In between is a rather ill-defined transitional state called the mesopic state in which retinal circuits express both rod and cone signals. The mesopic state is characterized by its dynamic and fluid nature: the rod and cone signals flowing through retinal networks are continually changing. Consequently, in the mesopic state the retinal output to the brain contained in the firing patterns of the ganglion cells consists of information derived from both rod and cone signals. Morphology, physiology, and psychophysics all contributed to an understanding that the two systems are not independent but interact extensively via both pooling and mutual inhibition. This review lays down a rationale for such rod-cone interactions in the vertebrate retinas. It suggests that the important functional role of rod-cone interactions is that they shorten the duration of the mesopic state. As a result, the retina is maintained in either in the (rod-dominated) high sensitivity photon counting mode or in the second mode, which emphasizes temporal transients and spatial resolution (the cone-dominated photopic state). Experimental evidence for pre- and postsynaptic mixing of rod and cone signals in the retina of the clawed frog, Xenopus, is shown together with the preeminent neuromodulatory role of both light and dopamine in controlling interactions between rod and cone signals. Dopamine is shown to be both necessary and sufficient to mediate light adaptation in the amphibian retina.

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.

Horizontal cell dynamics: what are the main factors?

The factors potentially determining the dynamics of horizontal cell (HC) responses are: (1) the rate of transmitter release (including its transient component) and removal; (2) the voltage non-linearity of HC non-synaptic membrane combined with its capacitance; and (3) the dynamics of feedback from HCs to photoreceptors. Using, in consecutive order, the models of an isolated HC, a HC with one or two synaptic inputs and a HC of chromatic type, we have analysed the relative importance of three factors in shaping HC responses to the light and electrical current. The most prominent effect on the shape of HC ON responses derives from the voltage-dependency of the non-synaptic membrane. The dynamics of synaptic transmission plays a leading role in shaping the OFF light responses. For depolarizing responses of C-type HCs, the key factor is the electrical feedback from L-type HCs, which provides not only the response of opposite polarity (to red light), but also the typical feedback delay.

Expression profiling of GABAA receptor beta-subunits in the rat retina

This study profiled the expression of the family of GABAA receptor beta-subunits in the adult rat retina. Using a combination of reverse transcriptase reaction followed by polymerase chain reaction (RT-PCR) with gene-specific primers, the expression of mRNAs encoding the beta 1, beta 2, and beta 3 subunits was first examined in the intact retina and then in separated retinal nuclear layers. However, it was found that a critical analysis had to be carried out at the level of the single cell in order to resolve the differential patterns of expression among the retinal cell types. When cells were isolated and identified following acute dissociation, RT-PCR revealed that individual rod photoreceptor cells expressed consistently the beta 1 and beta 2 messages while the bipolar cells expressed the beta 1 and beta 3 messages. Ganglion cells displayed considerable variability in beta-subunit expression, perhaps reflecting their functional and morphological heterogeneity in the retina. In contrast, the nonneuronal Mueller cells did not express any of the beta-subunit messages. These results indicate that the expression of GABAA receptor subunits is cell-type dependent. Furthermore, as the expression of other families of GABAA receptor subunits are profiled and the patterns of subunit assembly are better understood, our results raise the possibility that GABAA receptors with different subunit compositions can be expected to be coexpressed within a single retinal neuron.

Studies on the dopamine-dependent modulation of amino acid-gated currents in cone horizontal cells of the perch (Perca fluviatilis)

The whole-cell patch-clamp technique was employed to record membrane currents from isolated horizontal cells in culture. Concentration-response relationships for currents induced by L-glutamate, kainate and quisqualate were measured. Preincubation with dopamine changed the parameters of the concentration-response curves in a particular way for each agonist. The maximum currents induced by glutamate increased by 50-100% and the EC50-values were slightly shifted to higher values. The increase of kainate-induced maximum currents after dopamine incubation did not exceed 30% but the EC50-values were clearly shifted to lower concentrations. Quisqualate-induced maximum currents were not enhanced by dopamine preincubation but the EC50-values were shifted to lower concentrations. The dopamine-dependent modulation was affected by removal of magnesium and preincubation with concanavalin A and aniracetam. The concentration-response relation and the time-course of the dopamine effect on glutamate-induced currents is described.

Ionotropic non-N-methyl-D-aspartate agonists induce retraction of dendritic spinules from retinal horizontal cells

Horizontal cells invaginate the photoreceptors in the retina and form reciprocal synaptic connections in the cone pedicles. In fish retina the pattern of synaptic connections is plastic and modulated by the ambient light conditions. Numerous dendritic spinules protrude from the terminal horizontal-cell dendrites into the cone pedicle when the retina is light-adapted and are retracted during dark adaptation. The retraction of spinules can be induced during maintained illumination by an injection of the putative cone transmitter L-glutamate or its analogue kainic acid into the vitreous humor. The formation and the retraction of spinules have a time course of minutes. Activation of protein kinase C through phorbol esters initiates the formation of spinules, but the retraction has not yet been linked to a specific second messenger. Herein we report that physiological concentrations of the glutamate analogs quisqualic acid and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid induce retraction of spinules during maintained illumination. (+/-)-trans-1-Amino-1,3-cyclopentanedicarboxylic acid, an agonist for the metabotropic quisqualic acid receptor, was without effect on spinule retraction. N-Methyl-D-aspartate and L-2-amino-4-phosphonobutyric acid, agonists at other types of glutamate receptors, were also without any effect. The effects of the active agonists persisted when synaptic transmission was blocked. In the presence of the ionotropic quisqualate receptor antagonist 6-cyclo-7-nitro-quinoxaline-2,3-dione the effects of all active agonists were blocked. These results demonstrate that activation of ionotropic quisqualate receptors on the horizontal-cell membrane can induce dendritic spinule retraction, a process associated with dark adaptation.

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.

Glutamate antagonists that block hyperpolarizing bipolar cells increase the release of dopamine from turtle retina

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

Cyclic adenosine monophosphate as a second messenger in horizontal cell uncoupling in the teleost retina

The reduction in the receptive field of horizontal cells of the teleost Eugerres plumieri observed upon dopamine (DA) superfusion is thought to be due to cell uncoupling. The possible mechanisms by which activation of DA receptors modify the electric coupling between horizontal cells were studied in the present work. It was found that the effect of DA in different preparations is mediated by a modification of intracellular concentration of cAMP and H+. The effects of intracellular injection of cAMP and H+ were studied in retinal horizontal cells of the teleost E. plumieri. A triple microelectrode was used to inject the ion iontophoretically, to pass current pulses, and to record voltages from the same cell, while a fourth microelectrode was used to record voltages from a neighboring cell in the same retinal layer. Responses evoked by light spots and annuli were evaluated simultaneously. Coupling ratios between neighboring horizontal cells ranged from 0.22 to 0.45. The intercellular resistance (Rc), 0.5-3.5 x 10(6) ohms, and that of the remaining cell membrane resistance (Rm), 2.5-18 x 10(6) ohms, were calculated by means of a passive electrical model that has a hexagonal array. The microinjection of H+ with injection current from +5 to +30 nA for 40 to 100 sec led to temporary and reversible light response reduction. The coupling ratio between two impaled cells was reduced by about 30%, and intercellular resistance (Rc) increment was 320% while cell membrane resistance (Rm) did not change consistently. There was also a temporary and reversible Rm reduction (70-85%) and an Rc increment of 170-330% when cyclic adenosine monophosphate was iontophoretically injected with current from -30 to -40 nA for 50 to 170 sec. The coupling ratio between two impaled cells was reduced by about 40%, and light responses recorded from the injected cell showed a reduction in amplitude with the same time course as that of the resistive changes. The injection of Lucifer yellow into a horizontal cell under normal conditions always results in pronounced fluorescence for more distant cells; however, under constant injection of H+ or cAMP only the injected cell is fluorescent, which provides direct evidence of the reduction in the effectiveness of coupling between horizontal cells. The observed effects of intracellular H+ or cAMP injection correspond to the resistive changes in Rc and coupling ratio that occur in the horizontal cell network upon superfusion with a dopamine (DA) solution.

Junctions form between catfish horizontal cells in culture

Cone horizontal cells from the catfish retina extend out processes after a few days in culture that sometimes contact adjacent cone horizontal cells. Two types of specialized junctions were observed by electron microscopy along the newly formed contact areas. One junctional type consisted of prominent electron-dense material along and just under the plasma membrane of one or both of the contacting elements. Sometimes vesicle clusters were associated with these junctions. The other type of junction showed some electron-dense material along the membranes of both processes and patchy areas of close membrane apposition resembling gap junctions. In about half of the cases tested, electrical coupling was detected between cone horizontal cells that had made contact in culture. In no case was the coupling as tight as is typically found between horizontal cells that had formed gap junctions in vivo.

Responses of isolated white perch horizontal cells to changes in the concentration of photoreceptor transmitter agonists

Current and voltage responses elicited by increasing or decreasing the concentration of L-glutamate or its analog kainate around isolated cone horizontal cells were measured with patch pipettes using the whole cell recording configuration. Application of these photoreceptor transmitter agonists induced inward currents in voltage-clamp experiments (for negative holding potentials) and depolarizing responses in current-clamp experiments. Continuous exposure to either drug produced inward currents which were maintained for as long as superfusion with the drugs continued. Reducing the concentration of the agonists by pressure ejection of pulses of drug-free Ringer's solution onto the cells completely turned off the drug-induced currents. Under current-clamp conditions, pulses of control Ringer's elicited hyperpolarizing responses of large amplitude (40-80 mV). The data demonstrate the ability to simulate in vitro the horizontal cell's photoresponses and thus support the use of cultured cells as a model system for studying horizontal cell physiology and pharmacology.

Binding pattern of alpha-bungarotoxin on horizontal cells of a marine teleost retina

A conjugate of alpha-bungarotoxin and a fluorescent marker (fluorescein isothiocyanate) has been used to localize "nicotinic" acetylcholine receptors on neurons in the outer plexiform layer of marine teleost retina. Toxin binding was confined to bipolar cell dendrites and to intermediate horizontal cells. The arrangement of labeled horizontal cells appears irregular in the whole retina, with a peak density in the ventral and dorsal quandrants. Alpha-bungarotoxin receptors on horizontal cells differ from those on bipolar cells and from those on dendrites in the inner plexiform layer in their sensitivity to agonists and antagonists such as d-tubocurarine and nicotine. They constitute a different type of "nicotinic" receptor that probably has a different function.

Micro-electrode measurements and functional aspects of chloride activity in cyprinid fish retina: extracellular activity and intracellular activities of L- and C-type horizontal cells

Extracellular Cl- activity and intracellular Cl- activities of luminosity and biphasic-chromaticity type horizontal cells were measured in freshly isolated, non-superfused roach retinae using double-barrelled Cl- -sensitive micro-electrodes. The extracellular Cl- activity in dark-adapted retinae was found to have a surprisingly wide range (54-143 mM), although in a given preparation it was extremely constant. The mean intracellular Cl- activities of both types of horizontal cell were identical (47 mM), and this value was significantly greater than that required for "passive distribution" i.e. Cl- equilibrium potentials were 11-12 mV more positive than respective membrane resting potentials in the dark. In the presence of 10 microM dopamine, however, the difference between the Cl- equilibrium potential and the membrane resting potential was abolished, consistent with the hypothesis that dopamine increases Cl- conductance, presumably at the interplexiform cell synapse onto horizontal cells. In turn, it is suggested that a functional consequence of this pathway is to modulate the input impedances of the horizontal cells, and hence their sensitivity to light.

N-methyl-D-aspartate receptors coexist with kainate and quisqualate receptors on single isolated catfish horizontal cells

Horizontal cells enzymatically isolated from catfish retina were exposed to the putative neurotransmitters aspartate (Asp) or N-methyl-D-aspartate (NMDA). Under voltage clamp conditions, inward currents were recorded when the holding potential was more negative than zero and outward currents were recorded when the membrane potential was more positive than zero. The current voltage curve was highly non-linear in the range of membrane potential between -30 and -100 mV. This non-linearity was largely removed in zero magnesium solution. 2-Amino-phosphonovaleric acid selectively blocked Asp and NMDA responses. These response characteristics are consistent with the presence of NMDA receptors in these cells.

The self-regulating synapse: a functional role for the co-existence of neuroactive substances

Although the co-localizations of neuroactive substances, such as transmitters and peptides, in identified neurons is now a common histochemical phenomenon, the physiological roles and functional significance of such co-existence are largely unknown. Using the vertebrate retina as a model for the central nervous system, we have examined the relationship between co-existence and co-function. We propose here that the co-localization of neuroactive substances in a synaptic terminal provides the structural configuration to ensure the co-release of two or more predetermined substances into the same synaptic cleft, resulting in the capability of the presynaptic neuron to stringently regulate its own activities and output.

Neuropharmacological actions of kynurenic and quinolinic acids on horizontal cells of the isolated fish retina

We have studied the effects of naturally occurring metabolites of tryptophan, kynurenic and quinolinic acids, on the electrophysiological responses of retinal horizontal cells in the fish (Rutilus rutilus, the roach). Quinolinic acid usually hyperpolarizes the cells and reduces their light evoked responses (S-potentials) but on occasion, it causes a slight depolarization of the membrane potential. These actions are similar to those found with N-methyl-D-aspartate (NMDA) and our results are consistent with the proposal that it acts at NMDA binding sites. Kynurenic acid (greater than or equal to mM) invariably hyperpolarizes horizontal cells to their potassium Nernst equilibrium potential and, more significantly, blocks the depolarizing actions exerted on them by excitatory amino acids, such as kainic and quisqualic acids. We show that this action persists in the presence of the synaptic blocker, cobalt chloride, and thus is not mediated by chemical synaptic activity. Kynurenic acid does not reverse depolarization of horizontal cells by dopamine or gamma-aminobutyric acid, thus its inhibitory effects are selective to the actions of excitatory amino acids. Neither xanthurenic acid, a close structural analogue of kynurenic acid, nor quinolinic acid are effective in blocking depolarizations by excitatory amino acids.

GABA and GAD immunoreactivity of photoreceptor terminals in primate retina

Within the vertebrate retina, two types of photoreceptor cells--the rods and cones--transduce visual signals and convey this information through synapses with bipolar and horizontal cells. Although the neurotransmitter at these first-order synapses has not been identified, electrophysiological studies suggest that it might be excitatory. In the present study, however, we have found photoreceptor terminals in the rhesus monkey retina which are immunoreactive with antibodies to either gamma-aminobutyric acid (GABA) or L-glutamic acid decarboxylase (GAD, an enzyme involved in the synthesis of GABA). In the perifoveal region of the retina, approximately 25% of presynaptic profiles having ultrastructural characteristics of either rod or cone terminals are immunoreactive with one or the other antibody. This evidence for a putatively inhibitory neurotransmitter in photoreceptor terminals challenges present understanding of retinal synaptic function.

The effects of continuous superfusion of L-aspartate and L-glutamate on horizontal cells of the turtle retina

We have studied the effects of prolonged superfusion with L-aspartate and L-glutamate on the membrane potential and photoresponses recorded in luminosity type horizontal cells in the turtle retina using an everted eyecup preparation. These acidic amino acids produce effects which are a function of the past history of the impaled cell. Initial prolonged superfusions with 30 mM of either drug has no pronounced effect on the membrane potential and photoresponses of horizontal cells. Subsequent superfusions with either agent eventually produce depolarizations with reductions in the amplitudes of the light evoked responses. These effects, however, are transient; the horizontal cell rehyperpolarizes and the light evoked response grows with time. In a retina which had been stored at 4 degrees centrigrade for 20 hours, an initial superfusion with L-aspartate solution produced an immediate depolarization of the horizontal cell and complete suppression of the light evoked response for as long as the amino acid was present. The data are consistent with the existence of powerful amino acid uptake mechanisms operating at a number of sites within the inner and outer retina but also raise questions about the role of acidic amino acids in the outer plexiform layer of the turtle retina.

White perch horizontal cells in culture: methods, morphology and process growth

Four types of horizontal cells are recognized in primary cultures of the adult white perch retina. Immediately after isolation, all horizontal cell types show fine terminal processes. However, these fine processes disappear over the next 12-24 h and the cells typically round up partially. After 2-4 days in culture, new processes appear on the horizontal cells, and these usually grow substantially with time. By 14-21 days, processes several hundred micrometers in length are observed in the retinal culture dishes. Processes from different cells intertwine and contacts that appear synaptic in nature are observed.

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.

Effects of glycine and GABA on the ganglion cels of the retina of the skate Raja erinacea

Application of glycine and GABA caused inhibition of light-evoked activity from different populations of 'ON' center ganglion cells in the retina of the skate (Raja erinacea). Cells affected by glycine were not affected by GABA and vice-versa. Addition of 100 microM strychnine to the glycine-containing perfusate caused a resumption in light-evoked responses of the cells. The action of GABA was antagonized by 100 microM bicuculline methochloride, but not by picrotoxin. Autoradiography revealed the presence of [3H]glycine accumulating amacrine cells.

Evoked efflux of [3H]GABA from goldfish retina in the dark

H1 horizontal cells of goldfish retina probably are GABAergic and receive synaptic excitation from red cone photoreceptors in the dark. This study was designed to detect efflux of [3H]GABA from H1 cells by a physiological stimulus in order to obtain information regarding the identity of the red cone transmitter and obtain information on the role of dopaminergic interplexiform cells (DA-IPCs), the other synaptic input to H1 cells. Efflux of [3H]GABA was studied by biochemical analysis of perfused isolated retinas. Retinas were incubated in dim red light for 15 min in 0.72 microM [3H]GABA, rinsed for 30 min in red light and subjected to darkness under a variety of conditions. Radioactivity in the perfusate was determined by liquid scintillation spectroscopy. The findings are: 1. both L-glutamate and L-aspartate cause a dose-dependent efflux of [3H]GABA from H1 cells, 2. inclusion of 3.2 mM D-aspartate in the perfusion medium potentiates L-glutamate and totally inhibits L-aspartate, 3. retinas perfused in the standard Ringer do not show increased [3H]GABA efflux when placed in the dark, 4. when 3.2 mM D-aspartate is in the perfusion medium, there is significant dark-induced [3H]GABA efflux which is reduced with light onset, 5. 100 microM dopamine inhibits the dark-induced efflux of [3H]GABA. These results show that efflux of [3H]GABA from H1 cells can be detected under physiological conditions strongly suggesting that the H1 cell is GABAergic and, in addition, is subject to antagonistic inputs from red cones and DA-IPCs. Furthermore, since D-aspartate potentiates L-glutamate and inhibits L-aspartate, and is required for the detection of dark-induced efflux of [3H]GABA, it is unlikely that the transmitter for red cones is L-aspartate but more likely is L-glutamate.

Hyperpolarization of fish retinal horizontal cells by kainate and quisqualate

Kainic (KA) and quisqualic (QA) acids have a potent depolarizing action on a variety of neurones of the central nervous system, including retinal horizontal cells. We now report the novel finding that at low concentrations (1-3 microM), these 'excitatory' amino acids hyperpolarize horizontal cells of the fish retina. We show that the hyperpolarizing effects of both KA and QA are reversed by the gamma-aminobutyric acid (GABA) antagonist bicuculline, whereas a second GABA antagonist, picrotoxin, reverses the effects of KA, but not of QA. Neither GABA antagonist influences horizontal cell depolarization by 50 microM KA or 50 microM QA, thus the excitatory (depolarizing and inhibitory (hyperpolarizing) effects of the amino acids involve independent mechanisms. We provide evidence that the hyperpolarizing effects are not mediated by the dopaminergic pathways associated with retinal horizontal cells.

On the sensitivity of H1 horizontal cells of the carp retina to glutamate, aspartate and their agonists

Threshold concentrations for L-glutamate, L-aspartate and their agonists, kainate, quisqualate and N-methyl-D,L-aspartate, were determined for horizontal cells of the intact carp retina and horizontal cells that were enzymatically isolated and maintained in culture. Our results indicate that uptake or other mechanisms decrease the apparent sensitivity of these cells in the intact retina to L-glutamate by 25-200 times. Our findings also suggest that L-glutamate is more likely a photoreceptor transmitter than L-aspartate.

Electrophysiological effects of GABA on fish retinal horizontal cells are blocked by bicuculline but not by picrotoxin

We have studied electrophysiologically the actions of gamma-aminobutyric acid (GABA) and related pharmacological agents on fish retinal horizontal cells by recording intracellularly from isolated retinae perfused with Ringer containing the various drugs. We show that although GABA usually hyperpolarizes the membrane potential relative to its dark level, it sometimes and particularly at higher (greater than or equal to 5 mM) concentration produces membrane depolarization, with reduction in the light evoked responses (S-potentials) in both cases. These effects are reversed by bicuculline but not by picrotoxin, although both agents antagonize GABA in many other preparations [5, 25]. The GABA uptake blocker nipecotic acid [15] hyperpolarizes horizontal cells and reduces their light evoked responses, and again these effects are reversed by bicuculline but not by picrotoxin. beta-Alanine, which blocks glial GABA transport [29], and diaminobutyric acid (DABA), which blocks neuronal GABA transport [14, 29, 31], have effects similar to those of nipecotic acid. We discuss these actions of GABA and of the other related drugs and their differential sensitivity to bicuculline and picrotoxin.

Electrical coupling between horizontal cell bodies in the tiger salamander retina

We studied lateral interactions between horizontal cells in the tiger salamander retina in order to determine the underlying mode of synaptic transmission. Pairs of cells, visually identified by inspection of the in vitro retinal slices, were impaled with two separate electrodes and the transmission between them was examined by injecting current into one cell while recording the resulting voltage response from the other soma. It was found that coupling between horizontal cells remained intact and even increased somewhat during the light response and in the presence of cobalt or acetylcholine. It was also observed that a fluorescent dye injected into one cell would often migrate and stain neighbouring horizontal cells. These results suggest that synaptic transmission between horizontal cell bodies is mediated mainly by electrotonic conduction. The increase in apparent coupling observed under certain experimental conditions can be explained by the increase in input resistance of each horizontal cell.

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.

Connexions between retinal neurons with identified neurotransmitters

Dopaminergic and indoleamine accumulating neurons can be demonstrated both in the light and the electron microscopes. Considerable differences have been found between different animal species. There are two types of dopaminergic neurons, the interamacrine cells and the interplexiform cells. The interamacrine cells contact only other amacrine cells. They receive synapses from other amacrine cells which are likely to operate with, e.g. GABA or glycine as neurotransmitter. The dopamine turnover in the dopaminergic interamacrine cells is very rapidly activated by light. Dopaminergic interplexiform neurons are known only in teleost fish and New World monkeys. They have approximately the same contacts in the inner plexiform layer as the interamacrine cells, but, in addition, send processes to the outer plexiform layer and there contact both horizontal cells and bipolar cells. The function of the dopaminergic neurons has not been determined. The indoleamine accumulating amacrine neurons are in Cebus monkeys, cats and rabbits contacted by bipolar cells in dyads and form reciprocal synapses with them. They are also contacted by amacrine cells and make synapses on other with them. They are also contacted by amacrine cells and make synapses on other amacrine cells and on ganglion cells. The contacts are different in teleost fish, where the indoleamine accumulating cells mainly contact other amacrine cells only. The transmitter of the indoleamine accumulating neurons is debated in mammals but is most likely 5-hydroxytryptamine in other vertebrates.