Synaptic transmission to the horizontal cells in the retina of the larval tiger salamander

Role of acetylcholine in nitric oxide production in the salamander retina

Although acetylcholine is one of the most widely studied neurotransmitters in the retina, many questions remain about its downstream signaling mechanisms. In this study we initially characterized the cholinergic neurotransmitter system in the salamander retina by localizing a variety of cholinergic markers. We then examined the link between both muscarinic and nicotinic receptor activation and nitric oxide production by using immunocytochemistry for cyclic guanosine monophosphate (cGMP) as an indicator. We found a large increase in cGMP-like immunoreactivity (cGMP-LI) in the inner retina in response to muscarinic (but not nicotinic) receptor activation. Based on the amplification of mRNA transcripts, receptor immunocytochemistry, and the use of selective antagonists, we identified these receptors as M2 muscarinic receptors. Using double-labeling techniques, we established that these increases in cGMP-LI were seen in GABAergic but not cholinergic amacrine cells, and that the increases were blocked by inhibitors of nitric oxide production. The creation of nitric oxide in response to cholinergic receptor activation may provide a mechanism for modulating the well-known mutual interactions of acetylcholine-glycine-GABA in the inner retina. As GABA and glycine are the primary inhibitory neurotransmitters in the retina, signaling pathways that modulate their levels or release will have major implications for the processing of complex stimuli by the retina.

Cholinergic modulation of dopamine release and horizontal cell coupling in mudpuppy retina

The effects of cholinergic agonists and antagonists on electrical coupling between horizontal cells were studied in dark-adapted mudpuppy retinas. Carbachol and the nicotinic agonist 1,1-dimethyl-4-phenylpiperazinium (DMPP) uncoupled horizontal cells, but the muscarinic agonist oxotremorine did not. The uncoupling effects of carbachol and DMPP were blocked by the nicotinic antagonist D-tubocurarine and by the dopamine antagonist fluphenazine, indicating that carbachol uncoupled horizontal cells by stimulating dopamine release via nicotinic receptors. Carbachol also caused an increase in release of [3H]dopamine from retinas. D-Tubocurarine increased horizontal cell coupling, indicating that tonic cholinergic input was present in dark-adapted retinas. D-Tubocurarine did not reduce light-evoked uncoupling of horizontal cells, suggesting that cholinergic neurons are not an essential part of the direct pathway by which light causes an immediate increase in dopamine release.

Similar effects of carbachol and dopamine on neurons in the distal retina of the tiger salamander

Though there is considerable evidence that dopamine is an important retinal neuromodulator that mediates many of the changes in the properties of retinal neurons that are normally seen during light adaptation, the mechanism by which dopamine release is controlled remains poorly understood. In this paper, we present evidence which indicates that dopamine release in the retina of the tiger salamander, Ambystoma tigrinum, is driven excitatorily by a cholinergic input. We compared the effects of applying carbachol to those of dopamine application on the responses of rods, horizontal cells, and bipolar cells recorded intracellularly from the isolated, perfused retina of the tiger salamander. Micromolar concentrations of dopamine reduced the amplitudes of rod responses throughout the rods' operating range. The ratio of amplitudes of the cone-driven to rod-driven components of the responses of both horizontal and bipolar cells was increased by activation of both D1 and D2 dopamine receptors. Dopamine acted to uncouple horizontal cells and also off-center bipolar cells, the mechanism in the case of horizontal cells depending only upon activation of D1 receptors. Carbachol, a specific cholinomimetic, applied in five- to ten-fold higher concentrations, produced effects that were essentially identical to those of dopamine. These effects of carbachol were blocked by application of specific dopamine blockers, however, indicating that they are mediated secondarily by dopamine. We propose that the dopamine-releasing amacrine cells in the salamander are under the control of cells, probably amacrine cells, which secrete acetylcholine as their transmitter.

Modulation of transmission gain by protons at the photoreceptor output synapse

Synaptic transmission of the light response from photoreceptors to second-order cells of the retina was studied with the whole-cell patch-clamp technique in tiger salamander (Ambystoma tigrinum) retinal slices. Synaptic strength is modulated by extracellular pH in a striking manner: Light-sensitive postsynaptic currents in horizontal and bipolar cells were found to be exponential functions of pH, exhibiting an e-fold increase per 0.23 pH unit over the pH range from 7 to 8. Calcium channel currents in isolated photoreceptors were measured and also exhibited proton sensitivity. External alkalinization from pH 7 to 8 shifted the voltage dependence of channel activation negative by 12 mV. A model of the synaptic transfer function suggested that presynaptic Ca channels could be the primary sites of proton action. Increased Ca influx and transmitter release brought about by alkalinization give rise to larger postsynaptic currents. These results suggest that activity-dependent interstitial pH changes known to occur in the retina, while not alleviating signal clipping at this synapse, may provide an adaptative mechanism controlling gain at the photoreceptor output synapse.

Characterization of serine's inhibitory action on neurons in the mudpuppy retina

Experiments were performed in the superfused retina-eyecup of mudpuppies using intracellular electrophysiological techniques to evaluate the effects of serine on amacrine and ganglion cells. Serine was found to have a dose-dependent inhibitory effect mediated by the opening of chloride channels. Serine appears to act on a glycine receptor based on the observations that: (1) serine's effect is blocked by strychnine but not by bicuculline or picrotoxin, (2) in the presence of saturating glycine concentrations serine had no effect on membrane voltage or conductance, and (3) cells inhibited by serine were always sensitive to glycine, but not always sensitive to GABA. High pressure liquid chromatography measurements disclose that there is a high concentration of extracellular serine in the retina. The data indicate that serine could act as an inhibitory neurotransmitter.

Effects of calcium ions on L-type horizontal cells in the isolated turtle retina

A technique by which the retina can be isolated from the turtle eye is described. Scanning electron microscopy revealed morphological variability between preparations and also between regions of the same one. Large areas were often totally free of any pigment epithelial cells, yet contained a high proportion of photoreceptors with complete outer segments. However, adjacent regions may contain photoreceptors without outer segments or with fragmented ones. The physiological properties of the horizontal cells also demonstrated large variability between different preparations. In all cases, lowering calcium concentration from 2 mM to 0.1-0.5 mM depolarized the horizontal cells and augmented the amplitude of the maximum photoresponses. However, these effects were accompanied by changes in the photoresponse kinetics and by a reduction in the horizontal cell sensitivity to light. Moreover, prolonged exposure to low calcium induced permanent damage to the retina as was indicated by the reduction in the response amplitude after superfusion with 2 mM calcium solution had been resumed. The toxic effects of low calcium were most apparent when superfusion with 0.1-1.0 microM calcium concentration was performed. These solutions induced complex time-dependent effects on the resting potential of horizontal cells and on the amplitude and kinetics of the photoresponses. We conclude from these observations that the normal concentration of extracellular calcium in the turtle retina is in the 2 mM range.

Distribution of muscarinic acetylcholine receptors on processes of isolated retinal cells

Binding of propylbenzilylcholine mustard, a muscarinic acetylcholine receptor antagonist, to isolated retinal cells was examined with light microscopic autoradiography. Dissociation of the adult tiger salamander retina yielded identifiable rod, cone, horizontal, bipolar, amacrine/ganglion, and Müller cells. Preservation of fine structure was assessed with conventional electron microscopy. For all cell types, the plasmalemma was intact and free of adhering debris; in addition, presynaptic ribbon complexes were present in photoreceptor and bipolar axon terminals indicating that synaptic structures were retained. Specific binding to cell bodies and processes was analyzed separately by using morphometric and statistical techniques. The highest grain densities occurred on processes of amacrine/ganglion cells and axons and 2 degrees and 3 degrees dendrites of bipolar neurons. Bipolar cells, however, seemed to be a heterogeneous population because there was great variation in the density of binding sites on both their axons and distal dendrites. Intermediate levels of binding were found on bipolar 1 degree dendrites and horizontal cells. No specific binding was detected on Müller cells and most parts of photoreceptors. Comparisons between cells showed that grain densities were similar for bipolar axons and amacrine/ganglion cell processes but bipolar dendrites were richer in binding sites than horizontal cell dendrites. Thus, muscarinic receptors in the salamander retina are located on amacrine/ganglion, bipolar, and horizontal cells and primarily confined to the processes which compose the two synaptic layers. In the inner plexiform layer, muscarinic receptors reside on processes from all three inner retinal neurons: in the outer synaptic layer, receptors are only on second-order cells and are more numerous along bipolar than horizontal cell dendrites.

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.

Localization of putative GABAergic neurons in the larval tiger salamander retina by immunocytochemical and autoradiographic methods

Putative GABAergic neurons in the larval tiger salamander retina were localized by a comparative analysis of glutamate decarboxylase immunoreactivity (GAD-IR), GABA-like immunoreactivity (GABA-IR), and high-affinity 3H-GABA uptake at the light microscopical level. Preliminary data showed that all GAD-IR neurons were double labeled for GABA-IR. However, because the weak somatic labeling with GAD-IR, we could not determine if the converse were true. Neurons commonly labeled with GABA-IR and 3H-GABA uptake include horizontal cells, type I (outer) and type II (inner) bipolar cells, type I (inner) and type II (outer) amacrine cells, and cell bodies in the ganglion cell layer (GCL). In addition, interplexiform cells were identified with GABA-IR. The presence of GABA-IR ganglion cells was indicated by GABA-IR fibers in the optic fiber layer and optic nerve as well as by a GABA-IR cell in the GCL that included a labeled axon. The percentage of labeled somas in the inner nuclear layer (INL) compared to all cells in each layer was similar for the two methods: 30% in INL 1 (outer layer of somas), 15% in INL 2 (middle layer), 43-52% in INL 3 (inner layer), and about 21-26% in the GCL. Labeled processes were found in three bands in the inner plexiform layer, with the densest band located in the most proximal part. Postembedding labeling of 1-micron Durcupan resin sections for GABA-IR showed the same general pattern as obtained with 10-microns cryostat sections, with additional staining, however, of type II (inner) bipolar cell Landolt's clubs. Extensive colocalization of labeling was indicated, and we conclude that GABA-IR can serve as a valid and reliable marker for GABA-containing neurons in this retina and suggest that GABA serves as a transmitter for horizontal cells, several types of amacrine cell, a type of interplexiform cell, and perhaps a small percentage of type I and type II bipolar cells and ganglion cells.

Melatonin enhances horizontal cell sensitivity in salamander retina

Intracellular electrophysiological recording techniques were utilized to investigate the possible function of retinal melatonin in the larval tiger salamander. Endogenous retinal melatonin was present and appeared to bind a membrane-enriched fraction of the salamander retina, as determined by radioimmunoassay and receptor binding studies. Melatonin added through the perfusion bath to flat-mounted retinas resulted in a horizontal cell (HC) hyperpolarization of 10-20 mV. Additionally, the amplitude of HC responses to short test flashes increased in the presence of melatonin. Voltage-intensity plots revealed that application of 500 microM of melatonin caused an increase of the HC light sensitivity and this effect was reversible. These results suggest that melatonin synthesized and released during the dark period of the diurnal cycle may alter the sensitivity of second-order neurons at a time of day when photopic input is at its lowest level.

Subtype of excitatory amino acid receptor in cone horizontal cells of the carp retina as specified by reversal potential measurement technique

Effects of agonists of the excitatory amino acid (EAA) transmitters were examined in carp cone horizontal cells where glutamate (Glu) or aspartate (Asp) is believed to act as the transmitter released from the photoreceptors. Bath application of kainic (KA), quisqualic (QA) and N-methyl-D-aspartic (NMDA) acids produced little effect on cone cells, indicating that their effects act directly on the horizontal cells. KA and QA (100 microM for both) produced depolarizations in the horizontal cells. Their reversal potentials were measured by our novel technique which was developed to overcome a serious experimental disadvantage due to electrical coupling between horizontal cells. The retina was perfused with a modified Ringer solution which contained high-Ca2+,Ba2+, and some K+-channel blockers. A Ca2+ action potential having an overshoot was evoked in the horizontal cells when they were depolarized by application of the EAA. During the action potential, perfect potential uniformity was achieved throughout electrically coupled cells. Responses induced by KA and QA during the overshoot appeared in reversed polarities to those elicited at the resting state. Their reversal potentials were then estimated to be similar at around -6mV, and this value coincided with that of the Glu- or Asp-induced responses. On the other hand, effects of NMDA were diverse even though applied in the order of mM; some cells were hyperpolarized, but the others were little affected. These observations indicate that the EAA receptor of carp horizontal cells is KA/QA (non-NMDA) type.

Acetylcholine as a neurotransmitter in the vertebrate retina

The evidence for the existence of acetylcholine as a neurotransmitter in the vertebrate retina is reviewed. There is evidence for the existence of a cholinergic system in every retina studied to date; therefore, it appears that acetylcholine is both essential and ubiquitous at this level of the visual system. Particular attention is directed to descriptions of the possible functions of acetylcholine in the retina, and formation of testable models which will serve to elucidate some of the details of cholinergic neurotransmission in the retina.

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.

Interactions between enkephalin and gamma-aminobutyric acid in the larval tiger salamander retina

Both double-label and intracellular electrophysiological recording techniques were utilized to investigate the interactions between enkephalin and gamma-aminobutyric acid in the larval tiger salamander retina. Double-label studies revealed that the vast majority (greater than 96%) of enkephalin-immunostained amacrine cells also exhibit high affinity uptake of [3H]gamma-aminobutyric acid. Electrophysiological evidence demonstrated that morphine and gamma-aminobutyric acid exert opposite effects on a population of On-Off ganglion cells. gamma-Aminobutyric acid decreased the activity of these cells, while enkephalin increased their activity. These findings support the idea that opiate-mediated pathways inhibit GABAergic pathways in the vertebrate retina.

Excitatory amino acids have different effects on horizontal cells in eyecup and isolated retina

Horizontal cells in the mudpuppy eyecup responded to continuous superfusion with L-glutamate, L-aspartate, kainate and quisqualate with a transient depolarization and reduction of the light evoked responses. However, in isolated retina preparations, in which these substances were applied to the photoreceptor side of the retina, the effects were sustained as long as the agonists were present. These results suggest that the transient action of these agonists in eyecup preparations was due to the rapid development of an intraretinal diffusion barrier, and are consistent with the hypothesis that photoreceptors release an excitatory amino acid transmitter.

Effects of gamma-aminobutyric acid on cones and bipolar cells of the tiger salamander retina

The gamma-aminobutyric acid (GABA) system in the tiger salamander retina was studied using autoradiographic and electrophysiological techniques. A high-affinity uptake mechanism for GABA has been localized in about 60% of the horizontal cells and about 30% of the amacrine cells. Effects of exogeneously applied GABA on the membrane conductance of cones and hyperpolarizing bipolar cells (HBC) were examined using the two electrode current-clamp technique in the living retinal slices. In both cell types, 1 mM of GABA caused a conductance increase. In perfused eyecups, 2 mM of GABA selectivity abolished the surround response of the HBC and left the center response unchanged. These results are consistent with the notion that a population of horizontal cells and a population of amacrine cells in the salamander retina may use GABA as their neurotransmitter.

Horizontal cell signal is smaller with texture-like nonuniform patterns than with uniform fields of the same space-average illuminance

While measurement of the pertinent response as a function of flash intensity has been a standard procedure in vision research, we ask here what happens to horizontal cells in the vertebrate retina if the flash energy is varied through the density of a large number of small light spots rather than through the intensity of uniform illumination. The experiment described here demonstrates that the electrical response of horizontal cells in the turtle retina is consistently smaller with the present dot-density modulation than with the usual intensity modulation, even though the comparison is made when the mean irradiance per photoreceptor is equal in the two methods of modulation. Thus, the spatial summation, an important retinal function often thought to provide a base signal level for contrast detection, is affected significantly by how the visual pattern is structured at a level of microscopic dimension far smaller than the receptive field. The present finding, which seems to be a new form of area-intensity effect, can be explained if the dendritic membrane conductance of horizontal cells at each synaptic site increases at a progressively higher rate with decreases of the corresponding local illuminance. This possibility is discussed in the light of relevant photoreceptor response data, the presumed sigmoidal trend of the postsynaptic chemosensitivity and a simple electrical analog of contiguous horizontal cells.

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.

Muscarinic cholinergic receptors in the retina of the larval tiger salamander

The pharmacology and autoradiographic localization of muscarinic cholinergic receptors in retinal slices of the larval tiger salamander have been examined using the muscarinic antagonist [3H]propylbenzilylcholine mustard. Under the conditions of these experiments the binding of this ligand is irreversible. Saturation and maximum specific binding of 270 pM of ligand per gram protein are observed after an incubation of 1 h, and autoradiographic studies show that this does not reflect surface binding alone. Muscarinic but not nicotinic drugs suppress the binding of propylbenzilylcholine mustard at physiologically relevant concentrations; half-maximal suppression of binding by the muscarinic antagonists atropine sulfate and quinuclidinyl benzilate occurs, respectively, at 9.0 and 7.5 X 10(-10) M. Light microscopic autoradiography reveals the discrete localization of the ligand to the sites of synaptic contact, the retinal plexiform layers, predominantly the inner plexiform layer. The implications of the present study on current theories of cholinergic function in the vertebrate retina are discussed.

Responses of solitary retinal horizontal cells to L-glutamate and kainic acid are antagonized by D-aspartate

Solitary horizontal cells dissociated from goldfish retinas depolarized when exposed to micromolar doses of either L-glutamate or kainic acid. The responses to both of these agonists were antagonized by D-aspartate, and unaffected by L-aspartate, L-glutamic acid diethyl ester and folic acid. the results of the present study thus suggest that L-glutamate and kainic acid may produce depolarizations of horizontal cells by interacting with pharmacologically similar membrane receptors.

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.

Localization of cholinesterase activity in the outer plexiform layer of the larval tiger salamander retina

Acetylcholinesterase activity, which may be indicative of cholinergic synapses, has been ultrastructurally localized in the outer plexiform layer of the larval tiger salamander retina using the cholinesterase staining method of Karnovsky and Roots. In the presence of the butyrylcholinesterase inhibitor, ethopropazine, precipitate is localized to discrete 'stain laminae' about 300 nm in their largest dimension and bounded by a 10 nm wide electron-lucent gap. Stain laminae are predominantly found adjacent to horizontal cell axon terminals which were identified by serial section analysis. Stain precipitate has not been found associated with horizontal cell dendrites and in only one case was a lamina found adjacent to a bipolar cell dendrite. Intracellular deposits of cholinesterase stain precipitate were also observed. In some cases, stain is found adjacent to synaptic ribbons in photoreceptor terminals. Likely artifacts appear to have been ruled out by control experiments in which retinas either were not treated with cholinesterase stain or were treated in the presence of the specific acetylcholinesterase inhibitor, BW284C51. Thus, the stain laminae probably result from specific, highly localized areas of acetylcholinesterase activity. The implications of the present study on current theories of cholinergic function in the vertebrate retina are discussed.

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.

Light adaptation in luminosity horizontal cells in the turtle retina. Role of cellular coupling

Intracellular recordings were made from L-type horizontal cells in the retina of the turtle Clemmys caspica. The test flashes covered a retinal area smaller than the receptive field of the impaled cells. Increment responses were compared between two states of light adaptation that were equal in intensity but differed in spatial pattern. The peak amplitude of the response to a dim test flash was larger when measured during large-field background compared to the one obtained during small-field one. It is suggested that this phenomenon of surround enhancement is due to the cellular coupling between neighboring horizontal cells.

Sustained and transient synaptic inputs to on-off ganglion cells in the mudpuppy retina

Synaptic inputs to on-off ganglion cells in mudpuppy retina were studied by measuring current-voltage relations in darkness, during different phases of the response to light, and in the presence of 4 mM-Co2+. The addition of Co2+ to the bathing medium usually caused a hyperpolarization of the membrane potential in darkness and an increase in input resistance, indicating that on-off ganglion cells receive tonic excitatory synaptic input in darkness. Other results suggest that an additional synaptic input, with a reversal potential near the dark potential, may also be active in darkness. At the onset of a light stimulus in the receptive field centre all on-off ganglion cells responded with transient excitatory and inhibitory synaptic events, both of which were due to increases in conductance. Similar transient excitatory and inhibitory events occurred at the termination of the light stimulus. In about one-half of the on-off ganglion cells studied the synaptic activity during steady illumination was the same as in darkness. In the remaining cells steady illumination caused an increase in sustained inhibition.

A sign-reversing pathway from rods to double and single cones in the retina of the tiger salamander

Signal transmission between rods and cones was studied by passing current into a rod and recording the voltage response in a nearby double or single cone and vice versa. Two types of rod-cone interaction were found. Between immediately adjacent rods and cones, passage of current into either receptor elicited in the other receptor a sustained voltage response of the same sign as the injected current. These signals were still seen in the presence of Co2+, and are probably mediated by the electrical synapses which have been seen anatomically between adjacent rods and cones. In addition to this short-range sign-preserving interaction, passing current into a rod elicited a transient sign-inverted signal in cones up to at least 80 micron from the injected rod. No such response was seen in rods for current injection into cones. This signal was greatly reduced by Co2+ ions. Hyperpolarization of the cone to about -65 mV, with about 0.1 nA current, reversed this signal, which is presumed to be mediated by a chemical synaptic input to cones. Light flashes suppressed the sign-inverted signal for a period which was longer for brighter flashes. The time of reappearance of the signal was correlated with the return of the rod and horizontal cell potentials to their dark levels. This suppression could also be produced by an annulus of light which produced no light response in the receptors at the centre of the annulus, but which did polarize horizontal cells under the centre of the annulus. The wave form of the sign-inverted signal was similar to that produced in horizontal cells by current injection into rods, but of opposite sign. If an electrode was left in a cone for some time, the normal hyperpolarizing light response diminished, leaving a depolarizing response produced, presumably, by feed-back from horizontal cells. This signal was reversed when the cone was hyperpolarized with about 0.1 nA current. These data suggest that the sign-inverted response is mediated by feed-back from horizontal cells and, assuming that depolarization increases the rate of release of horizontal cell synaptic transmitter, then the feed-back transmitter opens channels in the cone membrane whose currents have a reversal potential around -65 mV.

Intracellular recording from identified photoreceptors and horizontal cells of the Xenopus retina

Intracellular recordings were made from rods, cones and horizontal cells of the Xenopus retina. The cells under study were identified by injection of the fluorescent dye, Lucifer yellow. Rod spectral sensitivity peaked near 524 nm, that of cones near 612 nm whereas horizontal cells reflected input from both these classes of photoreceptors. No intracellular recordings were made from blue-sensitive rods (lambda max = 445 nm) nor did this rod appear to provide an input to the horizontal cell. Under dark-adapted conditions, horizontal cells had a slow waveform, a Vmax less than or equal to 18 mV and were driven by 524 nm rods only. When light-adapted, horizontal cell responses were fast, Vmax was 30-40 mV and the responses reflected only 612 nm cone input. In the mesopic state rod and cone inputs to the horizontal cell interacted non-linearly: weak green backgrounds greatly enhanced the response to a superimposed red flash compared to the red flash response on a dark field. The length constant of the horizontal cell exceeded its dendritic arbor by 2-15 fold. All of the stained horizontal cells, however, possessed a long slender axon without a terminal but which emitted periodic short branches that appeared to contact receptors.

Rod and cone inputs to bipolar and horizontal cells of the Xenopus retina

This report summarizes some recent studies of the Xenopus retina in which intracellular recordings were made from photoreceptors, horizontal and bipolar cells. The studied cells were identified by injection of Lucifer yellow. Rod spectral sensitivity functions conformed to the density spectrum of a 524 nm pigment, those of cones to that of a 612 nm pigment. Horizontal cell responses reflected both these classes of photoreceptor input. Rod input evoked a slow waveform, with Vmax less than or equal to 18 mV, cone input a faster waveform with Vmax = 30-40 mV. In the mesopic state the horizontal response reflected both waveforms. Rod and cone inputs to the horizontal cells appeared not to act independently, in that a steady weak green background greatly enhanced the response to a superimposed red flash, but not the reverse. A third photoreceptor type (blue-sensitive rod, Y lambda max = 445 nm) provided input to a chromatic bipolar cell which was hyperpolarized by blue light and depolarized by red light. Such chromatic bipolars had broad areas of spatial integration and lacked center-surround organization.

Solitary horizontal cells in culture--II. A new tool for examining effects of photoreceptor neurotransmitter candidates

Membrane potentials of solitary horizontal cells dissociated from goldfish retinas were intracellularly measured while applying various acidic amino acids. L-glutamate and kainic acid depolarized solitary horizontal cells at micromolar doses. Neither L- nor D-aspartate produced any change in solitary horizontal cell resting potentials. Low-amplitude responses to L-glutamate showed no sign of desensitization. A steady, plateau-like, dose-dependent component of solitary horizontal cell responses to either L-glutamate or kainic acid, though obscured during its rising phase by action potentials, was always recorded during maintained agonist applications. Responses to either L-glutamate or kainic acid reversed in polarity at membrane potentials between 0 and -20 mV. Responses to L-glutamate collapsed reversibly when extracellular sodium ions were replaced by choline ions. Responses to either L-glutamate or kainic acid were antagonized by relatively high doses of D-aspartate. These results demonstrate that retinal horizontal cell chemosensitivity to acidic amino acids persists after dissociation, and in several respects resembles that found in several other preparations, including retinas in situ.

Sustained synaptic input to ganglion cells of mudpuppy retina

1. Intracellular responses were recorded from on-centre and off-centre ganglion cells in isolated eyecups of the mudpuppy, Necturus maculosus.2. Current-voltage relations were measured in darkness, during illumination of the receptive field centre, and after chemically mediated synaptic inputs were blocked by 4 mM-cobalt chloride.3. In on-centre cells the membrane potential in darkness was -56+/-6 mV (mean+/-S.D.). Addition of Co(2+) resulted in an average depolarization of 10 mV and an average decrease in conductance of 2.1 nS. These results suggest that in darkness on-centre cells are tonically inhibited by synaptic input which increases conductance and has a reversal potential more negative than the dark membrane potential. In off-centre cells the membrane potential in darkness was -46+/-5 mV. Addition of Co(2+) caused an average hyperpolarization of 6 mV and an average decrease in conductance of 1.5 nS. These results suggest that in darkness off-centre cells receive a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.4. In on-centre cells light causes a sustained depolarization. This response involves an increase in a tonic excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential.5. In off-centre cells, light causes a sustained hyperpolarization. This response involves an increase in a sustained inhibitory input which increases conductance and has a reversal potential more negative than the dark membrane potential.6. The depolarizing off-response of off-centre cells is associated with an increase in an excitatory input which increases conductance and has a reversal potential more positive than the dark membrane potential. This response may be due to a temporary increase in the excitatory input which is tonically active in darkness or may reflect an additional excitatory input.7. It is suggested that in both on- and off-centre ganglion cells the balance of sustained excitatory and inhibitory synaptic inputs determines the resting potential in darkness. Centre illumination alters the balance of these inputs, by increasing one and decreasing the other, to produce the characteristic sustained light responses.8. The possible presynaptic sources of the sustained excitatory and inhibitory inputs are discussed.

Selective potentiation of retinal horizontal cell responses to L-glutamate by D-aspartate

1. L-Glutamate and L-aspartate depolarize type H1 horizontal cells in the isolated retina of goldfish, but only at millimolar concentrations. 2. When applied in the presence of D-aspartate, L-glutamate depolarizes H1 cells at concentrations nearly 15-fold lower than when it is applied alone. The effects of L-aspartate were not potentiated by either D-aspartate or D-glutamate. 3. Since D-aspartate seems also to enhance the effect of the transmitter released by cone photoreceptors, these results are consistent with the possibility that L-glutamate is a neurotransmitter of cones.

Behaviour of the rod network in the tiger salamander retina mediated by membrane properties of individual rods

1. The spread of electrical signals between rods in the salamander retina was examined by passing current into one rod and recording the voltage responses in nearby rods. Rod network behaviour, measured in this way, was simulated from data on rod membrane properties gathered in voltage-clamp experiments on single isolated rods.2. The network voltage responses to square current pulses became smaller, more transient, and had a longer time-to-peak, for rods further away from the site of current injection. Depolarizing currents produced smaller responses than hyperpolarizing currents of the same magnitude.3. Neighbouring rods and cones were coupled less strongly than neighbouring rods.4. The response of the rod network to current injection was unaffected by 2 mm-aspartate(-), which eliminates transmission from receptors to horizontal cells.5. The input resistance of single isolated rods, measured at the resting potential, varied between 100 and 680 MOmega. The lower values were probably due to damage by the micro-electrodes. Electrical coupling was found to be very strong between the rod inner and outer segments.6. A strong ;instantaneous' outward rectification seen in isolated rods at potentials positive to -35 mV was reduced, but not abolished, by 15 mm-TEA.7. In normal solution, isolated rods exhibited a voltage- and time-dependent current, I(A), whose kinetics were approximated by a single first-order gating variable, and whose activation curve spanned the range between -40 and -80 mV. The time constant for the current varied with voltage and was 60-200 msec between -140 and -40 mV.8. A reversal potential for I(A) could not be found between -140 and -40 mV in normal solution, and the fully activated current, I(A), was approximately voltage-independent, with a magnitude of approximately 0.1 nA over this potential range.9. By several criteria, I(A) behaved as a single inward current activated by hyperpolarization. Pharmacological studies suggest, however, that it is the sum of at least two currents with very similar kinetics.10. Most isolated rods exhibited a very slow (tau approximately 3 sec) increase in net outward current on depolarizing beyond -35 mV. The magnitude of this current varied considerably between cells.11. Assuming that the rod network can be approximated by a square lattice of individual rods, resistively coupled together, the voltage-clamp data on isolated rods were used to predict the response of the network to current injection at one cell. The theoretical and observed network behaviour were in good agreement. The resistance coupling neighbouring rods was estimated to be approximately 300 MOmega. The current I(A) plays a major role in determining the behaviour of the rod network.12. The time-dependent current, I(A), is responsible for the peak-plateau wave form of the response to a bright flash. A current similar to I(A) could also account for the negative propagation velocity of the peak of the dim flash response, through the rod network of the turtle, observed by Detwiler, Hodgkin & McNaughton (1978).

Rod and cone signals in the horizontal cells of the tiger salamander retina

1. Intracellular recordings of horizontal cell responses to monochromatic lights of various wave-lengths and intensities were made in the retina of the larval tiger salamander in order to determine the contributions of rod and cone activities to horizontal cell responses. 2. Under conditions of extensive dark adaptation, and with dim light stimulation, the horizontal cell responses reflected mainly rod activity. In the light-adapted state or at high light intensities the cone contribution was dominant. 3. Bright adapting flashes selectively suppressed the rod component of horizontal cell responses. 4. Intracellular recordings from rods and cones showed that interactions between the two receptor types are very small and cannot account for the large rod--cone mixed input observed in horizontal cells. It is concluded that this input is mediated by direct connexions between receptors and horizontal cells.

Synaptic inputs to the ganglion cells in the tiger salamander retina

The postsynaptic potentials (PSPs) that form the ganglion cell light response were isolated by polarizing the cell membrane with extrinsic currents while stimulating at either the center or surround of the cell's receptive field. The time-course and receptive field properties of the PSPs were correlated with those of the bipolar and amacrine cells. The tiger salamander retina contains four main types of ganglion cell: "on" center, "off" center, "on-off", and a "hybrid" cell that responds transiently to center, but sustainedly, to surround illumination. The results lead to these inferences. The on-ganglion cell receives excitatory synpatic input from the on bipolars and that synapse is "silent" in the dark. The off-ganglion cell receives excitatory synaptic input from the off bipolars with this synapse tonically active in the dark. The on-off and hybrid ganglion cells receive a transient excitatory input with narrow receptive field, not simply correlated with the activity of any presynaptic cell. All cell types receive a broad field transient inhibitory input, which apparently originates in the transient amacrine cells. Thus, most, but not all, ganglion cell responses can be explained in terms of synaptic inputs from bipolar and amacrine cells, integrated at the ganglion cell membrane.

Electron microscopic localization of [125I]alpha-bungarotoxin binding sites in the outer plexiform layer of the goldfish retina

Light and electron microscope autoradiography were performed on goldfish (Carassius auratus) retinas incubated in [125I]labelled alpha-bungarotoxin. The toxin was bound preferentially to membrane receptors in the inner and outer plexiform layers. Binding was suppressed by 10(-5) M nicotine or 10(-5) M native alpha-bungarotoxin. Electron microscopic analysis of the outer plexiform layer (OPL) strongly suggested that alpha-bungarotoxin binding sites were located on small bipolar cell dendritic processes that invaginated rod and cone synaptic terminals, and on large bipolar cell dendritic processes more proximally situated in the OPL. Large horizontal cell processes in the OPL and horizontal cell processes that invaginated rod and cone synaptic terminals did not appear to be labelled.

Effects of catecholamines and related compounds on horizontal cells in the fish retina

The effects of catecholamines (CA) and certain related compounds in the superfusate were examined on the intracellularly recorded potential from horizontal cells in the fish (Eugerres plumieri) retina. The stimulated retinal area consisted of a central spot 1.0 mm in diameter and an annulus 2.0 mm in inner diameter and 4.0 mm in outer diameter; both forms of monochromatic stimuli were centered relative to the recording microelectrode. Each of the CA (dopamine, noradrenaline, and adrenaline) produced an analogous effect on the hyperpolarizing response of all types of horizontal cells. The depolarizing response of the C (R/G)-type cells was found to change variably with the CA. The effect of dopamine (DA) among the CA was most pronounced when they were used at an equivalent amount. With 10--50 microM, the action of DA was variable but in general its effect was to increase slightly both center and surround responses. In some cases, however, DA initially augmented the surround and reduced the center response. Large amounts of the CA (100--200 microM) augmented the center response and attenuated the surround response considerably; these reciprocal changes usually were associated with moderate depolarization of the cells (5--10 mV). Recovery then occurred in 15--20 minutes. These results indicate that the CA, up to a certain amount, do not directly affect the transmission from photoreceptors to horizontal cells, since the center response became larger. At the same time, the lateral propagation of an S-potential appears to be selectively affected by the CA, suggesting that the adrenergic system participates in this phenomenon. When an excess of these compounds (200-500 microM) was given, the cells were rapidly depolarized to near 0 MV and eventually the light-induced responses were abolished. Large amounts (5--10 mM) of metabolic products of the CA (DOPAC and VMA) were found to reduce the center response slightly more than the surround. alpha-Methylnoradrenaline, 5-hydroxydopamine, and serotonin also caused the same but less effect on horizontal cells as did DA. Reserpine and clonidine mimicked the CA effect only if these compounds had been preceded by repeated applications of one of the CA or if the retina had been pretreated with Marplan. Propranolol, haloperidol, and apomorphine affected neither the horizontal cell membrane potential nor the CA effect. However, phentolamine in large amounts (500 microM) markedly diminished the DA action. Therefore, alpha-adrenergic receptors appear to be involved in the CA-induced changes observed in the horizontal cell response.

Similarities in effects of acetylcholine and dopamine on horizontal cells in the fish retina

The effect of acetylcholine (ACh) on cone- and rod-connected horizontal cells (generating photopic and scotopic L-type S-potentials, respectively) in the fish (Eugerres plumieri) retina was compared with that of dopamine (DA), and some similarities were found in the effects. About two-thirds of the L-type horizontal cells examined were sensitive to a test solution containing ACh (10 mM in concentration) and BW-anticholinesterase (BW; 1.0 mM). ACh with BW augmented a center response and attenuated a surround response as did DA (0.2 mM), although the latter effect was more pronounced and longer-lasting. ACh with BW frequently produced oscillations of the horizontal cell membrane potential, as did DA with clonidine (alpha-adrenergic stimulant). Phentolamine (alpha-adrenergic blocker) interrupted both the effects of DA and ACh on the center and surround responses, while hexamethonium (cholinergic antagonist) appeared to interfere selectively with the effect of ACh. Therefore, the ACh-induced changes observed in the horizontal cell membrane potential are assumed to be mediated by the adrenergic system in the fish retina.

Effects of some amino acids on horizontal cells in the fish retina

Effects of some amino acids on horizontal cells were examined in the fish (Eugerres plumieri) retina. L-aspartate (5 mM in concentration) and L-glutamate (10 mM) consistently depolarized all types of horizontal cells, although the effect of L-aspartate was more pronounced. gamma-Aminobutyric acid (GABA; 10 mM) consistently hyperpolarized the cone-connected L- and C-type horizontal cells, whereas the effect on the rod-connected cells was slight or nonexistent. The hyperpolarizing effect of GABA on the cone-horizontal cells was markedly diminished by picrotoxin (0.1 mM). Usually, glycine (10 mM) showed slight and variable effects. It merely depolarized some cone-horizontal cells but weakly hyperpolarized some rod-horizontal cells. In a few cases, however, glycine began to drastically hyperpolarize the cone-horizontal cells in retinas that had been repeatedly treated with certain agents used. Taurine and beta-alanine (10 mM each) had negligible effects, although the former slightly depolarized and the latter merely hyperpolarized both cone- and rod-horizontal cells.

Transmission along and between rods in the tiger salamander retina

1. The electrical pathways that couple the rods and that link the outer segments of the rods to the coupled network, were evaluated. Two separate micro-electrodes were inserted into the inner or outer segments of the same or neighbouring rods under visual control. Current was passed through one electrode, and the resulting potential recorded with the other. 2. The input resistance, measured at the inner or outer segment in a rod in the network, is strongly outward rectifying. It is typically near 40 Momega when the membrane is hyperpolarized 10 mV or more by extrinsic current, less than 10 Momega when the membrane is depolarized by 5 mV or more, and near 30 Momega at the no-current level. 3. When current is injected into the outer segment, the response in the inner segment is nearly identical with that at the outer segment, suggesting that the resistance coupling the segments is not high relative to the input resistance of the rod in the network. 4. Under voltage clamp the light response current for a rod in the network is of constant magnitude for potential levels between -80 and -20 mV. This suggests that there is little or no measurable light elicited conductance change associated with the response, possibly a consequence of coupling between rods. 5. The rod response increases with increasing diameter of a concentric test flash up to about 200 micron, or about 16 rod diameters. 6. When current is injected into one rod, the response in its immediate neighbours is between a quarter and one tenth that recorded in the injected rod for all potential levels in the injected rod. 7. The membrane time constant, measured in a rod in the network, is proportional to the voltage-dependent input resistance at 0.16 msec/Momega. With assumptions about the geometry of the rod network this represents a membrane capacitance of 1.5 muF/cm2. 8. The data can be approximated by a network model of square array. The model predicts that: the outer segment contributes less than half the current for the total rod response, the membrane resistance of an individual rod is greater than twice the measured input resistance for the rod in the network, near 60 Momega, and the coupling resistance for each arm of the network is about 4 times the individual rod resistance, near 240 Momega.