Pharmacological properties of isolated fish horizontal cells

At least three distinct receptors for neurotransmitter substances are present on carp horizontal cells. Activation of two of the receptor types, by dopamine and vasoactive intestinal peptide respectively, results in the accumulation of cyclic AMP within the cells. Activation of the third receptor type, by L-glutamate or its analogues, causes a large, 60-80 mV depolarization of the cells. Similar glutamate receptors are found on skate horizontal cells, which also possess receptors specific for gamma-aminobutyric acid (GABA). Activation of the GABA receptors on skate horizontal cells also results in a large, 60-80 mV cell depolarization.

Effects of intraocular injections of 6-hydroxydopamine on dopamine-dependent cyclic AMP accumulation in intact pieces of carp retina

Dopamine-stimulated cyclic AMP accumulation was measured in intact pieces of carp retina following intraocular injections of the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA). This treatment is known to induce a selective destruction of dopaminergic nerve terminals, and in these experiments caused a 94% reduction in retinal dopamine content. However, dopamine-dependent cyclic AMP accumulation was essentially unaltered in retinas exposed to 6-OHDA, indicating that the dopamine receptors linked to adenylate cyclase in the carp retina are located mainly on postsynaptic elements, and not presynaptically on the dopaminergic terminals.

Calcium alters the sensitivity of intact horizontal cells to dopamine antagonists

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

Inner plexiform circuits in the carp retina: effects of cholinergic agonists, GABA, and substance P on the ganglion cells

Th effects on ganglion cell light responses and spontaneous activity of neurotransmitter candidates, applied by nebulizer spray and iontophoresis, were studied in the isolated carp retina. ACh, GABA, and substance P had strong effects on the ganglion cells; dopamine and the amino acids aspartate, glutamate, and glycine and only weak effects. ACh and substance P exerted their actions even when synaptic transmission was blocked by cobalt chloride, suggesting postsynaptic receptors for those agents on the ganglion cell membrane. The 3 amino acids and dopamine do not appear to act directly on the ganglion cells. The pharmacological sensitivity of ganglion cells was correlated with their physiological response type. About three-quarters of ON/OFF and half of other transiently responding ganglion cells were excited by micromolar concentrations of cholinergic agonists; most ON-center sustained ganglion cells were insensitive. The light response of some of the ACh-sensitive cells could be suppressed by cholinergic antagonists. Substance P generally excited ganglion cells with an ON-component in their light response. GABA inhibited cells of all response types, but affected least the OFF-center tonic cells. In view of these observations, and of corroborating histological evidence, we propose that ACh, GABA, and substance P are neurotransmitters that are released by amacrine cells and affect receptors located on ganglion cells.

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

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

Isolated horizontal cells from carp retina demonstrate dopamine-dependent accumulation of cyclic AMP

Horizontal cells of the carp retina were separated from other retinal cell types by using enzymatic dissociation and velocity sedimentation at unit gravity. Fractions containing horizontal cells were tested for their ability to accumulate cyclic AMP in the presence of various putative neurotransmitters. Micromolar concentrations of dopamine, when added in the presence of 3-isobutyl-1-methylxanthine, stimulated cyclic AMP accumulation in these isolated cells. The dopamine-dependent accumulation of cyclic AMP in intact isolated horizontal cells was blocked by nanomolar concentrations of dopamine antagonists such as haloperidol, (+)-butaclamol, and fluphenazine. The results indicate that there is a postsynaptic dopamine receptor on carp horizontal cells that is associated with adenylate cyclase [ATP pyrophosphate-lyase (cyclizing), EC 4.6.1.1].

Dopaminergic mechanisms in the teleost retina. I. Dopamine-sensitive adenylate cyclase in homogenates of carp retina; effects of agonists, antagonists, and ergots

A specific dopamine-sensitive adenylate cyclase has been identified in homogenates of the teleost (carp) retina. Maximal stimulation by 100 microM-dopamine resulted in a 5--10-fold increase in adenylate cyclase activity with half-maximal stimulation occurring at a concentration of 1 microM. l-Noradrenaline and l-adrenaline were some 10 times less potent than dopamine whilst the alpha- and beta-adrenoreceptor agonists, l-phenylephrine and dl-isoprenaline were inactive. Apomorphine elicited a partial stimulation of adenylate cyclase activity whilst various ergot alkaloids produced mixed agonist/antagonist responses. Dopamine-stimulated adenylate cyclase activity was potently antagonised by various neuroleptic drugs including fluphenazine, alpha-flupenthixol and alpha-piflutixol, and to a lesser extent by the butyrophenone derivatives haloperidol and spiperone. The benzamide derivatives, metoclopramide and sulpiride, together with the alpha- and beta-adrenoreceptor blocking agents, phentolamine and propranolol respectively were essentially inactive at blocking dopamine-stimulated adenylate cyclase activity. These data suggest the presence of a highly specific dopamine-sensitive adenylate cyclase in homogenates of teleost retina possessing similar pharmacological properties to the dopamine-sensitive adenylate cyclase observed in the mammalian central nervous system.

Dopaminergic mechanisms in the teleost retina. II. Factors affecting the accumulation of cyclic AMP in pieces of intact carp retina

The ability of dopamine, dopamine agonists, other proposed retinal neurotransmitters, depolarizing agents and light to stimulate adenylate cyclase activity in pieces of intact carp retina has been examined. The evidence indicates that a dopamine-sensitive adenylate cyclase is the only neurotransmitter activated adenylate cyclase in the carp retina. That is, only dopamine, or agents that activate dopamine receptors, appear to stimulate cyclic AMP synthesis in the retina. Depolarizing agents such as K+ or veratridine also increase retinal cyclic AMP levels, but apparently by releasing endogenous stores of dopamine. For example, the increase of retinal cyclic AMP levels induced by 45 mM-K+ is blocked by 5 mM-Co2+ or 100 microM-haloperidol, a dopamine antagonist. Flashing lights slightly increase cyclic AMP levels in the retina, an effect that is likewise abolished by haloperidol.

Effects of GABA and glycine on the distal cells of the cyprinid retina

Two putative retinal neurotransmitters, gamma-aminobutyric acid (GABA) and glycine, were applied to the isolated carp and goldfish retinas while intracellular recordings from horizontal cells and cones were made. At relatively low concentrations (0.1-1 mM), GABA consistently hyperpolarized dark-adapted H1 or L-type cone horizontal cells and cone photoreceptors, reduced light evoked responses and suppressed the on-transients. The effects of GABA on H1 horizontal cells were abolished when Co2+ was applied to the retina, indicating that GABA exerts its effects on the horizontal cells via the receptors. Thus GABA is likely to be involved in the feedback synapse between H1 horizontal cells and the cones. Low concentrations of glycine (0.1-1 mM), hyperpolarized a number of H2 or C-type horizontal cells and selectively abolished their depolarizing responses to red light; thus glycine may be involved in synaptic pathways which mediate or modulate the depolarizing responses to red flashes in C-type horizontal cells.

Fluorescence and electron microscopical observations on the amine-accumulating neurons of the cebus monkey retina

The organization of the Cebus monkey regina was analysed after the intraocular injection of 5,6-dihydroxytryptamine. This amine was taken up not only by the previously known dopaminergic neurons, but also by a set of indoleamine-accumulating neurons, whose processes are confined to the inner plexiform layer. The synaptic contacts of the dopaminergic neurons were analysed in the electron microscope after the processes of the indoleamine-accumulating neurons were destroyed by the intravitreal injection of the neurotoxic indoleamine, 5,7-dihydroxytryptamine. The subsequent injection of 5,6-dihydroxytryptamine induces certain changes in the dopaminergic neurons which accumulate the substance: electron-dense cores appear in the synaptic vesicles, and increased electron-density of mitochodrial and cellular membranes is often observed. The dopaminergic neurons were found to be presynaptic to amacrine cell perikarya and processes in the inner plexiform layer. In the outer plexiform layer they were presynaptic to both bipolar and horizontal cells, but they did not contact photoreceptors. The dopaminergic neurons received synapses only in the inner plexiform layer, from amacrine cell processes. It is inferred that in Cebus most dopaminergic neurons belong to a special class of retinal neuron, the interplexiform cells, which appear to transmit information centrifugally within the retina, from the inner to the outer plexiform layers. There are considerable similarities between the synaptology of the dopaminergic interplexiform neurons in the Cebus monkey and the goldfish retina, and the function of interplexiform neurons may therefore be similar in these two species.

Monoaminergic neurons of the mudpuppy retina

The mudpuppy retina was investigated with the histofluorescence method of Falck and Hillarp in normal animals and in animals injected intraocularly with alpha-methylnoradrenaline, 5,6-dihydroxytryptamine, or a combination of the two drugs. Catecholaminergic amacrine cells were found to form a thin layer of terminals at the border between the inner nuclear and the inner plexiform layers. Catecholaminergic interplexiform cells were not found. Indoleamine-accumulating amacrine cells were also observed. They are fifteen to twenty times more numerous than the catecholaminergic cells, and their terminals occur diffusely throughout the inner plexiform layer. In a number of eyes the majority of the indoleamine-accumulating terminals were eliminated with intraocular injections of the neurotoxin, 5,7-dihydroxytryptamine, but the reproducibility of this effect was not consistent. Intravitreal injections of 5,6 dihydroxytryptamine were used to label both types of neurons for electron microscopy. They were found to make conventional type synapses on amacrine cells and, less frequently, on bipolar cells.

Rotational matched spatial filter for biological pattern recognition

Randomly oriented microbiological patterns are recognized by rotating a matched spatial filter with an optical wedge prism. The filter is made from a composite input pattern with various sized objects to cover wider ranges of size variation in a species to be identified.

Photoreceptor coupling in retina of the toad, Bufo marinus. I. Anatomy

1. Red rod photoreceptors in the toad retina, which are known to be physiologically coupled, were examined for interreceptor contacts. 2. A dense network of large gap junctions was found between the inner segments of red rods, this being the only specialized site of contact that was observed between rods. Each red rod contacts an average of about four neighboring red rods with a junctional area of approximately 0.75 micrometer2. From freeze-fracture micrographs, the density of junctional particles was found to be 5 X 10(3)/micrometer2. 3. The large gap junctions were found only to connect red rods to each other in agreement with physiological data. Only small focal gap junctions were seen between red rods and cones and no junctions were observed between red and green rods. 4. It is concluded that these gap junctions are the site of coupling between the red rods and that the coupling is electrical.

The oscillatory potentials of the mudpuppy retina

The properties of the oscillatory potentials (OPs) of the mudpuppy electroretinogram (ERG) were studied and compared with the properties of the b-wave of the ERG and the proximal negative response (PNR). Both transretinal and intraretinal ERGs were recorded in response to full-field as well as 200 and 500 microgram spot illumination. The OPs differed in behavior from the b-wave in terms of voltage-response relations and the effects of repetitive stimuli. Thus the OPs appear to have a different origin from that of the b-wave. The laminar profile of the OPs was also compared with both the PNR and the b-wave. The OPs reverse in polarity as a function of retinal depth and therefore appear to reflect radial flows of currents within the retina. Thus the origin of the OPs seems different also from that of the PNR, which appears to represent tangential current flows around the amacrine cells. The earlier OPs arise more proximally within the retina than the later ones, suggesting that the individual oscillatory peaks are likely to have different origins. We propose that the OPs may represent feedback loops within retina. In support of this notion, it was found that the OPs were selectively depressed by GABA, glycine, glutamate, and dopamine. Acetylcholine and carbacholine did not affect the oscillatory responses, suggesting perhaps that the OPs are generated by inhibitory feedback synaptic circuits.

Generation of b-wave currents in the skate retina

Potassium kinetics within the skate retina were monitored extracellularly with K+-selective electrodes. Two sources of K+ efflux were detected in response to photic stimulation: one in the distal retina in the region of the outer plexiform layer, and the other at a more proximal location near the border between the inner nuclear and inner plexiform layers. The magnitude of the K+ efflux at these retinal depths was affected differently by spot and full-field illumination, suggesting that the two sources originate from different classes of neuron. There is evidence that both sources are associated with current sinks provided by the Müller cells, thereby establishing radial current paths along the lengths of these elements. We have proposed a model in which asymmetries in the magnitudes of these currents give rise to the b-wave of the electroretinogram. Extracellular field potentials recorded differentially at various retinal depths, and in response to changes in stimulus configuration, were consistent with predictions of the model.

L-aspartate: evidence for a role in cone photoreceptor synaptic transmission in the carp retina

A number of putative neurotransmitter substances and their antagonists were applied to the carp retina while intracellular recordings from L-type cone horizontal cells were made. Of all the substances tested, L-aspartate was found to be the most potent agent in depolarizing these horizontal cells in dark-adapted, partially light-adapted, and Co2+-treated retinas. Furthermore, DL-alpha-aminoadipate, an L-aspartate antagonist, blocked the effects of both the endogenous photoreceptor transmitter and exogenously applied L-aspartate on the horizontal cells. The results suggest that L-aspartate and the natural transmitter interact with the same population of postsynaptic receptors in the horizontal cell membrane.

The interplexiform cell system. I. Synapses of the dopaminergic neurons of the goldfish retina

Interplexiform cells are a class of retinal neuron that extends processes widely in both plexiform layers. In goldfish they contain dopamine and readily take up certain biogenic amines. Two of these amines, 6-hydroxyopamine (6-HDA) and 5, 6-dihydroxytryptamine (5,6-DHT), induce fine structural changes in the neurons that accumulate them, allowing the processes of the cells to be recognized by electron microscopy. Typically, the synaptic vesicles within the processes show electron-dense cores. The terminal cytoplasm may also show increased density, as may the cellular and cytoplasmic membranes, presumably an indication of degenerative changes induced by the drugs. 5, 6-DHT gives more readily observable changes than 6-HDA but labels both dopaminergic and indoleamine-accumulating neurons. The terminals of the indoleamine-accumulating terminals were therefore removed by intraocular injections of 5, 7-dihydroxytryptamine (5, 7-DHT) prior to the labelling with 5, 6-DHT. This procedure allowed an analysis of the dopaminergic terminals without interference by the terminals of the indoleamine-accumulating cells. The dopaminergic neurons were found to make synapses of the conventional type. In the outer plexiform layer they contacted both external horizontal cells and bipolar cell dendrites, but not hotoreceptor terminals or intermediate (rod) horizontal cells. No synapses onto the dopaminergic processes were found in the outer plexiform layer despite an extensive search. In the inner plexiform layer the dopaminergic processes were observed to be both pre- and postsynaptic to amacrine cells and their processes. No synaptic contacts between dopaminergic processes and bipolar cell terminals or ganglion cell dendrites were seen. We conclude that the dopaminergic interplexiform cells provide a centri­fugal pathway for information flow in the retina from inner to outer plexiform layer.

The interplexiform cell system. II. Effects of dopamine on goldfish retinal neurones

The effects of atomized solutions of dopamine and certain related com­pounds have been tested on the intracellularly recorded activity of receptor, horizontal, bipolar and amacrine cells in the goldfish retina. Dopamine depolarizes the cone L-type horizontal cells and reduces the amplitude of light-evoked responses. These effects on L-type horizontal cells are completely abolished by the α-adrenergie blocker, phentolamine, but only partially depressed by the β-blocker, propanolol. L-Dopa, noradrenalin, and serotonin do not have effects on L-type horizontal cells when applied at concentrations similar to those that cause maximal dopamine effects. The results suggest that the effects of dopamine on L-type horizontal cells are specific, and we propose that they mimic the effects of interplexiform cell activity. Dopamine has no effects on rod horizontal cells in goldfish and variable effects on C-type horizontal cells. On bipolar cells, dopamine alters the dark membrane potential, enhances the central response to light, and depresses the surround response. Dopamine also decreases the horizontal cell feedback evident in cone responses. Finally, dopamine strongly depolarizes the transient type of amacrine cells, but it has no significant effect on the sustained type of amacrine cells. Assuming that dopamine is the transmitter of interplexiform cells, we suggest that these neurons regulate lateral inhibitory effects mediated by L-type horizontal cells in the outer plexiform layer and transient amacrine cells in the inner plexiform layer. In addition, it appears as if interplexiform cells have specific effects on bipolar cells and are capable of regulating centre-surround antagonism in these cells. The net effect of interplexiform cell activity is to isolate the bipolars from the influence of the surround.

Visual pigment and photoreceptor sensitivity in the isolated skate retina

Photoreceptor potentials were recorded extracellularly from the aspartate-treated, isolated retina of the skate (Raja oscellata and R. erinacea), and the effects of externally applied retinal were studied both electrophysiologically and spectrophotometrically. In the absence of applied retinal, strong light adaptation leads to an irreversible depletion of rhodopsin and a sustained elevation of receptor threshold. For example, after the bleaching of 60% of the rhodopsin initially present in dark-adapted receptors, the threshold of the receptor response stabilizes at a level about 3 log units above the dark-adapted value. The application of 11-cis retinal to strongly light-adapted photoreceptors induces both a rapid, substantial lowering of receptor threshold and a shift of the entire intensity-response curve toward greater sensitivity. Exogenous 11-cis retinal also promotes the formation of rhodopsin in bleached photoreceptors with a time-course similar to that of the sensitization measured electrophysiologically. All-trans and 13-cis retinal, when applied to strongly light-adapted receptors, fail to promote either an increase in receptor sensitivity or the formation of significant amounts of light-sensitive pigment within the receptors. However, 9-cis retinal isin. These findings provide strong evidence that the regeneration of visual pigment in the photoreceptors directly regulates the process of photochemical dark adaptation.

Electrical and adaptive properties of rod photoreceptors in Bufo marinus. I. Effects of altered extracellular Ca2+ levels

The effects of altering extracellular Ca(2+) levels on the electrical and adaptive properties of toad rods have been examined. The retina was continually superfused in control (1.6 mM Ca(2+)) or test ringer's solutions, and rod electrical activity was recorded intracellularly. Low-calcium ringer's (10(-9)M Ca(2+)) superfused for up to 6 min caused a substantial depolarization of the resting membrane potential, an increase in light-evoked response amplitudes, and a change in the waveform of the light-evoked responses. High Ca(2+) ringer's (3.2 mM) hyperpolarized the cell membrane and decreased response amplitudes. However, under conditions of either low or high Ca(2+) superfusion for up to 6 min, in both dark-adapted and partially light-adapted states, receptor sensitivity was virtually unaffected; i.e., the V-log I curve for the receptor potential was always located on the intensity scale at a position predicted by the prevailing light level, not by Ca(2+) concentration. Thus, we speculate that cytosol Ca(2+) concentration is capable of regulating membrane potential levels and light-evoked response amplitudes, but not the major component of rod sensitivity. Low Ca(2+) ringer's also shortened the period of receptor response saturation after a bright but nonbleaching light flash, hence accelerating the onset of both membrane potential and sensitivity recovery during dark adaptation. Exposure of the retina to low Ca(2+) (10(-9)M) ringer's for long periods (7-15 min) caused dark-adapted rods to lose responsiveness. Response amplitudes gradually decreased, and the rods became desensitized. These severe conditions of low Ca(2+) caused changes in the dark-adapted rod that mimic those observed in rods during light adaptation. We suggest that loss of receptor sensitivity during prolonged exposure to low Ca(2+) ringer's results from a decrease of intracellular (intradisk) stores of Ca(2+); i.e., less Ca(2+) is thereby released per quantum catch.

Electrical and adaptive properties of rod photoreceptors in Bufo marinus. II. Effects of cyclic nucleotides and prostaglandins

Substances known to alter cyclic nucleotide levels in cells were applied to the isolated toad retina and effects on rod electrical and adaptive behavior were studied. The retina was continually superfused in control ringer's or ringer's containing one or a combination of drugs, and rod activity was recorded intracellularly. Superfusion with cGMP, Bu(2)GMP, isobutylmethylxanthine (IBMX; a phosphodiesterase inhibitor), or PGF(2alpha) (a prostaglandin) caused effects in rods that closely match those observed when extracellular Ca(2+) levels were lowered. For example, short exposures (up to 6 min) of the retina to these substances caused depolarization of the membrane potential, increase in response amplitudes, and some changes in waveform; but under dark-adapted or partially light-adapted conditions receptor sensitivity was virtually unaffected. That is, the position of the V-log I curve on the intensity axis was determined by the prevailing light level, not by drug level. These drugs, like lowered extracellular Ca(2+), also decreased the period of receptor saturation after a bright-adapting flash, resulting in an acceleration of the onset of membrane and sensitivity recovery during dark adaptation. Long-term (6-15 min) exposure of a dark-adapted retina to 5 mM IBMX or a combination of IBMX and cGMP caused a loss of response amplitude and a desensitization of the rods that was similar to that observed in rods after a long-term low Ca(2+) (10(-9)M) treatment. Application of high (3.2 mM) Ca(2+) to the retina blocked the effects of applied Bu(2)cGMP. PGE(1) superfusion mimicked the effects of increasing extracellular Ca(2+). The results show that increased cGMP and lowered Ca(2+) produce similar alterations in the electrical activity of rods. These findings suggest that Ca(2+) and cGMP are interrelated messengers. We speculate that low Ca(2+) may lead to increased intracellular cGMP, and/or that applied cGMP, and/or that applied cGMP may lower cytosol Ca(2+), perhaps by stimulating Ca(2+)- ATPase pumps in the outer segment.

Development of outer segments and synapses in the rabbit retina

The peripheral retina of rabbits aged 0 to 60 days was studied by electron microscopy. Ribbon and conventional synaptogenesis was studied with serial sections, and the density of synapses of the inner plexiform layer was measured on large (1,500 micrometer 2) montages. Photoreceptor and bipolar ribbon synapses seem to develop similarly in that processes of the prospective dyad or triad contact the presynaptic ribbon-containing terminal one at a time. No statistically significant difference in the lengths of ribbon lamellae was found at 11 and 30 days. Conventional synapses appear to result from the aggregation of synaptic vesicles on one side of junctions that first existed as symmetrical membrane densities without vesicles. The length of the synaptic membrane specialization constant between 0 and 30 days. The density of inner plexiform layer conventional synapses remains at a low and roughly constant level from 0 to 9 days, after which there is an abrupt increase to a plateau at about 20 days. After nine days the density of ribbon synapses also increases, with an initially steep time course similar to that of conventional synapses. All subcategories of synapse studied (amacrine-to-amacrine, amacrine-to-bipolar, serial, and reciprocal) participate in the general increase between 9 and 20 days. Functional circuits of the inner plexiform layer thus seem to be assembled primarily during the second and third weeks of life.

The proximal negative response and visual adaptation in the skate retina

The proximal negative response (PNR), a complex extracellular potential derived mainly from amacrine cell activity, was studied in the all-rod retina of the skate. Tetrodotoxin (10(-6) mg/ml) did not affect either the waveform or the latency of the response, indicating that the PNR reflects the graded, nonregenerative components of the amacrine cell potential. As regards its adaptive properties, the PNR exhibited both the extreme sensitivity to weak background light and the slow time course of light and dark adaptation that are characteristic of other responses from the proximal retina. Thus, the PNR, like the b-wave and ganglion cell discharge, appears to reflect adaptive processes located within the neural network of the inner retina.

Visual adaptation: effects of externally applied retinal on the light-adapted, isolated skate retina

Incubation with externally applied 11-cis retinal induces a marked increase of visual sensitivity within partially bleached skate photoreceptors. This activity of 11-cis retinal is duplicated by 9-cis retinal, but not by all-trans retinal. The sensitization of photoreceptors promoted by 11-cis and 9-cis retinal is accompanied by the formation of rhodopsin and isorhodopsin, respectively.

Interplexiform cells of the mammalian retina and their comparison with catecholamine-containing retinal cells

Retinal interplexiform cells have processes that branch within both the inner and outer plexiform layers. Their morphology is described from Golgi-preparations of cat, rhesus macaque and squirrel monkey retinae. Comparisons are made with similar cells, known to be catecholamine-containing, which have been observed histofluorometrically in the teleost fish and New World monkeys. It is concluded that there may be more than one pharmacological type of interplexiform cell. In addition an inner nuclear layer plexus of fibres is described for the first time from Golgi-material of the squirrel monkey’s retina. Electron microscopy reveals that this plexus synapses within the inner nuclear layer on to bipolar and amacrine cells. It is compared with the catecholamine-containing inner nuclear layer plexus of New World monkeys.

Intracellular recordings from gecko photoreceptors during light and dark adaptation

Intracellular recordings were obtained from rods in the Gekko gekko retina and the adaptation characteristics of their responses studied during light and dark adaptation. Steady background illumination induced graded and sustained hyperpolarizing potentials and compressed the incremental voltage range of the receptor. Steady backgrounds also shifted the receptor's voltage-intensity curve along the intensity axis, and bright backgrounds lowered the saturation potential of the receptor. Increment thresholds of single receptors followed Weber's law over a range of about 3.5 log units and then saturated. Most of the receptor sensitivity change in light derived from the shift of the voltage-intensity curve, only little from the voltage compression. Treatment of the eyecup with sodium aspartate at concentrations sufficient to eliminate the beta-wave of the electroretinogram (ERG) abolished initial transients in the receptor response, possibly indicating the removal of horizontal cell feedback. Aspartate treatment, however, did not significantly alter the adaptation characteristics of receptor responses, indicating that they derive from processes intrinsic to the receptors. Dark adaptation after a strongly adapting stimulus was similarly associated with temporary elevation of membrane potential, initial lowering of the saturation potential, and shift of the voltage-intensity curve. Under all conditions of adaptation studied, small amplitude responses were linear with light intensity. Further, there was no unique relation between sensitivity and membrane potential suggesting that receptor sensitivity is controlled at least in part by a step of visual transduction preceding the generation of membrane voltage change.

Synaptic organization of the amine-containing interplexiform cells of the goldfish and Cebus monkey retinas

Fluorescence microscopy has revealed a new type of amine-containing retinal neuron, the interplexiform cell, that extends processes in both plexiform layers. After intravitreal injection of 5,6-dihydroxytryptamine in goldfish and Cebus monkey, the processes of these cells can be identified by electron microscopy. In goldfish, the processes are pre- and postsynaptic to amacrine cells in the inner plexiform layer and presynaptic to bipolar and horizontal cells in the outer plexiform layer. Interplexiform cells thus provide an intraretinal centrifugal pathway from inner to outer plexiform layers.

Retinal mechanisms of visual adaptation in the skate

Electrical potentials were recorded from different levels within the skate retina. Comparing the adaptive properties of the various responses revealed that the isolated receptor potential and the S-potential always exhibited similar changes in sensitivity, and that the b-wave and ganglion-cell thresholds acted in concert. However, the two sets of responses behaved differently under certain conditions. For example, a dimly iluminated background that had no measurable effect on the senitivities of either of the distal responses, raised significantly the thresholds of both the b-wave and the ganglion cell responses. In addition, the rate of recovery during the early, "neural" phase of dark adaptation was significantly faster for the receptor and S-potentials than for the b-wave or ganglion cell discharge. These results indicate that there is an adaptive ("network") mechanism in the retina which can influence significantly b-wave and gaglion cell activity and which behaves independently of the receptors and horizontal cells. We conclude that visual adaptation in the skate retina is regulated by a combination of receptoral and network mechanisms.

Anatomical evidence for cone and rod-like receptors in the gray squirrel, ground squirrel, and prairie dog retinas

In the gray squirrel (Sciurus carolinensis), the prairie dog (Cynomys ludovicianus), and the Mexican and 13-line ground squirrels (Citellus mexicanus and C. tridecemlineatus) there exist two distinct classes of photo-receptors that have cone-like and rod-like anatomical features respectively. These two receptor classes were previously known to exist in the gray squirrel, but only the cone-like (C) receptor had been observed in the other species. We have now found small numbers of rod-like (R) receptors in the other species as well. R-receptors comprise about 40% of the receptors in the gray squirrel, 10% of the receptors in the prairie dog, and 4-5% of the receptors in the two species of ground squirrel. This paper describes certain light and electron microscopic features of these two receptor classes including their synaptic connections with second-order cells and with each other. We find that the C-receptor has a morphology and synaptic organization characteristic of other mammalian cones. However, the R-receptor differs from other mammalian rods in certain morphological respects, and its synaptic organization has both cone and rod characteristics as well as some unusual features.

Electrophysiological evidence for rod-like receptors in the gray squirrel, ground squirrel and prairie dog retinas

Spectral sensitivities of the gray squirrel, Mexican and 13-line ground squirrel and prairie dog were determined by electroretinography under both dark- and light-adapted conditions. The dark-adapted spectral sensitivity function obtained from intact eyes of these species peaks between 515-525 nm; however, when corrected for lens absorption or recorded from the lensless eye, it peaks near 500 nm and closely matches in shape a rhodopsin nomogram curve (lambda max equals 502 nm). Upon light adaptation all these retinas become relatively more sensitive to long-wave stimuli (i.e., they show a small Purkinje shift). The light-adapted spectral sensitivity function is broader than that obtained from the dark-adapted eye, especially toward the longer wavelengths. Weconclude that in all these species the dark-adapted spectral sensitivity is mediated by a single, rhodopsin-like photopigment and that light-adapted sensitivity is mediated by two (or more) photopigments.

Intracellular recordings from single rods and cones in the mudpuppy retina

Mudpuppy rod and cone responses differ both in time course of recovery and in absolute sensitivity. Rods are about 25 times more sensitive than cones and appear to generate a larger voltage per quantum absorbed. Comparison of mudpuppy receptor sensitivities to those of other vertebrates suggests that the difference in sensitivity between rods and cones may be a general phenomenon.

Adaptation in skate photoreceptors

Receptor potentials were recorded extracellularly from the all-rod retina of the skate after the application of sodium aspartate. This agent suppresses the responses of proximal elements, but leaves relatively unaffected the electrical activity of the photoreceptors (a-wave) and pigment epithelium (c-wave). Since the latter develops too slowly to interfere with the receptor response, it was possible to isolate receptor potentials and to compare their behavior in light and dark adaptation with earlier observations on the S-potential, b-wave, and ganglion cell discharge. The results show that the photoreceptors display the full complement of adaptational changes exhibited by cells proximal to the receptors. Thus, it appears that visual adaptation in the skate is governed primarily by the photoreceptors themselves. Of particular interest was the recovery of sensitivity in the presence of background fields that initially saturate the receptor potential. Analysis of this recovery phase indicates that a gain-control mechanism operates within the receptors, at a distal stage of the visual process.

Synapses onto different morphological types of retinal ganglion cells

The percentage of bipolar and amacrine synapses onto ganglion cell dendrites of the ground squirrel has been determined by electron microscopy of cells impregnated by the Golgi method. One group of ganglion cells has mainly amacrine input (approximately 97 percent); the other group has an approximately equal bipolar and amacrine input. Morphologically distinct types of ganglion cells usually have a consistent synaptic input, but exceptions may exist.

Neural organization of the median ocellus of the dragonfly. II. Synaptic structure

Two types of presumed synaptic contacts have been recognized by electron microscopy in the synaptic plexus of the median ocellus of the dragonfly. The first type is characterized by an electron-opaque, button-like organelle in the presynaptic cytoplasm, surrounded by a cluster of synaptic vesicles. Two postsynaptic elements are associated with these junctions, which we have termed button synapses. The second synaptic type is characterized by a dense cluster of synaptic vesicles adjacent to the presumed presynaptic membrane. One postsynaptic element is observed at these junctions. The overwhelming majority of synapses seen in the plexus are button synapses. They are found most commonly in the receptor cell axons where they synaptically contact ocellar nerve dendrites and adjacent receptor cell axons. Button synapses are also seen in the ocellar nerve dendrites where they appear to make synapses back onto receptor axon terminals as well as onto adjacent ocellar nerve dendrites. Reciprocal and serial synaptic arrangements between receptor cell axon terminals, and between receptor cell axon terminals and ocellar nerve dendrites are occasionally seen. It is suggested that the lateral and feedback synapses in the median ocellus of the dragonfly play a role in enhancing transients in the postsynaptic responses.

Neural organization of the median ocellus of the dragonfly. I. Intracellular electrical activity

Intracellular responses from receptors and postsynaptic units have been recorded in the median ocellus of the dragonfly. The receptors respond to light with a graded, depolarizing potential and a single, tetrodotoxin-sensitive impulse at "on." The postsynaptic units (ocellar nerve dendrites) hyperpolarize during illumination and show a transient, depolarizing response at "off." The light-evoked slow potential responses of the postsynaptic units are not altered by the application of tetrodotoxin to the ocellus. It appears, therefore, that the graded receptor potential, which survives the application of tetrodotoxin, is responsible for mediating synaptic transmission in the ocellus. Comparison of pre- and postsynaptic slow potential activity shows (a) longer latencies in postsynaptic units by 5-20 msec, (b) enhanced photosensitivity in postsynaptic units by 1-2 log units, and (c) more transient responses in postsynaptic units. It is suggested that enhanced photosensitivity of postsynaptic activity is a result of summation of many receptors onto the postsynaptic elements, and that transients in the postsynaptic responses are related to the complex synaptic arrangements in the ocellar plexus to be described in the following paper.

S-potentials in the skate retina. Intracellular recordings during light and dark adaptation

The S-potentials recorded intracellularly from the all-rod retina of the skate probably arise from the large horizontal cells situated directly below the layer of receptors. These cells hyperpolarize in response to light, irrespective of stimulus wavelength, and the responses in photopic as well as scotopic conditions were found to be subserved by a single photopigment with lambda(max) = 500 nm. The process of adaptation was studied by recording simultaneously the threshold responses and membrane potentials of S-units during both light and dark adaptation. The findings indicate that the sensitivity of S-units, whether measured upon steady background fields or in the course of dark adaptation, exhibits changes similar to those demonstrated previously for the ERG b-wave and ganglion cell discharge. However, the membrane potential level of the S-unit and its sensitivity to photic stimulation varied independently for all the adapting conditions tested. It appears, therefore, that visual adaptation in the skate retina occurs before the S-unit is reached, i.e., at the receptors themselves.