Lateral spread of S-potential components in the carp retina

Lateral interactions in the outer retina

Lateral interactions in the outer retina, particularly negative feedback from horizontal cells to cones and direct feed-forward input from horizontal cells to bipolar cells, play a number of important roles in early visual processing, such as generating center-surround receptive fields that enhance spatial discrimination. These circuits may also contribute to post-receptoral light adaptation and the generation of color opponency. In this review, we examine the contributions of horizontal cell feedback and feed-forward pathways to early visual processing. We begin by reviewing the properties of bipolar cell receptive fields, especially with respect to modulation of the bipolar receptive field surround by the ambient light level and to the contribution of horizontal cells to the surround. We then review evidence for and against three proposed mechanisms for negative feedback from horizontal cells to cones: 1) GABA release by horizontal cells, 2) ephaptic modulation of the cone pedicle membrane potential generated by currents flowing through hemigap junctions in horizontal cell dendrites, and 3) modulation of cone calcium currents (I(Ca)) by changes in synaptic cleft proton levels. We also consider evidence for the presence of direct horizontal cell feed-forward input to bipolar cells and discuss a possible role for GABA at this synapse. We summarize proposed functions of horizontal cell feedback and feed-forward pathways. Finally, we examine the mechanisms and functions of two other forms of lateral interaction in the outer retina: negative feedback from horizontal cells to rods and positive feedback from horizontal cells to cones.

Modulation of chromatic difference in receptive field size of H1 horizontal cells in carp retina: dopamine- and APB-sensitive mechanisms

Chromatic aspects of receptive field size in the H1 horizontal cell syncytium of the carp retina were investigated using spectral photostimuli (blue or red) presented in the form of either a pair of a small spot and annulus, or a narrow moving slit. In the light-adapted retina, the receptive field for the blue stimulus was found to be significantly smaller than that for the red, i.e. there was a chromatic difference in the receptive field size. During the course of dark adaptation, the overall receptive field size increased, but the chromatic difference decreased. Immediately after adaptation to bright light, the receptive field sizes were reduced significantly, but the chromatic difference increased, mainly due to a greater reduction in the receptive field for the blue stimulus. Application of dopamine (5 microM) to a dark-adapted retina gradually decreased the receptive field size for both colours, but the chromatic difference became larger, again due to a greater reduction in the receptive field size for the blue stimulus. 2-Amino-4-phosphonobutyrate (APB) applied to light-adapted retinae at a working concentration of 1 mM, greatly expanded the receptive field size and suppressed the chromatic difference due to the effect being greater for the receptive field for the blue stimulus. The effect of APB was slow and cumulative. On the other hand, intracellular injection of cGMP or dibutyryl-cGMP increased the chromatic difference in the receptive field size. It is suggested (i) that the chromatic difference in the receptive field size could be due to a cGMP-coupled, conductance-decreasing receptor mechanism activated by APB; and (ii) that the mechanism is associated with short-wavelength sensitive cone input to the H1 cells and operates in the light-adapted state of the retina.

Synaptic feedback, depolarization, and color opponency in cone photoreceptors

For some 20 years, synaptic feedback from horizontal cells to cones has often been invoked, more or less convincingly, in discussions of retinal action and vision. However, feedback in cones has proved to be rather complex and difficult to study experimentally. The mechanisms and consequences of feedback are therefore still only partly understood. This review attempts to assess the knowns and unknowns. The limitations of the evidence for feedback are reviewed to support the position that unequivocal evidence still largely rests on intracellular recording from cones. Of the three distinct types of depolarization observed in cones, the graded depolarization is taken as the fundamental manifestation of feedback. The evidence for the hypothesis that GABA is the neurotransmitter for feedback appears reasonably strong but several complications will have to be resolved to make the hypothesis more secure. There is evidence that feedback contributes to aspects of light adaptation and spatiotemporal processing of visual information. The contributions seem modest in magnitude. The role of feedback in shaping the color-opponent responses of retinal neurons is evaluated with particular emphasis on pharmacological studies, spatial and temporal aspects of the response of chromatic horizontal cells, and the enigmatic nature of depolarizations in blue- and green-sensitive cones. On this and other evidence, it is suggested that feedback may impress some detectable wavelength dependency in some cones but the dominant mechanisms for color opponency probably reside beyond the photoreceptors.

Dopamine decreases receptive field size of rod-driven horizontal cells in carp retina

Receptive field size of rod-driven horizontal cells (HCs) in the carp retina was measured by the spread of responses to the slit of light stimulus with changing the distance from the recording electrode and it was found to decay with a single exponential function. By perfusing 10 microM dopamine (DA) the length constant of rod-driven HCs was reduced to half and the response amplitude in the centre increased approximately two-fold, and the input resistance was markedly increased. This suggests that DA as a neuromodulator released from interplexiform cells could decouple the rod-driven HCs which had no direct synaptic contact with the interplexiform cells.

Light-dependent dynamics of gap junctions between horizontal cells in the retina of the crucian carp

The dynamics of gap junctions between outer horizontal cells or their axon terminals in the retina of the crucian carp were investigated during light and dark adaptation by use of ultrathin-section and freeze-fracture electron microscopy. Light adaptation was induced by red light, while dark adaptation took place under ambient dark conditions. The two principal findings were: (1) The density of connexons within an observed gap junction is high in dark-adapted retina, and low in light-adapted retina. This, respectively, may reflect the coupled and uncoupled state of the gap junction. (2) The size of individual gap junctions is larger in light- than in dark-adapted retinae. Whereas the overall area occupied by gap junctions is reduced with dark adaptation, the percentage of small and very small gap junctions increases dramatically. A lateral shift of connexons in the gap junctional membrane is strongly suggested by these reversible processes of densification and dispersion. Two additional possibilities of gap junction modulation are discussed: (1) the de novo formation of very small gap junctions outside the large ones in the first few minutes of dark adaptation, and (2) the rearrangement of a portion of the very large gap junctions. The idea that the cytoskeleton is involved in such modulatory processes is corroborated by thin-section observations.

Response properties of C-type horizontal cells in the retina of the bowfin

Intracellular recordings were obtained from biphasic- and triphasic-type horizontal cells (C cells) in the retina of the bowfin. For steady-state responses, both cell types displayed a linear stimulus-response function for responses up to at least 20% of maximum. In the linear range, responses to red/green mixtures were well predicted from the assumption that opposed inputs combine by simple summation. Action spectra were measured in the linear range for 30 biphasic and 12 triphasic cells. Biphasic cells showed their peak hyperpolarization near 530 nm and peak depolarization near 680 nm. Triphasic cells showed peak hyperpolarization near 450 nm, peak depolarization near 570 nm and small hyperpolarizing responses to deep red flashes (greater than 670 nm). The response to deep red test flashes was reduced by chromatic backgrounds which either depolarized or hyperpolarized the cell, in contrast to past findings in carp triphasic cells. In both classes of cells, the depolarizing input mechanism had a shorter latency than the hyperpolarizing mechanism, a result not previously observed in other fish retinas. Color opponency was maintained in both classes of C cells for stimuli of small diameter. The findings in bowfin and other species suggest that both feedback and direct pathways shape the depolarizing response of C cells.

Gap junctions between horizontal cells in the cyprinid fish alter rapidly their structure during light and dark adaptation

The dynamics of the structure of gap junctions between outer horizontal cells (HCs) and between their axonal terminals in the retina of the goldfish during light and dark adaptation is described by means of quantitative freeze-fracture replica examination. The light adaptation was performed in red light. In dark-adapted retinae the gap junctional connexons are arranged much more dense than in light-adapted retinae. The rearrangement during the first minutes of light adaptation proceeds faster than during the first minutes of dark adaptation. Since dark adaptation is accompanied by surround enhancement and presumably by coupling of HCs it is concluded that densification of HC gap junctions may correlate with coupling, and scattering of HC gap junctions with uncoupling of this type of electrotonic synapse.

Interaction between the soma and the axon terminal of retinal horizontal cells in Cyprinus carpio

Intracellular recordings were made from the monophasic horizontal cells of the carp retina which are known to respond with a sustained hyperpolarization to all visible monochromatic light. The receptive field of each subcellular structure, the soma and the axon terminal, was determined using a long narrow slit of light. Somata and axon terminals showed receptive fields that encompassed almost the entire retina. This observation suggests that each aggregate of the subcellular parts forms a synctial structure. However, with increasing distance from the slit, the response peak decayed more steeply in somata than in axon terminals. The spatial decline of the peak consisted of two exponential functions in somata, while a single exponential function in axon terminals. The length constant of the axon terminal was similar to the larger length constant revealed in the soma. This finding suggests an electrical communication at work between the soma and the axon terminal. A quantitative account was made in light of a discrete resistive network model which consists of a pair of syncytia coupled through connecting axons; one represents the contiguous layer of somata and the other the contiguous layer of axon terminals. Relevant response properties computed from the model analysis were in satisfactory agreement with experimental data. It was concluded that the soma and the axon terminal of the horizontal cell are electrically connected in the cyprinid retina.

Coupling between horizontal cells in the carp retina revealed by diffusion of Lucifer yellow

Electrical coupling among 4 different types of horizontal cells in Cyprinus carpio was examined from the diffusion of the intracellularly injected Lucifer yellow. The type of horizontal cells was identified by a spectral response and by a distinct morphology when the retina was viewed in flat mounts under a fluorescence microscope. Lucifer yellow diffused from the injected cell into surrounding cells, and in all of these preparations, diffusion was limited to horizontal cells of the same morphological type. Axons of horizontal cells were found to be coupled, and also at axons the coupling is likely to be limited to the same cell type.

Receptive field properties of the photopic luminosity horizontal cell of carp retina

The receptive field of the LEHC of the carp's retina is different when tested with red versus green stimuli. The sensitivities to 706 nm vs 519 nm flashes were compared for various size spots centered on the receptive field. Full summation (area times intensity equaling a constant at threshold) and greater than full summation were found to occur up to larger diameters of spots with red illumination than with green. A further test was made of the effect of constant background green (502 nm) illumination on the sensitivity to red vs green flashes. At all background intensities the sensitivity to red vs green flashes. At all background intensities the sensitivity to green flashes was reduced, but over an optimal range of background intensities the sensitivity to red flashes was increased. These findings are explained in terms of a previously proposed model of cone-LEHC connections.

A GABA antagonist, bicuculline, exerts its uncoupling action on external horizontal cells through dopamine cells in carp retina

External horizontal cells in isolated retinas of the carp (Cyprinus carpio) were intracellularly recorded and marked with the fluorescent dye Lucifer Yellow (LY). These cells are electrically coupled via gap junctions, so that the injected dye normally diffused to neighboring cells. A GABA antagonist, bicuculline (Bcc, 20 microM), applied to the vitreous fluid beneath the isolated retina, altered the spatial properties of light-induced responses, by increasing the amplitude of responses to central spots and decreasing that of those to distant spots. Bcc also restricted to injected dye to single recorded cells. These uncoupling actions of Bcc were similar to those of amphetamine (20 microM) or dopamine (10-20 microM) and were abolished by the presence of a dopaminergic blocker, haloperidol (20-40 microM). Both the actions of Bcc and amphetamine were not observed when applied to retinas deprived of dopaminergic cells by prior destruction with 6-hydroxydopamine. Therefore, Bcc exerts its uncoupling actions on external horizontal cells indirectly through the dopaminergic system in the carp retina.

Dopamine modulates S-potential amplitude and dye-coupling between external horizontal cells in carp retina

Horizontal cells in the fish retina are electrically coupled and possess gap junctions so that intracellularly injected dye normally diffuses freely to neighbouring cells. Applied dopamine (DA) alters the spatial properties of the horizontal cell responses to light, increasing the amplitude of photopic L-type S-potentials but decreasing their lateral spread. These effects have been attributed to the action of DA on horizontal cell membrane resistance, particularly at the gap junctions, and our present study on the carp retina agrees with this in showing that DA also restricts intracellular Lucifer yellow (LY) to single injected horizontal cells, an effect, like those of DA on the S-potentials, which is antagonized by the dopamine blocker haloperidol. In addition, we present evidence that dopaminergic interplexiform cells in fish normally function to regulate the spatial properties of responses in horizontal cells, possibly acting on their junctional resistance via a DA-receptor-mediated mechanism. Previous destruction of the interplexiform cells with 6-hydroxydopamine (6-OHDA) resulted in much reduced L-type S-potentials to centred lights but wider lateral spread of these responses, while the dye injected spread extensively to neighbouring cells. After 6-OHDA treatment, however, applied DA retained its normal activity, restoring large-amplitude, narrow receptive-field S-potentials and restricting LY to the injected cells, effects which were both closely mimicked by dibutyryl cyclic AMP.

Regional difference in density of monoamine-accumulating cells of carp and catfish retinas

By means of a histofluorescence technique, a comparative study was conducted on the regional density of dopaminergic (DA) and indoleamine-accumulating (IA) cells in carp (Cyprinus carpio) and catfish (Ictalurus punctatus) retinas. In order to enhance detection of fluorescent cells, noradrenaline (NA; 5.0 micrograms) or a mixture of NA (2.5 micrograms) and 5,6-dihydroxytryptamine (5,6-DHT; 2.5 micrograms) was intravitreally injected into the eyes 2-3 hr before enucleation. DA and IA cells were counted systematically in space on flat-mounted preparations. Both classes of cells were found to be distributed similarly in the two species of fish; the cell density is highest in the circumferential margin of the retina, and is slightly higher in a region dorsal to the optic disc than in the surrounding area. Differences in the distribution pattern of the cells between carp and catfish retinas were as follows: (a) the DA cell density is higher over the whole retinal field in carp (the mean density +/- SD = 34 +/- 16 cells/mm2) than in catfish (13 +/- 7 cells/mm2); (b) the region where the density is slightly higher than in the surrounding area is restricted to a small area immediately dorsolateral to the optic disc in carp, while it is relatively broadly placed dorsal to the optic discs, forming a horizontal band in catfish; (c) the density ratio of DA cells to IA cells is 1:1 in carp but 1:2 in catfish; and (d) catfish DA cells seem to be more irregular than carp DA cells in shape, size, dendritic arborization, uptake preference for monoamines intravitreally injected, and also in depth location seen in radial cryosections.