Lateral feedback from monophasic horizontal cells to cones in carp retina. I. Experiments

Electrical coupling and its channels

As the physiology of synapses began to be explored in the 1950s, it became clear that electrical communication between neurons could not always be explained by chemical transmission. Instead, careful studies pointed to a direct intercellular pathway of current flow and to the anatomical structure that was (eventually) called the gap junction. The mechanism of intercellular current flow was simple compared with chemical transmission, but the consequences of electrical signaling in excitable tissues were not. With the recognition that channels were a means of passive ion movement across membranes, the character and behavior of gap junction channels came under scrutiny. It became evident that these gated channels mediated intercellular transfer of small molecules as well as atomic ions, thereby mediating chemical, as well as electrical, signaling. Members of the responsible protein family in vertebrates-connexins-were cloned and their channels studied by many of the increasingly biophysical techniques that were being applied to other channels. As described here, much of the evolution of the field, from electrical coupling to channel structure-function, has appeared in the pages of the Journal of General Physiology.

Spectral responses in zebrafish horizontal cells include a tetraphasic response and a novel UV-dominated triphasic response

Zebrafish are tetrachromats with red (R, 570 nm), green (G, 480 nm), blue (B, 415 nm), and UV (U, 362 nm) cones. Although neurons in other cyprinid retinas are rich in color processing neural circuitry, spectral responses of individual neurons in zebrafish retina, a genetic model for vertebrate color vision, are yet to be studied. Using dye-filled sharp microelectrodes, horizontal cell voltage responses to light stimuli of different wavelengths and irradiances were recorded in a superfused eyecup. Spectral properties were assessed both qualitatively and quantitatively. Six spectral classes of horizontal cell were distinguished. Two monophasic response types (L1 and L2) hyperpolarized at all wavelengths. L1 sensitivities peaked at 493 nm, near the G cone absorbance maximum. Modeled spectra suggest equally weighted inputs from both R and G cones and, in addition, a "hidden opponency" from blue cones. These were classified as R-/G-/(b+). L2 sensitivities were maximal at 563 nm near the R cone absorbance peak; modeled spectra were dominated by R cones, with lesser G cone contributions. B and UV cone signals were small or absent. These are R-/g-. Four chromatic (C-type) horizontal cells were either depolarized (+) or hyperpolarized (-) depending on stimulus wavelength. These types are biphasic (R+/G-/B-) with peak excitation at 467 nm, between G and B cone absorbance peaks, UV triphasic (r-/G+/U-) with peak excitation at 362 nm similar to UV cones, and blue triphasic (r-/G+/B-/u-) and blue tetraphasic (r-/G+/B-/u+), with peak excitation at 409 and 411 nm, respectively, similar to B cones. UV triphasic and blue tetraphasic horizontal cell spectral responses are unique and were not anticipated in previous models of distal color circuitry in cyprinids.

No evidence of UV cone input to mono- and biphasic horizontal cells in the goldfish retina

Many animal species make use of ultraviolet (UV) light in a number of behaviors, such as feeding and mating. The goldfish (Carassius auratus) is among those with a UV photoreceptor and pronounced UV sensitivity. Little is known, however, about the retinal processing of this input. We addressed this issue by recording intracellularly from second-order neurons in the adult goldfish retina. In order to test whether cone-driven horizontal cells (HCs) receive UV cone inputs, we performed chromatic adaptation experiments with mono- and biphasic HCs. We found no functional evidence of a projection from the UV-sensitive cones to these neurons in adult animals. This suggests that goldfish UV receptors may contact preferentially triphasic HCs, which is at odds with the hypothesis that all cones contact all cone-driven HC types. However, we did find evidence of direct M-cone input to monophasic HCs, favoring the idea that cone-HC contacts are more promiscuous than originally proposed. Together, our results suggest that either UV cones have a more restricted set of post-synaptic partners than the other three cone types, or that the UV input to mono- and biphasic HCs is not very pronounced in adult animals.

Lateral gain control in the outer retina leads to potentiation of center responses of retinal neurons

The retina can function under a variety of adaptation conditions and stimulus paradigms. To adapt to these various conditions, modifications in the phototransduction cascade and at the synaptic and network levels occur. In this paper, we focus on the properties and function of a gain control mechanism in the cone synapse. We show that horizontal cells, in addition to inhibiting cones via a "lateral inhibitory pathway," also modulate the synaptic gain of the photoreceptor via a "lateral gain control mechanism." The combination of lateral inhibition and lateral gain control generates a highly efficient transformation. Horizontal cells estimate the mean activity of cones. This mean activity is subtracted from the actual activity of the center cone and amplified by the lateral gain modulation system, ensuring that the deviation of the activity of a cone from the mean activity of the surrounding cones is transmitted to the inner retina with high fidelity. Sustained surround illumination leads to an enhancement of the responses of transient ON/OFF ganglion cells to a flickering center spot. Blocking feedback from horizontal cells not only blocks the lateral gain control mechanism in the outer retina, but it also blocks the surround enhancement in transient ON/OFF ganglion cells. This suggests that the effects of the outer retinal lateral gain control mechanism are visible in the responses of ganglion cells. Functionally speaking, this result illustrates that horizontal cells are not purely inhibitory neurons but have a role in response enhancement as well.

Modeling the pre- and post-synaptic components involved in the synaptic modification between cones and horizontal cells in carp retina

In retinal synapses between cones and luminosity type horizontal cells (LHC), it was previously found in this laboratory that repetitive red flashes progressively strengthened the LHC's response to red flash, whereas weakened the LHC's response to green flash; repetitive green flash remarkably depressed the LHC's red response, but caused little changes in the cell's green response. However, the detailed mechanisms underlying these phenomena are not entirely clear. In the present study, based on an ion-channel model described mainly in the form of Hodgkin-Huxley equations, possible mechanisms of the short-term synaptic modification are investigated. The simulation results suggest that: (1) the auto-enhancement effect might be induced by the Ca2+-dependent process on the post-synaptic AMPA receptors, which could lead to changes of the ionic channel's properties; (2) the asymmetric response to red- and green-flashes and the mutual-chromatic suppression effects might be attributed to the regulatory effects on the presynaptic glutamate release.

Adaptive plasticity during the development of colour vision

Colour vision greatly enhances the discriminatory and cognitive capabilities of visual systems and is found in a great majority of vertebrates and many invertebrates. However, colour coding visual systems are confronted with the fact that the external stimuli are ambiguous because they are subject to constant variations of luminance and spectral composition. Furthermore, the transmittance of the ocular media, the spectral sensitivity of visual pigments and the ratio of spectral cone types are also variable. This results in a situation where there is no fixed relationship between a stimulus and a colour percept. Colour constancy has been identified as a powerful mechanism to deal with this set of problems; however, it is active only in a short-term time range. Changes covering longer periods of time require additional tuning mechanisms at the photoreceptor level or at postreceptoral stages of chromatic processing. We have used the trichromatic blue acara (Aequidens pulcher, Cichlidae) as a model system and studied retinal morphology and physiology, and visually evoked behaviour after rearing fish for 1-2 years under various conditions including near monochromatic lights (spectral deprivation) and two intensities of white light (controls). In general, long-term exposure to long wavelengths light had lesser effects than light of middle and short wavelengths. Within the cone photoreceptors, spectral deprivation did not change the absorption characteristics of the visual pigments. By contrast, the outer segment length of middle and long-wave-sensitive cones was markedly increased in the blue rearing group. Furthermore, in the same group, we observed a loss of 65% short-wave-sensitive cones after 2 years. These changes may be interpreted as manifestations of compensatory mechanisms aimed at restoring the balance between the chromatic channels. At the horizontal cell level, the connectivity between short-wave-sensitive cones and the H2 cone horizontal cells, and the spinule dynamics were both affected in the blue light group. This observation rules out the role of spinules as sites of chromatic feedback synapses. The light-evoked responses of H2 horizontal cells were also sensitive to spectral deprivation showing a shift of the neutral point towards short wavelengths in the blue rearing group. Interestingly, we also found an intensity effect because in the group reared in bright white light the neutral point was more towards longer wavelength than in the dim light group. Like the changes in the cones, the reactions of horizontal cells to spectral deprivation in the long wave domain can be characterised as compensatory. We also tested the spectral sensitivity of the various experimental groups of blue acara in visually evoked behaviour using the optomotor response paradigm. In this case, the changes in the relative spectral sensitivity were more complex and could not be explained by a simple extrapolation of the adaptive and compensatory processes in the outer retina. We conclude that the inner retina, and/or the optic tectum are also involved and react to the changes of the spectral environment. In summary, we have shown a considerable developmental plasticity in the colour vision system of the blue acara, where epigenetic adaptive processes at various levels of the visual system respond to the specific spectral composition of the surroundings and provide a powerful mechanism to ensure functional colour vision in different visual environments. We suggest that processes involving an active fine-tuning of the photoreceptors and the postreceptoral processing of chromatic information during ontogenetic development are a general feature of all colour vision systems. Such mechanisms would establish a functional balance between the various chromatic channels. This appears to be an essential condition for the cognitive systems to extract the relevant and stable information from the unstable and changing stimulus situation.

Neurotransmitter modulation of extracellular H+ fluxes from isolated retinal horizontal cells of the skate

Self-referencing H(+)-selective microelectrodes were used to measure extracellular H(+) fluxes from horizontal cells isolated from the skate retina. A standing H(+) flux was detected from quiescent cells, indicating a higher concentration of free hydrogen ions near the extracellular surface of the cell as compared to the surrounding solution. The standing H(+) flux was reduced by removal of extracellular sodium or application of 5-(N-ethyl-N-isopropyl) amiloride (EIPA), suggesting activity of a Na(+)-H(+) exchanger. Glutamate decreased H(+) flux, lowering the concentration of free hydrogen ions around the cell. AMPA/kainate receptor agonists mimicked the response, and the AMPA/kainate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) eliminated the effects of glutamate and kainate. Metabotropic glutamate agonists were without effect. Glutamate-induced alterations in H(+) flux required extracellular calcium, and were abolished when cells were bathed in an alkaline Ringer solution. Increasing intracellular calcium by photolysis of the caged calcium compound NP-EGTA also altered extracellular H(+) flux. Immunocytochemical localization of the plasmalemma Ca(2+)-H(+)-ATPase (PMCA pump) revealed intense labelling within the outer plexiform layer and on isolated horizontal cells. Our results suggest that glutamate modulation of H(+) flux arises from calcium entry into cells with subsequent activation of the plasmalemma Ca(2+)-H(+)-ATPase. These neurotransmitter-induced changes in extracellular pH have the potential to play a modulatory role in synaptic processing in the outer retina. However, our findings argue against the hypothesis that hydrogen ions released by horizontal cells normally act as the inhibitory feedback neurotransmitter onto photoreceptor synaptic terminals to create the surround portion of the centre-surround receptive fields of retinal neurones.

Metabotropic glutamate receptor-mediated hetero-synaptic interaction of red- and green-cone inputs to LHC of carp retina

The role of presynaptic metabotropic glutamate receptor (mGluR) on the interaction of red- and green-cone signals was investigated in luminosity-type horizontal cell (LHC) of isolated carp retina. It was found that a dim red background could enhance LHC's light response to green stimulus, and a dim green background was also able to increase the cell's response to red flash. Such mutual color enhancement was eliminated by application of groups II and III mGluR antagonist (S)-methyl-4-carboxyphenyl-glycine (MCPG). Furthermore, inhibition of glutamate uptake by using D-aspartate (D-Asp) or DL-threo-beta-hydroxy-aspartic acid (THA) completely blocked the mutual enhancement of color signals in LHC. However, the GABAergic feedback pathway in the outer retina was unnecessarily involved.

The dynamic characteristics of the feedback signal from horizontal cells to cones in the goldfish retina

1. The dynamic properties of the microcircuitry formed by cones and horizontal cells in the isolated goldfish retina were studied. Cones project to horizontal cells and horizontal cells feed back to cones via a relatively slow negative feedback pathway. 2. The time constant of the feedback signal in cones and of the effect this feedback signal had on the responses of second-order neurons was determined using whole-cell patch clamp and intracellular recording techniques. 3. It was found that the feedback signal in cones had a time constant of around 80 ms, whereas the time constant of the effect this feedback signal had on the second-order neurons ranged from 36 to 116 ms. This range of time constants can be accounted for by the non-linearity of the Ca(2+) current in the cones. In depolarized cones, the feedback-mediated response in second-order neurons had a similar time constant to that of the direct light response of the cone, whereas in hyperpolarized cones, the time constant of the feedback-mediated response in second-order neurons was considerably larger. 4. Further, it was shown that there was no delay in the feedback pathway. This is in contrast to what has been deduced from the response properties of second-order neurons. In one type of horizontal cell, the responses to red light were delayed relative to the responses to green light. This delay in the second-order neurons can be accounted for by the interaction of the direct light response of the medium-wavelength-sensitive cones (M-cones) with the feedback response of the M-cones received from the horizontal cells.

Dark- and light-induced changes in coupling between horizontal cells in mammalian retina

Retinal horizontal cells exhibit large receptive fields derived from their extensive electrical coupling by means of gap junctions. The conductance of these gap junctions seems to be regulated by dopamine acting through a cAMP-mediated cascade. There is now abundant evidence that extracellular dopamine levels vary with changes in ambient light intensity, suggesting that changes in the dark/light adaptational state of the retina can modulate coupling between horizontal cells. We studied this question in the mammalian retina by determining the effects of ambient light levels, in the form of changing background light intensity, on the coupling profiles of A- and B-type horizontal cells in the rabbit. Changes in coupling were assessed by measurements of the space constants of the syncytium formed by horizontal cells and the intercellular spread of the biotinylated tracer Neurobiotin. Our results indicate that dark-adapted horizontal cells show relatively weak coupling. However, presentation of background lights as dim as one-quarter log unit above rod threshold resulted in increases in both the averaged extent of tracer coupling and space constants of A- and B-type horizontal cells. Coupling expanded further as background light intensities were increased by 1-1.5 log units, after which additional light adaptation brought about an uncoupling of cells. Coupling reached its minimum at light intensities about 3 log units above rod threshold, after which, with further light adaptation, it stabilized at levels close to those seen in dark-adapted retinas. Our results indicate that electrical coupling between mammalian horizontal cells is modulated dramatically by changes in the adaptational state of the retina: coupling is maximized under dim ambient light conditions and diminishes as the retina is dark or light adapted from this level.

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.

Connectivity patterns of cone horizontal cells in blue acara (Aequidens pulcher, Cichlidae) reared in different light regimes

Two types of cone horizontal cells were identified morphologically in the retina of a trichromatic fish by fluorescent labelling with Lucifer Yellow and confocal laser scanning microscopy. H1 cells are located adjacent to the outer plexiform layer, have large somata, small dendritic fields, and contact all cone types. H2 cells are positioned vitread to the H1 cells, have small somata, and large dendritic fields. Their dendrites invaginate the synaptic pedicles of short wavelength sensitive single cones and show a significant preference for one of the spectrally different members of the double cones, presumably the middle wavelength sensitive member. We tested the impacts of different visual environments on the development of these connectivity patterns and found minor changes induced by rearing in white light of different intensities or monochromatic blue light.

Processing of color- and noncolor-coded signals in the gourami retina. I. Horizontal cells

There are two types of horizontal cells, the luminosity and the chromaticity cells, in the retina of the kissing gourami, Helostoma rudolfi. Luminosity cells occupy the outermost layer proximal to the receptor terminals, whereas chromaticity cells form a layer proximal to the layer of luminosity cells. Neither type of cell has axons. Responses were evoked by light from red and green light-emitting diodes. The two stimuli were modulated either by a pulsatile or a white-noise signal. The luminosity cell always produced a hyperpolarizing response. The chromaticity cell produced a hyperpolarizing response when stimulated by only one color. However, in the presence of a steady or modulated green input, a red stimulus produced a depolarizing response. Such chromaticity cells were similar to the (spectral) biphasic chromaticity horizontal cells observed in other retinae. The depolarizing phase of the red response was produced by the balance of intensity of the two inputs, red and green. We used white-noise methodology to identify the dynamics of the horizontal cell's modulation response by taking advantage of the fact that a Wiener kernel is a measure of a cell's incremental sensitivity, which includes its response dynamics. Under all conditions, a steady state modulation response by both luminosity and chromaticity cells always was related linearly to the input modulation. The average mean square error (MSE) of the model predicted by the first-order kernel was approximately 8% for both luminosity (n = 116) and chromaticity (n = 23) cells. In some cases, the MSE was a few percent even when the peak-to-peak response amplitude was nearly 30 mV. The ratio of inputs from red and green cones to both types of horizontal cells was variable; the major input for luminosity cells came from red cones, whereas the major input for chromaticity cells came from green cones. First-order kernels generated by the major input were robust in terms of waveform in the sense that the waveform remained unchanged whether or not there was a steady or modulated illumination by the opposing color. The results reported here do not address the question of the neural circuitry that generates horizontal cell responses, in particular, the depolarizing response. However, whatever that circuitry might be, the high degree of linearity of the modulation response by both types of cell under various stimulus conditions imposes restrictions on the performance of any proposed model as well as on mechanisms that underlie the generation of the horizontal cell response.

Horizontal cell spinule dynamics in fish are affected by rearing in monochromatic light

Blue acaras (Aequidens pulcher, Cichlidae) were reared for 1 yr in white or monochromatic "red", "green" and "blue" lights to study the function and control mechanisms of horizontal cell (HC) spinules in the synaptic pedicles of cones. Ratios of spinules per synaptic ribbon (S/R) were determined in tangential sections in both single and double cones. We found that the S/R ratios in light adapted retinae decreased with decreasing wavelength of the rearing light in all cone types. Conversely, there was an increasing number of incompletely formed spinules with the highest frequency in the blue light group. Dark adaptation resulted in the complete degradation of mature spinules. However, significant numbers of incompletely degraded spinules were observed in the group reared in blue light. Fish reared in blue light which were transferred to white light formed mature spinules when light adapted and still had vestigial spinules when dark adapted. The mechanisms of spinule formation and degradation and the control of spinule dynamics appear to be fully developed in fish reared in monochromatic light. However, long-term chromatic deprivation seems to induce a compensatory modulation of spinule dynamics. A working hypothesis is formulated that interprets the observed effects as manifestations of differences in the activition of dopaminergic interplexiform cells (light adapted) and the sensitivity to glutamate of HCs (dark adapted). Our findings are consistent with the hypothesis that spinules are involved in sign-inverting feedback transmission from HCs to cones.

Horizontal cells feed back to cones by shifting the cone calcium-current activation range

We studied feedback from horizontal cells to cones in isolated goldfish retinae and found that surround stimuli evoke an inward current and a slowly developing outward current. The surround-evoked currents are blocked by the glutamate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) and are, like horizontal cell responses, most effectively evoked by large stimuli. This indicates that the currents are caused by feedback from horizontal cells. The surround-evoked inward current is neither blocked by picrotoxin nor carried by chloride. Instead, it is carried by calcium, and it triggers a slowly developing calcium-dependent chloride current. We were unable to mimick the surround-evoked currents by modulating the extracellular GABA concentration. We conclude that when horizontal cells hyperpolarize they feed back to the cones by shifting the cone calcium-current activation range to more negative potentials. This type of feedback, directly targeted at the calcium current, scarcely influences the membrane potential of the receiving neuron, but effectively modulates its synaptic output.

The size of the horizontal cell receptive fields adapts to the stimulus in the light adapted goldfish retina

In this study the dynamic properties of goldfish horizontal cell (HC) receptive fields were evaluated. The size of HC receptive fields increases up to about 60 msec after stimulus onset, and then reduces to a smaller end value. They can therefore not adequately be described by the cable equation. Estimates of the length constant of the HC network based on the sustained responses are about 43% smaller than those based on the initial part of the response. This difference can be accounted for by feedback connections from HCs to cones because negative feedback reduces the receptive field size. The implication is that HCs are strongly coupled when the retina is stimulated more or less homogeneously but that they partly uncouple from the rest of the HC network when they are stimulated differently than the rest of the retina. The HCs thus generate a feedback signal based on the "local" stimulus properties. The size of the HC receptive fields depends on the spatial detail of the stimulus.

Ca(2+)-dependency of spinule plasticity at dendrites of retinal horizontal cells and its possible implication for the functional role of spinules

Calcium is involved in many aspects of synaptic plasticity and we have analyzed its involvement in spinule dynamics at retinal horizontal cell dendrites. We show here that in particular the retraction of spinules is a Ca(2+)-dependent process. Inhibiting calmodulin or CaMKII, blocked the retraction that was also impaired in low calcium Ringer. Changes of the cytosolic Ca(2+)-concentration through depletion of internal Ca(2+)-stores were without effect. This suggested that Ca(2+)-influx during dark adaption and subsequent activation of CaMKII is an important step for spinule retraction. Voltage dependent Ca(2+)-channels were not responsible for the Ca(2+)-influx, rather Ca2+ leaking through alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)/kainate-gated channels. This suggested a close local link between AMPA/kainate receptors and CaMKII indicating a possible postsynaptic function of spinules. The distribution of bound, omega-shaped vesicles within the cone pedicles and its dependence on artificial depolarization further supported the idea of a postsynaptic function of spinules.

Modulation of horizontal cell receptive fields in the light adapted goldfish retina

In the isolated goldfish retina, 700 nm background illumination increases the horizontal cell receptive field size, as measured with 565 nm slits of light, but decreases the receptive field size, when measured with 660 nm slits. These background-induced changes in receptive field size are absent when the depolarizing responses in bi- and triphasic horizontal cells are blocked by lowering the [Ca2+] in the Ringer's solution from 1.0 to 0.1 mM. These results cannot be explained by the linear properties of the horizontal cell layers, nor by slow adaptational processes, but are consistent with the concept that feedback from horizontal cells to cones modifies the horizontal cell receptive field properties.

Horizontal cells function normally in ethambutol-treated goldfish

Ethambutol, a tuberculostatic drug, induces red-green colour vision defects in man and goldfish. The ethambutol-induced red-green colour vision defect in goldfish was argued to originate in the retina because after ethambutol application: (1) inhibitive interactions in red-green (double) opponent ganglion cells are lost [Van Dijk & Spekreijse, 1982 (Investigative Ophthalmology and Visual Science, 24, 128-133); Wietsma & Spekreijse, 1992 (Investigative Ophthalmology and Visual Science Suppl., 33, 1032)] and (2) the depolarizing responses to red light in the biphasic horizontal cells are reduced. To account for these findings Spekreijse, Wietsma and Neumeyer [(1991) Vision Research, 31, 551-562] suggested that ethambutol induced dark adaptation in the retina. In this paper the dark adaptation hypothesis is tested with the following results: (1) ethambutol changes only transiently the receptive field size and spectral sensitivity of horizontal cells; (2) the spectral characteristics of horizontal cells do not change in long-term ethambutol-treated goldfish; (3) formation of spinules on horizontal cell dendrites in cone terminals, a parameter for light adaptation, remains unaffected. Therefore we conclude that ethambutol does not induce functional dark adaptation of horizontal cells and that the ethambutol-induced red-green colour vision deficiency does not originate in the horizontal cell layers.

Computational studies on the interaction between red cone and H1 horizontal cell

We propose an equivalent circuit model of a discrete formulation to describe the interaction between the red cone syncytium and the H1 horizontal cell syncytium in lower vertebrate retinas. Analytical solutions of the model provide intuitive understandings of spatio-temporal properties of light-induced responses in reference to membrane impedance, strength of chemical synapse and coupling resistance connecting neighbouring cells. Physiologically plausible values of these parameters are estimated using the solutions. Quantitative studies are made to elucidate the function of (1) the negative feedback from the H1 horizontal cell to the red cone, and (2) the resistance increase of H1 horizontal cell coupling by dopamine.

Spinule-type neurite outgrowth from horizontal cells during light adaptation in the carp retina: an actin-dependent process

Dendrites of horizontal cells in the carp retina which invaginate the cone pedicles form numerous spinules during light adaptation. We have analyzed the contribution of cytoskeletal elements to this process. Isolated horizontal cells and frozen sections were screened with phalloidin for the existence of F-actin. F-actin was present in all types of horizontal cells and particularly enriched in the distal parts of the dendrites. Electron microscopical analysis demonstrated that interruption of the F-actin polymerization with cytochalasin B inhibited the formation of spinules during light adaptation. The persistence of spinules was also affected. Cytochalasin B also prevented the light-independent, phorbol ester-induced formation of spinules. Cytochalasin B only affected the morphology of the lateral, spinule-forming dendrites of cone horizontal cells within the cone pedicles, leaving the central, non spinule-forming dendrites of cone horizontal cells and the processes of rod horizontal cells within rod spherules unaffected. Whereas cytochalasin B prevented the protrusion of spinules, the spinule-associated membrane densities were only slightly affected. The two main characteristics of spinules, protrusion and membrane densities are therefore independently regulated processes.

Color opponency in cone-driven horizontal cells in carp retina. Aspecific pathways between cones and horizontal cells

The spectral and dynamic properties of cone-driven horizontal cells in carp retina were evaluated with silent substitution stimuli and/or saturating background illumination. The aim of this study was to describe the wiring underlying the spectral sensitivity of these cells. We will present electrophysiological data that indicate that all cone-driven horizontal cell types receive input from all spectral cone types, and we will present evidence that all cone-driven horizontal cell types feedback to all spectral cone types. These two findings are the basis for a model for the spectral and dynamic behavior of all cone-driven horizontal cells in carp retina. The model can account for the spectral as well as the dynamic behavior of the horizontal cells. It will be shown that the strength of the feedforward and feedback pathways between a horizontal cell and a particular spectral cone type are roughly proportional. This model is in sharp contrast to the Stell model, where the spectral behavior of the three horizontal cell types is explained by a cascade of feedforward and feedback pathways between cones and horizontal cells. The Stell model accounts for the spectral but not for the dynamic behavior of the horizontal cells.

Synaptic inputs to turtle horizontal cells analyzed after blocking of gap junctions by intracellular injection of cyclic nucleotides

Intracellular injection of cAMP or cGMP into turtle horizontal cells significantly increased the input resistances, and the cells could thus be easily polarized by current injection, suggesting that the cyclic nucleotides blocked gap junctions between cells. Then, synaptic inputs onto triphasic chromaticity-type cells were analyzed. Hyperpolarizing and depolarizing light responses were all reduced with depolarizing current, and their polarities were reversed by further depolarization. Their reversal potentials coincided at around 0 mV. This level was the same as observed in luminosity-type and biphasic chromaticity-type cells, suggesting that the ionic mechanisms of synaptic transmission are common among horizontal cell types.

Bicuculline produces reversible red-green color blindness in goldfish, as revealed by monocular behavioral testing

The effect of the GABAA receptor antagonist bicuculline (methiodide) on goldfish wavelength discrimination was studied. This was done using a behavioral two alternative forced choice procedure at training wavelengths 500 and 600 nm, where goldfish wavelength discrimination is the best. During the experiments the goldfish could use only the eye that was intravitreally injected with bicuculline; the other eye was covered. In control experiments, to exclude systemic effects, the covering of the eyes was reversed. Bicuculline induced loss of wavelength discrimination ability around 600 nm, while this ability was not affected around 500 nm. The effect was reversible, since discrimination around 600 nm returned to normal within a day after injection. The results indicate that GABAA receptor mediated processes, like horizontal cell to cone feedback, play an important role in wavelength discrimination at the long wavelength part of the spectrum.

Spatial properties of horizontal cell responses in the cat retina

The spatial properties of horizontal cells in the cat retina have been studied by means of intracellular recordings in the optically intact, in situ, eye. The spread of potentials in the horizontal cell layer and the spatial summation properties have been investigated using "white light" stimuli in several different configurations. Area-response curves were measured with flashing spots carefully centered on the receptive field. The lateral spread of potentials was studied using long, narrow slits of light and circular spots of different sizes at different positions in the receptive field. Two-dimensional receptive field profiles showed that the receptive field structure was, both on a large scale and at higher resolutions, relatively homogeneous and isotropic. The size and shape of the receptive fields have been characterized by applying a simplified version of the model proposed by Naka and Rushton (1967) of electrical coupling in the horizontal cells layer in fish. Results show that the model describes the variation of response amplitude reasonably well both as a function of spot size and spot or slit position. However, deviations were found for area-response curves measured at higher light intensities and for receptive field profiles measured with relatively small spots. Furthermore, the estimated length constants resulting from the different experiments were not in agreement. Different model parameters were needed in order to describe the spatial summation properties and the lateral spread of potentials respectively. It is concluded that passive electrical spread and linear summation of potentials cannot account for the observed spatial properties.