Dynamics of turtle horizontal cell response

The small- and large-field (cone) horizontal cells produce similar dynamic responses to a stimulus whose mean luminance is modulated by a white-noise signal. Nonlinear components increase with an increase in the mean luminance and may produce a mean square error (MSE) of up to 15%. Increases in the mean luminance of the field stimulus bring about three major changes: the incremental sensitivity defined by the amplitude of the kernels decreases in a Weber-Fechner fashion; the waveforms of the kernels are transformed from monophasic (integrating) to biphasic (differentiating); the peak response time of the kernels becomes shorter and the cells respond to much higher-frequency inputs. The dynamics of the horizontal cell response also depend on the area of the retina stimulated. Smaller spots of light produce monophasic kernels of a longer peak response time. The presence of a steady background produces three major changes in the spot kernels: the kernel's amplitude becomes larger (incremental sensitivity increases); the peak response times become shorter; the waveform of the kernels changes in a fashion similar to that observed with an increase in the mean luminance of the field stimulus. A similar enhancement in the incremental sensitivity by a steady background has also been observed in catfish, which shows that this phenomenon is a common feature of the horizontal cells in the lower vertebrate retina.     

White noise analysis of a chromatic type horizontal cell in the Xenopus retina

The dynamics of color-coded signal transmission in the light-adapted Xenopus retina were studied by a combination of white noise (Wiener) analysis and simultaneous recordings from two types of horizontal cells: chromatic-type horizontal cells (C-HCs) are hyperpolarized by blue light and depolarized by red light, whereas luminosity-type horizontal cells (L-HCs) are hyperpolarized by all wave-lengths. The retina was stimulated by two superimposed fields of red and blue light modulated by two independent white noise signals, and the resulting intracellular responses were decomposed into red and blue components (first-order kernels). The first-order kernels predict the intracellular responses with a small degree of error (3.5-9.5% in terms of mean square error) under conditions where modulated responses exceeded 30 mV in amplitude peak-to-peak, thus demonstrating that both red and blue modulation responses are linear. Moreover, there is little or no interaction between the red- and blue-evoked responses; i.e., nearly identical first-order kernels were obtained for one color whether the other color was modulated or not. In C-HCs (but not L-HCs), there were consistent differences in the dynamics of the red and blue responses. In the C-HC, the cutoff frequency of the red response was higher than for the blue (approximately 12 vs 5 Hz), and the red kernel was more bandpass than the blue. In the L-HC, kernel waveform and cutoff frequencies were similar for both colors (approximately 12 Hz or greater), and the time-to-peak of the L-HC kernel was always shorter than either the red or blue C-HC kernel. These results have implications for the mechanisms underlying color coding in the distal retina, and they further suggest that nonlinear phenomena, such as voltage-dependent conductances in HCs, do not contribute to the generation of modulation responses under the experimental conditions used here.     

Dynamics of cockroach ocellar neurons

The incremental responses from the second-order neurons of the ocellus of the cockroach, Periplaneta americana, have been measured. The stimulus was a white-noise-modulated light with various mean illuminances. The kernels, obtained by cross-correlating the white-noise input against the resulting response, provided a measure of incremental sensitivity as well as of response dynamics. We found that the incremental sensitivity of the second-order neurons was an exact Weber-Fechner function; white-noise-evoked responses from second-order neurons were linear; the dynamics of second-order neurons remain unchanged over a mean illuminance range of 4 log units; the small nonlinearity in the response of the second-order neuron was a simple amplitude compression; and the correlation between the white-noise input and spike discharges of the second-order neurons produced a first-order kernel similar to that of the cell's slow potential. We conclude that signal processing in the cockroach ocellus is simple but different from that in other visual systems, including vertebrate retinas and insect compound eyes, in which the system's dynamics depend on the mean illuminance.     

Lateral feedback from monophasic horizontal cells to cones in carp retina. II. A quantitative model

About half of the monophasic horizontal cells in carp retina receive input from both red- and green-sensitive cones. Since the horizontal cells feed back to cones, the color and feedback pathways result in wavelength- and intensity-dependent changes of the dynamics and of the receptive field amplitude profile of the horizontal cell responses. In this paper we present a quantitative model that describes adequately the color and spatial coding and the dynamics of the responses from monophasic horizontal cells in carp. Lateral feedback plays a distinct role in this model.     

Dynamics of skate horizontal cells

The all-rod retina of the skate (Raja erinacea or R. oscellata) is known to have the remarkable capability of responding to incremental flashes superimposed on background intensities that initially block all light-evoked responses and are well above the level at which rods saturate in mixed rod/cone retinas. To examine further the unusual properties of the skate visual system, we have analyzed responses of their horizontal cells to intensity-modulated step, sinusoidal, and white-noise stimuli. We found that during exposures to mean intensities bright enough to block responses to incremental stimuli, decremental stimuli were also initially blocked. Thereafter, the horizontal cells underwent a slow recovery phase during which there was marked nonlinearity in their response properties. The cell first (within 2-3 min) responded to decrements in intensity and later (after greater than 10 min) became responsive to incremental stimuli. After adaptation to a steady state, however, the responses to intensity modulation were nearly linear over a broad range of modulation depths even at the brightest mean levels of illumination. Indeed, examination of the steady-state responses over a 5-log-unit range of mean intensities revealed that the amplitude of the white noise-evoked responses depended solely on contrast, and was independent of the retinal irradiance as the latter was increased from 0.02 to 20 muW/cm2; i.e., contrast sensitivity remained unchanged over this 1,000-fold increase in mean irradiance. A decrement from the mean as brief as 2 s, however, disturbed the steady state. Another unexpected finding in this all-rod retina concerns surround-enhancement, a phenomenon observed previously for cone-mediated responses of horizontal cells in the retinas of turtle and catfish. While exposure to annular illumination induced response compression and a pronounced sensitivity loss in response to incremental light flashes delivered to the dark central region, the cell's sensitivity showed a significant increase when tested with a white noise or sinusoidally modulated central spot. Unlike horizontal cells in other retinas studied thus far, however, response dynamics remained unchanged. Responses evoked either by a small spot (0.25-mm diam) or by a large field light covering the entire retina were almost identical in time course. This is in contrast with past findings from cone-driven horizontal cells whose response waveform (dynamics) was dependent upon the size of the retinal area stimulated.     

Spatiotemporal testing and modeling of catfish retinal neurons.

The responses of retinal neurons depend on the interaction of both temporal and spatial aspects of a light stimulus. We developed a linear spatiotemporal model of receptor and horizontal cell layers in the catfish retina based on reciprocal interactions between both layers and coupling within each. Horizontal cell transfer properties were measured experimentally using white-noise intensity modulated light spots of different diameters and were compared with analytical predictions based on the model. Good agreement was obtained with a reasonable choice of model space-constants and feedback parameters. Furthermore, the same set of parameter values determined from spot experiments enabled accurate prediction of experimental horizontal cell responses to traveling gratings. The proposed feedback connections from horizontal cells to receptors quicken the time-course of responses in both layers and sharpen receptive fields.     

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

The spatial and color coding of the monophasic horizontal cells were studied in light- and dark-adapted retinae. Slit displacement experiments revealed differences in integration area for the different cone inputs of the monophasic horizontal cells. The integration area measured with a 670-nm stimulus was larger than that measured with a 570-nm stimulus. Experiments in which the diameter of the test spot was varied, however, revealed at high stimulus intensities a larger summation area for 520-nm stimuli than for 670-nm stimuli. The reverse was found for low stimulus intensities. To investigate whether these differences were due to interaction between the various cone inputs to the monophasic horizontal cell, adaptation experiments were performed. It was found that the various cone inputs were not independent. Finally, some mechanisms for the spatial and color coding will be discussed.     

The nature of surround-induced depolarizing responses in goldfish cones

Cones in the vertebrate retina project to horizontal and bipolar cells and the horizontal cells feedback negatively to cones. This organization forms the basis for the center/surround organization of the bipolar cells, a fundamental step in the visual signal processing. Although the surround responses of bipolar cells have been recorded on many occasions, surprisingly, the underlying surround-induced responses in cones are not easily detected. In this paper, the nature of the surround-induced responses in cones is studied. Horizontal cells feed back to cones by shifting the activation function of the calcium current in cones to more negative potentials. This shift increases the calcium influx, which increases the neurotransmitter release of the cone. In this paper, we will show that under certain conditions, in addition to this increase of neurotransmitter release, a calcium-dependent chloride current will be activated, which polarizes the cone membrane potential. The question is, whether the modulation of the calcium current or the polarization of the cone membrane potential is the major determinant for feedback-mediated responses in second-order neurons. Depolarizing light responses of biphasic horizontal cells are generated by feedback from monophasic horizontal cells to cones. It was found that niflumic acid blocks the feedback-induced depolarizing responses in cones, while the shift of the calcium current activation function and the depolarizing biphasic horizontal cell responses remain intact. This shows that horizontal cells can feed back to cones, without inducing major changes in the cone membrane potential. This makes the feedback synapse from horizontal cells to cones a unique synapse. Polarization of the presynaptic (horizontal) cell leads to calcium influx in the postsynaptic cell (cone), but due to the combined activity of the calcium current and the calcium-dependent chloride current, the membrane potential of the postsynaptic cell will be hardly modulated, whereas the output of the postsynaptic cell will be strongly modulated. Since no polarization of the postsynaptic cell is needed for these feedback-mediated responses, this mechanism of synaptic transmission can modulate the neurotransmitter release in single synaptic terminals without affecting the membrane potential of the entire cell.     

White noise analysis of graded response in a wind-sensitive,nonspiking interneuron of the cockroach

1. A novel approach using a Gaussian white noise as stimulus is described which allowed quantitative analysis of neuronal responses in the cercal system of the cockroach, Periplaneta americana. Cerci were stimulated by air displacement which was modulated by a sinusoidal and a white noise signal. During the stimulation, intracellular recordings were made from a uniquely identifiable, nonspiking, local interneuron which locates within the terminal abdominal ganglion. The white noise stimulation was cross-correlated with the evoked response to compute first- and second-order kernels that could define the cell's response dynamics. 2. The interneuron, cell 101, has an exceptionally large transverse neurite that connects two asymmetrical dendritic arborizations located on both sides of the ganglion. 3. The first-order Wiener kernels in cell 101 were biphasic (differentiating). The waveforms of the kernels produced by the ipsilateral and contralateral stimulations were roughly mirror images of each other: the kernels produced by wind stimuli on the side ipsilateral to the cell body of the interneuron are initially depolarized and then hyperpolarized, whereas those on the other side are initially hyperpolarized. The polarity reversal occurred along the midline of the animal's body, and no well-defined kernel was produced by a stimulus directed head on or from the tail. 4. Mean square error (MSE) between the actual response and the model prediction suggests that the linear component in cell 101 comprises half of the cell's total response (MSEs for the linear models were about 50% at preferred directions), whereas the second-order, non-linear component is insignificant. The linear component of the wind-evoked response was bandpass with the preferred frequency of 70-90 Hz. 5. Accounting for a noise, we reasonably assumed that at high frequencies the graded response in cell 101 is linearly related to a modulation of the air displacement and sensitive to the rate of change of the signal (i.e., wind velocity) and the direction of its source. It is suggested that the dynamics of the first-order kernel simply reflect the dynamics of sensory receptors that respond linearly to wind stimulation.     

Modulation of synaptic transfer between retinal cones and horizontal cells by spatial contrast

We studied the influence of steady annular light on the kinetics and sensitivity of horizontal cell (HC) responses to modulation of the intensity of small concentric spots in the turtle retina. As shown by previous investigators, when the intensity of the annulus was equal to the mean spot intensity, spot response kinetics were the same as those for the modulation of spatially uniform light. Turning off the annulus attenuated dramatically high-frequency flicker sensitivity and enhanced somewhat low-frequency sensitivity. This phenomenon reflects a modulation of synaptic transfer between cones and second-order neurons that is mediated by cones, and it will be referred to as cone-mediated surround enhancement (CMSE). Our main results are as follows: (a) The change in test-spot response sensitivity and kinetics upon dimming a steady surrounding annulus is a consequence of the change in spatial contrast rather than change in overall light level. (b) Introduction of moderate contrast between the mean spot intensity and steady surrounding light intensity causes a marked change in spot response kinetics. (c) The dependence of spot response kinetics on surrounding light can be described by a phenomenological model in which the steady state gain and the time constant of one or two single-stage, low-pass filters increase with decreasing annular light intensity (d) The effect of surrounding light on spot responses of a given HC is not determined by change in the steady component of the membrane potential of that cell. (e) Light outside the receptive field of an HC can affect that cell's spot response kinetics. (f) In an expanding annulus experiment, the distance over which steady annular light affects spot response kinetics varies among HCs and can be quite different even between two cells with closely matched receptive field sizes. (g) The degree of CMSE is correlated with HC receptive field size. This correlation suggests that part of the enhancement mechanism is located in the HC. Taken together, our results suggest the involvement of the inner retina in CMSE.     

Regulation of Transpiration in Avena. Responses to White Light Steps

The transient responses in transpiration rate of the primary leaf of 8-days-old oat plants were studied when step changes in the white light irradiance were administered to the plants. A light or dark step was achieved by an instant increase or decrease, respectively, of the irradiance on the leaf surface. Usually the light step responses consisted of one rapid response (rise time 1–2 min) and one slow response (rise time 5–10 min). The dark step response had a fall time of about 10 min. At a high irradiance level and in CO₂-free air the slow response disappeared. This could mean that the slow response is CO₂-controlled. The rapid response was still present under these conditions and is therefore probably caused by other processes than the slow response. In long time experiments several cycles of a circadian rhythm (period 26–28 h) could be followed in the light step responses. Thus the rapid response was small at that time of the day when the slow response was most pronounced and vice versa. The endogenous nature of the rhythm was established in phase-shift experiments. It is concluded that experiments on stomatal dynamics must take circadian variations into account.     

Filter characteristics of cercal afferents in the cockroach

Summary The response dynamics of cercal afferents in the cockroach, Periplaneta americana, were determined by means of a cross-correlation technique using a Gaussian white noise modulation of wind as a stimulus. The white noise stimulus could evoke sustained firing activity in most of the afferents examined (Fig. 1). The spike discharges were unitized and then cross-correlated with the stimulus to compute 1st- and 2nd-order Weiner kernels. The Ist-order kernels from a total of 28 afferents were biphasic and closely matched the time differential of a pulse (Figs. 1, 3 and 4). The amplitude and waveform of the kernels depended on the stimulus angle in such a way that the kernels were the mirror image of those on the polar opposite side (Figs. 2 and 3). The 2nd-order kernels were also differential. They had 2 diagonal peaks and 2 off-diagonal valleys in a 2-dimensional plot with 2 time axes (Figs. 1, 5 and 6). This 4-eye configuration was basically invariant irrespective of the stimulus angle, although the kernels varied in amplitude when the stimulus angle was changed. The time between the peak and a following trough of the 1st-order kernel was constant and had a mean of 4.6±0.1 ms, whereas the time between 2 diagonal peaks of the 2nd-order kernels was 4.7±0.1 ms (Figs. 4 and 6), suggesting that wind receptors (filiform sensilla) on cerci act as a band-pass filter with a peak frequency of about 106 Hz. The peak time, however, varies from 2.3 to 6.9 ms in both kernels, which may reflect the spatial distribution of the corresponding hairs on the cercus. The summation of the 1st- (linear) and 2nd-order (nonlinear) models precisely predicted the timing of the spike firing (Fig. 8). Thus, these 2 lower-order kernels can totally characterize the response dynamics of the wind receptors. The nonlinear response explains the directional sensitivity of the sensory neurons, while the differentiating 1st-order kernel explains the velocity sensitivity of the neurons. The nonlinearity is a signal compression in which one of the diagonal peaks of the 2nd-order kernel always offsets the downward phase of the 1st-order kernel (Fig. 7) and obviously represents a half-wave rectification property of the wind receptors that are excited by hair movement in only one direction and inhibited by hair movement in the polar opposite direction.     

VA opsin,melanopsin, and an inherent light response within retinal interneurons

BACKGROUND: Although photoreception is best understood in rods and cones, it is increasingly clear that these are not the only photoreceptive cells of the vertebrate retina. While considerable attention has been paid to the role of melanopsin in the generation of intrinsic light sensitivity in the retinal ganglion cells of mammals, nothing is known about the photoreceptive capacity of the horizontal cells of the fish retina in which both VA opsin and melanopsin are expressed. As yet, there has been little more than speculation as to the physiological function of these opsins within local retinal circuit neurons. RESULTS: VA opsin and melanopsin have been isolated and localized within the well-characterized cyprinid retina of the roach (Rutilus rutilus). Parallel electrophysiological studies identified a novel subtype of horizontal cell (HC-RSD) characterized by a depolarizing response that fits an opsin photopigment with a lambda(max) of 477 nm. The HC-RSD cells mediate responses to light that are characterized by long integration times, well beyond those observed for rods and cones. Significantly, HC-RSD responses persist when the conventional photoreceptor inputs are saturated by background light. CONCLUSIONS: The syncytium of coupled horizontal cells has long been considered to provide a signal of overall retinal irradiance. Our data suggest that this light information is, at least in part, derived from a population of intrinsically photosensitive VA opsin and/or melanopsin horizontal cells.     

Light-Induced Changes in Photoreceptor Membrane Resistance and Potential in Gecko Retinas : II. Preparations with Active Lateral Interactions

The time-course of light-induced changes in membrane voltage and resistance were measured in single photoreceptors in eyecup preparations of Gekko gekko. A small circular stimulus directed toward the impaled receptor produced membrane hyperpolarization. Application of a steady annular light to the receptor periphery resulted in diminution of the receptor's response to the stimulus. The effects of illumination of the surrounding receptors were isolated by directing a small, steady desensitizing light to the impaled receptor and then applying a peripheral stimulus. Brief stimuli produced a transient decrease in resistance with rapid onset and offset, a time-course similar to that of the response diminution. For some cells a depolarization that coincided with the resistance decrease was seen. During illumination with prolonged stimuli the resistance decrease was followed by a slow increase. After offset resistance rose transiently above the original value and then returned slowly to its original value. The slow resistance changes were not accompanied by changes in membrane voltage. The response diminution, resistance decrease, and depolarization were not observed in retinas treated with aspartate or hypoxia. It is therefore concluded that these effects are mediated by horizontal cells. The diminution is achieved by shunting the receptor potential and may play a role in field adaptation.     

Distinct responses of cones and melanopsin-expressing retinal ganglion cells in the human electroretinogram

ABSTRACT: BACKGROUND: The discovery of the novel photoreceptor, melanopsin-expressing retinal ganglion cells (mRGCs), has raised researchers' interest in photoreceptive tasks performed by the mRGC, especially in non-image-forming visual functions. In a prior study, we investigated the mRGC response to light stimuli independent of rods and cones with the four-primary illumination system, which modulates stimulus levels to the mRGC and cones independently, and mRGC baseline responses were recorded in the electroretinogram (ERG). METHODS: In the present study, we used the same illumination system to compare independent responses of the mRGC and cones in five subjects (mean +/- SD age, 23.0 +/- 1.7 years). The ERG waveforms were examined as direct measurements of responses of the mRGCs and cones to stimulation (250 msec). Implicit times (the time taken to peaks) and peak values from 30 stimuli given to each subject were analyzed. RESULTS: Two distinct positive peaks appeared in the mRGC response, approximately 80 msec after the onset of the stimuli and 30 msec after their offset, while no such peaks appeared in the cone response. The response to the mRGC stimulus was significantly higher than that to the cone stimulus at ~80 msec (p < 0.05) and tended to be higher than the cone stimulus at ~280 msec (p = 0.08). CONCLUSIONS: Implicit time of the first peak was much longer than that to the b-wave and this delay might reflect mRGC's sluggish responses. This is the first report of amplitudes and implicit time in the ERG from the response of the mRGC that is independent of rods and cones and obtained using the four-primary illumination system.     

Model of the cone-horizontal cell circuit in the catfish retina

A model of the cone-L-type horizontal cell circuit of the catfish contains 3 stages. The outer segment consists of a compression factor producing the Naka-Rushton relationship between amplitude of response and intensity and 7 low-pass filters in tandem that produces an absolute delay of about 15 ms. The cone pedicle consists of an internal negative feedback circuit in series with a low-pass filter. The L-type horizontal cell acts as a linear low-pass filter and forms the external negative feedback circuit with the cone pedicle. The system shows peicewise linearity with the feedback gain of the external negative feedback circuit directly proportional to the dc level of the horizontal cell. Thus, at any given mean illuminance the impulse response of the cone and L-HC adequately defines the dynamics of the responses. The conversion of a slow monophasic to a faster biphasic impulse response due to either an increase in mean illuminace or use of a steady annulus results from the change in the characteristic equation as the effective value of the feedback gain changes. By proper adjustement of gains and time constants, the cone-L-HC circuit of the catfish retina simulates the experimental data.     

White-noise analysis of nonlinear behavior in an insect sensory neuron: Kernel and cascade approaches

A functional expansion was used to model the relationship between a Gaussian white noise stimulus current and the resulting action potential output in the single sensory neuron of the cockroach femoral tactile spine. A new precise procedure was used to measure the kernels of the functional expansion. Very similar kernel estimates were obtained from separate sections of the data produced by the same neuron with the same input noise power level, although some small time-varying effects were detectable in moving through the data. Similar kernel estimates were measured using different input noise power levels for a given cell, or when comparing different cells under similar stimulus conditions. The kernels were used to identify a model for sensory encoding in the neuron, comprising a cascade of dynamic linear, static nonlinear, and dynamic linear elements. Only a single slice of the estimated experimental second-order kernel was used in identifying the cascade model. However, the complete second-order kernel of the cascade model closely resembled the estimated experimental kernel. Moreover, the model could closely predict the experimental action potential train obtained with novel white noise inputs.     

Dynamics of the ganglion cell response in the catfish and frog retinas

Responses were evoked from ganglion cells in catfish and frog retinas by a Gaussian modulation of the mean luminance. An algorithm was devised to decompose intracellularly recorded responses into the slow and spike components and to extract the time of occurrence of a spike discharge. The dynamics of both signals were analyzed in terms of a series of first-through third-order kernels obtained by cross-correlating the slow (analog) or spike (discrete or point process) signals against the white-noise input. We found that, in the catfish, (a) the slow signals were composed mostly of postsynaptic potentials, (b) their linear components reflected the dynamics found in bipolar cells or in the linear response component of type-N (sustained) amacrine cells, and (c) their nonlinear components were similar to those found in either type-N or type-C (transient) amacrine cells. A comparison of the dynamics of slow and spike signals showed that the characteristic linear and nonlinear dynamics of slow signals were encoded into a spike train, which could be recovered through the cross-correlation between the white-noise input and the spike (point process signals. In addition, well-defined spike correlates could predict the observed slow potentials. In the spike discharges from frog ganglion cells, the linear (or first-order) kernels were all inhibitory, whereas the second-order kernels had characteristics of on-off transient excitation. The transient and sustained amacrine cells similar to those found in catfish retina were the sources of the nonlinear excitation. We conclude that bipolar cells and possibly the linear part of the type-N cell response are the source of linear, either excitatory or inhibitory, components of the ganglion cell responses, whereas amacrine cells are the source of the cells' static nonlinearity.     

Ultraviolet light provides a major input to non-image-forming light detection in mice

The change in irradiance at dawn and dusk provides the primary cue for the entrainment of the mammalian circadian pacemaker. Irradiance detection has been ascribed largely to melanopsin-based phototransduction [1-5]. Here we examine the role of ultraviolet-sensitive (UVS) cones in the modulation of circadian behavior, sleep, and suprachiasmatic nucleus (SCN) electrical activity. UV light exposure leads to phase-shifting responses comparable to those of white light. Moreover, UV light exposure induces sleep in wild-type and melanopsin-deficient (Opn4(-/-)) mice with equal efficacy. Electrical recordings from the SCN of wild-type mice show that UV light elicits irradiance-dependent sustained responses that are similar to those induced by white light, with characteristic fast transient components occurring at the light transitions. These responses are retained in Opn4(-/-) mice and preserved under saturating photopic conditions. The sensitivity of phase-shifting responses to UV light is unaffected by the loss of rods but is severely attenuated by the additional loss of cones. Our data show that UVS cones play an important role in circadian and sleep regulation in mice.     

Taurine blocks spontaneous cone contraction but not horizontal cell dark suppression in isolated goldfish retina

The objective of this study was to investigate the effects of taurine on cone retinomotor movements and the responses of cone-driven horizontal cells in dark-adapted teleost retina. In isolated goldfish retina preparations maintained in the dark, cones spontaneously contracted, and the responses of horizontal cells were suppressed. Addition of 5 mM taurine to the physiological solution blocked the spontaneous contraction of cones in the dark but did not block the dark-suppression of horizontal cell responses. These results indicate that the mechanism that leads to horizontal cell dark suppression is not sensitive to taurine. Although both cone retinomotor position and horizontal cell responsiveness are known to be modulated by dopamine, the present results do not support the hypothesis that taurine inhibits dopamine release in the dark because only spontaneous cone contraction was affected by taurine. These results also indicate that spontaneous cone contraction in the dark is not the cause of horizontal cell dark suppression because, in the presence of taurine, cones were elongated yet horizontal cell responses were still suppressed. Consequently, these results make it clear that horizontal cell dark suppression is not an artifact produced by incubating isolated teleost retina preparations in taurine-free physiological solution.