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Chromaticity type horizontal cells (C-type HCs) are the first retinal neurons exhibiting spectral information processing in cold-blooded vertebrates. The simple input of hyperpolarizing responses of cone photoreceptors is transformed in the C-type HCs into spectral opponent output. Nitric oxide (NO), a known background neuromodulator in the distal retina, was tested here for its effects upon spectral information processing by C-type HCs in the retina of turtle. Photoresponses were intracellularly recorded from C-type HCs, using light stimuli of different wavelength, applied over backgrounds of different wavelengths, and changing retinal NO level. Raising retinal level of NO in darkness by adding the precursor for its synthesis (l-Arginine) augmented the depolarizing photoresponses elicited by long-wavelength light stimuli, and reduced the hyperpolarizing photoresponses elicited by short-wavelength light stimuli. Lowering retinal level of NO by l-NAME, an inhibitor of NO synthesis, induced the opposite effects. However, the total voltage range of operation remained constant regardless of the level of NO. Qualitatively similar effects were observed under background illuminations regardless of background strength and wavelength. Altering retinal level of NO exerted a small effect upon the null wavelength. Our findings are consistent with the known effects of NO upon turtle distal retinal neurons, with the addition of NO strengthening the negative feedback pathway from L-type horizontal cells onto medium-wavelength cones.

Action spectra were obtained by intracellular recordings from horizontal cells of the retina of turtle ( Pseudemys scripta ). Dark-adapted L 1 type horizontal cells received their major input from either red cones or from blue and green cones. With chromatic adaptation, red-cone dominated L 1 cells could be further subdivided on the basis of receptor input. They received input from only red cones (L 1 -R), red and green cones (L 1 -RG) or all cones, red, green and blue (L 1 -RGB). L 1 -RG and L 1 -RGB cells most likely receive their green-cone input from double cones. C-type horizontal cells were found to be of two types, biphasic and triphasic. Biphasic cells were depolarized by red light and hyperpolarized by green light (C-R/GB). Chromatic adaptation showed that blue and green cones acted together and opposed the activity of red cones. Triphasic C-type cells depolarized to green light and hyperpolarized to blue and red light (C-G /BR). Thus, blue cones acted with red cones and opposed the action of green cones. From these results it is concluded that: (1) red cones are the main determinant in the turtle luminosity system. (2) green cones and blue cones have their major role in the chromaticity system, (3) these results are consistent with the feedback hypothesis for the formation of C-type responses in turtle horizontal cells.

The horizontal cells of the rabbit retina have been studied by light microscopy of Golgi-impregnated whole-mount retinas. The two types of horizontal cell of the rabbit retina are similar to the horizontal cells of the cat retina in most respects. However, the majority of the A-type horizontal cells of the rabbit have asymmetrical dendritic fields compared to the circular, symmetrical dendritic fields of this cell type in the cat. The A-type horizontal cells of the superior edge of the linear visual streak in the rabbit retina are the most strikingly asymmetric and most of them are elongated and oriented in a direction approximately parallel to the linear visual streak. Like H1 axon terminals of the turtle retina the oriented, elongated A-type horizontal cells of the rabbit visual streak region may play a role in the neurocircuitry which underlies orientation sensitive ganglion cells.

The scotopic spectral sensitivity curve ofChrysemys picta picta was determined, using an instrumental technique. The results showed a maximum in the region of 460–500 nm, with a second maximum at about 640 nm in the red. There was also a significant interaction between the effects of wavelength and intensity on the rate of response. At low intensities, the rate of response was highest in the blue (460–500 nm), but at higher intensities it was greatest in the red (about 640 nm). These results show that the turtle retina is not a single-mechanism retina, but contains at least two receptors. One of these, most sensitive in the red, agrees with electrophysiological data on related species, and is probably a result of the oil droplets shifting the photopic sensitivity into the red. The second receptor, which has a lower threshold, is maximally sensitive in the blue.

The c-wave of the ERG was thought to be rod specific. We monitor the extracellular K + , [K + ] o , in the photoreceptor layer of the turtle retina (1) by K + electrodes and (2) by intraceilular recording from pigment epithelial cells which act as K + electrodes. Light-induced changes in [K + ] o give rise to a cone specific “fast c-wave ” as shown (a) by light-dependent changes in [K + ] o with spectral sensitivities matching green- and red-sensitive cones and (b) by the presence of the “fast c-wave ” in the ERG when the b-wave is selectively depressed. We propose an analogue electrical model of the photoreceptors by means of which both the rod- and. cone-specific c-waves are accounted for.

We have studied the effects of prolonged superfusion with l -aspartate and l -glutamate on the membrane potential and photoresponses recorded in luminosity type horizontal cells in the turtle retina using an everted eyecup preparation. These acidic amino acids produce effects which are a function of the past history of the impaled cell. Initial prolonged superfusions with 30 mM of either drug has no pronounced effect on the membrane potential and photoresponses of horizontal cells. Subsequent superfusions with either agent eventually produce depolarizations with reductions in the amplitudes of the light evoked responses. These effects, however, are transient; the horizontal cell rehyperpolarizes and the light evoked response grows with time. In a retina which had been stored at 4 degrees centrigrade for 20 hours, an initial superfusion with l -aspartate solution produced an immediate depolarization of the horizontal cell and complete suppression of the light evoked response for as long as the amino acid was present. The data are consistent with the existence of powerful amino acid uptake mechanisms operating at a number of sites within the inner and outer retina but also raise questions about the role of acidic amino acids in the outer plexiform layer of the turtle retina.

The kinetics of synaptic transfer from photoreceptors to second order neurons have been determined in the retina of the snapping turtle. Analysis of the fluctuations resulting from the random release of cone transmitter molecules reveals that the action on postsynaptic channels of the depolarizing bipolar cells is prolonged by approximately 10 times compared to the action on the hyperpolarizing bipolar cells. Determination of the cone and rod to horizontal cell impulse response shows that the kinetics of the cone/HIAT synapse is approximately 8–10 times shorter than the rod/HIAT synapse. These results indicate that the cone driven, sign preserving synapses exhibit comparable temporal characteristics.

The spectral and spatial properties of the photoresponses of luminosity horizontal cells were studied intracellularly in the turtle retina. A bright, full field, short-wavelength stimulus elicited a response which hyperpolarized to an initial transient peak and then sagged to an intermediate level. This transient on response could be eliminated by either decreasing the retinal area illuminated or by changing the color of the stimulus from green to red. The mechanism underlying the transient on response had a large receptive field ( ~ 1.7 mm diameter) and a short-wavelength input, probably from green cones. These findings are consistent with the hypothesis that lateral interactions between chromaticity- and luminosity-type horizontal cells exist in the turtle retina.

The spread of potentials along electrically-coupled horizontal cell networks in the turtle retina was studied at different levels of illumination. The mean values of the stationary space constant, lambda st, measured by the steady-state responses to a bar of light moved along the retina, is about 0.13 mm for the LII horizontal cell response and 0.35 mm for the LI response. lambda st is quite stable in the physiological range of membrane potentials. The dynamic space constant, lambda (t), measured at different intervals after the onset of the light bar, transiently increased during the rising phase of the on-response. Such a transient change of lambda (t) is absent or less apparent for depolarizing responses to the offset of the light bar. Ba2+ ions strongly enhanced the transient increase of lambda (t) for hyperpolarizing responses without affecting lambda (t) for depolarizing responses. Possible mechanisms of the transient increase of the dynamic space constant are discussed.

The multi-input Wiener analysis technique has been used to evaluate the modes of chromatic interactions in the turtle retina using intracellular recording and staining. Receptor, horizontal and bipolar cells displayed linear chromatic interactions. More proximal neurons have predominantly nonlinear responses and displayed nonlinear chromatic interactions. These observations make it clear that there are chromatic interactions at the inner plexiform layer of the retina in addition to those of the outer plexiform layer and that modes of chromatic interaction at the two synaptic layers are significantly different.

Action spectra were obtained by intracellular recordings from bipolar cells of the retina of turtle Pseudemys scripta. All bipolar cells responded independently to spots and annuli. Bipolar cells could be broken down into two main categories which differed in the dependence of centersurround interactions on changes in wavelength. In the first category, center-surround antagonism occurred regardless of the wavelengths used. Spectral analysis of these cells showed that both the center and surround received their major input from red cones. With chromatic adaptation, green cone input was detected in the center response of some cells. In the second category, activation of the surround was antagonistic to the center for some wavelengths and synergistic for others. These cells had a color-opponent center of the R/G type imbedded in a non-opponent surround. The surround spectral sensitivity curve could be matched by a simple linear sum of the spectral sensitivities of red-cone-dominated L1 horizontal cells and C-R/GB horizontal cells. Theories concerning the formation of bipolar cell receptive fields are discussed. It is concluded that simple feedback from red cones to green cones via L1 horizontal cells cannot fully account for the chromatic properties of the bipolar cells reported in this study.

We have recorded the electrical responses to red light and simultaneously marked with procion yellow the cells that give the L- and C-type of S-potential in the turtle retina. The L-type response originates in a horizontal cell with predominantly thick dendrites. The C-type response originates in a horizontal cell with predominantly thin dendrites which are located sclered to those of the L-type response cell. The L- and C-type cells as seen in procion-yellow marking are compared with cells of toluidine-blue stained sections.

Three types of horizontal cell have been identified in the turtle retina by means of intracellular recording, procion-yellow intracellular dye injection and Golgi silver-chromate histological impregnation. (a) The L1-type has predominantly thick dendrites. Its cell body is in the inner nuclear layer. Its potential changes are of the luminosity type and the cell shows a large area effect. (b) The L2-type has 10 or more fine dendrites which are of uniform diameter from their origin in the soma. The cell body is located near the outer plexiform layer. Its potential changes are of the luminosity type and the cell shows a small area effect. (c) The C-type cell has about five dendrites that are relatively thick at their origin in the soma and become progressively finer. The cell body is in the inner nuclear layer. Its potential changes are of the chromaticity type and the cell shows a small area effect.