Nitric oxide (NO) activates soluble guanylyl cyclase (sGC) and the resulting increase in cyclic guanosine monophosphate (cGMP) is an important intracellular signalling pathway in the vertebrate retina. Immunocytochemical detection of cGMP following exposure to NO donors has proven an effective method of identifying cells that express sGC. While such an approach has proven useful for the study of several vertebrate retinas, it has not been applied to the well-characterized teleost retina. Therefore, in the present study, we have applied this approach to the retina of the goldfish (Carassius auratus). In the presence of the phosphodiesterase (PDE) inhibitor 3-isobutyl-1-methylxanthine (IBMX), incubation of goldfish eyecups in Ringer's solution containing (±)-S-nitroso-N-acetylpenicillamine (SNAP) increased cGMP-like immunoreactivity (cG-ir) in bipolar, horizontal, amacrine, and ganglion cells and in ganglion cell axons and optic nerve. Weak labeling was observed in horizontal cells but no change in cG-ir was noted within photoreceptors. The NO donor-stimulated increases of cG-ir in horizontal, bipolar, amacrine, and ganglion cells are consistent with known physiological effects of NO on these neurons. The physiological significance of NO action at the level of optic nerve is not known. The lack of an effect of SNAP on cG-ir in photoreceptors was unexpected, as there are known physiological actions of NO, mediated by cGMP, on these neurons. Although this may be due to insufficient sensitivity of immunolabeling, this result may indicate a difference between isoforms of sGC or cGMP PDE in these neurons, compared to neurons where exogenous NO increased cG-ir.

We investigated chromatic processing in the outer retina of the cichlid fish Aequidens pulcher. Intracellular recordings from cone-specific horizontal cells (CHCs) revealed that the two morphologically identified types (H1 and H2) also have different spectral responses. H1-L cells hyperpolarize to all wavelengths (“luminosity”). H2-Cb cells depolarize to long wavelengths and hyperpolarize to short wavelengths (“chromaticity”, biphasic). Furthermore, we verified by immunocytochemistry that H2-Cb cells of A. pulcher predominantly contact the middle-wavelength-sensitive (MWS) members of double cones. Developmental plasticity in the cone–CHC networks was induced by rearing fish under conditions of spectral deprivation and different levels of white light. H1-L spectral responses were unaffected by the rearing conditions. Different intensity levels of white light and deprivation of long wavelengths during rearing both induced changes in the spectral responses of H2-Cb. Deprivation of short and middle wavelengths had no effect. Our results indicate that spectral processing in the outer retina of fishes can be modulated in response to different visual experiences and suggest that developmental fine tuning of the color-vision system occurs at early levels of visual processing.

We model feedback from primary visual cortex to the dorsal lateral geniculate nucleus (dLGN). This feedback makes dLGN neurons sensitive to orientation discontinuity (Sillito et al., 1993; Cudeiro & Sillito, 1996). In the model, each dLGN neuron receives retinotopic input driven by layer 6 cortical neurons in a full set of orientation columns. Excitation is monosynaptic, while inhibition is through perigeniculate neurons and dLGN interneurons. The stimulus consists of drifting gratings, one within and the other outside a circular region centered over the receptive field of the model dLGN relay neuron we study. They appear as a single grating when they are aligned with equal contrast. The model reproduces experimental results showing an increasing inhibitory effect of feedback on the firing rate of dLGN neurons as the two gratings move towards the aligned position. Moreover, enhancement of dLGN cell center-surround antagonism by feedback is revealed by measuring the responses to drifting gratings inside a circular window, as a function of window radius. This effect is related to the observed length tuning of dLGN cells. Sensitivity to orientation discontinuity could be mediated in the model by feedback from either simple or complex cells. The model puts constraints on the feedback synaptic footprint and shows that its elongated shape does not play a crucial role in sensitivity to orientation discontinuity. The inhibitory component of feedback must predominate overall, but the feedback signal from a cortical neuron to a dLGN neuron with the same or nearby receptive-field center can be dominated by excitation. Predictions of the model include (1) robust stimuli for layer 6 cortical neurons give pronounced nonlinearities in the responses of dLGN neurons; (2) the sensitivity to orientation discontinuity at low contrast is twice that at high contrast.

The contribution of interocular orientation differences to depth perception, at either the neuronal or the psychophysical level, is unclear. To understand the responses of binocular neurons to orientation disparity, we extended the energy model of Ohzawa et al. (1990) to incorporate binocular differences in receptive-field orientation. The responses of the model to grating stimuli with interocular orientation differences were examined, along with the responses to random dot stereograms (RDS) depicting slanted surfaces. The responses to combinations of stimulus orientations in the two eyes were left–right separable, which means there was no consistent response to the binocular orientation difference. All existing neuronal data concerning orientation disparity can be well described by this type of model (even a version with no disparity selectivity). The disparity sensitive model is nonetheless sensitive to changes in RDS slant, although it requires narrow orientation bandwidth to produce substantial modulation. The disparity-insensitive model shows no selectivity to slant in this stimulus. Several modifications to the model were attempted to improve its selectivity for orientation disparity and/or slant. A model built by summing several disparity-sensitive models showed left–right inseparable responses, responding maximally to a consistent orientation difference. Despite this property, the selectivity for slant in RDS stimuli was no better than the simple disparity-selective model. The range of models evaluated here demonstrate that interocular orientation differences are neither necessary nor sufficient for signaling slant. In contrast, within the framework of the energy model, positional disparity sensitivity appears to be both necessary and sufficient.

Chromaticity (C-type) horizontal cells in the retina of cold-blooded vertebrates receive antagonistic inputs from cone photoreceptors of different spectral types leading to color opponency. The relative contribution of each spectral type of cones can be selectively altered by chromatic background illumination. Therefore, the spectral properties of C-type horizontal cells are expected to change when the intensity and color of ambient illumination are altered. In this study, we investigated the effects of chromatic background lights upon color opponency in Red/Green (RGH) and Yellow/Blue (YBH) C-type horizontal cells in the everted eyecup preparation of the turtle Mauremys caspica. Photoresponses were elicited by long-wavelength and short-wavelength light stimuli in the dark-adapted state and under conditions of chromatic background illumination. We found that the total voltage range, within which graded depolarizing and the hyperpolarizing photoresponses could be elicited, either increased or decreased depending upon the color of the background light. However, the maximal and minimal potential levels determined respectively by long-wavelength and short-wavelength light stimuli of supersaturating intensity remained unchanged, regardless of the wavelength and intensity of the background. These findings indicate that turtle C-type horizontal cells operate as push–pull devices. A sufficiently bright short-wavelength stimulus can push them all the way to the maximal hyperpolarizing level while a very bright long-wavelength stimulus can pull them towards the most depolarizing potential.

Observers performed red–green and yellow–blue hue cancellation tasks for a 0.8-deg circular test field on a dark surround, by manipulating the excitation level of one cone class while the other two classes were held constant. The results of the red–green judgments conformed to classical opponent color theory in that both L- and S-cone excitation levels were antagonistic to M-cone signals. The yellow–blue judgments revealed a nonmonotonic nonlinearity in which the S-cone signal could act either antagonistically or synergistically with M- and L-cone signals. These results demonstrate that fixed hue sensations should not be associated with a given class of cone, even at the level of opponent neural coding.

Rhodopsin phosphorylation is one of the key mechanisms of inactivation in vertebrate and invertebrate visual signal transduction. Here we report the cDNA cloning and protein characterization of a 70-kDa squid rhodopsin kinase, SQRK. The cDNA encoding the 70-kDa protein demonstrates high sequence identity with octopus rhodopsin kinase (92%) and mammalian β-adrenergic receptor kinases (63–65%), but only 33% similarity with bovine rhodopsin kinase, suggesting that invertebrate rhodopsin kinases may be structurally similar to β-adrenergic receptor kinases. This cDNA encodes three distinct modular domains: RGS, S/TKc, and PH domains. The native SQRK is an eye-specific protein that is only expressed in photoreceptor cells and the optic ganglion as determined by immunoblotting. Purified SQRK is able to phosphorylate both squid and bovine rhodopsin. Squid rhodopsin phosphorylation by purified SQRK was sensitive to both Mg2+ and GTPγS but was insensitive to Ca2+/CaM regulation. The ability of SQRK to phosphorylate rhodopsin was totally lost in the presence of SQRK-specific antibodies. Our results suggest that SQRK plays an important role in squid visual signal termination.

Flash stimulation elicits a visual evoked potential (VEP) as part of the electroencephalogram (EEG). This VEP is known to contain strong oscillatory activity around 120 Hz, which ceases 100 ms after the flash. It was unclear so far whether this time limit represents an averaging artifact due to loss of intertrial phase coherence or indicates a veridical cessation. Here we present results obtained from single-trial analysis of the EEG. These show that the oscillations exhibit virtually perfect phase locking and do in fact cease around 100 ms after the stimulus. Thus, the cessation of oscillatory activity in the VEP is not due to increasing intertrial phase jitter. Comparison with simultaneous retinal recordings exclude the possibility of direct crosstalk from the retina, but suggest that the oscillations are propagated from the retina to the cortex with a time lag of 48 ms.

Cadherins are homophilic cell adhesion molecules that control development of a variety of tissues and maintenance of adult structures. Although cadherins have been implicated in the development of the brain, including the visual system, in several vertebrate species, little is known of their role in zebrafish. In this study, we examined distribution of cadherin-2 (Cdh2, N-cadherin) in the visual system of developing and adult zebrafish using both immunocytochemical and in situ hybridization methods, and we compared Cdh2 distribution to that of the previously reported and closely related cadherin-4 (Cdh4, R-cadherin). As in other vertebrates, in zebrafish embryos Cdh2 was widely expressed in the early nervous system, but its expression became more restricted as development proceeded. Cdh4 was not detectable until later in development, at about the time when the first ganglion cells are generated. Cdh2 and Cdh4 were expressed in distinct regions of developing visual structures, including the lens. We hypothesize that the differential expression of these two cadherins in developing zebrafish visual structures reflects functionally different roles in the development of the vertebrate visual system.

The relatively sparse vasculature of the retina minimizes obstruction to incoming light, but also poses a challenge to fulfilling the metabolic demands of retinal neurons. An efficient process for distributing energy supplies to areas of need is likely to involve neuron-derived vasoactive signals. However, knowledge of the mechanisms by which capillary perfusion is regulated by neuron-to-vascular signaling is limited. Potential targets of vasoactive molecules released from nerve cells are the pericytes, which are positioned on the endothelial walls of microvessels and are thought to play a role in controlling the microcirculation. In this study, we assessed the effect of dopamine on pericyte physiology. Because dopaminergic neurites are closely associated with microvessels that express dopamine receptors, this molecule is a putative neuron-to-capillary signal, as well as neurotransmitter. We used the perforated-patch configuration of the patch-clamp technique to monitor the whole-cell currents of pericytes located on microvessels freshly isolated from the adult rat retina. In 43% (58/134) of the sampled pericytes, we found that dopamine reversibly activated a hyperpolarizing current, which increased the membrane potential by 19 ± 1 mV. This dopamine-induced current was inhibited by the ATP-sensitive potassium (KATP) channel blocker, glibenclamide. Consistent with a signaling pathway involving D1 dopamine receptors, adenylate cyclase and protein kinase A (PKA), the selective D1 antagonist, SCH23390, inhibited the hyperpolarizing effect of dopamine; the activator of adenylate cyclase, forskolin, mimicked the dopaminergic effect, and H89, which inhibits PKA, significantly reduced the hyperpolarization induced by dopamine. Taken together, our experiments indicate that a mechanism involving D1 dopamine receptors, adenylate cyclase, and PKA activates KATP currents in retinal pericytes. Our observations support the hypothesis that, in addition to being a neuromodulator, dopamine also serves as a signal linking neuronal activity with the function of the pericyte-containing microvasculature.

A sinusoidal mask grating oriented orthogonally to and superimposed onto an optimally oriented base grating reduces a cortical neuron's response amplitude. The spatial selectivity of cross-orientation suppression (XOR) has been described, so for this paper we investigated the temporal properties of XOR. We recorded from single striate cortical neurons (n = 72) in anesthetized and paralyzed cats. After quantifying the spatial and temporal characteristics of each cell's excitatory response to a base grating, we measured the temporal-frequency tuning of XOR by systematically varying the temporal frequency of a mask grating placed at a null orientation outside of the cell's excitatory orientation domain. The average preferred temporal frequency of the excitatory response of the neurons in our sample was 3.8 (± 1.5 S.D.) Hz. The average cutoff frequency for the sample was 16.3 (± 1.7) Hz. The average preferred temporal frequency (7.0 ± 2.6 Hz) and cutoff frequency (20.4 ± 6.9 Hz) of the XOR were significantly higher. The differences averaged 1.1 (± 0.6) octaves for the peaks and 0.3 (± 0.4) octaves for the cutoffs. The XOR mechanism's preference for high temporal frequencies suggests a possible extrastriate origin for the effect and could help explain the low-pass temporal-frequency response profile displayed by most striate cortical neurons.

Recent studies imaging nitric oxide (NO) production in the retina have indicated a much wider distribution of NO production than would be suggested by previous light-microscopic localizations of neuronal nitric oxide synthase (nNOS). To help resolve this discrepancy, the present study analyzed the ultrastructural localization of nNOS-like immunoreactivity (-LI) in all layers of the retina. In the ellipsoids of rod photoreceptors and the accessory elements of double cones, nNOS-LI was associated with some atypical mitochondria. In the outer plexiform layer, nNOS-LI was in some postsynaptic horizontal and bipolar cell processes at photoreceptor ribbon synapses. In some amacrine and ganglion cell somata, nNOS-LI was diffusely localized in the cytoplasm and associated with the endoplasmic reticulum. In the inner plexiform layer, nNOS-LI diffusely filled some amacrine cell processes, while in other amacrine cells nNOS-LI was selectively localized at the presynaptic specializations of conventional synapses. Neuronal NOS-LI was also found at membrane specializations in bipolar cell terminals that were distinct from their normal ribbon synapses. Finally, some nNOS-LI was found in mitochondria in Müller cells. The diverse subcellular localizations of nNOS-LI indicates that NO may play distinct functional roles in many retinal cells, which correlates well with the widespread NO production found in previous NO imaging studies.

The mosaics of S-cones and the neurons to which they are connected are relatively well characterized, so the S-cone system is a good vehicle for exploring how the sampling of the retinal image controls visual performance. We used an interferometer to measure the grating acuity of the S-cone system in the fovea and at a range of eccentricities out to 20 deg. We also developed a simple model observer that, by assuming only that cone pathways are noisy and that signals are subject to eccentricity-dependent postreceptoral pooling, predicts the measured acuities from the sampling properties of the S-cone mosaic. The amount of pooling required to explain performance is consistent with that suggested by anatomical and physiological measurements.

Cell responses to drifting Cartesian (parallel) and non-Cartesian (concentric, radial, and hyperbolic) stimuli were recorded in and beyond the classical receptive field (CRF) in the lateral geniculate nucleus (LGN), V1, and V2 of anesthetized monkeys. Many cells were equally responsive to Cartesian and non-Cartesian, especially concentric, gratings. Around 15% of cells in each area were significantly more responsive to concentric compared to parallel gratings; however, cells significantly more responsive to parallel compared to concentric gratings were more numerous in the cortex. While many cells responded to hyperbolic and radial gratings, few were most responsive to these gratings. Cell selectivity decreased for Cartesian and increased for non-Cartesian gratings from V1 to V2 and the relative response varied as a function of stimulus extent with respect to the CRF. Complex, nonoriented, nondirectional cells with a low aspect ratio (AR) responded best to non-Cartesian gratings. These results cannot be fully explained using Gabor linear/energy models of simple and complex receptive fields (RFs) although such models predict some cells to respond equally to Cartesian and non-Cartesian gratings. Cells significantly more responsive to non-Cartesian gratings can be accounted for by CRF selectivity influenced by modulation from the nonclassical receptive field (nCRF). The present study shows that Cartesian/non-Cartesian selectivity is not an emergent property of V4 cells but is present at all levels of early visual processing being subserved by a subset of cells with specific tuning properties.

Kang Derwent and Linsenmeier (2000) reported that the a- and b-waves of the cat electroretinogram (ERG) are resistant to hypoxemia, at least for PaO2 above 40 mm Hg. The a-wave may be resistant because the photoreceptor can increase glycolysis during hypoxemia. This hypothesis was tested by making the animal hypoglycemic, which should minimize the ability of the photoreceptor to switch to glycolysis. The ERG of dark-adapted anesthetized cats was recorded between an Ag/AgCl electrode in the vitreous humor and a reference electrode near the eye. Responses to bright flashes of diffuse white light were recorded at 3-min intervals during hypoxic episodes at three levels of blood glucose. The moderate hypoglycemia (50 to 70 mg/dl) had a relatively small effect on the a-wave amplitude. Furthermore, the a-wave amplitude increased transiently by 16 ± 7% within 30–40 min of the start of severe hypoglycemia (20 to 40 mg/dl), before recovering to near normal. Severe hypoglycemia alone decreased the b-wave amplitude by 15 ± 13%. Combined hypoxemia and hypoglycemia decreased the b-wave amplitude more than the a-wave amplitude. At all levels of blood glucose, b-wave decreases began at a higher PaO2 than the a-wave changes. For both the a- and b-waves, hypoxemic effects began at higher PaO2 when the animal was hypoglycemic. The increased sensitivity to hypoxemia during severe hypoglycemia suggests that a switch from oxidative to glycolytic metabolism ordinarily protects the retina from moderate hypoxemia. Because the a- and b-waves were affected to different degrees and at different PaO2s, it is unlikely that inner retinal changes are caused completely by changes in the photoreceptor.

Mechanisms that mediate the calcium influx in mammalian horizontal cells were studied. Horizontal cells (HCs) enzymatically dissociated from the rabbit retina were recorded by the whole-cell configuration of the patch-clamp technique and by calcium image ratioespectrophotometry of Fura-2 loaded cells. AMPA-preferring glutamate receptors were shown to permeate Ca2+ in mammalian HCs by ionic substitution experiments. Furthermore, after blocking the L-type calcium current with nifedipine (100 μM), calcium current through the AMPA-preferring glutamate receptors was measured. Calcium image ratioespectrophotometry was performed on the dissociated HCs in order to determine the changes in the intracellular calcium ([Ca2+]i). Fura-2 microspectrophotometry showed that in HCs, K+-induced cell depolarization promoted an increase in [Ca2+]i, mediated by the L-type calcium channels, since it was abolished in the presence of nifedipine. The increase in [Ca2+]i upon cell depolarization was observed throughout each cell; however, it was maximal at the cell soma. Activation of glutamate receptors in dissociated HCs by glutamate, AMPA or kainate promoted an increase in [Ca2+]i. This increase in [Ca2+]i was abolished in nominally Ca2+-free solution (0 mM Ca2+); in contrast, nifedipine decreased the glutamate-induced influx of calcium in ca. 50%. The present study demonstrates that calcium may permeate through glutamate receptors expressed in HCs of the rabbit retina.

In the probed-sinewave paradigm—used to study the dynamics of light adaptation—a small probe of light is superimposed on a sinusoidally flickering background. Detection threshold for the probe is measured at various times with respect to the flickering background. Here we present such stimuli using three methods: monoptic (the probe and the flickering background are presented to the same eye), dichoptic (the probe is presented to one eye and the flickering background is presented to the other eye), and binocular (the probe and the flickering background are both presented to both eyes). The results suggest that the processing associated with detecting the probe is primarily in the retina (or any place with monocular input). However, the results also suggest a slight amount of processing in the cortex (or any place with binocular input), particularly at the higher frequency of flickering background used here (9.4 Hz vs. 1.2 Hz). A simple schematic model with three ocular-dominance channels is consistent with the results.