Reversal potentials of color opponent responses in horizontal cells of the carp retina

Histochemistry of NADPH-diaphorase--a marker for neuronal nitric oxide synthase--in the carp retina

The nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase histochemical technique was used as a marker to assess the distribution of nitric oxide synthase activity in the carp retina. NADPH-diaphorase activity was found to be present in photoreceptors (rods and cones), horizontal cells, amacrine cells, bipolar cells, Müller cells and ganglion cells. Staining was most prominent in the photoreceptor ellipsoids but was not confined to any particular subtype. The density of the staining within the inner plexiform layer (IPL) was determined by image analysis. There was a broad peak of activity in each sublamina of the IPL, but sublamina b appeared to be relatively more heavily stained. The results taken together suggest that the nitric oxide signalling system could have a broader involvement in retinal function than previously thought. Furthermore, nitric oxide may have a novel mode of action in the retina whereby it could be effective on cells (photoreceptors) that also synthesize it.

Long wavelength-middle wavelength cone interaction under no background in the electroretinogram of the cynomolgus monkey

We recorded electroretinograms from anesthetized cynomolgus monkeys for 30 monochromatic (400-700 nm) full-field stimuli with five different stimulation levels presented without a background light. Waveforms were analyzed by means of principal component analysis to investigate the influence of stimulus intensity on long- and middle-wavelength cone interaction. With increasing levels of stimulus intensity, the middle-wavelength cone system showed a slight peak shift and a noticeable increase in response, while the long-wavelength cone system showed an obvious change in peak wavelength rather than response. In addition, spectral responses in the long-wavelength cone system increased over the red region, whereas they decreased over the yellow region. On the other hand, responses in the middle wavelength cone system increased over the yellow region much more than over the green region. These complicated changes are thought to indicate the induction of long- and middle-wavelength cone interaction by stimulus light.

Synaptic feedback, depolarization, and color opponency in cone photoreceptors

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