EFFECTS OF NITRIC-OXIDE ON THE HORIZONTAL CELL NETWORK AND DOPAMINE RELEASE IN THE CARP RETINA

In the teleost retina the intercellular messenger nitric oxide can be synthesized by several cell types including cone photoreceptors and H1 horizontal cells, indicating a modulatory role within the outer plexi form layer, the first stage of the visual information processing. Ther efore, the aim of this study was to elucidate the effects of nitric ox ide on the physiology of cone horizontal cells in the intact retina. T he nitric oxide donor sodium nitroprusside (0.5-2.5 mM) enhanced the l ight responsiveness of cone horizontal cells and reduced the degree of electrical coupling in the network Furthermore, the spread of intrace llularly injected Lucifer Yellow was restricted. The effects on light responsiveness and electrical coupling were qualitatively mimicked by 8-bromo-cGMP (0.5 mM) and could not be achieved by ferrocyanide (1 mM) , the byproduct of nitric oxide liberation from nitroprusside. The eff ects of NO on the responsiveness of horizontal cells may be due to an action on green- and red-sensitive cones. Nitroprusside (0.1 mM) dimin ished the K+-stimulated release of endogenous dopamine by 50%, whereas the basal dopamine release was not affected, indicating that the effe cts on electrotonic horizontal cell coupling were not elicited by an N O-induced release of dopamine. With respect to the morphologic plastic ity of the cone-horizontal cell synapse the inhibitor of endogenous ni tric oxide synthesis L-nitroarginine (0.1 mM) had no influence on the formation or retraction of spinules. These results show that NO affect s the responsiveness and coupling of the horizontal cell network in a dopamine-independent way. (C) 1997 Elsevier Science Ltd.