gamma-Aminobutyric acid (GABA) is considered to be the most important inhibitory neurotransmitter in the central nervous system, including the retina. It has been shown that nitric oxide (NO) can influence the physiology of all retinal neuronal types, by mechanisms including modulation of GABA release. However, until now, there have been no data concerning the effects on endogenous GABA release of NO produced by cells in the intact retina. In the present study, we have investigated how NO production induced by drugs influences the release of endogenous GABA in cells of the intact retina of mature chicken. Retinas were exposed to different drugs that affect NO production, and GABA remaining in the tissue was detected by immunohistochemical procedures. A specific nNOS inhibitor (7-NI) reduced the number of GABA+amacrine cells and cells in the ganglion cell layer (GCL) by 33% and 41%, respectively. A GABA transporter inhibitor blocked this effect. L-arginine (100 microM), the precursor of NO, induced increases of 62% and 34% in the number of GABA+amacrine cells and GCL cells, respectively. A sodium (Na(+))-free solution, 7-NI and a PKG inhibitor prevented the effect of L-arginine (100 microM). However, a higher concentration of L-arginine (1mM) induced a 35% reduction in the number of GABA+cells by a Na(+)-dependent mechanism that was restricted to the GCL population. NMDA, which stimulates NO production, increased GABA release as indicated by 53% and 38% reductions in the number of GABA+amacrine cells and GCL cells, respectively. This effect was blocked by 7-NI only in GCL cells. We conclude that basal NO production and moderate NO production (possibly induced by L-arginine; 100 microM) inhibit basal GABA release from amacrine cells and GCL cells. However, NMDA or L-arginine (1mM) induce a NO-dependent increase in GABA release in GCL cells, possibly by stimulating higher NO production.

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