NITRIC-OXIDE INHIBITS HYPOTHALAMIC LUTEINIZING-HORMONE-RELEASING HORMONE-RELEASE BY RELEASING GAMMA-AMINOBUTYRIC-ACID

Nitric oxide synthase (NOS) containing neurons, termed NOergic neurons , occur in various regions of the hypothalamus, including the median e minence-arcuate region, which plays an important role in controlling t he release of luteinizing hormone-releasing hormone (LHRH). We examine d the effect of NO on release of gamma-aminobutyric acid (GABA) from m edial basal hypothalamic (MBH) explants incubated in vitro. Sodium nit roprusside (NP) (300 mu M), a spontaneous releaser of NO, doubled the release of GABA. This release was significantly reduced by incubation of the tissue with hemoglobin, a scavenger of NO, whereas hemoglobin a lone had no effect on the basal release of GABA. Elevation of the pota ssium concentration (40 mM) in the medium increased GABA release 15-fo ld; this release was further augmented by NP. Hemoglobin blocked the i ncrease in GABA release induced by NP but had no effect on potassium-i nduced release, suggesting that the latter is not related to NO. As in the case of hemoglobin, N-G-monomethyl-L-arginine (NMMA), a competiti ve inhibitor of NOS, had no effect on basal release of GABA, which ind icates again that NO is not significant to basal GABA release. However , NMMA markedly inhibited the release of GABA induced by high potassiu m, which indicates that NO plays a role in potassium-induced release o f GABA. In conditions in which the release of GABA was substantially a ugmented, there was a reduction in GABA tissue stores as well, suggest ing that synthesis of GABA in these conditions did not keep up with re lease of the amine. Although NO released GABA, there was no effect of the released GABA on NO production, for incubation of MBH explants wit h GABA had no effect on NO release as measured by [C-14]citrulline pro duction. To determine whether GABA had any effect on the release of LH RH from these MBH explants, GABA was incubated with the tissue and the effect on LHRH release was determined. GABA (10(-5) or 10(-6) M) indu ced a 70% decrease in the release of LHRH, indicating that in the male rat GABA inhibits the release of this hypothalamic peptide. This inhi bition in LHRH release induced by GABA was blocked by NMMA (300 mu M), which indicates that GABA converts the stimulatory effect of NO on LH RH release into an inhibitory one, presumably via GABA receptors, whic h activate chloride channels that hyperpolarize the cell. Previous res ults have indicated that norepinephrine stimulates release of NO from the NOergic neurons, which then stimulates the release of LHRH. The cu rrent results indicate that the NO released also induces release of GA BA, which then inhibits further LHRH release. Thus, in vivo the norepi nephrinergic-driven pulses of LHRH release may be terminated by GABA r eleased from GABAergic neurons via NO.