Nitric oxide synthesized by gonadotropin-releasing hormone neurons is a mediator of N-methyl-D-aspartate (NMDA)-induced GnRH secretion

N-Methyl-D-aspartate (NMDA) directly stimulates gonadotropin-releasing hormone (GnRH) neurons to secrete GnRH. It is not known if this stimulatory effect of NMDA is mediated by NO. Northern blot analysis of the immortalized hypothalamic GnRH neuronal cells (GT1-1) mRNA with a neuronal NOS cDNA revealed this clonal cell line expressed neuronal NOS transcripts as a single 10.5-kb band. Immunoblot analysis of GT1-1 proteins with anti-neuronal NOS serum showed that the GT1-1 cells contain neuronal NOS. GT1-1 cells were used to study the effects of NO and NMDA on GnRH release. L-Arginine (10^-2M), a precursor of NO enhances basal GnRH secretion. Both oxyhemoglobin (Hb)(10^{- 6}-10^-4M), a NO scavenger and N(ω)-nitro-L-arginine (NNA)(10^-3,10^{- 2}M), a NOS inhibitor and inactivator block basal as well as NMDA-induced GnRH release. Sodium nitroprusside (SNP) (10^-4, 10^-3M), a NO donor stimulates GnRH release, an effect inhibited by Hb. Incubation of GT1-1 cells in Ca²⁺-free medium abolished the stimulatory effect of NMDA on GnRH release. In contrast, incubation in medium with increasing concentrations of Ca²⁺ enhances basal GnRH release as well as augments NMDA-mediated GnRH release. The results demonstrate that L-arginine-NO pathway is functional in the GT1-1 cells and an increase in intracellular Ca²⁺ [Ca²⁺](i) following NMDA receptor activation activates NOS to generate NO. We conclude that endogenous NO mediates, at least in part, basal as well as NMDA-stimulated GnRH release and may play a role as an intercellular messenger in synchronizing pulsatile GnRH release.