N-methyl-D-aspartate (NMDA) receptors mediate the release of gonadotropin-releasing hormone (GnRH) by NMDA in a hypothalamic GnRH neuronal cell line (GT1-1).

Abstract

Previous studies demonstrated that an excitatory amino acid analog, N-methyl-D-aspartate (NMDA), stimulates GnRH secretion in the rat, prepubertal primate, and ovine fetus at the hypothalamic level. It is not known if this stimulatory effect of NMDA is mediated directly on the GnRH neurosecretory neuron. A hypothalamic GnRH neuronal cell line (GT1-1) provided a useful model system to study the effect of NMDA on GnRH release by both superfusion and static incubation techniques. Studies with GT1-1 cells indicate that GnRH neurons exhibit spontaneous autorhythmicity and function intrinsically as a neuronal oscillator for the synchronous discharge of GnRH pulses. In static incubation studies, 10(-4) and 10(-3) M NMDA increased GnRH release, whereas 10(-6), 10(-5), and 10(-2) M NMDA had no effect. A competitive NMDA receptor antagonist, AP-5 (10(-4)-10(-2) M), and a noncompetitive NMDA receptor antagonist, MK-801 (10(-12)-10(-5) M), inhibited the action of NMDA. Superfusion of GT1-1 cells after a 90-min control period followed by either continuous NMDA or intermittent NMDA (10(-4) and 10(-3) M) for 90 min increased GnRH pulse amplitude by 100-400%, but had no effect on the interpulse interval (17 min by Cluster); 10(-6), 10(-5), and 10(-2) M NMDA had no effect on either pulse amplitude or interpulse interval. MK-801 (10(-5) M) attenuated the stimulatory effect of NMDA on GnRH pulse amplitude. Incubation in glycine-free and high magnesium medium abolished the action of NMDA on GnRH release. Hybridization analysis of GT1-1 mRNA with an NMDA R1 receptor cDNA showed that this pure neuronal cell line expressed NMDA receptor transcripts observed as a 4.2-kilobase band. The results demonstrate that NMDA stimulates GnRH neurons directly to secrete GnRH through their NMDA receptors by increasing pulse amplitude without affecting pulse frequency.