Pulsatile release of gonadotropin-releasing hormone from hypothalamic explants is restrained by blockade of N-methyl-D,L-aspartate receptors

Abstract

We have shown previously that N-methyl-D,L-aspartate (NMDA) and kainate, two neuroexcitatory amino acids acting through distinct receptors, may induce the release of GnRH from hypothalamic explants. However, that effect could have no physiological significance, since very high concentrations (50 mM) of NMDA and kainate were required. Here, using agents blocking the activation of receptors to neuroexcitatory amino acids, we evaluated their possible physiological involvement in the pulsatile release of GnRH from the hypothalamus of 50-day-old male rats in vitro. In control conditions (10 nM glycine and 1 mM mg2+), the release of GnRH in 7.5-min fractions collected for 2-4 h showed an obvious pulsatile pattern. The mean (+/- 1 SD) interval between pulses, identified by PULSAR program, was 34.3 +/- 11.4 min. The stimulation of GnRH release by NMDA (50 mM) added to the medium for 7.5 min could be blocked reversibly in the presence of MK-801 (100 microM) using medium without glycine or enriched with Mg2+ (2 mM). The endogenous pulses of GnRH secretion were abolished in the presence of MK-801 or using increased Mg2+ concentrations as well as in the absence of glycine. In contrast, pulsatile release of GnRH was not affected in the presence of 6,7-dinitroquinoxaline-2,3-dione (0.1 mM), a selective inhibitor of kainate and quisqualate receptors which suppressed the increase in GnRH release induced by kainate (50 mM) without affecting the response to NMDA. These data indicate that the physiological mechanism of pulsatile GnRH secretion in the hypothalamus may involve endogenous neuroexcitatory factors acting through NMDA-sensitive receptors.