ENDOGENOUS GLUTAMATE INVOLVEMENT IN PULSATILE SECRETION OF GONADOTROPIN-RELEASING-HORMONE - EVIDENCE FROM EFFECT OF GLUTAMINE AND DEVELOPMENTAL-CHANGES

The secretion of GnRH can be stimulated by glutamate (GLU) and GLU ago nists, whereas GLU receptor antagonists inhibit GnRH. Using 6-diazo-5- oxo-L-norleucine (DON), an inhibitor of glutaminase, we aimed to study the involvement of endogenous GLU in GnRH secretion through the effec ts of impaired GLU biosynthesis from its precursor glutamine (GLN). Gn RH secretion by hypothalamic explants of male rats, aged 15 and 50 day s, was compared, because the frequency of spontaneous GnRH secretory p ulses showed a 2-fold increase between those two ages. Using explants of 50-day-old rats, GLN elicited GnRH secretion in a similar dose-rela ted manner as GLU. DON prevented GLN-evoked secretion of GnRH, whereas the effect of GLU was not altered. DON also markedly inhibited sponta neous pulsatile secretion of GnRH and the secretory response to veratr idine, a Na+ channel opener. The inhibitory effect of DON on veratridi ne-evoked secretion of GnRH was directly related to the duration of ex posure to DON and the frequency of GnRH secretory episodes. Using expl ants of 15-day-old rats, GLN could elicit GnRH release, although this response was lower than GLU-evoked secretion of GnRH. The DON concentr ations required for inhibition of veratridine-evoked secretion of GnRH were lower at 15 days than at 50 days. These data indicate that 1) GL U biosynthesis from GLN is a prerequisite to the physiological mechani sm of pulsatile GnRH secretion; and 2) inhibition of veratridine- or G LN-induced secretion of GnRH requires higher DON concentrations after the onset of puberty than before. This suggests that glutaminase, the enzyme controlling GLU biosynthesis from GLN, shows increased activity after the onset of puberty when the frequency of pulsatile GnRH secre tion is increased as well.