Maturation of the hypothalamic control of pulsatile gonadotropin-releasing hormone secretion at onset of puberty. I. Increased activation of N-methyl-D-aspartate receptors.

In the male rat the timing of puberty can be estimated by the rapid increase in testicular weight occurring between 25-50 days of age. We found that elongated spermatids, the most mature germ cells identified using flow cytometry, were first seen at 25 days (4% of the testicular cells), while an adult proportion (63%) was attained by 45 days of age. We have shown previously that hypothalamic explants could release GnRH in a pulsatile fashion at a frequency increasing around the age of 25 days, thus consistent with the time of onset of puberty. Since pulsatile GnRH secretion could be suppressed by MK-801, a noncompetitive antagonist of N-methyl-D-aspartate (NMDA) receptor activation, we postulated that an increased activation of those receptors could be involved in the neuroendocrine mechanism that activates pulsatile GnRH secretion at the onset of puberty. Such a concept was supported by the NMDA-induced release of GnRH, which was observed using 1 mM NMDA at 25 days, while a dose of 20-50 mM was required at 15 or 50 days of age. MK-801 could provide an index of NMDA receptor activation, since the antagonistic effect of MK-801 is use dependent. This particular property was confirmed by the inability of MK-801 (5 pM) to block the depolarization (veratridine)-induced release of GnRH in the presence of 0.001 mM NMDA, while partial or complete suppression was obtained in the presence of 0.1 and 10 mM NMDA, respectively. Using explants obtained at 5, 10, 15, 20, 25, 30, 35, and 50 days of age, the lowest concentrations of MK-801 that blocked the veratridine-induced release of GnRH were, respectively, 10(7), 10(7), 10(7), 10(3), 10, 10(2), 10(4), and 10(8) pM. In contrast, there was no age-related difference in sensitivity to the inhibitory effect of Mg2+, a noncompetitive NMDA receptor antagonist which is not use dependent. The pulsatile secretion of GnRH occurred at a similar frequency at 25 and 50 days of age (4.7 and 5.4 pulses/3.5 h, respectively) but it was suppressed by a lower MK-801 concentration at 25 days (10(4) pM) than at 50 days (10(8) pM). These data indicate that the NMDA receptors involved in the control of pulsatile GnRH secretion are markedly and transiently activated around the time of onset of puberty in the male rat.