Early prepubertal ontogeny of pulsatile gonadotropin-releasing hormone (GnRH) secretion: I. Inhibitory autofeedback control through prolyl endopeptidase degradation of GnRH

GnRH((1-5)), a subproduct resulting from degradation of GnRH by prolyl endo peptidase (PEP) and endopeptidase 24.15 (EP24.15) was known to account for an inhibitory autofeedback of GnRH secretion through an effect at the N-met hyl-D-aspartate (NMDA) receptors. This study aimed at determining the possi ble role of such a mechanism in the early developmental changes in frequenc y of pulsatile GnRH secretion. Using retrochiasmatic explants from fetal ma le rats (day 20-21 of gestation), no GnRH pulses could be observed in vitro , whereas pulses occurred at a mean interval of 86 min from the day of birt h onwards. This interval decreased steadily until day 25 (39 min), during t he period preceding the onset of puberty. Based on GnRH((1-10)) or GnRH((1- 9)) degradation and GnRH((1-5)) generation after incubation with hypothalam ic extracts, EP24.15 activity did not change with age, whereas PEP activity was maximal at days 5-10 and decreased subsequently until day 50. These ch anges were consistent with the ontogenetic variations in PEP messenger RNAs (mRNAs) quantitated using RT-PCR. Using fetal explants, the NMDA-evoked re lease of GnRH was potentiated in a dose-dependent manner by bacitracin, a c ompetitive PEP inhibitor and the desensitization to the NMDA effect was pre vented using 2 mM of bacitracin. At day 5, a higher bacitracin concentratio n of 20 mM was required for a similar effect. Pulsatile GnRH secretion from fetal explants was not caused to occur using bacitracin or Fmoc-Prolyl-Pyr rolidine-2-nitrile (Fmoc-Pro-PyrrCN), a noncompetitive PEP inhibitor. At po stnatal days 5 and 15, a significant acceleration of pulsatility was obtain ed using 1 mu M Of Fmoc-Pro-PyrrCN or 2 mM of bacitracin. At 25 and 50 days , a lower bacitracin concentration of 20 mu M was effective as well in incr easing the frequency of GnRH pulsatility. We conclude that the GnRH inhibit ory autofeedback resulting from degradation of the peptide is operational i n the fetal hypothalamus but does not explain the absence of pulsatile GnRH secretion at that early age. After birth, PEP activity is high and may acc ount for the low frequency of pulsatility. The potency of that effect decre ases before the onset of puberty and may contribute to the acceleration of GnRH pulsatility.