Duality of glutamatergic and gabaergic control of pulsatile GnRH secretion by rat hypothalamic explants .2. Reduced NR2C- and GABA(A)-receptor-mediated inhibition at initiation of sexual maturation

N-methyl-D-aspartate (NMDA) receptors and gamma-aminobutyric acid (GABA) receptors are involved in the mechanism of pulsatile gonadotrophin-releasing hormone (GnRH) secretion. The aim of this study was to elucidate the role of those receptors in the acceleration of pulsatile GnRH secretion seen at onset of puberty. Using hypothalamic explants from prepubertal (15 days), early pubertal (25 days) and adult (50 days) male rats, we studied the effects of pharmacological antagonists and antisense oligodeoxynucleotides on GnRH release evoked by NMDA and GABA receptor agonists as well as the interval between spontaneous GnRH secretory pulses. At the three studied ages, the muscimol-evoked release of GnRH is similarly inhibited by the GABA(A) receptor antagonist bicuculline. In contrast, the frequency of pulsatility is stimulated by bicuculline as indicated by a reduction of the mean GnRH interpulse interval from 60 to 40 min and such an effect is seen at 15 days only. The GnRH interpulse interval is also reduced by GABA, receptor antisense oligodeoxynucleolides at 15 days while no effects are seen at 25 days. At the three studied ages, the NMDA-evoked release of GnRH and the GnRH interpulse interval are similarly inhibited by 100 or 500 mu M of the NMDA receptor antagonist 7-chlorokynurenic acid (7CK). These effects are consistent with the increase of GnRH interpulse interval caused by NR2A antisense oligodeoxynucleotides at 15 days (86 vs 64 min in controls) as well as 25 days (44 vs 36 min). A low (5 mu M) concentration of 7CK does not result in any effect except a reduction of GnRH interpulse interval which is seen at 15 days only. A similar reduction of GnRH interpulse interval is obtained using NR2C antisense oligodeoxynucleotides at 15 days (50 vs 64 min in controls) while no effects are seen at 25 days (35 vs 36 min). At 25 days, muscimol can prevent the developmental increase in frequency of pulsatile GnRH secretion. In summary, pulsatile GnRH secretion by the prepubertal hypothalamus characteristically involves an inhibition mediated through GABA(A) receptors and the NR2C subunit of NMDA receptors. Based on these data, we propose a model for the mechanism of the onset of puberty which involves the disappearance or inactivation of GABAergic neurons located in the retrochiasmatic hypothalamus and expressing the NR2C subtype of NMDA receptors.