EFFECT OF OVARIAN HORMONES ON THE HYPOTHALAMIC EXCITATORY AMINO-ACIDSSYSTEM DURING SEXUAL-MATURATION IN FEMALE RATS

The present experiments were designed to study in female rats during s exual maturation: (1) the hypothalamic release of aspartate (Asp), glu tamate (Glu) and glycine (Gly) which are the excitatory amino acids (E AAs) involved in NMDA neurotransmission and of taurine (Tau), a putati ve inhibitory amino acid of GnRH secretion; (2) the relationships betw een the effect of estrogen-progesterone (EP) on the release of these E AAs and the secretion of gonadotropins, and (3) the effect of hypothal amic NMDA receptor stimulation on EAAs release by the hypothalamus as well as the effect of EP on this release. The release of EAAs by the a nterior preoptic and medial-basal hypothalamic areas (APOA-MBH) is sig nificantly higher in peripubertal than in prepubertal rats (p<0.01). E P treatment in prepubertal rats (16 days of age) decreased LH and FSH plasmatic levels and also the in vitro release of Asp, Glu, Gly and Ta u. Contrary to the observations in prepubertal rats, in 30-day-old per ipubertal rats the ovarian hormones significantly (p<0.01) increased t he levels of LH and FSH as well as the release to the medium of these amino acids. In prepubertal rats, the addition of NMDA to the incubati on media significantly increased the release of Asp and of Glu. The NM DA receptor agonist did not modify the hypothalamic release of Gly and Tau to the incubation media. On the other hand, pretreatment with EP did not modify either the Asp and Glu release response to NMDA or the lack of response on Gly and Tau. In peripubertal rats NMDA also signif icantly increased the in vitro release of Asp and Glu, but at this age pretreatment with EP significantly potentiated the release of these a mino acids induced by NMDA. On the other hand, the ovarian hormones in duced Gly and Tau release by NMDA, this not being observed in control rats. In conclusion, the present results indicate that during sexual m aturation the in vitro release of Asp, Glu and Gly by APOA-MBH increas es and Tau release decreases. EP pretreatment of prepubertal rats decr eases the hypothalamic release of EAAs as well as the serum LH and FSH levels but increases both EAAs release and plasmatic gonadotropin in peripubertal rats. On the basis of these results it is proposed that t he increase in EAAs release by the hypothalamus is directly connected with the onset of puberty and that the maturation of the positive feed back effect of ovarian hormones on gonadotropin secretion is related t o the maturation of the capacity of EP to increase hypothalamic EAAs. Before this maturational event EP inhibits EAAs release as well as gon adotropin release (prepubertal rats). NMDA receptor stimulation leads to a positive mechanism which increases the release of Asp and Glu fro m APOA-MBH both in prepubertal and peripubertal rats, but EP potentiat es this mechanism only in peripubertal rats. This could be an addition al neuroendocrine mechanism involved in the increase of gonadotropin d uring sexual maturation which induces the onset of puberty and the pre ovulatory discharge of these pituitary hormones.