Autocrine regulation of gonadotropin-releasing hormone secretion in cultured hypothalamic neurons

Episodic hormone secretion is a characteristic feature of the hypothalamo-p ituitary-gonadal system, in which the profile of gonadotropin release from pituitary gonadotrophs reflects the pulsatile secretory activity of GnRH-pr oducing neurons in the hypothalamus. Pulsatile release of GnRH is also evid ent in vitro during perifusion of immortalized GnRH neurons (GT1-7 cells) a nd cultured fetal hypothalamic cells, which continue to produce bioactive G nRH for up to 2 months. Such cultures, as well as hypothalamic tissue from adult rats, express GnRH receptors as evidenced by the presence of high-aff inity GnRH binding sites and GnRH receptor transcripts. Furthermore, indivi dual GnRH neurons coexpress GnRH and GnRH receptors as revealed by double i mmunostaining of hypothalamic cultures. In static cultures of hypothalamic neurons and GT1-7 cells, treatment with the GnRH receptor antagonist, [D-pG lu(1), D-Phe(2), D-Trp(3,6)]GnRH caused a prominent increase in GnRH releas e. In perifused hypothalamic cells and GT1-7 cells, treatment with the GnRH receptor agonist, des-Gly(10)-[D-Ala(6)] GnRH N-ethylamide, reduced the fr equency and increased the amplitude of pulsatile GnRH release, as previousl y observed in GT1-7 cells. In contrast, exposure to the GnRH antagonist ana logs abolished pulsatile secretion and caused a sustained and progressive i ncrease in GnRH release. These findings have demonstrated that GnRH recepto rs are expressed in hypothalamic GnRH neurons, and that receptor activation is required for pulsatile GnRH release in vitro. The effects of GnRH agoni st and antagonist analogs on neuropeptide release are consistent with the o peration of an ultrashort-loop autocrine feedback mechanism that exerts bot h positive and negative actions that are necessary for the integrated contr ol of GnRH secretion from the hypothalamus.