Endothelins represent a family of recently discovered peptides that are secreted from endothelial cells cultured in vitro and depict potent vasoactive properties. Of the three endothelin (ET) genes identified, ET-3 seems to be the member of the family most abundant in the brain. Since ET- 3 immunoreactivity and receptors are present in the hypothalamus, the peptide could play a role in the modulation of neuroendocrine functions. In this respect, it has been recently shown that ET-3 may enhance LH secretion from pituitary cells in vitro, indicating a role for ET-3 in the regulation of reproductive functions. In these studies, we evaluate whether ET-3 participates in the central regulation of gonadal functions by controlling LHRH release. For this purpose, we analyzed the effects of the peptide on LHRH secretion using two experimental models: arcuate nucleus- median eminence (AN-ME) fragments incubated in vitro and immortalized LHRH neurons (GT1-7) incubated in a perfusion paradigm. ET-3 stimulated LHRH secretion from both AN-ME fragments incubated in a static system and GT1-7 cells in perfusion. In AN-ME incubations, ET-3-induced LHRH release appears to require PGE2 synthesis. ET-3-evoked LHRH secretion was concomitant with an increase in PGE2 release into the incubation medium; in addition, inhibition of PG synthesis with indomethacin blocked ET-3-induced secretion of both LHRH and PGE2. Furthermore, the effects of ET-3 on LHRH release were enhanced by potassium-evoked depolarization. This enhancement could not be explained by a simple additive effect of both secretagogues, indicating that the activation of other messenger systems may be required for obtaining a complete activation of the ET-3-related actions on LHRH terminals. Our data indicate that endothelins could be considered as modulators of LHRH secretion and that PGE2 is a possible intracellular mediator in these effects.
