Galanin is a paracrine inhibitor of gonadotroph function in the female rat

Recent evidence suggests that pituitary galanin synthesized in the lactotroph is a paracrine regulator of lactotroph proliferation and PRL secretion and that these effects are mediated via a pituitary-specific galanin receptor, GAL-R2(orig.). At this receptor subtype, the galanin fragment 3-29 is fully active, in contrast to both the cloned GAL-R1 and GAL-R2, at which this fragment is inactive. Since paracrine communication has been demonstrated between pituitary gonadotrophs and lactotrophs, we investigated the hypothesis that galanin is also a paracrine regulator of gonadotroph function. Galanin attenuated LHRH-stimulated LH release in a dose-dependent manner in monodispersed rat anterior pituitaries harvested at proestrus (LHRH 100 nM, 10.7 +/- 0.2 ng/ml(-1) x 4 h vs. LHRH 100 nM + 1 microM porcine galanin (pGal), 7.0 +/- 0.2 ng/ml(-1) x 4 h; P < 0.01; i.e. 37% reduction). Galanin had similar suppressive effects on FSH release. Galanin, also dose-dependently, attenuated the LHRH-stimulated LH release from perifused proestrous rat pituitary fragments. pGal (1 microM) reduced the stimulated LH release by 80%, [area under the curve (AUC), LHRH 100 nM, 713 +/- 149 vs. LHRH 100 nM + 1 microM pGal, 131 +/- 7 ng/min x ml(-1) x 4 h; P < 0.02]. In addition, galanin 3-29, the specific GAL-R2(orig.) receptor agonist, inhibited LHRH-stimulated LH release from perifused proestrous rat pituitary fragments [AUC, LHRH 100 nM, 642 +/- 77 ng/min x ml(-1) vs. LHRH 100 nM + pGal 1-29, 206 +/- 44 ng/min x ml(-1) (P < 0.02); and LHRH 100 nM + pGal 3-29, 310 +/- 19 ng/min x ml(-1) (P < 0.02)]. Immunoblockade with specific galanin antiserum potentiated the LHRH-stimulated release of LH by 48% from perifused proestrous rat pituitary fragments (AUC, LHRH 100 nM + galanin antiserum, 721 +/- 65 ng/min x ml(-1) vs. LHRH 100 nM alone or with nonimmune antiserum, 489 +/- 33 ng/min x ml(-1) or 545 +/- 46 ng/min x ml(-1), P < 0.05). This data suggests that galanin may act as a paracrine agent via the pituitary-specific GAL-R2(orig.) to inhibit gonadotroph function.