Pituitary adenylate cyclase-activating polypeptide effects in pituitary cells: modulation by gonadotropin-releasing hormone in alpha T3-1 cells.

Pituitary adenylate cyclase-activating polypeptide (PACAP) acts via type I receptors in the pituitary to stimulate cAMP production. Gonadotropes are likely target cells for PACAP action, and we have recently shown alpha T3-1 cells, a clonal gonadotrope-derived cell line, to be PACAP responsive. Here we have explored the influence of GnRH on PACAP action in alpha T3-1 cells and show that PACAP38-stimulated cAMP production is inhibited by GnRH in both the presence and the absence of a phosphodiesterase inhibitor. This effect appears not to be Ca++ mediated but is mimicked by protein kinase C activation with phorbol 12-myristate 13-acetate. However, GnRH and phorbol 12-myristate 13-acetate do not inhibit binding of [125I]PACAP27 to intact alpha T3-1 cells, nor do they inhibit forskolin- or cholera toxin-stimulated cAMP accumulation, implying that the inhibitory effects are exerted at early stages in the PACAP receptor signaling pathway but distal to receptor occupancy. When cells were preincubated with PACAP38, extensive washing failed to prevent the stimulatory effect of the polypeptide presumably because of the slow rate of receptor-ligand dissociation. However, when the time course of PACAP38-stimulated effects on intracellular cAMP was assessed, the stimulatory effect of PACAP38 could be rapidly reversed by GnRH addition, and the inhibitory effect of GnRH was rapidly be reversed by a GnRH receptor antagonist. The data provide the first demonstration of cross-talk between phospholipase C and adenylate cyclase-activating peptides in gonadotrope-derived cells and establish the potential for hormonal modulation of PACAP action. We suggest that this inhibitory effect of GnRH might enable the releasing hormone to control the kinetics of cAMP signaling in gonadotropes in vivo.