Prostaglandin E 2 releases luteinizing hormone-releasing hormone from the female juvenile hypothalamus through a Ca 2+-dependent, calmodulin-independent mechanism

Abstract

Prostaglandin E 2 (PGE 2) has been implicated as a mediator of norepinephrine-induced luteinizing hormone-releasing hormone (LHRH) release from the hypothalamus. The present experiments were undertaken to examine the hypothesis that mobilization of Ca 2+ from intracellular stores and cyclic AMP (cAMP) formation are involved in this effect. Incubation of median eminence (ME) nerve terminals from juvenile rats in Ca 2+-free Krebs-Ringer bicarbonate buffer reduced, but failed to prevent the stimulatory effect of PGE 2 on LHRH release. None of 5 calmodulin antagonists or a blocker of calmodulin-dependent kinase affected the LHRH response to PGE 2. In contrast, inhibition of intracellular Ca 2+ mobilization with TMB-8 or dantrolene in Ca 2+-free medium prevented the LHRH releasing effect of PGE 2. Similarly to PGE 2, the stimulatory effect of the Ca 2+ ionophore A 23187 on LHRH release was not affected by inhibition of calmodulin activity. This ,however, blocked the increase in PGE 2 formation induced by the ionophore. PGE 2 evoked a dose-related increase in cAMP accumulation in Ca 2+-containing medium and this effect was inhibited both by blockers of intracellular Ca 2+ mobilization and by calmodulin antagonists. Surprisingly, removal of extracellular Ca 2+ increased basal cAMP levels in the incubation medium without affecting LHRH release; PGE 2 induced a further increase in cAMP which was prevented by inhibition of intracellular Ca 2+ translocation. Stimulation of adenylate cyclase activity with forskolin (F) resulted in similar increases in cAMP levels both in the presence and absence of extracellular Ca 2+. However, F failed to evoke LHRH release in Ca 2+-free medium. The results indicate that: (a) the stimulatory effect of PGE 2 on LHRH release involves mobilization of intracellular Ca 2+ but not the participation of calmodulin; (b) the formation of PGE 2 itself is calmodulin-dependent; (c) PGE 2 stimulates cAMP formation through a calmodulin-dependent mechanism that requiied translocation of intracellular (membrane?) Ca 2+; (d) the cAMP system of a population nerve terminals different from that responsive to PGE 2 is normally subjected to a Ca 2+-dependent inhibitory control; and (e) although PGE 2 is a potent stimulator of cAMP formation in the ME and edo genously produced cAMP can induce LHRH release, in the absence of extracellular Ca 2+ PGE 2 stimulates LHRH release in a cAMP-independent manner.