Prostaglandin E₂ releases luteinizing hormone-releasing hormone from the female juvenile hypothalamus through a Ca²⁺-dependent, calmodulin-independent mechanism

Prostaglandin E₂ (PGE₂) 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 Ca²⁺ from intracellular stores and cyclic AMP (cAMP) formation are involved in this effect. Incubation of median eminence (ME) nerve terminals from juvenile rats in Ca²⁺-free Krebs-Ringer bicarbonate buffer reduced, but failed to prevent the stimulatory effect of PGE₂ on LHRH release. None of 5 calmodulin antagonists or a blocker of calmodulin-dependent kinase affected the LHRH response to PGE₂. In contrast, inhibition of intracellular Ca²⁺ mobilization with TMB-8 or dantrolene in Ca²⁺-free medium prevented the LHRH releasing effect of PGE₂. Similarly to PGE₂, the stimulatory effect of the Ca²⁺ ionophore A 23187 on LHRH release was not affected by inhibition of calmodulin activity. This,however, blocked the increase in PGE₂ formation induced by the ionophore. PGE₂ evoked a dose-related increase in cAMP accumulation in Ca²⁺-containing medium and this effect was inhibited both by blockers of intracellular Ca²⁺ mobilization and by calmodulin antagonists. Surprisingly, removal of extracellular Ca²⁺ increased basal cAMP levels in the incubation medium without affecting LHRH release; PGE₂ induced a further increase in cAMP which was prevented by inhibition of intracellular Ca²⁺ translocation. Stimulation of adenylate cyclase activity with forskolin (F) resulted in similar increases in cAMP levels both in the presence and absence of extracellular Ca²⁺. However, F failed to evoke LHRH release in Ca²⁺-free medium. The results indicate that: (a) the stimulatory effect of PGE₂ on LHRH release involves mobilization of intracellular Ca²⁺ but not the participation of calmodulin; (b) the formation of PGE₂ itself is calmodulin-dependent; (c) PGE₂ stimulates cAMP formation through a calmodulin-dependent mechanism that requiied translocation of intracellular (membrane?) Ca²⁺; (d) the cAMP system of a population nerve terminals different from that responsive to PGE₂ is normally subjected to a Ca²⁺-dependent inhibitory control; and (e) although PGE₂ is a potent stimulator of cAMP formation in the ME and edo genously produced cAMP can induce LHRH release, in the absence of extracellular Ca²⁺ PGE₂ stimulates LHRH release in a cAMP-independent manner.