Activation of two different but complementary biochemical pathways stimulates release of hypothalamic luteinizing hormone-releasing hormone

Evidence exists that a norepinephrine/prostaglandin E₂ (PGE₂)/cAMP pathway is involved in the regulation of luteinizing hormone-releasing hormone (LHRH) secretion. The aim of the present experiments was to determine if release of LHRH from the immature rat hypothalamus could also be stimulated by activation of protein kinase C. Median eminences from 28-day-old female rats were incubated in vitro with either dioctanoylglycerol (a synthetic diacylglycerol that selectively activates protein kinase C in intact cells) or 4β-phorbol 12β-myristate 13α-acetate (another protein kinase C activator). Both agents increased LHRH release, the response to dioctanoylglycerol being more pronounced than that to the phrobol ester. This direct activation of protein kinase C was not accompanied by changes in PGE₂ formation. Activation of the PGE₂/cAMP pathway by either norepinephrine, PGE₂, or forskolin (a stimulator of adenylate cyclase) increased LHRH release. Dioctanoylglycerol or phorbol ester in conjunction with either norepinephrine, PGE₂, or forskolin resulted in an additive effect on LHRH release suggesting coexistence of both pathways. Phospholipase C, which activates protein kinase C via formation of diacylglycerol, increased the release of both LHRH and PGE₂. This suggests that an increase in endogenous phospholipase C activity caused by neurotransmitter inputs may lead to both activation of protein kinase C and PGE₂ formation. Blockade of cyclooxygenase activity by indomethacin obliterated phospholipase C-induced PGE₂ release. The same treatment reduced the LHRH response by only 50% indicating that protein kinase C activation can cause LHRH release in the absence of PGE₂ synthesis. It is suggested that the median eminence of the rat possesses a protein kinase C-dependent pathways that is coupled positively to LHRH release and complements PGE₂/cAMP-dependent mechanisms. Norepinephrine, however, does not appear to be the neurotransmitter responsible for activating the protein kinase C pathway. Simultaneous activation of both pathways may provide a mechanism by which a large increase in LHRH secretion occurs, such as in the afternoon of first proestrus.