Alpha-melanocyte-stimulating hormone inhibits corticotropin-releasing factor release by blocking protein kinase C

Cytokine-induced release of corticotropin-releasing factor (CRF) from hypothalamic explants in vitro can be inhibited by femtomolar concentrations of alpha-melanocyte-stimulating hormone (alpha-MSH). Because the mechanism of the anticytokine action of alpha-MSH remains unknown, we examined if the peptide inhibits CRF release by interference with various steps in the activation of CRF release. Previous studies have shown that CRF release is induced by activation of phospholipase A2 (PLA2). Therefore, we examined the effect of alpha-MSH on the action of melittin (MEL), a PLA2 activator. After 60 min preincubation in Krebs-Ringer bicarbonate buffer, medial basal hypothalami were incubated for 30 min with Krebs-Ringer bicarbonate buffer or MEL with or without alpha-MSH (10(-11) to 10(-16) M). CRF release into the incubation medium was measured by RIA. As reported previously none of the alpha-MSH concentrations used changed basal CRF release nor did any concentration of alpha-MSH significantly alter CRF release induced by MEL (10 micrograms/ml). Thus, alpha-MSH alters cytokine-induced CRF release at a step unrelated to the activation of PLA2. Because activation of PLA2 requires an increase in intracellular calcium ion (Ca2+) concentrations, we evaluated the effect of alpha-MSH on the release of CRF induced by a high concentration of potassium (56 mM). This concentration of potassium induced a 3.5-fold increase in CRF release that was not affected by alpha-MSH. Protein kinase C (PKC) stimulates CRF release. Consequently, we examined the effect of alpha-MSH on CRF release induced by phorbol myristate acetate (PMA), which in the presence of Ca2+ stimulates PKC.(ABSTRACT TRUNCATED AT 250 WORDS)