Protein kinase C activators stimulate betaendorphin secretion from hypothalamic cells

Abstract

Relatively little is known about the regulation of secretion of hypothalamic beta-endorphin, the potent opioid that is believed to play a variety of physiological roles in brain. Previous work has shown that arginine vasopressin (AVP), which acts in brain primarily via activation of the phosphoinositol (PI) second messenger system, stimulates secretion of hypothalamic beta-endorphin. To test the hypothesis that activators of protein kinase C (PKC), which is activated following PI hydrolysis, stimulates secretion of beta-endorphins from hypothalamus, we studied the separate effects of stimulators of PKC including phorbol ester 12-myristate-13-acetate (PMA) and 1-oleolyl-2-acetyl glycerol (OAG — a diacyl glycerol analogue) on secretion of immunoreactive (IR-) beta-endorphin (measured by RIA) from dissociated fetal rat hypothalamic cell cultures. We also studied AVP and angiotensin II (Ang II), hypothalamic peptides which activate the PI second messenger pathway, and interactions of PMA and forskolin (FSK), an activator of the cyclic AMP/protein kinase A (PKA) pathway. PMA, OAG, AVP, and Ang II stimulated IR-beta-endorphin secretion. The stimulatory effect of both PMA and FSK on IR-beta-endorphin secretion was greater than that of PMA or FSK alone and was essentially additive. From these results we conclude that 1. (a) hypothalamic beta-endorphin secretion can be stimulated by activating PKC; 2. (b) AVP and Ang II are potential physiological regulators of hypothalamic beta-endorphin secretion; 3. (c) coupling of PKC activation and stimulation of hypothalamic IR-beta-endorphin secretion which is presumed at maturity (adulthood) originates during early development (fetal life); 4. (d) stimulating hypothalamic beta-endorphin secretion via PKC activation utilizes a distinct, different mechanism than stimulation of hypothalamic beta-endorphin secretion via PKA activation.