Effects of morphine and beta-endorphin on basal and elevated plasma levels of alpha-MSH and vasopressin.

Morphine induced an increase of plasma α-MSH levels and a decrease of AVP levels after peripheral or intracerebroventricular administration. This increase of α-MSH levels and decrease of AVP levels after morphine treatment was observed in non-stimulated animals as well as in rats in which the hormone levels were elevated by water deprivation or by administration of hypertonic saline. These latter effects of morphine on plasma levels of α-MSH and AVP could be blocked by simultaneous administration of naltrexone.β-Endorphin also increased plasma α-MSH levels and lowered plasma AVP levels. From these effects only the increase of the plasma α-MSH level and not the decrease of plasma AVP could be blocked by naltrexone. Moreover PLG treatment was ineffective with respect to the endorphin-induced decrease in plasma AVP, but it partly blocked the increase of plasma α-MSH when this tripeptide was given in combination with β-endorphin.