Inhibition of tolerance to the pharmacological effects of human beta-endorphin by prolyl-leucyl-glycinamide and cyclo(leucylglycine) in the rat.

Acute intraventricular administration of human beta-endorphin (15 microgram) produced analgesia, hypothermia and catalepsy in male Sprague-Dawley rats. Injections of beta-endorphin given every 8 hr for 3 days resulted in the development of tolerance to all of the above mentioned pharmacological effects. Tolerance developed rapidly to the hypothermic effect and less rapidly to the analgesic and cataleptic effects. After the third or the fourth injection of beta-endorphin, pronounced hyperthermia, rather than hypothermia, was observed. After seven or eight injections of beta-endorphin, tolerance to the analgesic effect was complete and the cataleptic effect was reduced to 50% of the original. Daily s.c. administration of Pro-Leu-Gly-NH2 (MIF) or cyclo(Leu-Gly) (2 mg/kg each) blocked the development of tolerance to the analgesic and cataleptic effects of beta-endorphin. The hyperthermic effect of beta-endorphin in beta-endorphin-tolerant rats was partially blocked by both MIF and cyclo(Leu-Gly). Multiple injections of MIF or cyclo(Leu-Gly) did not alter beta-endorphin-induced analgesia, catalepsy and hypothermia in rats which were given repeated intraventricular injections of saline. Since MIF is a naturally occurring peptide of hypothalamic origin, these studies suggest that the hypothalamus may be an important site in regulating the pharmacological effects of chronically administered endogenous opiates.