Effect of ACTH on opiate inhibition of oxytocin release

beta-Endorphin (200ng), dynorphin1-17 (2 micrograms) and morphine (4 micrograms) suppressed the osmotically-evoked release of oxytocin in anaesthetized lactating rats. ACTH1-24 (0.2-3 micrograms) administered intraventricularly 3 min before beta-endorphin or morphine attenuated their inhibitory action, but when applied afterwards was never seen to reverse their effects although this was achieved with naloxone (lmg X kg-1, i.v.). ACTH however, was ineffective against dynorphin. At higher doses an opiate-like action of ACTH was observed. Thus ACTH may act as a partial antagonist and at higher doses as a partial agonist of the opiate receptor for which it is known to have an affinity. It is suggested that suppression of oxytocin release by endogenous beta-endorphin could be modified by ACTH with which it may be co-released.