Endogenous opioid inhibition of the release of oxytocin from the isolated rat neurohypophysis during high-frequency stimulation of the pituitary stalk

Abstract

Isolated rat neurohypophyses were superfused in vitro and the release of vasopressin and oxytocin determined by radioimmunoassays. Hormone release was evoked by short trains of electrical stimulation (10 s), at frequencies from 15 to 100 Hz. The release of oxytocin evoked by stimulation up to 80 Hz was increased 2- to 3-fold in the presence of naloxone. The evoked release of vasopressin was not significantly affected by naloxone. In conclusion, opioid inhibition of oxytocin release operates also under stimulation conditions which mimic the electrical activity (i.e. short bursts of high frequencies) of physiologically activated (e.g. by suckling) oxytocinergic neurons.