Inhibition by Dopamine of Oxytocin Release from Isolated Posterior Lobe of the Hypophysis of the Rat; Disinhibitory Effect of β-Endorphin/Enkephalin

Abstract

Oxytocin [O] release from isolated posterior lobe of the hypophysis of untreated rats and rats pretreated with .alpha.-methyl-p-tyrosine (.alpha.-MPT) was studied. the amount of O released under resting conditions, in response to ouabain was much higher in those preparations which were pretreated with .alpha.-MPT. Dopamine failed to affect the resting release in tissue taken from control rats but it significantly reduced O secretion in tissue dissected from dopamine-deficient rats. Opioid peptides, .beta.-endorphin or D-Ala2-Pro5-enkephalinamide enhanced O rlease from isolated neural lobe of the hypophysis dissected from untreated rats, but they failed to enhance significantly the release from posterior lobe of rats which were pretreated by .alpha.-MPT. Naloxone prevented the effect of the opioid peptides, and by itself significantly reduced O release. Dopamine stored in, and released from, the neural lobe may inhibit O secretion; O release may be continously controlled by an endogenous opioid peptide:opioid peptides may exert their effect via a disinhibitory phenomenon; they remove the inhibitory effect of dopamine on O release possibly by inhibiting dopamine release.