Inhibition of dopamine of oxytocin release from isolated posterior lobe of the hypophysis of the rat; disinhibitory effect of beta-endorphin/enkephalin.
The release of oxytocin from isolated posterior lobe of the hypophysis of untreated rats and rats pretreated with alpha-methyl-p-tyrosine (alpha-MPT) has been studied. The amount of oxytocin released under resting conditions, in response to ouabain was much higher in those preparations which had been pretreated with alpha-MPT. Dopamine failed to affect the resting release in tissue taken from control rats but it significantly reduced the secretion of oxytocin in tissue dissected from dopamine-deficient rats. Opioid peptides, beta-endorphin or D-Ala2-Pro5-enkephalinamide enhanced the release of oxytocin 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 had been pretreated by alpha-MPT. Naloxone prevented the effect of the opioid peptides, and by itself significantly reduced the release of oxytocin. The data suggest that (i) dopamine stored in, and released from, the neural lobe may inhibit the secretion of oxytocin; (ii) the release of oxytocin 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 oxytocin release possibly by inhibiting the release of dopamine.
D000818:Animals; D004298:Dopamine / Q000494:pharmacology*; D004723:Endorphins / Q000494:pharmacology*; D004745:Enkephalins / Q000494:pharmacology*; D005260:Female; D008297:Male; D008781:Methyltyrosines / Q000494:pharmacology; D010121:Oxytocin / Q000037:antagonists & inhibitors* / Q000378:metabolism; D010904:Pituitary Gland, Posterior / Q000378:metabolism*; D051381:Rats
D004723:Endorphins; D004745:Enkephalins; D008781:Methyltyrosines; D010121:Oxytocin; D004298:Dopamine