Prolactin release and milk removal induced by suckling and milking in lactating ewes is prevented by L-DOPA treatment

― The effect of L-DOPA on milk removal and on prolactin release during suckling or milking was studied in lactating ewes.Various doses of L-DOPA (25, 50, 100 and 200 mg per ani- mal) were injected iv 30 min before the suckling or milking period.Control ewes were injected with 0.9% NaCl solution only.Milking induced a significant long-lasting release of prolactin.An inhibition of milk removal was obtained with the dose of 200 mg of L-DOPA.An inhibition of prolactin secretion was observed related to the dose of drug administered.The inhibitory effect of 200 mg of L -DOPA on the secretion of prolactin after milking lasted for about 120 min, and thereafter a significant increase in serum prolactin level occurred.This increase in serum prolactin was not due to a &dquo;rebound&dquo; effect of L-DOPA, since the milking stimulus had to be present to induce the delayed increase in prolactin.Doses of 25 or 50 mg of L-DOPA prevented the surge of prolactin observed im- mediately after mitking, but a long-lasting release of prolactin was obtained thereafter.The inhibitory effect of L-DOPA on prolactin release could be overridden by the suckling or milking stimuli accor- ding to the dose administered.The suckling stimulus was more effective than milking in overriding the inhibitory effect of the low dose of L -DOPA.The results indicate that milk removal and prolactin release induced by milking or suckling in lactating ewes is inhibited by an increase in monoamines at the hypothalamic-hypophyseal level. * Correspondence and reprints.ficative du taux de la prolactinémie, indépendante d'un phénomène de «rebond» éventuellement conséquence de l 5 njection de L-DOPA, puisque pour observer ce phénomène, il faut qu il y ait eu sti- mulation mammaire.Les doses de 25 et 50 mg de L-DOPA inhibent la décharge de prolactine réflexe immédiatement après la traite uniquement dans sa composante d'amplitude sans en modifier la durée.L effet inhibiteur de l'administration de L-DOPA sur la décharge de prolactine peut être modulé en fonction des doses de L-DOPA injectées par le stimulus résultant de la traite ou de la tétée.A cet égard, la tétée s'est révélée plus efficace que la traite.Cette expérimentation démontre que la quantité de lait obtenue après une traite ou une tétée, ainsi que la décharge de prolactine concomitante peuvent être inhibées par une augmentation de monoamines au niveau hypothalamo-hypophysaire.