[The formation and regulation of the lactotrophic function of the hypophysis in the prenatal period of rat development. II. Dopamine inhibitory control of prolactin secretion].

Using the technique of radioimmunoassay, we studied the secretion of prolactin and its control by dopaminergic system in 22-day-old rat fetuses under normal conditions and after pharmacological inhibition of dopamine receptors. In order to elucidate the origin of prolactin and dopamine participating in this process, we used decapitation and encephalectomy of fetuses in utero. Decapitation of fetuses did not result in any changes of baseline prolactin secretion into blood in males and insignificantly decreased it in females as compared with nonoperated controls. We conclude that prolactin detected in blood plasma of nonoperated fetuses does not originate in the pituitary, and any prolactin synthesized in the pituitary is not secreted into blood. Inhibition of dopamine receptors in decapitated fetuses did not result in any changes of prolactin level in blood. This provided evidence that in nonoperated fetuses, it is pituitary prolactin which is secreted in response to haloperidol, while the secretion of nonpituitary prolactin is not controlled by dopamine. Encephalectomy increased prolactin level in plasma and resulted in a drastic decrease of its level in the pituitary. The block of dopamine receptors did not affect the level of prolactin in blood plasma or pituitary of encephalectomized fetuses. We conclude that the inhibitory dopaminergic control of prolactin secretion by the pituitary during the prenatal period is accomplished just as in adult animals by dopaminergic neurons of hypothalamus.