ABSENCE OF SHORT-LOOP AUTOREGULATION OF PROLACTIN DURING LATE PREGNANCY IN THE RAT

Animals bearing intrahypothalamic anterior pituitary (AP) grafts exhib it a central hyperprolactinemia, and thus, serve as a model for the st udy of short-loop feedback regulation of prolactin (PRL) secretion. We investigated the effects of intrahypothalamic AP grafts on PRL secret ion during late pregnancy (n = 7). A further group of five rats was in jected with the dopamine agonist bromocriptine during late pregnancy a nd five control rats were treated with the bromocriptine-vehicle only. A nocturnal surge in plasma PRL concentrations was observed in the ve hicle-injected control animals, peaking at 212 +/- 11 ng/ml at 0300 h on the day of parturition. Despite the central hyperprolactinemia due to the grafts, a similar PRL surge was observed in grafted animals, pe aking at 205 +/- 35 ng/ml at 0300 h on the day of parturition. Bromocr iptine treatment completely blocked the nocturnal surge of PRL. These results suggest that short-loop feedback autoregulation of PRL secreti on becomes less responsive or nonfunctional in the last 24 h of pregna ncy in the rat. This apparent change in sensitivity of the autofeedbac k mechanism may be an important physiological mechanism to allow the h ypersecretion of PRL during lactation.