Time-dependent changes in beta-endorphin-stimulated prolactin release during pregnancy

Endogenous opioid peptides (EOPs) stimulate prolactin (PRL) release in various physiological conditions in the rat. Moreover, EOPs are essential in initiating and maintaining the nocturnal PRL surges that occur over the first half of gestation in the rat. The purpose of this study was to investigate the potential role of the opioid beta-endorphin (beta-End) in mediating the nocturnal PRL surges. Day 8 pregnant rats received an infusion of 2.5, 10, 25 or 100 ng/microliters/min beta-End intracerebroventricularly (i.c.v.) for 15 min at 12.00 h, an intersurge period. PRL increased in a dose-dependent manner and from this, the largest dose was used in subsequent experiments to ensure maximal opioid receptor stimulation. The next experiment defined the temporal sensitivity of the neuroendocrine system regulating PRL surges to exogenous beta-End. Day 8 pregnant rats showed dramatic PRL responses to beta-End when given at midnight (presurge) or 12.00 h (intersurge), but greatly attenuated responses at 02.00 h (early surge) and 04.00 h (late surge). Animals treated at 06.00 h (postsurge) showed recovered responsiveness to beta-End. To determine what may account for the significantly lower PRL increases to beta-End during the surge, day 8 pregnant rats received 100 mg/microliters/min beta-End i.c.v. for 15 min at 10.00 h, and then again at 12.00 h. All animals showed PRL increases greater than 1,140 ng/ml at 10.00 h, but the subsequent response to beta-End at 12.00 h was reduced by 70%. In another experiment, beta-End was infused at midnight and the animals were monitored for a subsequent endogenous nocturnal PRL surge.(ABSTRACT TRUNCATED AT 250 WORDS)