Semicircadian Rhythm in Plasma Levels of Prolactin During Early Gestation in the Rat

Abstract

Blood was collected following decapitation of 280 Sprague-Dawley rats with 4- day estrous cycles, which were killed at 3 hr intervals from 3 AM of proestrus to midnight of day 4 of gestation (day after finding of sperm in the vaginal smear was considered day 1 of gestation). Groups of rats also were killed at 30 min intervals from 2–7 PM of proestrus. Plasma was stored frozen until analysed by radioimmunoassay for prolactin. The plasma concentrations of prolactin showed a distinct semicircadian rhythm during the first 4 days of gestation. Peaks of prolactin were observed late in the periods of light and dark with nadirs at noon and between 9 PM and midnight. The proestrus peak of prolactin was present, but extended 12 hr longer than the interval reported for unmated rats. A peak of prolactin also was observed on the afternoon of estrus. (Endocrinology90: 1125, 1972)