Two daily surges of prolactin secretion in the immature female rat.

Serum concentrations of PRL were determined at 3-h intervals during a 24-h period in 27- to 43-day-old female rats maintained on a 14-h light, 10-h dark schedule. Blood samples for analysis by RIA were obtained either by decapitation or from intraatrial cannulae implanted 1-2 days before the experiment. Two peaks of serum PRL were observed in the 27- to 33-day-old rats. In the animals in which blood was obtained after decapitation, one peak occurred at 1800 h and the other occurred at 0600 h; in animals with intraatrial cannulae, one PRL peak was at 1400-1700 h and the other was at 0200-0500 h. PRL surges were constantly separated from each other by 9-12 h. The 42- to 43-day-old rats, a few days before puberty, showed a diurnal peak at 1800 h but no nocturnal peak in the cannulation experiment. The data demonstrate the existence of two daily surges of PRL secretion in the immature female rat.