Two Daily Surges of Prolactin Secretion in the Immature Female Rat

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Abstract
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-dayold 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.