The present study investigated whether prolactin (Prl) itself acts as a physiological factor in the control of nocturnal Prl surges in pseudopregnant (PSP) rats. Intracerebroventricular (ICV) injection of ovine Prl at doses of 0.1, 1 and 10 micrograms/rat given 1 h before the start of the nocturnal Prl surge inhibited its occurrence completely and dose-dependently. The same inhibition of nocturnal Prl surge was observed 4 and 1 h before, and even 1.5 h after, the start of the nocturnal Prl surge. Prl concentrations in the cerebrospinal fluid (CSF) increased twice a day, the timings of which were similar to those of diurnal and nocturnal surges of Prl in PSP rats. Changes of Prl concentrations in the CSF correlated closely with those of plasma Prl concentrations. ICV injection of antiserum to rat Prl in the first half period of nocturnal Prl surges did not modify the elevation pattern of plasma Prl concentrations. Inhibition of nocturnal Prl surge produced by ICV injection of ovine Prl was overcome by subcutaneous administration of pimozide (1 mg/kg). These results suggest that Prl itself may act as an important physiological modulatory factor in the control mechanism(s) of nocturnal Prl surge in PSP rats. Prl may inhibit further discharge of Prl by a feedback mechanism through the hypothalamic dopaminergic system within a very short time period.
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