Two Daily Surges of Prolactin Secretion Preserved after Ovariectomy in the Immature Rat

Abstract

Recently we reported that the immature female rat spontaneously expressed twodaily prolactin (PRL) surges; one surge at 1800h (diurnal) and the other at 0600h (nocturnal), but the nocturnal surge ceased occurring as puberty approached. In the present experiment, the effects of ovariectomy and administration of ovarian hormones on the two surges were investigated. Ovariectomy at the age of 23 days did notabolish the two surges occurring on day 29 to 30 and their timing and magnitudewere not different from those in the sham-ovariectomized rat. In the intact femalerat at the ages of 29-30 days, an injection of 2.5μg estradiol benzoate at 1200h progressively elevated PRL levels, whereas 0.5mg progesterone injected at 1200 and 2400h increased the magnitude of diurnal and nocturnal surges. The results indicate that the mechanism for two daily PRL surges in the immature female rat is capable of functioning independently of the ovarian activity, although the latter may exert certain modulatory influence.