Ovarian Steroid Modulation of Prolactin Surges in Cervically Stimulated Ovariectomized Rats*

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Abstract
PRL [prolactin] secretion induced by a cervical stimulation (CS) in the intact rat is characterized by a daily diurnal surge (1900 h) which recurs for 10 days and a daily nocturnal surge (0500 h) which lasts for 11 days. In ovariectomized (OV) rats, CS induces attenuated PRL surges which last for only 6 days. This study describes the effect of SC Silastic implants of estradiol (E2) or progesterone (P) on the magnitude and persistence of PRL surges on OV-CS rats. Placement of P at 1500 h and CS at 1900 h day 0 (D-O) resulted in nocturnal surges of PRL on D-2 which were significantly greater than those of blank-implanted rats. Diurnal surges of PRL on D-2 were equivalent in P and blank-implanted rats. Placement of E2 at 0800 h on the day before CS at 1900 h on D-0 resulted in an enhanced dirunal surge and extinction of the nocturnal surge of PRL on D-2 compared to blank-implanted rats. In the absence of CS, OV rats presented diurnal but not nocturnal surges of PRL in response to E2. When steroid implantation was delayed until D-6 in OV-CS rats, P prolonged the nocturnal and E2 prolonged the dirunal stages of PRL to D-10. The 2 surges of PRL resulting from CS are differentially responsive to steroids: elevated levels of P accentuate the nocturnal surge while elevated levels of E2 accentuate the diurnal and inhibit the nocturnal surge.