Evidence for a rapid in Vivo Effect on Estradiol-17β on Prolactin Secretion in Ovariectomized Rats

Abstract

Repeated intraarterial injections of synthetic thryrotropin releasing hormone (TRH, 1 ug/rat) increased plasma prolactin levels 4 hours, after a single subcutaneous injection of 10 ug estradiol-17β (E₂-17β) in rats ovariectomized 1, 2 or 4 weeks and at 2 hours after E₂-17β injection in rats ovariectomized for 6 weeks. The effect of TRH was still present at 24 but not 48 hours after estradiol treatment. TRH-induced increases in plasma prolactin were similar in groups of rats treated with 10 ug E₂-17β (s.c.) or implanted with 0.5 cm Silastic capsules of crystalline E₂-17β (s.c.) wheareas smaller, yet significan, TRH-induced increases in plasma prolactin were observed in rats injected s.c. with 1.0 ug E₂-17β. Single intraarterial injections of TRH at 4 or 8 hours after E₂-17β treatment induced increases in plasma prolactin similar in magnitude to those observed at the same times after E₂-17β in rats given repeated TRH injections. No effect of TRH was observed in ovariectomized rats given sesame oil and E₂-17β treatment did not influence plasma prolactin in rats given saline instead of TRH. Intraarterial administration of serotonin creatinine sulfate (5-HT, 10 mg/kg body weight) induced marked increases in plasma prolactin in rats ovariectomized for 4 weeks which were potentiated at 2 and 6 hours after E₂-17β (10 ug) treatment. The data show that estradiol has a fairly rapid stimulatory effect on plasma levels of prolactin induced by two different secretagogues but the exact site and mechanism of action remain unresolved.