Evidence for a rapid in vivo effect on estradiol-17 beta on prolactin secretion in ovariectomized rats

Repeated intraarterial injections of synthetic thryrotropin releasing hormone (TRH, 1 microgram/rat) increased plasma prolactin levels 4 hours after a single subcutaneous injection of 10 micrograms estradiol-17 beta (E2-17 beta) in rats ovariectomized 1, 2 or 4 weeks and at 2 hours after E2-17 beta 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 micrograms E2-17 beta (s.c.) or implanted with 0.5 cm Silastic capsules of crystalline E2-17 beta (s.c.) whereas smaller, yet significant, TRH-induced increases in plasma prolactin were observed in rats injected s.c. with 1.0 microgram E2-17 beta. Single intraarterial injections of TRH at 4 or 8 hours after E2-17 beta treatment induced increases in plasma prolactin similar in magnitude to those observed at the same times after E2-17 beta in rats given repeated TRH injections. No effect of TRH was observed in ovariectomized rats given sesame oil and E2-17 beta 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 E2-17 beta (10 micrograms) 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.