Effects of 17β-Estradiol on Serum Prolactin Levels and on Prolactin Responses to Thyrotropin-Releasing Hormone in Female Rhesus Monkeys*

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Abstract
Daily changes in serum PRL in response to 17β-estradiol (E2) treatment were determined in long and short term ovariectomized (ovx) and cycling rhesus monkeys. The mean serum PRL in the long term ovx monkeys (9.0 ± 0.4 ng/ml) was lower than in intact monkeys (11.4 ± 0.4 ng/ml), but it rose significantly to 11.4 ± 0.8 ng/ml after serum E2 had been raised to 81.7 ± 0.8 pg/ml for a week by sc implantation of E2-filled Silastic capsules. Further weekly increments in circulating E2 produced smaller increases in serum PRL. During week 6, when E2 levels had been raised to 212.7 ± 9.5 pg/ml, the mean serum PRL was 70% higher than during the pretreatment period. In the face of weekly increases in serum E2, PRL remained unaffected in the intact animals and gave only weak responses in the short term ovx group. When serum E2 was decreased progressively by weekly removal of E2 implants, serum PRL in the long term ovx group remained unchanged during the first 3 weeks but increased by 20% when serum E2 had declined to 46.9 ± 3.8 pg/ ml. Similarly, increases in serum PRL were observed in the short term ovx monkeys during the period of declining E2 levels, whereas no significant changes were noted in the intact animals. PRL responses to TRH challenges were markedly attenuated by ovariectomy, were restored to preovariectomy levels by E2 treatment, and were most pronounced after the end of E2 treatment. Thus, E2 influences PRL secretion in primates, and the nature of this influence is determined by the duration and level of circulating estrogen and the gonadal status of the animal. The paradoxical finding that PRL release is enhanced first by estrogen treatment and then by estrogen withdrawal suggests a complex mechanism of estrogen action, perhaps at multiple sites.