Effect of Estrogen on Serum Prolactin Levels in Rhesus Monkeys After Hypophyseal Stalk Transection1

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Abstract
The effect of estradiol (E2) on prolactin (PRL) secretion was compared in ovariectomized (OVX) and ovariectomized-stalk transected (OVX-ST) monkeys. After an 8-day period during which basal serum PRL levels were established, all monkeys were provided with a Silastic implant containing crystalline estradiol. Blood samples for PRL assay were collected daily for the 15 days that the implants were in place, and at 1- or 2-day intervals after implant removal. Basal serum PRL levels were, on the average, 6.7-fold higher in OVX-ST monkeys than in OVX animals. Estrogen evoked a rapid and sustained elevation of circulating PRL in OVX-ST monkeys, but had no effect on OVX monkeys. In the OVX-ST group, serum PRL concentrations rose significantly above baseline within 24 h after initiating E2 treatment, reached a peak within 7 days and then returned to pretreatment levels within 6 days after removal of the E2 stimulus. During the final 8 days of E2 treatment, circulating PRL levels were 210% of pretreatment values. The degree to which E2 stimulated PRL secretion in OVX-ST monkeys was inversely correlated (r = −0.88) with pretreatment serum PRL concentrations. These data indicate that the hypothalamus exerts primarily an inhibitory influence on PRL secretion in monkeys, and that this inhibition is sufficient to negate the potential enhancement of PRL release induced by E2 at the pituitary level.