Autofeedback effects of prolactin on basal, suckling-induced, and proestrous secretion of prolactin.

Subcutaneous injections of ovine prolactin (oPRL, 4 mg/kg) were utilized to study the negative feedback effect of PRL on its own secretion in lactating and 4-day estrous cycling female rats. Basal PRL secretion in the 10-day postpartum lactating rat is not suppressed by acute or 48 hr of exposure to oPRL. In contrast, basal PRL secretion on the morning of proestrus is inhibited following 48 hr of exposure to oPRL. Thus, the lactating rat appears to be unresponsive to the negative feedback action of PRL compared with the adult female rat in regards to basal PRL secretion. The suckling-induced PRL response is partially suppressed by exposure to oPRL in 10-day postpartum lactating rats. Considering the areas under the PRL response curves, the suckling-induced release is blunted by 6 (decreased 49%), 12 (decreased 46%), and 48 (decreased 77%) hr of oPRL exposure compared to controls. In contrast, in cycling animals, 48 hr of exposure to elevated oPRL levels dramatically abolished the proestrous PRL surge. Elevated PRL levels, by a direct action on the brain and/or indirectly by altering ovarian function, inhibit or override the hypothalamic mechanism(s) mediating the proestrous PRL surge while having a lesser effect on the mechanism(s) mediating the suckling-induced PRL response. Taken together, the present data point to probable differences in the mechanism(s) mediating basal, suckling-induced, and the proestrous secretion of prolactin.