Changes in pituitary prolactin responsiveness to TRH during pregnancy.

Prolactin plasma concentration during pregnancy was determined in rats treated with thyrotropin-releasing hormone (TRH). Day 0 of pregnancy was defined as the day sperm were first found in the vagina. All blood samples were obtained in unanesthetized rats which had previously received a cannula in the right common carotid. On Day 8 of pregnancy, plasma prolactin concentrations reached a peak between 2400 and 0800 hr (lights on from 0600 to 1800 hr). Injection of TRH (1 microgram/kg body wt) via the carotid artery increased plasma prolactin levels within 5 min. The largest increase occurred when TRH was given during the prolactin surge, whereas much smaller effects were found when TRH was given at the beginning or after the end of the surge period. Thus, the sensitivity of the prolactin cell to TRH appears to be the greatest when the secretory activity of the cell is high. It was then determined whether there was any change in the sensitivity of the prolactin cell to TRH after the prolactin surges had disappeared at midpregnancy. Injection of TRH between 1100 and 1200 hr increased prolactin less on Day 12 than on Day 8 of pregnancy. Since placental lactogen (PL) levels in the plasma are high on Day 12 compared to Day 8, and are inhibitory to prolactin secretion, it was reasoned that PL may be the factor which caused the reduced sensitivity to TRH. However, hysterectomy on Day 11 failed to increase the pituitary responsiveness to TRH the next day. In summary, these data indicate that the pituitary responsiveness to factors that stimulate prolactin, such as TRH, varies with relation to the time of pregnancy or presence of the nocturnal surge. What cellular mechanism is responsible for these sensitivity changes is not known.