Changes of pituitary thyrotropin releasing hormone (TRH) receptor level and prolactin response to TRH during the rat estrous cycle.

The plasma PRL and TSH responses to TRH injected iv at different stages of the estrous cycle in normal rats under Surital anesthesia were maximal during the afternoon of proestrus and morning of estrus and lowest on diestrus I. As calculated from the areas under the plasma response curves, a 10-fold difference was found between the maximal and minimal PRL responses while a 2-fold difference was measured for TSH. The plasma PRL and TSH responses to TRH showed a correlation with the binding of [3H]TRH to anterior pituitary gland, a 3-fold difference being observed between the minimal binding measured on the morning of diestrus II and the maximal value found on the evening of proestrus. Contrary to findings with LHRH and LH, repeated injections of a small dose (10 ng) of TRH in the afternoon of proestrus abolished PRL and TSH responses to subsequent injection of the neurohormone.