The proestrous surge of prolactin enhances sexual receptivity in the rat.

The influence of the proestrous surge of prolactin (Prl) on expression of feminine sexual behavior (lordosis) has been investigated. In the first experiment, proestrous rats were treated with a dopamine agonist, bromocriptine (CB-154; 100 micrograms at 1200, 1300, and 1600 h), which blocked the proestrous surge of Prl without affecting the preovulatory surge of luteinizing hormone. Such animals displayed depressed lordosis quotients (LQs) when compared to control animals at 2000 h on proestrus. However, in CB-154-treated animals given ovine Prl (10 I.U.) at 1400 h on proestrus, LQs were restored to control levels. Ovariectomized (OVX) rats primed with estradiol benzoate (EB; 2 micrograms for 2 days) produced surges of Prl that were similar in timing to those of proestrous. Once again, CB-154 treatment blocked this Prl surge and significantly depressed the LQ, whereas replacement with ovine Prl returned the LQ to control values. These results suggest that the Prl surge facilitates the expression of lordosis. Ovariectomized/adrenalectomized (ADX) and OVX/sham ADX rats were treated with EB (2 micrograms for 2 days) and tested for lordosis on Day 3. Adrenalectomized rats responded with lower levels of lordosis than did sham controls. Administration of progesterone (P4) to ADX rats on Day 3 enhanced the LQ compared to sham ADX values. CB-154 was ineffective in reversing the enhanced LQ, indicating that Prl may be acting through stimulation of adrenal progestins. These data taken together suggest that the proestrous surge of Prl contributes to the normal expression of feminine sexual behavior on proestrus.