Progesterone facilitates lordosis, but not LH release, in estradiol pulse-primed male rats

Gonadectomized (GDX) male and female rats received repeated cycles of estradiol pulses, which prime animals of both sexes to display progesterone-facilitated lordosis. One, three and six treatment cycles of estradiol pulses followed by progesterone induced progressively higher luteinizing hormone (LH) surges in GDX females. Six treatment cycles of estradiol pulses alone (i.e., without subsequent progesterone treatment) induced a small but significant (approximately 4 ng/ml) increase in LH levels in GDX females. In contrast, GDX males never produced LH surges in response to estradiol pulses alone or estradiol pulses followed by progesterone, regardless of the number of hormone treatment cycles. Thus, female patterns of steroid-induced sexual receptivity and LH release are not inextricably linked, as steroid treatments sufficient for induction of lordosis do not stimulate LH secretion in adult male rats. These data also suggest that the neural system governing the LH surge might be more firmly sexually differentiated than that responsible for sexual receptivity in rats.