Sex differences in response to discrete estradiol injections.

ABSTRACT

Developmental effects of perinatal androgens render adult male rats refractory to the activation of feminine sexual behavior by estradiol (E2) and progesterone (P). Recent evidence suggested that fluctuating levels of systemic E2, which are thought to approximate the ovarian secretion under physiological conditions, may reverse this insensitivity to E2 and, particularly, to the synergistic effects of P in male rats of the Wistar strain. We examined whether this hormonal regimen would reverse this insensitivity in Sprague-Dawley rats. Gonadectomized animals received two injections of E2 (1 microgram per injection) 12 hr apart at 0900 and 2100 hr followed by P (0.5 mg) or oil, at 35 hr, and a mating behavior test, at 38 hr, subsequent to the initial E2 administration. This treatment was repeated four times at 4-day intervals. The inability of Sprague-Dawley male rats to respond to E2 and P was unaffected by this pattern of exposure to exogenous E2. Receptivity scores, lordosis quotients, and proceptivity were negligible in males, and significantly less than that displayed by females. In addition, the levels of sexual receptivity and proceptivity were facilitated by the availability of P following E2 in females, but not in males. The present findings fail to support a general hypothesis that "discontinuous" E2 stimulation, achieved by two spaced injections of this hormone, reverses developmental determinants of sex differences in responsiveness to hormones mediating female sexual behaviors.