Individual differences in sexual responsiveness to estrogen and progesterone in ovariectomized rats

Abstract

Individual differences among 83 ovariectomized rats in behavioral responsiveness to estrogen were measured by scoring the quality of sexual receptivity induced by injections of estradiol benzoate (EB) and progesterone (P). The P dose remained constant but the quantity of EB administered was systematically reduced over successive weeks until lordosis behavior could no longer be elicited. This EB dose was considered threshold. This sequence of weekly hormone injections and receptivity tests was repeated to assess the reliability of our procedures. Animals had thresholds of either 2.0, 1.0 or 0.5 [mu]g/kg EB on both tests; the correlation between threshold values on the two tests was high (r=0.66; p<0.001). Sixty-two females were used to determine the facilitating effects of various quantities of P following EB treatment. Subgroups were tested after the E alone and again after one of 6 P doses. Zero, 20, 50 and 100 [mu]g P failed to elevate receptivity scores significantly; both 250 and 500 [mu]g P had significant facilitating effects. The results demonstrated that individual differences in EB sensitivity can be measured reliably, and a further analysis also suggests similar individual differences in P responsiveness. Our threshold determination procedures provide a useful technique for measuring the effects of various experimental manipulations on the hormone sensitivity of brain mechanisms which regulate estrous behavior.