Ovarian steroids and modulation of morphine-induced analgesia and catalepsy in female rats.

The influence of ovarian steroids on modulation of antinociceptive and cataleptic responses to morphine in female rats was evaluated. The sensitivity of the animals to morphine varied at different stages of the estrous cycle. The responses of postpartum and ovariectomized rats to morphine was attenuated. The test doses of estradiol-17 beta or progesterone, either alone or in combination, did not alter this attenuated morphine sensitivity. Testosterone, however, sensitized post-partum as well as ovariectomized rats to morphine. Unlike progesterone, 17-alpha-hydroxy progesterone antagonized testosterone. Collectively these data implicate ovarian testosterone as a physiological modulator of actions of morphine in female rats.