Body temperature and temperature gradients: changes during the estrous cycle and in response to ovarian steroids

Rectal temperature, body weight, and vaginal membrane condition were monitored in female guinea pigs in order to determine the pattern and magnitude of changes in these variables across the estrous cycle and as a result of treatment with ovarian hormones. A thermistor probe was used to measure rectal temperatures at depths of 4 and 8 cm and to determine the temperature gradient between these two sites. Data collected across the estrous cycles of 18 intact females were aligned by day of vaginal membrane rupture, the estimated time of ovulation. Compared to measurements during the midluteal phase of the estrous cycle, rectal temperatures increased significantly one to three days prior to vaginal membrane rupture. In addition, a significant periovular decline was observed in the temperature gradient and in body weight. Treatment of ovariectomized females with 3 micrograms estradiol benzoate for 3 days (EB) followed by a single injection of 0.4 mg progesterone on the 4th day (P) led to biphasic changes in temperature. Compared to a control group which received injections of 0.2 ml corn oil vehicle for 4 consecutive days (OIL), EB significantly increased rectal temperature measured at the depth of 4 cm, but had no significant effect on measurements taken at a depth of 8 cm. The temperature difference between these two sites also decreased significantly. Rectal temperatures at both 4 cm and 8 cm sites dropped significantly following injection of progesterone. Ovarian hormones therefore have significant effects on body temperature of guinea pigs, effects which, in combination with other evidence, lead us to suggest that some of the observed temperature variations may be related to estrogen-induced changes in peripheral vasodilation or other mechanisms of heat loss.