Influence of exogenous and endogenous estrogen on thermoregulatory responses to mild heat and the interaction with light and dark phases.

The present study aimed to determine the influence of estradiol (E(2)) and the interaction with circadian phases on thermoregulatory responses to mild heat in female rats. Heat loss and production during 3-h exposure to the environment at an ambient temperature of 28-34 °C were assessed by measuring abdominal temperature (T(abd)), tail skin temperature, and oxygen consumption in ovariectomized rats with and without E(2) replacement (OVX + E(2) and OVX, respectively) and in control rats in the proestrus (P) and diestrus (D) phases. In the light phase, T(abd) remained unchanged in all groups. T(abd) increased in the dark phase, but was lower in the OVX + E(2) and P groups than in the OVX and D groups. Oxygen consumption decreased at 34 °C, but to a lesser extent in the OVX + E(2) group than in the OVX group. These results suggest that E(2) activates thermoregulation in mild heat in the dark phase.