Independent effects of estradiol on water and food intake.

Six experiments examined the effects of estradiol on ingestive behaviors of guinea pigs. Estradiol treatment was found to reduce water intake independently of its actions on food intake and body weight. In the first experiment, minimum intake and body weight of intact female guinea pigs coincided with rupture of the vaginal membrane, the estimated time of ovulation. In a second experiment, injections of 3 micrograms of estradiol benzoate per day to ovariectomized females significantly depressed food intake, water intake, and body weight, compared with oil injections. The ratio of water intake per gram of food intake did not change significantly during the estrous cycle or following estradiol injections, results suggesting that the reduced drinking might be a consequence of the reduced feeding. However, reducing food rations to 30% below ad lib levels in Experiment 3 by itself had no significant effect on drinking. In Experiment 4, therefore, ovariectomized females were first placed on a food ration 30% below ad lib levels and then injected daily with either 3 micrograms of estradiol benzoate or oil. Compared with oil injections, these estradiol injections significantly reduced water intake, while food intake did not decline significantly. In these experiments, the reduction in food intake was therefore neither a sufficient nor a necessary condition for the estradiol-induced suppression of water intake. The last two experiments verified that estradiol has independent actions on feeding. The daily water ration was reduced to 30% below ad lib levels in Experiment 5, with no significant effect on food intake. In the sixth experiment, the water ration was first reduced to 30% below ad lib levels, and then the ovariectomized females were injected with either oil or 3 micrograms of estradiol benzoate per day. With this reduced water ration, the estradiol significantly suppressed food intake while producing only minimal and insignificant changes in water intake. These findings established that estradiol can independently influence water intake and food intake in the guinea pig and thereby indicate that estradiol operates through di different mechanisms to produce these two effects.