Abstract

Like leptin and insulin, estrogen decreases body weight and food intake in male rats; levels of these three hormones vary as a function of fat stores. Thus, it was of interest to determine whether the effect of estrogen on brain stimulation reward (BSR) mimics that of leptin and insulin. The results did not support this prediction: estradiol increased the reward effectiveness of the stimulation in all subjects. The second experiment investigated this relationship by measuring the effect of estradiol implantation on body weight and food intake in ad libitum fed and food restricted male rats. It was found that estrogen further decreased body weight in subjects that had been food-restricted to a level below that of ad libitum fed subjects treated with estrogen. This decrease in body weight is likely due to an increase in energy expenditure. Once ad libitum access to food was restored, the estrogen-treated rats increased their food intake and body weight. This implies that it would be possible to hold constant the weight of food-restricted, estrogen-treated, male rats by adjusting their daily ration. Thus, it should be feasible to investigate the effect of estrogen on BSR in food-restricted male rats whose body weight is held constant. In this way, a direct action of estrogen on the neural substrate for BSR could be distinguished from an indirect influence due to changes in body weight.