Sex differences in the activational effects of gonadal hormones on food intake and body weight.

Estrogens have been shown to decrease, and androgens to increase body weight (BWt) of guinea pigs (GPs). The magnitude of the BWt sex difference shown by intact adult GPs is due primarily to these concurrent, or activational, effects of gonadal steroids. However, a small but significant sex difference in BWt persists in animals gonadectomized at birth, indicating that early hormonal exposure may permanently influence certain steroid sensitive weight regulatory mechanisms in the two sexes. Three experiments were therefore designed to investigate the short term effects of estradiol and testosterone on food intake (FI) and BWt of gonadectomized adult male and female GPs. In the first experiment, GPs gonadectomized in adulthood were given a single injection of 6 micrograms estradiol benzoate (EB). Although EB treatment reduced FI and BWt of both females and males, significantly larger reductions occurred in females. In the second experiment, GPs gonadectomized at birth received treatments of oil or 2 micrograms EB for 7 days. EB treatment also produced significantly larger effects on FI and BWt in the neonatally gonadectomized females. The third experiment involved GPs gonadectomized as adults who were injected with either oil or 1 mg/day testosterone propionate in oil (TP) for 32 days. Compared to changes in oil injected controls, TP produced significantly larger increases in male BWt than female BWt. Therefore, although GPs show only minor sex differences in BWt which might relate to prenatal gonadal hormonal exposure, significant sex differences remain in their responsiveness to the activational effects of gonadal steroids on FI and BWt in adulthood.