Sex differences in the binding of type I and type II corticosteroid receptors in rat hippocampus

Binding parameters of soluble Type I and Type II receptors were assessed in hippocampus of adult, adrenalectomized, male and female rats. No sex differences in the number of either Type I or Type II receptors could be demonstrated between gonadally intact animals. When females treated with 17 beta-estradiol benzoate (10 micrograms/day) were compared with males, a statistically significant reduction in Type II receptors was observed in the females; progesterone produced no further decrease in receptor numbers. The amount of tissue-associated corticosteroid-binding globulin in gonadally intact animals (perfused with dextran-saline) was twice as great in females as males. Sex-dependent differences in these gonadally intact rats were found in the affinity, measured as the dissociation constant (Kd), of both the Type I and Type II receptors. For both receptors, affinity in cytosols from females was reduced. The difference for the Type II receptor was slight, but the Kd value of the Type I receptor was several-fold higher in females. The difference in affinity was evident with both natural and synthetic steroid ligands. There appears to be little, if any, difference in affinity between the hippocampal Type I and the Type II receptors in females. This suggests that the occupancy of Type I receptors in females is substantially less than that of males at low circulating concentrations of corticosteroids.