Effects of estradiol replacement in ovariectomized rats on conditioned avoidance responses and other behaviors

The acquisition of conditioned avoidance responses along with spontaneous behaviors were studied in ovariectomized rats. Fourteen days after ovariectomy, they were injected subcutaneously with one of the following doses of estradiol benzoate: 0.2, 2 or 20 microgram/rat. Behavioral tests were applied 3, 24, 48 or 72 hours after estradiol treatment. Although estradiol 2 microgram/rat induced a decrease in acquisition of conditioned avoidance responses at all times tested, this effect was maximum at 48 h. Estradiol 0.2, and 20 microgram/kg decreased and stimulated, respectively, the acquisition performance, as tested 3 h after injection. All doses increased global motility and rearing behavior. This hypermotility disappeared at 24 h, but it was observed again at 48 and 72 h after estradiol 0.2 and 20 microgram/rat. The hormone also induced an increase in head shaking and a decrease in grooming. Although the behavioral changes are more significant in presence of very low serum levels of estradiol, they seem to be triggered by the previous increase in the estradiol levels. The possible sites and mechanisms of action of estradiol on behavior are discussed.