Effects of Silastic progesterone implants on activity cycles and steroid levels in ovariectomized and intact female rats.

Abstract We have previously shown that although Silastic implants of progesterone reduce the amount of running of animals living in activity wheels, progesterone-treated animals continue to show perodic fluctuations or peaks in activity. We hypothesized that although progesterone treatment inhibited estrous cycles, ovaries of animals treated with Silastic implants of progesterone continued to secrete estradiol in amounts adequate to stimulate moderate levels of running. In the present study we tested this hypothesis by removing ovaries from progesterone-treated animals and comparing their running behavior and steroid levels to progesterone-treated animals who received sham ovariectomies. Although progesterone treatment significantly inhibited running activity, removal of ovaries in progesterone-treated animals further suppressed running activity. In addition, both estradiol and progesterone levels were significantly reduced following removal of ovaries in progesterone-treated animals. We conclude that although Silastic progesterone implants inhibit normal ovarian and estrous activity cycles, ovaries produce sufficient estradiol to stimulate running behavior.