Preoptic implants of estradiol increase wheel running but not the open field activity of female rats

Abstract

In order to localize the neural substrate for estrogen induction of increases in activity, bilateral implants of dilute estradiol (in concentrations of 1:10, 1:100, and 1:250) were made into the medial preoptic area (n=5), the anterior hypothalamus (n=8), the ventromedial hypothalamus (n=6), and the posterior hypothalamus (n=4) of ovariectomized female rats housed in running wheels. The same animals were also tested for open field activity and lordosis. The preoptic area was the most effective site tested for estrogenic stimulation of running wheel activity, with 3 5 animals responding consistently to estradiol application. 3 8 animals with cannulae in the anterior hypothalamus (AHA) responded with increased in activity measured in the running wheel on at least two out of three estradiol treatments. No animals with cannula placements posterior to the AHA exhibited consistent running wheel responses to estradiol. No estrogen-induced increase in open field activity was found, regardless of cannula location or concentration of estradiol; however, there was a tendency for the number of squares entered during the later tests to decrease in the ventromedial hypothalamic nucleus group and in the subgroup of animals not showing estrogen-sensitive wheel running in the anterior hypothalamic area group. Only the ventromedial hypothalamic estradiol treatments stimulated lordosis behavior. These results support earlier reports suggesting that the medial preoptic area (and possibly the adjacent anterior hypothalamic area) is the critical brain site for estrogenic stimulation of running wheel activity. Since estradiol treatments effective in stimulating running in an activity wheel did not increase locomotion in the open field, it is possible that estrogen effects on open field measures of activity are regulated separately.