Estradiol-17β Reduces Number of Ovulations in Adult Rats: Direct Action on the Ovary? 2

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Abstract
The specific role of estrogen and other steroids in folliculogenesis is unclear since both inhibitory and stimulatory effect have been described. Atresia of the preovulatory follicle was induced when estradiol-17.beta. (E2) or progesterone was administered peripherally in rhesus monkeys, presumably due to a direct effect at the ovarian level. Whether a similar direct action of E2 and other steroids occurs in rats was investigated. Minicapsules of Silastic containing E2, progesterone or dihydrotestosterone in amounts of 12.5-100% mixed with cholesterol, were placed unilaterally under the ovarian bursa on the morning of metestrus in rats having 4-days cycles. At autopsy on the morning of estrus, the number of oocytes ovulated from treated and untreated contralateral ovaries was determined. Ovaries treated with E2 averaged 3.1 .+-. 0.4 oocytes while untreated ovaries in the same animals averaged 6.4 .+-. 0.4 oocytes (P < 0.001 by paired t test, n = 20). Results were similar for all amounts of E2 used and serum levels of E2 were not elevated at autopsy by this local treatment. Cholesterol alone did not alter the number of oocytes. Results of similar experiments with progestrone and dihydrotestosterone were less conclusive than for E2. In additional trials, ovaries were treated with E2 as above, and preovulatory follicles were explanted on the morning of proestrus to determine their steroidogenic capability in vitro. Follicles from treated ovaries released somewhat less E2 and progesterone into luteinizing hormone (LH)-free medium than follicles from untreated ovaries, but not when LH was added to the medium. Since only ovaries treated with E2 were affected, and since local treatment did not increase circulating amounts of E2, apparently was responsible for reducing the number of oocytes ovulated by a direct action on the ovary. The mechanism for this effect is yet unclear.