MATING STIMULATES ESTRADIOL PRODUCTION BY OVARIES OF THE MUSK SHREW (SUNCUS-MURINUS)

Asian musk shrews (Suncus murinus) are induced ovulators, but exhibit no cyclic changes in reproductive structures or in sexual behavior. Mating behavior is induced by contact with a male. To determine if mating induces changes in ovarian steroidogenesis, ovaries removed from unmated animals and at 3, 10, 15, and 36 h after mating were cultured for 4 h in the presence or absence of gonadotropins (LH + FSH, 1-mu-g/ml). Histological analysis revealed no obvious changes in follicular size or appearance at the end of culture in ovaries cultured at 3 and 10 h post-mating, as compared with ovaries from unmated shrews, and mating did not stimulate any discernable changes in steroid secretion in these two groups. However, at the end of the culture period, ovulation had occurred or was occurring in ovaries from 35% of the animals ovariectomized at 15 h after mating, and corpora lutea (CLs) were present in 39% of ovarian pairs obtained 36 h after mating. At 15 h post-mating, ovaries with ovulations secreted three times more estradiol than did ovaries that showed no evidence of stimulation by mating, but there were no differences in testosterone or progesterone production. In contrast, ovaries isolated 36 h post-mating with CLs secreted dramatically more of all three steroids than ovaries without CLs (23, 13, and 52 times more estradiol, testosterone, and progesterone, respectively). These data are consistent with plasma concentrations of estradiol at the time of ovariectomy, which were twice as high at both 15 and 36 h after mating, in animals whose ovaries showed evidence of ovarian stimulation. Gonadotropins increased the accumulation of estradiol, testosterone, and progesterone in medium of ovaries cultured at 0, 3, 10, and 15 h after mating. In ovaries cultured 36 h after mating, gonadotropins increased the secretion of estradiol and testosterone only by ovaries lacking CLs and did not increase progesterone production in the presence or absence of CLs. In summary, the results suggest that in musk shrews increases in estradiol secretion are important for preovulatory and/or ovulatory processes, that the CL secretes progesterone, estradiol, and testosterone, and that the ovaries are responsive to gonadotropins at least until the time of ovulation, but become refractory by the early luteal phase. Therefore, in a very general way, the patterns of follicular and luteal steroidogenesis in shrews are not unlike those of other mammals. However, the timing of periovulatory changes in steroidogenesis and in responses to gonadotropins appears to be dramatically different from most other mammalian species, including other induced ovulators.