DEFEMINIZATION OF THE REPRODUCTIVE RESPONSE TO PHOTOPERIOD OCCURS EARLY IN PRENATAL DEVELOPMENT IN THE SHEEP

Photoperiod times the transition to sexual maturity in many seasonal b reeders. In male and female sheep, photoperiod influences the timing o f puberty differentially. Whereas in females, age at sexual maturity i s highly dependent on photoperiod, puberty in males begins at the same age regardless of day length. We have determined that this sex differ ence is due to the organizing action of androgens during prenatal deve lopment. In the present investigation, we studied when during gestatio n (term: similar to 150 days) androgens defeminize the reproductive re sponse to photoperiod. We compared the age at sexual maturity in femal e lambs treated with testosterone prenatally from Days 30 to 76 (Early , n = 7) or 89 to 135 (Late, n = 8) to that of normal males (n = 8) an d normal females (n = 7). To reveal differential responsiveness to pho toperiod, all lambs were maintained from birth under constant long day s (16L:8D), a treatment that inhibits puberty in normal females. The a ge at the pubertal LH rise was determined in a standardized experiment al model (lambs gonadectomized and treated with estradiol). As expecte d in the long day photoperiod, only 1 of 7 normal females had a pubert al rise in LH. In contrast, all males increased LH secretion by 6.7 +/ - 0.6 wk. Similarly, in the Early group, a sustained increase in LH oc curred in all females, but this was delayed relative to the increase i n the males (16.8 +/- 1.7 wk; p < 0.001). The Late group had LH patter ns similar to those of the normal females, with only 3 of 8 females ha ving sustained elevations in LH. These data suggest that a ''critical period'' for the defeminization of the reproductive response to photop eriod occurs early in prenatal development. In addition, it appears th at this critical period and the period for defeminization of the surge mode of gonadotropin secretion occur at similar stages in development . When challenged with an acute increase in estradiol, all normal and Late androgenized females responded with an LH surge. In contrast, non e of the males and only 1 of 7 Early females produced a robust respons e to estradiol.