Ovarian hormones can organize the rat corpus callosum in adulthood

The rat corpus callosum (CC) is larger in males than females, and is respon sive to hormone manipulations during development. Previous data suggest tha t CC sensitivity to testosterone ends by postnatal day 8 (P8). In contrast, responsivity to ovarian hormones extends as late as P25. The current serie s of experiments investigates whether ovarian hormone effects on the callos um are permanent and whether CC sensitivity to ovarian hormones extends bey ond P25. We found that P70 ovariectomy (Ovx) did not affect callosal size, suggesting that ovarian hormone exposure sometime prior to P70 is sufficien t to feminize the CC, and that once the callosum is feminized, the effects can not be reversed. We also found that P25 ovariectomy enlarged, or defemi nized, adult female CC, whereas ovary transfer starting on P55 or P70 count eracted this enlarging effect, resulting in feminized adult CC. Thus, altho ugh a previously feminized callosum is not affected by P70 ovarian hormone removal, a not-yet feminized callosum can still be feminized after P70. The se findings indicate that there is flexibility in the developmental window within which the female brain is responsive to the active feminization proc ess initiated by ovarian hormones. (C) 2000 Elsevier Science B.V. All right s reserved.