Ovarian hormones can organize the rat corpus callosum in adulthood

The rat corpus callosum (CC) is larger in males than females, and is responsive to hormone manipulations during development. Previous data suggest that CC sensitivity to testosterone ends by postnatal day 8 (P8). In contrast, responsivity to ovarian hormones extends as late as P25. The current series of experiments investigates whether ovarian hormone effects on the callosum are permanent and whether CC sensitivity to ovarian hormones extends beyond P25. We found that P70 ovariectomy (Ovx) did not affect callosal size, suggesting that ovarian hormone exposure sometime prior to P70 is sufficient 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 defeminized, adult female CC, whereas ovary transfer starting on P55 or P70 counteracted this enlarging effect, resulting in feminized adult CC. Thus, although a previously feminized callosum is not affected by P70 ovarian hormone removal, a not-yet feminized callosum can still be feminized after P70. These findings indicate that there is flexibility in the developmental window within which the female brain is responsive to the active feminization process initiated by ovarian hormones.