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.