Female corpus callosum development and adult learning and memory processes: Parameters governing the effects of ovarian hormones

Date of Completion

January 2000


Biology, Neuroscience|Psychology, Psychobiology|Psychology, Cognitive|Health Sciences, Human Development




This laboratory has previously shown that the male rat corpus callosum (CC) is larger than the female's, and that, in both sexes, gonadal hormone exposure is necessary for normal CC development. Several experiments in this dissertation evaluated the temporal parameters guiding ovarian hormone effects on female CC morphology. ^ Postnatal day 25 (P25), but not P70, ovariectomy (Ovx) resulted in enlarged adult CC, thereby suggesting that CC feminization depends upon ovarian hormones, is irreversible, and ends between P25 and P70. ^ Although callosal feminization is not reversed by P70 ovarian hormone removal, it is affected by P70 ovarian hormone replacement. In three experiments, P70 ovary transfer (OvT) or estrogen replacement counteracted the enlarging effects of P25 Ovx. Further, neonatal estrogen blockade before P25 Ovx prevented normal CC responsiveness to P70 OvT and estrogen replacement. ^ Previous work showed that neonatal Ovx effects are first expressed between P90 and P110. This suggests that P25 Ovx females did not exhibit CC enlargement, beyond that of normal growth, at the time of P70 OvT. Since P25 Ovx effects are expressed by P130, we gave OvT on P130 to investigate whether CC feminization could occur after enlargement has begun. Results showed that P130 OvT still counteracted the callosal size increase due to P25 Ovx. ^ Taken together, these results suggest that ovarian hormone effects on the female CC are irreversible, sensitivity to these effects extends at least through P130, and normal CC responsiveness to adult ovarian hormones depends upon previous estrogen exposure. ^ Other experiments using a water version of the radial-arm demonstrated that female rats and mice exhibited superior working memory and inferior reference memory compared to males; that female rats exhibited more vicarious-trial-and-error behavior than males; and, that estradiol supplementation to adult Ovx females decreased perseverative and working memory errors as the memory load increased. ^ Collectively, these findings indicate that ovarian hormones actively influence the female brain from at least the neonatal period, continuing through adulthood, resulting in the normal female phenotype. ^