Gonadal steroid organization and behavioral correlates of corpus callosum morphology in the rat

Date of Completion

January 1996

Keywords

Biology, Anatomy|Biology, Neuroscience|Psychology, Psychobiology|Biology, Animal Physiology|Psychology, Physiological

Degree

Ph.D.

Abstract

This laboratory has shown endogenous gonadal hormones to play a role in determining the sexual dimorphism (M $>$ F) in the midsagittal size of the adult rat corpus callosum (CC). The first set of experiments were designed to more clearly define the sensitive period of hormone action. In the female, chronic estrogen replacement on Day 25 to rats ovariectomized on Day 12 was found to reverse the enlarging effect of ovariectomy on CC size. Callosal size was not affected by fluctuating hormone levels associated with the estrous cycle, nor by ovariectomy in adulthood (Day 78), pointing to an organizational role of ovarian steroids in CC development. These data also show that the female CC remains responsive to ovarian input late in development, with a sensitive period of hormone action ending between Days 25 and 78. Also in the female, it was shown that ovariectomy on Day 12 increased callosal size regardless of early handling conditions. This contrasts the effect Day 4 androgen treatment, which has previously been shown to increase callosal size in handled, but not nonhandled females. In the male, castration on Day 1 prior to the two-hour postnatal testosterone surge had no effect on CC size. Since blockade of androgens from gestational Day 17 through adulthood has previously been found to reduce male CC size to that of the female, the organization of the male CC can be attributed to the action of prenatal androgens. A second set of experiments investigated behavioral correlates of regional callosal size. A positive relationship was found between the posterior midbody of the CC and Morris maze performance in handled male rats. A negative association was also found between the posterior portion of the splenium and activity levels in handled females. In the last experiment, sex differences in callosal ultrastructure were investigated. Electron microscopic examination of the genu revealed a greater number of unmyelinated axons in females, whereas there was no sex difference in the number of myelinated axons, nor in the size of either axon type. ^

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