| There are many sexual dimorphisms in the rat brain that are not directly related to reproduction and it is unclear when many of these differences develop as well as how they may be affected by aging. The primary visual cortex (Oc1) and splenium of the corpus callosum (which contains axons projecting from Oc1) are both sexually dimorphic in adulthood, with male rats having more neurons in Oc1 (Reid & Juraska, 1992a) and a greater number of myelinated axons in the splenium (Kim, Ellman, & Juraska, 1996) compared to females. Previous work has demonstrated effects of ovarian hormones on neuron number in Oc1 (Nunez, Sodhi, & Juraska, 2002) and on the size of the corpus callosum (Bimonte et al., 2000), suggesting that sex differences may not be present prior to puberty. To determine the effects of age on sex differences in Oc1, neuron number was examined in male and female rats at day 20 (pre-weaning), day 35 (peripuberty), and day 90 (adult). Sex differences were not present at day 20 or 35 but were seen at day 90 as a result of increases in neuron number, which may be smaller in females due to inhibitory actions of ovarian hormones. Similar effects of ovarian hormone exposure were seen in the splenium. Females were ovariectomized before puberty (day 20) and examined in adulthood to determine the effects of ovarian hormones on axons within the splenium. Animals that underwent ovariectomy prior to puberty had more myelinated axons compared to sham animals, suggesting that ovarian hormones inhibit myelination within the corpus callosum.;Effects of aging were also investigated in Oc1 and the splenium. Examination of neuron number in aged animals (19-22 months) found that Oc1 was significantly impacted during aging, with both males and females losing neurons (18-20%) between day 90 and old age, maintaining the sex differences in neuron number. However, examination of the splenium in adult (day 120), middle age (12-13 months) and aged (18-26 months) animals showed no age-related deterioration. Instead, the splenium increased in size between adulthood and old age, demonstrating the regionally-specific effects of age in the rat. |
