Anterior commissure and corpus callosum size increase disproportionately with advancing age in rats.

Michael Noonan, Katherine Sion, Michael Viksjo & Katherine Wescott

Presented at the Society for Neuroscience, Los Angeles, November, 1998.

Introduction

The Corpus Callosum and Anterior Commissure are large bundles of fibers which interconnect the two cerebral hemispheres. Considerable interest has been shown in recent years in the question of whether these two commissures vary in size between the sexes. Last year, we published a paper which reported a male-greater-than-female size difference for the AC, even when the sexes were equated for overall brain size. At that time, we hypothesized that the sex difference in the AC would be greater for its anterior limb than for its posterior limb, and that this difference would vary with age. In the present study we investigated these questions in a large sample of rats.

Procedures

Our subjects were 214 long-evans rats divided among 5 age groups. All subjects had been previously utilized in other, unrelated behavioral investigations, but were physiologically untreated. For this study, following intracardial perfusion, frozen sagittal sections of their brains were taken beginning at the midline and stained for myelin. The total areas encompassed by the CC, ACa, and ACp were assessed separately in the first intact section. Representative sections are presented in Figs 1 and 2.

Results

Sex. Male and females differed significantly in overall brain weight, and this male-greater-than-female sex difference was true at each age when analyzed separately (Fig. 3). There was also a significant sex by age interaction reflecting the fact that while the males continued to show an increase in overall brain size with age, females showed a slight decrease in size beyond one year of age. Concomitant with the overall size difference, the commissures were likewise larger in male than in female brains when these areas were analyzed without adjustment.

However when the commissure sizes were adjusted for variations in overall brain size (whether using ratio, regression, or log-transform methods), there were no significant sex differences for either commissure taken as a whole (CC or AC), or for either subdivision (ACa, ACp). This also held true when each age was analyzed separately. Similarly, there were no sex by age interactions for any adjusted measure of commissure size, nor were any significant sex differences found for any ratio relating CC, ACa, and ACp.

Age. The effect of age proved to be significant in all analyses of adjusted commissure size (regardless of the method used to adjust for overall brain size). This reflected the fact that for all three commissural divisions (CC, ACa, ACp) the areas increased in size with age beyond that attributable to increases in overall brain size. This effect was primarily due to increases in adjusted commissure size in our oldest age group. This was true for both sexes.

Our last figure depicts three regression lines which best fit the slope of our commissure area measures with age (centered around zero). The rate of increase with age is greatest for the corpus callosum, next greatest for ACa, and least for ACp.

Discussion

We feel that the very large sample of rats contained in the present study should put to rest the question of whether sex differences in commissure size exist in this strain of rat. The commissures do not vary in size between the sexes when adjusted for difference in overall brain size.

For both sexes, the commissures increase in size with advancing age at a rate beyond that shown for the brain overall. We are presently endeavoring to determine whether these increases in size are accompanied by a concomitant increase in the number of fibers.

 

Contact Info: Michael Noonan, PhD, Canisius College , 2001 Main St., Buffalo, NY 14208                                                                             noonan@canisius.edu