Males, but not females, lose tyrosine hydroxylase fibers in the medial prefrontal cortex and are impaired on a delayed alternation task during aging

Abstract

The structure of the prefrontal cortex (PFC) is particularly vulnerable to the effects of aging, and behaviors mediated by the PFC are impaired during aging in both humans and animals. In male rats, behavioral deficits have been correlated with a decrease in dopaminergic functioning. However, studies have found that anatomical changes associated with aging are sexually dimorphic, with males experiencing greater age-related loss than females. The present study investigated the effects of sex and aging on performance of a delayed alternation t-maze, a task mediated by the medial prefrontal cortex (mPFC), and on tyrosine hydroxylase (TH) immunoreactivity in this brain region using adult (7 months) and aged (21 months) male and female F344 rats. There was a sex by age interaction in performance of the delayed alternation task such that adult males performed better than aged males, but aged females were not different than adult females. Adult males performed better than adult females across all delays; however, this sex difference was reversed during aging and aged males performed worse than aged females. In addition, TH immunoreactivity decreased during aging in layers 2/3 in the male, but not female mPFC. Thus females were less sensitive to the effects of aging on the prefrontal dopaminergic system and on performance of a delayed alternation task. These effects may be due to decreases in testosterone in aging males, as well as the protective effects of ovarian hormones, which continue to be secreted after cessation of the estrous cycle in aging females.

Figure 1

A. Tyrosine hydroxylase immunostained fibers in a Z-stacked image that has been compressed using Axiovision software. Image pixel density (the percent of the image in black) was measured in two ways: as a binary image (B), taking thickness into account, and as a skeletonized image (C), reducing the thickness to 1 pixel wide.

Figure 2

TH immunostained fibers in layer 1(A) layers 2/3 (B) and layers 5/6 (C) that illustrate the differences between the layers.

Figure 3

Average number of correct alternations across all delays (mean+ SEM). There was an effect of age (p =.001), and an interaction between age and sex (p = .001). Adult males made more correct choices than adult females across all delays. Aged females made more correct choices than aged males across all delays. ** p <.02, * p <.05

Figure 4

Total number of correct alternations at each delay (mean+ SEM). Adult males performed better than aged males at all delays except the five minute delay. Performance in aged females did not differ from adult females at any of the delays. There was also an effect of delay (p <.01) such that increasing the inter-trial delay reduced performance. * p<.05

Figure 5

The density of TH fibers in Layer 1 (mean+ SEM). In skeletonized images (A), there was an effect of sex (p < .04) and an age × sex interaction (p <.03). Aged females had higher TH pixel densities than aged males and adult females. In binary images (B) there was a trend for an age × sex interaction (p <.07). ** p <.02, * p <.05

Figure 6

The density of TH fibers in layers 2/3 (mean+ SEM). In skeletonized images (A), there was a significant age × sex interaction (p < 0.02). Aged males had lower TH pixel densities than adult males and aged females. Aged females were not different than adult females. In binary images (B), there was a significant age × sex interaction (p < 0.03). Aged males had lower TH fiber densities than adult males and aged females. ** p <.01, * p<.05

Figure 7

The density of TH fibers in layers 5/6 (mean+ SEM). In skeletonized images (A), there was an effect of sex (p < 0.05). In binary images (B), there was a trend for an effect of sex (p < 0.07).