Effects of testosterone and sex on the cytoarchitecture and the GAP-43 and tau-1 immunoreactive innervation of the medial nucleus of the amygdala

Freake, Darren John (1994) Effects of testosterone and sex on the cytoarchitecture and the GAP-43 and tau-1 immunoreactive innervation of the medial nucleus of the amygdala. Masters thesis, Memorial University of Newfoundland.

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Abstract

The posterior dorsal division of the medial nucleus of the amygdala (MePD) is a target of gonadal hormone action. It is sexually dimorphic, and many of its neurons contain estrogen and androgen receptors. In adult male rats, its cytoarchitecture depends on testosterone (T), as the volume of MePD is reduced by 27% at 8 weeks post-castration (Malsbury and McKay, 1994). Experiment 1 of this thesis was based on the hypothesis that this atrophic response to castration includes the retraction of axons within MePD. If this hypothesis is correct, T replacement should stimulate axonal growth within MePD. In experiment 1, castrated male rats were given T, and two protein markers of axonal growth, GAP-43 and tau-1, were examined using immunocytochemistry. -- Several recent studies have found sex differences in GAP-43 mRNA levels in the ventromedial nucleus of the hypothalamus (Shughrue and Dorsa, 1993a) and the frontal cortex (Lustig et al., 1993; Lustig et al., 1991) of adult rats. Based on these findings, experiment 2 investigated the possibility of sex differences in GAP-43 and/or tau-1 staining in the region of MePD. -- Four groups of adult males were examined in experiment 1. Eight weeks after castration, controls received empty silastic capsules sc for either 2 (group CC-2) or 8 days (group CC-8), while T-treated males received T-filled silastic capsules sc for either 2 (group C+T-2) or 8 days (group C+T-8), before being sacrificed. Alternate series of brain sections were stained for GAP-43, tau-1, and with cresyl violet. The relative optical densities (RODs) of GAP-43 and tau-1 staining in the region of MePD and part of the cerebral cortex were measured. The size of MePD was measured using cresyl violet-stained sections. As staining for both GAP-43 and tau-1 was darker in the region of MePD than in surrounding areas, the area of this darker staining was also measured. The results showed that T-treatment did not augment the density of GAP-43 or tau-1 immunoreactivity in the region of MePD or the cerebral cortex. However, it did increase the size of MePD, since the area of MePD in cresyl-stained sections was significantly larger in group C+T-8 than in group CC-8. -- In experiment 2, brain sections of untreated adult males and diestrous females were stained for GAP-43, tau-1 or with cresyl violet as in experiment 1. Area and ROD measures were acquired as in experiment 1. No sex differences were found in the density of GAP-43 or tau-1 staining in the region of MePD or the cerebral cortex. However, there was a marked sex difference (male > female) in the size of MePD as well as in the areas of dense staining for GAP-43 and tau-1 in this region. -- When data from castrated and intact males from experiment 1 and 2 were compared, it was found that castration did not reduce the density of GAP-43 or tau-1 staining in the region of MePD or the cerebral cortex. Castration did, however, significantly reduce the size of MePD as well as the areas of dense GAP-43 and tau-1 staining in this region. -- In conclusion, the present experiments demonstrate that although T-treatment for 8 days is sufficient to produce a growth response within MePD in castrated male rats, this is not accompanied by any increase in the average density of staining for GAP-43 or tau-1 in the region of MePD. However, the size of MePD and the areas covered by the dense GAP-43 and tau-1 staining in the region of MePD are reduced by castration. Castration probably decreases the size of MePD at least partly by shrinking the dendritic trees of its neurons. This is probably accompanied by a loss innervation of MePD dendrites by GAP-43 and tau-1 immunoreactive axons. The reason for the lack of effect of castration on the average density of staining is not known. It may be that castration reduces both the synthesis and degradation of GAP-43 and tau-1, resulting in no change in their levels within the region where MePD dendrites are still present.

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