Evaluation of the effects of testosterone and luteinizing hormone on regulation of β-amyloid in male 3xTg-AD mice

Abstract

During normal aging, men experience a significant decline in testosterone levels and a compensatory elevation in levels of gonadotropin luteinizing hormone (LH). Both low testosterone and elevated LH have been identified as significant risk factors for the development of Alzheimer's disease (AD) in men. It is unclear whether changes in testosterone or LH primarily underlie the relationship with AD, and therefore may be a more suitable therapeutic target. To examine this issue, we compared levels of β-amyloid (Aβ) immunoreactivity in male 3xTg-AD mice under varying experimental conditions associated with relatively low or high levels of testosterone and/or LH. In gonadally intact mice, Aβ accumulation increased after treatment with the gonadotropin-releasing hormone agonist leuprolide, which inhibits the hypothalamic-pituitary-gonadal (HPG) axis and reduces both testosterone and LH levels. In gonadectomized (GDX) mice with low testosterone and high LH, we also observed increased Aβ levels. Treatment of GDX mice with testosterone significantly reduced Aβ levels. In contrast, leuprolide did not significantly decrease Aβ levels and moreover, inhibited the Aβ-lowering effect of testosterone. Evaluation of hippocampal-dependent behavior revealed parallel findings, with performance in GDX mice improved by testosterone but not leuprolide. These data suggest that Aβ-lowering actions of testosterone are mediated directly by androgen pathways rather than indirectly via regulation of LH and the HPG axis. These findings support the clinical evaluation of androgen therapy in the prevention and perhaps treatment of AD in hypogonadal men.

Figure 1

Comparison of the effects of GnRH agonist leuprolide acetate (LA) and gonadectomy (GDX) on Aβ accumulation and spontaneous alternation behavior in male 3xTg-AD mice. After a 4 mo treatment period, levels of Aβ load in subiculum (A) and CA1 region of hippocampus (B) of male 3xTg-AD mice were quantified in the following treatment groups: Sham GDX, GDX, and Sham + LA. Data show mean Aβ load values (± SEM). (C) Prior to sacrifice, male 3xTgAD mice in all groups were assesses for spontaneous alternation behavior (SAB) in the Y-maze. Data show mean alternation percentage (±SEM). * Denotes p < 0.05 in comparison to Sham GDX (Sham) group.

Figure 2

Hormone regulation of Aβ in subiculum of male 3xTg-AD mice. Representative high magnification photomicrographs show Aβ immunoreactivity in male 3xTg-AD mice in the following treatment groups: Sham GDX (A), GDX+PL (B), GDX+T (C), GDX+Leu (D), and GDX+Leu + T (E). Scale bar = 100µm. (F) Levels of Aβ immunoreactive load in subiculum were quantified across groups. Data show mean Aβ load values (± SEM). * Denotes p < 0.05 in comparison to Sham GDX (Sham) group.

Figure 3

Hormone regulation of Aβ in hippocampus CA1 of male 3xTg-AD mice. Representative high magnification photomicrographs show Aβ immunoreactivity in male 3xTg-AD mice in the following treatment groups: Sham GDX (A), GDX+PL (B), GDX+T (C), GDX+Leu (D), and GDX+Leu + T (E). Scale bar = 100µm. (F) Levels of Aβ immunoreactive load in CA1 of hippocampus were quantified across groups. Data show mean Aβ load values (± SEM). * Denotes p < 0.05 in comparison to Sham GDX (Sham) group.

Figure 4

Hormone regulation of working memory behavior. Hippocampal-dependent working memory performance in male 3xTgAD mice was measured using spontaneous alternation behavior in a Y-maze in the following treatment groups: Sham GDX, GDX+PL, GDX+T, GDX+Leu, and GDX+Leu + T. Data show % alternations (± SEM). * Denotes p < 0.05 in comparison to Sham GDX (Sham) group.