The Anxiolytic and Antidepressant-like Effects of Testosterone and Estrogen in Gonadectomized Male Rats

Abstract

Background: While the influence of testosterone levels on vulnerability to affective disorders is not straightforward, research suggests this hormone may confer some degree of resiliency in men. We recently demonstrated a role for the dentate gyrus in mediating testosterone's protective effects on depressive-like behavior in gonadectomized male rats. Here, testosterone may exert its effects through androgen receptor-mediated mechanisms or via local aromatization to estradiol.

Methods: Gonadectomized male rats were implanted with a placebo, testosterone, or estradiol pellet, and subsequent protective anxiolytic- and antidepressant-like effects of testosterone and its aromatized metabolite, estradiol, were then investigated in the open field and sucrose preference tests, respectively. Moreover, their influence on gene expression in the hippocampus was analyzed by genome-wide complementary DNA microarray analysis. Finally, the contribution of testosterone's aromatization within the dentate gyrus was assessed by local infusion of the aromatase inhibitor fadrozole, whose efficacy was confirmed by liquid chromatography-tandem mass spectrometry.

Results: Both hormones had antidepressant-like effects associated with a substantial overlap in transcriptional regulation, particularly in synaptic plasticity- and mitogen-activated protein kinase pathway-related genes. Further, chronic aromatase inhibition within the dentate gyrus blocked the protective effects of testosterone.

Conclusions: Both testosterone and estradiol exhibit anxiolytic- and antidepressant-like effects in gonadectomized male rats, while similarly regulating critical mediators of these behaviors, suggesting common underlying mechanisms. Accordingly, we demonstrated that testosterone's protective effects are mediated, in part, by its aromatization in the dentate gyrus. These findings thus provide further insight into a role for estradiol in mediating the protective anxiolytic- and antidepressant-like effects of testosterone.

Keywords: Anxiety; Aromatase; Depression; Estradiol; Hippocampus; Testosterone.

Figure 1

Testosterone or estrogen replacement prevents gonadectomy-induced anhedonia. While gonadectomized rats receiving a placebo pellet (GDX) have a sucrose preference lower than sham-operated animals, gonadectomized rats with testosterone (GDX+T) or estrogen (GDX+E) replacement exhibit similar levels compared to sham-operated animals (Sham). * p < 0.05 vs. SHAM, GDX+T and GDX+E, Fisher’s PLSD post-hoc test.

Figure 2

Testosterone or estrogen replacements induce a similar profile of gene expression in the dorsal hippocampus of gonadectomized rats. (2A) The Venn diagram represents the genes uniquely regulated by testosterone replacement (left section, 1 327 genes), estradiol replacement (right section, 1 209 genes), or commonly regulated by both treatments (intersection, 2 302 genes). Only the genes with a minimum fold-change of 1.5 were considered. (2B) Heatmap representation of hierarchical clustering of gene expression levels. Rats receiving testosterone (GDX+T) or estradiol (GDX+E) replacements exhibit a similar profile of regulation compared to gonadectomized animals treated with a placebo pellet (GDX).

Figure 3

Aromatase inhibition in the dorsal dentate gyrus blocks the anxiolytic- and antidepressant-like effects of testosterone replacement. (3A,B) When infused with saline (GDX+T+SAL), gonadectomized rats receiving testosterone replacement spent more time in the center of an open field than gonadectomized rats receiving a placebo pellet and infused with saline (GDX+SAL), but not when infused with the aromatase inhibitor fadrozole (GDX+FAD+T). ** p < 0.01 vs. GDX+SAL, * p < 0.05 vs. GDX+FAD and GDX+FAD+T, Fisher’s PLSD post-hoc test. (3C) None of the treatments affected rats’ locomotion in the open field. (3D) Testosterone replacement increased sucrose preference of gonadectomized rats infused with saline, but not fadrozole. (3E) Representation of individual values depicted in panel 3C, highlighting the greater variability observed following treatment with the aromatase inhibitor fadrozole, compared to animals infused with saline. ** p < 0.01 vs. GDX+SAL, * p < 0.05 vs. GDX+FAD+T, † p = 0.0503 vs. GDX + FAD, Mann-Whitney post-hoc test.

Figure 4

Fadrozole effectively inhibits local conversion of testosterone to estradiol in the dorsal hippocampus. (4A) Representation of the infusion sites (black circles) on The Rat Brain Atlas (Paxinos and Watson), ranging from −3.60 mm to −4.52 mm from Bregma. (4B) Significantly elevated levels of testosterone were measured in the dorsal hippocampus of testosterone-supplemented male rats receiving fadrozole infusions compared to those receiving saline infusions. Conversely, low to undetectable levels of testosterone were observed in placebo-treated rats, regardless of local aromatase inhibition.