Mice with the testicular feminization mutation demonstrate a role for androgen receptors in the regulation of anxiety-related behaviors and the hypothalamic–pituitary–adrenal axis

Abstract

Testosterone (T) appears to play a role in anxiety and sensorimotor gating in rodents, but whether T acts through the androgen receptor (AR) to influence these behaviors is less clear. We compared adult genetic male mice with the testicular feminization mutation (Tfm), which lack functional ARs, to wild type male littermates (wt males) on an assay of sensorimotor gating (prepulse inhibition of the acoustic startle response; PPI) and several tests thought to reflect anxiety: open field exposure, novel object exposure, elevated plus maze (EPM), and light/dark (LD) box. PPI was similar between groups, but indices of anxiety in the novel object and LD box tests were increased in Tfm males with no significant differences found in the open field or EPM. Since Tfm male mice have decreased circulating T, the same tests were conducted in mice that were gonadectomized (wt males) or sham-operated (Tfm males) as adults and supplemented with T or nothing (B). While T treatment reduced indices of anxiety in the novel object and LD box tests in wt males, it was ineffective in Tfm males. Increased indices of anxiety in Tfm males appear to be related to hyper-activation of the hypothalamic–pituitary–adrenal axis since levels of the stress hormone corticosterone were elevated in Tfm males compared to wt males at baseline and at several time points after exposure to a novel object. These findings demonstrate that ARs influence anxiety and stress responses in mice.

Introduction

In humans, gonadal hormones have been implicated in the development and maintenance of several mental disorders including anxiety, depression, and schizophrenia. Women suffer from anxiety disorders and depression more frequently than do men (Weinstock, 1999, Kornstein, 1997, Wilhelm et al., 1998), and variations in the ovarian hormone estrogen appear to contribute to the etiology of these disorders in women (Arpels, 1996, Yazici et al., 2003). Evidence in animal models supports a role of estrogen in mood disorders. In both male and female rodents, estrogens have been shown to decrease depressive and anxiety-related behavior (Frye and Lacey, 2001, Walf and Frye, 2005), particularly through activation of the estrogen receptor (ER) isoform ERβ (Lund et al., 2005, Imwalle et al., 2005).

Emerging evidence suggests that androgens also contribute to mood disorders. Boys and girls with low T levels show increased indices of depression and anxiety compared to those with high T (Granger et al., 2003). Furthermore, in hypogonadal and aging men who have low levels of circulating T, mood disorders are increased (Kaminetsky, 2005, Lund et al., 1999, Amore, 2005, Eskelinen et al., 2007, Cooper and Ritchie, 2000). T treatment of these individuals can decrease symptoms of depression and anxiety (Kumano, 2007, Kaminetsky, 2005, Cooper and Ritchie, 2000).

Androgens have also been shown to play a role in anxiety-related behavior in rodents, with an increase in T generally correlated with decreased indices of anxiety (Frye et al., 2008, Bing et al., 1998, Bitran et al., 1993). Reducing androgen levels via gonadectomy in male rodents increases anxiety and fear-related behaviors (Adler et al., 1999, Bitran et al., 1993, Frye et al., 2008, Frye and Edinger, 2004). Supplementation with T also decreases indices of anxiety in several rodent models (Frye et al., 2008, Aikey et al., 2002, Edinger and Frye, 2005). As in hypogonadal men, diminished testicular production of T in mice is associated with anxiogenesis and depressive behaviors (Umehara et al., 2006). In rodents, aging, which has also been correlated with increased anxiety-related behaviors (Koprowska et al., 2004, Boguszewski and Zagrodzka, 2002), is accompanied by a decline in T. A recent study suggests that anxiety-related behavior in aged male mice can be reduced by administration of androgens (Frye et al., 2008).

One mechanism through which androgens may influence anxiety-related behavior is through the regulation of the hypothalamic–pituitary–adrenal (HPA) axis. While HPA axis activation is generally adaptive, hyper-activation may be maladaptive and has been associated with increased indices of anxiety and depression in both humans and rodents (Scott and Dinan, 1998, Landgraf et al., 1999, Lund et al., 2005). In rodents, administration of T, and its metabolites dihydrotestosterone (DHT) and 5alpha-androstane, 3beta,17beta-diol (3β-diol), can suppress the normal rise of stress hormones (adrenocorticotropic hormone (ACTH) from the pituitary and corticosterone from the adrenal cortex) following exposure to a stressful situation (Handa et al., 1994, Lund et al., 2004, Lund et al., 2005, Lund et al., 2006), suggesting androgens can influence HPA axis activity, and in turn, may potentially influence anxiety-related behaviors.

Gonadal hormones, particularly estrogens, have also been hypothesized to play a neuroprotective role in schizophrenia, which may contribute to the later onset and milder symptoms of this disease in women compared to men (Häfner et al., 1998, Castle et al., 1995). One common characteristic in people with schizophrenia is a deficit in sensorimotor gating, the capacity to filter sensory, motor, and cognitive information (Kodsi and Swerdlow, 1994). This reduction in sensorimotor gating may be linked to symptoms of schizophrenia such as information processing deficits, disorders of thought and auditory hallucinations (Perry and Braff, 1994, Kumari et al., 2008). In an operational measure of sensorimotor gating, prepulse inhibition of the acoustic startle response (PPI; Swerdlow et al., 1996), people with schizophrenia show reduced PPI indicating a deficit in sensorimotor gating (Braff et al., 1992). This deficit is alleviated by treatment with atypical antipsychotic medications (Kumari et al., 2002).

Animal models suggest that gonadal hormones can also influence sensorimotor gating. In female rats, PPI varies depending on the phase of the estrus cycle (Koch, 1998) and administration of estrogen to ovariectomized females increases PPI (van den Buuse and Eikelis, 2001). T has also been shown to facilitate PPI in male and female rats (Gogos and van den Buuse, 2003), however, another recent study suggests that in male rats PPI is unaffected by treatment with androgens or estrogens (Turvin et al., 2007). In mice the role of gonadal hormones in PPI has been less studied, and the roles of specific hormone receptors, particularly the AR, in modulating PPI are largely unknown, although studies in aromatase knockout (ArKO) males suggest that estrogenic metabolites of androgen acting through ERs can influence PPI (van den Buuse et al., 2003, Gogos et al., 2006).

One model for exploring the role of the AR in the brain and behavior is testicular feminization mutant (Tfm) mice (Zuloaga et al., 2008). Tfm mice have a mutation in the AR gene that involves a single nucleotide deletion (Charest et al., 1991), which introduces a reading frame shift and premature stop codon, resulting in a shortened transcript and essentially no AR protein (He et al., 1991, Monks et al., 2007). Consequently these mice have virtually no sensitivity to androgen through the AR (Drews, 1998). Since this trait is X-linked, only genetic males (XY) are wholly androgen insensitive. To further investigate the role of the AR in anxiety-related behavior, regulation of the HPA axis, and sensorimotor gating in males, we compared behavioral and physiological responses in wt and Tfm male mice. Because Tfm male mice have significantly decreased circulating T levels (Jones et al., 2003) behavior was also assessed in mice provided with exogenous T in adulthood. These studies indicate that the AR regulates the HPA axis and plays a role in many behaviors associated with anxiety in mice, but plays little if any role in regulating PPI.