Gonadal steroid hormone receptors in the medial amygdala contribute to experience-dependent changes in stress vulnerability

Abstract

Social experience can generate neural plasticity that changes how individuals respond to stress. Winning aggressive encounters alters how animals respond to future challenges and leads to increased plasma testosterone concentrations and androgen receptor (AR) expression in the social behavior neural network. In this project, our aim was to identify neuroendocrine mechanisms that account for changes in stress-related behavior following the establishment of dominance relationships over a two-week period. We used a Syrian hamster model in which acute social defeat stress increases anxiety-like responses in a conditioned defeat test in males and in a social avoidance test in females. First, we administered flutamide, an AR antagonist, via intraperitoneal injections daily during the establishment of dominance relationships in male hamsters. We found that pharmacological blockade of AR prevented a reduction in conditioned defeat in dominant males and blocked an upregulation of AR in the posterior dorsal medial amygdala (MePD) and posterior ventral medial amygdala (MePV), but not in the ventral lateral septum. Next, we administered flutamide into the posterior aspects of the medial amygdala (MeP) prior to acute social defeat stress or prior to conditioned defeat testing in males. We found that pharmacological blockade of AR in the MeP prior to social defeat, but not prior to testing, increased the conditioned defeat response in dominant males and did not alter behavior in subordinates. Finally, we developed a procedure to establish dominance relationships in female hamsters and investigated status-dependent changes in plasma steroid hormone concentrations, estrogen receptor alpha (ERα) immunoreactivity, and defeat-induced social avoidance. We found that dominant female hamsters showed reduced social avoidance regardless of social defeat exposure as well as increased ERα expression in the MePD, but no status-dependent changes in the concentration of plasma steroid hormones. Overall, these findings suggest that achieving and maintaining stable social dominance leads to sex-specific neural plasticity in the MeP that underlies status-dependent changes in stress vulnerability.

Keywords: Aggression; Androgen receptors; Estrogen receptors; Medial amygdala; Social defeat; Social dominance; Stress.

Fig. 1.

Effects of flutamide injections during development of dominance relationships. A) Timeline of drug treatments, dominance encounters, social defeat stress, and conditioned defeat (CD) testing. During CD testing we quantified the duration of B) submissive and defensive, C) aggressive, D) affiliative, and E) nonsocial behavior. Dominant (Dom) animals treated with flutamide (Flu) showed increased submissive and defensive behavior compared to dominants treated with vehicle (Veh). There was also a main effect of social dominance on aggressive behavior. No effects of flutamide treatment were found in subordinates (Sub) or no status (NS) animals. F) Photomicrographs showing representative images of androgen receptor (AR) immunoreactivity (IR) in posterior dorsal and posterior ventral medial amygdala (MePD and MePV, respectively, 10x magnification, scale bar = 200 μm, OT: optic tract). The number of AR-positive cells were quantified in the G) MePD and H) MePV. Flutamide-treated dominants showed fewer AR-positive cells compared to vehicle-treated dominants in both the MePD and MePV. There were no effects of flutamide treatment on AR immunoreactivity in subordinates (Sub) or no status (NS) animals. Asterisks (*) indicate a significant difference between treatment groups (p < .05). n = 7-10 animals per group.

Fig. 2.

Effects of flutamide microinjection into posterior aspects of the medial amygdala (MeP). A) Timeline of dominance encounters and flutamide (Flu) or vehicle (Veh) injections prior to social defeat or conditioned defeat testing. B) The location of microinjections is shown using illustrations adapted from a hamster stereotaxic atlas. Black circles indicate injection sites within the MePD or MePV, while open circles indicate injection sites for anatomical controls. Circles also indicate more than one microinjection. We injected flutamide into the MeP prior to social defeat and quantified C) submissive and defensive, D) aggressive, E) affiliative, and F) nonsocial behavior at conditioned defeat testing. Flutamide treatment increased the duration of submissive and defensive behavior in dominants (Dom) but not subordinates (Sub). We injected flutamide into the MeP prior to conditioned defeat testing and quantified the duration of G) submissive and defensive, H) aggressive, I) affiliative, and J) nonsocial behavior. Flutamide treatment did not alter the conditioned defeat response, although there was a main effect of dominance status on submissive and defensive behavior and affiliative behavior. Asterisks (*) indicate a significant difference between treatment groups (p < .05). n = 8-10 animals per group.

Fig. 3.

Effects of social dominance on gonadal steroid receptors and responses to social defeat stress in female hamsters. A) Timeline for dominance encounters, blood collection, social defeat stress, and social avoidance testing. Following social defeat stress, animals received social avoidance testing and we quantified B) time spent in the interaction zone, C) the interaction ratio, D) time spent in the far zone, and E) the frequency of flank marks. We found that dominant (Dom) animals spent more time in the interaction zone compared to subordinates (Sub) and no status (NS) animals following social defeat stress. In addition, in animals not exposed to social defeat stress, dominants spent more time in the interaction zone than subordinates. Further, we found a main effect of social dominance on the interaction ratio and a main effect of social defeat on time spent in the far zone. Also, there was an interaction of dominance status and social defeat in the number of flank marks. F) Photomicrographs showing representative images of estrogen receptor alpha (ERα) immunoreactivity (IR) in posterior dorsal and posterior ventral medial amygdala (MePD and MePV, respectively, 10x magnification, scale bar = 200 μm, OT: optic tract). The number of ERα-positive cells were quantified in the G) MePD and H) MePV. We found the dominant animals showed more ERα-positive cells in the MePD compared to subordinates and no status animals. We also quantified the number of cells showing androgen receptor (AR) immunoreactivity but found no significant difference in the I) MePD or J) MePV. Asterisks (*) indicate a significant difference between treatment groups (p < .05). n = 8-12 animals per group.