Activation of androgen receptor-expressing neurons in the posterior medial amygdala is associated with stress resistance in dominant male hamsters

Abstract

Social stress is a negative emotional experience that can increase fear and anxiety. Dominance status can alter the way individuals react to and cope with stressful events. The underlying neurobiology of how social dominance produces stress resistance remains elusive, although experience-dependent changes in androgen receptor (AR) expression is thought to play an essential role. Using a Syrian hamster (Mesocricetus auratus) model, we investigated whether dominant individuals activate more AR-expressing neurons in the posterior dorsal and posterior ventral regions of the medial amygdala (MePD, MePV), and display less social anxiety-like behavior following social defeat stress compared to subordinate counterparts. We allowed male hamsters to form and maintain a dyadic dominance relationship for 12 days, exposed them to social defeat stress, and then tested their approach-avoidance behavior using a social avoidance test. During social defeat stress, dominant subjects showed a longer latency to submit and greater c-Fos expression in AR+ cells in the MePD/MePV compared to subordinates. We found that social defeat exposure reduced the amount of time animals spent interacting with a novel conspecific 24 h later, although there was no effect of dominance status. The amount of social vigilance shown by dominants during social avoidance testing was positively correlated with c-Fos expression in AR+ cells in the MePV. These findings indicate that dominant hamsters show greater neural activity in AR+ cells in the MePV during social defeat compared to their subordinate counterparts, and this pattern of neural activity correlates with their proactive coping response. Consistent with the central role of androgens in experience-dependent changes in aggression, activation of AR+ cells in the MePD/MePV contributes to experience-dependent changes in stress-related behavior.

Keywords: Aggression; Androgen receptor; Medial amygdala; Proactive coping; Social defeat; Social dominance; Social vigilance.

Figure 1.

Diagram of experimental design. Using a resident-intruder format, animals were exposed to the same partner in 12 daily dominance encounters to establish dominant-subordinate relationships. No status animals remained in their home cage and were handled daily. Then, 48 hours after the final dominance encounter, subjects were exposed to social defeat stress in a series of 3, 5-minute encounters in which they were placed in the home cage of a larger, more aggressive hamster. No status animals were either exposed to social defeat stress, or they were placed in the empty cage of three separate resident aggressors and called handled controls. In Experiment 1, animals were euthanized for brain collection 90 minutes after social defeat. In Experiment 2, animals were euthanized for brain collection 90 minutes after social avoidance (SA) testing. SA testing consisted of an acclimation phase with an empty box and testing phase with a box containing an unfamiliar adult.

Figure 2.

Representative images of AR+ and c-Fos+ cells in a dominant and subordinate pair. A) Coronal image showing the location of the posterior medial amygdala. B) Representative confocal image of AR+, c-Fos+, and double-labeled cells. C) Images were collected in the dorsal and ventral portions of the posterior medial amygdala (MePD and MePV). The majority of c-Fos expression in the MePV was located in medial aspects near the optic tract. D-F) Representative MePD images collected at 20x magnification with a 300μm × 300μm clip region show AR+ cells, c-Fos+ cells, and double-labeled cells in dominant and subordinate subjects (G-I). Pink arrows indicate double-labeled cells.

Figure 3.

Mutual aggression and adjusted latency to submit during the first social defeat encounter. Dominant animals showed a greater duration of mutual aggression compared to subordinates and no status (NS) animals in A) Experiment 1 and C) Experiment 2. Dominants also were slower to submit after receiving aggression compared to subordinates and NS animals in B) Experiment 1 and D) Experiment 2. * Indicates p < 0.05.

Figure 4.

Patterns of AR and c-Fos expression during social defeat. A, D) Dominants and subordinates showed more AR+ cells in the MePD and MePV than no status (NS) and handled control animals. B) All subjects that were defeated showed more c-Fos+ cells in the MePD compared to handled controls. In addition, dominants had more c-Fos+ cells in the MePD compared to subordinates and no status animals. C) In the MePD, social defeat increased the proportion of AR+ cells showing c-Fos immunoreactivity compared to handled controls. In addition, dominants showed a greater proportion of MePD AR+ cells with c-Fos immunoreactivity compared to subordinates. E) In the MePV, dominants showed more c-Fos+ cells compared to handled controls. F) Subordinates showed a smaller proportion of MePV AR+ cells with c-Fos immunoreactivity compared to dominants and no status animals. * Indicates p < 0.05.

Figure 5.

Regressing patterns of AR and c-Fos expression on behavior at social defeat stress. A) Dominance status did not interact with MePD c-Fos expression to predict latency to submit. B) MePV c-Fos expression was positively associated with latency to submit in dominant animals. C) Dominance status did not interact with the proportion of MePD AR+ cells with c-Fos immunoreactivity to predict latency to submit. D) In contrast, the proportion of MePV AR+ cells with c-Fos immunoreactivity was positively associated with latency to submit in dominants. * Indicates p < 0.05.

Figure 6.

Approach-avoidance behavior in Experiment 2. A) During the acclimation phase there were no differences in interaction time with an empty box. There was an effect of social defeat on interaction time during the testing phase, as handled controls spent more time interacting with an unfamiliar adult compared to those exposed to social defeat stress. B) Handled controls had a significantly greater interaction ratio than animals exposed to social defeat. C) Animals exposed to social defeat spent more time engaged in social vigilance compared to handled controls. Also, dominants displayed greater social vigilance than no status (NS) animals and handled controls. * Indicates p < 0.05.

Figure 7.

Patterns of AR and c-Fos expression in Experiment 2. A, D) There were no differences in AR expression in the MePD and MePV. B) There were no differences in c-Fos expression in the MePD. E) Subordinate animals showed significantly fewer c-Fos+ cells in the MePV compared to no status (NS) animals and handled controls. C) In the MePD, handled controls had a greater proportion of AR+ cells with c-Fos immunoreactivity compared to dominants and subordinates. Also, subordinates showed a significantly smaller proportion of MePD AR+ cells with c-Fos immunoreactivity than NS animals. F) In the MePV, subordinates showed a significantly smaller proportion of AR+ cells with c-Fos immunoreactivity compared to dominants, NS animals, and handled controls. * Indicates p < 0.05.

Figure 8.

Regression analysis on social vigilance during social interaction testing and the proportion of double-labeled cells. A) There was no association between the proportion of AR+ cells in the MePD showing c-Fos immunoreactivity and duration of social vigilance. B) The proportion of AR+ cells in the MePV showing c-Fos immunoreactivity was more strongly associated with social vigilance in dominant animals than in subordinate and no status (NS) animals. * Indicates p < 0.05.