Characterizing the behavioral and neuroendocrine features of susceptibility and resilience to social stress

Abstract

Evaluating and coping with stressful social events as they unfold is a critical strategy in overcoming them without long-lasting detrimental effects. Individuals display a wide range of responses to stress, which can manifest in a variety of outcomes for the brain as well as subsequent behavior. Chronic Social Defeat Stress (CSDS) in mice has been widely used to model individual variation following a social stressor. Following a course of repeated intermittent psychological and physical stress, mice diverge into separate populations of social reactivity: resilient (socially interactive) and susceptible (socially avoidant) animals. A rich body of work reveals distinct neurobiological and behavioral consequences of this experience that map onto the resilient and susceptible groups. However, the range of factors that emerge over the course of defeat have not been fully described. Therefore, in the current study, we focused on characterizing behavioral, physiological, and neuroendocrine profiles of mice in three separate phases: before, during, and following CSDS. We found that following CSDS, traditional read-outs of anxiety-like and depression-like behaviors do not map on to the resilient and susceptible groups. By contrast, behavioral coping strategies used during the initial social stress encounter better predict which mice will eventually become resilient or susceptible. In particular, mice that will emerge as susceptible display greater escape behavior on Day 1 of social defeat than those that will emerge as resilient, indicating early differences in coping mechanisms used between the two groups. We further show that the social avoidance phenotype in susceptible mice is specific to the aggressor strain and does not generalize to conspecifics or other strains, indicating that there may be features of threat discrimination that are specific to the susceptible mice. Our findings suggest that there are costs and benefits to both the resilient and susceptible outcomes, reflected in their ability to cope and adapt to the social stressor.

Keywords: Avoidance; Chronic social defeat stress; Coping behaviors; Predictive traits; Social; Threat discrimination.

Fig. 1

Experimental Schematic: Behavioral and endocrine profiling following chronic social defeat stress (CSDS) 1A. Schematic timeline of the 3 timepoints (pre-, during, and post-defeat) used for behavioral phenotyping and endocrine profiling. Each timepoint includes the overall category of experiments conducted. Superscripts indicate the cohorts of mice used for each assay. 1B. Social Interaction (SI) ratios of all 4 cohorts of mice combined. Defeat (n = 80; mean = 1.096, sd = ±0.877) and Control (n = 77; mean = 1.856, sd = ±0.934). Colors: red dash = SI ratio of 1, yellow = susceptible, blue = resilient, grey = control.

Fig. 2

Pre-stress: Basal social interaction and corticosterone levels did not predict resilience or susceptibility post social defeat. 2 A. Schematic timeline of pre-post SI test and AM/PM corticosterone (CORT) sampling. 2 B. In control mice, SI Ratios displayed before the rotation did not correlate with SI ratios measured after the rotation. 2C. In defeated mice, SI ratios measured before defeat did not correlate with SI ratios displayed after defeat. 2D. CORT ELISA showing diurnal patterns of CORT before and after CSDS/CR. For all groups, the diurnal CORT patterns remained stable throughout the experiment, with higher PM CORT. For either pre- or post-CSDS/CR timepoints, there were no differences in CORT levels between groups. Significance Codes: n.s. = not significant. Colors: grey = control, blue = resilient, yellow = susceptible.

Fig. 3

During Stress: Mice that end up susceptible used escape as a coping behavior 3A. Schematic timeline indicating observation of coping behaviors during Day 1 and Day 10 of defeat. 3 B. Average % of freezing, escape, and fight behaviors displayed on Day 1 and Day 10, according to resilient and susceptible phenotype. 3C. i. Ratio of # of fights/total behaviors observed on Day 1 and Day 10 of defeat. Overall, fight behaviors decreased from Day 1 to Day 10, but levels of fighting were similar between resilient and susceptible groups. 3C. ii. Fight ratios measured on Day 1 of defeat were positively correlated with SI ratios measured post-defeat. 3D. i. Ratio of # of escapes/total behaviors observed on Day 1 and Day 10 of defeat. Escape behaviors were higher overall in susceptible compared to resilient mice and also higher overall on Day 10 compared to Day 1.3D. ii. Escape ratios measured on Day 1 of defeat were negatively correlated with SI ratios measured post-defeat. 3D. ii. Escape contingency table illustrating a higher proportion of susceptible mice escape compared to resilient on Day 1.3 E. i. Replication of Escape analysis in an additional cohort presented a trend for susceptible mice to escape more compared to resilient. 3 E. ii. Escape contingency table in replicate cohort illustrating a higher proportion of susceptible mice escape compared to resilient on Day 1. Significance Codes: Group Main Effect α, Day Main Effect β, *p < 0.05. Colors: blue = resilient, yellow = susceptible.

Fig. 4

Post-Stress: Characterization of Behavioral Outcomes. 4 A. Schematic timeline of assessing traditional read-outs of anxiety-like (Open field, OF) and depression-like (Forced swim test (FST) behavior tests. 4 B. Cumulative duration (%) of time spent in the open area of the OF was overall decreased by defeat. 4C. Resilient mice spent less time immobile compared to susceptible mice. 4D. Schematic timeline of Von Frey (VF) testing. 4 E. In the VF test, susceptible mice had increased pain sensitivity compared to controls. 4 F. Lower SI ratios were correlated with lower force (g) filaments used to elicit a response in the VF test (increased pain sensitivity). Significance Codes: Group Main Effect α, *p < 0.05, **p < 0.01. Colors: grey = control, blue = resilient, yellow = susceptible.

Fig. 5

Susceptible mice gained less weight and mounted a blunted CORT response compared to resilient mice. 5 A. Schematic timeline of body weight and Forced Interaction Test (FIT) CORT measurements. 5 B. FIT-induced CORT (ng/mL) levels increased in resilient mice compared to controls. 5C. In defeated mice, lower FIT CORT (ng/mL) levels correlated with lower SI Ratios. 5D. Change in BW (Day 10 – Day 1) indicates that susceptible mice had reduced weight gain compared to resilient and control mice. 5 E. In defeated mice, higher SI ratios were significantly correlated with larger weight gains over the course of the experiment.Significance Codes: Group Main Effect α, *p < 0.05, **p < 0.01. Colors: grey = control, blue = resilient, yellow = susceptible.

Fig. 6

Susceptibility in the SI test was specific to strain used during defeat 6A. Schematic timeline of strain-specificity social interaction testing. 6 B. When tested with the aggressor strain (CD1) as the social partner, susceptible mice had reduced SI ratios compared to the other groups, indicating social avoidance. However, susceptible mice displayed similar SI ratios compared to controls and resilient mice when placed with a C56BL/6 J (BL6). 6C. When tested with a Black Swiss (BS), susceptible mice displayed SI ratios similar to control and resilient mice. Susceptible mice only exhibited social avoidance when a CD1 was the social partner in the SI test. Resilient mice displayed lower SI ratios compared to controls in the CD1 interaction but remained above 1 (socially interactive). Significance Codes: Group Main Effect α, Interaction Effect Ι, *p < 0.05, **p < 0.01, p<****0.0001. Colors: red dash = SI ratio of 1, grey = control, blue = resilient, yellow = susceptible.