Susceptibility and resilience to chronic social defeat stress in adolescent male mice: No correlation between social avoidance and sucrose preference

Abstract

Psychosocial stress is the major form of stress faced by children and adolescents and is an important risk factor for the development of mental illnesses. Chronic social defeat stress (CSDS) is a preclinical mouse model that induces an entire spectrum of phenotypes with similar interindividual variability as seen in humans. Following CSDS, adult male mice have been characterized as being either susceptible or resilient to emotional stress on the basis of their social interactions, which was reported to be highly correlated with sucrose preference (SP) when measured after the last defeat episode. We studied adolescent male C57BL/6 mice (30 days old) for susceptibility and resilience to social avoidance, anhedonia and anxiety-like behaviors, body weight change and basal blood corticosterone concentrations after 10 days of CSDS. Defeated adolescents showed reduced SP, reduced social interaction time (with an unknown adolescent male from their same strain), reduced weight gain and higher basal blood corticosterone concentration when compared to nondefeated mice. Only a small proportion of defeated adolescents were either totally susceptible (20%) or totally resilient (30%) in both the SP and social avoidance tests. The remaining defeated mice had a distinct behavioral impairment - susceptible in one test and resilient in the other. Surprisingly, behaviorally resilient defeated adolescents were the most affected population in terms of both endocrine/physiological outcomes. These findings illustrate that, contrary to prior assumptions in adults, the CSDS responses are more complex and singular in adolescents, and caution should be taken for the correct interpretation of those phenotypes. We propose a better characterization of social defeat stress responses as a critical step to advance our understanding of the mechanisms behind stress resilience that translate to human experience.

Keywords: Anhedonia; Animal model; Anxiety; Bullying; Depression; Juvenile; Mood disorders.

Fig. 1

Experimental design. Timeline showing all the steps of the experimental manipulations. Thirty-day-old C57BL/6 male mice were exposed daily to chronic defeat stress (CSDS) from an aggressive CD-1 mouse during 5 min of physical interaction (occurring in the afternoons), followed by 24 h of protected cohabitation with the aggressor (threat period) over 10 days. Control adolescents were paired in similar cages as the experimental mice (and at the same time) but were placed in different compartments; thus, they were protected from physical interactions. All the pairing compositions were changed every day. All adolescent mice were subjected to an overnight sucrose preference (SP) test before (basal) and on the 7th, 8th, 9th and 10th nights of the CSDS period (with measurements performed the next morning). The elevated plus maze (EPM) test and social approach-avoidance (SAA) test were performed in the morning on the last two days of the CSDS, respectively.

Fig. 2

Sucrose test parameters in male adolescent mice after chronic social defeat stress. The results represent the mean of four days of measurement. A) No difference in total liquid intake (sucrose + water; mL) was observed between the control (n = 10) and defeated groups (n = 20). B) Sucrose preference in the defeated group was significantly lower than that in the control group. C) Vertical scatterplot depicting the distribution of sucrose preference for control, resilient and susceptible mice. The dashed line represents 74% preference for sucrose over water, which corresponds to two standard deviations from the control mean and was taken as the criterion for susceptibility. Only susceptible mice (n = 9) displayed anhedonia, as measured by a reduction in sucrose preference. The mean preference for sucrose shown by resilient (n = 11) mice did not significantly differ from that shown by control mice. Mean ± SEM; *p < 0.05.

Fig. 3

Social avoidance behavior in male adolescent mice after chronic social defeat stress. A) No differences in the latency to interact with an unfamiliar adolescent C57BL/6 mouse were observed between the control and defeated groups. B) Defeated mice showed social avoidance spending significantly less time interacting with the social target. C) Vertical scatterplot showing the interaction time for the control, resilient and susceptible groups. The dashed line represents two standard deviations from the control mean, which is ~70 s. Susceptible mice (n = 9) spent less time engaged in social interaction than controls. The time spent in social interaction by resilient (n = 11) mice did not significantly differ from controls (n = 10). Mean ± SEM; *p < 0.05.

Fig. 4

Relationship between sucrose preference and interaction time parameters of socially defeated adolescent male mice and clustering observation. Sucrose preference and interaction time values of experimental mice (n = 20) were transformed into z-scores using control animal data. Defeated adolescents were clustered in subgroups according to resilience and susceptibility to each parameter. No correlation was observed between sucrose preference and interaction time scores (r² = 0.019; p > 0.05). The dashed lines represent the separation criterion value for resilience and susceptibility according to each parameter.

Fig. 5

Anxiety-like behaviors in the EPM of male adolescent mice after chronic social defeat stress. (A) Time spent in the open arms (s), B) the number of entries in the open arms in relation to open + closed arms, and C) the distance traveled in the open arms (cm) in the five total minutes the mice spent in the EPM. There were no differences between the control (n = 10) and the defeated (n = 20) groups (p > 0.05). Mean ± SEM.

Fig. 6

Anxiety-like behaviors in the EPM of male adolescent mice after chronic social stress according to susceptibility and resilience to the SP and SAA tests. Susceptible animals in the sucrose preference test (n = 9) did not differ from controls (n = 10) and resilient mice (n = 11) in the time spent in the open arms (s), the number of entries in open arms (over total entries), and the distance traveled in the open arms (cm) (p > 0.05; Fig. 6A). Susceptible animals in the social approach-avoidance test (n = 9) did not differ from controls (n = 10) and resilient mice (n = 11) in any parameter from the EPM test (p > 0.05; Fig. 6B). Mean ± SEM.

Fig. 7

Body weight gain in male adolescent mice after chronic social defeat stress. (A) Defeated animals (n = 20) gained significantly less weight over the 10-day period compared to controls (n = 10). (B) Segregation of resilient and susceptible defeated mice according to the SP or SAA tests. Resilient groups in both subsets gained significantly less body weight when compared to controls. (C) Segregation of defeated mice according to both SP and SAA resiliency-susceptibility spectra. In total, resilient mice (to both SP and SAA parameters - blue) had less body weight gain than controls. Resilient animals exclusively in the SP (green) or exclusively in the SAA (purple), and susceptible to both SP and SAA parameters (red). Mean ± SEM; *p < 0.05.

Fig. 8

Serum corticosterone concentrations (ng/mL) in male adolescent mice 24 h after the last episode of CSDS. (A) Defeated mice (n = 20) exhibited increased levels of blood corticosterone compared to controls (n = 10). (B) No difference in blood corticosterone concentration was observed among the control, susceptible (n = 9) and resilient (n = 11) groups when susceptibility and resilience were based on the SP test results, but resilient mice according to the SAA test results showed increased corticosterone when compared to the control group (p < 0.05). Mean ± SEM.