Synergistic consequences of early-life social isolation and chronic stress impact coping and neural mechanisms underlying male prairie vole susceptibility and resilience

Abstract

Chronic stress can be challenging, lead to maladaptive coping strategies, and cause negative mental and physical health outcomes. Early-life adversity exposes developing young to physical or psychological experiences that risks surpassing their capacity to effectively cope, thereby impacting their lifetime physical and mental wellbeing. Sensitivity to stressful events, like social isolation, has the potential to magnify stress-coping. Chronic stress through social defeat is an established paradigm that models adverse early-life experiences and can trigger enduring alterations in behavioral and neural phenotypes. To assess the degree to which stress resilience and sensitivity stemming from early-life chronic stress impact sociability, we exposed male prairie voles to chronic social defeat stress (CSDS) during adolescence. We simultaneously exposed subjects to either social isolation (CSDS+Isol) or group housing (CSDS+Soc) during this crucial time of development. On PND41, all subjects underwent a social approach test to examine the immediate impact of isolation, CSDS, or their combined effects on sociability. Unlike the CSDS+Isol group which primarily displayed social avoidance, the CSDS+Soc group was split by individuals exhibiting susceptible or resilient stress phenotypes. Notably, the Control+Soc and CSDS+Soc animals and their cage-mates significantly gained body weight between PND31 and PND40, whereas the Control+Isol and CSDS+Isol animals did not. These results suggest that the effects of early-life stress may be mitigated by having access to social support. Vasopressin, oxytocin, and opioids and their receptors (avpr1a, oxtr, oprk1, oprm1, and oprd1) are known to modulate social and stress-coping behaviors in the lateral septum (LS). Therefore, we did an mRNA expression analysis with RT-qPCR of the avpr1a, oxtr, oprk1, oprm1, and oprd1 genes to show that isolation and CSDS, or their collective influence, can potentially differentially bias sensitivity of the LS to early-life stressors. Collectively, our study supports the impact and dimensionality of early-life adversity because the type (isolation vs. CSDS), duration (acute vs. chronic), and combination (isolation + CSDS) of stressors can dynamically alter behavioral and neural outcomes.

Keywords: chronic social defeat stress (CSDS); comfort-seeking; gene concordance; lateral septum; social isolation; social support; stress resilience.

Figure 1

Experimental design. (A) Graphical representation of the experimental timeline. Male prairie voles were standard-reared between PND0-30. In adolescence (PND31), voles from each group experienced either control conditions (Control: light orange or light green voles) or 10 days of chronic social defeat stress (CSDS: dark orange or dark green voles). After the first social defeat session, half of the voles remained in social isolation (Control+Isol and CSDS+Isol) and the other half remained pair housed with a same-sex cage-mate (Control+Soc and CSDS+Soc). Scale signs indicate that body weights of subjects and cage-mates were measured before and after CSDS, PND31 and PND40, respectively. Subjects and their cage-mates were assessed for social avoidance in the social approach test (SAT), before they were immediately sacrificed for brain tissue harvesting and gene expression analysis. (B) Diagram representing the CSDS procedure. The CSDS+Isol (dark orange) and CSDS+Soc (dark green) groups experienced CSDS, while the Control+Isol (light orange) and Control+Soc groups (light green) were placed into an empty and clean cage. (C) Diagram representing the social approach test (SAT) when the stimulus is absent (i.e., habituation phase) and when the stimulus is present (i.e., testing phase). All groups were assessed in the SAT.

Figure 2

Effects of social isolation and CSDS on body weight and body weight gain. (A) Body weight measured before the first social defeat session on PND31 and after the last social defeat session on PND41. (B) Body weight gain (body weight on PND40 – body weight on PND31). Color scheme follows Figure 1 where orange = isolated; green = group-housed; dark colors = chronic social defeat; light colors = control (no chronic social defeat). Data are presented as mean ± SEM and dots represent individual data. *p < 0.05; ***p < 0.01.

Figure 3

Effects of acute and chronic social defeat stress on comfort-seeking and consoling behaviors. Duration of comfort-seeking behavior by the Control+Soc (light green bars) or CSDS+Soc (dark green bars) subjects or duration of consoling behavior by the subjects' corresponding cage-mates (light purple bars for Control+Soc; dark purple bars for CSDS+Soc) after the first social defeat session (PND31) and last social defeat session (PND40). Data are presented as mean ± SEM and dots represent individual data. *p < 0.05; **p < 0.01.

Figure 4

Effects of social isolation and CSDS on social approach. (A) Latency (s) to approach the stimulus. Data are presented as mean ± SEM and dots represent individual data. (B) Social interaction (SI) ratios were calculated for all subjects (= the time spent in social zone with the stimulus present divided by the time spent in the social zone with the stimulus absent). Dotted lines indicate SI ratios at 0.9 and 1.1, with red transparent panel indicating a socially avoidant phenotype (SI ratios < 0.9) and blue transparent panel indicating a prosocial phenotype (SI ratios >1.1). Color scheme follows Figure 1 where orange = isolated; green = group-housed; dark colors = chronic social defeat; light colors = control (no chronic social defeat). Data presented as mean ± SEM and dots represent individual data. *p < 0.05; **p < 0.01.

Figure 5

Effects of social isolation and CSDS on gene expression in the lateral septum. (A) avpr1a, (B) oxtr, (C) oprk1, (D) oprm1, and (E) oprd1 mRNA levels in the LS are not significantly associated with vulnerability or resilience to isolation of social stress. Color scheme follows Figure 1 where orange = isolated; green = group-housed; dark colors = chronic social defeat; light colors = control (no chronic social defeat). Data are presented as mean ± SEM and dots represent individual data.

Figure 6

Correlograms separated by group. Pair-wise Pearson correlations with false discovery rate were calculated for mRNA expression and all correlations plotted for the (A) CSDS+Isol, (B) Control+Isol, (C) CSDS+Soc, and (D) Control+Soc subjects. Positive correlations are displayed in blue and negative correlations in red color. Color intensity and the size of the circle are proportional to the correlation coefficients. In the right side of each correlogram, the legend color shows the correlation coefficients and the corresponding colors. Statistically non-significant correlations are left blank. The FDR adjusted alpha was α < 0.01 for Control+Isol, α < 0.005 for Control+Soc, α < 0.035 for CSDS+Isol, and α < 0.005 for CSDS-Soc (Benjamini and Hochberg, 1995).