Global Patterns of Gene Expression Associated with Vulnerability and Resilience to Social Defeat

While stressful life events are an important cause of psychopathology such as Post-Traumatic Stress Disorder (PTSD) and Major Depressive Disorder (MDD), most individuals exposed to chronic stress remain psychologically unperturbed. The molecular mechanisms such variations in stress resilience remain to be explored. Utilizing a mouse social defeat paradigm as a model for chronic social stress exposure, we segregated socially defeated c57bl/6J mice into Susceptible and Unsusceptible groups based on scores of social avoidance, and these groups differ along multiple behavioral and physiological domains. To explore neuroplastic changes within the mesolimbic dopamine circuit that may explain variations in susceptibility, we examined global patterns of gene expression within the VTA (ventral tegmental area) and the NAc (nucleus accumbens). Tissue punches of NAc and VTA were obtained from either non-defeated control, Susceptible and Unsusceptible mice approximately 48 hours after the cessation of social defeat stress. Data from Illumina sentrix gene expression microarrays revealed three main findings: (1) the total number of genes significantly up or downregulated was considerably higher within the NAc, (2) the Unsusceptible phenotype was associated with the regulation of a much higher number of genes, and (3) as compared to the NAc, the VTA displayed far fewer genes that were identically regulated in both Susceptible and Unsusceptible groups. Taken together, these data strongly suggest that the VTA is an important substrate for transcriptional changes that can be correlated with variations in Susceptibility. In addition, they suggest that changes in anxiety (found in both Susceptible and Unsusceptible groups) versus changes in reward behavior (found only in Susceptilbe mice) can be explained by gene expression changes within the mesolimbic dopamine circuit. Keywords: neural stress response This microarray experiment involved 18 arrays that were run simultaneously. The experimental design can be captured in factorial format. 3 behavioral groups: Control (C), Susceptible (S) and Unsusceptible (U) 2 anatomical structures: VTA and NAc 3 biological replicates for each condition Each biological replicate consisted of tissue pooled from four animals. Biological replicates within each condition (e.g., Unsusceptible NAc) displayed comparable avoidance scores. Data analysis and conclusions were derived from pairwise comparisons of C Vs. S and C Vs. U.