Such epigenetic regulation of stress responses is illustrated by our work in the chronic social defeat model of depression. Under chronic social stress, susceptible mice manifest a depressionlike syndrome, while resilient ones avoid most of these deleterious symptoms despite being subjected to the same level of stress. We have shown that such susceptibility vs resilience is controlled in part by changes in histone acetylation and a repressive form of histone methylation –dimethylation of Lys9 of histone H3 (H3K9me2)-in nucleus accumbens. Global levels of histone acetylation are increased, and his adaptation promotes a resilient outcome because HDAC inhibition specifically in this brain region opposes susceptibility and induces antidepressantlike responses. In contrast, global increases in H3K9me2 are pathological: local knockout or inhibition of the HMT that mediates this mark promotes susceptibility, whereas its activation promotes resilience. We have used genomewide measures to identify numerous genes that display alterations in these histone modifications, which provide unprecedented insight into the molecular basis of susceptibility vs resilience.
There is also strong support for the importance of epigenetic modifications occurring early in life and driving lifelong vulnerability to stress. In landmark experiments in rats by Weaver and colleagues, offspring of mothers that received low levels of maternal care/grooming (LG) showed increased stress reactivity and anxiety-related behavior in adulthood compared with those that received high levels of maternal care/grooming (HG). LG rats show decreased hippocampal glucocorticoid receptor (GR) mRNA expression and corresponding decreased H3K9 acetylation and increased DNA methylation around the GR gene promoter compared with HG animals. Such differences in GR expression help determine the maternal care delivered by these animals, thereby perpetuating patterns of LG vs HG behavior from generation to generation. It is likely that epigenetic regulation of many additional genes is also involved. This form of transgenerational transmission of stress responsiveness is thus behaviorally transmitted and does not represent true epigenetic inheritance.