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.
Studies of epigenetic mechanisms in stress models throughout life are revealing fundamentally new insight into the range of genes and biochemical pathways that are altered within specific brain regions to govern risk for depression and the reversal of symptoms under antidepressant treatment. Such knowledge can now drive efforts to identify novel antidepressant medications by moving well beyond the current focus on monoamine transporters and receptors.