Despite abundant expression of DNA methyltransferases (Dnmt’s) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We find that Dnmt3a expression is regulated in mouse nucleus accumbens (NAc) by chronic cocaine and chronic social defeat stress. Moreover, NAc specific manipulations that block DNA methylation potentiate cocaine reward and exert antidepressant-like effects, whereas NAc specific Dnmt3a overexpression attenuates cocaine reward and is pro-depressant. On a cellular level, we show that chronic cocaine selectively increases thin dendritic spines on NAc neurons and that DNA methylation is both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli.

In contrast to the vast literature on stress effects on the brain, relatively little is known about the molecular mechanisms of resilience, the ability of some individuals to escape the deleterious effects of stress. Here we show that the transcription factor, ΔFosB, mediates an essential mechanism of resilience in mice. Induction of ΔFosB in the nucleus accumbens, a key brain reward region, in response to chronic social defeat stress is both necessary and sufficient for resilience. ΔFosB induction also is required for the ability of the standard antidepressant, fluoxetine, to reverse behavioral pathology induced by social defeat. ΔFosB produces these effects through the induction of the GluR2 AMPA glutamate receptor subunit, which decreases the responsiveness of nucleus accumbens neurons to glutamate, and through other synaptic proteins. Together, these findings establish a novel molecular pathway underlying both resilience and antidepressant action.

BACKGROUND The nucleus accumbens is a critical mediator of depression-related outcomes to social defeat stress. Previous studies demonstrate distinct neuroplasticity adaptations in the two medium spiny neuron (MSN) subtypes, those enriched in dopamine receptor D1 versus dopamine receptor D2, in reward and reinforcement leading to opposing roles for these MSNs in these behaviors. However, the distinct roles of nucleus accumbens MSN subtypes, in depression, remain poorly understood. METHODS Using whole-cell patch clamp electrophysiology, we examined excitatory input to MSN subtypes and intrinsic excitability measures in D1-green fluorescent protein and D2-green fluorescent protein bacterial artificial chromosome transgenic mice that underwent chronic social defeat stress (CSDS). Optogenetic and pharmacogenetic approaches were used to bidirectionally alter firing of D1-MSNs or D2-MSNs after CSDS or before a subthreshold social defeat stress in D1-Cre or D2-Cre bacterial artificial chromosome transgenic mice. RESULTS We demonstrate that the frequency of excitatory synaptic input is decreased in D1-MSNs and increased in D2-MSNs in mice displaying depression-like behaviors after CSDS. Enhancing activity in D1-MSNs results in resilient behavioral outcomes, while inhibition of these MSNs induces depression-like outcomes after CSDS. Bidirectional modulation of D2-MSNs does not alter behavioral responses to CSDS; however, repeated activation of D2-MSNs in stress naïve mice induces social avoidance following subthreshold social defeat stress. CONCLUSIONS Our studies uncover novel functions of MSN subtypes in depression-like outcomes. Notably, bidirectional alteration of D1-MSN activity promotes opposite behavioral outcomes to chronic social stress. Therefore, targeting D1-MSN activity may provide novel treatment strategies for depression or other affective disorders.

Here, we characterized behavioral abnormalities induced by prolonged social isolation in adult rodents. Social isolation induced both anxiety-and anhedonia-like symptoms and decreased cAMP response element-binding protein (CREB) activity in the nucleus accumbens shell (NAcSh). All of these abnormalities were reversed by chronic, but not acute, antidepressant treatment. However, although the anxiety phenotype and its reversal by antidepressant treatment were CREB-dependent, the anhedonia-like symptoms were not mediated by CREB in NAcSh. We found that decreased CREB activity in NAcSh correlated with increased expression of certain K + channels and reduced electrical excitability of NAcSh neurons, which was sufficient to induce anxiety-like behaviors and was reversed by chronic antidepressant treatment. Together, our results describe a model that distinguishes anxiety-and depression-like behavioral phenotypes, establish a selective role of NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript decreased CREB activity in NAcSh in anxiety-like behavior, and provide a mechanism by which antidepressant treatment alleviates anxiety symptoms after social isolation.Depression and anxiety are common forms of mental illness in the general population. Although they are classified as distinct syndromes by the Diagnostic and Statistical Manual (American Psychiatric Association), symptoms of depression and anxiety often occur together and to widely varying extents in different subtypes of the illnesses. Despite the importance of these clinical phenomena, very little is known about the distinctions between depression-and anxiety-like symptoms in animal models 1 . Models of 'active' stress, such as foot shock, restraint stress, social defeat and learned helplessness, produce depression-and anxiety-like phenotypes; the molecular mechanisms of these models have been extensively studied, but specific molecular mediators of depression versus anxiety symptoms have not yet been described [2][3][4] . Even less well studied, however, is a 'passive' model of stress and social isolation in adulthood, which, as with active stress, mimics aspects of human depression and anxiety 5,6 . This lack of attention is unfortunate, as social isolation would appear to be particularly relevant to certain subtypes of human depression and anxiety disorders 7,8 .Although social isolation has been studied, most models to date have focused on adulthood behaviors after isolation rearing early in life, either as pups or adolescents, which is a very different model than adulthood social isolation 5 . Reports on adulthood isolation provide evidence for a strong anxiety-like phenotype 9,10 , an increase in alcohol intake 11 , modulation of responses to rewarding stimuli 9,10,12 , changes in circadian rhythms 13 and a dampening in running-induced neurogenesis 14 . Although reports on changes in neurochemistry are often conflicting, there appears to be decreased serotonergic and noradrenergic function and metabolism in several brain regi...