Ventral tegmental area (VTA) dopamine (DA) neurons in the brain’s reward circuit play a crucial role in mediating stress responses1–4 including determining susceptibility vs. resilience to social stress-induced behavioural abnormalities5. VTA DA neurons exhibit two in vivo patterns of firing: low frequency tonic firing and high frequency phasic firing6–8. Phasic firing of the neurons, which is well known to encode reward signals6,7,9, is upregulated by repeated social defeat stress, a highly validated mouse model of depression5,8,10–13. Surprisingly, this pathophysiological effect is seen in susceptible mice only, with no change in firing rate apparent in resilient individuals5,8. However, direct evidence linking—in real-time—DA neuron phasic firing in promoting the susceptible (depression-like) phenotype is lacking. Here, we took advantage of the temporal precision and cell type- and projection pathway-specificity of optogenetics to demonstrate that enhanced phasic firing of these neurons mediates susceptibility to social defeat stress in freely behaving mice. We show that optogenetic induction of phasic, but not tonic, firing, in VTA DA neurons of mice undergoing a subthreshold social defeat paradigm rapidly induced a susceptible phenotype as measured by social avoidance and decreased sucrose preference. Optogenetic phasic stimulation of these neurons also quickly induced a susceptible phenotype in previously resilient mice that had been subjected to repeated social defeat stress. Furthermore, we show differences in projection pathway-specificity in promoting stress susceptibility: phasic activation of VTA neurons projecting to the nucleus accumbens (NAc), but not to the medial prefrontal cortex (mPFC), induced susceptibility to social defeat stress. Conversely, optogenetic inhibition of the VTA-NAc projection induced resilience, while inhibition of the VTA-mPFC projection promoted susceptibility. Overall, these studies reveal novel firing pattern- and neural circuit-specific mechanisms of depression.

Decreased medial prefrontal cortex (mPFC) neuronal activity is associated with social defeat-induced depression-and anxiety-like behaviors in mice. However, the molecular mechanisms underlying the decreased mPFC activity and its prodepressant role remain unknown. We show here that induction of the transcription factor ⌬FosB in mPFC, specifically in the prelimbic (PrL) area, mediates susceptibility to stress. ⌬FosB induction in PrL occurred selectively in susceptible mice after chronic social defeat stress, and overexpression of ⌬FosB in this region, but not in the nearby infralimbic (IL) area, enhanced stress susceptibility. ⌬FosB produced these effects partly through induction of the cholecystokinin (CCK)-B receptor: CCKB blockade in mPFC induces a resilient phenotype, whereas CCK administration into mPFC mimics the anxiogenic-and depressant-like effects of social stress. We previously found that optogenetic stimulation of mPFC neurons in susceptible mice reverses several behavioral abnormalities seen after chronic social defeat stress. Therefore, we hypothesized that optogenetic stimulation of cortical projections would rescue the pathological effects of CCK in mPFC. After CCK infusion in mPFC, we optogenetically stimulated mPFC projections to basolateral amygdala or nucleus accumbens, two subcortical structures involved in mood regulation. Stimulation of corticoamygdala projections blocked the anxiogenic effect of CCK, although no effect was observed on other symptoms of social defeat. Conversely, stimulation of corticoaccumbens projections reversed CCK-induced social avoidance and sucrose preference deficits but not anxiogenic-like effects. Together, these results indicate that social stress-induced behavioral deficits are mediated partly by molecular adaptations in mPFC involving ⌬FosB and CCK through cortical projections to distinct subcortical targets.

Although chronic cocaine-induced changes in dendritic spines on nucleus accumbens (NAc) neurons have been correlated with behavioral sensitization, the molecular pathways governing these structural changes, and their resulting behavioral effects, are poorly understood. The transcription factor, nuclear factor B (NFB), is rapidly activated by diverse stimuli and regulates expression of many genes known to maintain cell structure. Therefore, we evaluated the role of NFB in regulating cocaine-induced dendritic spine changes on medium spiny neurons of the NAc and the rewarding effects of cocaine. We show that chronic cocaine induces NFB-dependent transcription in the NAc of NFB-Lac transgenic mice. This induction of NFB activity is accompanied by increased expression of several NFB genes, the promoters of which show chromatin modifications after chronic cocaine exposure consistent with their transcriptional activation. To study the functional significance of this induction, we used viral-mediated gene transfer to express either a constitutively active or dominant-negative mutant of Inhibitor of B kinase (IKKca or IKKdn), which normally activates NFB signaling, in the NAc. We found that activation of NFB by IKKca increases the number of dendritic spines on NAc neurons, whereas inhibition of NFB by IKKdn decreases basal dendritic spine number and blocks the increase in dendritic spines after chronic cocaine. Moreover, inhibition of NFB blocks the rewarding effects of cocaine and the ability of previous cocaine exposure to increase an animal's preference for cocaine. Together, these studies establish a direct role for NFB pathways in the NAc to regulate structural and behavioral plasticity to cocaine.

Repeated cocaine administration increases the dendritic arborization of nucleus accumbens neurons, but the underlying signaling events remain unknown. Here, we show that repeated cocaine negatively regulates the active form of Rac1, a small GTPase that controls actin remodeling in other systems. We show further, using viral-mediated gene transfer, that overexpression of a dominant negative mutant of Rac1, or local knockout of Rac1 from floxed Rac1 mice, is sufficient to increase the density of immature dendritic spines on nucleus accumbens neurons, whereas overexpression of a constitutively active Rac1 mutant, or light activation of a photoactivatible form of Rac1, blocks the ability of repeated cocaine to produce this effect. Downregulation of Rac1 activity in nucleus accumbens likewise promotes behavioral responses to cocaine, with Rac1 activation producing the opposite effect. These findings establish an important role for Rac1 signaling in mediating structural and behavioral plasticity to cocaine.

SUMMARY While the abuse of opiate drugs continues to rise, the neuroadaptations that occur with long-term drug exposure remain poorly understood. We describe here a series of chronic morphine-induced adaptations in ventral tegmental area (VTA) dopamine neurons, which are mediated via downregulation of AKT-mTORC2 (mammalian target of rapamycin complex-2). Chronic opiates decrease the size of VTA dopamine neurons in rodents, an effect seen in humans as well, and concomitantly increase the excitability of the cells but decrease dopamine output to target regions. Chronic morphine decreases mTORC2 activity, and overexpression of Rictor, a component of mTORC2, prevents morphine-induced changes in cell morphology and activity. Further, local knock-out of Rictor in VTA decreases DA soma size and reduces rewarding responses to morphine, consistent with the hypothesis that these adaptations represent a mechanism of reward tolerance. Together, these findings demonstrate a novel role for AKT-mTORC2 signaling in mediating neuroadaptations to opiate drugs of abuse.