Significance Depression and anxiety have been linked to increased inflammation. However, we do not know if inflammatory status predates onset of disease or whether it contributes to depression symptomatology. We report preexisting individual differences in the peripheral immune system that predict and promote stress susceptibility. Replacing a stress-naive animal’s peripheral immune system with that of a stressed animal increases susceptibility to social stress including repeated social defeat stress (RSDS) and witness defeat (a purely emotional form of social stress). Depleting the cytokine IL-6 from the whole body or just from leukocytes promotes resilience, as does sequestering IL-6 outside of the brain. These studies demonstrate that the emotional response to stress can be generated or blocked in the periphery, and offer a potential new form of treatment for stress disorders.

Enhanced glutamatergic transmission in the nucleus accumbens (NAc), a region critical for reward and motivation, has been implicated in the pathophysiology of depression; however, the afferent source of this increased glutamate tone is not known. The NAc receives glutamatergic inputs from the medial prefrontal cortex (mPFC), ventral hippocampus (vHIP) and basolateral amygdala (AMY). Here, we demonstrate that glutamatergic vHIP afferents to NAc regulate susceptibility to chronic social defeat stress (CSDS). We observe reduced activity in vHIP in mice resilient to CSDS. Furthermore, attenuation of vHIP-NAc transmission by optogenetic induction of long-term depression is pro-resilient, whereas acute enhancement of this input is pro-susceptible. This effect is specific to vHIP afferents to the NAc, as optogenetic stimulation of either mPFC or AMY afferents to the NAc is pro-resilient. These data indicate that vHIP afferents to NAc uniquely regulate susceptibility to CSDS, highlighting an important, novel circuit-specific mechanism in depression.

Background It is well known that exposure to severe stress increases the risk for developing mood disorders. However, most chronic stress models in rodents involve at least some form of physically experiencing traumatic events. Methods This study assessed the effects of a novel social stress paradigm that is insulated from the effects of physical stress. Specifically, adult male C57BL/6J mice were exposed to either emotional (ES) or physical stress (PS) for ten minutes per day for ten days. ES mice were exposed to the social defeat of a PS mouse by a larger more aggressive CD-1 mouse from the safety of an adjacent compartment. Results Like PS mice, ES mice exhibited a range of depression- and anxiety-like behaviors both 24 hr and 1 month after the stress. Increased levels of serum corticosterone, part of the stress response, accompanied these behavioral deficits. Based on prior work which implicated gene expression changes in the ventral tegmental area (a key brain reward region) in the PS phenotype, we compared genome-wide mRNA expression patterns in this brain region of ES and PS mice using RNA-seq. We found significant overlap between these conditions, which suggests several potential gene targets for mediating the behavioral abnormalities observed. Conclusions Together, these findings demonstrate that witnessing traumatic events is a potent stress in adult male mice capable of inducing long-lasting neurobiological perturbations.

The transcription factor deltaFosB (⌬FosB), induced in nucleus accumbens (NAc) by chronic exposure to drugs of abuse, has been shown to mediate sensitized responses to these drugs. However, less is known about a role for ⌬FosB in regulating responses to natural rewards. Here, we demonstrate that two powerful natural reward behaviors, sucrose drinking and sexual behavior, increase levels of ⌬FosB in the NAc. We then use viral-mediated gene transfer to study how such ⌬FosB induction influences behavioral responses to these natural rewards. We demonstrate that overexpression of ⌬FosB in the NAc increases sucrose intake and promotes aspects of sexual behavior. In addition, we show that animals with previous sexual experience, which exhibit increased ⌬FosB levels, also show an increase in sucrose consumption. This work suggests that ⌬FosB is not only induced in the NAc by drugs of abuse, but also by natural rewarding stimuli. Additionally, our findings show that chronic exposure to stimuli that induce ⌬FosB in the NAc can increase consumption of other natural rewards.

There is a strong link between tobacco consumption and mood disorders. It has been suggested that afflicted individuals smoke to manage mood, however, there is evidence indicating that tobacco consumption can induce negative mood. This study was designed to investigate whether nicotine exposure during adolescence influences emotionality/behavioral functioning later in life. Adolescent (postnatal days [PD] 30–44) male rats were treated with twice-daily injections of nicotine (0, 0.16, 0.32, or 0.64 mg/kg) for 15 consecutive days, and their behavioral reactivity to various behavioral paradigms (the elevated plus-maze [EPM], sucrose preference, locomotor activity in the open field [OFT], and forced swim test [FST] was assessed 24 h (short-term) or 1-month (long-term) after exposure. Separate groups of adult rats received nicotine (0.32 mg/kg) to control for age-dependent effects. We report that nicotine exposure during adolescence — but not adulthood — leads to a depression-like state manifested in decreased sensitivity to natural reward (sucrose), and enhanced sensitivity to stress- (FST) and anxiety-eliciting situations (EPM) later in life. Our data show that behavioral dysregulation can emerge 1-week after drug cessation, and that a single day of nicotine exposure during adolescence can be sufficient to precipitate a depression-like state in adulthood. We further demonstrate that these deficits can be normalized by subsequent nicotine (0.32 mg/kg) or antidepressant (i.e., Fluoxetine or Bupropion; 10 mg/kg) treatment in adulthood. These data suggest that adolescent exposure to nicotine results in a negative emotional state rendering the organism significantly more vulnerable to the adverse effects of stress. Within this context, our findings, together with others indicating that nicotine exposure during adolescence enhances risk for addiction later in life, could serve as a potential model of comorbidity.