Social defeat in adolescent mice increases vulnerability to alcohol consumption

The recovery trajectory of adolescent social defeat stress-induced behavioral, (1)H-MRS metabolites and myelin changes in Balb/c mice

Adolescent exposure to social stress precipitates emotion-related disorders and affects the development and function of medial prefrontal cortex (mPFC). However, this adversity-induced behavioral and neurological changes remain not fully explored. Adolescent Balb/c mice were subjected to intermittent social defeat stress during postnatal days 28 to 42. Proton magnetic resonance spectroscopy (¹H-MRS) measurements, behavioral tests and immunohistochemistry were performed one day or 3 weeks after the last stress episode. Defeated mice exhibited hypoactivity and social avoidance with the latter lasting into the early adulthood, while the anxiety level was unchanged. Social defeat experience lead to temporary decreases in the levels of total creatines (Cr + pCr) and Glx (Glu + Gln), but a delayed increase of N- acetylaspartate (NAA) levels. These alternations were accompanied with a persistent reduction of myelin basic protein expression although the number of mature oligodendrocyte did not change. These findings provide evidence that adolescent adverse social experience permanently impairs the emotion-related behavioral performance and induces biochemical and molecular changes in the brain which at least lasts into early adulthood, thus enhancing our understanding of the neurobiology of social defeat stress. Our finding also implicates that NAA signals on MRS may reflect myelin status.

Paternal preconception ethanol exposure blunts hypothalamic-pituitary-adrenal axis responsivity and stress-induced excessive fluid intake in male mice

A growing number of environmental insults have been shown to induce epigenetic effects that persist across generations. For instance, paternal preconception exposures to ethanol or stress have independently been shown to exert such intergenerational effects. Since ethanol exposure is a physiological stressor that activates the hypothalamic-pituitary-adrenal (HPA) axis, we hypothesized that paternal ethanol exposure would impact stress responsivity of offspring. Adult male mice were exposed to chronic intermittent vapor ethanol or control conditions for 5 weeks before being mated with ethanol-naïve females to produce ethanol (E)- and control (C)-sired offspring. Adult male and female offspring were tested for plasma corticosterone (CORT) levels following acute restraint stress and the male offspring were further examined for stress-evoked 2-bottle choice ethanol-drinking. Paternal ethanol exposure blunted plasma CORT levels following acute restraint stress selectively in male offspring; females were unaffected. In a stress-evoked ethanol-drinking assay, there was no effect of stress on ethanol consumption. However, C-sired males exhibited increased total fluid intake (polydipsia) in response to stress while E-sired males were resistant to this stress-induced phenotype. Taken together, these data suggest that paternal ethanol exposure imparts stress hyporesponsivity to male offspring.

Long-term effects of repeated social stress on the conditioned place preference induced by MDMA in mice

Previous studies have demonstrated that social defeat stress increases the rewarding effects of psychostimulant drugs such as cocaine and amphetamine. In the present study we evaluated the long-term effects of repeated social defeat (RSD) on the rewarding effects of ±3,4-methylenedioxymethamphetamine (MDMA) hydrochloride in the conditioned place preference (CPP) paradigm. Adolescent and young adult mice were exposed to four episodes of social defeat (on PND 29-40 and PND 47-56, respectively) and were conditioned three weeks later with 1.25 or 10mg/kg i.p. of MDMA (experiment 1). The long-term effects of RSD on anxiety, social behavior and cognitive processes were also evaluated in adult mice (experiment 2). RSD during adolescence enhanced vulnerability to priming-induced reinstatement in animals conditioned with 1.25mg/kg of MDMA and increased the duration of the CPP induced by the 10mg/kg of MDMA. The latter effect was also observed after RSD in young adult mice, as well as an increase in anxiety-like behavior, an alteration in social interaction (reduction in attack and increase in avoidance/flee and defensive/submissive behaviors) and an impairment of maze learning. These results support the idea that RSD stress increases the rewarding effects of MDMA and induces long-term alterations in anxiety, learning and social behavior in adult mice. Thus, exposure to stress may increase the vulnerability of individuals to developing MDMA dependence, which is a factor to be taken into account in relation to the prevention and treatment of this disorder.