Abstract
Emotional contagion refers to the sharing of emotional states between individuals and can cause depressive behaviors in healthy persons who live with depressed individuals. Negative emotional contagion has been observed in animal models, but the vast majority of studies are short-term and bear little resemblance to long-term human relationships. Thus, the first aim of this study was to establish an animal model of stress-induced negative emotional contagion that develops across time and between pairs. To accomplish this, we tested the hypothesis that sedentary male rats that cohabitate for five weeks with a stress-exposed female will exhibit a depression-like phenotype that is observable on behavioral and physiological measures. In addition, drawing from a comprehensive literature that describes the beneficial effects of prior exercise on stress-related behavior, we tested our second hypothesis that in males that were paired with a stressed female, prior voluntary exercise will diminish the impact of negative emotional contagion. We found that pair housing a healthy male with a stressed female led to emotional contagion; males gained less body weight, were anhedonic, demonstrated heightened anxiety-like behavior, had lower serum brain-derived neurotrophic factor (BDNF) levels, had decreased hippocampal BDNF-stimulated tyrosine receptor kinase B (TrkB) signaling and had increased pro-inflammatory cytokine expression in the hippocampus. For the most part, the five-week exercise window that occurred prior to pair housing had few effects in non-stress paired rats, but had partial, yet substantial protective effects in rats that were pair-housed with a stressed female. Specifically, stress-paired, exercised rats showed less depressive-like behavior, had partially preserved hippocampal BDNF-stimulated TrkB signaling, had normalized serum BDNF concentration, and had hippocampal cytokine and immediate early gene levels that were equivalent to controls. These preclinical findings introduce a new model of negative emotional contagion between dyads of male-female rats and support the view that inclusion of exercise programs would be beneficial for persons that may, in the future, be susceptible to negative emotional contagion.