Abstract
Psychological chronic stress is an important risk factor for major depressive disorder, of which consequences have been widely studied in rodent models. This work aimed at describing a pig model of chronic stress based on social isolation, environmental impoverishment and unpredictability. Three groups of animals of both sexes were constituted. Two were exposed to the psychosocial stressors while receiving (SF, n = 12) or not (SC, n = 22) the antidepressant fluoxetine, and a third group (NSC, n = 22) remained unstressed. Animals were observed in home pens and during dedicated tests to assess resignation and anxiety-like behaviors. Brain structure and function were evaluated via proton MRS and fMRI. Hippocampal molecular biology and immunodetection of cellular proliferation (Ki67+) and neuron maturation (DCX+) in the dentate gyrus were also performed. Salivary cortisol, fecal short-chain fatty acids (SCFAs), and various plasmatic and intestinal biomarkers were analyzed. Compared to NSC, SC animals showed more resignation (p = 0.019) and had a higher level of salivary cortisol (p = 0.020). SC brain responses to stimulation by a novel odor were lower, similarly to their hippocampal neuronal density (p = 0.015), cellular proliferation (p = 0.030), and hippocampal levels of BDNF and 5-HT1AR (p = 0.056 and p = 0.007, respectively). However, the number of DCX+ cells was higher in the ventral dentate gyrus in this group (p = 0.025). In addition, HOMA-IR was also higher (p < 0.001) and microbiota fermentation activity was lower (SCFAs, SC/NSC: p < 0.01) in SC animals. Fluoxetine partially or totally reversed several of these effects. Exposure to psychosocial stressors in the pig model induced effects consistent with the human and rodent literature, including resignation behavior and alterations of the HPA axis and hippocampus. This model opens the way to innovative translational research exploring the mechanisms of chronic stress and testing intervention strategies with good face validity related to human.
Highlights
Major depressive disorder is a major cause of disability worldwide [“Depression: let’s talk” says World Health Organization [WHO] (2017)], as depression tops list of causes of ill health
We measured parameters classically depicted in rodent chronic stress models and in human patients, and extended our investigations to the gut physiology and microbiota activity, as well as to functional brain responses to sensory stimulation
Stressed animals demonstrated a higher variability in the Novelty-Suppressed Feeding (NSF) test, and females from the SC group showed a trend for a higher latency to feed, suggesting limited impacts of our model on anxiety
Summary
Major depressive disorder is a major cause of disability worldwide [“Depression: let’s talk” says World Health Organization [WHO] (2017)], as depression tops list of causes of ill health. One of the leading risk factor for depression is psychological chronic stress (Caspi, 2003), which includes a notion of inescapability and/or uncontrollability (Richter-Levin and Xu, 2018) In this context, several psychological chronic stress models have been developed in rodents, especially the chronic unpredictable mild stress (CMS) model. Rodent models are widely used in biomedical research Their simplicity of breeding, feeding and handling has permitted a quick development of many strains with a high genetic homogeneity, enabling a low variability and high statistical power. Their wide use has enabled the quick development of many specific calibrated tests or models related to the different mood disorders. This leads to a relatively good face validity and predictive validity, two of the criteria usually discussed in animal models of psychiatric disorders (Dzirasa and Covington, 2012)
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