Social defeat induces depressive-like states and microglial activation without involvement of peripheral macrophages

Michael L Lehmann,Dragan Maric,Miles Herkenham,Hannah A Cooper

doi:10.1186/s12974-016-0672-x

Michael L Lehmann, Dragan Maric + Show 2 more

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https://doi.org/10.1186/s12974-016-0672-x

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Abstract

BackgroundWe are interested in the causal interactions between psychological stress and activity within different compartments of the immune system. Psychosocial stress has been reported to not only alter microglia morphology but also produce anxiety-like and depressive-like effects by triggering CNS infiltration of macrophages from the periphery. We sought to test these phenomena in a somewhat different but standardized model of chronic social defeat (SD) stress.MethodsWe used a paradigm of dyadic home pairing of dominant and subordinate mice that has been validated to induce powerful anxiety-like and depressive-like effects manifested by behavior assessed in social tasks. We administered the SD stress for 3 days (acute SD) or 14 days (chronic SD) and looked for monocyte entry into the brain by three independent means, including CD45 activation states assessed by flow cytometry and tracking fluorescently tagged peripheral cells from Ccr2wt/rfp and Ubcgfp/gfp reporter mice. We further characterized the effects of SD stress on microglia using quantitative morphometric analysis, ex vivo phagocytosis assays, flow cytometry, and immunochemistry.ResultsWe saw no evidence of stress-induced macrophage entry after acute or chronic defeat stress. In comparison, brain infiltration of peripheral cells did occur after endotoxin administration. Furthermore, mutant mice lacking infiltrating macrophages due to CCR2 knockout developed the same degree of chronic SD-induced depressive behavior as wildtype mice. We therefore focused more closely on the intrinsic immune cells, the microglia. Using Cx3cr1wt/gpf microglial reporter mice, we saw by quantitative methods that microglial morphology was not altered by stress at either time point. However, chronic SD mice had elevated numbers of CD68hi microglia examined by flow cytometry. CD68 is a marker for phagocytic activity. Indeed, these cells ex vivo showed elevated phagocytosis, confirming the increased activation status of chronic SD microglia. Finally, acute SD but not chronic SD increased microglial proliferation, which occurred selectively in telencephalic stress-related brain areas.ConclusionsIn the SD paradigm, changes in CNS-resident microglia numbers and activation states might represent the main immunological component of the psychosocial stress-induced depressive state.

Highlights

We are interested in the causal interactions between psychological stress and activity within different compartments of the immune system
Behavior after acute social defeat (SD) and chronic SD The urine scent marking (USM) and social interaction (SI) tests were used to measure hedonic drive and sociability; declines in these behaviors are maladaptive responses, and they occur coincidently with anxiety-like and depressive-like behaviors measured in open field, light/dark box, elevated zero maze, sucrose preference test, forced swim test, and tail suspension test [35,36,37,38]
Cx3cr1wt/gfp mice exposed to chronic SD showed significant reductions in marking preference (F(2,23) = 7.26, p < 0.005) compared to HC and acute SD Cx3cr1wt/gfp mice in the USM task (Fig. 1a, b)

Summary

Introduction

We are interested in the causal interactions between psychological stress and activity within different compartments of the immune system. Psychosocial stress has been reported to alter microglia morphology and produce anxiety-like and depressive-like effects by triggering CNS infiltration of macrophages from the periphery. Psychosocial stressors, which contribute to the development of affective disorders in humans, induce central and peripheral immune pathway signaling that is increasingly thought to be relevant to the pathophysiology of depression [1,2,3,4]. Stressors and associated neural activity in limbic brain areas, via activation of the sympathetic nervous system (SNS) and hypothalamic-pituitary-adrenal (HPA) axis, can alter the activation states of peripheral immune cells and trigger the release of proinflammatory cytokines that are thought to be pro-depressive through an incompletely understood central action [4,5,6]. The entry of macrophages into the brain has recently been reported in several stress models [9,10,11]

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