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Depression triggered by harmful stress during adolescence is a common problem that can affect mental health. To date, the mechanisms underlying this type of depression remain unclear. One mechanism for the promotion of depression by chronic stress in adulthood is the loss of hippocampal microglia. Since deleterious stress in adolescence also activates microglia, we investigated the dynamic changes of microglia in the hippocampus in mice exposed to chronic unpredictable stress (CUS) in adolescence. Our results showed that 12 days of CUS stimulation in adolescence induced typical depression-like behaviors in adult mice, which were accompanied by a significant decrease and dystrophy of microglia in the dentate gyrus of the hippocampus. Further analysis showed that this decrease in microglia was mediated by the initial response of microglia to unpredictable stress in the dentate gyrus of the hippocampus and their subsequent apoptosis. Blocking the initial response of microglia to unpredictable stress by pretreatment with minocycline was able to prevent apoptosis and microglial decline as well as the development of depression-like behaviors in adult mice induced by adolescent CUS. Moreover, administration of lipopolysaccharide (LPS) or macrophage-colony stimulatory factor (M-CSF), two drugs that reversed microglia decline in the dentate gyrus, ameliorated the depression-like behaviors induced by CUS stimulation in adolescence. These findings reveal a novel mechanism for the development of depression-like behaviors in animals triggered by deleterious stress in adolescence and suggest that reversing microglial decline in the hippocampus may be a hopeful strategy for the treatment of depression triggered by deleterious stress in adolescence., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier B.V. All rights reserved.)

Depression triggered by early-life stress has begun to attract wide attention due to its severe symptoms and poor treatment outcomes. However, the pathophysiological mechanism for this type of depression remains unclear. Recently, we and others reported that different types of chronic stress induce a significant loss of hippocampal microglia, which is mediated by an initial activation of these microglia. Since early-life stress also promotes microglial activation, we investigated the dynamic changes in hippocampal microglia in mice suffering from depression induced by early social isolation (ESI). Results showed that 8 days of ESI induced depressive-like behaviors in a tail suspension test, forced swim test, sucrose preference test, and open field test, and it also induced a loss and dystrophy of hippocampal microglia. We found that this ESI-induced loss of hippocampal microglia was mediated by both microglial activation and apoptosis. This was demonstrated by the following results: (i) 1 day of ESI induced an obvious activation of hippocampal microglia followed by their apoptosis, and (ii) the blockade of the initial activation of hippocampal microglia by minocycline pretreatment suppressed the ESI-induced apoptosis and loss as well as ESI-induced depressive-like behavior. Lipopolysaccharide (LPS) and macrophage colony-stimulating factor (M-CSF), two activators of microglia, almost completely reversed ESI-induced depressive-like behavior by promoting microglial proliferation in the hippocampus. These results reveal an etiological role of hippocampal microglial loss in ESI-induced depression and demonstrate that the restoration of microglial homeostasis in the hippocampus may serve as a therapeutic strategy for depression induced by early-life stress., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Recently, the loss and dystrophy of hippocampal microglia induced by chronic unpredictable stress (CUS) has been reported to mediate the development of major depression in mice whose microglial cells were labeled with enhanced green fluorescent protein-conjuncted-CX3C receptor type 1. However, whether this happens in endogenous microglia with no genetic intervention remains unclear. Here, we addressed this issue in mice treated with different types of chronic stresses, including the CUS, chronic restraint stress (CRS) and chronic social defeat stress (CSDS). Results showed that the cellular numbers, process lengths, soma areas and activation markers of endogenous hippocampal but not cortical microglia, were markedly reduced by CUS, CRS and CSDS treatment. Administration of mice with two classical stimulators of microglia, lipopolysaccharide (LPS) or macrophage colony-stimulating factor (M-CSF), reversed the CUS-, CRS- and CSDS-induced reductions in endogenous hippocampal microglial numbers, and also improved the CUS-, CRS- or CSDS-induced behavioral abnormalities, including the increases in the immobile time in the forced swimming test and tail suspension test, the inhibition of sucrose preference, and the decrease in the time spent in the center of open field. Furthermore, inhibition of the initial activation of hippocampal microglia by minocycline pretreatment also reversed the reduction in hippocampal microglial numbers as well as the behavioral abnormalities induced by CUS, CRS and CSDS treatment. These results provide compelling evidences to show that different types of chronic stresses can trigger the loss of endogenous hippocampal microglia and restoration of microglial numbers may have therapeutic values in major depression.