Microglia depletion ameliorates neuroinflammation, anxiety-like behavior, and cognitive deficits in a sex-specific manner in Rev-erbα knockout mice.

The circadian system is an evolutionarily adaptive system that synchronizes biological and physiological activities within the body to the 24 h oscillations on Earth. At the molecular level, circadian clock proteins are transcriptional factors that regulate the rhythmic expression of genes involved in numerous physiological processes such as sleep, cognition, mood, and immune function. Environmental and genetic disruption of the circadian clock can lead to pathology. For example, global deletion of the circadian clock gene Rev-erbα (RKO) leads to hyperlocomotion, increased anxiety-like behaviors, and cognitive impairments in male mice; however, the mechanisms underlying behavioral changes remain unclear. Here we hypothesized that RKO alters microglia function leading to neuroinflammation and altered mood and cognition, and that microglia depletion can resolve neuroinflammation and restore behavior. We show that microglia depletion (CSF1R inhibitor, PLX5622) in 8-month-old RKO mice ameliorated hyperactivity, memory impairments, and anxiety/risky-like behaviors. RKO mice exhibited striking increases in expression of pro-inflammatory cytokines (e.g., IL-1ß and IL-6). Surprisingly, these increases were only fully reversed by microglia depletion in the male but not female RKO hippocampus. In contrast, male RKO mice showed greater alterations in microglial morphology and phagocytic activity than females. In both sexes, microglia depletion reduced microglial branching and decreased CD68 production without altering astrogliosis. Taken together, we show that male and female RKO mice exhibit unique perturbations to the neuroimmune system, but microglia depletion is effective at rescuing aspects of behavioral changes in both sexes. These results demonstrate that microglia are involved in Rev-erbα-mediated changes in behavior and neuroinflammation.

Circadian Regulation of the Neuroimmune Environment Across the Lifespan: From Brain Development to Aging.

Circadian clocks confer 24-h periodicity to biological systems, to ultimately maximize energy efficiency and promote survival in a world with regular environmental light cycles. In mammals, circadian rhythms regulate myriad physiological functions, including the immune, endocrine, and central nervous systems. Within the central nervous system, specialized glial cells such as astrocytes and microglia survey and maintain the neuroimmune environment. The contributions of these neuroimmune cells to both homeostatic and pathogenic demands vary greatly across the day. Moreover, the function of these cells changes across the lifespan. In this review, we discuss circadian regulation of the neuroimmune environment across the lifespan, with a focus on microglia and astrocytes. Circadian rhythms emerge in early life concurrent with neuroimmune sculpting of brain circuits and wane late in life alongside increasing immunosenescence and neurodegeneration. Importantly, circadian dysregulation can alter immune function, which may contribute to susceptibility to neurodevelopmental and neurodegenerative diseases. In this review, we highlight circadian neuroimmune interactions across the lifespan and share evidence that circadian dysregulation within the neuroimmune system may be a critical component in human neurodevelopmental and neurodegenerative diseases.

Chronic circadian phase advance in male mice induces depressive-like responses and suppresses neuroimmune activation.

Altered working and sleeping schedules during the COVID-19 pandemic likely impact our circadian systems. At the molecular level, clock genes form feedback inhibition loops that control 24-hr oscillations throughout the body. Importantly, core clock genes also regulate microglia, the brain resident immune cell, suggesting circadian regulation of neuroimmune function. To assess whether circadian disruption induces neuroimmune and associated behavioral changes, we mimicked chronic jetlag with a chronic phase advance (CPA) model. 32 adult male C57BL/6J mice underwent 6-hr light phase advance shifts every 3 light/dark cycles (CPA) 14 times or were maintained in standard light/dark cycles (control). CPA mice showed higher behavioral despair but not anhedonia in forced swim and sucrose preferences tests, respectively. Changes in behavior were accompanied by altered hippocampal circadian genes in CPA mice. Further, CPA suppressed expression of brain-derived neurotrophic factor (BDNF) and pro-inflammatory cytokine interleukin-1 beta in the hippocampus. Plasma corticosterone concentrations were elevated by CPA, suggesting that CPA may suppress neuroimmune pathways via glucocorticoids. These results demonstrate that chronic circadian disruption alters mood and neuroimmune function, which may have implications for shift working populations such as frontline health workers.

Light at night during development in mice has modest effects on adulthood behavior and neuroimmune activation.

Exposure to light at night (LAN) can disrupt the circadian system, thereby altering neuroimmune reactivity and related behavior. Increased exposure to LAN affects people of all ages - and could have particularly detrimental effects during early-life and adolescence. Despite this, most research on the behavioral and physiological effects of LAN has been conducted in adult animals. Here we evaluated the effects of dim LAN during critical developmental windows on adulthood neuroimmune function and affective/sickness behaviors. Male and female C57BL/6 J mice were exposed to dim LAN [12:12 light (150 lx)/dim (15 lx) cycle] during early life (PND10-24) or adolescence (PND30-44) [control: 12:12 light (150 lx)/dark (0 lx) cycle]. Behaviors were assessed during juvenile (PND 42-44) and adult (PND60) periods. Contrary to our hypothesis, juvenile mice that were exposed to dim LAN did not exhibit changes in anxiety- or depressive-like behaviors. By adulthood, adolescent LAN-exposed female mice showed a modest anxiety-like phenotype in one behavioral task but not another. Adolescent LAN exposure also induced depressive-like behavior in a forced swim task in adulthood in both male and female mice. Additionally, developmental LAN exacerbated the hippocampal cytokine response (IL-1ß) following peripheral LPS in female, but not male mice. These results suggest female mice may be more susceptible to developmental LAN than male mice: LAN female mice had a modest anxiety-like phenotype in adulthood, and upon LPS challenge, higher hippocampal IL-1ß expression. Taken together, developmental LAN exposure in mice promotes a modest increase in susceptibility to anxiety- and depressive-like symptoms.

Pharmacological Properties of δ-Opioid Receptor-Mediated Behaviors: Agonist Efficacy and Receptor Reserve.

δ-Opioid receptor (δ-receptor) agonists produce antihyperalgesia, antidepressant-like effects, and convulsions in animals. However, the role of agonist efficacy in generating different δ-receptor-mediated behaviors has not been thoroughly investigated. To this end, efficacy requirements for δ-receptor-mediated antihyperalgesia, antidepressant-like effects, and convulsions were evaluated by comparing the effects of the partial agonist BU48 and the full agonist SNC80 and changes in the potency of SNC80 after δ-receptor elimination. Antihyperalgesia was measured in a nitroglycerin-induced thermal hyperalgesia assay. An antidepressant-like effect was evaluated in the forced swim test. Mice were observed for convulsions after treatment with SNC80 or the δ-opioid receptor partial agonist BU48. Ligand-induced G protein activation was measured by [35S]guanosine 5'-O-[γ-thio]triphosphate binding in mouse forebrain tissue, and δ-receptor number was measured by [3H]D-Pen2,5-enkephalin saturation binding. BU48 produced antidepressant-like effects and convulsions but antagonized SNC80-induced antihyperalgesia and G protein activation. The potency of SNC80 was shifted to the right in δ-receptor heterozygous knockout mice and naltrindole-5'-isothiocyanate-treated mice, and the magnitude of potency shift differed across assays, with the largest shift occurring in the thermal hyperalgesia assay, followed by the forced swim test and then convulsion observation. Naltrindole antagonized these SNC80-induced behaviors with similar potencies, suggesting that these effects are mediated by the same type of δ-receptor. These data suggest that δ-receptor-mediated behaviors display a rank order of efficacy requirement, with antihyperalgesia having the highest requirement, followed by antidepressant-like effects and then convulsions. These findings further our understanding of the pharmacological mechanisms mediating the in vivo effects of δ-opioid receptor agonists. SIGNIFICANCE STATEMENT: δ-Opioid receptor (δ-receptor) agonists produce antihyperalgesia, antidepressant-like effects, and convulsions in animal models. This study evaluates pharmacological properties, specifically the role of agonist efficacy and receptor reserve, underlying these δ-receptor-mediated behaviors. These data suggest that δ-receptor-mediated behaviors display a rank order of efficacy requirement, with antihyperalgesia having the highest requirement, followed by antidepressant-like effects and then convulsions.