Chronic hM3Dq signaling in microglia ameliorates neuroinflammation in male mice

Basal forebrain cholinergic systems as circuits through which traumatic stress disrupts emotional memory regulation

Contextual and spatial systems facilitate changes in emotional memory regulation brought on by traumatic stress. Cholinergic basal forebrain (chBF) neurons provide input to contextual/spatial systems and although chBF neurons are important for emotional memory, it is unknown how they contribute to the traumatic stress effects on emotional memory. Clusters of chBF neurons that project to the prefrontal cortex (PFC) modulate fear conditioned suppression and passive avoidance, while clusters of chBF neurons that project to the hippocampus (Hipp) and PFC (i.e. cholinergic medial septum and diagonal bands of Broca (chMS/DBB neurons) are critical for fear extinction. Interestingly, neither Hipp nor PFC projecting chMS/DBB neurons are critical for fear extinction. The retrosplenial cortex (RSC) is a contextual/spatial memory system that receives input from chMS/DBB neurons, but whether this chMS/DBB-RSC circuit facilitates traumatic stress effects on emotional memory remain unexplored. Traumatic stress leads to neuroinflammation and the buildup of reactive oxygen species. These two molecular processes may converge to disrupt chBF circuits enhancing the impact of traumatic stress on emotional memory.

Tuning neural circuits and behaviors by microglia in the adult brain

Microglia are the primary immune cells of the CNS, contributing to both inflammatory damage and tissue repair in neurological disorder. In addition, emerging evidence highlights the role of homeostatic microglia in regulating neuronal activity, interacting with synapses, tuning neural circuits, and modulating behaviors. Herein, we review how microglia sense and regulate neuronal activity through synaptic interactions, thereby directly engaging with neural networks and behaviors. We discuss current studies utilizing microglial optogenetic and chemogenetic approaches to modulate adult neural circuits. These manipulations of microglia across different CNS regions lead to diverse behavioral consequences. We propose that spatial heterogeneity of microglia-neuron interaction lays the groundwork for understanding diverse functions of microglia in neural circuits and behaviors.

Microglia in neuroimmunopharmacology and drug addiction

Drug addiction is a chronic and debilitating disease that is considered a global health problem. Various cell types in the brain are involved in the progression of drug addiction. Recently, the xenobiotic hypothesis has been proposed, which frames substances of abuse as exogenous molecules that are responded to by the immune system as foreign "invaders", thus triggering protective inflammatory responses. An emerging body of literature reveals that microglia, the primary resident immune cells in the brain, play an important role in the progression of addiction. Repeated cycles of drug administration cause a progressive, persistent induction of neuroinflammation by releasing microglial proinflammatory cytokines and their metabolic products. This contributes to drug addiction via modulation of neuronal function. In this review, we focus on the role of microglia in the etiology of drug addiction. Then, we discuss the dynamic states of microglia and the correlative and causal evidence linking microglia to drug addiction. Finally, possible mechanisms of how microglia sense drug-related stimuli and modulate the addiction state and how microglia-targeted anti-inflammation therapies affect addiction are reviewed. Understanding the role of microglia in drug addiction may help develop new treatment strategies to fight this devastating societal challenge.

Chemogenetic approaches reveal dual functions of microglia in seizures

Microglia are key players in maintaining brain homeostasis and exhibit phenotypic alterations in response to epileptic stimuli. However, it is still relatively unknown if these alterations are pro- or anti-epileptic. To unravel this dilemma, we employed chemogenetic manipulation of microglia using the artificial Gi-Dreadd receptor within a kainic acid (KA) induced murine seizure model. Our results indicate that acute Gi-Dreadd activation with Clozapine-N-Oxide can reduce seizure severity. Additionally, we observed increased interaction between microglia and neuronal soma, which correlated with reduced neuronal hyperactivity. Interestingly, prolonged activation of microglial Gi-Dreadds by repeated doses of CNO over 3 days, arrested microglia in a less active, homeostatic-like state, which associated with increased neuronal loss after KA induced seizures. RNAseq analysis revealed that prolonged activation of Gi-Dreadd interferes with interferon β signaling and microglia proliferation. Thus, our findings highlight the importance of microglial Gi signaling not only during status epilepticus (SE) but also within later seizure induced pathology.

Microglia facilitate and stabilize the response to general anesthesia via modulating the neuronal network in a brain region-specific manner

General anesthesia leads to a loss of consciousness and an unrousable state in patients. Although general anesthetics are widely used in clinical practice, their underlying mechanisms remain elusive. The potential involvement of nonneuronal cells is unknown. Microglia are important immune cells in the central nervous system (CNS) that play critical roles in CNS function and dysfunction. We unintentionally observed delayed anesthesia induction and early anesthesia emergence in microglia-depleted mice. We found that microglial depletion differentially regulates neuronal activities by suppressing the neuronal network of anesthesia-activated brain regions and activating emergence-activated brain regions. Thus, microglia facilitate and stabilize the anesthesia status. This influence is not mediated by dendritic spine plasticity. Instead, it relies on the activation of microglial P2Y12 and subsequent calcium influx, which facilitates the general anesthesia response. Together, we elucidate the regulatory role of microglia in general anesthesia, extending our knowledge of how nonneuronal cells modulate neuronal activities.

Obesity-associated microglial inflammatory activation paradoxically improves glucose tolerance

Hypothalamic gliosis associated with high-fat diet (HFD) feeding increases susceptibility to hyperphagia and weight gain. However, the body-weight-independent contribution of microglia to glucose regulation has not been determined. Here, we show that reducing microglial nuclear factor κB (NF-κB) signaling via cell-specific IKKβ deletion exacerbates HFD-induced glucose intolerance despite reducing body weight and adiposity. Conversely, two genetic approaches to increase microglial pro-inflammatory signaling (deletion of an NF-κB pathway inhibitor and chemogenetic activation through a modified Gq-coupled muscarinic receptor) improved glucose tolerance independently of diet in both lean and obese rodents. Microglial regulation of glucose homeostasis involves a tumor necrosis factor alpha (TNF-α)-dependent mechanism that increases activation of pro-opiomelanocortin (POMC) and other hypothalamic glucose-sensing neurons, ultimately leading to a marked amplification of first-phase insulin secretion via a parasympathetic pathway. Overall, these data indicate that microglia regulate glucose homeostasis in a body-weight-independent manner, an unexpected mechanism that limits the deterioration of glucose tolerance associated with obesity.

Chemogenetic manipulation of CX3CR1+ cells transiently induces hypolocomotion independent of microglia

Chemogenetic approaches using Designer Receptors Exclusively Activated by Designer Drugs (DREADD, a family of engineered GPCRs) were recently employed in microglia. Here, we used Cx3cr1CreER/+:R26hM4Di/+ mice to express Gi-DREADD (hM4Di) on CX3CR1+ cells, comprising microglia and some peripheral immune cells, and found that activation of hM4Di on long-lived CX3CR1+ cells induced hypolocomotion. Unexpectedly, Gi-DREADD-induced hypolocomotion was preserved when microglia were depleted. Consistently, specific activation of microglial hM4Di cannot induce hypolocomotion in Tmem119CreER/+:R26hM4Di/+ mice. Flow cytometric and histological analysis showed hM4Di expression in peripheral immune cells, which may be responsible for the hypolocomotion. Nevertheless, depletion of splenic macrophages, hepatic macrophages, or CD4+ T cells did not affect Gi-DREADD-induced hypolocomotion. Our study demonstrates that rigorous data analysis and interpretation are needed when using Cx3cr1CreER/+ mouse line to manipulate microglia.

Microglia modulate general anesthesia through P2Y12 receptor

General anesthesia (GA) is an unconscious state produced by anesthetic drugs, which act on neurons to cause overall suppression of neuronal activity in the brain. Recent studies have revealed that GA also substantially enhances the dynamics of microglia, the primary brain immune cells, with increased process motility and territory surveillance. However, whether microglia are actively involved in GA modulation remains unknown. Here, we report a previously unrecognized role for microglia engaging in multiple GA processes. We found that microglial ablation reduced the sensitivity of mice to anesthetics and substantially shortened duration of loss of righting reflex (LORR) or unconsciousness induced by multiple anesthetics, thereby promoting earlier emergence from GA. Microglial repopulation restored the regular anesthetic recovery, and chemogenetic activation of microglia prolonged the duration of LORR. In addition, anesthesia-accompanying analgesia and hypothermia were also attenuated after microglial depletion. Single-cell RNA sequencing analyses showed that anesthesia prominently affected the transcriptional levels of chemotaxis and migration-related genes in microglia. By pharmacologically targeting different microglial motility pathways, we found that blocking P2Y₁₂ receptor (P2Y₁₂R) reduced the duration of LORR of mice. Moreover, genetic ablation of P2Y₁₂R in microglia also promoted quicker recovery in mice from anesthesia, verifying the importance of microglial P2Y₁₂R in anesthetic regulation. Our work presents the first evidence that microglia actively participate in multiple processes of GA through P2Y₁₂R-mediated signaling and expands the non-immune roles of microglia in the brain.

Proinflammatory activation of microglia in the cerebellum hyperexcites Purkinje cells to trigger ataxia

Specific medications to combat cerebellar ataxias, a group of debilitating movement disorders characterized by difficulty with walking, balance and coordination, are still lacking. Notably, cerebellar microglial activation appears to be a common feature in different types of ataxic patients and rodent models. However, direct evidence that cerebellar microglial activation in vivo is sufficient to induce ataxia is still lacking. Here, by employing chemogenetic approaches to manipulate cerebellar microglia selectively and directly, we found that specific chemogenetic activation of microglia in the cerebellar vermis directly leads to ataxia symptoms in wild-type mice and aggravated ataxic motor deficits in 3-acetylpyridine (3-AP) mice, a classic mouse model of cerebellar ataxia. Mechanistically, cerebellar microglial proinflammatory activation induced by either chemogenetic M3D(Gq) stimulation or 3-AP modeling hyperexcites Purkinje cells (PCs), which consequently triggers ataxia. Blockade of microglia-derived TNF-α, one of the most important proinflammatory cytokines, attenuates the hyperactivity of PCs driven by microglia. Moreover, chemogenetic inhibition of cerebellar microglial activation or suppression of cerebellar microglial activation by PLX3397 and minocycline reduces the production of proinflammatory cytokines, including TNF-α, to effectively restore the overactivation of PCs and alleviate motor deficits in 3-AP mice. These results suggest that cerebellar microglial activation may aggravate the neuroinflammatory response and subsequently induce dysfunction of PCs, which in turn triggers ataxic motor deficits. Our findings thus reveal a causal relationship between proinflammatory activation of cerebellar microglia and ataxic motor symptoms, which may offer novel evidence for therapeutic intervention for cerebellar ataxias by targeting microglia and microglia-derived inflammatory mediators.

Adolescent ethanol drinking promotes hyperalgesia, neuroinflammation and serotonergic deficits in mice that persist into adulthood

Adolescent alcohol use can permanently alter brain function and lead to poor health outcomes in adulthood. Emerging evidence suggests that alcohol use can predispose individuals to pain disorders or exacerbate existing pain conditions, but the underlying neural mechanisms are currently unknown. Here we report that mice exposed to adolescent intermittent access to ethanol (AIE) exhibit increased pain sensitivity and depressive-like behaviors that persist for several weeks after alcohol cessation and are accompanied by elevated CD68 expression in microglia and reduced numbers of serotonin (5-HT)-expressing neurons in the dorsal raphe nucleus (DRN). 5-HT expression was also reduced in the thalamus, anterior cingulate cortex (ACC) and amygdala as well as the lumbar dorsal horn of the spinal cord. We further demonstrate that chronic minocycline administration after AIE alleviated hyperalgesia and social deficits, while chemogenetic activation of microglia in the DRN of ethanol-naïve mice reproduced the effects of AIE on pain and social behavior. Chemogenetic activation of microglia also reduced tryptophan hydroxylase 2 (Tph2) expression and was negatively correlated with the number of 5-HT-immunoreactive cells in the DRN. Taken together, these results indicate that microglial activation in the DRN may be a primary driver of pain, negative affect, and 5-HT depletion after AIE.

SKF96365 impedes spinal glutamatergic transmission-mediated neuropathic allodynia

Spinal nerve injury causes mechanical allodynia and structural imbalance of neurotransmission, which were typically associated with calcium overload. Store-operated calcium entry (SOCE) is considered crucial elements-mediating intracellular calcium homeostasis, ion channel activity, and synaptic plasticity. However, the underlying mechanism of SOCE in mediating neuronal transmitter release and synaptic transmission remains ambiguous in neuropathic pain. Neuropathic rats were operated by spinal nerve ligations. Neurotransmissions were assessed by whole-cell recording in substantia gelatinosa. Immunofluorescence staining of STIM1 with neuronal and glial biomarkers in the spinal dorsal horn. The endoplasmic reticulum stress level was estimated from qRT-PCR. Intrathecal injection of SOCE antagonist SKF96365 dose-dependently alleviated mechanical allodynia in ipsilateral hind paws of neuropathic rats with ED₅₀ of 18 μg. Immunofluorescence staining demonstrated that STIM1 was specifically and significantly expressed in neurons but not astrocytes and microglia in the spinal dorsal horn. Bath application of SKF96365 inhibited enhanced miniature excitatory postsynaptic currents in a dosage-dependent manner without affecting miniature inhibitory postsynaptic currents. Mal-adaption of SOCE was commonly related to endoplasmic reticulum (ER) stress in the central nervous system. SKF96365 markedly suppressed ER stress levels by alleviating mRNA expression of C/EBP homologous protein and heat shock protein 70 in neuropathic rats. Our findings suggested that nerve injury might promote SOCE-mediated calcium levels, resulting in long-term imbalance of spinal synaptic transmission and behavioral sensitization, SKF96365 produces antinociception by alleviating glutamatergic transmission and ER stress. This work demonstrated the involvement of SOCE in neuropathic pain, implying that SOCE might be a potential target for pain management.

Chemogenetic activation of VGLUT3-expressing neurons decreases movement

Vesicular glutamate transporters (VGLUTs) are responsible for the storage of glutamate into secretory vesicles. The VGLUT3 isoform is mainly expressed in neurons that secrete other classical neurotransmitters, including the cholinergic interneurons in the striatum, and VGLUT3-expressing neurons often secrete two distinct neurotransmitters. VGLUT3 is discretely distributed throughout the brain and is found in subpopulations of spinal cord interneurons, in subset of neurons in the dorsal root ganglion, and in Merkel cells. Mice with a global loss of VGLUT3 are hyperactive and the modulation of specific VGLUT3-expressing circuits can lead to changes in movement. In this study, we tested the hypothesis that increased activity of VGLUT3-expressing neurons is associated with decreased movement. Using a mouse line expressing excitatory designer receptor exclusively activated by designer drugs (hM3Dq-DREADD) on VGLUT3-expressing neurons, we showed that activation of hM3Dq signalling acutely decreased locomotor activity. This decreased locomotion was likely not due to circuit changes mediated by glutamate nor acetylcholine released from VGLUT3-expressing neurons, as activation of hM3Dq signalling in mice that do not release glutamate or acetylcholine from VGLUT3-expressing neurons also decreased locomotor activity. This suggests that other neurotransmitters are likely driving this hypoactive phenotype. We used these mouse lines to compare the effects of DREADD agonists in vivo. We observed that clozapine-N-oxide (CNO), clozapine, compound 21 and perlapine show small differences in the speed at which they prompt behavioural responses but the four of them are selective DREADD ligands.

Chemogenetic and Optogenetic Manipulations of Microglia in Chronic Pain

Chronic pain relief remains an unmet medical need. Current research points to a substantial contribution of glia-neuron interaction in its pathogenesis. Particularly, microglia play a crucial role in the development of chronic pain. To better understand the microglial contribution to chronic pain, specific regional and temporal manipulations of microglia are necessary. Recently, two new approaches have emerged that meet these demands. Chemogenetic tools allow the expression of designer receptors exclusively activated by designer drugs (DREADDs) specifically in microglia. Similarly, optogenetic tools allow for microglial manipulation via the activation of artificially expressed, light-sensitive proteins. Chemo- and optogenetic manipulations of microglia in vivo are powerful in interrogating microglial function in chronic pain. This review summarizes these emerging tools in studying the role of microglia in chronic pain and highlights their potential applications in microglia-related neurological disorders.

The DREADDful Hurdles and Opportunities of the Chronic Chemogenetic Toolbox

The chronic character of chemogenetics has been put forward as one of the assets of the technique, particularly in comparison to optogenetics. Yet, the vast majority of chemogenetic studies have focused on acute applications, while repeated, long-term neuromodulation has only been booming in the past few years. Unfortunately, together with the rising number of studies, various hurdles have also been uncovered, especially in relation to its chronic application. It becomes increasingly clear that chronic neuromodulation warrants caution and that the effects of acute neuromodulation cannot be extrapolated towards chronic experiments. Deciphering the underlying cellular and molecular causes of these discrepancies could truly unlock the chronic chemogenetic toolbox and possibly even pave the way for chemogenetics towards clinical application. Indeed, we are only scratching the surface of what is possible with chemogenetic research. For example, most investigations are concentrated on behavioral read-outs, whereas dissecting the underlying molecular signature after (chronic) neuromodulation could reveal novel insights in terms of basic neuroscience and deregulated neural circuits. In this review, we highlight the hurdles associated with the use of chemogenetic experiments, as well as the unexplored research questions for which chemogenetics offers the ideal research platform, with a particular focus on its long-term application.

Cholinergic blockade of neuroinflammation – from tissue to RNA regulators

Inflammatory stimuli and consequent pro-inflammatory immune responses may facilitate neurodegeneration and threaten survival following pathogen infection or trauma, but potential controllers preventing these risks are incompletely understood. Here, we argue that small RNA regulators of acetylcholine (ACh) signaling, including microRNAs (miRs) and transfer RNA fragments (tRFs) may tilt the balance between innate and adaptive immunity, avoid chronic inflammation and prevent the neuroinflammation-mediated exacerbation of many neurological diseases. While the restrictive permeability of the blood–brain barrier (BBB) protects the brain from peripheral immune events, this barrier can be disrupted by inflammation and is weakened with age. The consequently dysregulated balance between pro- and anti-inflammatory processes may modify the immune activities of brain microglia, astrocytes, perivascular macrophages, oligodendrocytes and dendritic cells, leading to neuronal damage. Notably, the vagus nerve mediates the peripheral cholinergic anti-inflammatory reflex and underlines the consistent control of body–brain inflammation by pro-inflammatory cytokines, which affect cholinergic functions; therefore, the disruption of this reflex can exacerbate cognitive impairments such as attention deficits and delirium. RNA regulators can contribute to re-balancing the cholinergic network and avoiding its chronic deterioration, and their activities may differ between men and women and/or wear off with age. This can lead to hypersensitivity of aged patients to inflammation and higher risks of neuroinflammation-driven cholinergic impairments such as delirium and dementia following COVID-19 infection. The age- and sex-driven differences in post-transcriptional RNA regulators of cholinergic elements may hence indicate new personalized therapeutic options for neuroinflammatory diseases.

Sickness and the social brain: How the immune system regulates behavior across species

Many instances of sickness critically involve the immune system. The immune system talks to the brain in a bi-directional loop. This discourse affords the immune system immense control, such that it can influence behavior and optimize recovery from illness. These behavioral responses to infection are called sickness behaviors and can manifest in many ways, including changes in mood, motivation, or energy. Fascinatingly, most of these changes are conserved across species, and most organisms demonstrate some form of sickness behaviors. One of the most interesting sickness behaviors, and not immediately obvious, is altered sociability. Here, we discuss how the immune system impacts social behavior, by examining the brain regions and immune mediators involved in this process. We first outline how social behavior changes in response to infection in various species. Next, we explore which brain regions control social behavior and their evolutionary origins. Finally, we describe which immune mediators establish the link between illness and social behavior, in the context of both normal development and infection. Overall, we hope to make clear the striking similarities between the mechanisms that facilitate changes in sociability in derived and ancestral vertebrate, as well as invertebrate, species.

Long-term selective stimulation of transplanted neural stem/progenitor cells for spinal cord injury improves locomotor function

In cell transplantation therapy for spinal cord injury (SCI), grafted human induced pluripotent stem cell-derived neural stem/progenitor cells (hiPSC-NS/PCs) mainly differentiate into neurons, forming synapses in a process similar to neurodevelopment. In the developing nervous system, the activity of immature neurons has an important role in constructing and maintaining new synapses. Thus, we investigate how enhancing the activity of transplanted hiPSC-NS/PCs affects both the transplanted cells themselves and the host tissue. We find that chemogenetic stimulation of hiPSC-derived neural cells enhances cell activity and neuron-to-neuron interactions in vitro. In a rodent model of SCI, consecutive and selective chemogenetic stimulation of transplanted hiPSC-NS/PCs also enhances the expression of synapse-related genes and proteins in surrounding host tissues and prevents atrophy of the injured spinal cord, thereby improving locomotor function. These findings provide a strategy for enhancing activity within the graft to improve the efficacy of cell transplantation therapy for SCI.

Emerging roles for microglia and microbiota in the development of social circuits

Social withdrawal is a core component of the behavioral response to infection. This fact points to a deep evolutionary and biologic relationship between the immune system and the social brain. Indeed, a large body of literature supports such an intimate connection. In particular, immune activation during the perinatal period has been shown to have long-lasting consequences for social behavior, but the neuroimmune mechanisms by which this occurs are only partially understood. Microglia, the resident immune cells of the brain, influence the formation of neural circuits by phagocytosing synaptic and cellular elements, as well as by releasing chemokines and cytokines. Intriguingly, microbiota, especially those that reside within the gut, may also influence brain development via the release of metabolites that travel to the brain, by influencing vagal nerve signaling, or by modulating the host immune system. Here, I will review the work suggesting important roles for microglia and microbiota in social circuit formation during development. I will then highlight avenues for future work in this area, as well as technological advances that extend our capacity to ask mechanistic questions about the relationships between microglia, microbiota, and the social brain.

Abnormalities in the composition of the gut microbiota in mice after repeated administration of DREADD ligands

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are known as genetically modified G-protein-coupled receptors (GPCRs), which can be activated by synthetic ligands such as clozapine N-oxide (CNO) and DREADD agonist 21 (compound 21: C21). The brain-gut-microbiota axis has a crucial role in bidirectional interactions between the brain and the gastrointestinal microbiota. In this study, we investigated whether repeated administration of CNO or C21 could influence the gut microbiota and short-chain fatty acids (SCFAs) in feces of adult mice. Repeated administration of CNO or C21 as drinking water did not alter the α- and β-diversity of gut microbiota in mice compared with control mice. However, we found significant changes in relative abundance for several bacteria in the CNO (or C21) group at the taxonomic level compared to the control group. The linear discriminant analysis effect size (LEfSe) algorithm distinguished the family Prevotellaceae, the genus Anaerocolumna, the genus Prevotella, and the genus Frisingicoccus, these four specific microbial markers for the CNO group relative to the control group. In addition, the LEfSe algorithm identified the family Clostridiaceae, the genus Faecalicatena and the genus Marinisporobacter, these three bacteria of different taxonomic as potential microbial markers for the C21 group relative to the control group. In contrast, repeated administration of CNO (or C21) did not alter SCFAs in feces samples of adult mice. The data suggest that repeated administration of CNO or C21 contributes to an unusual organization of the gut microbiota in adult mice. Therefore, abnormalities in the composition of gut microbiota by repeated dosing of DREADD ligands should be taken into consideration for behavioral and biological functions in rodents treated with DREADD ligands.

Chemogenetic Activation of CX3CR1-Expressing Spinal Microglia Using Gq-DREADD Elicits Mechanical Allodynia in Male Mice

It is important to investigate the sex-dependent roles of microglia in pain hypersensitivity as reactive microglia within the spinal dorsal horn (DH) have been reported to be pivotal in neuropathic pain induction in male rodents upon nerve injury. Here, we aimed at determining the role of sex differences in the behavioral and functional outcomes of the chemogenetic activation of spinal microglia using Gq-designer receptors exclusively activated by designer drugs (Gq-DREADD) driven by the microglia-specific Cx3cr1 promoter. CAG-LSL-human Gq-coupled M3 muscarinic receptors (hM3Dq)-DREADD mice were crossed with CX3C chemokine receptor 1 (CX3CR1)-Cre mice, and immunohistochemistry images revealed that hM3Dq was selectively expressed on Iba1⁺ microglia, but not on astrocytes and neurons. Intrathecal (i.t.) administration of clozapine-N-oxide (CNO) elicited mechanical allodynia exclusively in male mice. Furthermore, the reactive microglia-dominant molecules that contributed to pain hypersensitivity in CX3CR1-hM3Dq were upregulated in mice of both sexes. The degree of upregulation was greater in male than in female mice. Depletion of spinal microglia using pexidartinib (PLX3397), a colony stimulating factor-1 receptor inhibitor, alleviated the male CX3CR1-hM3Dq mice from pain hypersensitivity and compromised the expression of inflammatory molecules. Thus, the chemogenetic activation of spinal microglia resulted in pain hypersensitivity in male mice, suggesting the sex-dependent molecular aspects of spinal microglia in the regulation of pain.

Chemogenetic manipulation of microglia inhibits neuroinflammation and neuropathic pain in mice

Microglia play an important role in the central sensitization and chronic pain. However, a direct connection between microglial function and pain development in vivo remains incompletely understood. To address this issue, we applied chemogenetic approach by using CX₃CR1^creER/+:R26^LSL-hM4Di/+ transgenic mice to enable expression of inhibitory Designer Receptors Exclusively Activated by Designer Drugs (Gi DREADD) in microglia. We found that microglial Gi DREADD activation inhibited spinal nerve transection (SNT)-induced microglial reactivity as well as chronic pain in both male and female mice. Gi DREADD activation downregulated the transcription factor interferon regulatory factor 8 (IRF8) and its downstream target pro-inflammatory cytokine interleukin 1 beta (IL-1β). Using in vivo spinal cord recording, we found that activation of microglial Gi DREADD attenuated synaptic transmission following SNT. Our results demonstrate that microglial Gi DREADD reduces neuroinflammation, synaptic function and neuropathic pain after SNT. Thus, chemogenetic approaches provide a potential opportunity for interrogating microglial function and neuropathic pain treatment.

Deletion of muscarinic acetylcholine receptor 3 in microglia impacts brain ischemic injury

Microglia are the immune cells of the brain and become activated during any type of brain injury. In the middle cerebral artery occlusion (MCAo) model, a mouse model for ischemic stroke, we have previously shown that microglia and invaded monocytes upregulate the expression of the muscarinic acetylcholine receptor 3 (M3R) in the ischemic lesion. Here we tested whether this upregulation has an impact on the pathogenesis of MCAo. We depleted the m3R receptor in microglia, but not in circulating monocytes by giving tamoxifen to CX3CR1-CreERT^+/+M3R^flox/flox (M3RKO^mi) animals 3 weeks prior to MCAo. We found that M3RKO^mi male mice had bigger lesions, more pronounced motor deficits after one week and cognitive deficits after about one month compared to control males. The density of Iba1⁺ cells was lower in the lesions of M3RKO male mice in the early, but not in the late disease phase. In females, these differences were not significant. By giving tamoxifen 1 week prior to MCAo, we depleted m3R in microglia and in circulating monocytes (M3RKO^mi/mo). Male M3RKO^mi/mo did not differ in lesion size, but had a lower survival rate, showed motor deficits and a reduced accumulation of Iba1⁺ positive cells into the lesion site. In conclusion, our data suggest that the upregulation of m3R in microglia and monocytes in stroke has a beneficial effect on the clinical outcome in male mice.

Sickness and the Social Brain: Love in the Time of COVID

As a highly social species, inclusion in social networks and the presence of strong social bonds are critical to our health and well-being. Indeed, impaired social functioning is a component of numerous neuropsychiatric disorders including depression, anxiety, and substance use disorder. During the current COVID-19 pandemic, our social networks are at risk of fracture and many are vulnerable to the negative consequences of social isolation. Importantly, infection itself leads to changes in social behavior as a component of "sickness behavior." Furthermore, as in the case of COVID-19, males and females often differ in their immunological response to infection, and, therefore, in their susceptibility to negative outcomes. In this review, we discuss the many ways in which infection changes social behavior-sometimes to the benefit of the host, and in some instances for the sake of the pathogen-in species ranging from eusocial insects to humans. We also explore the neuroimmune mechanisms by which these changes in social behavior occur. Finally, we touch upon the ways in which the social environment (group living, social isolation, etc.) shapes the immune system and its ability to respond to challenge. Throughout we emphasize how males and females differ in their response to immune activation, both behaviorally and physiologically.

Taenia larvae possess distinct acetylcholinesterase profiles with implications for host cholinergic signalling

Larvae of the cestodes Taenia solium and Taenia crassiceps infect the central nervous system of humans. Taenia solium larvae in the brain cause neurocysticercosis, the leading cause of adult-acquired epilepsy worldwide. Relatively little is understood about how cestode-derived products modulate host neural and immune signalling. Acetylcholinesterases, a class of enzyme that breaks down acetylcholine, are produced by a host of parasitic worms to aid their survival in the host. Acetylcholine is an important signalling molecule in both the human nervous and immune systems, with powerful modulatory effects on the excitability of cortical networks. Therefore, it is important to establish whether cestode derived acetylcholinesterases may alter host neuronal cholinergic signalling. Here we make use of multiple techniques to profile acetylcholinesterase activity in different extracts of both Taenia crassiceps and Taenia solium larvae. We find that the larvae of both species contain substantial acetylcholinesterase activity. However, acetylcholinesterase activity is lower in Taenia solium as compared to Taenia crassiceps larvae. Further, whilst we observed acetylcholinesterase activity in all fractions of Taenia crassiceps larvae, including on the membrane surface and in the excreted/secreted extracts, we could not identify acetylcholinesterases on the membrane surface or in the excreted/secreted extracts of Taenia solium larvae. Bioinformatic analysis revealed conservation of the functional protein domains in the Taenia solium acetylcholinesterases, when compared to the homologous human sequence. Finally, using whole-cell patch clamp recordings in rat hippocampal brain slice cultures, we demonstrate that Taenia larval derived acetylcholinesterases can break down acetylcholine at a concentration which induces changes in neuronal signalling. Together, these findings highlight the possibility that Taenia larval acetylcholinesterases can interfere with cholinergic signalling in the host, potentially contributing to pathogenesis in neurocysticercosis.

Infection of the human nervous system with larvae of the parasite Taenia solium is a significant cause of acquired epilepsy worldwide. Despite this, the precise cellular and molecular mechanisms underlying epileptogenesis in neurocysticercosis remain unclear. Acetylcholinesterases are a family of enzymes widely produced by helminthic parasites. These enzymes facilitate the breakdown of acetylcholine, which is also a major neurotransmitter in the human nervous system. If T. solium larvae produce acetylcholinesterases, this could potentially disrupt host cholinergic signalling, which may in turn contribute to seizures and epilepsy. We therefore set out to investigate the presence and activity of acetylcholinesterases in T. solium larvae, as well as in Taenia crassiceps larvae, a species commonly used as a model parasite in neurocysticercosis research. We found that both T. crassiceps and T. solium larvae produce acetylcholinesterases with substantial activity and that the functional protein domains in the Taenia solium acetylcholinesterases have great similarity to those of human acetylcholinesterases. We further demonstrate that the acetylcholinesterase activity in the products of these parasites is sufficient to break down acetylcholine at a concentration which induces changes in neuronal signalling in an ex vivo brain slice model. This study provides evidence that Taenia larvae produce acetylcholinesterases and that these can potentially interfere with cholinergic signalling in the host and contribute to pathogenesis in neurocysticercosis.