Trust your gut, lest thou be anxious.

Can gut-residing bacteria influence mood and anxiety? And can targeting bacteria-produced metabolites reduce anxiety? Based on two Nature and Nature Medicine papers, the answers to these questions are likely yes. Needham, Campbell, and colleagues identified bacteria that enhance anxiety-like behaviors in mice and ways to mitigate anxiety in autistic patients.

The neuroimmunology of social-stress-induced sensitization.

Myriad clinical findings provide links between chronic stressors, inflammation, and mood disorders. Furthermore, traumatic or chronic exposure to psychological stressors may promote stress sensitization, in which individuals have long-term complications, including increased vulnerability to subsequent stressors. Post-traumatic stress disorder (PTSD) is a clinically relevant example of stress sensitization. PTSD alters neuronal circuitry and mood; however, the mechanisms underlying long-term stress sensitization within this disorder are unclear. Rodent models of chronic social defeat recapitulate several key physiological, immunological, and behavioral responses associated with psychological stress in humans. Repeated social defeat (RSD) uniquely promotes the convergence of neuronal, central inflammatory (microglial), and peripheral immune (monocyte) pathways, leading to prolonged anxiety, social withdrawal, and cognitive impairment. Moreover, RSD promotes stress sensitization, in which mice are highly sensitive to subthreshold stress exposure and recurrence of anxiety weeks after the cessation of stress. Therefore, the purpose of this Review is to discuss the influence of social-defeat stress on the immune system that may underlie stress sensitization within three key cellular compartments: neurons, microglia, and monocytes. Delineating the mechanisms of stress sensitization is critical in understanding and treating conditions such as PTSD.

The Immune-Mind Connection.

How does stress promote anxiety? In this issue of Cell, Fan et al. report that immune cells have a direct role in this process. They show that chronic stress promotes mitochondrial fission in CD4+ T cells, causing increased synthesis of xanthine, which acts on the brain and induces anxiety-like behaviors.

Stress-Induced Metabolic Disorder in Peripheral CD4+ T Cells Leads to Anxiety-like Behavior.

Physical or mental stress leads to neuroplasticity in the brain and increases the risk of depression and anxiety. Stress exposure causes the dysfunction of peripheral T lymphocytes. However, the pathological role and underlying regulatory mechanism of peripheral T lymphocytes in mood disorders have not been well established. Here, we show that the lack of CD4+ T cells protects mice from stress-induced anxiety-like behavior. Physical stress-induced leukotriene B4 triggers severe mitochondrial fission in CD4+ T cells, which further leads to a variety of behavioral abnormalities including anxiety, depression, and social disorders. Metabolomic profiles and single-cell transcriptome reveal that CD4+ T cell-derived xanthine acts on oligodendrocytes in the left amygdala via adenosine receptor A1. Mitochondrial fission promotes the de novo synthesis of purine via interferon regulatory factor 1 accumulation in CD4+ T cells. Our study implicates a critical link between a purine metabolic disorder in CD4+ T cells and stress-driven anxiety-like behavior.

Targeting Peripheral Somatosensory Neurons to Improve Tactile-Related Phenotypes in ASD Models.

Somatosensory over-reactivity is common among patients with autism spectrum disorders (ASDs) and is hypothesized to contribute to core ASD behaviors. However, effective treatments for sensory over-reactivity and ASDs are lacking. We found distinct somatosensory neuron pathophysiological mechanisms underlie tactile abnormalities in different ASD mouse models and contribute to some ASD-related behaviors. Developmental loss of ASD-associated genes Shank3 or Mecp2 in peripheral mechanosensory neurons leads to region-specific brain abnormalities, revealing links between developmental somatosensory over-reactivity and the genesis of aberrant behaviors. Moreover, acute treatment with a peripherally restricted GABAA receptor agonist that acts directly on mechanosensory neurons reduced tactile over-reactivity in six distinct ASD models. Chronic treatment of Mecp2 and Shank3 mutant mice improved body condition, some brain abnormalities, anxiety-like behaviors, and some social impairments but not memory impairments, motor deficits, or overgrooming. Our findings reveal a potential therapeutic strategy targeting peripheral mechanosensory neurons to treat tactile over-reactivity and select ASD-related behaviors.

Disruption of the Na+/K+-ATPase-purinergic P2X7 receptor complex in microglia promotes stress-induced anxiety.

Na+/K+-ATPase (NKA) plays an important role in the central nervous system. However, little is known about its function in the microglia. Here, we found that NKAα1 forms a complex with the purinergic P2X7 receptor (P2X7R), an adenosine 5'-triphosphate (ATP)-gated ion channel, under physiological conditions. Chronic stress or treatment with lipopolysaccharide plus ATP decreased the membrane expression of NKAα1 in microglia, facilitated P2X7R function, and promoted microglia inflammatory activation via activation of the NLRP3 inflammasome. Accordingly, global deletion or conditional deletion of NKAα1 in microglia under chronic stress-induced aggravated anxiety-like behavior and neuronal hyperexcitability. DR5-12D, a monoclonal antibody that stabilizes membrane NKAα1, improved stress-induced anxiety-like behavior and ameliorated neuronal hyperexcitability and neurogenesis deficits in the ventral hippocampus of mice. Our results reveal that NKAα1 limits microglia inflammation and may provide a target for the treatment of stress-related neuroinflammation and diseases.

An Amygdala-Hippocampus Subnetwork that Encodes Variation in Human Mood.

Human brain networks that encode variation in mood on naturalistic timescales remain largely unexplored. Here we combine multi-site, semi-chronic, intracranial electroencephalography recordings from the human limbic system with machine learning methods to discover a brain subnetwork that correlates with variation in individual subjects' self-reported mood over days. First we defined the subnetworks that influence intrinsic brain dynamics by identifying regions that showed coordinated changes in spectral coherence. The most common subnetwork, found in 13 of 21 subjects, was characterized by ß-frequency coherence (13-30 Hz) between the amygdala and hippocampus. Increased variability of this subnetwork correlated with worsening mood across these 13 subjects. Moreover, these subjects had significantly higher trait anxiety than the 8 of 21 for whom this amygdala-hippocampus subnetwork was absent. These results demonstrate an approach for extracting network-behavior relationships from complex datasets, and they reveal a conserved subnetwork associated with a psychological trait that significantly influences intrinsic brain dynamics and encodes fluctuations in mood.

Anatomically Defined and Functionally Distinct Dorsal Raphe Serotonin Sub-systems.

The dorsal raphe (DR) constitutes a major serotonergic input to the forebrain and modulates diverse functions and brain states, including mood, anxiety, and sensory and motor functions. Most functional studies to date have treated DR serotonin neurons as a single population. Using viral-genetic methods, we found that subcortical- and cortical-projecting serotonin neurons have distinct cell-body distributions within the DR and differentially co-express a vesicular glutamate transporter. Further, amygdala- and frontal-cortex-projecting DR serotonin neurons have largely complementary whole-brain collateralization patterns, receive biased inputs from presynaptic partners, and exhibit opposite responses to aversive stimuli. Gain- and loss-of-function experiments suggest that amygdala-projecting DR serotonin neurons promote anxiety-like behavior, whereas frontal-cortex-projecting neurons promote active coping in the face of challenge. These results provide compelling evidence that the DR serotonin system contains parallel sub-systems that differ in input and output connectivity, physiological response properties, and behavioral functions.

Meningeal γδ T cells regulate anxiety-like behavior via IL-17a signaling in neurons.

Interleukin (IL)-17a has been highly conserved during evolution of the vertebrate immune system and widely studied in contexts of infection and autoimmunity. Studies suggest that IL-17a promotes behavioral changes in experimental models of autism and aggregation behavior in worms. Here, through a cellular and molecular characterization of meningeal γδ17 T cells, we defined the nearest central nervous system-associated source of IL-17a under homeostasis. Meningeal γδ T cells express high levels of the chemokine receptor CXCR6 and seed meninges shortly after birth. Physiological release of IL-17a by these cells was correlated with anxiety-like behavior in mice and was partially dependent on T cell receptor engagement and commensal-derived signals. IL-17a receptor was expressed in cortical glutamatergic neurons under steady state and its genetic deletion decreased anxiety-like behavior in mice. Our findings suggest that IL-17a production by meningeal γδ17 T cells represents an evolutionary bridge between this conserved anti-pathogen molecule and survival behavioral traits in vertebrates.

Integrated Control of Predatory Hunting by the Central Nucleus of the Amygdala.

Superior predatory skills led to the evolutionary triumph of jawed vertebrates. However, the mechanisms by which the vertebrate brain controls predation remain largely unknown. Here, we reveal a critical role for the central nucleus of the amygdala in predatory hunting. Both optogenetic and chemogenetic stimulation of central amygdala of mice elicited predatory-like attacks upon both insect and artificial prey. Coordinated control of cervical and mandibular musculatures, which is necessary for accurately positioning lethal bites on prey, was mediated by a central amygdala projection to the reticular formation in the brainstem. In contrast, prey pursuit was mediated by projections to the midbrain periaqueductal gray matter. Targeted lesions to these two pathways separately disrupted biting attacks upon prey versus the initiation of prey pursuit. Our findings delineate a neural network that integrates distinct behavioral modules and suggest that central amygdala neurons instruct predatory hunting across jawed vertebrates.

A Cortical Circuit for Sexually Dimorphic Oxytocin-Dependent Anxiety Behaviors.

The frequency of human social and emotional disorders varies significantly between males and females. We have recently reported that oxytocin receptor interneurons (OxtrINs) modulate female sociosexual behavior. Here, we show that, in male mice, OxtrINs regulate anxiety-related behaviors. We demonstrate that corticotropin-releasing-hormone-binding protein (CRHBP), an antagonist of the stress hormone CRH, is specifically expressed in OxtrINs. Production of CRHBP blocks the CRH-induced potentiation of postsynaptic layer 2/3 pyramidal cell activity of male, but not female, mice, thus producing an anxiolytic effect. Our data identify OxtrINs as critical for modulation of social and emotional behaviors in both females and males and reveal a molecular mechanism that acts on local medial prefrontal cortex (mPFC) circuits to coordinate responses to OXT and CRH. They suggest that additional studies of the impact of the OXT/OXTR and CRHBP/CRH pathways in males and females will be important in development of gender-specific therapies.

Hypothalamic Agrp neurons drive stereotypic behaviors beyond feeding.

The nervous system evolved to coordinate flexible goal-directed behaviors by integrating interoceptive and sensory information. Hypothalamic Agrp neurons are known to be crucial for feeding behavior. Here, however, we show that these neurons also orchestrate other complex behaviors in adult mice. Activation of Agrp neurons in the absence of food triggers foraging and repetitive behaviors, which are reverted by food consumption. These stereotypic behaviors that are triggered by Agrp neurons are coupled with decreased anxiety. NPY5 receptor signaling is necessary to mediate the repetitive behaviors after Agrp neuron activation while having minor effects on feeding. Thus, we have unmasked a functional role for Agrp neurons in controlling repetitive behaviors mediated, at least in part, by neuropeptidergic signaling. The findings reveal a new set of behaviors coupled to the energy homeostasis circuit and suggest potential therapeutic avenues for diseases with stereotypic behaviors.

WNT signalling control by KDM5C during development affects cognition.

Although KDM5C is one of the most frequently mutated genes in X-linked intellectual disability1, the exact mechanisms that lead to cognitive impairment remain unknown. Here we use human patient-derived induced pluripotent stem cells and Kdm5c knockout mice to conduct cellular, transcriptomic, chromatin and behavioural studies. KDM5C is identified as a safeguard to ensure that neurodevelopment occurs at an appropriate timescale, the disruption of which leads to intellectual disability. Specifically, there is a developmental window during which KDM5C directly controls WNT output to regulate the timely transition of primary to intermediate progenitor cells and consequently neurogenesis. Treatment with WNT signalling modulators at specific times reveal that only a transient alteration of the canonical WNT signalling pathway is sufficient to rescue the transcriptomic and chromatin landscapes in patient-derived cells and to induce these changes in wild-type cells. Notably, WNT inhibition during this developmental period also rescues behavioural changes of Kdm5c knockout mice. Conversely, a single injection of WNT3A into the brains of wild-type embryonic mice cause anxiety and memory alterations. Our work identifies KDM5C as a crucial sentinel for neurodevelopment and sheds new light on KDM5C mutation-associated intellectual disability. The results also increase our general understanding of memory and anxiety formation, with the identification of WNT functioning in a transient nature to affect long-lasting cognitive function.

Metabolic shift induced by systemic activation of T cells in PD-1-deficient mice perturbs brain monoamines and emotional behavior.

T cells reorganize their metabolic profiles after being activated, but the systemic metabolic effect of sustained activation of the immune system has remained unexplored. Here we report that augmented T cell responses in Pdcd1-/- mice, which lack the inhibitory receptor PD-1, induced a metabolic serum signature characterized by depletion of amino acids. We found that the depletion of amino acids in serum was due to the accumulation of amino acids in activated Pdcd1-/- T cells in the lymph nodes. A systemic decrease in tryptophan and tyrosine led to substantial deficiency in the neurotransmitters serotonin and dopamine in the brain, which resulted in behavioral changes dominated by anxiety-like behavior and exacerbated fear responses. Together these data indicate that excessive activation of T cells causes a systemic metabolomic shift with consequences that extend beyond the immune system.

Control of stress-induced persistent anxiety by an extra-amygdala septohypothalamic circuit.

The extended amygdala has dominated research on the neural circuitry of fear and anxiety, but the septohippocampal axis also plays an important role. The lateral septum (LS) is thought to suppress fear and anxiety through its outputs to the hypothalamus. However, this structure has not yet been dissected using modern tools. The type 2 CRF receptor (Crfr2) marks a subset of LS neurons whose functional connectivity we have investigated using optogenetics. Crfr2(+) cells include GABAergic projection neurons that connect with the anterior hypothalamus. Surprisingly, we find that these LS outputs enhance stress-induced behavioral measures of anxiety. Furthermore, transient activation of Crfr2(+) neurons promotes, while inhibition suppresses, persistent anxious behaviors. LS Crfr2(+) outputs also positively regulate circulating corticosteroid levels. These data identify a subset of LS projection neurons that promote, rather than suppress, stress-induced behavioral and endocrinological dimensions of persistent anxiety states and provide a cellular point of entry to LS circuitry.

Gut enterochromaffin cells drive visceral pain and anxiety.

Gastrointestinal (GI) discomfort is a hallmark of most gut disorders and represents an important component of chronic visceral pain1. For the growing population afflicted by irritable bowel syndrome, GI hypersensitivity and pain persist long after tissue injury has resolved2. Irritable bowel syndrome also exhibits a strong sex bias, afflicting women three times more than men1. Here, we focus on enterochromaffin (EC) cells, which are rare excitable, serotonergic neuroendocrine cells in the gut epithelium3-5. EC cells detect and transduce noxious stimuli to nearby mucosal nerve endings3,6 but involvement of this signalling pathway in visceral pain and attendant sex differences has not been assessed. By enhancing or suppressing EC cell function in vivo, we show that these cells are sufficient to elicit hypersensitivity to gut distension and necessary for the sensitizing actions of isovalerate, a bacterial short-chain fatty acid associated with GI inflammation7,8. Remarkably, prolonged EC cell activation produced persistent visceral hypersensitivity, even in the absence of an instigating inflammatory episode. Furthermore, perturbing EC cell activity promoted anxiety-like behaviours which normalized after blockade of serotonergic signalling. Sex differences were noted across a range of paradigms, indicating that the EC cell-mucosal afferent circuit is tonically engaged in females. Our findings validate a critical role for EC cell-mucosal afferent signalling in acute and persistent GI pain, in addition to highlighting genetic models for studying visceral hypersensitivity and the sex bias of gut pain.

Cardiogenic control of affective behavioural state.

Emotional states influence bodily physiology, as exemplified in the top-down process by which anxiety causes faster beating of the heart1-3. However, whether an increased heart rate might itself induce anxiety or fear responses is unclear3-8. Physiological theories of emotion, proposed over a century ago, have considered that in general, there could be an important and even dominant flow of information from the body to the brain9. Here, to formally test this idea, we developed a noninvasive optogenetic pacemaker for precise, cell-type-specific control of cardiac rhythms of up to 900 beats per minute in freely moving mice, enabled by a wearable micro-LED harness and the systemic viral delivery of a potent pump-like channelrhodopsin. We found that optically evoked tachycardia potently enhanced anxiety-like behaviour, but crucially only in risky contexts, indicating that both central (brain) and peripheral (body) processes may be involved in the development of emotional states. To identify potential mechanisms, we used whole-brain activity screening and electrophysiology to find brain regions that were activated by imposed cardiac rhythms. We identified the posterior insular cortex as a potential mediator of bottom-up cardiac interoceptive processing, and found that optogenetic inhibition of this brain region attenuated the anxiety-like behaviour that was induced by optical cardiac pacing. Together, these findings reveal that cells of both the body and the brain must be considered together to understand the origins of emotional or affective states. More broadly, our results define a generalizable approach for noninvasive, temporally precise functional investigations of joint organism-wide interactions among targeted cells during behaviour.

Central regulation of stress-evoked peripheral immune responses.

Stress-linked psychiatric disorders, including anxiety and major depressive disorder, are associated with systemic inflammation. Recent studies have reported stress-induced alterations in haematopoiesis that result in monocytosis, neutrophilia, lymphocytopenia and, consequently, in the upregulation of pro-inflammatory processes in immunologically relevant peripheral tissues. There is now evidence that this peripheral inflammation contributes to the development of psychiatric symptoms as well as to common co-morbidities of psychiatric disorders such as metabolic syndrome and immunosuppression. Here, we review the specific brain and spinal regions, and the neuronal populations within them, that respond to stress and transmit signals to peripheral tissues via the autonomic nervous system or neuroendocrine pathways to influence immunological function. We comprehensively summarize studies that have employed retrograde tracing to define neurocircuits linking the brain to the bone marrow, spleen, gut, adipose tissue and liver. Moreover, we highlight studies that have used chemogenetic or optogenetic manipulation or intracerebroventricular administration of peptide hormones to control somatic immune responses. Collectively, this growing body of literature illustrates potential mechanisms through which stress signals are conveyed from the CNS to immune cells to regulate stress-relevant behaviours and comorbid pathophysiology.

Microbiota modulate behavioral and physiological abnormalities associated with neurodevelopmental disorders.

Neurodevelopmental disorders, including autism spectrum disorder (ASD), are defined by core behavioral impairments; however, subsets of individuals display a spectrum of gastrointestinal (GI) abnormalities. We demonstrate GI barrier defects and microbiota alterations in the maternal immune activation (MIA) mouse model that is known to display features of ASD. Oral treatment of MIA offspring with the human commensal Bacteroides fragilis corrects gut permeability, alters microbial composition, and ameliorates defects in communicative, stereotypic, anxiety-like and sensorimotor behaviors. MIA offspring display an altered serum metabolomic profile, and B. fragilis modulates levels of several metabolites. Treating naive mice with a metabolite that is increased by MIA and restored by B. fragilis causes certain behavioral abnormalities, suggesting that gut bacterial effects on the host metabolome impact behavior. Taken together, these findings support a gut-microbiome-brain connection in a mouse model of ASD and identify a potential probiotic therapy for GI and particular behavioral symptoms in human neurodevelopmental disorders.

Glucocorticoid rhythm renders female mice more daring.

Glucocorticoids, which have been implied in mood modulation, display robust diurnal oscillations in the blood. But does their circadian rhythm regulate mood swings? Ikeda et al. now identify a paracrine signaling pathway in the adrenal cortex that potentiates the daily amplitude of plasma glucocorticoids and renders female mice braver.