Spinal astrocytes in superficial laminae gate brainstem descending control of mechanosensory hypersensitivity

Remote limb ischemic preconditioning alleviated spinal cord injury through inhibiting proinflammatory immune response and promoting the survival of spinal neurons

STUDY DESIGN

Experimental animal study.

OBJECTIVES

To investigate the protective effect of remote limb ischemia preconditioning (RLPreC) on traumatic spinal cord injury (SCI) and explore the underlying biological mechanisms using RNA sequencing.

SETTING

China Rehabilitation Science Institute; Beijing; China.

METHODS

spinal cord injury was induced in mice using a force of 0.7 N. RLPreC treatment was administered. Motor function, pain behavior, and gene expression were assessed.

RESULTS

RLPreC treatment significantly improved motor function and reduced pain-like behavior in SCI mice. RNA-Seq analysis identified 5247 differentially expressed genes (DEGs). GO analysis revealed enrichment of immune response, inflammatory signaling, and synaptic transmission pathways among these DEGs. KEGG analysis indicated suppression of inflammation and promotion of synapse-related pathways.

CONCLUSIONS

RLPreC is a promising therapeutic strategy for improving motor function and alleviating pain after traumatic SCI. RNA-Seq analysis provides insights into potential therapeutic targets and warrants further investigation.

Sigma-1 receptor targeting inhibits connexin 43 based intercellular communication in chronic neuropathic pain

BACKGROUND AND OBJECTIVE

Neuropathic pain is a chronic condition characterized by aberrant signaling within the somatosensory system, affecting millions of people worldwide with limited treatment options. Herein, we aim at investigating the potential of a sigma-1 receptor (σ1R) antagonist in managing neuropathic pain.

METHODS

A Chronic Constriction Injury (CCI) model was used to induce neuropathic pain. The potential of (+)-MR200 was evaluated following daily subcutaneous injections of the compound. Its mechanism of action was confirmed by administration of a well-known σ1R agonist, PRE084.

RESULTS

(+)-MR200 demonstrated efficacy in protecting neurons from damage and alleviating pain hypersensitivity in CCI model. Our results suggest that (+)-MR200 reduced the activation of astrocytes and microglia, cells known to contribute to the neuroinflammatory process, suggesting that (+)-MR200 may not only address pain symptoms but also tackle the underlying cellular mechanism involved. Furthermore, (+)-MR200 treatment normalized levels of the gap junction (GJ)-forming protein connexin 43 (Cx43), suggesting a reduction in harmful intercellular communication that could fuel the chronicity of pain.

CONCLUSIONS

This approach could offer a neuroprotective strategy for managing neuropathic pain, addressing both pain symptoms and cellular processes driving the condition. Understanding the dynamics of σ1R expression and function in neuropathic pain is crucial for clinical intervention.

Potential role of remimazolam in alleviating bone cancer pain in mice via modulation of translocator protein in spinal astrocytes

Bone cancer pain (BCP) is a complex clinical challenge, with current treatments often falling short of providing adequate relief. Remimazolam, a benzodiazepine receptor agonist recognized for its anxiolytic effects, has emerged as a potential agent in managing BCP. This study explores the analgesic properties of remimazolam and its interaction with the translocator protein (TSPO), previously known as the peripheral benzodiazepine receptor, in spinal astrocytes. In the context of BCP, previous research has indicated that TSPO expression in spinal astrocytes may serve a protective regulatory function in neuropathic pain models. Building on this, the BCP mice received various doses of remimazolam on the 15th day post-inoculation, and pain behavior was assessed over time. The results showed that BCP induced an upregulation of TSPO and astrocyte activation in the spinal dorsal horn, alongside increased extracellular signal-regulated kinase (ERK) signaling and inflammatory cytokine expression. Remimazolam administration resulted in a dose-dependent reduction of pain behaviors, which corresponded with a decrease in both ERK pathway activation and inflammatory factor expression. This suggests that remimazolam's analgesic effects are mediated through its action as a TSPO agonist, leading to the attenuation of neuroinflammation and pain signaling pathways. Importantly, the analgesic effects of remimazolam were reversed by the TSPO antagonist PK11195, underscoring the pivotal role of TSPO in the drug's mechanism of action. This reversal also reinstated the heightened levels of ERK activity and inflammatory mediators, further confirming the involvement of TSPO in the modulation of these pain-related processes. These findings open new avenues for the therapeutic management of bone cancer pain, positioning remimazolam as a promising candidate for further investigation and development.

Exploring the role of spinal astrocytes in the onset of hyperalgesic priming signals in acid-induced chronic muscle pain

Hyperalgesic priming, a form of pain plasticity initiated by initial injury, leads to heightened sensitivity to subsequent noxious stimuli, contributing to chronic pain development in animals. While astrocytes play active roles in modulating synaptic transmission in various pain models, their specific involvement in hyperalgesic priming remains elusive. Here, we show that spinal astrocytes are essential for hyperalgesic priming formation in a mouse model of acid-induced muscle pain. We observed spinal astrocyte activation 4 h after initial acid injection, and inhibition of this activation prevented chronic pain development upon subsequent acid injection. Chemogenetic activation of spinal astrocytes mimicked the first acid-induced hyperalgesic priming. We also demonstrated that spinal phosphorylated extracellular regulated kinase (pERK)-positive neurons were mainly vesicular glutamate transporter-2 positive (Vglut2+) neurons after the first acid injection, and inhibition of spinal pERK prevented astrocyte activation. Furthermore, pharmacological inhibition of astrocytic glutamate transporters glutamate transporter-1 and glutamate-aspartate transporter abolished the hyperalgesic priming. Collectively, our results suggest that pERK activation in Vglut2+ neurons activate astrocytes through astrocytic glutamate transporters. This process eventually establishes hyperalgesic priming through spinal D-serine. We conclude that spinal astrocytes play a crucial role in the transition from acute to chronic pain.

Aged females unilaterally hypersensitize, lack descending inhibition, and overexpress alpha1D adrenergic receptors in a murine posttraumatic chronic pain model

ABSTRACT

Vulnerability to chronic pain is found to depend on age and sex. Most patients with chronic pain are elderly women, especially with posttraumatic pain after bone fracture that prevails beyond the usual recovery period and develops into a complex regional pain syndrome (CRPS). There, a distal bone fracture seems to initiate a pathophysiological process with unknown mechanism. To investigate whether sex, age, and alpha adrenergic receptors also contribute to a CRPS-like phenotype in animals, we performed experiments on tibia-fractured mice. Those mice commonly are resilient to the development of a CRPS-like phenotype. However, we found them to be vulnerable to long-lasting pain after distal bone fracture when they were of old age. These mice expressed mechanical and thermal hypersensitivity, as well as weight-bearing and autonomic impairment following bone trauma, which persisted over 3 months. Site-specific and body side-specific glycinergic and α1D-noradrenergic receptor expression in the spinal cord and the contralateral locus coeruleus were misbalanced. Aged female tibia-fractured mice lost descending noradrenergic inhibition and displayed enhanced spinal activity on peripheral pressure stimuli. Together, changes in the noradrenergic, hence, glycinergic system towards excitation in the pain pathway-ascending and descending-might contribute to the development or maintenance of long-lasting pain. Conclusively, changes in the noradrenergic system particularly occur in aged female mice after trauma and might contribute to the development of long-lasting pain. Our data support the hypothesis that some patients with chronic pain would benefit from lowering the adrenergic/sympathetic tone or antagonizing α1(D).

Dock4 contributes to neuropathic pain by regulating spinal synaptic plasticity in mice

Introduction

Neuropathic pain (NP) conditions arising from injuries to the nervous system due to trauma, disease, or neurotoxins are chronic, severe, debilitating, and exceedingly difficult to treat. However, the mechanisms of NP are not yet clear. Here we explored the role of Dock4, an atypical Rac1 GEF, in the development of NP.

Methods

Mechanical allodynia was assessed as paw withdrawal threshold by a dynamic plantar aesthesiometer. Immunofluorescence staining was conducted to investigate the expression and localization of Dock4, Rac1 and GluN2B. Quantitative analysis of Dock4, Rac1 and GluN2B were determined by qRT-PCR and Western blot assay. Spontaneous excitatory and inhibitory postsynaptic currents in spinal cord slices were examined using whole cell patch clam. Dendritic spine remodeling and synaptogenesis were detected in cultured dorsal spinal neurons.

Results and discussion

We found that SNL caused markedly mechanical allodynia accompanied by increase of Dock4, GTP-Rac1and GluN2B, which was prevented by knockdown of Dock4. Electrophysiological tests showed that SNL facilitated excitatory synaptic transmission, however, this was also inhibited by Dock RNAi-LV. Moreover, knockdown of Dock4 prevented dendritic growth and synaptogenesis.

Conclusion

In summary, our data indicated that Dock4 facilitated excitatory synaptic transmission by promoting the expression of GluN2B at the synaptic site and synaptogenesis, leading to the occurrence of NP.

Pannexin-1 channel inhibition alleviates opioid withdrawal in rodents by modulating locus coeruleus to spinal cord circuitry

Opioid withdrawal is a liability of chronic opioid use and misuse, impacting people who use prescription or illicit opioids. Hyperactive autonomic output underlies many of the aversive withdrawal symptoms that make it difficult to discontinue chronic opioid use. The locus coeruleus (LC) is an important autonomic centre within the brain with a poorly defined role in opioid withdrawal. We show here that pannexin-1 (Panx1) channels expressed on microglia critically modulate LC activity during opioid withdrawal. Within the LC, we found that spinally projecting tyrosine hydroxylase (TH)-positive neurons (LCspinal) are hyperexcitable during morphine withdrawal, elevating cerebrospinal fluid (CSF) levels of norepinephrine. Pharmacological and chemogenetic silencing of LCspinal neurons or genetic ablation of Panx1 in microglia blunted CSF NE release, reduced LC neuron hyperexcitability, and concomitantly decreased opioid withdrawal behaviours in mice. Using probenecid as an initial lead compound, we designed a compound (EG-2184) with greater potency in blocking Panx1. Treatment with EG-2184 significantly reduced both the physical signs and conditioned place aversion caused by opioid withdrawal in mice, as well as suppressed cue-induced reinstatement of opioid seeking in rats. Together, these findings demonstrate that microglial Panx1 channels modulate LC noradrenergic circuitry during opioid withdrawal and reinstatement. Blocking Panx1 to dampen LC hyperexcitability may therefore provide a therapeutic strategy for alleviating the physical and aversive components of opioid withdrawal.

Modulating Neural Circuits of Pain in Preclinical Models: Recent Insights for Future Therapeutics

Chronic pain is a pathological state defined as daily pain sensation over three consecutive months. It affects up to 30% of the general population. Although significant research efforts have been made in the past 30 years, only a few and relatively low effective molecules have emerged to treat chronic pain, with a considerable translational failure rate. Most preclinical models have focused on sensory neurotransmission, with particular emphasis on the dorsal horn of the spinal cord as the first relay of nociceptive information. Beyond impaired nociceptive transmission, chronic pain is also accompanied by numerous comorbidities, such as anxiety-depressive disorders, anhedonia and motor and cognitive deficits gathered under the term "pain matrix". The emergence of cutting-edge techniques assessing specific neuronal circuits allow in-depth studies of the connections between "pain matrix" circuits and behavioural outputs. Pain behaviours are assessed not only by reflex-induced responses but also by various or more complex behaviours in order to obtain the most complete picture of an animal's pain state. This review summarises the latest findings on pain modulation by brain component of the pain matrix and proposes new opportunities to unravel the mechanisms of chronic pain.

Norepinephrine Signals Through Astrocytes To Modulate Synapses

Locus coeruleus (LC)-derived norepinephrine (NE) drives network and behavioral adaptations to environmental saliencies by reconfiguring circuit connectivity, but the underlying synapse-level mechanisms are elusive. Here, we show that NE remodeling of synaptic function is independent from its binding on neuronal receptors. Instead, astrocytic adrenergic receptors and Ca2+ dynamics fully gate the effect of NE on synapses as the astrocyte-specific deletion of adrenergic receptors and three independent astrocyte-silencing approaches all render synapses insensitive to NE. Additionally, we find that NE suppression of synaptic strength results from an ATP-derived and adenosine A1 receptor-mediated control of presynaptic efficacy. An accompanying study from Chen et al. reveals the existence of an analogous pathway in the larval zebrafish and highlights its importance to behavioral state transitions. Together, these findings fuel a new model wherein astrocytes are a core component of neuromodulatory systems and the circuit effector through which norepinephrine produces network and behavioral adaptations, challenging an 80-year-old status quo.

A method for selective and efficient isolation of gray matter astrocytes from the spinal cord of adult mice

A growing body of evidence indicates intra- and inter-regional heterogeneity of astrocytes in the brain. However, because of a lack of an efficient method for isolating astrocytes from the spinal cord, little is known about how much spinal cord astrocytes are heterogeneous in adult mice. In this study, we developed a new method for isolating spinal astrocytes from adult mice using a cold-active protease from Bacillus licheniformis with an astrocyte cell surface antigen-2 (ACSA-2) antibody. Using fluorescence-activated cell sorting, isolated spinal ACSA-2+ cells were divided into two distinct populations, ACSA-2high and ACSA-2low. By analyzing the expression of cell-type marker genes, the ACSA-2high and ACSA-2low populations were identified as astrocytes and ependymal cells, respectively. Furthermore, ACSA-2high cells had mRNAs encoding genes that were abundantly expressed in the gray matter (GM) but not white matter astrocytes. By optimizing enzymatic isolation procedures, the yield of GM astrocytes also increased. Therefore, our newly established method enabled the selective and efficient isolation of GM astrocytes from the spinal cord of adult mice and may be useful for bulk- or single-cell RNA-sequencing under physiological and pathological conditions.

Sigma-1 Receptor Inhibition Reduces Mechanical Allodynia and Modulate Neuroinflammation in Chronic Neuropathic Pain

Neuropathic pain is one of the most debilitating forms of chronic pain, resulting from an injury or disease of the somatosensory nervous system, which induces abnormal painful sensations including allodynia and hyperalgesia. Available treatments are limited by severe side-effects and reduced efficacy in the chronic phase of the disease. Sigma-1 receptor (σ1R) has been identified as a chaperone protein, which modulate opioid receptors activities and the functioning of several ion channels, exerting a role in pain transmission. As such, it represents a druggable target to treat neuropathic pain. This study aims at investigating the therapeutic potential of the novel compound (+)-2R/S-LP2, a σ1R antagonist, in reducing painful behaviour and modulating the neuroinflammatory environment. We showed that repeated administration of the compound significantly inhibited mechanical allodynia in neuropathic rats, increasing the withdrawal threshold as compared to CCI-vehicle rats. Moreover, we found that (+)-2R/S-LP2-mediated effects resolve the neuroinflammatory microenvironment by reducing central gliosis and pro-inflammatory cytokines expression levels. This effect was coupled with a significant reduction of connexin 43 (Cx43) expression levels and gap junctions/hemichannels mediated microglia-to-astrocyte communication. These results suggest that inhibition of σ1R significantly attenuates neuropathic pain chronicization, thus representing a viable effective strategy.

Anterior cingulate cortex projections to the dorsal medial striatum underlie insomnia associated with chronic pain

Chronic pain often leads to the development of sleep disturbances. However, the precise neural circuit mechanisms responsible for sleep disorders in chronic pain have remained largely unknown. Here, we present compelling evidence that hyperactivity of pyramidal neurons (PNs) in the anterior cingulate cortex (ACC) drives insomnia in a mouse model of nerve-injury-induced chronic pain. After nerve injury, ACC PNs displayed spontaneous hyperactivity selectively in periods of insomnia. We then show that ACC PNs were both necessary for developing chronic-pain-induced insomnia and sufficient to mimic sleep loss in naive mice. Importantly, combining optogenetics and electrophysiological recordings, we found that the ACC projection to the dorsal medial striatum (DMS) underlies chronic-pain-induced insomnia through enhanced activity and plasticity of ACC-DMS dopamine D1R neuron synapses. Our findings shed light on the pivotal role of ACC PNs in developing chronic-pain-induced sleep disorders.

Laminar-selective spinal astrocyte population capable of converting tactile information into nociceptive in rats

We previously identified a spinal astrocyte population that expresses hairy and enhancer of split 5 (Hes5) and is selectively present in superficial laminae in mice. However, it was unclear whether such astrocyte heterogeneity is commonly observed across species. Using adeno-associated viral (AAV) vectors incorporating a rat Hes5 promotor (AAV-Hes5P), we found that AAV-Hes5P-captured astrocytes were selectively located in the superficial laminae in rats. Furthermore, activation of AAV-Hes5P+ astrocytes elicited allodynia-like behavior and increased c-FOS+ cells in the superficial laminae. Thus, laminar-selective Hes5+ astrocytes are conserved beyond species and have the capability to convert tactile information to nociceptive.

Anterior cingulate cross-hemispheric inhibition via the claustrum resolves painful sensory conflict

The anterior cingulate cortex (ACC) responds to noxious and innocuous sensory inputs, and integrates them to coordinate appropriate behavioral reactions. However, the role of the projections of ACC neurons to subcortical areas and their influence on sensory processing are not fully investigated. Here, we identified that ACC neurons projecting to the contralateral claustrum (ACC→contraCLA) preferentially respond to contralateral mechanical sensory stimulation. These sensory responses were enhanced during attending behavior. Optogenetic activation of ACC→contraCLA neurons silenced pyramidal neurons in the contralateral ACC by recruiting local circuit fast-spiking interneuron activation via an excitatory relay in the CLA. This circuit activation suppressed withdrawal behavior to mechanical stimuli ipsilateral to the ACC→contraCLA neurons. Chemogenetic silencing showed that the cross-hemispheric circuit has an important role in the suppression of contralateral nociceptive behavior during sensory-driven attending behavior. Our findings identify a cross-hemispheric cortical-subcortical-cortical arc allowing the brain to give attentional priority to competing innocuous and noxious inputs.

Dopamine and noradrenaline activate spinal astrocyte endfeet via D1-like receptors

Astrocytes, the most abundant glial cells in the central nervous system, respond to a wide variety of neurotransmitters binding to metabotropic receptors. Here, we investigated the intracellular calcium responses of spinal cord astrocytes to dopamine and noradrenaline, two catecholamines released by specific descending pathways. In a slice preparation from the spinal cord of neonatal mice, puff application of dopamine resulted in intracellular calcium responses that remained in the endfeet. Noradrenaline induced stronger responses that also started in the endfeet but spread to neighbouring compartments. The intracellular calcium responses were unaffected by blocking neuronal activity or inhibiting various neurotransmitter receptors, suggesting a direct effect of dopamine and noradrenaline on astrocytes. The intracellular calcium responses induced by noradrenaline and dopamine were inhibited by the D1 receptor antagonist SCH 23390. We assessed the functional consequences of these astrocytic responses by examining changes in arteriole diameter. Puff application of dopamine or noradrenaline resulted in vasoconstriction of spinal arterioles. However, blocking D1 receptors or manipulating astrocytic intracellular calcium levels did not abolish the vasoconstrictions, indicating that the observed intracellular calcium responses in astrocyte endfeet were not responsible for the vascular changes. Our findings demonstrate a compartmentalized response of spinal cord astrocytes to catecholamines and expand our understanding of astrocyte-neurotransmitter interactions and their potential roles in the physiology of the central nervous system.

Neuron-astrocyte metabolic coupling facilitates spinal plasticity and maintenance of inflammatory pain

Long-lasting pain stimuli can trigger maladaptive changes in the spinal cord, reminiscent of plasticity associated with memory formation. Metabolic coupling between astrocytes and neurons has been implicated in neuronal plasticity and memory formation in the central nervous system, but neither its involvement in pathological pain nor in spinal plasticity has been tested. Here we report a form of neuroglia signalling involving spinal astrocytic glycogen dynamics triggered by persistent noxious stimulation via upregulation of the Protein Targeting to Glycogen (PTG) in spinal astrocytes. PTG drove glycogen build-up in astrocytes, and blunting glycogen accumulation and turnover by Ptg gene deletion reduced pain-related behaviours and promoted faster recovery by shortening pain maintenance in mice. Furthermore, mechanistic analyses revealed that glycogen dynamics is a critically required process for maintenance of pain by facilitating neuronal plasticity in spinal lamina 1 neurons. In summary, our study describes a previously unappreciated mechanism of astrocyte-neuron metabolic communication through glycogen breakdown in the spinal cord that fuels spinal neuron hyperexcitability.

Understanding of Spinal Wide Dynamic Range Neurons and Their Modulation on Pathological Pain

The spinal dorsal horn (SDH) transmits sensory information from the periphery to the brain. Wide dynamic range (WDR) neurons within this relay site play a critical role in modulating and integrating peripheral sensory inputs, as well as the process of central sensitization during pathological pain. This group of spinal multi-receptive neurons has attracted considerable attention in pain research due to their capabilities for encoding the location and intensity of nociception. Meanwhile, transmission, processing, and modulation of incoming afferent information in WDR neurons also establish the underlying basis for investigating the integration of acupuncture and pain signals. This review aims to provide a comprehensive examination of the distinctive features of WDR neurons and their involvement in pain. Specifically, we will examine the regulation of diverse supraspinal nuclei on these neurons and analyze their potential in elucidating the mechanisms of acupuncture analgesia.

Astrocytes in Pain Perception: A Systems Neuroscience Approach

Astrocytes play an active role in the function of the brain integrating neuronal activity and regulating back neuronal dynamic. They have recently emerged as active contributors of brain's emergent properties such as perceptions. Here, we analyzed the role of astrocytes in pain perception from the lens of systems neuroscience, and we do this by analyzing how astrocytes encode nociceptive information within brain processing areas and how they are key regulators of the internal state that determines pain perception. Specifically, we discuss the dynamic interactions between astrocytes and neuromodulators, such as noradrenaline, highlighting their role in shaping the level of activation of the neuronal ensemble, thereby influencing the experience of pain. Also, we will discuss the possible implications of an "Astro-NeuroMatrix" in the integration of pain across sensory, affective, and cognitive dimensions of pain perception.

Optogenetic inhibition of spinal inhibitory neurons facilitates mechanical responses of spinal wide dynamic range neurons and causes mechanical hypersensitivity

Inhibitory interneurons in the spinal dorsal horn (DH) play a major role in regulating innocuous and noxious information. Reduction in inhibitory synaptic transmission is thought to contribute to the development of touch-evoked pain (allodynia), a common symptom of neuropathic pain. However, it is not fully understood how inhibitory neurons in the DH regulate sensory responses in surrounding neurons and modulate sensory transmission. In this study, we established a novel experimental method to analyze temporal activity of DH neurons during the optogenetically induced disinhibition state by combining extracellular recording and optogenetics. We investigated how specific and temporally restricted dysfunction of DH inhibitory neurons affected spinal neuronal activities evoked by cutaneous mechanical stimulation. In behavioral experiments, the specific and temporally restricted spinal optogenetic suppression of DH inhibitory neurons induced mechanical hypersensitivity. Furthermore, this manipulation enhanced the mechanical responses of wide dynamic range (WDR) neurons, which are important for pain transmission, in response to brush and von Frey stimulation but not in response to nociceptive pinch stimulation. In addition, we examined whether a neuropathic pain medication, mirogabalin, suppressed these optogenetically induced abnormal pain responses. We found that mirogabalin treatment attenuated the abnormal firing responses of WDR neurons and mechanical hypersensitivity. These results suggest that temporally restricted and specific reduction of spinal inhibitory neuronal activity facilitates the mechanical responses of WDR neurons, resulting in neuropathic-like mechanical allodynia which can be suppressed by mirogabalin. Our optogenetic methods could be useful for developing novel therapeutics for neuropathic pain.

Compromised trigemino-coerulean coupling in migraine sensitization can be prevented by blocking beta-receptors in the locus coeruleus

BACKGROUND

Migraine is a disabling neurological disorder, characterized by recurrent headaches. During migraine attacks, individuals often experience sensory symptoms such as cutaneous allodynia which indicates the presence of central sensitization. This sensitization is prevented by oral administration of propranolol, a common first-line medication for migraine prophylaxis, that also normalized the activation of the locus coeruleus (LC), considered as the main origin of descending noradrenergic pain controls. We hypothesized that the basal modulation of trigeminal sensory processing by the locus coeruleus is shifted towards more facilitation in migraineurs and that prophylactic action of propranolol may be attributed to a direct action in LC through beta-adrenergic receptors.

METHODS

We used simultaneous in vivo extracellular recordings from the trigeminocervical complex (TCC) and LC of male Sprague-Dawley rats to characterize the relationship between these two areas following repeated meningeal inflammatory soup infusions. Von Frey Hairs and air-puff were used to test periorbital mechanical allodynia. RNAscope and patch-clamp recordings allowed us to examine the action mechanism of propranolol.

RESULTS

We found a strong synchronization between TCC and LC spontaneous activities, with a precession of the LC, suggesting the LC drives TCC excitability. Following repeated dural-evoked trigeminal activations, we observed a disruption in coupling of activity within LC and TCC. This suggested an involvement of the two regions' interactions in the development of sensitization. Furthermore, we showed the co-expression of alpha-2A and beta-2 adrenergic receptors within LC neurons. Finally propranolol microinjections into the LC prevented trigeminal sensitization by desynchronizing and decreasing LC neuronal activity.

CONCLUSIONS

Altogether these results suggest that trigemino-coerulean coupling plays a pivotal role in migraine progression, and that propranolol's prophylactic effects involve, to some extent, the modulation of LC activity through beta-2 adrenergic receptors. This insight reveals new mechanistic aspects of LC control over sensory processing.

Trans-segmental imaging in the spinal cord of behaving mice

Spinal cord circuits play crucial roles in transmitting pain, but the underlying activity patterns within and across spinal segments in behaving mice have remained elusive. We developed a wearable widefield macroscope with a 7.9-mm2 field of view, ~3- to 4-μm lateral resolution, 2.7-mm working distance and <10-g overall weight and show that highly localized painful mechanical stimuli evoke widespread, coordinated astrocyte excitation across multiple spinal segments.

The dual role of microglia in neuropathic pain after spinal cord injury: Detrimental and protective effects

Spinal cord injury (SCI) is a debilitating condition that is frequently accompanied by neuropathic pain, resulting in significant physical and psychological harm to a vast number of individuals globally. Despite the high prevalence of neuropathic pain following SCI, the precise underlying mechanism remains incompletely understood. Microglia are a type of innate immune cell that are present in the central nervous system (CNS). They have been observed to have a significant impact on neuropathic pain following SCI. This article presents a comprehensive overview of recent advances in understanding the role of microglia in the development of neuropathic pain following SCI. Specifically, the article delves into the detrimental and protective effects of microglia on neuropathic pain following SCI, as well as the mechanisms underlying their interconversion. Furthermore, the article provides a thorough overview of potential avenues for future research in this area.

A conceptual framework for astrocyte function

The participation of astrocytes in brain computation was hypothesized in 1992, coinciding with the discovery that these cells display a form of intracellular Ca2+ signaling sensitive to neuroactive molecules. This finding fostered conceptual leaps crystalized around the idea that astrocytes, once thought to be passive, participate actively in brain signaling and outputs. A multitude of disparate roles of astrocytes has since emerged, but their meaningful integration has been muddied by the lack of consensus and models of how we conceive the functional position of these cells in brain circuitry. In this Perspective, we propose an intuitive, data-driven and transferable conceptual framework we coin 'contextual guidance'. It describes astrocytes as 'contextual gates' that shape neural circuitry in an adaptive, state-dependent fashion. This paradigm provides fresh perspectives on principles of astrocyte signaling and its relevance to brain function, which could spur new experimental avenues, including in computational space.

Cell type-specific dissection of sensory pathways involved in descending modulation

Decades of research have suggested that stimulation of supraspinal structures, such as the periaqueductal gray (PAG) and rostral ventromedial medulla (RVM), inhibits nocifensive responses to noxious stimulation through a process known as descending modulation. Electrical stimulation and pharmacologic manipulations of the PAG and RVM identified transmitters and neuronal firing patterns that represented distinct cell types. Advances in mouse genetics, in vivo imaging, and circuit tracing methods, in addition to chemogenetic and optogenetic approaches, allowed the characterization of the cells and circuits involved in descending modulation in further detail. Recent work has revealed the importance of PAG and RVM neuronal cell types in the descending modulation of pruriceptive as well as nociceptive behaviors, underscoring their roles in coordinating complex behavioral responses to sensory input. This review summarizes how new technical advances that enable cell type-specific manipulation and recording of neuronal activity have supported, as well as expanded, long-standing views on descending modulation.

Sensory ataxia and cardiac hypertrophy caused by neurovascular oxidative stress in chemogenetic transgenic mouse lines

Oxidative stress is associated with cardiovascular and neurodegenerative diseases. Here we report studies of neurovascular oxidative stress in chemogenetic transgenic mouse lines expressing yeast D-amino acid oxidase (DAAO) in neurons and vascular endothelium. When these transgenic mice are fed D-amino acids, DAAO generates hydrogen peroxide in target tissues. DAAO-TGCdh5 transgenic mice express DAAO under control of the putatively endothelial-specific Cdh5 promoter. When we provide these mice with D-alanine, they rapidly develop sensory ataxia caused by oxidative stress and mitochondrial dysfunction in neurons within dorsal root ganglia and nodose ganglia innervating the heart. DAAO-TGCdh5 mice also develop cardiac hypertrophy after chronic chemogenetic oxidative stress. This combination of ataxia, mitochondrial dysfunction, and cardiac hypertrophy is similar to findings in patients with Friedreich's ataxia. Our observations indicate that neurovascular oxidative stress is sufficient to cause sensory ataxia and cardiac hypertrophy. Studies of DAAO-TGCdh5 mice could provide mechanistic insights into Friedreich's ataxia.

One immune system plays many parts: The dynamic role of the immune system in chronic pain and opioid pharmacology

The transition from acute to chronic pain is an ongoing major problem for individuals, society and healthcare systems around the world. It is clear chronic pain is a complex multidimensional biological challenge plagued with difficulties in pain management, specifically opioid use. In recent years the role of the immune system in chronic pain and opioid pharmacology has come to the forefront. As a highly dynamic and versatile network of cells, tissues and organs, the immune system is perfectly positioned at the microscale level to alter nociception and drive structural adaptations that underpin chronic pain and opioid use. In this review, we highlight the need to understand the dynamic and adaptable characteristics of the immune system and their role in the transition, maintenance and resolution of chronic pain. The complex multidimensional interplay of the immune system with multiple physiological systems may provide new transformative insight for novel targets for clinical management and treatment of chronic pain. This article is part of the Special Issue on "Opioid-induced changes in addiction and pain circuits".

Identification of brain-to-spinal circuits controlling the laterality and duration of mechanical allodynia in mice

Mechanical allodynia (MA) represents one prevalent symptom of chronic pain. Previously we and others have identified spinal and brain circuits that transmit or modulate the initial establishment of MA. However, brain-derived descending pathways that control the laterality and duration of MA are still poorly understood. Here we report that the contralateral brain-to-spinal circuits, from Oprm1 neurons in the lateral parabrachial nucleus (lPBNOprm1), via Pdyn neurons in the dorsal medial regions of hypothalamus (dmHPdyn), to the spinal dorsal horn (SDH), act to prevent nerve injury from inducing contralateral MA and reduce the duration of bilateral MA induced by capsaicin. Ablating/silencing dmH-projecting lPBNOprm1 neurons or SDH-projecting dmHPdyn neurons, deleting Dyn peptide from dmH, or blocking spinal κ-opioid receptors all led to long-lasting bilateral MA. Conversely, activation of dmHPdyn neurons or their axonal terminals in SDH can suppress sustained bilateral MA induced by lPBN lesion.

The Similar and Distinct Roles of Satellite Glial Cells and Spinal Astrocytes in Neuropathic Pain

Preclinical studies have identified glial cells as pivotal players in the genesis and maintenance of neuropathic pain after nerve injury associated with diabetes, chemotherapy, major surgeries, and virus infections. Satellite glial cells (SGCs) in the dorsal root and trigeminal ganglia of the peripheral nervous system (PNS) and astrocytes in the central nervous system (CNS) express similar molecular markers and are protective under physiological conditions. They also serve similar functions in the genesis and maintenance of neuropathic pain, downregulating some of their homeostatic functions and driving pro-inflammatory neuro-glial interactions in the PNS and CNS, i.e., "gliopathy". However, the role of SGCs in neuropathic pain is not simply as "peripheral astrocytes". We delineate how these peripheral and central glia participate in neuropathic pain by producing different mediators, engaging different parts of neurons, and becoming active at different stages following nerve injury. Finally, we highlight the recent findings that SGCs are enriched with proteins related to fatty acid metabolism and signaling such as Apo-E, FABP7, and LPAR1. Targeting SGCs and astrocytes may lead to novel therapeutics for the treatment of neuropathic pain.

Multiplex translaminar imaging in the spinal cord of behaving mice

While the spinal cord is known to play critical roles in sensorimotor processing, including pain-related signaling, corresponding activity patterns in genetically defined cell types across spinal laminae have remained challenging to investigate. Calcium imaging has enabled cellular activity measurements in behaving rodents but is currently limited to superficial regions. Here, using chronically implanted microprisms, we imaged sensory and motor-evoked activity in regions and at speeds inaccessible by other high-resolution imaging techniques. To enable translaminar imaging in freely behaving animals through implanted microprisms, we additionally developed wearable microscopes with custom-compound microlenses. This system addresses multiple challenges of previous wearable microscopes, including their limited working distance, resolution, contrast, and achromatic range. Using this system, we show that dorsal horn astrocytes in behaving mice show sensorimotor program-dependent and lamina-specific calcium excitation. Additionally, we show that tachykinin precursor 1 (Tac1)-expressing neurons exhibit translaminar activity to acute mechanical pain but not locomotion.

GPR55 contributes to neutrophil recruitment and mechanical pain induction after spinal cord compression in mice

Pain transmission and processing in the nervous system are modulated by various biologically active substances, including lysophospholipids, through direct and indirect actions on the somatosensory pathway. Lysophosphatidylglucoside (LysoPtdGlc) was recently identified as a structurally unique lysophospholipid that exerts biological actions via the G protein-coupled receptor GPR55. Here, we demonstrated that GPR55-knockout (KO) mice show impaired induction of mechanical pain hypersensitivity in a model of spinal cord compression (SCC) without the same change in the models of peripheral tissue inflammation and peripheral nerve injury. Among these models, only SCC recruited peripheral inflammatory cells (neutrophils, monocytes/macrophages, and CD3⁺ T-cells) in the spinal dorsal horn (SDH), and GPR55-KO blunted these recruitments. Neutrophils were the first cells recruited to the SDH, and their depletion suppressed the induction of SCC-induced mechanical hypersensitivity and inflammatory responses in compressed SDH. Furthermore, we found that PtdGlc was present in the SDH and that intrathecal administration of an inhibitor of secretory phospholipase A₂ (an enzyme required for producing LysoPtdGlc from PtdGlc) reduced neutrophil recruitment to compressed SDH and suppressed pain induction. Finally, by screening compounds from a chemical library, we identified auranofin as a clinically used drug with an inhibitory effect on mouse and human GPR55. Systemically administered auranofin to mice with SCC effectively suppressed spinal neutrophil infiltration and pain hypersensitivity. These results suggest that GPR55 signaling contributes to the induction of inflammatory responses and chronic pain after SCC via the recruitment of neutrophils and may provide a new target for reducing pain induction after spinal cord compression, such as spinal canal stenosis.

The role of astrocytes in behaviors related to emotion and motivation

Astrocytes are present throughout the brain and intimately interact with neurons and blood vessels. Three decades of research have shown that astrocytes reciprocally communicate with neurons and other non-neuronal cells in the brain and dynamically regulate cell function. Astrocytes express numerous receptors for neurotransmitters, neuromodulators, and cytokines and receive information from neurons, other astrocytes, and other non-neuronal cells. Among those receptors, the main focus has been G-protein coupled receptors. Activation of G-protein coupled receptors leads to dramatic changes in intracellular signaling (Ca²⁺ and cAMP), which is considered a form of astrocyte activity. Methodological improvements in measurement and manipulation of astrocytes have advanced our understanding of the role of astrocytes in circuits and have begun to reveal unexpected functions of astrocytes in behavior. Recent studies have suggested that astrocytic activity regulates behavior flexibility, such as coping strategies for stress exposure, and plays an important role in behaviors related to emotion and motivation. Preclinical evidence suggests that impairment of astrocytic function contributes to psychiatric diseases, especially major depression. Here, we review recent progress on the role of astrocytes in behaviors related to emotion and motivation.

IP3R1-dependent astrocyte calcium signaling in chronic itch

Astrocytes, the most abundant type of glial cell, are electrically non-excitable cells that use intracellular calcium (Ca²⁺) for functional regulation. Changes in intracellular Ca²⁺ concentration play important roles in the central nervous system (CNS), as they are involved in the release of gliotransmitters and the control of extracellular ion concentrations, thereby affecting the regulation of neuronal excitability, CNS homeostasis, and behavior. Intracellular calcium mobilization in astrocytes is known to be mediated via inositol 1,4,5-trisphosphate receptors (IP₃Rs), particularly IP₃R2, and its association with CNS pathogenesis has been widely reported. In addition, the existence of IP₃R2-independent calcium signaling has recently been postulated; however, the detailed mechanisms and its role in astrocyte functions and CNS pathogenesis are still poorly understood. In this paper, we describe the putative mechanisms underlying IP₃R1-dependent calcium signaling in astrocytes and its effects on the reactive state, compare this signaling with IP₃R2-dependent calcium signaling, and discuss its contribution to chronic itch-like behavior.

Encoding of inflammatory hyperalgesia in mouse spinal cord

ABSTRACT

Inflammation modifies the input-output properties of peripheral nociceptive neurons such that the same stimulus produces enhanced nociceptive firing. This increased nociceptive output enters the superficial dorsal spinal cord (SDH), an intricate neuronal network composed largely of excitatory and inhibitory interneurons and a small percentage of projection neurons. The SDH network comprises the first central nervous system network integrating noxious information. Using in vivo calcium imaging and a computational approach, we characterized the responsiveness of the SDH network in mice to noxious stimuli in normal conditions and investigated the changes in SDH response patterns after acute burn injury-induced inflammation. We show that the application of noxious heat stimuli to the hind paw of naïve mice results in an overall increase in SDH network activity. Single-cell response analysis reveals that 70% of recorded neurons increase or suppress their activity, while ∼30% of neurons remain nonresponsive. After acute burn injury and the development of inflammatory hyperalgesia, application of the same noxious heat stimuli leads to the activation of previously nonresponding neurons and desuppression of suppressed neurons. We further demonstrate that an increase in afferent activity mimics the response of the SDH network to noxious heat stimuli under inflammatory conditions. Using a computational model of the SDH network, we predict that the changes in SDH network activity result in overall increased activity of excitatory neurons, amplifying the output from SDH to higher brain centers. We suggest that during acute local peripheral inflammation, the SDH network undergoes dynamic changes promoting hyperalgesia.

Voltage-gated calcium channel subunit α2δ-1 in spinal dorsal horn neurons contributes to aberrant excitatory synaptic transmission and mechanical hypersensitivity after peripheral nerve injury

Neuropathic pain, an intractable pain symptom that occurs after nerve damage, is caused by the aberrant excitability of spinal dorsal horn (SDH) neurons. Gabapentinoids, the most commonly used drugs for neuropathic pain, inhibit spinal calcium-mediated neurotransmitter release by binding to α₂δ-1, a subunit of voltage-gated calcium channels, and alleviate neuropathic pain. However, the exact contribution of α₂δ-1 expressed in SDH neurons to the altered synaptic transmission and mechanical hypersensitivity following nerve injury is not fully understood. In this study, we investigated which types of SDH neurons express α₂δ-1 and how α₂δ-1 in SDH neurons contributes to the mechanical hypersensitivity and altered spinal synaptic transmission after nerve injury. Using in situ hybridization technique, we found that Cacna2d1, mRNA coding α₂δ-1, was mainly colocalized with Slc17a6, an excitatory neuronal marker, but not with Slc32a1, an inhibitory neuronal marker in the SDH. To investigate the role of α₂δ-1 in SDH neurons, we used clustered regularly interspaced short palindromic repeats (CRISPR)-Cas9 system and showed that SDH neuron-specific ablation of Cacna2d1 alleviated mechanical hypersensitivity following nerve injury. We further found that excitatory post-synaptic responses evoked by electrical stimulation applied to the SDH were significantly enhanced after nerve injury, and that these enhanced responses were significantly decreased by application of mirogabalin, a potent α₂δ-1 inhibitor, and by SDH neuron-specific ablation of Cacna2d1. These results suggest that α₂δ-1 expressed in SDH excitatory neurons facilitates spinal nociceptive synaptic transmission and contributes to the development of mechanical hypersensitivity after nerve injury.

Identification of Spinal Inhibitory Interneurons Required for Attenuating Effect of Duloxetine on Neuropathic Allodynia-like Signs in Rats

Neuropathic pain is a chronic pain condition that occurs after nerve damage; allodynia, which refers to pain caused by generally innocuous stimuli, is a hallmark symptom. Although allodynia is often resistant to analgesics, the antidepressant duloxetine has been used as an effective therapeutic option. Duloxetine increases spinal noradrenaline (NA) levels by inhibiting its transporter at NAergic terminals in the spinal dorsal horn (SDH), which has been proposed to contribute to its pain-relieving effect. However, the mechanism through which duloxetine suppresses neuropathic allodynia remains unclear. Here, we identified an SDH inhibitory interneuron subset (captured by adeno-associated viral (AAV) vectors incorporating a rat neuropeptide Y promoter; AAV-NpyP⁺ neurons) that is mostly depolarized by NA. Furthermore, this excitatory effect was suppressed by pharmacological blockade or genetic knockdown of α_1B-adrenoceptors (ARs) in AAV-NpyP⁺ SDH neurons. We found that duloxetine suppressed Aβ fiber-mediated allodynia-like behavioral responses after nerve injury and that this effect was not observed in AAV-NpyP⁺ SDH neuron-selective α_1B-AR-knockdown. These results indicate that α_1B-AR and AAV-NpyP⁺ neurons are critical targets for spinal NA and are necessary for the therapeutic effect of duloxetine on neuropathic pain, which can support the development of novel analgesics.

Hevin/Sparcl1 drives pathological pain through spinal cord astrocyte and NMDA receptor signaling

High endothelial venule protein/SPARC-like 1 (hevin/Sparcl1) is an astrocyte-secreted protein that regulates synapse formation in the brain. Here we show that astrocytic hevin signaling plays a critical role in maintaining chronic pain. Compared with WT mice, hevin-null mice exhibited normal mechanical and heat sensitivity but reduced inflammatory pain. Interestingly, hevin-null mice have faster recovery than WT mice from neuropathic pain after nerve injury. Intrathecal injection of WT hevin was sufficient to induce persistent mechanical allodynia in naive mice. In hevin-null mice with nerve injury, adeno-associated-virus-mediated (AAV-mediated) re-expression of hevin in glial fibrillary acidic protein-expressing (GFAP-expressing) spinal cord astrocytes could reinstate neuropathic pain. Mechanistically, hevin is crucial for spinal cord NMDA receptor (NMDAR) signaling. Hevin-potentiated N-Methyl-D-aspartic acid (NMDA) currents are mediated by GluN2B-containing NMDARs. Furthermore, intrathecal injection of a neutralizing Ab against hevin alleviated acute and persistent inflammatory pain, postoperative pain, and neuropathic pain. Secreted hevin that was detected in mouse cerebrospinal fluid (CSF) and nerve injury significantly increased CSF hevin abundance. Finally, neurosurgery caused rapid and substantial increases in SPARCL1/HEVIN levels in human CSF. Collectively, our findings support a critical role of hevin and astrocytes in the maintenance of chronic pain. Neutralizing of secreted hevin with monoclonal Ab may provide a new therapeutic strategy for treating acute and chronic pain and NMDAR-medicated neurodegeneration.

Astrocytes in Chronic Pain: Cellular and Molecular Mechanisms

Chronic pain is challenging to treat due to the limited therapeutic options and adverse side-effects of therapies. Astrocytes are the most abundant glial cells in the central nervous system and play important roles in different pathological conditions, including chronic pain. Astrocytes regulate nociceptive synaptic transmission and network function via neuron–glia and glia–glia interactions to exaggerate pain signals under chronic pain conditions. It is also becoming clear that astrocytes play active roles in brain regions important for the emotional and memory-related aspects of chronic pain. Therefore, this review presents our current understanding of the roles of astrocytes in chronic pain, how they regulate nociceptive responses, and their cellular and molecular mechanisms of action.

Spinal cord astrocytes regulate myocardial ischemia-reperfusion injury

Astrocytes play a key role in the response to injury and noxious stimuli, but its role in myocardial ischemia-reperfusion (I/R) injury remains largely unknown. Here we determined whether manipulation of spinal astrocyte activity affected myocardial I/R injury and the underlying mechanisms. By ligating the left coronary artery to establish an in vivo I/R rat model, we observed a 1.7-fold rise in glial fibrillary acidic protein (GFAP) protein level in spinal cord following myocardial I/R injury. Inhibition of spinal astrocytes by intrathecal injection of fluoro-citrate, an astrocyte inhibitor, decreased GFAP immunostaining and reduced infarct size by 29% relative to the I/R group. Using a Designer Receptor Exclusively Activated by Designer Drugs (DREADD) chemogenetic approach, we bi-directionally manipulated astrocyte activity employing GFAP promoter-driven Gq- or Gi-coupled signaling. The Gq-DREADD-mediated activation of spinal astrocytes caused transient receptor potential vanilloid 1 (TRPV1) activation and neuropeptide release leading to a 1.3-fold increase in infarct size, 1.2-fold rise in serum norepinephrine level and higher arrhythmia score relative to I/R group. In contrast, Gi-DREADD-mediated inhibition of spinal astrocytes suppressed TRPV1-mediated nociceptive signaling, resulting in 35% reduction of infarct size and 51% reduction of arrhythmia score from I/R group, as well as lowering serum norepinephrine level from 3158 ± 108 to 2047 ± 95 pg/mL. Further, intrathecal administration of TRPV1 or neuropeptide antagonists reduced infarct size and serum norepinephrine level. These findings demonstrate a functional role of spinal astrocytes in myocardial I/R injury and provide a novel potential therapeutic approach targeting spinal cord astrocytes for the prevention of cardiac injury.

Ventrolateral Periaqueductal Gray Astrocytes Regulate Nociceptive Sensation and Emotional Motivation in Diabetic Neuropathic Pain

Diabetic neuropathic pain (DNP) is a diabetes complication experienced by many patients. Ventrolateral periaqueductal gray (vlPAG) neurons are essential mediators of the descending pain modulation system, yet the role of vlPAG astrocytes in DNP remains unclear. The present study applied a multidimensional approach to elucidate the role of these astrocytes in DNP. We verified the activation of astrocytes in different regions of the PAG in male DNP-model rats. We found that only astrocytes in the vlPAG exhibited increased growth. Furthermore, we described differences in vlPAG astrocyte activity at different time points during DNP progression. After the 14th day of modeling, vlPAG astrocytes exhibited obvious activation and morphologic changes. Furthermore, activation of Gq-designer receptors exclusively activated by a designer drug (Gq-DREADDs) in vlPAG astrocytes in naive male rats induced neuropathic pain-like symptoms and pain-related aversion, whereas activation of Gi-DREADDs in vlPAG astrocytes in male DNP-model rats alleviated sensations of pain and promoted pain-related preference behavior. Thus, bidirectional manipulation of vlPAG astrocytes revealed their potential to regulate pain. Surprisingly, activation of Gi-DREADDs in vlPAG astrocytes also mitigated anxiety-like behavior induced by DNP. Thus, our results provide direct support for the hypothesis that vlPAG astrocytes regulate diabetes-associated neuropathic pain and concomitant anxiety-like behavior.SIGNIFICANCE STATEMENT Many studies examined the association between the ventrolateral periaqueductal gray (vlPAG) and neuropathic pain. However, few studies have focused on the role of vlPAG astrocytes in diabetic neuropathic pain (DNP) and DNP-related emotional changes. This work confirmed the role of vlPAG astrocytes in DNP by applying a more direct and robust approach. We used chemogenetics to bidirectionally manipulate the activity of vlPAG astrocytes and revealed that vlPAG astrocytes regulate DNP and pain-related behavior. In addition, we discovered that activation of Gi-designer receptors exclusively activated by a designer drug in vlPAG astrocytes alleviated anxiety-like behavior induced by DNP. Together, these findings provide new insights into DNP and concomitant anxiety-like behavior and supply new therapeutic targets for treating DNP.

The Role of Neuro-Immune Interactions in Chronic Pain: Implications for Clinical Practice

Chronic pain remains a public health problem and contributes to the ongoing opioid epidemic. Current pain management therapies still leave many patients with poorly controlled pain, thus new or improved treatments are desperately needed. One major challenge in pain research is the translation of preclinical findings into effective clinical practice. The local neuroimmune interface plays an important role in the initiation and maintenance of chronic pain and is therefore a promising target for novel therapeutic development. Neurons interface with immune and immunocompetent cells in many distinct microenvironments along the nociceptive circuitry. The local neuroimmune interface can modulate the activity and property of the neurons to affect peripheral and central sensitization. In this review, we highlight a specific subset of many neuroimmune interfaces. In the central nervous system, we examine the interface between neurons and microglia, astrocytes, and T lymphocytes. In the periphery, we profile the interface between neurons in the dorsal root ganglion with T lymphocytes, satellite glial cells, and macrophages. To bridge the gap between preclinical research and clinical practice, we review the preclinical studies of each neuroimmune interface, discuss current clinical treatments in pain medicine that may exert its action at the neuroimmune interface, and highlight opportunities for future clinical research efforts.

Microglia Regulate Blood-Brain Barrier Integrity via MiR-126a-5p/MMP9 Axis during Inflammatory Demyelination

Blood-brain barrier (BBB) impairment is an early prevalent feature of multiple sclerosis (MS), and remains vital for MS progression. Microglial activation precedes BBB disruption and cellular infiltrates in the brain of MS patients. However, little is known about the function of microglia in BBB impairment. Here, microglia acts as an important modulator of BBB integrity in inflammatory demyelination. Microglial depletion profoundly ameliorates BBB impairment in experimental autoimmune encephalomyelitis (EAE). Specifically, miR-126a-5p in microglia is positively correlated with BBB integrity in four types of MS plaques. Mechanistically, microglial deletion of miR-126a-5p exacerbates BBB leakage and EAE severity. The protective effect of miR-126a-5p is mimicked and restored by specific inhibition of MMP9 in microglia. Importantly, Auranofin, an FDA-approved drug, is identified to protect BBB integrity and mitigate EAE progression via a microglial miR-126a-5p dependent mechanism. Taken together, microglia can be manipulated to protect BBB integrity and ameliorate inflammatory demyelination. Targeting microglia to regulate BBB permeability merits consideration in therapeutic interventions in MS.

A perspective on astrocyte regulation of neural circuit function and animal behavior

Studies over the past two decades have demonstrated that astrocytes are tightly associated with neurons and play pivotal roles in neural circuit development, operation, and adaptation in health and disease. Nevertheless, precisely how astrocytes integrate diverse neuronal signals, modulate neural circuit structure and function at multiple temporal and spatial scales, and influence animal behavior or disease through aberrant excitation and molecular output remains unclear. This Perspective discusses how new and state-of-the-art approaches, including fluorescence indicators, opto- and chemogenetic actuators, genetic targeting tools, quantitative behavioral assays, and computational methods, might help resolve these longstanding questions. It also addresses complicating factors in interpreting astrocytes' role in neural circuit regulation and animal behavior, such as their heterogeneity, metabolism, and inter-glial communication. Research on these questions should provide a deeper mechanistic understanding of astrocyte-neuron assemblies' role in neural circuit function, complex behaviors, and disease.

Switch of serotonergic descending inhibition into facilitation by a spinal chloride imbalance in neuropathic pain

Descending control from the brain to the spinal cord shapes our pain experience, ranging from powerful analgesia to extreme sensitivity. Increasing evidence from both preclinical and clinical studies points to an imbalance toward descending facilitation as a substrate of pathological pain, but the underlying mechanisms remain unknown. We used an optogenetic approach to manipulate serotonin (5-HT) neurons of the nucleus raphe magnus that project to the dorsal horn of the spinal cord. We found that 5-HT neurons exert an analgesic action in naïve mice that becomes proalgesic in an experimental model of neuropathic pain. We show that spinal KCC2 hypofunction turns this descending inhibitory control into paradoxical facilitation; KCC2 enhancers restored 5-HT-mediated descending inhibition and analgesia. Last, combining selective serotonin reuptake inhibitors (SSRIs) with a KCC2 enhancer yields effective analgesia against nerve injury-induced pain hypersensitivity. This uncovers a previously unidentified therapeutic path for SSRIs against neuropathic pain.

Distinct brainstem to spinal cord noradrenergic pathways inversely regulate spinal neuronal activity

Brainstem to spinal cord noradrenergic pathways include a locus coeruleus origin projection and diffuse noxious inhibitory controls. While both pathways are traditionally viewed as exerting an inhibitory effect on spinal neuronal activity, the locus coeruleus was previously shown to have a facilitatory influence on thermal nocioception according to the subpopulation of coerulean neurons activated. Coupled with knowledge of its functional modular organisation and the fact that diffuse noxious inhibitory controls are not expressed in varied animal models of chronicity, we hypothesized a regulatory role for the locus coeruleus on non-coerulean, discrete noradrenergic cell group(s). We implemented locus coeruleus targeting strategies by microinjecting canine adenovirus encoding for channelrhodopsin-2 under a noradrenaline-specific promoter in the spinal cord (retrogradely labelling a coeruleospinal module) or the locus coeruleus itself (labelling the entire coerulean module). Coeruleospinal module optoactivation abolished diffuse noxious inhibitory controls (two-way ANOVA, P < 0.0001), which were still expressed following locus coeruleus neuronal ablation. We propose that the cerulean system interacts with, but does not directly govern, diffuse noxious inhibitory controls. This mechanism may underlie the role of the locus coeruleus as a 'chronic pain generator'. Pinpointing the functionality of discrete top-down pathways is crucial for understanding sensorimotor modulation in health and disease.

Profiling of fatty acid metabolism in the dorsal root ganglion after peripheral nerve injury

Peripheral nerve injury (PNI) induces neuronal hyperexcitability, which underlies neuropathic pain. The emergence of RNA sequencing technologies has enabled profiling of transcriptional changes in pathological conditions. However, these approaches do not provide information regarding metabolites such as lipids that are not directly encoded by genes. Fatty acids (FAs) are some of the essential lipids in mammalian organisms and are mainly stored as membrane phospholipids. In response to various biological stimuli, FAs are rapidly released and converted into several mediators, such as eicosanoids and docosanoids. FAs themselves or their metabolites play important roles in physiology and pathology. In this study, using a comprehensive lipidomic analysis of FA metabolites, 152 species were measured in the dorsal root ganglia of mice at multiple time points after PNI. We found that PNI increased the ω-6 FA metabolites produced by cyclooxygenases but not those produced by lipoxygenases or cytochrome P450 enzymes in the dorsal root ganglia. In contrast, ω-3 FA metabolites biosynthesized by any enzyme transiently increased after nerve injury. Overall, these findings provide a new resource and valuable insights into PNI pathologies, including pain and nerve regeneration.

Reactive astrocytes in pain neural circuit pathogenesis

Reactive astrocytes are commonly activated in the spinal dorsal horn (SDH) of various animal models of pathological pain. Previous investigations suggest an association between astrogliosis and pain pathogenesis. However, our understanding of the mechanisms underlying astrogliosis activation and the contributions of reactive astrocytes to pain neural circuit malfunction is rudimentary. This short review highlights recent advances in these areas.

Activation of locus coeruleus-spinal cord noradrenergic neurons alleviates neuropathic pain in mice via reducing neuroinflammation from astrocytes and microglia in spinal dorsal horn

Background

The noradrenergic neurons of locus coeruleus (LC) project to the spinal dorsal horn (SDH), and release norepinephrine (NE) to inhibit pain transmission. However, its effect on pathological pain and the cellular mechanism in the SDH remains unclear. This study aimed to explore the analgesic effects and the anti-neuroinflammation mechanism of LC-spinal cord noradrenergic pathway (LC:SC) in neuropathic pain (NP) mice with sciatic chronic constriction injury.

Methods

The Designer Receptors Exclusively Activated by Designer Drugs (DREADD) was used to selectively activate LC:SC. Noradrenergic neuron-specific retro–adeno-associated virus was injected to the spinal cord. Pain threshold, LC and wide dynamic range (WDR) neuron firing, neuroinflammation (microglia and astrocyte activation, cytokine expression), and α2AR expression in SDH were evaluated.

Results

Activation of LC:SC with DREADD increased the mechanical and thermal nociceptive thresholds and reduced the WDR neuron firing. LC:SC activation (daily, 7 days) downregulated TNF-α and IL-1β expression, upregulated IL-4 and IL-10 expression in SDH, and inhibited microglia and astrocytes activation in NP mice. Immunofluorescence double staining confirmed that LC:SC activation decreased the expression of cytokines in microglia of the SDH. In addition, the effects of LC:SC activation could be reversed by intrathecal injection of yohimbine. Immunofluorescence of SDH showed that NE receptor α2B-AR was highly expressed in microglia in CCI mice.

Conclusion

These findings indicate that selective activation of LC:SC alleviates NP in mice by increasing the release of NE and reducing neuroinflammation of astrocytes and microglia in SDH.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02489-9.

Sculpting Astrocyte Diversity through Circuits and Transcription

Astrocytes are the most abundant glial cell in the central nervous system and occupy a wide range of roles that are essential for brain function. Over the past few years, evidence has emerged that astrocytes exhibit cellular and molecular heterogeneity, raising the possibility that subsets of astrocytes are functionally distinct and that transcriptional mechanisms are involved in encoding this prospective diversity. In this review, we focus on three emerging areas of astrocyte biology: region-specific circuit regulation, molecular diversity, and transcriptional regulation. This review highlights our nascent understanding of how molecular diversity is converted to functional diversity of astrocytes through the lens of brain region-specific circuits. We articulate our understanding of how transcriptional mechanisms regulate this diversity and key areas that need further exploration to achieve the overarching goal of a functional taxonomy of astrocytes in the brain.