Brainstem local microglia induce whisker map plasticity in the thalamus after peripheral nerve injury

The dorsal column nuclei scale mechanical sensitivity in naive and neuropathic pain states

During pathological conditions, tactile stimuli can aberrantly engage nociceptive pathways leading to the perception of touch as pain, known as mechanical allodynia. The brain stem dorsal column nuclei integrate tactile inputs, yet their role in mediating tactile sensitivity and allodynia remains understudied. We found that gracile nucleus (Gr) inhibitory interneurons and thalamus-projecting neurons are differentially innervated by primary afferents and spinal inputs. Functional manipulations of these distinct Gr neuronal populations bidirectionally shifted tactile sensitivity but did not affect noxious mechanical or thermal sensitivity. During neuropathic pain, Gr neurons exhibited increased sensory-evoked activity and asynchronous excitatory drive from primary afferents. Silencing Gr projection neurons or activating Gr inhibitory neurons in neuropathic mice reduced tactile hypersensitivity, and enhancing inhibition ameliorated paw-withdrawal signatures of neuropathic pain and induced conditioned place preference. These results suggest that Gr activity contributes to tactile sensitivity and affective, pain-associated phenotypes of mechanical allodynia.

Behavioral adaptations after unilateral whisker denervation

The rodent whisker system provides an excellent model to study experience dependent plasticity in neural morphology, circuitry, and behavior. Rodents use bilateral whisker sensation to gather information about their environment. Unilateral whisker denervation disrupts whisker circuitry but its impact on task specific behavior is largely unknown. Adult mice with unilateral whisker denervation display a preference to using the intact whisker set to inspect objects, but do not have altered open field navigation. An object localization task requiring only the intact whisker set did not detect any change in performance, but gap crossing was impaired after unilateral whisker denervation. Finally, chronic whisker denervation led to increased anxiety-like behavior which was rescued by training on the gap cross task. These findings indicate that mice use behavioral strategies to adapt to life with only one set of intact whiskers.

Peripheral and central innervation pattern of mechanosensory neurons in the trigeminal ganglion

The trigeminal ganglion (TG) comprises primary sensory neurons responsible for orofacial sensations, subsequently projecting to the trigeminal nuclei in the brainstem. However, the circuit basis of nasal mechanosensation is not well characterized. Here we elucidate the anatomical organization of both peripheral and central projections of the TG. We found that the non-peptidergic nociceptor, MAS-related G protein-coupled receptor member D positive (MrgprD+) neurons in the TG densely innervate the nasal mucosa, whereas the low-threshold mechanoreceptors subtypes rarely innervate the nasal mucosa. We also identified the central projection pattern of the mechanosensory neurons in TG. The tyrosine kinase receptor C positive (TrkC+) neurons, tyrosine kinase receptor B positive (TrkB+) and tyrosine hydroxylase positive (TH+) neurons project to multiple subregions of brainstem trigeminal complex and solitary nucleus. In contrast, the MrgprD+ neurons only densely project to outer edge of Sp5C. In addition, we further determined the ascending pathway of the TG neurons. Taken together, our study demonstrates the peripheral and central projection pattern of mechanosensory neurons in the TG, which provides a basis for the future functional studies.

Intranasal Treatment with Cannabinoid 2 Receptor Agonist HU-308 Ameliorates Cold Sensitivity in Mice with Traumatic Trigeminal Neuropathic Pain

Post-traumatic trigeminal neuropathy (PTTN) is a sensory abnormality caused by injury to the trigeminal nerve during orofacial surgery. However, existing analgesics are ineffective against PTTN. Abnormal microglial activation in the caudal part of the spinal trigeminal nucleus caudal part (Sp5C), where the central trigeminal nerve terminals reside, plays an important role in PTTN pathogenesis. Therefore, regulating microglial activity in Sp5C appears to be an important approach to controlling pain in PTTN. Cannabinoid receptor 2 (CB2) is expressed in immune cells including microglia, and its activation has anti-inflammatory effects. The current study demonstrates that the repeated intranasal administration of CB2 agonist HU-308 ameliorates the infraorbital nerve cut (IONC)-induced hyperresponsiveness to acetone (cutaneous cooling). The therapeutic efficacy of oral HU-308 was found to be less pronounced in alleviating cold hypersensitivity in IONC mice compared to intranasal administration, indicating the potential advantages of the intranasal route. Furthermore, repeated intranasal administration of HU-308 suppressed the activation of Sp5C microglia in IONC mice. Additionally, pretreatment with the CB2 antagonist, SR 144528, significantly blocked the anti-nociceptive effect of repeated intranasal administration of HU-308 on cold hypersensitization in IONC mice. These data suggest that the continuous stimulation of CB2 ameliorates PTTN-induced pain via the inhibition of microglial activation. Thus, CB2 agonists are potential candidates for novel therapeutic agents against PTTN.

Gene-environment interaction elicits dystonia-like features and impaired translational regulation in a DYT-TOR1A mouse model

DYT-TOR1A dystonia is the most common monogenic dystonia characterized by involuntary muscle contractions and lack of therapeutic options. Despite some insights into its etiology, the disease's pathophysiology remains unclear. The reduced penetrance of about 30% suggests that extragenetic factors are needed to develop a dystonic phenotype. In order to systematically investigate this hypothesis, we induced a sciatic nerve crush injury in a genetically predisposed DYT-TOR1A mouse model (DYT1KI) to evoke a dystonic phenotype. Subsequently, we employed a multi-omic approach to uncover novel pathophysiological pathways that might be responsible for this condition. Using an unbiased deep-learning-based characterization of the dystonic phenotype showed that nerve-injured DYT1KI animals exhibited significantly more dystonia-like movements (DLM) compared to naive DYT1KI animals. This finding was noticeable as early as two weeks following the surgical procedure. Furthermore, nerve-injured DYT1KI mice displayed significantly more DLM than nerve-injured wildtype (wt) animals starting at 6 weeks post injury. In the cerebellum of nerve-injured wt mice, multi-omic analysis pointed towards regulation in translation related processes. These observations were not made in the cerebellum of nerve-injured DYT1KI mice; instead, they were localized to the cortex and striatum. Our findings indicate a failed translational compensatory mechanisms in the cerebellum of phenotypic DYT1KI mice that exhibit DLM, while translation dysregulations in the cortex and striatum likely promotes the dystonic phenotype.

A microglial activation cascade across cortical regions underlies secondary mechanical hypersensitivity to amputation

Neural mechanisms underlying amputation-related secondary pain are unclear. Using in vivo two-photon imaging, three-dimensional reconstruction, and fiber photometry recording, we show that a microglial activation cascade from the primary somatosensory cortex of forelimb (S1FL) to the primary somatosensory cortex of hindlimb (S1HL) mediates the disinhibition and subsequent hyperexcitation of glutamatergic neurons in the S1HL (S1HLGlu), which then drives secondary mechanical hypersensitivity development in ipsilateral hindpaws of mice with forepaw amputation. Forepaw amputation induces rapid S1FL microglial activation that further activates S1HL microglia via the CCL2-CCR2 signaling pathway. Increased engulfment of GABAergic presynapses by activated microglia stimulates S1HLGlu neuronal activity, ultimately leading to secondary mechanical hypersensitivity of hindpaws. It is widely believed direct neuronal projection drives interactions between distinct brain regions to prime specific behaviors. Our study reveals microglial interactions spanning different subregions of the somatosensory cortex to drive a maladaptive neuronal response underlying secondary mechanical hypersensitivity at non-injured sites.

Functional and structural synaptic remodeling mechanisms underlying somatotopic organization and reorganization in the thalamus

The somatosensory system organizes the topographic representation of body maps, termed somatotopy, at all levels of an ascending hierarchy. Postnatal maturation of somatotopy establishes optimal somatosensation, whereas deafferentation in adults reorganizes somatotopy, which underlies pathological somatosensation, such as phantom pain and complex regional pain syndrome. Here, we focus on the mouse whisker somatosensory thalamus to study how sensory experience shapes the fine topography of afferent connectivity during the critical period and what mechanisms remodel it and drive a large-scale somatotopic reorganization after peripheral nerve injury. We will review our findings that, following peripheral nerve injury in adults, lemniscal afferent synapses onto thalamic neurons are remodeled back to immature configuration, as if the critical period reopens. The remodeling process is initiated with local activation of microglia in the brainstem somatosensory nucleus downstream to injured nerves and heterosynaptically controlled by input from GABAergic and cortical neurons to thalamic neurons. These fruits of thalamic studies complement well-studied cortical mechanisms of somatotopic organization and reorganization and unveil potential intervention points in treating pathological somatosensation.

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".

Peripheral nerve injury elicits microstructural and neurochemical changes in the striatum and substantia nigra of a DYT-TOR1A mouse model with dystonia-like movements

The relationship between genotype and phenotype in DYT-TOR1A dystonia as well as the associated motor circuit alterations are still insufficiently understood. DYT-TOR1A dystonia has a remarkably reduced penetrance of 20-30%, which has led to the second-hit hypothesis emphasizing an important role of extragenetic factors in the symptomatogenesis of TOR1A mutation carriers. To analyze whether recovery from a peripheral nerve injury can trigger a dystonic phenotype in asymptomatic hΔGAG3 mice, which overexpress human mutated torsinA, a sciatic nerve crush was applied. An observer-based scoring system as well as an unbiased deep-learning based characterization of the phenotype showed that recovery from a sciatic nerve crush leads to significantly more dystonia-like movements in hΔGAG3 animals compared to wildtype control animals, which persisted over the entire monitored period of 12 weeks. In the basal ganglia, the analysis of medium spiny neurons revealed a significantly reduced number of dendrites, dendrite length and number of spines in the naïve and nerve-crushed hΔGAG3 mice compared to both wildtype control groups indicative of an endophenotypical trait. The volume of striatal calretinin+ interneurons showed alterations in hΔGAG3 mice compared to the wt groups. Nerve-injury related changes were found for striatal ChAT+, parvalbumin+ and nNOS+ interneurons in both genotypes. The dopaminergic neurons of the substantia nigra remained unchanged in number across all groups, however, the cell volume was significantly increased in nerve-crushed hΔGAG3 mice compared to naïve hΔGAG3 mice and wildtype littermates. Moreover, in vivo microdialysis showed an increase of dopamine and its metabolites in the striatum comparing nerve-crushed hΔGAG3 mice to all other groups. The induction of a dystonia-like phenotype in genetically predisposed DYT-TOR1A mice highlights the importance of extragenetic factors in the symptomatogenesis of DYT-TOR1A dystonia. Our experimental approach allowed us to dissect microstructural and neurochemical abnormalities in the basal ganglia, which either reflected a genetic predisposition or endophenotype in DYT-TOR1A mice or a correlate of the induced dystonic phenotype. In particular, neurochemical and morphological changes of the nigrostriatal dopaminergic system were correlated with symptomatogenesis.

FDA-Approved Kinase Inhibitors in Preclinical and Clinical Trials for Neurological Disorders

Cancers and neurological disorders are two major types of diseases. We previously developed a new concept termed "Aberrant Cell Cycle Diseases" (ACCD), revealing that these two diseases share a common mechanism of aberrant cell cycle re-entry. The aberrant cell cycle re-entry is manifested as kinase/oncogene activation and tumor suppressor inactivation, which are hallmarks of both tumor growth in cancers and neuronal death in neurological disorders. Therefore, some cancer therapies (e.g., kinase inhibition, tumor suppressor elevation) can be leveraged for neurological treatments. The United States Food and Drug Administration (US FDA) has so far approved 74 kinase inhibitors, with numerous other kinase inhibitors in clinical trials, mostly for the treatment of cancers. In contrast, there are dire unmet needs of FDA-approved drugs for neurological treatments, such as Alzheimer's disease (AD), intracerebral hemorrhage (ICH), ischemic stroke (IS), traumatic brain injury (TBI), and others. In this review, we list these 74 FDA-approved kinase-targeted drugs and identify those that have been reported in preclinical and/or clinical trials for neurological disorders, with a purpose of discussing the feasibility and applicability of leveraging these cancer drugs (FDA-approved kinase inhibitors) for neurological treatments.

The Role of Microglia in Neuroinflammation of the Spinal Cord after Peripheral Nerve Injury

Peripheral nerve injuries induce a pronounced immune reaction within the spinal cord, largely governed by microglia activation in both the dorsal and ventral horns. The mechanisms of activation and response of microglia are diverse depending on the location within the spinal cord, type, severity, and proximity of injury, as well as the age and species of the organism. Thanks to recent advancements in neuro-immune research techniques, such as single-cell transcriptomics, novel genetic mouse models, and live imaging, a vast amount of literature has come to light regarding the mechanisms of microglial activation and alluding to the function of microgliosis around injured motoneurons and sensory afferents. Herein, we provide a comparative analysis of the dorsal and ventral horns in relation to mechanisms of microglia activation (CSF1, DAP12, CCR2, Fractalkine signaling, Toll-like receptors, and purinergic signaling), and functionality in neuroprotection, degeneration, regeneration, synaptic plasticity, and spinal circuit reorganization following peripheral nerve injury. This review aims to shed new light on unsettled controversies regarding the diversity of spinal microglial-neuronal interactions following injury.

Distinct nociception processing in the dysgranular and barrel regions of the mouse somatosensory cortex

Nociception, a somatic discriminative aspect of pain, is, like touch, represented in the primary somatosensory cortex (S1), but the separation and interaction of the two modalities within S1 remain unclear. Here, we show spatially distinct tactile and nociceptive processing in the granular barrel field (BF) and adjacent dysgranular region (Dys) in mouse S1. Simultaneous recordings of the multiunit activity across subregions revealed that Dys neurons are more responsive to noxious input, whereas BF neurons prefer tactile input. At the single neuron level, nociceptive information is represented separately from the tactile information in Dys layer 2/3. In contrast, both modalities seem to converge on individual layer 5 neurons of each region, but to a different extent. Overall, these findings show layer-specific processing of nociceptive and tactile information between Dys and BF. We further demonstrated that Dys activity, but not BF activity, is critically involved in pain-like behavior. These findings provide new insights into the role of pain processing in S1.

The processing of nociception in the somatosensory cortex (S1) has yet to be fully understood. Here, the authors demonstrate that the dysgranular region in S1 has an affinity for nociception and is critically involved in pain-like behavior.

Neuroplasticity related to chronic pain and its modulation by microglia

Neuropathic pain is often chronic and can persist after overt tissue damage heals, suggesting that its underlying mechanism involves the alteration of neuronal function. Such an alteration can be a direct consequence of nerve damage or a result of neuroplasticity secondary to the damage to tissues or to neurons. Recent studies have shown that neuroplasticity is linked to causing neuropathic pain in response to nerve damage, which may occur adjacent to or remotely from the site of injury. Furthermore, studies have revealed that neuroplasticity relevant to chronic pain is modulated by microglia, resident immune cells of the central nervous system (CNS). Microglia may directly contribute to synaptic remodeling and altering pain circuits, or indirectly contribute to neuroplasticity through property changes, including the secretion of growth factors. We herein highlight the mechanisms underlying neuroplasticity that occur in the somatosensory circuit of the spinal dorsal horn, thalamus, and cortex associated with chronic pain following injury to the peripheral nervous system (PNS) or CNS. We also discuss the dynamic functions of microglia in shaping neuroplasticity related to chronic pain. We suggest further understanding of post-injury ectopic plasticity in the somatosensory circuits may shed light on the differential mechanisms underlying nociceptive, neuropathic, and nociplastic-type pain. While one of the prominent roles played by microglia appears to be the modulation of post-injury neuroplasticity. Therefore, future molecular- or genetics-based studies that address microglia-mediated post-injury neuroplasticity may contribute to the development of novel therapies for chronic pain.

Electrophysiological and anatomical characterization of synaptic remodeling in the mouse whisker thalamus

In the central nervous system, developmental and pathophysiologic conditions cause a large-scale reorganization of functional connectivity of neural circuits. Here, by using a mouse model for peripheral sensory nerve injury, we present a protocol for combined electrophysiological and anatomical techniques to identify neural basis of synaptic remodeling in the mouse whisker thalamus. Our protocol provides comprehensive approaches to analyze both structural and functional components of synaptic remodeling.

For complete details on the use and execution of this protocol, please refer to Ueta and Miyata, (2021).

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The infraorbital nerve cut for preparing a peripheral nerve injury mouse model

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Pressure or iontophoretic drug application via stereotaxic injection

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Assessing functional synaptic remodeling by whole-cell patch-clamp in acute slices

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Immunohistochemical identification of structural synaptic remodeling