Neuronal circuitry for pain processing in the dorsal horn

Spatial transcriptomics combined with single-nucleus RNA sequencing reveals glial cell heterogeneity in the human spinal cord

JOURNAL/nrgr/04.03/01300535-202511000-00032/figure1/v/2024-12-20T164640Z/r/image-tiff Glial cells play crucial roles in regulating physiological and pathological functions, including sensation, the response to infection and acute injury, and chronic neurodegenerative disorders. Glial cells include astrocytes, microglia, and oligodendrocytes in the central nervous system, and satellite glial cells and Schwann cells in the peripheral nervous system. Despite the greater understanding of glial cell types and functional heterogeneity achieved through single-cell and single-nucleus RNA sequencing in animal models, few studies have investigated the transcriptomic profiles of glial cells in the human spinal cord. Here, we used high-throughput single-nucleus RNA sequencing and spatial transcriptomics to map the cellular and molecular heterogeneity of astrocytes, microglia, and oligodendrocytes in the human spinal cord. To explore the conservation and divergence across species, we compared these findings with those from mice. In the human spinal cord, astrocytes, microglia, and oligodendrocytes were each divided into six distinct transcriptomic subclusters. In the mouse spinal cord, astrocytes, microglia, and oligodendrocytes were divided into five, four, and five distinct transcriptomic subclusters, respectively. The comparative results revealed substantial heterogeneity in all glial cell types between humans and mice. Additionally, we detected sex differences in gene expression in human spinal cord glial cells. Specifically, in all astrocyte subtypes, the levels of NEAT1 and CHI3L1 were higher in males than in females, whereas the levels of CST3 were lower in males than in females. In all microglial subtypes, all differentially expressed genes were located on the sex chromosomes. In addition to sex-specific gene differences, the levels of MT-ND4 , MT2A , MT-ATP6 , MT-CO3 , MT-ND2 , MT-ND3 , and MT-CO2 in all spinal cord oligodendrocyte subtypes were higher in females than in males. Collectively, the present dataset extensively characterizes glial cell heterogeneity and offers a valuable resource for exploring the cellular basis of spinal cord-related illnesses, including chronic pain, amyotrophic lateral sclerosis, and multiple sclerosis.

Twelve-Month Clinical Trial Results of a Novel, Dorsal Horn Dendrite Stimulation Waveform for Chronic Neuropathic Low Back Pain

OBJECTIVES

The aim of this study was to evaluate the effectiveness and safety of a novel subperception spinal cord stimulation (SCS) waveform paradigm designed to target the dorsal horn dendrites for treating chronic neuropathic low back pain (LBP). The final 12-month results are reported here.

MATERIALS AND METHODS

Twenty-seven participants were implanted with a commercial SCS system. Devices were programmed to deliver the waveform (frequency 100 Hz, pulse width 1000 μsec, T9-T10 disk bipole) at decreasing stimulation perception threshold amplitudes (80%, 60%, then 40%) over a 14-week period. Participants were blinded to the program settings. Participants then received their preferred program for further evaluation at 26 and 52 weeks after activation. Outcome measures included back pain score (visual analogue scale [VAS]), Brief Pain Inventory (BPI), EuroQol 5-Dimension 5-Level (EQ-5D-5L), 36-Item Short Form Health Survey (SF-36), treatment satisfaction, and clinician global impression of change (CGIC).

RESULTS

At 52 weeks (n = 24), the responder rate (≥50% pain relief) was 65.6%, and the high-responder rate (≥80% pain relief) was 56.5%. The mean change from baseline in pain VAS was -43.94 mm (95% CI -57.89, -30.00; p < 0.001) and mean pain relief was 64.69% ± 39.43%. BPI and SF-36 scores remained significantly improved (p ≤ 0.001). EQ-5D-5L index and EuroQoL-VAS further improved, and 87.0% of participants met the minimum clinically important difference for the EQ-5D-5L index. Treatment satisfaction was 83%, and 91% of participants had a CGIC rating of "much improved" or above. No serious study-related adverse events were reported.

CONCLUSIONS

The 12-month trial results show sustained improvements in pain, quality of life, and health-related outcomes. This novel subperception dorsal horn dendrite SCS approach seems a safe and promising treatment option for patients with chronic neuropathic LBP. The open-source availability of this waveform on commercial SCS platforms allows widespread patient access. Further evaluation seems warranted.

CLINICAL TRIAL REGISTRATION

The Clinicaltrials.gov registration number for the study is ACTRN12618000647235 (anzctr.org.au).

Visualizing the modulation of neurokinin 1 receptor-positive neurons in the superficial dorsal horn by spinal cord stimulation in vivo

ABSTRACT

Spinal cord stimulation (SCS) is an effective modality for pain treatment, yet its underlying mechanisms remain elusive. Neurokinin 1 receptor-positive (NK1R + ) neurons in spinal lamina I play a pivotal role in pain transmission. To enhance our mechanistic understanding of SCS-induced analgesia, we investigated how different SCS paradigms modulate the activation of NK1R + neurons, by developing NK1R-Cre;GCaMP6s transgenic mice and using in vivo calcium imaging of superficial NK1R + neurons under anesthesia (1.5% isoflurane). Neurokinin 1 receptor-positive neurons in the lumbar spinal cord (L4-5) showed a greater activation by electrical test stimulation (TS, 3.0 mA, 1 Hz) at the hindpaw at 2 weeks after tibia-sparing nerve injury (SNI-t) than in naïve mice. Spinal cord stimulation was then delivered through a bipolar plate electrode placed epidurally at L1-2 level. The short-term 50-Hz high-intensity SCS (80% motor threshold [MoT], 10 minutes) induced robust and prolonged inhibition of NK1R + neuronal responses to TS in both naïve and SNI-t mice. The 30-minute 50-Hz and 900-Hz SCS applied at moderate intensity (50% MoT) also significantly inhibited neuronal responses in SNI-t mice. However, at low intensity (20% MoT), the 30-minute 900-Hz SCS only induced persistent neuronal inhibition in naïve mice, but not in SNI-t mice. In conclusion, both 10-minute high-intensity SCS and 30-minute SCS at moderate intensity inhibit the activation of superficial NK1R + neurons, potentially attenuating spinal nociceptive transmission. Furthermore, in vivo calcium imaging of NK1R + neurons provides a new approach for exploring the spinal neuronal mechanisms of pain inhibition by neuromodulation pain therapies.

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.

A septo-hypothalamic-medullary circuit directs stress-induced analgesia

Stress is a potent modulator of pain. Specifically, acute stress due to physical restraint induces stress-induced analgesia (SIA). However, where and how acute stress and pain pathways interface in the brain are poorly understood. Here, we describe how the dorsal lateral septum (dLS), a forebrain limbic nucleus, facilitates SIA through its downstream targets in the lateral hypothalamic area (LHA) of mice. Taking advantage of transsynaptic viral-genetic, optogenetic, and chemogenetic techniques, we show that the dLS→LHA circuitry is sufficient to drive analgesia and is required for SIA. Furthermore, our results reveal that the dLS→LHA pathway is opioid-dependent and modulates pain through the pro-nociceptive neurons in the rostral ventromedial medulla (RVM). Remarkably, we found that the inhibitory dLS neurons are recruited specifically when the mice struggle to escape under restraint and, in turn, inhibit excitatory LHA neurons. As a result, the RVM neurons downstream of LHA are disengaged, thus suppressing nociception. Together, we delineate a poly-synaptic pathway that can transform escape behavior in mice under restraint to acute stress into analgesia.

Central projections of nociceptive input originating from the low back and limb muscle in rats

Since clinical features of chronic muscle pain originating from the low back and limbs are different (higher prevalence and broader/duller sensation of low back muscle pain than limb muscle pain), spinal and/or supraspinal projection of nociceptive information could differ between the two muscles. We tested this hypothesis using c-Fos immunohistochemistry combined with retrograde-labeling of dorsal horn (DH) neurons projecting to ventrolateral periaqueductal grey (vlPAG) or ventral posterolateral nucleus of the thalamus (VPL) by fluorogold (FG) injections into the vlPAG or VPL. C-Fos expression in the DH was induced by injecting 5% formalin into the multifidus (MF, low back) or gastrocnemius-soleus (GS, limb) muscle. A double-labeled DH neuron showing both c-Fos-immunoreactive nucleus and retrogradely transported FG in the cytoplasm was considered as a nociceptive projection neuron. Consistent with DH somatotopy for proximal vs. distal cutaneous inputs, DH neurons with MF input were located in the most lateral area of laminae I - II (segments Th12 - L5), while those with GS input were located in the middle area of laminae I - II (L3 - L5). DH neurons projecting to the vlPAG were located in superficial DH, while those projecting to VPL were located in deep DH. Supraspinal projection derived from more spinal segments for MF input than for GS input. These data suggest that nociceptive input from low back muscles is integrated more in craniocaudal direction than for limb muscles, and that these signals are then forwarded to both PAG and thalamus and contribute to the different nature of muscle pain arising from the low back and limbs.

An atlas of neuropathic pain-associated molecular pathological characteristics in the mouse spinal cord

Peripheral nerve injury (PNI)-induced neuropathic pain (NP) is a severe disease with high prevalence in clinics. Gene reprogramming and tissue remodeling in the dorsal root ganglia (DRG) and spinal cord (SC) drive the development and maintenance of neuropathic pain (NP). However, our understanding of the NP-associated spatial molecular processing landscape of SC and the non-synaptic interactions between DRG neurons and SC cells remains limited. We here integrate spatial transcriptomics (ST) with single-nucleus RNA-sequencing (snRNA-seq) and bulk RNA-sequencing (bulk RNA-seq) to characterize regional pathological heterogeneity of the SC under NP conditions. First, the SC of NP mice manifests unique spatial atlases of genes, cell populations, cell-cell cross-talks, signaling pathways, and transcriptional regulatory networks compared to sham mice. We further report that injured DRG sensory neurons and the corresponding ventral horn of the SC show similar expression patterns after PNI. In addition, for the first time, we systematically exhibit "cross-talk omics" between the DRG neurons and SC dorsal horn neurons and glial cells, indicating an altered communication profile under NP conditions. Together, our findings decode the spatial and cellular heterogeneity of molecular pathological mechanisms underlying NP, providing a foundation for designing therapeutic targets for this disorder.

Mechanisms for dysregulation of excitatory-inhibitory balance underlying allodynia in dorsal horn neural subcircuits

Chronic pain is a wide-spread condition that is debilitating and expensive to manage, costing the United States alone around $600 billion in 2010. In a common symptom of chronic pain called allodynia, non-painful stimuli produce painful responses with highly variable presentations across individuals. While the specific mechanisms remain unclear, allodynia is hypothesized to be caused by the dysregulation of excitatory-inhibitory (E-I) balance in pain-processing neural circuitry in the dorsal horn of the spinal cord. In this work, we analyze biophysically-motivated subcircuit structures that represent common motifs in neural circuits in laminae I-II of the dorsal horn. These circuits are hypothesized to be part of the neural pathways that mediate two different types of allodynia: static and dynamic. We use neural firing rate models to describe the activity of populations of excitatory and inhibitory interneurons within each subcircuit. By accounting for experimentally-observed responses under healthy conditions, we specify model parameters defining populations of subcircuits that yield typical behavior under normal conditions. Then, we implement a sensitivity analysis approach to identify the mechanisms most likely to cause allodynia-producing dysregulation of the subcircuit's E-I signaling. We find that disruption of E-I balance generally occurs either due to downregulation of inhibitory signaling so that excitatory neurons are "released" from inhibitory control, or due to upregulation of excitatory neuron responses so that excitatory neurons "escape" their inhibitory control. Which of these mechanisms is most likely to occur, the subcircuit components involved in the mechanism, and the proportion of subcircuits exhibiting the mechanism can vary depending on the subcircuit structure. These results suggest specific hypotheses about diverse mechanisms that may be most likely responsible for allodynia, thus offering predictions for the high interindividual variability observed in allodynia and identifying targets for further experimental studies on the underlying mechanisms of this chronic pain symptom.

Marine-derived bioactive compounds for neuropathic pain: pharmacology and therapeutic potential

Neuropathic pain, a challenging condition often associated with diabetes, trauma, or chemotherapy, impairs patients' quality of life. Current treatments often provide inconsistent relief and notable adverse effects, highlighting the urgent need for safer and more effective alternatives. This review investigates marine-derived bioactive compounds as potential novel therapies for neuropathic pain management. Marine organisms, including fungi, algae, cone snails, sponges, soft corals, tunicates, and fish, produce a diverse range of secondary metabolites with significant pharmacological properties. These include peptides (e.g., conopeptides, piscidin 1), non-peptides (e.g., guanidinium toxins, astaxanthin, docosahexaenoic acid, fucoidan, apigenin, fumagillin, aaptamine, flexibilide, excavatolide B, capnellenes, austrasulfones, lemnalol), and crude extracts (e.g., Spirulina platensis, Dunaliella salina, Cliothosa aurivilli). These compounds exhibit diverse mechanisms of action, such as modulating ion channels (e.g., transient receptor potential channels, voltage-gated sodium, calcium, and potassium channels, and G protein-coupled inwardly rectifying potassium channels), interacting with cell-surface receptors (e.g., nicotinic acetylcholine, NMDA, kainate, GABAB​, and neurotensin receptors), inhibiting norepinephrine transporters, reducing oxidative stress, and attenuating neuroinflammation. These effects collectively contribute to alleviating nerve degeneration and symptoms of neuropathic pain, including hyperalgesia, allodynia, and associated psychomotor disturbances. Marine-derived bioactive compounds represent promising alternatives to conventional neuropathic pain treatments, to advance their development and assess their integration into neuropathic pain management strategies.

A novel approach to completely alleviate peripheral neuropathic pain in human patients: insights from preclinical data

Neuropathic pain is a pervasive health concern worldwide, posing significant challenges to both clinicians and neuroscientists. While acute pain serves as a warning signal for potential tissue damage, neuropathic pain represents a chronic pathological condition resulting from injury or disease affecting sensory pathways of the nervous system. Neuropathic pain is characterized by long-lasting ipsilateral hyperalgesia (increased sensitivity to pain), allodynia (pain sensation in response to stimuli that are not normally painful), and spontaneous unprovoked pain. Current treatments for neuropathic pain are generally inadequate, and prevention remains elusive. In this review, we provide an overview of current treatments, their limitations, and a discussion on the potential of capsaicin and its analog, resiniferatoxin (RTX), for complete alleviation of nerve injury-induced neuropathic pain. In an animal model of neuropathic pain where the fifth lumbar (L5) spinal nerve is unilaterally ligated and cut, resulting in ipsilateral hyperalgesia, allodynia, and spontaneous pain akin to human neuropathic pain. The application of capsaicin or RTX to the adjacent uninjured L3 and L4 nerves completely alleviated and prevented mechanical and thermal hyperalgesia following the L5 nerve injury. The effects of this treatment were specific to unmyelinated fibers (responsible for pain sensation), while thick myelinated nerve fibers (responsible for touch and mechanoreceptor sensations) remained intact. Here, we propose to translate these promising preclinical results into effective therapeutic interventions in humans by direct application of capsaicin or RTX to adjacent uninjured nerves in patients who suffer from neuropathic pain due to peripheral nerve injury, following surgical interventions, diabetic neuropathy, trauma, vertebral disc herniation, nerve entrapment, ischemia, postherpetic lesion, and spinal cord injury.

Neurochemical mechanism of muscular pain: Insight from the study on delayed onset muscle soreness

We reviewed fundamental studies on muscular pain, encompassing the characteristics of primary afferent fibers and neurons, spinal and thalamic projections, several muscular pain models, and possible neurochemical mechanisms of muscle pain. Most parts of this review were based on data obtained from animal experiments, and some researches on humans were also introduced. We focused on delayed-onset muscle soreness (DOMS) induced by lengthening contractions (LC), suitable for studying myofascial pain syndromes. The muscular mechanical withdrawal threshold (MMWT) decreased 1-3 days after LC in rats. Changing the speed and range of stretching showed that muscle injury seldom occurred, except in extreme conditions, and that DOMS occurred in parameters without muscle damage. The B2 bradykinin receptor-nerve growth factor (NGF) route and COX-2-glial cell line-derived neurotrophic factor (GDNF) route were involved in the development of DOMS. The interactions between these routes occurred at two levels. A repeated-bout effect was observed in MMWT and NGF upregulation, and this study showed that adaptation possibly occurred before B2 bradykinin receptor activation. We have also briefly discussed the prevention and treatment of DOMS.

Alteration of serotonin release response in the central nucleus of the amygdala to noxious and non-noxious mechanical stimulation in a neuropathic pain model rat

Previously, we found that serotonin (5-HT) release in the central nucleus of the amygdala (CeA) of anesthetized rats decreases in response to innocuous stroking of the skin, irrespective of stimulus laterality, but increases in response to noxious pinching applied to a hindlimb contralateral to the 5-HT measurement site. The aim of the present study was to determine whether intra-CeA 5-HT release responses to cutaneous stimulation were altered in an animal model of neuropathic pain induced by ligation of the left L5 spinal nerve. In anesthetized neuropathic pain model rats, stroking of the left hindlimb increased 5-HT release in the CeA, whereas stroking of the right hindlimb decreased it. Meanwhile, pinching of the left hindlimb increased intra-CeA 5-HT release irrespective of stimulus laterality. In conclusion, the present study demonstrated that intra-CeA 5-HT release responses to cutaneous stimulation are altered in an animal model of neuropathic pain.

Tachykinin signaling in the right parabrachial nucleus mediates early-phase neuropathic pain development

The lateral parabrachial nucleus (PBN) is critically involved in neuropathic pain modulation. However, the cellular and molecular mechanisms underlying this process remain largely unknown. Here, we report that in mice, the right-sided, but not the left-sided, PBN plays an essential role in the development of hyperalgesia following nerve injury, irrespective of the injury side. Spino-parabrachial pathways targeting the right-sided PBN display short-term facilitation, and right-sided PBN neurons exhibit an increase in the excitability and activity after nerve injury. Inhibiting Tacr1-positive neurons, blocking Tacr1-encoding tachykinin 1 receptor (NK1R), or knocking down the Tacr1 gene in the right-sided, rather than left-sided, PBN alleviates neuropathic pain-induced sensory hypersensitivity. Additionally, the right-sided PBN plays a critical role in the development of hyperalgesia during the early phase of neuropathic pain. These results highlight the essential role of NK1R in the lateralized modulation of neuropathic pain by the PBN, providing new insights into the mechanisms underlying neuropathic pain.

Anesthetic- and Analgesic-Related Drugs Modulating Both Voltage-Gated Na+ and TRP Channels

Nociceptive information is transmitted by action potentials (APs) through primary afferent neurons from the periphery to the central nervous system. Voltage-gated Na+ channels are involved in this AP production, while transient receptor potential (TRP) channels, which are non-selective cation channels, are involved in receiving and transmitting nociceptive stimuli in the peripheral and central terminals of the primary afferent neurons. Peripheral terminal TRP vanilloid-1 (TRPV1), ankylin-1 (TRPA1) and melastatin-8 (TRPM8) activation produces APs, while central terminal TRP activation enhances the spontaneous release of L-glutamate from the terminal to spinal cord and brain stem lamina II neurons that play a pivotal role in modulating nociceptive transmission. There is much evidence demonstrating that chemical compounds involved in Na+ channel (or nerve AP conduction) inhibition modify TRP channel functions. Among these compounds are local anesthetics, anti-epileptics, α2-adrenoceptor agonists, antidepressants (all of which are used as analgesic adjuvants), general anesthetics, opioids, non-steroidal anti-inflammatory drugs and plant-derived compounds, many of which are involved in antinociception. This review mentions the modulation of Na+ channels and TRP channels including TRPV1, TRPA1 and TRPM8, both of which modulations are produced by pain-related compounds.

The Effect of Spinal Cord Stimulation on Spinal Dorsal Horn Lipid Expression in Experimental Painful Diabetic Polyneuropathy: A Matrix-Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry Imaging Study

OBJECTIVES

Diabetes-induced peripheral nerve fiber damage can cause painful diabetic polyneuropathy (PDPN), induced by central sensitization through proinflammatory processes in the spinal dorsal horn. Disturbances in spinal dorsal horn lipid metabolism play a major role in proinflammatory regulation. Conventional (Con)-spinal cord stimulation (SCS) is an alternative treatment for pain relief in PDPN, whereas differential target multiplexed (DTM)-SCS could be more effective than Con-SCS, specifically targeting the spinal inflammatory response. We hypothesize that Con- and DTM-SCS differentially affect lipid metabolism in the spinal cord of PDPN animals. To study pain relief mechanisms, we analyzed lipid expression in the spinal dorsal horn using matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry imaging (MSI).

MATERIAL AND METHODS

Diabetes was induced through streptozotocin (STZ) injection in 28 rats, of which 12 developed PDPN. These and four nondiabetic animals (sham STZ) were implanted with a quadripolar lead and stimulated with Con-, DTM-, or Sham-SCS for 48 hours. Mechanical sensitivity was assessed using Von Frey filaments after 24 and 48 hours. After 48 hours of SCS, the spinal cord was collected, and lipids were analyzed using MALDI-TOF MSI.

RESULTS

STZ-induced hypersensitivity in the hind paws was reduced by Con- and DTM-SCS. PDPN induction decreased the expression of a glycosphingolipid in laminae 3 of the spinal dorsal horn. After 48 hours of Con- and DTM-SCS, expression levels of several lipids in the spinal dorsal horn decreased, including (HexCer 36:1;O, 40:1;O3), diacylglycerophosphocholines (PC 36:1, 38:6, 40:5), and diacylglycerophosphoserines (PS 36:4).

CONCLUSIONS

Both Con- and DTM-SCS provide pain relief and decrease spinal dorsal horn lipid expression of PDPN animals, highlighting the complex effects of SCS on the spinal cord physiology. STZ-induced PDPN has a limited effect on lipid expression in the spinal dorsal horn.

An Open-Source Tool for Investigation of Differential RNA Expression Between Spinal Cord Cells of Male and Female Mice

Chronic pain is a highly debilitating condition that differs by type, prevalence, and severity between men and women. To uncover the molecular underpinnings of these differences, it is critical to analyze the transcriptomes of spinal cord pain-processing networks for both sexes. Despite several recently published single-nucleus RNA-sequencing (snRNA-seq) studies on the function and composition of the mouse spinal cord, a gene expression analysis investigating the differences between males and females has yet to be performed. Here, we combined data from three different large-scale snRNA-seq studies, which used sex-identified adult mice. Using SeqSeek, we classified more than 37,000 unique viable cells within predicted cell types with the use of machine learning. We then utilized DESeq2 to identify significant differentially expressed genes (DEGs) between males and females in a variety of cell populations, including superficial dorsal horn (SDH) neurons. We found a large number of DEGs between males and females in all cells, in neurons, and in SDH neurons of the mouse spinal cord, with a greater level of differential expression in inhibitory SDH neurons compared to excitatory SDH neurons. The results of these analyses are available on an open-source web-app: https://justinbellavance.shinyapps.io/snRNA_Visualization/. Lastly, we used gene set enrichment analysis to identify sex-enriched pathways from our previously identified DEGs. Through this, we have identified specific genetic players within the rodent spinal cord that diverge between males and females, which may underlie reported sex differences in spinal nociceptive mechanisms and pain processing.

The Antinociceptive Effects and Sex-Specific Neurotransmitter Modulation of Metformin in a Mouse Model of Fibromyalgia

Fibromyalgia (FM) is a chronic and debilitating condition characterized by diffuse pain, often associated with symptoms such as fatigue, cognitive disturbances, and mood disorders. Metformin, an oral hypoglycemic agent, has recently gained attention for its potential benefits beyond glucose regulation. It has shown promise in alleviating neuropathic and inflammatory pain, suggesting that it could offer a novel approach to managing chronic pain conditions like FM. This study aimed to further explore metformin's analgesic potential by evaluating its effects in an experimental FM model induced by reserpine in both male and female mice. After the administration of 200 mg/kg metformin to male and female mice, the FM-related symptoms were assessed, including mechanical allodynia, thermal hyperalgesia, and depressive-like behaviors. A histological examination of the thalamus, hippocampus, and spinal cord was conducted using haematoxylin and eosin staining. The neurotransmitter and proinflammatory cytokines levels were measured in the brains and spinal cords. Our results have shown that metformin treatment for seven days significantly reversed these FM-like symptoms, reducing pain sensitivity and improving mood-related behaviors in both the male and female mice. Additionally, metformin exhibited neuroprotective effects, mitigating reserpine-induced damage in the hippocampus, thalamus, and spinal cord. It also significantly lowered the levels of the proinflammatory cytokine interleukin 1-beta (IL-1β) in the brain and spinal cord. Notably, metformin modulated the neurotransmitter levels differently between the sexes, decreasing glutamate and increasing serotonin and norepinephrine in the male mice, but not in the females. These findings underscore metformin's potential as an alternative therapy for FM, with sex-specific differences suggesting distinct mechanisms of action.

Converging inputs compete at the lateral parabrachial nuclei to dictate the affective-motivational responses to cold pain

ABSTRACT

The neural mechanisms of the affective-motivational symptoms of chronic pain are poorly understood. In chronic pain, our innate coping mechanisms fail to provide relief. Hence, these behaviors are manifested at higher frequencies. In laboratory animals, such as mice and rats, licking the affected areas is a behavioral coping mechanism and it is sensitized in chronic pain. Hence, we have focused on delineating the brain circuits mediating licking in mice with chemotherapy-induced peripheral neuropathy (CIPN). Mice with CIPN develop intense cold hypersensitivity and lick their paws upon contact with cold stimuli. We studied how the lateral parabrachial nucleus (LPBN) neurons facilitate licking behavior when mice are exposed to noxious thermal stimuli. Taking advantage of transsynaptic viral, optogenetic, and chemogenetic strategies, we observed that the LPBN neurons become hypersensitive to cold in mice with CIPN and facilitate licks. Furthermore, we found that the expression of licks depends on competing excitatory and inhibitory inputs from the spinal cord and lateral hypothalamus (LHA), respectively. We anatomically traced the postsynaptic targets of the spinal cord and LHA in the LPBN and found that they synapse onto overlapping populations. Activation of this LPBN population was sufficient to promote licking due to cold allodynia. In sum, our data indicate that the nociceptive inputs from the spinal cord and information on brain states from the hypothalamus impinge on overlapping LPBN populations to modulate their activity and, in turn, regulate the elevated affective-motivational responses in CIPN.

C-LTMRs evoke wet dog shakes via the spinoparabrachial pathway

Many hairy mammals perform rapid oscillations of their body, called wet dog shakes, to remove water and irritants from their back hairy skin. The somatosensory mechanisms that underlie this behavior are unclear. We report that Piezo2-dependent mechanosensation mediates wet dog shakes evoked by water or oil droplets applied to back hairy skin of mice. Unmyelinated C-fiber low-threshold mechanoreceptors (C-LTMRs) were activated by oil droplets, and their optogenetic activation elicited wet dog shakes. Ablation of C-LTMRs attenuated this behavior. Moreover, C-LTMRs synaptically couple to spinoparabrachial neurons, and optogenetically inhibiting spinoparabrachial neuron synapses and excitatory neurons in the parabrachial nucleus impaired both oil droplet- and C-LTMR-evoked wet dog shakes. Thus, a C-LTMR-spinoparabrachial pathway promotes wet dog shakes for removal of water and mechanical irritants from back hairy skin.

Neuroanatomy of the nociceptive system: From nociceptors to brain networks

This chapter reviews the neuroanatomy of the nociceptive system and its functional organization. We describe three main compartments of the nervous system that underlie normal nociception and the resulting pain percept: Peripheral, Spinal Cord, and Brain. We focus on how ascending nociceptive processing streams traverse these anatomical compartments, culminating in the multidimensional experience of pain. We also describe neuropathic pain conditions, in which nociceptive processing is abnormal, not only because of the primary effects of a lesion or disease affecting peripheral nerves or the central nervous system (CNS), but also due to secondary effects on ascending pathways and brain networks. We discuss how the anatomical components (circuits/networks) reorganize under various etiologies of neuropathic pain and how these changes can give rise to pathological pain states.

Behavioral and neural correlates of diverse conditioned fear responses in male and female rats

Pavlovian fear conditioning is a widely used tool that models associative learning in rodents. For decades the field has used predominantly male rodents and focused on a sole conditioned fear response: freezing. However, recent work from our lab and others has identified darting as a female-biased conditioned response, characterized by an escape-like movement across a fear conditioning chamber. It is also accompanied by a behavioral phenotype: Darters reliably show decreased freezing compared to Non-darters and males and reach higher velocities in response to the foot shock ("shock response"). However, the relationship between shock response and conditioned darting is not known. This study investigated if this link is due to differences in general processing of aversive stimuli between Darters, Non-darters and males. Across a variety of modalities, including corticosterone measures, the acoustic startle test, and sensitivity to thermal pain, Darters were found not to be more reactive or sensitive to aversive stimuli, and, in some cases, they appear less reactive to Non-darters and males. Analyses of cFos activity in regions involved in pain and fear processing following fear conditioning identified discrete patterns of expression among Darters, Non-darters, and males exposed to low and high intensity foot shocks. The results from these studies further our understanding of the differences between Darters, Non-darters and males and highlight the importance of studying individual differences in fear conditioning as indicators of fear state.

Periaqueductal gray connectivity in spinal cord injury-induced neuropathic pain

BACKGROUND AND PURPOSE

Neuropathic pain (NP) is a debilitating condition following spinal cord injury (SCI). The role of periaqueductal gray (PAG) in NP development following SCI remains underexplored. Using resting-state functional MRI (rsfMRI), our study aimed to demonstrate the alterations in functional connectivity (FC) of PAG in NP following SCI.

METHODS

Ten SCI patients (SCI + NP, n = 7, and SCI - NP, n = 3), alongside 10 healthy controls (HCs), were enrolled. rsfMRI was conducted followed by seed-to-voxel analysis using PAG as the seed region and then group-based analysis comprising three groups (SCI + NP, SCI - NP, and HC). Age and gender were considered as confounding variables.

RESULTS

Compared to HCs, SCI + NP demonstrated decreased FC between PAG and right insula, right frontal orbital cortex, right pallidum, dorsal raphe nucleus (DRN), red nuclei (RN), substantia nigra (SN), and ventral posterolateral (VPL) thalamic nuclei. Compared to SCI - NP, SCI + NP demonstrated increased FC between PAG and posterior cingulate cortex (PCC), hippocampus, cerebellar vermis lobules IV and V, and thalamic structures (posterior and lateral pulvinar, the mediodorsal nuclei, and the ventral lateral nuclei). Additionally, decreased FC between the PAG and VPL, geniculate bodies, intralaminar nuclei of thalamus, DRN, RN, SN, and prefrontal cortex was observed in this comparison.

CONCLUSIONS

Altered FC between PAG and right anterior insula, VPL, DRN, RN, SN, cerebellar vermis lobules IV and V, frontal cortex, and PCC was associated with NP sequelae of SCI. Additionally, SCI was independently associated with decreased FC between PAG and right posterior insula, cerebellar lobules IV and V, and cerebellar vermis lobules III, IV, and V.

Glycine Transporter 1 Inhibitors Minimize the Analgesic Tolerance to Morphine

Opioid analgesic tolerance (OAT), among other central side effects, limits opioids' indispensable clinical use for managing chronic pain. Therefore, there is an existing unmet medical need to prevent OAT. Extrasynaptic N-methyl D-aspartate receptors (NMDARs) containing GluN2B subunit blockers delay OAT, indicating the involvement of glutamate in OAT. Glycine acts as a co-agonist on NMDARs, and glycine transporters (GlyTs), particularly GlyT-1 inhibitors, could affect the NMDAR pathways related to OAT. Chronic subcutaneous treatments with morphine and NFPS, a GlyT-1 inhibitor, reduced morphine antinociceptive tolerance (MAT) in the rat tail-flick assay, a thermal pain model. In spinal tissues of rats treated with a morphine-NFPS combination, NFPS alone, or vehicle-comparable changes in µ-opioid receptor activation, protein and mRNA expressions were seen. Yet, no changes were observed in GluN2B mRNA levels. An increase was observed in glycine and glutamate contents of cerebrospinal fluids from animals treated with a morphine-NFPS combination and morphine, respectively. Finally, GlyT-1 inhibitors are likely to delay MAT by mechanisms relying on NMDARs functioning rather than an increase in opioid efficacy. This study, to the best of our knowledge, shows for the first time the impact of GlyT-1 inhibitors on MAT. Nevertheless, future studies are required to decipher the exact mechanisms.

Photobiomodulation transiently increases the spontaneous firing in the superficial layer of the rat spinal dorsal horn

The therapeutic benefits of photobiomodulation (PBM) in pain management, although well documented, are accompanied by concerns about potential risks, including pain, particularly at higher laser intensities. This study investigated the effects of laser intensity on pain perception using behavioral and electrophysiological evaluations in rats. Our results show that direct laser irradiation of 1000 mW/cm2 to the sciatic nerve transiently increases the frequency of spontaneous firing in the superficial layer without affecting the deep layer of the spinal dorsal horn, and this effect reverses to pre-irradiation levels after irradiation. Interestingly, laser irradiation at 1000 mW/cm2, which led to an increase in spontaneous firing, did not prompt escape behavior. Furthermore, a significant reduction in the time to initiate escape behavior was observed only at 9500 mW/cm2 compared to 15, 510, 1000, and 4300 mW/cm2. This suggests that 1000 mW/cm2, the laser intensity at which an increase in spontaneous firing was observed, corresponds to a stimulus that did not cause pain. It is expected that a detailed understanding of the risks and mechanisms of PBM from a neurophysiological perspective will lead to safer and more effective use of PBM.

Slack potassium channels in spinal dorsal horn neurons control neuropathic pain and acute itch

ABSTRACT

The sodium-activated potassium channel Slack (KNa1.1, Kcnt1) plays a critical role in tuning neuronal excitability. Previous studies have revealed that Slack is expressed in neurons of the superficial dorsal horn of the spinal cord. However, the precise role of Slack in spinal dorsal horn neurons is unclear. In this study, we used mice in which Slack is conditionally ablated in spinal dorsal horn neurons (Lbx1-Slack-/- mice) and analyzed their behaviors in various models of pain and itch. Lbx1-Slack-/- mice exhibited increased neuropathic pain behavior after peripheral nerve injury but normal responses in a model of inflammatory pain. Unexpectedly, Lbx1-Slack-/- mice demonstrated increased scratching after intradermal injection of chloroquine, LY344864, and histamine. Moreover, neuromedin B receptors are coexpressed with Slack in the dorsal horn, and scratching after intrathecal delivery of neuromedin B was increased in Lbx1-Slack-/- mice. Our study provides in vivo evidence that Slack expressed in spinal dorsal horn neurons inhibits nerve injury-induced allodynia and acute itch induced by various pruritogens.

Disentangling the spinal mechanisms of illusory heat and burning sensations in the thermal grill illusion

ABSTRACT

The thermal grill illusion (TGI), a phenomenon in which the juxtaposition of innocuous warm and cold temperatures on the skin elicits a burning sensation, offers a unique perspective to how pain occurs in response to harmless stimuli. We investigated the role of the spinal cord in the generation of the TGI across 2 experiments (total n = 80). We applied heat and cold stimuli to dermatomes, areas of skin innervated by a single spinal nerve, that mapped onto adjacent or nonadjacent spinal segments. Enhanced warm and burning ratings during the TGI were observed when cold and warm stimuli were confined within the same dermatome. Furthermore, we found the spatial organisation of warm and cold stimuli within and across dermatomes affected TGI perception. Perceived warmth and burning intensity increased when the cold stimulus projected to the segment more caudal to the warm stimulus, whereas perceived cold during the TGI decreased compared with the opposite spatial arrangement. This suggests that the perception of TGI is enhanced when cold afferents are projected to spinal segments positioned caudally in relation to those receiving warm afferents. Our results indicate distinct interaction of sensory pathways based on the segmental arrangement of afferent fibres and are consistent with current interpretations of the spread and integration of thermosensory information along the spinal cord.

Heterogeneity of synaptic NMDA receptor responses within individual lamina I pain-processing neurons across sex in rats and humans

Excitatory glutamatergic NMDA receptors (NMDARs) are key regulators of spinal pain processing, and yet the biophysical properties of NMDARs in dorsal horn nociceptive neurons remain poorly understood. Despite the clinical implications, it is unknown whether the molecular and functional properties of synaptic NMDAR responses are conserved between males and females or translate from rodents to humans. To address these translational gaps, we systematically compared individual and averaged excitatory synaptic responses from lamina I pain-processing neurons of adult Sprague-Dawley rats and human organ donors, including both sexes. By combining patch-clamp recordings of outward miniature excitatory postsynaptic currents with non-biased data analyses, we uncovered a wide range of decay constants of excitatory synaptic events within individual lamina I neurons. Decay constants of synaptic responses were distributed in a continuum from 1-20 ms to greater than 1000 ms, suggesting that individual lamina I neurons contain AMPA receptor (AMPAR)-only as well as GluN2A-, GluN2B- and GluN2D-NMDAR-dominated synaptic events. This intraneuronal heterogeneity in AMPAR- and NMDAR-mediated decay kinetics was observed across sex and species. However, we discovered an increased relative contribution of GluN2A-dominated NMDAR responses at human lamina I synapses compared with rodent synapses, suggesting a species difference relevant to NMDAR subunit-targeting therapeutic approaches. The conserved heterogeneity in decay rates of excitatory synaptic events within individual lamina I pain-processing neurons may enable synapse-specific forms of plasticity and sensory integration within dorsal horn nociceptive networks. KEY POINTS: Synaptic NMDA receptors (NMDARs) in spinal dorsal horn nociceptive neurons are key regulators of pain processing, but it is unknown whether their functional properties are conserved between males and females or translate from rodents to humans. In this study, we compared individual excitatory synaptic responses from lamina I pain-processing neurons of male and female adult Sprague-Dawley rats and human organ donors. Individual lamina I neurons from male and female rats and humans contain AMPA receptor-only as well as GluN2A, GluN2B- and GluN2D-NMDAR-dominated synaptic events. This may enable synapse-specific forms of plasticity and sensory integration within dorsal horn nociceptive networks. Human lamina I synapses have an increased relative contribution of GluN2A-dominated NMDAR responses compared with rodent synapses. These results uncover a species difference relevant to NMDAR subunit-targeting therapeutic approaches.

Electroacupuncture Relieves Neuropathic Pain via Adenosine 3 Receptor Activation in the Spinal Cord Dorsal Horn of Mice

Neuropathic pain (NPP) is a devastating and unbearable painful condition. As prevailing treatment strategies have failed to mitigate its complications, there remains a demand for effective therapies. Electroacupuncture (EA) has proved a potent remedial strategy in NPP management in humans and mammals. However, past studies have investigated the underlying mechanism of the analgesic effects of EA on NPP, focusing primarily on adenosine receptors in peripheral tissues. Herein, we elucidate the role of the adenosine (Adora-3) signaling pathway in mediating pain relief through EA in the central nervous system, which is obscure in the literature and needs exploration. Specific pathogen-free (SPF) male adult mice (C57BL/6 J) were utilized to investigate the effect of EA on adenosine metabolism (CD73, ADA) and its receptor activation (Adora-3), as potential mechanisms to mitigate NPP in the central nervous system. NPP was induced via spared nerve injury (SNI). EA treatment was administered seven times post-SNI surgery, and lumber (L4-L6) spinal cord was collected to determine the molecular expression of mRNA and protein levels. In the spinal cord of mice, following EA application, the expression results revealed that EA upregulated (p < 0.05) Adora-3 and CD73 by inhibiting ADA expression. In addition, EA triggered the release of adenosine (ADO), which modulated the nociceptive responses and enhanced neuronal activation. Meanwhile, the interplay between ADO levels and EA-induced antinociception, using an Adora-3 agonist and antagonist, showed that the Adora-3 agonist IB-MECA significantly increased (p < 0.05) nociceptive thresholds and expression levels. In contrast, the antagonist MRS1523 exacerbated neuropathic pain. Furthermore, an upregulated effect of EA on Adora-3 expression was inferred when the Adora-3 antagonist was administered, and the EA treatment increased the fluorescent intensity of Adora-3 in the spinal cord. Taken together, EA effectively modulates NPP by regulating the Adora-3 signaling pathway under induced pain conditions. These findings enhance our understanding of NPP management and offer potential avenues for innovative therapeutic interventions.

Andrographolide exhibits antinociceptive effects in neuropathic rats via inhibiting class Ⅱ MHC associated response and regulating synaptic plasticity

BACKGROUND

Neuropathic pain (NP) due to nerve injury, disrupts neural plasticity by triggering the release of inflammatory mediators. Alongside the hypothesis that neuro-inflammation contributes to this disruption, Andrographolide (Andro), a traditional bioactive compound derived from Andrographis paniculata, has garnered attention for its potent anti-inflammatory properties. However, whether Andro could ameliorate NP by regulating neuroinflammation remains unknown.

PURPOSE

This study aimed to investigate whether and how Andro regulates neuroinflammation and alleviates NP.

METHODS

The analgesic effects of Andro on NP were evaluated using both the spinal nerve ligation (SNL) and formalin rat models. A combination of network pharmacology, RNA sequencing, and experimental validation was employed to elucidate the underlying mechanism behind Andro's analgesic effects. Additionally, various techniques such as functional ultrasound, immunohistochemistry, quantitative real-time polymerase chain reaction (qPCR), patch clamp, and electron microscopy were employed to investigate the specific neural cell types, neural functions, and changes in neural plasticity influenced by Andro.

RESULTS

Network pharmacology analysis unveiled the crucial roles played by shared targets of Andro and pain in regulating pain-related inflammation, including microglia activation, neuroinflammation, immune modulation, and synaptic transmission. Furthermore, we confirmed Andro's superior efficacy in pain relief compared to the traditional analgesic drug, Gabapentin. In these models, Andro was observed to modulate the haemodynamic response triggered by SNL. Transcriptome analysis and molecular docking studies indicated the involvement of major histocompatibility complex class II (MHCII) genes (Db1, Da, and Bb). Electron microscopy revealed improvements in synaptic ultrastructure, and electrophysiological investigations showed a selective reduction in glutamatergic transmission in neuropathic rats after following Andro treatment. The integration of systems pharmacology analysis and biological validation collectively demonstrated that the mechanism of pain relief involves immune modulation, enhancement of synaptic plasticity, and precise regulation of excitatory neurotransmission.

CONCLUSION

In conclusion, this study has demonstrated that Andro, by targeting MHCII genes, may serve as a promising therapeutic candidate for neuropathic pain.

Parabrachial neurons promote nociplastic pain

The parabrachial nucleus (PBN) in the dorsal pons responds to bodily threats and transmits alarm signals to the forebrain. Parabrachial neuron activity is enhanced during chronic pain, and inactivation of PBN neurons in mice prevents the establishment of neuropathic, chronic pain symptoms. Chemogenetic or optogenetic activation of all glutamatergic neurons in the PBN, or just the subpopulation that expresses the Calca gene, is sufficient to establish pain phenotypes, including long-lasting tactile allodynia, that scale with the extent of stimulation, thereby promoting nociplastic pain, defined as diffuse pain without tissue inflammation or nerve injury. This review focuses on the role(s) of molecularly defined PBN neurons and the downstream nodes in the brain that contribute to establishing nociplastic pain.

The efficacy of the analgesic GlyT2 inhibitor, ORG25543, is determined by two connected allosteric sites

Glycine Transporter 2 (GlyT2) inhibitors have shown considerable potential as analgesics for the treatment of neuropathic pain but also display considerable side effects. One potential source of side effects is irreversible inhibition. In this study, we have characterized the mechanism of ORG25543 inhibition of GlyT2 by first considering three potential ligand binding sites on GlyT2-the substrate site, the vestibule allosteric site and the lipid allosteric site. The three sites were tested using a combination of molecular dynamics simulations and analysis of the inhibition of glycine transport of a series point mutated GlyT2 using electrophysiological methods. We demonstrate that the lipid allosteric site on GlyT2 is the most likely binding site for ORG25543. We also demonstrate that cholesterol derived from the cell membrane can form specific interactions with inhibitor-bound transporters to form an allosteric network of regulatory sites. These observations will guide the future design of GlyT2 inhibitors with the objective of minimising on-target side effects and improving the therapeutic window for the treatment of patients suffering from neuropathic pain.

Targeting spinal mechanistic target of rapamycin complex 2 alleviates inflammatory and neuropathic pain

The development and maintenance of chronic pain involves the reorganization of spinal nociceptive circuits. The mechanistic target of rapamycin complex 2 (mTORC2), a central signaling hub that modulates both actin-dependent structural changes and mTORC1-dependent mRNA translation, plays key roles in hippocampal synaptic plasticity and memory formation. However, its function in spinal plasticity and chronic pain is poorly understood. Here we show that pharmacological activation of spinal mTORC2 induces pain hypersensitivity, whereas its inhibition, using downregulation of the mTORC2-defining component Rictor, alleviates both inflammatory and neuropathic pain. Cell-type-specific deletion of Rictor showed that the selective inhibition of mTORC2 in a subset of excitatory neurons impairs spinal synaptic potentiation and alleviates inflammation-induced mechanical and thermal hypersensitivity, and nerve injury-induced heat hyperalgesia. The ablation of mTORC2 in inhibitory interneurons strongly alleviated nerve injury-induced mechanical hypersensitivity. Our findings reveal the role of mTORC2 in chronic pain and highlight its cell-type-specific functions in mediating pain hypersensitivity in response to peripheral inflammation and nerve injury.

Behavioral and neural correlates of diverse conditioned fear responses in male and female rats

Pavlovian fear conditioning is a widely used tool that models associative learning in rodents. For decades the field has used predominantly male rodents and focused on a sole conditioned fear response: freezing. However, recent work from our lab and others has identified darting as a female-biased conditioned response, characterized by an escape-like movement across a fear conditioning chamber. It is also accompanied by a behavioral phenotype: Darters reliably show decreased freezing compared to Non-darters and males and reach higher velocities in response to the foot shock ("shock response"). However, the relationship between shock response and conditioned darting is not known. This study investigated if this link is due to differences in general processing of aversive stimuli between Darters, Non-darters and males. Across a variety of modalities, including corticosterone measures, the acoustic startle test, and sensitivity to thermal pain, Darters were found not to be more reactive or sensitive to aversive stimuli, and, in some cases, they appear less reactive to Non-darters and males. Analyses of cFos activity in regions involved in pain and fear processing following fear conditioning identified discrete patterns of expression among Darters, Non-darters, and males exposed to low and high intensity foot shocks. The results from these studies further our understanding of the differences between Darters, Non-darters and males and highlight the importance of studying individual differences in fear conditioning as indicators of fear state.

Oleic acid released by sensory neurons inhibits TRPV1-mediated thermal hypersensitivity via GPR40

Noxious stimuli activate nociceptive sensory neurons, causing action potential firing and the release of diverse signaling molecules. Several peptides have already been identified to be released by sensory neurons and shown to modulate inflammatory responses and inflammatory pain. However, it is still unclear whether lipid mediators can be released upon sensory neuron activation to modulate intercellular communication. Here, we analyzed the lipid secretome of capsaicin-stimulated nociceptive neurons with LC-HRMS, revealing that oleic acid is strongly released from sensory neurons by capsaicin. We further demonstrated that oleic acid inhibits capsaicin-induced calcium transients in sensory neurons and reverses bradykinin-induced TRPV1 sensitization by a calcineurin (CaN) and GPR40 (FFAR1) dependent pathway. Additionally, oleic acid alleviated zymosan-mediated thermal hypersensitivity via the GPR40, suggesting that the capsaicin-mediated oleic acid release from sensory neurons acts as a protective and feedback mechanism, preventing sensory neurons from nociceptive overstimulation via the GPR40/CaN/TRPV1-axis.

IRF7 overexpression alleviates CFA-induced inflammatory pain by inhibiting nuclear factor-κB activation and pro-inflammatory cytokines expression in rats

BACKGROUND

It is known that nerve signals arising from sites of inflammation lead to persistent changes in the spinal cord and contribute to the amplification and persistence of pain. Nevertheless, the underlying mechanisms have not yet been completely elucidated. We identified differentially expressed genes in the lumbar (L4-L6) segment of the spinal cord from complete Freund's adjuvant (CFA) rats compared to control animals via high throughput sequencing. Based on differential gene expression analysis, we selected interferon regulatory factor 7 (IRF7) for follow-up experiments to explore its antinociceptive potential.

METHODS

An animal model of inflammatory pain was induced by intraplantar injection of CFA. We evaluated the effects of adeno-associated viral (AAV)-mediated overexpression of IRF7 in the spinal cord on pain-related behavior after CFA injection. Moreover, the activation of the nuclear factor-κB (NF-κB) and the expression of inflammatory cytokines were investigated to understand the underlying mechanisms related to the contribution of IRF7 to inflammatory pain.

RESULTS

CFA intraplantar injection caused a significant decrease in the level of spinal IRF7, which is mainly expressed in the dorsal horn neurons and astrocytes. Moreover, IRF7 overexpression significantly attenuated pain-related behaviors, as well as the activity of NF-κB/p65 and the production of interleukin-1β (IL-1β), interleukin-6 (IL-6), and tumor necrosis factor-α (TNF-α) in the spinal cord of CFA rats.

CONCLUSIONS

Our data indicated that spinal IRF7 plays an important role in the regulation of inflammatory pain. Thus, IRF7 overexpression at the spinal cord level might represent a potential target for the treatment of inflammatory pain.

Metabolomic and lipidomic profiling of the spinal cord in type 2 diabetes mellitus rats with painful neuropathy

In this paper we investigated lipid and metabolite changes in diabetic neuropathy, using untargeted lipidomics and metabolomics analyses of the spinal cords from streptozotocin-treated diabetic rats.170 metabolites and 45 lipids were dysregulated in the painful diabetic neuropathy (PDN) phase. Pathway enrichment analysis revealed perturbations in starch and sucrose, tryptophan, pyrimidine, cysteine and methionine, thiamine, tyrosine, and nucleotides. The disturbance of tyrosine, tryptophan, methionine, triacylglycerol, and phosphatidylethanolamine metabolism indicated that pathological mechanisms in the PDN involved energy metabolism, oxidative stress, and neural reparative regeneration. These revelations offered potential biomarkers for PDN and enriched the comprehension of the complex molecular mechanisms characterizing PDN, establishing a solid foundation for subsequent inquiries into neural convalescence and recovery after PDN.

Spinal Nmur2-positive Neurons Play a Crucial Role in Mechanical Itch

The dorsal spinal cord is crucial for the transmission and modulation of multiple somatosensory modalities, such as itch, pain, and touch. Despite being essential for the well-being and survival of an individual, itch and pain, in their chronic forms, have increasingly been recognized as clinical problems. Although considerable progress has been made in our understanding of the neurochemical processing of nociceptive and chemical itch sensations, the neural substrate that is crucial for mechanical itch processing is still unclear. Here, using genetic and functional manipulation, we identified a population of spinal neurons expressing neuromedin U receptor 2 (Nmur2+) as critical elements for mechanical itch. We found that spinal Nmur2+ neurons are predominantly excitatory neurons, and are enriched in the superficial laminae of the dorsal horn. Pharmacogenetic activation of cervical spinal Nmur2+ neurons evoked scratching behavior. Conversely, the ablation of these neurons using a caspase-3-based method decreased von Frey filament-induced scratching behavior without affecting responses to other somatosensory modalities. Similarly, suppressing the excitability of cervical spinal Nmur2+ neurons via the overexpression of functional Kir2.1 potassium channels reduced scratching in response to innocuous mechanical stimuli, but not to pruritogen application. At the lumbar level, pharmacogenetic activation of these neurons evoked licking and lifting behaviors. However, ablating these neurons did not affect the behavior associated with acute pain. Thus, these results revealed the crucial role of spinal Nmur2+ neurons in mechanical itch. Our study provides important insights into the neural basis of mechanical itch, paving the way for developing novel therapies for chronic itch. PERSPECTIVE: Excitatory Nmur2+ neurons in the superficial dorsal spinal cord are essential for mechanical but not chemical itch information processing. These spinal Nmur2+ neurons represent a potential cellular target for future therapeutic interventions against chronic itch. Spinal and supraspinal Nmur2+ neurons may play different roles in pain signal processing.

MicroRNA let-7b enhances spinal cord nociceptive synaptic transmission and induces acute and persistent pain through neuronal and microglial signaling

ABSTRACT

Secreted microRNAs (miRNAs) have been detected in various body fluids including the cerebrospinal fluid, yet their direct role in regulating synaptic transmission remains uncertain. We found that intrathecal injection of low dose of let-7b (1 μg) induced short-term (<24 hours) mechanical allodynia and heat hyperalgesia, a response that is compromised in Tlr7-/- or Trpa1-/- mice. Ex vivo and in vivo calcium imaging in GCaMP6-report mice revealed increased calcium signal in spinal cord afferent terminals and doral root ganglion/dorsal root ganglia neurons following spinal perfusion and intraplantar injection of let-7b. Patch-clamp recordings also demonstrated enhanced excitatory synaptic transmission (miniature excitatory postsynaptic currents [EPSCs]) in spinal nociceptive neurons following let-7b perfusion or optogenetic activation of axonal terminals. The elevation in spinal calcium signaling and EPSCs was dependent on the presence of toll-like receptor-7 (TLR7) and transient receptor potential ion channel subtype A1 (TRPA1). In addition, endogenous let-7b is enriched in spinal cord synaptosome, and peripheral inflammation increased let-7b in doral root ganglion/dorsal root ganglia neurons, spinal cord tissue, and the cerebrospinal fluid. Notably, let-7b antagomir inhibited inflammatory pain and inflammation-induced synaptic plasticity (EPSC increase), suggesting an endogenous role of let-7b in regulating pain and synaptic transmission. Furthermore, intrathecal injection of let-7b, at a higher dose (10 μg), induced persistent mechanical allodynia for >2 weeks, which was abolished in Tlr7-/- mice. The high dose of let-7b also induced microgliosis in the spinal cord. Of interest, intrathecal minocycline only inhibited let-7b-induced mechanical allodynia in male but not female mice. Our findings indicate that the secreted microRNA let-7b has the capacity to provoke pain through both neuronal and glial signaling, thereby establishing miRNA as an emerging neuromodulator.

Pharmacology of PACAP and VIP receptors in the spinal cord highlights the importance of the PAC1 receptor

BACKGROUND AND PURPOSE

The spinal cord is a key structure involved in the transmission and modulation of pain. Pituitary adenylate cyclase-activating peptide (PACAP) and vasoactive intestinal peptide (VIP), are expressed in the spinal cord. These peptides activate G protein-coupled receptors (PAC1, VPAC1 and VPAC2) that could provide targets for the development of novel pain treatments. However, it is not clear which of these receptors are expressed within the spinal cord and how these receptors signal.

EXPERIMENTAL APPROACH

Dissociated rat spinal cord cultures were used to examine agonist and antagonist receptor pharmacology. Signalling profiles were determined for five signalling pathways. The expression of different PACAP and VIP receptors was then investigated in mouse, rat and human spinal cords using immunoblotting and immunofluorescence.

KEY RESULTS

PACAP, but not VIP, potently stimulated cAMP, IP1 accumulation and ERK and cAMP response element-binding protein (CREB) but not Akt phosphorylation in spinal cord cultures. Signalling was antagonised by M65 and PACAP6-38. PACAP-27 was more effectively antagonised than either PACAP-38 or VIP. The patterns of PAC1 and VPAC2 receptor-like immunoreactivity appeared to be distinct in the spinal cord.

CONCLUSIONS AND IMPLICATIONS

The pharmacological profile in the spinal cord suggested that a PAC1 receptor is the major functional receptor subtype present and thus likely mediates the nociceptive effects of the PACAP family of peptides in the spinal cord. However, the potential expression of both PAC1 and VPAC2 receptors in the spinal cord highlights that these receptors may play differential roles and are both possible therapeutic targets.

Neph1 is required for neurite branching and is negatively regulated by the PRRXL1 homeodomain factor in the developing spinal cord dorsal horn

The cell-adhesion molecule NEPH1 is required for maintaining the structural integrity and function of the glomerulus in the kidneys. In the nervous system of Drosophila and C. elegans, it is involved in synaptogenesis and axon branching, which are essential for establishing functional circuits. In the mammalian nervous system, the expression regulation and function of Neph1 has barely been explored. In this study, we provide a spatiotemporal characterization of Neph1 expression in mouse dorsal root ganglia (DRGs) and spinal cord. After the neurogenic phase, Neph1 is broadly expressed in the DRGs and in their putative targets at the dorsal horn of the spinal cord, comprising both GABAergic and glutamatergic neurons. Interestingly, we found that PRRXL1, a homeodomain transcription factor that is required for proper establishment of the DRG-spinal cord circuit, prevents a premature expression of Neph1 in the superficial laminae of the dorsal spinal cord at E14.5, but has no regulatory effect on the DRGs or on either structure at E16.5. By chromatin immunoprecipitation analysis of the dorsal spinal cord, we identified four PRRXL1-bound regions within the Neph1 introns, suggesting that PRRXL1 directly regulates Neph1 transcription. We also showed that Neph1 is required for branching, especially at distal neurites. Together, our work showed that Prrxl1 prevents the early expression of Neph1 in the superficial dorsal horn, suggesting that Neph1 might function as a downstream effector gene for proper assembly of the DRG-spinal nociceptive circuit.

Targeting TANK-binding kinase 1 attenuates painful diabetic neuropathy via inhibiting microglia pyroptosis

BACKGROUND

Painful diabetic neuropathy (PDN) is closely linked to inflammation, which has been demonstrated to be associated with pyroptosis. Emerging evidence has implicated TANK-binding kinase 1 (TBK1) in various inflammatory diseases. However, it remains unknown whether activated TBK1 causes hyperalgesia via pyroptosis.

METHODS

PDN mice model of type 1 or type 2 diabetic was induced by C57BL/6J or BKS-DB mice with Lepr gene mutation. For type 2 diabetes PDN model, TBK1-siRNA, Caspase-1 inhibitor Ac-YVAD-cmk or TBK1 inhibitor amlexanox (AMX) were delivered by intrathecal injection or intragastric administration. The pain threshold and plantar skin blood perfusion were evaluated through animal experiments. The assessments of spinal cord, dorsal root ganglion, sciatic nerve, plantar skin and serum included western blotting, immunofluorescence, ELISA, and transmission electron microscopy.

RESULTS

In the PDN mouse model, we found that TBK1 was significantly activated in the spinal dorsal horn (SDH) and mainly located in microglia, and intrathecal injection of chemically modified TBK1-siRNA could improve hyperalgesia. Herein, we described the mechanism that TBK1 could activate the noncanonical nuclear factor κB (NF-κB) pathway, mediate the activation of NLRP3 inflammasome, trigger microglia pyroptosis, and ultimately induce PDN, which could be reversed following TBK1-siRNA injection. We also found that systemic administration of AMX, a TBK1 inhibitor, could effectively improve peripheral nerve injury. These results revealed the key role of TBK1 in PDN and that TBK1 inhibitor AMX could be a potential strategy for treating PDN.

CONCLUSIONS

Our findings revealed a novel causal role of TBK1 in pathogenesis of PDN, which raises the possibility of applying amlexanox to selectively target TBK1 as a potential therapeutic strategy for PDN.

Communicating pain: emerging axonal signaling in peripheral neuropathic pain

Peripheral nerve damage often leads to the onset of neuropathic pain (NeuP). This condition afflicts millions of people, significantly burdening healthcare systems and putting strain on families' financial well-being. Here, we will focus on the role of peripheral sensory neurons, specifically the Dorsal Root Ganglia neurons (DRG neurons) in the development of NeuP. After axotomy, DRG neurons activate regenerative signals of axons-soma communication to promote a gene program that activates an axonal branching and elongation processes. The results of a neuronal morphological cytoskeleton change are not always associated with functional recovery. Moreover, any axonal miss-targeting may contribute to NeuP development. In this review, we will explore the epidemiology of NeuP and its molecular causes at the level of the peripheral nervous system and the target organs, with major focus on the neuronal cross-talk between intrinsic and extrinsic factors. Specifically, we will describe how failures in the neuronal regenerative program can exacerbate NeuP.

Differential modulation of corticomotor excitability in older compared to young adults following a single bout of strength -exercise

Evidence shows corticomotor plasticity diminishes with age. Nevertheless, whether strength-training, a proven intervention that induces corticomotor plasticity in younger adults, also takes effect in older adults, remains untested. This study examined the effect of a single-session of strength-exercise on corticomotor plasticity in older and younger adults. Thirteen older adults (72.3 ± 6.5 years) and eleven younger adults (29.9 ± 6.9 years), novice to strength-exercise, participated. Strength-exercise involved four sets of 6-8 repetitions of a dumbbell biceps curl at 70-75% of their one-repetition maximum (1-RM). Muscle strength, cortical, corticomotor and spinal excitability, before and up to 60-minutes after the strength-exercise session were assessed. We observed significant changes over time (p < 0.05) and an interaction between time and age group (p < 0.05) indicating a decrease in corticomotor excitability (18% p < 0.05) for older adults at 30- and 60-minutes post strength-exercise and an increase (26% and 40%, all p < 0.05) in younger adults at the same time points. Voluntary activation (VA) declined in older adults immediately post and 60-minutes post strength-exercise (36% and 25%, all p < 0.05). Exercise had no effect on the cortical silent period (cSP) in older adults however, in young adults cSP durations were shorter at both 30- and 60- minute time points (17% 30-minute post and 9% 60-minute post, p < 0.05). There were no differences in short-interval cortical inhibition (SICI) or intracortical facilitation (ICF) between groups. Although the corticomotor responses to strength-exercise were different within groups, overall, the neural responses seem to be independent of age.

Translational control in the spinal cord regulates gene expression and pain hypersensitivity in the chronic phase of neuropathic pain

Sensitization of spinal nociceptive circuits plays a crucial role in neuropathic pain. This sensitization depends on new gene expression that is primarily regulated via transcriptional and translational control mechanisms. The relative roles of these mechanisms in regulating gene expression in the clinically relevant chronic phase of neuropathic pain are not well understood. Here, we show that changes in gene expression in the spinal cord during the chronic phase of neuropathic pain are substantially regulated at the translational level. Downregulating spinal translation at the chronic phase alleviated pain hypersensitivity. Cell-type-specific profiling revealed that spinal inhibitory neurons exhibited greater changes in translation after peripheral nerve injury compared to excitatory neurons. Notably, increasing translation selectively in all inhibitory neurons or parvalbumin-positive (PV+) interneurons, but not excitatory neurons, promoted mechanical pain hypersensitivity. Furthermore, increasing translation in PV+ neurons decreased their intrinsic excitability and spiking activity, whereas reducing translation in spinal PV+ neurons prevented the nerve injury-induced decrease in excitability. Thus, translational control mechanisms in the spinal cord, particularly in inhibitory neurons, play a role in mediating neuropathic pain hypersensitivity.

Topography of the GLP-1/GLP-1 receptor system in the spinal cord of male mice

Glucagon-like peptide-1 receptor (GLP-1R) agonists are now commonly used to treat type 2 diabetes and obesity. GLP-1R signaling in the spinal cord has been suggested to account for the mild tachycardia caused by GLP-1R agonists, and may also be involved in the therapeutic effects of these drugs. However, the neuroanatomy of the GLP-1/GLP-1R system in the spinal cord is still poorly understood. Here we applied in situ hybridization and immunohistochemistry to characterize this system, and its relation to cholinergic neurons. GLP-1R transcript and protein were expressed in neuronal cell bodies across the gray matter, in matching distribution patterns. GLP-1R-immunolabeling was also robust in dendrites and axons, especially in laminae II-III in the dorsal horn. Cerebrospinal fluid-contacting neurons expressed GLP-1R protein at exceedingly high levels. Only small subpopulations of cholinergic neurons expressed GLP-1R, including a subset of sympathetic preganglionic neurons at the rostral tip of the intermediolateral nucleus. GLP-1 axons innervated all regions where GLP-1R neurons were distributed, except laminae II-III. Scattered preproglucagon (Gcg) mRNA-expressing neurons were identified in the cervical and lumbar enlargements. The results will facilitate further studies on how GLP-1 regulates the sympathetic system and other autonomic and somatic functions via the spinal cord.

Functional remodeling of presynaptic voltage-gated calcium channels in superficial layers of the dorsal horn during neuropathic pain

N- and P/Q-type voltage-gated Ca2+ channels are critical for synaptic transmission. While their expression is increased in the dorsal root ganglion (DRG) neuron cell bodies during neuropathic pain conditions, less is known about their synaptic remodeling. Here, we combined genetic tools with 2-photon Ca2+ imaging to explore the functional remodeling that occurs in central presynaptic terminals of DRG neurons during neuropathic pain. We imaged GCaMP6s fluorescence responses in an ex vivo spinal cord preparation from mice expressing GCaMP6s in Trpv1-Cre lineage nociceptors. We show that Ca2+ transient amplitude is increased in central terminals of these neurons after spared nerve injury, and that this increase is mediated by both N- and P/Q-type channels. We found that GABA-B receptor-dependent inhibition of Ca2+ transients was potentiated in the superficial layer of the dorsal horn. Our results provide direct evidence toward nerve injury-induced functional remodeling of presynaptic Ca2+ channels in Trpv1-lineage nociceptor terminals.

Frequency-Dependent Neural Modulation of Dorsal Horn Neurons by Kilohertz Spinal Cord Stimulation in Rats

Kilohertz high-frequency spinal cord stimulation (kHF-SCS) is a rapidly advancing neuromodulatory technique in the clinical management of chronic pain. However, the precise cellular mechanisms underlying kHF-SCS-induced paresthesia-free pain relief, as well as the neural responses within spinal pain circuits, remain largely unexplored. In this study, using a novel preparation, we investigated the impact of varying kilohertz frequency SCS on dorsal horn neuron activation. Employing calcium imaging on isolated spinal cord slices, we found that extracellular electric fields at kilohertz frequencies (1, 3, 5, 8, and 10 kHz) induce distinct patterns of activation in dorsal horn neurons. Notably, as the frequency of extracellular electric fields increased, there was a clear and significant monotonic escalation in neuronal activity. This phenomenon was observed not only in superficial dorsal horn neurons, but also in those located deeper within the dorsal horn. Our study demonstrates the unique patterns of dorsal horn neuron activation in response to varying kilohertz frequencies of extracellular electric fields, and we contribute to a deeper understanding of how kHF-SCS induces paresthesia-free pain relief. Furthermore, our study highlights the potential for kHF-SCS to modulate sensory information processing within spinal pain circuits. These insights pave the way for future research aimed at optimizing kHF-SCS parameters and refining its therapeutic applications in the clinical management of chronic pain.

C-LTMRs mediate wet dog shakes via the spinoparabrachial pathway

Mammals perform rapid oscillations of their body- "wet dog shakes" -to remove water and irritants from their back hairy skin. The somatosensory mechanisms underlying this stereotypical behavior are unknown. We report that Piezo2-dependent mechanosensation mediates wet dog shakes evoked by water or oil droplets applied to hairy skin of mice. Unmyelinated low-threshold mechanoreceptors (C-LTMRs) were strongly activated by oil droplets and their optogenetic activation elicited wet dog shakes. Ablation of C-LTMRs attenuated this behavior. Moreover, C-LTMRs synaptically couple to spinoparabrachial (SPB) neurons, and optogenetically inhibiting SPB neuron synapses and excitatory neurons in the parabrachial nucleus impaired both oil droplet- and C-LTMR-evoked wet dog shakes. Thus, a C-LTMR- spinoparabrachial pathway mediates wet dog shakes for rapid and effective removal of foreign particles from back hairy skin.

Mechanisms for dysregulation of excitatory-inhibitory balance underlying allodynia in dorsal horn neural subcircuits

Chronic pain is a wide-spread condition that is debilitating and expensive to manage, costing the United States alone around $600 billion in 2010. In a common type of chronic pain called allodynia, non-painful stimuli produce painful responses with highly variable presentations across individuals. While the specific mechanisms remain unclear, allodynia is hypothesized to be caused by the dysregulation of excitatory-inhibitory (E-I) balance in pain-processing neural circuitry in the dorsal horn of the spinal cord. In this work, we analyze biophysically-motivated subcircuit structures that represent common motifs in neural circuits in layers I-II of the dorsal horn. These circuits are hypothesized to be part of the neural pathways that mediate two different types of allodynia: static and dynamic. We use neural firing rate models to describe the activity of populations of excitatory and inhibitory interneurons within each subcircuit. By accounting for experimentally-observed responses under healthy conditions, we specify model parameters defining populations of subcircuits that yield typical behavior under normal conditions. Then, we implement a sensitivity analysis approach to identify the mechanisms most likely to cause allodynia-producing dysregulation of the subcircuit's E-I signaling. We find that disruption of E-I balance generally occurs either due to downregulation of inhibitory signaling so that excitatory neurons are "released" from inhibitory control, or due to upregulation of excitatory neuron responses so that excitatory neurons "escape" their inhibitory control. Which of these mechanisms is most likely to occur, the subcircuit components involved in the mechanism, and the proportion of subcircuits exhibiting the mechanism can vary depending on the subcircuit structure. These results suggest specific hypotheses about diverse mechanisms that may be most likely responsible for allodynia, thus offering predictions for the high interindividual variability observed in allodynia and identifying targets for further experimental studies on the underlying mechanisms of this chronic pain condition.

Deep sequencing of Phox2a nuclei reveals five classes of anterolateral system neurons

The anterolateral system (ALS) is a major ascending pathway from the spinal cord that projects to multiple brain areas and underlies the perception of pain, itch, and skin temperature. Despite its importance, our understanding of this system has been hampered by the considerable functional and molecular diversity of its constituent cells. Here, we use fluorescence-activated cell sorting to isolate ALS neurons belonging to the Phox2a-lineage for single-nucleus RNA sequencing. We reveal five distinct clusters of ALS neurons (ALS1-5) and document their laminar distribution in the spinal cord using in situ hybridization. We identify three clusters of neurons located predominantly in laminae I-III of the dorsal horn (ALS1-3) and two clusters with cell bodies located in deeper laminae (ALS4 and ALS5). Our findings reveal the transcriptional logic that underlies ALS neuronal diversity in the adult mouse and uncover the molecular identity of two previously identified classes of projection neurons. We also show that these molecular signatures can be used to target groups of ALS neurons using retrograde viral tracing. Overall, our findings provide a valuable resource for studying somatosensory biology and targeting subclasses of ALS neurons.