Cholesterol-dependent LXR transcription factor activity represses pronociceptive effects of estrogen in sensory neurons and pain induced by myelin basic protein fragments

Background

A bioactive myelin basic protein (MBP) fragment, comprising MBP84-104, is released in sciatic nerve after chronic constriction injury (CCI). Intraneural injection (IN) of MBP84-104 in an intact sciatic nerve is sufficient to induce persistent neuropathic pain-like behavior via robust transcriptional remodeling at the injection site and ipsilateral dorsal root ganglia (DRG) and spinal cord. The sex (female)-specific pronociceptive activity of MBP84-104 associates with sex-specific changes in cholesterol metabolism and activation of estrogen receptor (ESR)1 signaling.

Methods

In male and female normal and post-CCI rat sciatic nerves, we assessed: (i) cholesterol precursor and metabolite levels by lipidomics; (ii) MBP84-104 interactors by mass spectrometry of MBP84-104 pull-down; and (iii) liver X receptor (LXR)α protein expression by immunoblotting. To test the effect of LXRα stimulation on IN MBP84-104-induced mechanical hypersensitivity, the LXRα expression was confirmed along the segmental neuraxis, in DRG and spinal cord, followed by von Frey testing of the effect of intrathecally administered synthetic LXR agonist, GW3965. In cultured male and female rat DRGs exposed to MBP84-104 and/or estrogen treatments, transcriptional effect of LXR stimulation by GW3965 was assessed on downstream cholesterol transporter Abc, interleukin (IL)-6, and pronociceptive Cacna2d1 gene expression.

Results

CCI regulated LXRα ligand and receptor levels in nerves of both sexes, with cholesterol precursors, desmosterol and 7-DHC, and oxysterol elevated in females relative to males. MBP84-104 interacted with nuclear receptor coactivator (Ncoa)1, known to activate LXRα, injury-specific in nerves of both sexes. LXR stimulation suppressed ESR1-induced IL-6 and Cacna2d1 expression in cultured DRGs of both sexes and attenuated MBP84-104-induced pain in females.

Conclusion

The injury-released bioactive MBP fragments induce pronociceptive changes by selective inactivation of nuclear transcription factors, including LXRα. By Ncoa1 sequestration, bioactive MBP fragments render LXRα function to counteract pronociceptive activity of estrogen/ESR1 in sensory neurons. This effect of MBP fragments is prevalent in females due to high circulating estrogen levels in females relative to males. Restoring LXR activity presents a promising therapeutic strategy in management of neuropathic pain induced by bioactive MBP.

Sonic hedgehog restrains the ubiquitin-dependent degradation of SP1 to inhibit neuronal/glial senescence associated phenotypes in chemotherapy-induced peripheral neuropathy via the TRIM25-CXCL13 axis

INTRODUCTION

Chemotherapy-induced peripheral neuropathy (CIPN) is a common complication that affects an increasing number of cancer survivors. However, the current treatment options for CIPN are limited. Paclitaxel (PTX) is a widely used chemotherapeutic drug that induces senescence in cancer cells. While previous studies have demonstrated that Sonic hedgehog (Shh) can counteract cellular dysfunction during aging, its role in CIPN remains unknown.

OBJECTIVES

Herein, the aim of this study was to investigate whether Shh activation could inhibits neuronal/glial senescence and alleviates CIPN.

METHODS

We treated ND7/23 neuronal cells and RSC96 Schwann cells with two selective Shh activators (purmorphamine [PUR] and smoothened agonist [SAG]) in the presence of PTX. Additionally, we utilized a CIPN mouse model induced by PTX injection. To assess cellular senescence, we performed a senescence-associated β-galactosidase (SA-β-gal) assay, measured reactive oxygen species (ROS) levels, and examined the expression of P16, P21, and γH2AX. To understand the underlying mechanisms, we conducted ubiquitin assays, LC-MS/MS, H&E staining, and assessed protein expression through Western blotting and immunofluorescence staining.

RESULTS

In vitro, we observed that Shh activation significantly alleviated the senescence-related decline in multiple functions included SA-β-gal activity, expression of P16 and P21, cell viability, and ROS accumulation in DRG sensory neurons and Schwann cells after PTX exposure. Furthermore, our in vivo experiments demonstrated that Shh activation significantly reduced axonal degeneration, demyelination, and improved nerve conduction. Mechanistically, we discovered that PTX reduced the protein level of SP1, which was ubiquitinated by the E3 ligase TRIM25 at the lysine 694 (K694), leading to increased CXCL13 expression, and we found that Shh activation inhibited PTX-induced neuronal/glial senescence and CIPN through the TRIM25-SP1-CXCL13 axis.

CONCLUSION

These findings provide evidence for the role of PTX-induced senescence in DRG sensory neurons and Schwann cells, suggesting that Shh could be a potential therapeutic target for CIPN.

Widespread latent hyperactivity of nociceptors outlasts enhanced avoidance behavior following incision injury

Nociceptors with somata in dorsal root ganglia (DRGs) exhibit an unusual readiness to switch from an electrically silent state to a hyperactive state of tonic, nonaccommodating, low-frequency, irregular discharge of action potentials (APs). Ongoing activity (OA) during this state is present in vivo in rats months after spinal cord injury (SCI), and has been causally linked to SCI pain. OA induced by various neuropathic conditions in rats, mice, and humans is retained in nociceptor somata after dissociation and culturing, providing a powerful tool for investigating its mechanisms and functions. An important question is whether similar nociceptor OA is induced by painful conditions other than neuropathy. The present study shows that probable nociceptors dissociated from DRGs of rats subjected to postsurgical pain (induced by plantar incision) exhibit OA. The OA was most apparent when the soma was artificially depolarized to a level within the normal range of membrane potentials where large, transient depolarizing spontaneous fluctuations (DSFs) can approach AP threshold. This latent hyperactivity persisted for at least 3 weeks, whereas behavioral indicators of affective pain - hindpaw guarding and increased avoidance of a noxious substrate in an operant conflict test - persisted for 1 week or less. An unexpected discovery was latent OA in neurons from thoracic DRGs that innervate dermatomes distant from the injured tissue. The most consistent electrophysiological alteration associated with OA was enhancement of DSFs. Potential in vivo functions of widespread, low-frequency nociceptor OA consistent with these and other findings are to amplify hyperalgesic priming and to drive anxiety-related hypervigilance.

Cortexin® Ameliorates High Glucose-Induced Neuropathy in Cultured Rat Sensory Neurons

INTRODUCTION

The aim of this study was to investigate whether Cortexin®, a brain peptide-containing agent, has any mitigating effect on high glucose-induced neuropathy, using primary cultured rat sensory neurons.

MATERIALS AND METHODS

Dorsal root ganglia (DRG) were excised from decapitated adult rats. Individual neurons were isolated following enzymatic and mechanical procedures. Cells were seeded on E-plate® with gold microelectrodes and maintained in conventional culture media in a CO2 incubator at 37°C. After allowing for 24 h for cell adhesion and recovery from acute enzymatic trauma, neurons were exposed to high glucose (HpG) in the absence and presence of different concentrations of Cortexin® (2-40 µg/mL). Neuroprotective effects were followed with the Real-Time Cell Analyzer® by utilizing measurement of Cell Index, a parameter representing cell viability, cell attachment, and neurite outgrowth.

RESULTS

Exposure of DRGs to HpG (50 mm) caused a rapid and sustained decrease in the mean area under the curve (AUC, values derived from time vs. Cell Index curve) compared to the mean AUC values in normoglycemic (NG) wells. Co-treatment with Cortexin® attenuated this HpG-induced effect, in a concentration-dependent manner (NG: 1.00 ± 0.00 vs. HG: 0.18 ± 0.02, p < 0.05; and HpG + Cortexin® [40 µg/mL]: 0.66 ± 0.17, p = 0.002 versus HpG). In normoglycemic conditions, Cortexin® treatment led to a concentration-dependent increase in the mean AUC values.

CONCLUSIONS

Data from this in vitro study suggest that Cortexin® has potential neuroprotective effects against chronic hyperglycemic insult in rat sensory neurons. Our results warrant further in vivo studies and may have clinical implications for diabetes-associated peripheral neuropathy.

Effects of Semaphorin3A on the growth of sensory and motor neurons

After peripheral nerve injury, motor and sensory axons can regenerate, but the inaccurate reinnervation of the target leads to poor functional recovery. Schwann cells (SCs) express sensory and motor phenotypes associated with selective regeneration. Semaphorin 3A (Sema3A) is an axonal chemorepellent that plays an essential role in axon growth. SCs can secret Sema3A, and Sema3A presents a different expression pattern at the proximal and distal ends of injured sensory and motor nerves. Hence, in our study, the protein expression and secretion of Sema3A in sensory and motor SCs and the expression of its receptor Neuropilin-1 (Nrp1) in dorsal root ganglia (DRG) sensory neurons (SNs) and spinal cord motor neurons (MNs) were detected by Western blot and ELISA. The effect of Sema3A at different concentrations on neurite growth of sensory and motor neurons was observed by immunostaining. Also, by blocking the Nrp1 receptor on neurons, the effect of Sema3A on neurite growth was observed. Finally, we observed the neurite growth of sensory and motor neurons cocultured with Sema3A siRNA transfected SCs by immunostaining. The results suggested that the expression and secretion of Sema3A in sensory SCs are more significant than that in motor SCs, and the expression of its receptor Nrp1 in SNs is higher than in MNs. Sema3A could inhibit the neurite growth of sensory and motor neurons via Nrp1, and Sema3A has a more substantial effect on the neurite growth of SNs. These data provide evidence that SC-secreted Sema3A might play a role in selective regeneration by a preferential effect on SNs.

Repeated activation of Trpv1-positive sensory neurons facilitates tumor growth associated with changes in tumor-infiltrating immune cells

It is considered that sensory neurons extend into the tumor microenvironment (TME), which could be associated with tumor growth. However, little is known about how sensory signaling could promote tumor progression. In this study, chemogenetic activation of transient receptor potential vanilloid 1 (Trpv1)-positive sensory neurons (C-fibers) by the microinjection of AAV-hSyn-FLEX-hM3Dq-mCherry into the sciatic nerve dramatically increased tumor volume in tumor-bearing Trpv1-Cre mice. This activation in Trpv1::hM3Dq mice that had undergone tumor transplantation significantly reduced the population of tumor-infiltrating CD4⁺ T cells and increased the mRNA level of the M2-macrophage marker, CX3C motif chemokine receptor 1 (Cx3cr1) in immunosuppressive cells, such as tumor-associated macrophages (TAMs) and tumor-infiltrating monocytic myeloid-derived suppressor cells (M-MDSCs). Under these conditions, we found a significant correlation between the decreased expression of the M1-macrophage marker Tnf and tumor volume. These findings suggest that repeated activation of Trpv1-positive sensory neurons may facilitate tumor growth along with changes in tumor-infiltrating immune cells.

Cancer aggravation due to persistent pain signals with the increased expression of pain-related mediators in sensory neurons of tumor-bearing mice

A growing body of evidence suggests that intractable pain reduces both the quality of life and survival in cancer patients. In the present study, we evaluated whether chronic pain stimuli could directly affect cancer pathology using tumor-bearing mice. For this purpose, we used two different models of chronic pain in mice, neuropathic pain and persistent postsurgical pain, with Lewis lung carcinoma (LLC) as tumor cells. We found that tumor growth was dramatically promoted in these pain models. As well as these pain models, tumor growth of LLC, severe osteosarcoma (AXT) and B16 melanoma cells was significantly promoted by concomitant activation of sensory neurons in AAV6-hM3Dq-injected mice treated with the designer drug clozapine-N-oxide (CNO). Significant increases in mRNA levels of vascular endothelial growth factor-A (Vegfa), tachykinin precursor 1 (Tac1) and calcitonin-related polypeptide alpha (Calca) in the ipsilateral side of dorsal root ganglion of AAV6-hM3Dq-injected mice were observed by concomitant activation of sensory neurons due to CNO administration. Moreover, in a model of bone cancer pain in which mice were implanted with AXT cells into the right femoral bone marrow cavity, the survival period was significantly prolonged by repeated inhibition of sensory neurons of AAV6-hM4Di-injected mice by CNO administration. These findings suggest that persistent pain signals may promote tumor growth by the increased expression of sensory-located peptides and growth factors, and controlling cancer pain may prolong cancer survival.

An induced pluripotent stem cell-based model identifies molecular targets of vincristine neurotoxicity

To model peripheral nerve degeneration and investigate molecular mechanisms of neurodegeneration, we established a cell system of induced pluripotent stem cell (iPSC)-derived sensory neurons exposed to vincristine, a drug that frequently causes chemotherapy-induced peripheral neuropathy. Sensory neurons differentiated from iPSCs exhibit distinct neurochemical patterns according to the immunocytochemical phenotypes, and gene expression of peripherin (PRPH, hereafter referred to as Peri) and neurofilament heavy chain (NEFH, hereafter referred to as NF). The majority of iPSC-derived sensory neurons were PRPH positive/NEFH negative, i.e. Peri(+)/NF(-) neurons, whose somata were smaller than those of Peri(+)/NF(+) neurons. On exposure to vincristine, projections from the cell body of a neuron, i.e. neurites, were degenerated quicker than somata, the lethal concentration to kill 50% (LC50) of neurites being below the LC50 for somata, consistent with the clinical pattern of length-dependent neuropathy. We then examined the molecular expression in the MAP kinase signaling pathways of, extracellular signal-regulated kinases 1/2 (MAPK1/3, hereafter referred to as ERK), p38 mitogen-activated protein kinases (MAPK11/12/13/14, hereafter referred to as p38) and c-Jun N-terminal kinases (MAPK8/9/10, hereafter referred to as JNK). Regarding these three cascades, only phosphorylation of JNK was upregulated but not that of p38 or ERK1/2. Furthermore, vincristine-treatment resulted in impaired autophagy and reduced autophagic flux. Rapamycin-treatment reversed the effect of impaired autophagy and JNK activation. These results not only established a platform to study peripheral degeneration of human neurons but also provide molecular mechanisms for neurodegeneration with the potential for therapeutic targets.

Pheromone sensing in Drosophila requires support cell-expressed Osiris 8

Background

The nose of most animals comprises multiple sensory subsystems, which are defined by the expression of different olfactory receptor families. Drosophila melanogaster antennae contain two morphologically and functionally distinct subsystems that express odorant receptors (Ors) or ionotropic receptors (Irs). Although these receptors have been thoroughly characterized in this species, the subsystem-specific expression and roles of other genes are much less well-understood.

Results

Here we generate subsystem-specific transcriptomic datasets to identify hundreds of genes, encoding diverse protein classes, that are selectively enriched in either Or or Ir subsystems. Using single-cell antennal transcriptomic data and RNA in situ hybridization, we find that most neuronal genes—other than sensory receptor genes—are broadly expressed within the subsystems. By contrast, we identify many non-neuronal genes that exhibit highly selective expression, revealing substantial molecular heterogeneity in the non-neuronal cellular components of the olfactory subsystems. We characterize one Or subsystem-specific non-neuronal molecule, Osiris 8 (Osi8), a conserved member of a large, insect-specific family of transmembrane proteins. Osi8 is expressed in the membranes of tormogen support cells of pheromone-sensing trichoid sensilla. Loss of Osi8 does not have obvious impact on trichoid sensillar development or basal neuronal activity, but abolishes high sensitivity responses to pheromone ligands.

Conclusions

This work identifies a new protein required for insect pheromone detection, emphasizes the importance of support cells in neuronal sensory functions, and provides a resource for future characterization of other olfactory subsystem-specific genes.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01425-w.

Preferential regeneration and collateral dynamics of motor and sensory neurons after nerve injury in mice

Specificity in regeneration after peripheral nerve injuries is a major determinant of functional recovery. Unfortunately, regenerating motor and sensory axons rarely find their original pathways to reinnervate appropriate target organs. Although a preference of motor axons to regenerate towards the muscle has been described, little is known about the specificity of the heterogeneous sensory populations. Here, we propose the comparative study of regeneration in different neuron subtypes. Using female and male reporter mice, we assessed the regenerative preference of motoneurons (ChAT-Cre/Ai9), proprioceptors (PV-Cre/Ai9), and cutaneous mechanoreceptors (Npy2r-Cre/Ai9). The femoral nerve of these animals was transected above the bifurcation and repaired with fibrin glue. Regeneration was assessed by applying retrograde tracers in the distal branches of the nerve 1 or 8 weeks after the lesion and counting the retrotraced somas and the axons in the branches. We found that cutaneous mechanoreceptors regenerated faster than other populations, followed by motoneurons and, lastly, proprioceptors. All neuron types had an early preference to regenerate into the cutaneous branch whereas, at long term, all neurons regenerated more through their original branch. Finally, we found that myelinated neurons extend more regenerative sprouts in the cutaneous than in the muscle branch of the femoral nerve and, particularly, that motoneurons have more collaterals than proprioceptors. Our findings reveal novel differences in regeneration dynamics and specificity, which indicate distinct regenerative mechanisms between neuron subtypes that can be potentially modulated to improve functional recovery after nerve injury.

Comprehensive phenotyping of cutaneous afferents reveals early-onset alterations in nociceptor response properties, release of CGRP, and hindpaw edema following spinal cord injury

Spinal cord injury (SCI) is a complex syndrome that has profound effects on patient well-being, including the development of medically-resistant chronic pain. The mechanisms underlying SCI pain have been the subject of thorough investigation but remain poorly understood. While the majority of the research has focused on changes occurring within and surrounding the site of injury in the spinal cord, there is now a consensus that alterations within the peripheral nervous system, namely sensitization of nociceptors, contribute to the development and maintenance of chronic SCI pain. Using an ex vivo skin/nerve/DRG/spinal cord preparation to characterize afferent response properties following SCI, we found that SCI increased mechanical and thermal responding, as well as the incidence of spontaneous activity (SA) and afterdischarge (AD), in below-level C-fiber nociceptors 24 hr following injury relative to naïve controls. Interestingly, the distribution of nociceptors that exhibit SA and AD are not identical, and the development of SA was observed more frequently in nociceptors with low heat thresholds, while AD was found more frequently in nociceptors with high heat thresholds. We also found that SCI resulted in hindpaw edema and elevated cutaneous calcitonin gene-related peptide (CGRP) concentration that were not observed in naïve mice. These results suggest that SCI causes a rapidly developing nociceptor sensitization and peripheral inflammation that may contribute to the early emergence and persistence of chronic SCI pain.

Systematic expression profiling of Dpr and DIP genes reveals cell surface codes in Drosophila larval motor and sensory neurons

In complex nervous systems, neurons must identify their correct partners to form synaptic connections. The prevailing model to ensure correct recognition posits that cell-surface proteins (CSPs) in individual neurons act as identification tags. Thus, knowing what cells express which CSPs would provide insights into neural development, synaptic connectivity, and nervous system evolution. Here, we investigated expression of Dpr and DIP genes, two CSP subfamilies belonging to the immunoglobulin superfamily, in Drosophila larval motor neurons (MNs), muscles, glia and sensory neurons (SNs) using a collection of GAL4 driver lines. We found that Dpr genes are more broadly expressed than DIP genes in MNs and SNs, and each examined neuron expresses a unique combination of Dpr and DIP genes. Interestingly, many Dpr and DIP genes are not robustly expressed, but are found instead in gradient and temporal expression patterns. In addition, the unique expression patterns of Dpr and DIP genes revealed three uncharacterized MNs. This study sets the stage for exploring the functions of Dpr and DIP genes in Drosophila MNs and SNs and provides genetic access to subsets of neurons.

Neuro-immune crosstalk and food allergy: Focus on enteric neurons and mucosal mast cells

The nervous system and the immune system individually play important roles in regulating the processes necessary to maintain physiological homeostasis, respond to acute stress and protect against external threats. These two regulating systems for maintaining the living body had often been assumed to function independently. Allergies develop as a result of an overreaction of the immune system to substances that are relatively harmless to the body, such as food, pollen and dust mites. Therefore, it has been generally supposed that the development and pathogenesis of allergies can be explained through an immunological interpretation. Recently, however, neuro-immune crosstalk has attracted increasing attention. Consequently, it is becoming clear that there is close morphological proximity and physiological and pathophysiological interactions between neurons and immune cells in various peripheral tissues. Thus, researchers are now beginning to appreciate that neuro-immune interactions may play a role in tissue homeostasis and the pathophysiology of immune-mediated disease, but very little information is available on the molecular basis of these interactions. Mast cells are a part of the innate immune system implicated in allergic reactions and the regulation of host-pathogen interactions. Mast cells are ubiquitous in the body, and these cells are often found in close proximity to nerve fibers in various tissues, including the lamina propria of the intestine. Mast cells and neurons are thought to communicate bidirectionally to modulate neurophysiological effects and mast cell functions, which suggests that neuro-immune interactions may be involved in the pathology of allergic diseases.

Intramuscular injection of a plasmid DNA vector expressing hepatocyte growth factor (HGF) ameliorated pain symptoms by controlling the expression of pro-inflammatory cytokines in the dorsal root ganglion

Hepatocyte growth factor (HGF) is a secretory protein that is involved in various biological activities such as angiogenesis, neuroprotection, and anti-inflammatory effects. Intramuscular injection of an HGF-encoding plasmid DNA (pCK-HGF-X7) has been shown to produce pain-relieving effects in a rodent model and patients with neuropathic pain.To further investigate the underlying mechanism, we investigated the anti-inflammatory effects of HGF in the context of neuropathic pain. Consistent with previous data, intramuscular injection of pCK-HGF-X7 showed pain relieving effects up to 8 weeks and pharmacological blockade of the c-Met receptor hindered this effect, which suggest that the analgesic effect was c-Met receptor-dependent. At the histological level, macrophage infiltration in the dorsal root ganglion (DRG) was significantly decreased in the pCK-HGF-X7 injected group. Moreover, HGF treatment significantly downregulated the LPS-mediated induction of pro-inflammatory cytokines in primary cultured DRG neurons. Taken together, these data suggest that HGF-encoding plasmid DNA attenuates neuropathic pain via controlling the expression of pro-inflammatory cytokines.

Neuronal FcεRIα directly mediates ocular itch via IgE-immune complex in a mouse model of allergic conjunctivitis

Background

Classical understanding of allergic conjunctivitis (ACJ) suggests that ocular itch results from a mast cell-dependent inflammatory process. However, treatments that target inflammatory mediators or immune cells are often unsatisfying in relieving the stubborn itch symptom. This suggests that additional mechanisms are responsible for ocular itch in ACJ. In this study, we aim to determine the role of neuronal FcεRIa in allergic ocular itch.

Methods

Calcium imaging was applied to observe the effect of IgE-immune complex in trigeminal neurons. Genomic FcεRIa knockout mice and adeno-associated virus (AAV) mediated sensory neuron FcεRIa knockdown mice were used in conjunction with behavioral tests to determine ocular itch. In addition, immunohistochemistry, Western blot and quantitative RT-PCR were used for in vitro experiments.

Results

We found that FcεRIα was expressed in a subpopulation of conjunctiva sensory neurons. IgE-IC directly activated trigeminal neurons and evoked acute ocular itch without detectible conjunctival inflammation. These effects were attenuated in both a global FcεRIa-knockout mice and after sensory neuronal-specific FcεRIa-knockdown in the mouse trigeminal ganglion. In an ovalbumin (OVA) induced murine ACJ model, FcεRIα was found upregulated in conjunctiva-innervating CGRP+ sensory neurons. Sensory neuronal-specific knockdown of FcεRIa significantly alleviated ocular itch in the ACJ mice without affecting the immune cell infiltration and mast cell activation in conjunctiva. Although FcεRIα mRNA expression was not increased by IgE in dissociated trigeminal ganglion neurons, FcεRIα protein level was enhanced by IgE in a cycloheximide-resistance manner, with concordant enhancement of neuronal responses to IgE-IC. In addition, incremental sensitization gradually enhanced the expression of FcεRIα in small-sized trigeminal neurons and aggravated OVA induced ocular itch.

Conclusions

Our study demonstrates that FcεRIα in pruriceptive neurons directly mediates IgE-IC evoked itch and plays an important role in ocular itch in a mouse model of ACJ. These findings reveal another axis of neuroimmune interaction in allergic itch condition independent to the classical IgE-mast cell pathway, and might suggest novel therapeutic strategies for the treatment of pruritus in ACJ and other immune-related disorders.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-022-02417-x.

Cellular models of pain: New technologies and their potential to progress preclinical research

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Human sensory neurons can reduce the translational gap in analgesic development.

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Access to dorsal root ganglion (hDRG) neurons is increasing.

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Diverse sensory neuron subtypes can now be generated via stem cell technology.

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Advances of these technologies will improve our understanding of human nociception.

In vitro models fill a vital niche in preclinical pain research, allowing detailed study of molecular pathways, and in the case of humanised systems, providing a translational bridge between in vivo animal models and human patients. Significant advances in cellular technology available to basic pain researchers have occurred in the last decade, including developing protocols to differentiate sensory neuron-like cells from stem cells and greater access to human dorsal root ganglion tissue. In this review, we discuss the use of both models in preclinical pain research: What can a human sensory neuron in a dish tell us that rodent in vivo models cannot? How similar are these models to their endogenous counterparts, and how should we judge them? What limitations do we need to consider? How can we leverage cell models to improve translational success? In vitro human sensory neuron models equip pain researchers with a valuable tool to investigate human nociception. With continual development, consideration for their advantages and limitations, and effective integration with other experimental strategies, they could become a driving force for the pain field's advancement.

Neuronal Dual Leucine Zipper Kinase Mediates Inflammatory and Nociceptive Responses in Cyclophosphamide-Induced Cystitis

Interstitial cystitis is associated with neurogenic inflammation and neuropathic bladder pain. Dual leucine zipper kinase (DLK) expressed in sensory neurons is implicated in neuropathic pain. We hypothesized that neuronal DLK is involved in the regulation of inflammation and nociceptive behavior in cystitis. Mice deficient in DLK in sensory neurons (cKO) were generated by crossing DLK floxed mice with mice expressing Cre recombinase under Advillin promoter. Cystitis was induced by cyclophosphamide (CYP) administration in mice. Nociceptive behavior, bladder inflammation, and pathology were assessed following cystitis induction in control and cKO mice. The role of DLK in CYP-induced cystitis was further determined by pharmacological inhibition of DLK with GNE-3511. Deletion of neuronal DLK attenuated CYP-induced pain-like nociceptive behavior and suppressed histamine release from mast cells, neuronal activation in the spinal cord, and bladder pathology. Mice deficient in neuronal DLK also showed reduced inflammation induced by CYP and reduced c-Jun activation in the dorsal root ganglia (DRG). Pharmacological inhibition of DLK with GNE-3511 recapitulated the effects of neuronal DLK depletion in CYP treatment mice. Our study suggests that DLK is a potential target for the treatment of neuropathic pain and bladder pathology associated with cystitis.

Inhibitory effect of polysulfide, an endogenous sulfur compound, on oxidative stress-induced TRPA1 activation

Polysulfide (PS), an endogenous sulfur compound, generated by oxidation of hydrogen sulfide, has a stimulatory action on the nociceptive TRPA1 channel. TRPA1 is also activated by reactive oxygen species such as hydrogen peroxide (H₂O₂) produced during inflammation. Here, we examined the effect of PS on H₂O₂-induced responses in native and heterologously expressed TRPA1 using a cell-based calcium assay. We also carried out behavioral experiments in vivo. In mouse sensory neurons, H₂O₂ elicited early TRPA1-dependent and late TRPA1-independent increases of [Ca²⁺]_i. The former was suppressed by the pretreatment with PS. In cells heterologously expressed TRPA1, PS suppressed [Ca²⁺]_i responses to H₂O₂. Simultaneous measurement of [Ca²⁺]_i and the intracellular PS level revealed that scavenging effect of PS was not related to the inhibitory effect. Removal of extracellular Ca²⁺, a calmodulin inhibitor and dithiothreitol attenuated the inhibitory effect of PS. Pretreatment with PS diminished nociceptive behaviors induced by H₂O₂. The present data suggest that PS suppresses oxidative stress-induced TRPA1 activation due to cysteine modification and Ca²⁺/calmodulin signaling. Thus, endogenous sulfurs may have regulatory roles in nociception via functional changes in TRPA1 under inflammatory conditions.

Establishing Myelinating Cocultures Using Human iPSC-Derived Sensory Neurons to Investigate Axonal Degeneration and Demyelination

Complex signaling between Schwann cells and axons are vital for peripheral neuron development, myelination, and repair. The interaction between these two cell types can be modeled in vitro by coculturing rodent Schwann cells and neurons together. These have in the past been used with great success to help unravel the bidirectional signaling mechanisms that lead to Schwann cell proliferation and myelination. To provide more translatable potential, we have developed myelinating cocultures using human, induced pluripotent stem cell (iPSC)-derived neurons. Under the right conditions, the human neurons are efficiently myelinated by rat Schwann cells, demonstrating successful cross-species signaling. This chapter describes all the necessary steps to generate these myelinating cocultures and methods to investigate and quantify various aspects of myelination. The myelinating cocultures can be maintained in excellent health for over 1 year, facilitating their use to study developmental or chronic disease processes. With this in mind, we have used the cocultures to model a sensory neuropathy which displays clinically with both axonal and demyelinating features. In the cocultures, we found evidence of extensive axonal degeneration and demyelination demonstrated by axonal swelling and fragmentation, and myelin disintegration. The myelinating cocultures can therefore be used to study complex, human disease processes that result in both axonal and myelin-associated degenerative processes.

A Chemotherapy-Induced Peripheral Neuropathy Model in Drosophila melanogaster

Peripheral neuropathies are one of the largest categories of neurodegenerative diseases. To investigate their mechanisms, many in vitro and in vivo models can be employed. Here we present a protocol for the induction of chemotherapy-induced peripheral neuropathy (CIPN) in the Drosophila melanogaster (fruit fly) model system. Using a clinically relevant degeneration initiator, paclitaxel (taxol), it is possible to model many aspects of axon and dendrite degeneration while in a genetically tractable, in vivo system. In this protocol, we feed larval stage Drosophila neurotoxic chemotherapy drugs during the duration of larval development, followed by dissection and imaging of genetically labeled sensory axons and dendrites. Both axons and dendrites degenerate with taxol exposure. Our protocol should facilitate the adoption and expansion of this model to include other neurotoxic compounds.

In Vivo Imaging of Anterograde and Retrograde Axonal Transport in Rodent Peripheral Nerves

Axonal transport, which is the process mediating the active shuttling of a variety cargoes from one end of an axon to the other, is essential for the development, function, and survival of neurons. Impairments in this dynamic process are linked to diverse nervous system diseases and advanced ageing. It is thus essential that we quantitatively study the kinetics of axonal transport to gain an improved understanding of neuropathology as well as the molecular and cellular mechanisms regulating cargo trafficking. One of the best ways to achieve this goal is by imaging individual, fluorescent cargoes in live systems and analyzing the kinetic properties of their progression along the axon. We have therefore developed an intravital technique to visualize different organelles, such as signaling endosomes and mitochondria, being actively transported in the axons of both motor and sensory neurons in live, anesthetized rodents. In this chapter, we provide step-by-step instructions on how to deliver specific organelle-targeting, fluorescent probes using several routes of administration to image individual cargoes being bidirectionally transported along axons within the exposed sciatic nerve. This method can provide detailed, physiologically relevant information on axonal transport, and is thus poised to elucidate mechanisms regulating this process in both health and disease.

Hepatocyte growth factor is necessary for efficient outgrowth of injured peripheral axons in in vitro culture system and in vivo nerve crush mouse model

Hepatocyte growth factor (HGF) is a neurotrophic factor and its role in peripheral nerves has been relatively unknown. In this study, biological functions of HGF and its receptor c-met have been investigated in the context of regeneration of damaged peripheral nerves. Axotomy of the peripheral branch of sensory neurons from embryonic dorsal root ganglia (DRG) resulted in the increased protein levels of HGF and phosphorylated c-met. When the neuronal cultures were treated with a pharmacological inhibitor of c-met, PHA665752, the length of axotomy-induced outgrowth of neurite was significantly reduced. On the other hand, the addition of recombinant HGF proteins to the neuronal culture facilitated axon outgrowth. In the nerve crush mouse model, the protein level of HGF was increased around the injury site by almost 5.5-fold at 24 h post injury compared to control mice and was maintained at elevated levels for another 6 days. The amount of phosphorylated c-met receptor in sciatic nerve was also observed to be higher than control mice. When PHA665752 was locally applied to the injury site of sciatic nerve, axon outgrowth and injury mediated induction of cJun protein were effectively inhibited, indicating the functional involvement of HGF/c-met pathway in the nerve regeneration process. When extra HGF was exogenously provided by intramuscular injection of plasmid DNA expressing HGF, axon outgrowth from damaged sciatic nerve and cJun expression level were enhanced. Taken together, these results suggested that HGF/c-met pathway plays important roles in axon outgrowth by directly interacting with sensory neurons and thus HGF might be a useful tool for developing therapeutics for peripheral neuropathy.

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In in vitro primary eDRGs, axotomy-induced HGF/c-met pathway enhanced the neurite outgrowth process.

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Nerve injury induced the expression of HGF, consequently leading to the activation of c-met in peripheral axons.

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HGF/c-met pathway played an important role in the regeneration process of injured peripheral nerves.

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Additional supply of HGF, in the form of plasmid DNA, enhanced the regeneration of damaged peripheral nerves.

The novel TRPA1 antagonist BI01305834 inhibits ovalbumin-induced bronchoconstriction in guinea pigs

Background

Asthma is a chronic respiratory disease in which the nervous system plays a central role. Sensory nerve activation, amongst others via Transient Receptor Potential Ankyrin 1 (TRPA1) channels, contributes to asthma characteristics including cough, bronchoconstriction, mucus secretion, airway hyperresponsiveness (AHR) and inflammation. In the current study, we evaluated the efficacy of the novel TRPA1 antagonist BI01305834 against AHR and inflammation in guinea-pig models of asthma.

Methods

First, a pilot study was performed in a guinea-pig model of allergic asthma to find the optimal dose of BI01305834. Next, the effect of BI01305834 on (1) AHR to inhaled histamine after the early and late asthmatic reaction (EAR and LAR), (2) magnitude of EAR and LAR and (3) airway inflammation was assessed. Precision-cut lung slices and trachea strips were used to investigate the bronchoprotective and bronchodilating-effect of BI01305834. Statistical evaluation of differences of in vivo data was performed using a Mann–Whitney U test or One-way nonparametric Kruskal–Wallis ANOVA, for ex vivo data One- or Two-way ANOVA was used, all with Dunnett’s post-hoc test where appropriate.

Results

A dose of 1 mg/kg BI01305834 was selected based on AHR and exposure data in blood samples from the pilot study. In the subsequent study, 1 mg/kg BI01305834 inhibited AHR after the EAR, and the development of EAR and LAR elicited by ovalbumin in ovalbumin-sensitized guinea pigs. BI01305834 did not inhibit allergen-induced total and differential cells in the lavage fluid and interleukin-13 gene expression in lung homogenates. Furthermore, BI01305834 was able to inhibit allergen and histamine-induced airway narrowing in guinea-pig lung slices, without affecting histamine release, and reverse allergen-induced bronchoconstriction in guinea-pig trachea strips.

Conclusions

TRPA1 inhibition protects against AHR and the EAR and LAR in vivo and allergen and histamine-induced airway narrowing ex vivo, and reverses allergen-induced bronchoconstriction independently of inflammation. This effect was partially dependent upon histamine, suggesting a neuronal and possible non-neuronal role for TRPA1 in allergen-induced bronchoconstriction.

Morphological elucidation of short-chain fatty acid receptor GPR41-positive enteric sensory neurons in the colon of mice with dextran sulfate sodium-induced colitis

Although the etiology of inflammatory bowel disease (IBD) remains unclear, it has generally been accepted that abnormalities in the intestinal immune system and dysbiosis of the gut microbiota are involved in the pathology of IBD. Recently, short-chain fatty acids (SCFAs) produced by gut microbiota were reported to maintain intestinal homeostasis through their receptors, such as GPR41. However, there are contradictory reports about the role of GPR41 in intestinal inflammation. Consequently, the roles of GPR41 in dysbiosis induced by intestinal inflammation remain unclear. Thus, we investigated the distribution of GPR41 in the colonic mucosa of mice with dextran sulfate sodium (DSS)-induced colitis. GPR41-immunoreactive fibrous structures were observed in the colonic lamina propria and muscularis layer of normal mice. In addition, GPR41-immunoreactive fibrous structures partly colocalized with calcitonin gene-related peptide (CGRP; a neurotransmitter of cholinergic enteric sensory neurons)-immunoreactive nerve fibers in the colonic lamina propria, indicating that GPR41 is expressed in cholinergic intrinsic sensory neurons. Furthermore, both GPR41-immunoreactivities and CGRP-immunoreactivities were significantly increased in the lamina propria of the colon in mice with DSS-induced colitis. Interestingly, GPR41-immunoreactivities were often found in close proximity to F4/80⁺ macrophages in the colonic mucosa of normal mice, and their frequency was elevated in the colonic mucosa of mice with DSS-induced colitis. Therefore, the crosstalk between SCFA-sensing intrinsic sensory neurons and macrophages might be involved in the pathology of acute colitis.

Effects of optogenetic activation of the enteric nervous system on gastrointestinal motility in mouse small intestine

BACKGROUND AND AIMS

Recently, it was demonstrated that optogenetics could be used to stimulate enteric calretinin neurons, leading to increased colonic transit in vitro and in vivo. The aim of the current study was to determine if similar approaches could be used to stimulate the isolated mouse small intestine, with the aim of potentially also improving transit in the small bowel.

METHODS

Cre-Lox recombination was used to generate transgenic mice expressing the light-sensitive ion channel channelrhodopsin-2 (ChR2) in calretinin neurons.

RESULTS

Spontaneous migrating motor complexes were recorded from isolated terminal small intestine in both Cal^Cre+ mice expressing ChR2 in calretinin-expressing neurons and experimental controls, Cal^Cre-. Trains of blue light pulses (20 ms, 5 Hz, 20s) evoked a brief local contraction of circular muscle, but never a premature MMC, irrespective of light intensity (1-40 mV/mm²) or the region of ileum stimulated. However, MMCs were readily evoked by calretinin neuron activation in colon, consistent with our previous study. Light-evoked contractions of the terminal small bowel were hexamethonium-resistant (300 μM), but blocked by tetrodotoxin (0.6 μM). Light-evoked smooth muscle contraction did not change the pacemaker frequency underlying MMCs.

CONCLUSION

Focal illumination of the small intestine does not appear as effective at inducing propulsive motor activity as has been demonstrated in the colon of the same colony mice. This study suggests caution should be exercised when assuming optogenetic technology is equally effective at increasing GI transit in the small intestine as in the large intestine of mice.

Mini-review - Sodium channels and beyond in peripheral nerve disease: Modulation by cytokines and their effector protein kinases

Peripheral neuropathy is associated with enhanced activity of primary afferents which is often manifested as pain. Voltage-gated sodium channels (VGSCs) are critical for the initiation and propagation of action potentials and are thus essential for the transmission of the noxious stimuli from the periphery. Human peripheral sensory neurons express multiple VGSCs, including Nav1.7, Nav1.8, and Nav1.9 that are almost exclusively expressed in the peripheral nervous system. Distinct biophysical properties of Nav1.7, Nav1.8, and Nav1.9 underlie their differential contributions to finely tuned neuronal firing of nociceptors, and mutations in these channels have been associated with several inherited human pain disorders. Functional characterization of these mutations has provided additional insights into the role of these channels in electrogenesis in nociceptive neurons and pain sensation. Peripheral tissue damage activates an inflammatory response and triggers generation and release of inflammatory mediators, which can act through diverse signaling cascades to modulate expression and activity of ion channels including VGSCs, contributing to the development and maintenance of pathological pain conditions. In this review, we discuss signaling pathways that are activated by pro-nociceptive inflammatory mediators that regulate peripheral sodium channels, with a specific focus on direct phosphorylation of these channels by multiple protein kinases.

Numerical analysis of conduction velocity/path relationships in a crustacean sensory neuron

Experimental observations of the axonal conduction velocities of sensory neurons associated with near-field sensilla on the cephalothorax of the crayfish Procambarus clarkii indicate that neurons supplying sensilla farther from their connections with the central nervous system exhibit higher overall impulse conduction velocities. The conduction velocity/distance relationship is best described by an exponentially rising, asymptotic curve. A numerical model for regional variations in impulse conduction velocity in these sensory neurons was developed, based upon neuronal morphological metrics and physiological data. The predicted relationship between conduction velocity and length of conduction pathway in the model was compared to experimental data from 88 sensory neurons associated with thoracic near-field receptor sensilla, in which both the mean conduction velocity and the length of the conduction pathway for each neuron were known. Curves fitted to the conduction velocity versus distance relationship in the two cases were similar, although not congruent. Chi-square statistics comparing the curves predict that the curves are similar at the 0.005 probability level, suggesting that the numerical model's variations in axonal morphology can satisfactorily account for the observed conduction velocity-distance relationship in these sensory neurons.

Reduction of extracellular sodium evokes nociceptive behaviors in the chicken via activation of TRPV1

Transient receptor potential vanilloid 1 (TRPV1), a non-selective cation channel, is mainly expressed in nociceptive primary sensory neurons. Sensitivity of TRPV1 to several stimuli is known to vary among species, specifically, the avian orthologue is nearly insensitive to capsaicin. Extracellular sodium ions ([Na⁺]_o) regulate TRPV1 activity in mammals, but their regulatory role on chicken TRPV1 (cTRPV1) is unknown. Here, we focused on the actions of capsaicin and low [Na⁺]_o on cTRPV1 activity. In chicken dorsal root ganglion (cDRG) neurons, capsaicin elicited [Ca²⁺]_i increases, but its effective concentration was much higher than those in mammals. Low [Na⁺]_o evoked [Ca²⁺]_i increases in cDRG neurons in a decreasing [Na⁺]_o-dependent manner and the complete removal of [Na⁺]_o (0Na) produced maximal effects. The population of 0Na-sensitive neurons was mostly overlapped with those of proton- and capsaicin-sensitive ones. Low [Na⁺]_o synergistically potentiated the capsaicin- and proton-induced TRPV1 activation in cDRG neurons. In HEK293 cells expressing cTRPV1 (cTRPV1-HEK), capsaicin elicited [Ca²⁺]_i increases with an EC₅₀ of 11.8 µM, and low [Na⁺]_o also did. Well-defined mammalian TRPV1 antagonists hardly suppressed cTRPV1 activation by low [Na⁺]_o. 0Na evoked outwardly rectified currents in cTRPV1-HEK. Mutagenesis analyses revealed a possible interaction of [Na⁺]_o with the proton-binding sites of cTRPV1. The administration of capsaicin and 0Na to chick eyes elicited pain-related behaviors. These results suggest that low [Na⁺]_o is capable of activating cTRPV1 in vitro, resulting in pain in vivo. Our data demonstrate that characterization of the cTRPV1 function is important to understand activation mechanisms of TRPV1.

A video protocol for rapid dissection of mouse dorsal root ganglia from defined spinal levels

OBJECTIVE

Dorsal root ganglia (DRG) are heterogeneous assemblies of assorted sensory neuron cell bodies found in bilateral pairs at every level of the spinal column. Pseudounipolar afferent neurons convert external stimuli from the environment into electrical signals that are retrogradely transmitted to the spinal cord dorsal horn. To do this, they extend single axons from their DRG-resident somas that then bifurcate and project both centrally and distally. DRG can be dissected from mice at embryonic stages and any age post-natally, and have been extensively used to study sensory neuron development and function, response to injury, and pathological processes in acquired and genetic diseases. We have previously published a step-by-step dissection method for the rapid isolation of post-natal mouse DRG. Here, the objective is to extend the protocol by providing training videos that showcase the dissection in fine detail and permit the extraction of ganglia from defined spinal levels.

RESULTS

By following this method, the reader will be able to swiftly and accurately isolate specific lumbar, thoracic, and cervical DRG from mice. Dissected ganglia can then be used for RNA/protein analyses, subjected to immunohistochemical examination, and cultured as explants or dissociated primary neurons, for in-depth investigations of sensory neuron biology.

Lysosomal Hydrolase Cathepsin D Non-proteolytically Modulates Dendritic Morphology in Drosophila

The main lysosomal protease cathepsin D (cathD) is essential for maintaining tissue homeostasis via its degradative function, and its loss leads to ceroid accumulation in the mammalian nervous system, which results in progressive neurodegeneration. Increasing evidence implies non-proteolytic roles of cathD in regulating various biological processes such as apoptosis, cell proliferation, and migration. Along these lines, we here showed that cathD is required for modulating dendritic architecture in the nervous system independent of its traditional degradative function. Upon cathD depletion, class I and class III arborization (da) neurons in Drosophila larvae exhibited aberrant dendritic morphology, including over-branching, aberrant turning, and elongation defects. Re-introduction of wild-type cathD or its proteolytically-inactive mutant dramatically abolished these morphological defects. Moreover, cathD knockdown also led to dendritic defects in the adult mushroom bodies, suggesting that cathD-mediated processes are required in both the peripheral and central nervous systems. Taken together, our results demonstrate a critical role of cathD in shaping dendritic architecture independent of its proteolytic function.

Neuronal selectivity of botulinum neurotoxins

Botulinum neurotoxins (BoNTs) are highly potent toxins responsible for a severe disease, called botulism. They are also efficient therapeutic tools with an increasing number of indications ranging from neuromuscular dysfunction to hypersecretion syndrome, pain release, depression as well as cosmetic application. BoNTs are known to mainly target the motor-neurons terminals and to induce flaccid paralysis. BoNTs recognize a specific double receptor on neuronal cells consisting of gangliosides and synaptic vesicle protein, SV2 or synaptotagmin. Using cultured neuronal cells, BoNTs have been established blocking the release of a wide variety of neurotransmitters. However, BoNTs are more potent in motor-neurons than in the other neuronal cell types. In in vivo models, BoNT/A impairs the cholinergic neuronal transmission at the motor-neurons but also at neurons controlling secretions and smooth muscle neurons, and blocks several neuronal pathways including excitatory, inhibitory, and sensitive neurons. However, only a few reports investigated the neuronal selectivity of BoNTs in vivo. In the intestinal wall, BoNT/A and BoNT/B target mainly the cholinergic neurons and to a lower extent the other non-cholinergic neurons including serotonergic, glutamatergic, GABAergic, and VIP-neurons. The in vivo effects induced by BoNTs on the non-cholinergic neurons remain to be precisely investigated. We report here a literature review of the neuronal selectivity of BoNTs.

HSV Mutant Generation and Dual Detection Methods for Gaining Insight into Latent/Lytic Cycles In Vivo

Two important components of a useful strategy to examine viral gene function, regulation, and pathogenesis in vivo are (1) a highly efficient protocol to generate viral mutants that limits undesired mutation and retains full replication competency in vivo, and (2) an efficient system to detect and quantify viral promoter activity and gene expression in rare cells in vivo and to gain insight into the surrounding tissue environment. Our strategy and protocols for generating, characterizing, and employing HSV viral promoter/reporter mutants in vivo are provided in this two-part chapter.

elegans

Nervous system development is instructed by genetic programs and refined by distinct mechanisms that couple neural activity to gene expression. How these processes are integrated remains poorly understood. Here, we report that the regulated release of insulin-like peptides (ILPs) during development of the Caenorhabditis elegans nervous system accomplishes such an integration. We find that the p38 MAP kinase PMK-3, which is required for the differentiation of chemosensory BAG neurons, limits an ILP signal that represses expression of a BAG neuron fate. ILPs are released from BAGs themselves in an activity-dependent manner during development, indicating that ILPs constitute an autocrine signal that regulates the differentiation of BAG neurons. Expression of a specialized neuronal fate is, therefore, coordinately regulated by a genetic program that sets levels of ILP expression during development, and by neural activity, which regulates ILP release. Autocrine signals of this kind might have general and conserved functions as integrators of deterministic genetic programs with activity-dependent mechanisms during neurodevelopment.

SuperCLEM: an accessible correlative light and electron microscopy approach for investigation of neurons and glia in vitro

The rapid evolution of super-resolution light microscopy has narrowed the gap between light and electron microscopy, allowing the imaging of molecules and cellular structures at high resolution within their normal cellular and tissue context. Multimodal imaging approaches such as correlative light electron microscopy (CLEM) combine these techniques to create a tool with unique imaging capacity. However, these approaches are typically reserved for specialists, and their application to the analysis of neural tissue is challenging. Here we present SuperCLEM, a relatively simple approach that combines super-resolution fluorescence light microscopy (FLM), 3D electron microscopy (3D-EM) and rendering into 3D models. We demonstrate our workflow using neuron-glia cultures from which we first acquire high-resolution fluorescent light images of myelinated axons. After resin embedding and re-identification of the region of interest, serially aligned EM sections are acquired and imaged using a serial block face scanning electron microscope (SBF-SEM). The FLM and 3D-EM datasets are then combined to render 3D models of the myelinated axons. Thus, the SuperCLEM imaging pipeline is a useful new tool for researchers pursuing similar questions in neuronal and other complex tissue culture systems.

Summary: We developed an advanced, accessible and adaptable correlative light and electron microscopy pipeline for study of neuron-glia cultures and demonstrate its power by deriving accurate 3D models of myelinated axons.

[Neuronal Correlation Between Acupoint BL 23 and the Adrenal Gland in Rats]

OBJECTIVE

To explore the neuroanatomical basis of acupoint-visceral correlation by studying the distribution of the neurons associated with acupoint "Shenshu" (BL 23) area and adrenal gland in rats.

METHODS

AF 488-CTB and AF 594-CTB were injected into the left side of BL 23 area and adrenal gland in the same rat respectively. Three days after injection, the dorsal root ganglions (DRG), sympathetic chain, and spinal cord were dissected out from the perfused rats. The neuronal labeling with AF 488/594-CTB was directly observed on the sections under a fluorescent microscope or a laser scanning confocal microscope.

RESULTS

All neural labeling was observed in the injection side. The sensory neurons associated with both acupoint BL 23 and adrenal gland distributed from thoracic (T) 10 to lumbar (L) 2 DRG with high concentration in T 12-T 13 and T 11-T 12, respectively, in which some of them were simultaneously labeled with both AF 488/594-CTB and located in T 12-L 1 DRG. For the sympathetic innervation, the postganglionic neurons correlated with BL 23 and adrenal gland were labeled with AF 488/594-CTB separately in the sympathetic chain at the lumbar segments, while the labeled preganglionic neurons were only observed at the lateral horn of T 11-T 13 spinal segments in the cases of adrenal gland. In addition, the labeled motor neurons were mainly detected in the spinal ventral horn at cervical (C) 7-C 8 and T 11-L 1 segments.

CONCLUSION

These results indicate that there are segmental correlation between BL 23 and adrenal gland on the sensory and sympathetic innervations, and this correlation might be a neural pathway for modulating the function role of adrenal gland through BL 23 needling.

Using stochastic resonance and strength training as part of a rehabilitation programme for recurrent low back pain treatment: a case study

Low back pain (LBP) is a common disabling health problem that can cause decreased spine proprioception. Stochastic resonance (SR) can influence detection performance, besides improving patients with significant sensory deficits, but have not been thoroughly tested for LBP. This study aimed to examine the application of SR therapy (SRT) and strength training for LBP treatment. The subject was a resistance-trained male in his early thirties. His back pain was unbearable after a strength training session. Standard pain relief alleviated the pain but the LBP developed at a similar intensity after 4 weeks. SRT (4–5 sets ×90 sec, 30-sec rest interval, supine position) was prescribed along with other exercises for 3 weeks (phase 1), and followed by tailor-made strength training for 16 weeks (phase 2). The Oswestry Disability Index was 66.7% (interpreted as “crippled”) prior to first SRT, and reduced to minimal levels of 15.6% and 6.7% after four and seven SRT sessions, respectively. Similarly, pain intensity was ranging from 5 to 9 (distracting-severe) of the Numeric Rating Scale (NRS-11) prior to the first session but this was reduced considerably after four sessions (NRS-11: 0–1). During phase 2, the patient performed without complaining of LBP, two repetitions of bench press exercise at a load intensity of 1.2 his body weight and attained 4 min of plank stabilisation. This LBP management strategy has a clinically meaningful effect on pain intensity, disability, and functional mobility, by receding the recurrent distracting to severe LBP.

FK506 (tacrolimus) causes pain sensation through the activation of transient receptor potential ankyrin 1 (TRPA1) channels

FK506 (tacrolimus) is an immunosuppressant widely used as an ointment in the treatment of atopic dermatitis. However, local application of FK506 can evoke burning sensations in atopic dermatitis patients, and its mechanisms are unknown. In this study, we found that FK506 activates transient receptor potential ankyrin 1 (TRPA1) channels. In Ca2+-imaging experiments, increases in intracellular Ca2+ concentrations ([Ca2+]i) by FK506 were observed in HEK293T cells expressing hTRPA1 or hTRPM8. FK506-induced currents were observed in HEK293T cells expressing hTRPA1 or mTRPA1, but less or not at all in cells expressing hTRPV1 or hTRPM8 using a patch-clamp technique. FK506 also evoked single-channel opening of hTRPA1 in an inside-out configuration. FK506-induced [Ca2+]i increases were also observed in TRPA1-expressing mouse primary sensory neurons. Furthermore, injection of FK506 evoked licking or biting behaviors and these behaviors were almost abolished in TRPA1 knockout mice. These results indicate that FK506 might cause pain sensations through TRPA1 activation.

Innervation of the tibial epiphysis through the intercondylar foramen

The periosteum and mineralized bone are innervated by nerves that sense pain. These include both myelinated and unmyelinated neurons with either free nerve endings or bearing nociceptors. Parasympathetic and sympathetic autonomic nerves also innervate bone. However, little is known about the route sensory nerves take leaving the epiphyses of long bones at the adult knee joint. Here, we used transgenic mice that express fluorescent Venus protein in Schwann cells (Sox10-Venus mice) to visualize myelinated and unmyelinated nerves in the tibial epiphysis. Immunofluorescence to detect a pan-neuronal marker and the sensory neuron markers calcitonin gene-related peptide (CGRP) and tropomyosin receptor kinase A (TrkA) also revealed Schwann cell-associated sensory neurons. Foramina in the intercondylar area of the tibia were conserved between rodents and primates. Venus-labeled fibers were detected within bone marrow of the proximal epiphysis, exited through foramina along with blood vessels in the intercondylar area of the tibia, and joined Venus-labeled fibers of the synovial membrane and meniscus. These data suggest that innervation of the subchondral plate and trabecular bone within the tibial epiphysis carries pain signals from the knee joint to the brain through intercondylar foramina.

Imaging in vivo dynamics of sensory axon responses to CNS injury

Axons in the adult mammalian brain and spinal cord fail to regenerate upon lesion. In vivo imaging serves as a tool to investigate the immediate response of axons to injury and how the same injured axons behave over time. Here, we describe the dynamic changes that injured sensory axons undergo and methods of imaging them in vivo. First, we explain how sensory axons in the dorsal column of the adult mouse spinal cord respond to axotomy. Then, we highlight practical considerations for implementing two-photon based in vivo imaging of these axons. Finally, we describe future directions for this technique, including the possibility of in vivo imaging of subcellular dynamics within the axon.

Identification of molecular targets for toxic action by persulfate, an industrial sulfur compound

Persulfate salts are broadly used as industrial chemicals and exposure to them causes occupational asthma, occupational rhinitis and contact dermatitis. However, the mechanisms underlying these toxic actions are not fully elucidated. Transient receptor potential (TRP) vanilloid 1 (V1), ankyrin 1 (A1) and melastatin 8 (M8) are non-selective cation channels preferentially expressing sensory neurons. These channels are known to be involved in respiratory and skin diseases. In the present study, we investigated the effects of sodium persulfate on these TRP channels. In wild-type mouse sensory neurons, persulfate evoked [Ca²⁺]_i increases that were inhibited by removal of extracellular Ca²⁺ or blockers of TRPA1 but not by those of TRPV1 and TRPM8. Persulfate failed to evoke [Ca²⁺]_i responses in neurons from TRPA1(-/-) mice, but did evoke them in neurons from TRPV1(-/-) mice. In HEK 293 cells expressing mouse TRPA1 (mTRPA1-HEK), persulfate induced [Ca²⁺]_i increases. Moreover, in HEK 293 cells expressing mouse TRPV1 (mTRPV1-HEK), a high concentration of persulfate also evoked [Ca²⁺]_i increases. Similar [Ca²⁺]_i responses were observed in HEK 293 cells expressing human TRPA1 and human TRPV1. Current responses were also elicited by persulfate in mTRPA1- and mTRPV1-HEK. Analysis using mutated channels revealed that persulfate acted on electrophilic agonist-sensitive cysteine residues of TRPA1, and it indirectly activated TRPV1 due to the external acidification, because of the disappearance of [Ca²⁺]_i responses in acid-insensitive mTRPV1 mutant. These results demonstrate that persulfate activates nociceptive TRPA1 and TRPV1 channels. It is suggested that activation of these nociceptive channels may be involved in respiratory and skin injuries caused by exposure to this industrial sulfur compound. Thus, selective TRPA1 and TRPV1 channel blockers may be effective to remedy persulfate-induced toxic actions.

Constitutive activity of transient receptor potential vanilloid type 1 triggers spontaneous firing in nerve growth factor-treated dorsal root ganglion neurons of rats

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We examined the role of TRPV1 in the generation of spontaneous APs in NGF-treated cultured DRG neurons of rats.

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Spontaneous firing in the on-cell configuration was abolished by TRPV1 antagonists capsazepine and BCTC.

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Chronic treatment with NGF induced capsazepine- and BCTC-sensitive cation conductance.

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NGF-induced cation conductance through TRPV1 causes spontaneous firing.

Dorsal root ganglion (DRG) neurons cultured in the presence of nerve growth factor (NGF, 100 ng/ml) often show a spontaneous action potential. Underlying mechanisms of this spontaneous firing were examined using the patch clamp technique. The spontaneous firing in the on-cell configuration was abolished by a decrease in the Na⁺ concentration and by the TRPV1 antagonists capsazepine (10 μM) and BCTC (1 μM). These responses were accompanied by hyperpolarization of the resting potential. The holding current observed in neurons voltage clamped at –60 mV in the whole-cell configuration was significantly larger in the neurons that fired spontaneously, indicating that these neurons had an additional cation conductance that caused depolarization and triggered action potentials. The holding current in the firing neurons was decreased by extracellular Na⁺ reduction, capsazepine and BCTC. The amplitudes of the capsazepine- or BCTC-sensitive component of the holding current in the spontaneously firing neurons were ten times as large as those recorded in the other neurons showing no spontaneous firing. However, the amplitudes of the current responses to capsaicin (1 μM) were not different regardless of the presence of spontaneous firing or treatment with NGF. These results indicate that chronic NGF treatment of cultured DRG neurons in rats induces a constitutively active cation conductance through TRPV1, which depolarizes the neurons and triggers spontaneous action potentials in the absence of any stimuli. Since NGF in the DRG is reported to increase after nerve injury, this NGF-mediated regulation of TRPV1 may be a cause of the pathogenesis of neuropathic pain.

Peripheral gabapentin regulates mosquito allergy-induced itch in mice

The antipruritic activity of gabapentin, an anticonvulsant, was studied in a mouse model of allergic itch. In mice sensitized by an extract of the salivary glands of the mosquito (ESGM), an intradermal injection of ESGM elicited scratching and increased peripheral nerve firing. Oral or intradermal administration of gabapentin at the ESGM injection site inhibited ESGM-induced scratching and peripheral nerve firing. However, gabapentin did not affect histamine-induced scratching. The distributions of immunoreactivity to the voltage-dependent calcium channel α₂δ-1 subunit, a site of gabapentin action, and the histamine H₁ receptor differed in the mouse dorsal root ganglia. The α₂δ-1 subunit was mainly found in neurons that were 15-20 µm in diameter, whereas the H₁ receptor was mainly in 20-30 µm neurons. In addition, α₂δ-1 subunit immunoreactivity co-localized with that of transient receptor potential vanilloid 1 (TRPV1). These results suggest that gabapentin regulates allergic itch by acting on the calcium channel α₂δ-1 subunit in peripheral TRPV1-positive neurons.

Deletion of the insulin receptor in sensory neurons increases pancreatic insulin levels

Insulin is known to have neurotrophic properties and loss of insulin support to sensory neurons may contribute to peripheral diabetic neuropathy (PDN). Here, genetically-modified mice were generated in which peripheral sensory neurons lacked the insulin receptor (SNIRKO mice) to determine whether disrupted sensory neuron insulin signaling plays a crucial role in the development of PDN and whether SNIRKO mice develop symptoms of PDN due to reduced insulin neurotrophic support. Our results revealed that SNIRKO mice were euglycemic and never displayed significant changes in a wide range of sensorimotor behaviors, nerve conduction velocity or intraepidermal nerve fiber density. However, SNIRKO mice displayed elevated serum insulin levels, glucose intolerance, and increased insulin content in the islets of Langerhans of the pancreas. These results contribute to the growing idea that sensory innervation of pancreatic islets is key to regulating islet function and that a negative feedback loop of sensory neuron insulin signaling keeps this regulation in balance. Our results suggest that a loss of insulin receptors in sensory neurons does not lead to peripheral nerve dysfunction. The SNIRKO mice will be a powerful tool to investigate sensory neuron insulin signaling and may give a unique insight into the role that sensory neurons play in modifying islet physiology.

Differential effects of lipopolysaccharide on mouse sensory TRP channels

Acute neurogenic inflammation and pain associated to bacterial infection have been traditionally ascribed to sensitization and activation of sensory nerve afferents secondary to immune cell stimulation. However, we recently showed that lipopolysaccharides (LPS) directly activate the Transient Receptor Potential channels TRPA1 in sensory neurons and TRPV4 in airway epithelial cells. Here we investigated whether LPS activates other sensory TRP channels expressed in sensory neurons. Using intracellular Ca²⁺ imaging and patch-clamp we determined the effects of LPS on recombinant TRPV1, TRPV2, TRPM3 and TRPM8, heterologously expressed in HEK293T cells. We found that LPS activates TRPV1, although with lower potency than for TRPA1. Activation of TRPV1 by LPS was not affected by mutations of residues required for activation by electrophilic agents or by diacylglycerol and capsaicin. On the other hand, LPS weakly activated TRPM3, activated TRPM8 at 25 °C, but not at 35 °C, and was ineffective on TRPV2. Experiments performed in mouse dorsal root ganglion (DRG) neurons revealed that genetic ablation of Trpa1 did not abolish the responses to LPS, but remain detected in 30% of capsaicin-sensitive cells. The population of neurons responding to LPS was dramatically lower in double Trpa1/Trpv1 KO neurons. Our results show that, in addition to TRPA1, other TRP channels in sensory neurons can be targets of LPS, suggesting that they may contribute to trigger and regulate innate defenses against gram-negative bacterial infections.

Involvement of nociceptive transient receptor potential channels in repellent action of pulegone

Pulegone, one of avian repellents, is used to prevent the economic loss caused by birds. Chemical repellents often evoke unpleasant sensations and sensory irritation resulting in avoidance under some circumstances. It is recognized that some TRP channels expressing sensory neurons are related to nociception. Here we determined the molecular mechanisms of the repellent action of pulegone using isolated chicken sensory neurons and heterologous expression system. Pulegone increased the intracellular Ca²⁺ concentration ([Ca²⁺]_i) in chicken sensory neurons. There were two types of neurons exhibiting different sensitivity to pulegone. One was responded to it at low concentrations and the other at high concentrations. Pharmacological analyses revealed that the former was predominantly mediated by TRP melastatin 8 (TRPM8), and the latter by both TRP ankyrin 1 (TRPA1) and TRPM8. An activation of both channels by pulegone was also determined using heterologously expression system. At high concentrations, pulegone suppressed chicken TRPM8 but not chicken TRPA1. The intraplantar injection of pulegone in chicks caused pain-related behaviors that were attenuated by TRPA1 antagonist. These results indicate that pulegone stimulates both TRPM8 and TRPA1 channel in chicken sensory neurons and suppresses the former but not the latter at high concentrations. Together, these data suggest that the molecular target for the repellent action of pulegone in avian species is nociceptive TRPA1.

Visceral pain - Novel approaches for optogenetic control of spinal afferents

Painful stimuli arising within visceral organs are detected by peripheral nerve endings of spinal afferents, whose cell bodies are located in dorsal root ganglia (DRG). Recent technical advances have made it possible to reliably expose and inject single DRG with neuronal tracers or viruses in vivo. This has facilitated, for the first time, unequivocal identification of different types of spinal afferent endings in visceral organs. These technical advances paved the way for a very exciting series of in vivo experiments where individual DRG are injected to facilitate opsin expression (e.g. Archaerhodopsin). Organ-specific expression of opsins in sensory neurons may be achieved by retrograde viral transduction. This means activity of target-specific populations of sensory neurons, within single DRG, can be modulated by optogenetic photo-stimulation. Using this approach we implanted micro light-emitting diodes (micro-LEDs) adjacent to DRG of interest, thereby allowing focal DRG-specific control of visceral and/or somatic afferents in conscious mice. This is vastly different from broad photo-illumination of peripheral nerve endings, which are dispersed over much larger surface areas across an entire visceral organ; and embedded deep within multiple anatomical layers. Focal DRG photo-stimulation also avoids the potential that wide-field illumination of the periphery could inadvertently activate other closely apposed organs, or co-activate different classes of axons in the same organ (e.g. enteric and spinal afferent endings in the gut). It is now possible to selectively control nociceptive and/or non-nociceptive pathways to specific visceral organs in vivo, using wireless optogenetics and micro-LEDs implanted adjacent to DRG, for targeted photo-stimulation.

Accessibility of axonal G protein coupled mu-opioid receptors requires conceptual changes of axonal membrane targeting for pain modulation

The mechanisms of axonal trafficking and membrane targeting are well established for sodium channels, which are the principle targets for perineurally applied local anaesthetics. However, they have not been thoroughly investigated for G protein coupled receptors such as mu-opioid receptors (MOR). Focusing on these axonal mechanisms, we found that axonal MOR functionality is quite distinct in two different pain states, i.e. hindpaw inflammation and nerve injury. We observed axonal membrane MOR binding and functional G protein coupling exclusively at sites of CCI nerve injury. Moreover at these axonal membrane sites, MOR exhibited extensive co-localization with the membrane proteins SNAP and Na/K-ATPase as well as NGF-dependent enhanced lipid rafts and L1CAM anchoring proteins. Silencing endogenous L1CAM with intrathecal L1CAM specific siRNA, disrupting lipid rafts with the perineurial cholesterol-sequestering agent MβCD, as well as suppressing NGF receptor activation with the perineurial NGF receptor inhibitor K252a abrogated MOR axonal membrane integration, functional coupling, and agonist-elicited antinociception at sites of nerve injury. These findings suggest that local conceptual changes resulting from nerve injury are required for the establishment of functional axonal membrane MOR. Axonal integration and subsequent accessibility of functionally coupled MOR are of great relevance particularly for patients suffering from severe pain due to nerve injury or tumour infiltration.

Gap junctions, pannexins and pain

Enhanced expression and function of gap junctions and pannexin (Panx) channels have been associated with both peripheral and central mechanisms of pain sensitization. At the level of the sensory ganglia, evidence includes augmented gap junction and pannexin1 expression in glial cells and neurons in inflammatory and neuropathic pain models and increased synchrony and enhanced cross-excitation among sensory neurons by gap junction-mediated coupling. In spinal cord and in suprapinal areas, evidence is largely limited to increased expression of relevant proteins, although in several rodent pain models, hypersensitivity is reduced by treatment with gap junction/Panx1 channel blocking compounds. Moreover, targeted modulation of Cx43 expression was shown to modulate pain thresholds, albeit in somewhat contradictory ways, and mice lacking Panx1 expression globally or in specific cell types show depressed hyperalgesia. We here review the evidence for involvement of gap junctions and Panx channels in a variety of animal pain studies and then discuss ways in which gap junctions and Panx channels may mediate their action in pain processing. This discussion focusses on spread of signals among satellite glial cells, in particular intercellular Ca²⁺ waves, which are propagated through both gap junction and Panx1-dependent routes and have been associated with the phenomenon of spreading depression and the malady of migraine headache with aura.

Regulation of T-type Ca2+ channel expression by herpes simplex virus-1 infection in sensory-like ND7 cells

Infection of sensory neurons by herpes simplex virus (HSV)-1 disrupts electrical excitability, altering pain sensory transmission. Because of their low threshold for activation, functional expression of T-type Ca²⁺ channels regulates various cell functions, including neuronal excitability and neuronal communication. In this study, we have tested the effect of HSV-1 infection on the functional expression of T-type Ca²⁺ channels in differentiated ND7-23 sensory-like neurons. Voltage-gated Ca²⁺ currents were measured using whole cell patch clamp recordings in differentiated ND7-23 neurons under various culture conditions. Differentiation of ND7-23 cells evokes a significant increase in T-type Ca²⁺ current densities. Increased T-type Ca²⁺ channel expression promotes the morphological differentiation of ND7-23 cells and triggers a rebound depolarization. HSV-1 infection of differentiated ND7-23 cells causes a significant loss of T-type Ca²⁺ channels from the membrane. HSV-1 evoked reduction in the functional expression of T-type Ca²⁺ channels is mediated by several factors, including decreased expression of Ca_v3.2 T-type Ca²⁺ channel subunits and disruption of endocytic transport. Decreased functional expression of T-type Ca²⁺ channels by HSV-1 infection requires protein synthesis and viral replication, but occurs independently of Egr-1 expression. These findings suggest that infection of neuron-like cells by HSV-1 causes a significant disruption in the expression of T-type Ca²⁺ channels, which can results in morphological and functional changes in electrical excitability.

Redox-guided axonal regrowth requires cyclic GMP dependent protein kinase 1: Implication for neuropathic pain

Cyclic GMP-dependent protein kinase 1 (PKG1) mediates presynaptic nociceptive long-term potentiation (LTP) in the spinal cord and contributes to inflammatory pain in rodents but the present study revealed opposite effects in the context of neuropathic pain. We used a set of loss-of-function models for in vivo and in vitro studies to address this controversy: peripheral neuron specific deletion (SNS-PKG1^-/-), inducible deletion in subsets of neurons (SLICK-PKG1^-/-) and redox-dead PKG1 mutants. In contrast to inflammatory pain, SNS-PKG1^-/- mice developed stronger neuropathic hyperalgesia associated with an impairment of nerve regeneration, suggesting specific repair functions of PKG1. Although PKG1 accumulated at the site of injury, its activity was lost in the proximal nerve due to a reduction of oxidation-dependent dimerization, which was a consequence of mitochondrial damage in injured axons. In vitro, PKG1 deficiency or its redox-insensitivity resulted in enhanced outgrowth and reduction of growth cone collapse in response to redox signals, which presented as oxidative hotspots in growing cones. At the molecular level, PKG1 deficiency caused a depletion of phosphorylated cofilin, which is essential for growth cone collapse and guidance. Hence, redox-mediated guidance required PKG1 and consequently, its deficiency in vivo resulted in defective repair and enhanced neuropathic pain after nerve injury. PKG1-dependent repair functions will outweigh its signaling functions in spinal nociceptive LTP, so that inhibition of PKG1 is no option for neuropathic pain.

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Axonal injury leads mitochondrial damage.

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The loss of signaling ROS is associated with a reduction of redox-dependent autoactivation of PKG1.

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Loss of PKG1 impairs peripheral nerve regeneration and aggravates neuropathic pain in mice.

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Oxidative hot spots are generated in spiky growth cones and trigger growth cone collapse via PKG1.

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Malfunctioning of this redox-PKG1 guided growth cone collapse leads to aberrant outgrowth.