Probes for INS

Epidermal keratinocytes express various purinergic 2 receptors that play an essential role in cell growth, differentiation, and proliferation. In the conditions of injury, concentrations of extracellular adenosine triphosphate (ATP) may dramatically increase due to cell damage and inflammatory processes. In this situation activation of purinergic signaling in keratinocytes could act as a double-edged sword contributing to skin regeneration or cell apoptosis. As the role of keratinocytes in transducing and modulating nociceptive stimuli has been increasingly appreciated in recent years, the aim of the present study was to evaluate whether peripheral nerve injury affects purinergic signaling in keratinocytes. Spared nerve injury (SNI), a classical model of peripheral neuropathic pain, was induced in mice. The injury was induced by sparing of the tibial nerve, and ligation and cut of the sural and common peroneal nerves. Keratinocytes were isolated and cultured on Days 2-4 post-injury and ATP-mediated calcium responses in keratinocytes were examined by confocal imaging. On average, the number of keratinocytes that responded to ATP with an increase in intracellular calcium gradient as well as the magnitude of the peak response was not significantly different between sham and SNI groups. However, significantly less delay in ATP-induced increase in intracellular calcium concentration was observed in keratinocytes in SNI group compared to sham. Selective pharmacological inhibition of keratinocyte response to ATP indicated a major role of P2 × 4 receptors in the modulation of calcium homeostasis in SNI. Our results indicate that epidermal purinergic signaling undergoes dramatic changes following peripheral nerve injury that may contribute to injury-induced mechanical hypersensitivity.

Many patients with sickle cell disease (SCD) suffer from chronic pain, the underlying causes of which are unclear. Recent 16s ribosomal RNA sequencing studies revealed differences in the number and types of bacteria in the gastrointestinal tract of patients and mouse models of SCD relative to controls, but it is unclear if or how these changes contribute to symptomology. In these experiments, we used transgenic SCD mice to determine the extent to which disease related gut dysbiosis contributes to persistent pain. Reflexive pain behaviors were first measured in SCD mice following longitudinal probiotic or antibiotic treatment. Vehicle-treated SCD mice displayed significant mechanical allodynia relative to vehicle-treated wildtype mice, and antibiotic treatment further exacerbated mechanical allodynia in both genotypes. In contrast, probiotic treatment completely reversed persistent touch hypersensitivity in SCD mice. Persistent touch pain was also transiently reversed in SCD mice following fecal material transplant from healthy mice. In complementary experiments, wildtype recipient mice developed cold and touch hypersensitivity that persisted for several weeks after fecal material transplant from SCD donors. Using whole-cell patch clamp recordings, we further determined that these behavioral observations were accompanied by altered intrinsic plasticity in a select class of nodose ganglia sensory neurons, the peripheral terminals of which are well positioned to detect sensory information in the gut. Nodose ganglia neurons isolated from animals that received sickle cell fecal material transplants were hyperexcitable relative to those isolated from animals that received control fecal material transplants. These data are the first to suggest that disease-related gut dysbiosis induces pain through changes in vagal nerve activity. Ongoing studies are examining specific bacterial populations and/or metabolites responsible for these functional changes in order to develop novel therapeutics for chronic SCD pain management. Grant support from National Institutes of Health grants K99HL155791 and R01NS070711.

The loss of GABAergic inhibition is a mechanism that underlies neuropathic pain. Therefore, rescuing the GABAergic inhibitory tone through the activation of GABA A receptors is a strategy to reduce neuropathic pain. This study was designed to elucidate the function of the spinal α 6 -containing GABA A receptor in physiological conditions and neuropathic pain in female and male rats. Results show that α 6 -containing GABA A receptor blockade or transient α 6 -containing GABA A receptor knockdown induces evoked hypersensitivity and spontaneous pain in naive female rats. The α 6 subunit is expressed in IB4 + and CGRP + primary afferent neurons in the rat spinal dorsal horn and dorsal root ganglia but not astrocytes. Nerve injury reduces α 6 subunit protein expression in the central terminals of the primary afferent neurons and dorsal root ganglia, whereas intrathecal administration of positive allosteric modulators of the α 6 -containing GABA A receptor reduces tactile allodynia and spontaneous nociceptive behaviors in female, but not male, neuropathic rats and mice. Overexpression of the spinal α 6 subunit reduces tactile allodynia and restores α 6 subunit expression in neuropathic rats. Positive allosteric modulators of the α 6 -containing GABA A receptor induces a greater antiallodynic effect in female rats and mice compared with male rats and mice. Finally, α 6 subunit is expressed in humans. This receptor is found in CGRP + and P2X3 + primary afferent fibers but not astrocytes in the human spinal dorsal horn. Our results suggest that the spinal α 6 -containing GABA A receptor has a sex-specific antinociceptive role in neuropathic pain, suggesting that this receptor may represent an interesting target to develop a novel treatment for neuropathic pain.

Pain and itch are distinct sensations arousing evasion and compulsive desire for scratching, respectively. It's unclear whether they could invoke different neural networks in the brain. Here, we use the type 1 herpes simplex virus H129 strain to trace the neural networks derived from two types of dorsal root ganglia (DRG) neurons: one kind of polymodal nociceptors containing galanin (Gal) and one type of pruriceptors expressing neurotensin (Nts). The DRG microinjection and immunosuppression were performed in transgenic mice to achieve a successful tracing from specific types of DRG neurons to the primary sensory cortex. About one-third of nuclei in the brain were labeled. More than half of them were differentially labeled in two networks. For the ascending pathways, the spinothalamic tract was absent in the network derived from Nts-expressing pruriceptors, and the two networks shared the spinobulbar projections but occupied different subnuclei. As to the motor systems, more neurons in the primary motor cortex and red nucleus of the somatic motor system participated in the Gal-containing nociceptor-derived network, while more neurons in the nucleus of the solitary tract (NST) and the dorsal motor nucleus of vagus nerve (DMX) of the emotional motor system was found in the Nts-expressing pruriceptor-derived network. Functional validation of differentially labeled nuclei by c-Fos test and chemogenetic inhibition suggested the red nucleus in facilitating the response to noxious heat and the NST/DMX in regulating the histamine-induced scratching. Thus, we reveal the organization of neural networks in a DRG neuron type-dependent manner for processing pain and itch.

Opioid signaling has been shown to be critically important in the neuromodulation of sensory circuits in the superficial spinal cord. Agonists of the mu-opioid receptor (MOR) elicit itch, whereas agonists of the kappa-opioid receptor (KOR) have been shown to inhibit itch. Despite the clear roles of MOR and KOR for the modulation itch, whether the delta-opioid receptor (DOR) is involved in the regulation of itch remained unknown. Here, we show that intrathecal administration of DOR agonists suppresses chemical itch and that intrathecal application of DOR antagonists is sufficient to evoke itch. We identify that spinal enkephalin neurons co-express neuropeptide Y (NPY), a peptide previously implicated in the inhibition of itch. In the spinal cord, DOR overlapped with both the NPY receptor (NPY1R) and KOR, suggesting that DOR neurons represent a site for convergent itch information in the dorsal horn. Lastly, we found that neurons co-expressing DOR and KOR showed significant Fos induction following pruritogen-evoked itch. These results uncover a role for DOR in the modulation of itch in the superficial dorsal horn. Perspective: This article reveals the role of the delta-opioid receptor in itch. Intrathecal administration of delta agonists suppresses itch whereas the administration of delta antagonists is sufficient to induce itch. These studies highlight the importance of delta-opioid signaling for the modulation of itch behaviors, which may represent new targets for the management of itch disorders.

Visceral hypersensitivity (VH) is commonly cited as a major driver of chronic abdominal pain in “functional” gastrointestinal disorders (e.g., irritable bowel syndrome) where persistent and/or recurrent abdominal pain is the primary unifying symptom regardless of any alterations in bowel habits. The complexity of VH is in part influenced by genetic factors and individual differences in gut microbiome composition, yet specific mechanisms that generate VH remain incompletely understood. Correspondingly, current treatments to primarily focus on symptom management rather than targeting physiological mechanisms responsible for generating VH. We have begun to examine the role of genetic susceptibility and microbiome response dynamics in VH development using a preclinical model of intracolonic zymosan (ZYM) administration in which there are strain differences to VH susceptibility. Preliminary data reveals differential susceptibility between ZYM-induced VH in two closely related C57BL/6 sub strains, one from Taconic Biosciences (C57BL/6NTac) and the other from Jackson Laboratory (C57BL/6J). We have identified a VH candidate gene that encodes the arginine-vasopressin receptor 1A (AVPR1A) protein. We have further observed dynamic strain differences in the location and composition of the gut microbiome in response to ZYM corresponding to VH susceptibility. Ongoing studies are focused on teasing apart the potential bidirectional relationship(s) between genetic susceptibility and host-microbiome interactions in the etiology of VH. Identifying underlying mechanisms that drive VH would provide novel targets for pharmacological intervention and decrease reliance on opioids, which are prescribed at a significantly higher rate to patients who report abdominal pain with no accompanying structural disease. Grant support from R21 NS104789/NS/NINDS (KMB), R03 NS096454/NS/NINDS (KMB), Rita Allen Foundation Award in Pain (KMB), P20GM103418 (EEY and KMB), and a K-INBRE recruitment startup package.

Chronic-pain is a debilitating illness that has become exceedingly widespread with currently limited treatments. Differences in the molecular signature of nociceptors, have been demonstrated between human and the commonly-used mouse model, suggesting functional differences in detection and transmission of noxious-stimuli. Therefore, direct understanding of pain-physiology in humans is required for pain treatment. This could be facilitated by studying humans carrying deleterious genetic mutations affecting pain sensation. The transient receptor potential vanilloid 1 (TRPV1) channel is associated with several body-functions, in particular, noxious-heat detection and inflammatory-pain. Reports of adverse effects in human trials have hinder the clinical development of TRPV1 antagonists as novel pain relievers. Hence, studies on the functional roles of TRPV1, which currently rely mainly on evidences obtained from rodents, should be extended to humans. Here, we examined humans carrying a unique missense mutation in TRPV1, rendering the channel non-functional. The affected individual demonstrated lack of aversion towards capsaicin and elevated heat-pain threshold. Surprisingly, he showed elevated cold-pain threshold and extensive neurogenic inflammatory flare and pain-responses following application of the TRPA1 channel-activator, mustard-oil. Our study provides the first direct evidence for pain-related functional-changes linked to TRPV1 in humans, which is a prime target in the development of novel pain-relievers.

Although itch and pain have many similarities, they are completely different in perceptual experience and behavioral response. In recent years, we have a deep understanding of the neural pathways of itch sensation transmission. However, there are few reports on the role of non-neuronal cells in itch. Microglia are known to play a key role in chronic neuropathic pain and acute inflammatory pain. It is still unknown whether microglia are also involved in regulating the transmission of itch sensation. In the present study, we used several kinds of transgenic mice to specifically deplete CX3CR1+ central microglia and peripheral macrophages together (whole depletion), or selectively deplete central microglia alone (central depletion). We observed that the acute itch responses to histamine, compound 48/80 and chloroquine were all significantly reduced in mice with either whole or central depletion. Spinal c-fos mRNA assay and further studies revealed that histamine and compound 48/80, but not chloroquine elicited primary itch signal transmission from DRG to spinal npr1- and somatostatin-positive neurons relied on microglial CX3CL1-CX3CR1 pathway. Our results indicated that central microglia were involved in multiple types of acute chemical itch transmission, while the underlying mechanisms for histamine dependent and non-dependent itch transmission were different that the former required the CX3CL1-CX3CR1 signal pathway.

This volume contains experimental approaches that are currently revolutionizing our understanding of the neurobiology of pain. The chapters cover many cutting-edge methods including the identification of gene expression profiles, transcriptomes or translatomes, from individual cells or defined groups of cells in rodents and primates;  the electrophysiological investigation of human tissues, such as human dorsal root ganglion neurons; ways to assess modality response profiles of neurons using calcium imaging in vitro and in vivo; and somatosensory behaviors in rodents using high-speed videography and machine learning.  In the _Neuromethods_ series style, the chapters include detailed advice from specialists to obtain successful results in your laboratory.

Following tissue injury, latent sensitization (LS) of nociceptive signaling can persist indefinitely, kept in remission by compensatory µ-opioid receptor constitutive activity (MORCA) in the dorsal horn of the spinal cord. To demonstrate LS, we conducted plantar incision in mice and then waited 3-4 weeks for hypersensitivity to resolve. At this time (remission), systemic administration of the opioid receptor antagonist/inverse agonist naltrexone reinstated mechanical and heat hypersensitivity. We first tested the hypothesis that LS extends to serotonergic neurons in the rostral ventral medulla (RVM) that convey pronociceptive input to the spinal cord. We report that in male and female mice, hypersensitivity was accompanied by increased Fos expression in serotonergic neurons of the RVM, abolished upon chemogenetic inhibition of RVM 5-HT neurons, and blocked by intrathecal injection of the 5-HT3R antagonist ondansetron; the 5-HT2AR antagonist MDL-11,939 had no effect. Second, to test for MORCA, we microinjected the MOR inverse agonist CTAP and/or neutral opioid receptor antagonist 6β-naltrexol. Intra-RVM CTAP produced mechanical hypersensitivity at both hindpaws. 6β-naltrexol had no effect by itself, but blocked CTAP-induced hypersensitivity. This indicates that MORCA, rather than an opioid ligand-dependent mechanism, maintains LS in remission. We conclude that incision establishes LS in descending RVM 5-HT neurons that drives pronociceptive 5-HT3R signaling in the dorsal horn, and this LS is tonically opposed by MORCA in the RVM. The 5-HT3 receptor is a promising therapeutic target for the development of drugs to prevent the transition from acute to chronic post-surgical pain.Significance statementSurgery leads to latent pain sensitization and a compensatory state of endogenous pain control that is maintained long after tissue healing. Here we show that either chemogenetic inhibition of serotonergic neuron activity in the rostral ventromedial medulla (RVM), or pharmacological inhibition of 5-HT3 receptor signaling at the spinal cord blocks behavioral signs of post-surgical latent sensitization. We conclude that µ-opioid receptor constitutive activity (MORCA) in the RVM opposes descending serotonergic facilitation of LS, and that the 5-HT3 receptor is a promising therapeutic target for the development of drugs to prevent the transition from acute to chronic post-surgical pain.

Neuropathic pain is amongst the most common non-communicable disorders and the poor effectiveness of current treatment is an unmet need. Although pain is a universal experience, there are significant inter-individual phenotypic differences. Developing models that can accurately recapitulate the clinical pain features is crucial to better understand underlying pathophysiological mechanisms and find innovative treatments. Current data from heterologous expression systems that investigate properties of specific molecules involved in pain signaling, and from animal models, show limited success with their translation into the development of novel treatments for pain. This is in part because they do not recapitulate the native environment in which a particular molecule functions, and due to species-specific differences in the properties of several key molecules that are involved in pain signaling. The limited availability of post-mortem tissue, in particular dorsal root ganglia (DRG), has hampered research using human cells in pre-clinical studies. Human induced-pluripotent stem cells (iPSCs) have emerged as an exciting alternative platform to study patient-specific diseases. Sensory neurons that are derived from iPSCs (iPSC-SNs) have provided new avenues towards elucidating peripheral pathophysiological mechanisms, the potential for development of personalized treatments, and as a cell-based system for high-throughput screening for discovering novel analgesics. Nevertheless, reprogramming and differentiation protocols to obtain nociceptors have mostly yielded immature homogenous cell populations that do not recapitulate the heterogeneity of native sensory neurons. To close the gap between native human tissue and iPSCs, alternative strategies have been developed. We will review here recent developments in differentiating iPSC-SNs and their use in pre-clinical translational studies. Direct conversion of stem cells into the cells of interest has provided a more cost- and time-saving method to improve reproducibility and diversity of sensory cell types. Furthermore, multi-cellular strategies that mimic in vivo microenvironments for cell maturation, by improving cell contact and communication (co-cultures), reproducing the organ complexity and architecture (three-dimensional organoid), and providing iPSCs with the full spatiotemporal context and nutrients needed for acquiring a mature phenotype (xenotransplantation), have led to functional sensory neuron-like systems. Finally, this review touches on novel prospective strategies, including fluorescent-tracking to select the differentiated neurons of relevance, and dynamic clamp, an electrophysiological method that allows direct manipulation of ionic conductances that are missing in iPSC-SNs.

The development of physical dependence and addiction disorders due to misuse of opioid analgesics is a major concern with pain therapeutics. We developed a mouse model of oxycodone exposure and subsequent withdrawal in the presence or absence of chronic neuropathic pain. Oxycodone withdrawal alone triggered robust gene expression adaptations in the nucleus accumbens, medial prefrontal cortex and ventral tegmental area, with numerous genes and pathways selectively affected by oxycodone withdrawal in mice with peripheral nerve injury. Pathway analysis predicted that histone deacetylase (HDAC) 1 is a top upstream regulator in opioid withdrawal in nucleus accumbens and medial prefrontal cortex. The novel HDAC1/HDAC2 inhibitor, Regenacy Brain Class I HDAC Inhibitor (RBC1HI), attenuated behavioral manifestations of oxycodone withdrawal, especially in mice with neuropathic pain. These findings suggest that inhibition of HDAC1/HDAC2 may provide an avenue for patients with chronic pain who are dependent on opioids to transition to non-opioid analgesics.