The peripheral terminals of primary nociceptive neurons play an essential role in pain detection mediated by membrane receptors like TRPV1, a molecular sensor of heat and capsaicin. However, the ...contribution of central terminal TRPV1 in the dorsal horn to chronic pain has not been investigated directly. Combining primary sensory neuron-specific GCaMP3 imaging with a trigeminal neuropathic pain model, we detected robust neuronal hyperactivity in injured and uninjured nerves in the skin, soma in trigeminal ganglion, and central terminals in the spinal trigeminal nucleus. Extensive TRPV1 hyperactivity was observed in central terminals innervating all dorsal horn laminae. The central terminal TRPV1 sensitization was maintained by descending serotonergic (5-HT) input from the brainstem. Central blockade of TRPV1 or 5-HT/5-HT3A receptors attenuated central terminal sensitization, excitatory primary afferent inputs, and mechanical hyperalgesia in the territories of injured and uninjured nerves. Our results reveal central mechanisms facilitating central terminal sensitization underlying chronic pain.
A new study shows that astrocytes are involved in the development of chronic itch in a mouse model. This is dependent on upregulation of lipocalin 2 (LCN2) by the transcription factor STAT-3 and ...astrogliosis.
According to current dogma, there is little or no ongoing neurogenesis in the fully developed adult enteric nervous system. This lack of neurogenesis leaves unanswered the question of how enteric ...neuronal populations are maintained in adult guts, given previous reports of ongoing neuronal death. Here, we confirm that despite ongoing neuronal cell loss because of apoptosis in the myenteric ganglia of the adult small intestine, total myenteric neuronal numbers remain constant. This observed neuronal homeostasis is maintained by new neurons formed in vivo from dividing precursor cells that are located within myenteric ganglia and express both Nestin and p75NTR, but not the pan-glial marker Sox10. Mutation of the phosphatase and tensin homolog gene in this pool of adult precursors leads to an increase in enteric neuronal number, resulting in ganglioneuromatosis, modeling the corresponding disorder in humans. Taken together, our results show significant turnover and neurogenesis of adult enteric neurons and provide a paradigm for understanding the enteric nervous system in health and disease.
Itch-specific neurons have been sought for decades. The existence of such neurons has been doubted recently as a result of the observation that itch-mediating neurons also respond to painful stimuli. ...We genetically labeled and manipulated MrgprA3(+) neurons in the dorsal root ganglion (DRG) and found that they exclusively innervated the epidermis of the skin and responded to multiple pruritogens. Ablation of MrgprA3(+) neurons led to substantial reductions in scratching evoked by multiple pruritogens and occurring spontaneously under chronic itch conditions, whereas pain sensitivity remained intact. Notably, mice in which TRPV1 was exclusively expressed in MrgprA3(+) neurons exhibited itch, but not pain, behavior in response to capsaicin. Although MrgprA3(+) neurons were sensitive to noxious heat, activation of TRPV1 in these neurons by noxious heat did not alter pain behavior. These data suggest that MrgprA3 defines a specific subpopulation of DRG neurons mediating itch. Our study opens new avenues for studying itch and developing anti-pruritic therapies.
Neuropathic pain is a refractory disease characterized by maladaptive changes in gene transcription and translation in the sensory pathway. Long noncoding RNAs (lncRNAs) are emerging as new players ...in gene regulation, but how lncRNAs operate in the development of neuropathic pain is unclear. Here we identify a conserved lncRNA, named Kcna2 antisense RNA, for a voltage-dependent potassium channel mRNA, Kcna2, in first-order sensory neurons of rat dorsal root ganglion (DRG). Peripheral nerve injury increased Kcna2 antisense RNA expression in injured DRG through activation of myeloid zinc finger protein 1, a transcription factor that binds to the Kcna2 antisense RNA gene promoter. Mimicking this increase downregulated Kcna2, reduced total voltage-gated potassium current, increased excitability in DRG neurons and produced neuropathic pain symptoms. Blocking this increase reversed nerve injury-induced downregulation of DRG Kcna2 and attenuated development and maintenance of neuropathic pain. These findings suggest endogenous Kcna2 antisense RNA as a therapeutic target for the treatment of neuropathic pain.
Due to the complex circuitry and plethora of cell types involved in somatosensation, it is becoming increasingly important to be able to observe cellular activity at the population level. In ...addition, since cells rely on an intricate variety of extracellular factors, it is important to strive to maintain the physiological environment. Many electrophysiological techniques require the implementation of artificially-produced physiological environments and it can be difficult to assess the activity of many cells simultaneously. Moreover, imaging Ca
2+
transients using Ca
2+
-sensitive dyes often requires
in vitro
preparations or
in vivo
injections, which can lead to variable expression levels. With the development of more sensitive genetically-encoded Ca
2+
indicators (GECIs) it is now possible to observe changes in Ca
2+
transients in large populations of cells at the same time. Recently, groups have used a GECI called GCaMP to address fundamental questions in somatosensation. Researchers can now induce GCaMP expression in the mouse genome using viral or gene knock-in approaches and observe the activity of populations of cells in the pain pathway such as dorsal root ganglia (DRG), spinal neurons, or glia. This approach can be used
in vivo
and thus maintains the organism’s biological integrity. The implementation of GCaMP imaging has led to many advances in our understanding of somatosensation. Here, we review the current findings in pain research using GCaMP imaging as well as discussing potential methodological considerations.
Capsaicin-sensitive sensory C-fibers derived from vagal ganglia innervate the visceral organs, and respond to inflammatory mediators and noxious stimuli. These neurons play an important role in ...maintenance of visceral homeostasis, and contribute to the symptoms of visceral inflammatory diseases. Vagal sensory neurons are located in two ganglia, the jugular ganglia (derived from the neural crest), and the nodose ganglia (from the epibranchial placodes). The functional difference, especially in response to immune mediators, between jugular and nodose neurons is not fully understood. In this study, we microscopically isolated murine nodose and jugular capsaicin-sensitive / Trpv1-expressing C-fiber neurons and performed transcriptome profiling using ultra-low input RNA sequencing. RNAseq detected genes with significantly differential expression in jugular and nodose neurons, which were mostly involved in neural functions. Transcriptional regulators, including Cited1, Hoxb5 and Prdm12 showed distinct expression patterns in the two C-fiber neuronal populations. Common and specific expression of immune receptor proteins was characterized in each neuronal type. The expression of immune receptors that have received little or no attention from vagal sensory biologists is highlighted including receptors for certain chemokines (CXCLs), interleukins (IL-4) and interferons (IFNα, IFNγ). Stimulation of immune receptors with their cognate ligands led to activation of the C-fibers in isolated functional assays.
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Pseudo-allergic reactions-adverse, non-immunologic, anaphylaxis-like sudden onset reactions mediated through an IgE-independent pathway—are activated by various basic compounds and ...occur at least as frequently as IgE-mediated reactions to drugs. A large family of G protein coupled receptors (Mas-related genes; Mrgprs) is closely related to pseudo-allergies. However, few therapies can directly target pseudo-allergies and related Mrgprs. Saikosaponin A (SSA) is effective in the treatment of passive cutaneous anaphylaxis (PCA), adjuvant arthritis, and delayed hypersensitiveness. In this study, we investigated the anti-pseudo-allergy effect of SSA and its underlying mechanism. We examined the effect of SSA on both IgE-independent and IgE-dependent responses using PCA and active systemic anaphylaxis models, as well as in vitro-cultured mast cells. We also evaluated whether the anti-allergy effect is related to Mrgprs by using in vitro Mrgprx2-expressing HEK293 cells. SSA dose dependently suppressed compound 48/80 (C48/80)-induced PCA and mast cell degranulation in mice. When SSA and C48/80 were administered together through the vein, C48/80-induced systemic anaphylaxis did not occur, and C48/80-induced shock ratio decreased dose-dependently upon SSA treatment. However, SSA did not affect IgE-dependent allergy. When administered topically 24 h before antigen challenge, Evans blue leakage and paw swelling were induced in the SSA-treated group and the vehicle group. Our in vitro studies revealed that SSA reduced C48/80-induced calcium flux and suppressed degranulation in LAD2 cells. SSA could also dose-dependently inhibit C48/80-induced Mrgprx2-expressing HEK293 cell activation. As a conclusion, SSA could inhibits IgE-independent allergy, but not IgE-dependent allergy, and this effect involves the Mrgprx2 pathway. This study provided a new sight on pseudo-allergy and its therapy.
Spinal pain is a major clinical problem, however, its origins and underlying mechanisms remain unclear. Here we report that in mice, osteoclasts induce sensory innervation in the porous endplates ...which contributes to spinal hypersensitivity in mice. Sensory innervation of the porous areas of sclerotic endplates in mice was confirmed. Lumbar spine instability (LSI), or aging, induces spinal hypersensitivity in mice. In these conditions, we show that there are elevated levels of PGE2 which activate sensory nerves, leading to sodium influx through Na
1.8 channels. We show that knockout of PGE2 receptor 4 in sensory nerves significantly reduces spinal hypersensitivity. Inhibition of osteoclast formation by knockout Rankl in the osteocytes significantly inhibits LSI-induced porosity of endplates, sensory innervation, and spinal hypersensitivity. Knockout of Netrin-1 in osteoclasts abrogates sensory innervation into porous endplates and spinal hypersensitivity. These findings suggest that osteoclast-initiated porosity of endplates and sensory innervation are potential therapeutic targets for spinal pain.
In humans, intradermal administration of β-alanine (ALA) and bovine adrenal medulla peptide 8-22 (BAM8-22) evokes the sensation of itch. Currently, it is unknown which human dorsal root ganglion ...(DRG) neurons express the receptors of these pruritogens, MRGPRD and MRGPRX1, respectively, and which cutaneous afferents these pruritogens activate in primate. In situ hybridization studies revealed that MRGPRD and MRGPRX1 are co-expressed in a subpopulation of TRPV1+ human DRG neurons. In electrophysiological recordings in nonhuman primates (
), subtypes of polymodal C-fiber nociceptors are preferentially activated by ALA and BAM8-22, with significant overlap. When pruritogens ALA, BAM8-22, and histamine, which activate different subclasses of C-fiber afferents, are administered in combination, human volunteers report itch and nociceptive sensations similar to those induced by a single pruritogen. Our results provide evidence for differences in pruriceptive processing between primates and rodents, and do not support the spatial contrast theory of coding of itch and pain.
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