Molecular and cellular mechanisms that initiate pain and itch

TRPV4 Channel Signaling in Macrophages Promotes Gastrointestinal Motility via Direct Effects on Smooth Muscle Cells

Intestinal macrophages are critical for gastrointestinal (GI) homeostasis, but our understanding of their role in regulating intestinal motility is incomplete. Here, we report that CX3C chemokine receptor 1-expressing muscularis macrophages (MMs) were required to maintain normal GI motility. MMs expressed the transient receptor potential vanilloid 4 (TRPV4) channel, which senses thermal, mechanical, and chemical cues. Selective pharmacologic inhibition of TRPV4 or conditional deletion of TRPV4 from macrophages decreased intestinal motility and was sufficient to reverse the GI hypermotility that is associated with chemotherapy treatment. Mechanistically, stimulation of MMs via TRPV4 promoted the release of prostaglandin E2 and elicited colon contraction in a paracrine manner via prostaglandin E receptor signaling in intestinal smooth muscle cells without input from the enteric nervous system. Collectively, our data identify TRPV4-expressing MMs as an essential component required for maintaining normal GI motility and provide potential drug targets for GI motility disorders.

Sensitization of C-fiber nociceptors in mice with sickle cell disease is decreased by local inhibition of anandamide hydrolysis

Chronic pain and hyperalgesia, as well as pain resulting from episodes of vaso-occlusion, are characteristic features of sickle cell disease (SCD) and are difficult to treat. Since there is growing evidence that increasing local levels of endocannabinoids can decrease hyperalgesia, we examined the effects of URB597, a fatty acid amide hydrolase (FAAH) inhibitor, which blocks the hydrolysis of the endogenous cannabinoid anandamide, on hyperalgesia and sensitization of cutaneous nociceptors in a humanized mouse model of SCD. Using homozygous HbSS-BERK sickle mice, we determined the effects of URB597 on mechanical hyperalgesia and on sensitization of C-fiber nociceptors in vivo. Intraplantar administration of URB597 (10 μg in 10 μL) decreased the frequency of withdrawal responses evoked by a von Frey monofilament (3.9 mN bending force) applied to the plantar hind paw. This was blocked by the CB1 receptor antagonist AM281 but not by the CB2 receptor antagonist AM630. Also, URB597 decreased hyperalgesia in HbSS-BERK/CB2R sickle mice, further confirming the role of CB1 receptors in the effects produced by URB597. Electrophysiological recordings were made from primary afferent fibers of the tibial nerve in anesthetized mice. The proportion of Aδ- and C-fiber nociceptors that exhibited spontaneous activity and responses of C-fibers to mechanical and thermal stimuli were greater in HbSS-BERK sickle mice as compared to control HbAA-BERK mice. Spontaneous activity and evoked responses of nociceptors were decreased by URB597 via CB1 receptors. It is suggested that enhanced endocannabinoid activity in the periphery may be beneficial in alleviating chronic pain associated with SCD.

Neuro-immune interactions in allergic diseases: novel targets for therapeutics

Recent studies have highlighted an emerging role for neuro-immune interactions in mediating allergic diseases. Allergies are caused by an overactive immune response to a foreign antigen. The peripheral sensory and autonomic nervous system densely innervates mucosal barrier tissues including the skin, respiratory tract and gastrointestinal (GI) tract that are exposed to allergens. It is increasingly clear that neurons actively communicate with and regulate the function of mast cells, dendritic cells, eosinophils, Th2 cells and type 2 innate lymphoid cells in allergic inflammation. Several mechanisms of cross-talk between the two systems have been uncovered, with potential anatomical specificity. Immune cells release inflammatory mediators including histamine, cytokines or neurotrophins that directly activate sensory neurons to mediate itch in the skin, cough/sneezing and bronchoconstriction in the respiratory tract and motility in the GI tract. Upon activation, these peripheral neurons release neurotransmitters and neuropeptides that directly act on immune cells to modulate their function. Somatosensory and visceral afferent neurons release neuropeptides including calcitonin gene-related peptide, substance P and vasoactive intestinal peptide, which can act on type 2 immune cells to drive allergic inflammation. Autonomic neurons release neurotransmitters including acetylcholine and noradrenaline that signal to both innate and adaptive immune cells. Neuro-immune signaling may play a central role in the physiopathology of allergic diseases including atopic dermatitis, asthma and food allergies. Therefore, getting a better understanding of these cellular and molecular neuro-immune interactions could lead to novel therapeutic approaches to treat allergic diseases.

Sensory TRP channels contribute differentially to skin inflammation and persistent itch

Although both persistent itch and inflammation are commonly associated with allergic contact dermatitis (ACD), it is not known if they are mediated by shared or distinct signaling pathways. Here we show that both TRPA1 and TRPV1 channels are required for generating spontaneous scratching in a mouse model of ACD induced by squaric acid dibutylester (SADBE), a small molecule hapten, through directly promoting the excitability of pruriceptors. TRPV1 but not TRPA1 channels protect the skin inflammation, as genetic ablation of TRPV1 function or pharmacological ablation of TRPV1-positive sensory nerves promotes cutaneous inflammation in the SADBE-induced ACD. Our results demonstrate that persistent itch and inflammation are mediated by distinct cellular and molecular mechanisms in a mouse model of ACD. Identification of distinct roles of TRPA1 and TRPV1 in regulating itch and inflammation may provide new insights into the pathophysiology and treatment of chronic itch and inflammation in ACD patients.

Allergic contact dermatitis is associated both with persistent itch and inflammation, but it is not known if these are mediated by shared signaling pathways. The authors show that persistent itch requires both TRPA1 and TRPV1, while TRPV1 has a protective role against skin inflammation in mice.

High-fat diet increases pain behaviors in rats with or without obesity

Obesity is associated with increased risk for chronic pain. Basic mechanisms for this association are poorly understood. Using a milder version of a radicular pain model, local inflammation of the dorsal root ganglion (DRG), we observed marked increases in mechanical and cold allodynia in rats of both sexes that were maintained on a high-fat diet (HFD) for 6 weeks prior to DRG inflammation. Notably, this increase in pain-related behaviors occurred in both Long-Evans and Sprague-Dawley rats despite the fact that the 6-week HFD exposure induced obesity (e.g., increased insulin, leptin, weight, and percent body fat) in the Long-Evans, but not Sprague-Dawley, strains. This suggested that HFD, rather than obesity per se, increased pain behaviors. Increased pain behaviors were observed even after a much shorter (1 week) exposure to the HFD but the effect was smaller. HFD also increased behavioral responses and paw swelling to paw injection of complete Freund’s adjuvant, a model of peripheral inflammatory pain. No change was detected in plasma cytokine levels in HFD rats. However, increased macrophage infiltration of the DRG was observed in response to the HFD, absent any pain model. The results suggest that HFD can increase pain even when it does not cause obesity.

Transient receptor potential vanilloid 4-expressing macrophages and keratinocytes contribute differentially to allergic and nonallergic chronic itch

BACKGROUND

Chronic itch is a highly debilitating symptom that underlies many medical disorders with no universally effective treatments. Although unique neuronal signaling cascades in the sensory ganglia and spinal cord have been shown to critically promote the pathogenesis of chronic itch, the role of skin-associated cells remains poorly understood.

OBJECTIVE

We sought to examine the cutaneous mechanisms underlying transient receptor potential vanilloid 4 (TRPV4)-mediated allergic and nonallergic chronic itch.

METHODS

Expression of TRPV4 in chronic itch and healthy control skin preparations was examined by using real-time RT-PCR. Trpv4^eGFP mice were used to study the expression and function of TRPV4 in the skin by means of immunofluorescence staining, flow cytometry, calcium imaging, and patch-clamp recordings. Genetic and pharmacologic approaches were used to examine the role and underlying mechanisms of TRPV4 in mouse models of dry skin-associated chronic itch and spontaneous scratching associated with squaric acid dibutylester-induced allergic contact dermatitis.

RESULTS

TRPV4 is selectively expressed by dermal macrophages and epidermal keratinocytes in mice. Lineage-specific deletion of TRPV4 in macrophages and keratinocytes reduces allergic and nonallergic chronic itch in mice, respectively. Importantly, TRPV4 expression is significantly increased in skin biopsy specimens from patients with chronic idiopathic pruritus in comparison with skin from healthy control subjects. Moreover, TRPV4-dependent chronic itch requires 5-hydroxytryptamine (5-HT) signaling secondary to activation of distinct 5-HT receptors in mice with allergic and those with nonallergic chronic itch conditions.

CONCLUSION

Our study reveals previously unrecognized mechanisms by which TRPV4-expressing epithelial and immune cells in the skin critically and dynamically mediate chronic itch and unravels novel targets for therapeutics in the setting of chronic itch.

The cell biology of acute itch

Itch, the irritation we feel and the relief that comes from scratching, is an evolutionary warning system and defense against harmful environmental agents. Although once considered a subtype of pain, itch is now recognized as a unique sense, with its own distinct physiology and cell receptors. Here, we discuss recent advances in our understanding of itch and the molecular players that mediate this sensory modality.

TNF-α/TNFR1 Signaling is Required for the Full Expression of Acute and Chronic Itch in Mice via Peripheral and Central Mechanisms

Increasing evidence suggests that cytokines and chemokines play crucial roles in chronic itch. In the present study, we evaluated the roles of tumor necrosis factor-alpha (TNF-α) and its receptors TNF receptor subtype-1 (TNFR1) and TNFR2 in acute and chronic itch in mice. Compared to wild-type (WT) mice, TNFR1-knockout (TNFR1-KO) and TNFR1/R2 double-KO (DKO), but not TNFR2-KO mice, exhibited reduced acute itch induced by compound 48/80 and chloroquine (CQ). Application of the TNF-synthesis inhibitor thalidomide and the TNF-α antagonist etanercept dose-dependently suppressed acute itch. Intradermal injection of TNF-α was not sufficient to evoke scratching, but potentiated itch induced by compound 48/80, but not CQ. In addition, compound 48/80 induced TNF-α mRNA expression in the skin, while CQ induced its expression in the dorsal root ganglia (DRG) and spinal cord. Furthermore, chronic itch induced by dry skin was reduced by administration of thalidomide and etanercept and in TNFR1/R2 DKO mice. Dry skin induced TNF-α expression in the skin, DRG, and spinal cord and TNFR1 expression only in the spinal cord. Thus, our findings suggest that TNF-α/TNFR1 signaling is required for the full expression of acute and chronic itch via peripheral and central mechanisms, and targeting TNFR1 may be beneficial for chronic itch treatment.

Centipede venom peptide SsmTX-I with two intramolecular disulfide bonds shows analgesic activities in animal models

Pain is a major symptom of many diseases and results in enormous pressures on human body or society. Currently, clinically used analgesic drugs, including opioids and nonsteroidal anti-inflammatory drugs, have adverse reactions, and thus, the development of new types of analgesic drug candidates is urgently needed. Animal venom peptides have proven to have potential as new types of analgesic medicine. In this research, we describe the isolation and characterization of an analgesic peptide from the crude venom of centipede, Scolopendra subspinipes mutilans. The amino acid sequence of this peptide was identical with SsmTX-I that was previously reported as a specific Kv2.1 ion channel blocker. Our results revealed that SsmTX-I was produced by posttranslational processing of a 73-residue prepropeptide. The intramolecular disulfide bridge motifs of SsmTX-I was Cys1-Cys3 and Cys2-Cys4. Functional assay revealed that SsmTX-I showed potential analgesic activities in formalin-induced paw licking, thermal pain, and acetic acid-induced abdominal writhing mice models. Our research provides the first report of cDNA sequences, disulfide motif, successful synthesis, and analgesic potential of SsmTX-I for the development of pain-killing drugs. It indicates that centipede peptide toxins could be a treasure trove for the search of novel analgesic drug candidates. Copyright © 2017 European Peptide Society and John Wiley & Sons, Ltd.

Antinociceptive activity and mechanism of action of hydroalcoholic extract and dichloromethane fraction of Amphilophium crucigerum seeds in mice

ETHNOPHARMACOLOGICAL RELEVANCE

The medicinal plant generally known as monkey's comb (Amphilophium crucigerum) has been popularly described for the treatment of neuropathic and inflammatory pain, specially seeds preparations.

AIM OF THE STUDY

The goal of the present study was to evaluate the antinociceptive effect of the crude extract (Crd) and dichloromethane fraction (Dcm) of A. crucigerum seeds, and investigate the involvement of transient receptor potential vanilloid 1 (TRPV1) receptor in this effect.

MATERIALS AND METHODS

Male Swiss mice were used in this study. The effects of Crd and Dcm was tested on capsaicin-induced Ca²⁺ influx or the specific binding of [³H]-resiniferatoxin. Moreover, after treatment with Crd or Dcm, animals were exposed to acute pain (hot water tail-flick and capsaicin intraplantar test) or chronic pain models (injection of complete Freund's adjuvant or partial ligation of the sciatic nerve). Acute adverse effects were also noted: locomotor activity, corporal temperature, hepatic or renal damage, gastrointestinal transit alteration, and ulcerogenic activity.

RESULTS

The oral administration of Crd or Dcm resulted in an antinociceptive effect in the hot water tail-flick (48°C) and capsaicin intraplantar tests. Furthermore, these preparations exhibited antinociceptive and anti-inflammatory effects in a chronic inflammatory pain model, and antinociceptive effects in a neuropathic pain model. Moreover, Crd and Dcm reduced capsaicin-induced Ca²⁺ influx and diminished the [³H]-resiniferatoxin specific binding to spinal cord membranes. Acute adverse events were not found with Crd or Dcm administration.

CONCLUSION

In conclusion, our results support the analgesic effect of A. crucigerum and suggest the presence of compounds that may act as TRPV1 antagonists.

The Use of DREADDs to Deconstruct Behavior

A central goal in understanding brain function is to link specific cell populations to behavioral outputs. In recent years, the selective targeting of specific neural circuits has been made possible with the development of new experimental approaches, including chemogenetics. This technique allows for the control of molecularly defined subsets of cells through engineered G protein-coupled receptors (GPCRs), which have the ability to activate or silence neuronal firing. Through chemogenetics, neural circuits are being linked to behavioral outputs at an unprecedented rate. Further, the coupling of chemogenetics with imaging techniques to monitor neural activity in freely moving animals now makes it possible to deconstruct the complex whole-brain networks that are fundamental to behavioral states. In this review, we highlight a specific chemogenetic application known as DREADDs (designer receptors exclusively activated by designer drugs). DREADDs are used ubiquitously to modulate GPCR activity in vivo and have been widely applied in the basic sciences, particularly in the field of behavioral neuroscience. Here, we focus on the impact and utility of DREADD technology in dissecting the neural circuitry of various behaviors including memory, cognition, reward, feeding, anxiety and pain. By using DREADDs to monitor the electrophysiological, biochemical, and behavioral outputs of specific neuronal types, researchers can better understand the links between brain activity and behavior. Additionally, DREADDs are useful in studying the pathogenesis of disease and may ultimately have therapeutic potential.

The microbiota-gut-brain axis and its potential therapeutic role in autism spectrum disorder

Autism spectrum disorder (ASD) is a series of neurodevelopmental disorders that are characterized by deficits in both social and cognitive functions. Although the exact etiology and pathology of ASD remain unclear, a disorder of the microbiota-gut-brain axis is emerging as a prominent factor in the generation of autistic behaviors. Clinical studies have shown that gastrointestinal symptoms and compositional changes in the gut microbiota frequently accompany cerebral disorders in patients with ASD. A disturbance in the gut microbiota, which is usually induced by a bacterial infection or chronic antibiotic exposure, has been implicated as a potential contributor to ASD. The bidirectional microbiota-gut-brain axis acts mainly through neuroendocrine, neuroimmune, and autonomic nervous mechanisms. Application of modulators of the microbiota-gut-brain axis, such as probiotics, helminthes and certain special diets, may be a promising strategy for the treatment of ASD. This review mainly discusses the salient observations of the disruptions of the microbiota-gut-brain axis in the pathogenesis of ASD and reveals its potential therapeutic role in autistic deficits.

Eact, a small molecule activator of TMEM16A, activates TRPV1 and elicits pain- and itch-related behaviours

BACKGROUND AND PURPOSE

TMEM16A, also known as anoctamin 1 channel, is a member of the Ca(2+)-activated chloride channels family and serves as a heat sensor in the primary nociceptors. Eact is a recently discovered small molecule activator of the TMEM16A channel. Here, we asked if Eact produces pain- and itch-related responses in vivo and investigated the cellular and molecular basis of Eact-elicited responses in dorsal root ganglia (DRG) neurons.

EXPERIMENTAL APPROACH

We employed behavioural testing combined with pharmacological inhibition and genetic ablation approaches to identify transient receptor potential vanilloid 1 (TRPV1) as the prominent mediator for Eact-evoked itch- or pain-related responses. We investigated the effects of Eact on TRPV1 and TMEM16A channels expressed in HEK293T cells and in DRG neurons isolated from wild type and Trpv1(-/-) mice using Ca(2+) imaging and patch-clamp recordings. We also used site-directed mutagenesis to determine the molecular basis of Eact activation of TRPV1.

KEY RESULTS

Administration of Eact elicited both itch- and pain-related behaviours. Unexpectedly, the Eact-elicited behavioural responses were dependent on the function of TRPV1, as shown by pharmacological inhibition and genetic ablation studies. Eact activated membrane currents and increased intracellular free Ca(2+) in both TRPV1-expressing HEK293T cells and isolated DRG neurons in a TRPV1-dependent manner. Eact activation of the TRPV1 channel was severely attenuated by mutations disrupting the capsaicin-binding sites.

CONCLUSIONS AND IMPLICATIONS

Our results suggest that Eact activates primary sensory nociceptors and produces both pain and itch responses mainly through direct activation of TRPV1 channels.

Corneal Dendritic Cell Density Is Associated with Subbasal Nerve Plexus Features, Ocular Surface Disease Index, and Serum Vitamin D in Evaporative Dry Eye Disease

Dry eye disease (DED) has evolved into a major public health concern with ocular discomfort and pain being responsible for significant morbidity associated with DED. However, the etiopathological factors contributing to ocular pain associated with DED are not well understood. The current IVCM based study investigated the association between corneal dendritic cell density (DCD), corneal subbasal nerve plexus (SBNP) features, and serum vitamin D and symptoms of evaporative dry eye (EDE). The study included age and sex matched 52 EDE patients and 43 heathy controls. A significant increase in the OSDI scores (discomfort subscale) was observed between EDE (median, 20.8) and control (median, 4.2) cohorts (P < 0.001). Similarly, an increase in DCD was observed between EDE (median, 48.1 cells/mm²) patients and controls (median, 5.6 cells/mm²) (P < 0.001). A significant decrease in SBNP features (corneal nerve fiber length, fiber density, fiber width, total branch density, nerve branch density, and fiber area) was observed in EDE patients with OSDI score >23 (P < 0.05). A positive correlation was observed between DCD and OSDI discomfort subscale (r = 0.348; P < 0.0003) and SBNP features. An inverse correlation was observed between vitamin D and OSDI scores (r = −0.332; P = 0.0095) and DCD with dendritic processes (r = −0.322; P = 0.0122). The findings implicate DCD, SBNP features, and vitamin D with EDE symptoms.

The effect of kinin B1 receptor on chronic itching sensitization

Background

Altered kallikrein-related peptidase activity and bradykinin are associated with skin disorders in humans and mice under chronic inflammation conditions. The bradykinin B1 receptor (B1R), also known as one of the G-protein-coupled receptor family and usually absent in intact tissues and upregulated during tissue injury, is responsible for vasodilation, capillary permeability, nociceptor sensitization, and pain; it is indispensable for physiopathological progress in chronic inflammation conditions, but its roles and effectors in the itching sensation of the allergic contact dermatitis model are poorly defined.

Results

We focused on incurable itching in a diphenylcyclopropenone (DCP) chronic inflammation experimental model. Preventive treatment with the B1R antagonist R892 significantly suppressed spontaneous scratching, while the B2R selective antagonist did not. B1R expression in the skin tissues of this model was detected using a quantitative, real-time polymerase chain reaction, Western blotting, and immunohistochemistry; B1R mRNA and protein levels were increased compared with a sham-treated control group. A higher B1R IHC staining signal was observed in the keratinocytes in DCP-treated mice compared with a vehicle-treated group, so we studied the B1R function when superimposed on a protease-activated receptor 2 (PAR2) background, establishing B1R as a pivotal mediator of PAR2 function in HaCaT cell lines.

Conclusion

Our data provide evidence that B1R facilitates the chronic itching sensation related to keratinocytes in a DCP-treated chronic inflammation experimental model.