Ca2+ and calpain mediate capsaicin-induced ablation of axonal terminals expressing transient receptor potential vanilloid 1

Slow depolarizing electrical stimuli reveal differential time courses of nociceptor recovery after prolonged topical capsaicin in human skin

BACKGROUND

We examined de-functionalization and temporal functional recovery of C-nociceptor evoked pain after topical 8% capsaicin applied for 4 consecutive days.

METHODS

Capsaicin and placebo patches were applied to human forearm skin (n = 14). Cold, warmth and heat pain thresholds, pain NRS to electrical and thermal (48°C, 5 s) stimuli and axon reflex flare were recorded weekly for 49 days. Mechanical and heat sensitive ('polymodal') nociceptors were activated by single electrical half-period sinusoidal pulses (0.5 s, 1 Hz). Mechanical and heat insensitive ('silent') nociceptors were activated by 4 Hz sinusoidal stimuli.

RESULTS

Capsaicin abolished heat pain. Sensation to electrical sinusoidal stimulation was reduced but never abolished during the treatment. Pain to electrical 1 Hz 'polymodal' nociceptor stimulation took longer to recover than pain ratings to 4 Hz 2.5 s sinusoidal stimulation activating 'polymodal' and 'silent' nociceptors (35 vs. 21 days). Heat pain was indifferent to placebo from day 21-49. Axon reflex flare was abolished during capsaicin and only recovered to ~50% even after 49 days.

CONCLUSIONS

Capsaicin abolishes heat transduction at terminal nociceptive endings, whereas small-diameter axons sensitive to sinusoidal electrical stimulation can still be activated. 1 Hz depolarizing stimuli evoke burst discharges, as demonstrated before, and recover slower after capsaicin than single pulses induced by 4 Hz. The difference in recovery suggests differential time course of functional regeneration for C-nociceptor sub-types after capsaicin. All sensations recovered completely within 7 weeks in healthy subjects. Our findings contrast analgesia lasting for months in spontaneous neuropathic pain patients treated with 8% capsaicin.

SIGNIFICANCE

Sinusoidal electrical stimulation can still activate small diameter axons desensitized to heat after 4 consecutive days of topical 8% capsaicin application and reveals differential temporal functional regeneration of C-nociceptor sub-types. Electrical sinusoidal stimulation may detect such axons that no longer respond to heat stimuli in neuropathic skin.

Capsaicin-induced Ca2+ overload and ablation of TRPV1-expressing axonal terminals for comfortable tumor immunotherapy

As a common malignancy symptom, cancer pain significantly affects patients' quality of life. Approximately 60%-90% of patients with advanced cancer experience debilitating pain. Therefore, a comprehensive treatment system that combines cancer pain suppression and tumor treatment could provide significant benefits for these patients. Here, we designed a manganese oxide (MnO2)/Bovine serum albumin (BSA)/polydopamine (PDA) composite nanoplatform internally loaded with capsaicin for cancer pain suppression and immunotherapy. MBD&C nanoparticles (NPs) can ablate tumor-innervated sensory nerve fibers via Transient receptor potential vanilloid 1 (TRPV1) channels, thereby reducing the pain caused by various inflammatory mediators. The ablation of TRPV1+ nerve terminals can also decrease the secretion of calcitonin gene-related peptide (CGRP) and substance P (SP) in sensory nerve fibers, thus reducing the tumor pain and inhibit tumor progression. MBD&C can promote calcium influx by activating overexpressed TRPV1 channels on the tumor membrane surface, thereby achieving cancer immunotherapy induced by endogenous Ca2+ overloading. In addition, MnO2 NPs can alleviate tumor hypoxia and mitigate the immunosuppressive tumor microenvironment (TME). Ultimately, this treatment system with dual capabilities of inhibiting tumor growth and relieving cancer pain makes comfortable tumor therapy feasible and paves the way for the development of patient-centered approaches to cancer treatment in the future.

The dual role of TRPV1 in peripheral neuropathic pain: pain switches caused by its sensitization or desensitization

The transient receptor potential vanilloid 1 (TRPV1) channel plays a dual role in peripheral neuropathic pain (NeuP) by acting as a "pain switch" through its sensitization and desensitization. Hyperalgesia, commonly resulting from tissue injury or inflammation, involves the sensitization of TRPV1 channels, which modulates sensory transmission from primary afferent nociceptors to spinal dorsal horn neurons. In chemotherapy-induced peripheral neuropathy (CIPN), TRPV1 is implicated in neuropathic pain mechanisms due to its interaction with ion channels, neurotransmitter signaling, and oxidative stress. Sensitization of TRPV1 in dorsal root ganglion neurons contributes to CIPN development, and inhibition of TRPV1 channels can reduce chemotherapy-induced mechanical hypersensitivity. In diabetic peripheral neuropathy (DPN), TRPV1 is involved in pain modulation through pathways including reactive oxygen species and cytokine production. TRPV1's interaction with TRPA1 channels further influences chronic pain onset and progression. Therapeutically, capsaicin, a TRPV1 agonist, can induce analgesia through receptor desensitization, while TRPV1 antagonists and siRNA targeting TRPV1 show promise in preclinical studies. Cannabinoid modulation of TRPV1 provides another potential pathway for alleviating neuropathic pain. This review summarizes recent preclinical research on TRPV1 in association with peripheral NeuP.

Identification of compounds that cause axonal dieback without cytotoxicity in dorsal root ganglia explants and intervertebral disc cells with potential to treat pain via denervation

Low back pain, knee osteoarthritis, and cancer patients suffer from chronic pain. Aberrant nerve growth into intervertebral disc, knee, and tumors, are common pathologies that lead to these chronic pain conditions. Axonal dieback induced by capsaicin (Caps) denervation has been FDA-approved to treat painful neuropathies and knee osteoarthritis but with short-term efficacy and discomfort. Herein, we propose to evaluate pyridoxine (Pyr), vincristine sulfate (Vcr) and ionomycin (Imy) as axonal dieback compounds for denervation with potential to alleviate pain. Previous literature suggests Pyr, Vcr, and Imy can cause undesired axonal degeneration, but no previous work has evaluated axonal dieback and cytotoxicity on adult rat dorsal root ganglia (DRG) explants. Thus, we performed axonal dieback screening using adult rat DRG explants in vitro with Caps as a positive control and assessed cytotoxicity. Imy inhibited axonal outgrowth and slowed axonal dieback, while Pyr and Vcr at high concentrations produced significant reduction in axon length and robust axonal dieback within three days. DRGs treated with Caps, Vcr, or Imy had increased DRG cytotoxicity compared to matched controls, but overall cytotoxicity was minimal and at least 88% lower compared to lysed DRGs. Pyr did not lead to any DRG cytotoxicity. Further, neither Pyr nor Vcr triggered intervertebral disc cell death or affected cellular metabolic activity after three days of incubation in vitro. Overall, our findings suggest Pyr and Vcr are not toxic to DRGs and intervertebral disc cells, and there is potential for repurposing these compounds for axonal dieback compounds to cause local denervation and alleviate pain.

Transient Receptor Potential Vanilloid-1 Channels Facilitate Axonal Degeneration of Corneal Sensory Nerves in Dry Eye

Corneal nerve impairment contributes significantly to dry eye disease (DED) symptoms and is thought to be secondary to corneal epithelial damage. Transient receptor potential vanilloid-1 (TRPV1) channels abound in corneal nerve fibers and respond to inflammation-derived ligands, which increase in DED. TRPV1 overactivation promotes axonal degeneration in vitro, but whether it participates in DED-associated corneal nerve dysfunction is unknown. To explore this, DED was surgically induced in wild-type and TRPV1-knockout mice, which developed comparable corneal epithelial damage and reduced tear secretion. However, corneal mechanosensitivity decreased progressively only in wild-type DED mice. Sensitivity to capsaicin (TRPV1 agonist) increased in wild-type DED mice, and consistently, only this strain displayed DED-induced pain signs. Wild-type DED mice exhibited nerve degeneration throughout the corneal epithelium, whereas TRPV1-knockout DED mice only developed a reduction in the most superficial nerve endings that failed to propagate to the deeper subbasal corneal nerves. Pharmacologic TRPV1 blockade reproduced these findings in wild-type DED mice, whereas CD4+ T cells from both strains were equally pathogenic when transferred, ruling out a T-cell-mediated effect of TRPV1 deficiency. These data show that ocular desiccation triggers superficial corneal nerve damage in DED, but proximal propagation of axonal degeneration requires TRPV1 expression. Local inflammation sensitized TRPV1 channels, which increased ocular pain. Thus, ocular TRPV1 overactivation drives DED-associated corneal nerve impairment.

The Afferent Function of Adipose Innervation

Adipose tissue innervation is critical for regulating metabolic and energy homeostasis. While the sympathetic efferent innervation of fat is well characterized, the role of sensory or afferent innervation remains less explored. This article reviews previous work on adipose innervation and recent advances in the study of sensory innervation of adipose tissues. We discuss key open questions, including the physiological implications of adipose afferents in homeostasis as well as potential cross talk with sympathetic neurons, the immune system, and hormonal pathways. We also outline the general technical challenges of studying dorsal root ganglia innervating fat, along with emerging technologies that may overcome these barriers. Finally, we highlight areas for further research to deepen our understanding of the afferent function of adipose innervation.

Nociceptor mechanisms underlying pain and bone remodeling via orthodontic forces: toward no pain, big gain

Orthodontic forces are strongly associated with pain, the primary complaint among patients wearing orthodontic braces. Compared to other side effects of orthodontic treatment, orthodontic pain is often overlooked, with limited clinical management. Orthodontic forces lead to inflammatory responses in the periodontium, which triggers bone remodeling and eventually induces tooth movement. Mechanical forces and subsequent inflammation in the periodontium activate and sensitize periodontal nociceptors and produce orthodontic pain. Nociceptive afferents expressing transient receptor potential vanilloid subtype 1 (TRPV1) play central roles in transducing nociceptive signals, leading to transcriptional changes in the trigeminal ganglia. Nociceptive molecules, such as TRPV1, transient receptor potential ankyrin subtype 1, acid-sensing ion channel 3, and the P2X3 receptor, are believed to mediate orthodontic pain. Neuropeptides such as calcitonin gene-related peptides and substance P can also regulate orthodontic pain. While periodontal nociceptors transmit nociceptive signals to the brain, they are also known to modulate alveolar bone remodeling in periodontitis. Therefore, periodontal nociceptors and nociceptive molecules may contribute to the modulation of orthodontic tooth movement, which currently remains undetermined. Future studies are needed to better understand the fundamental mechanisms underlying neuroskeletal interactions in orthodontics to improve orthodontic treatment by developing novel methods to reduce pain and accelerate orthodontic tooth movement-thereby achieving "big gains with no pain" in clinical orthodontics.

Membrane-anchored calpains - hidden regulators of growth and development beyond plants?

Calpains are modulatory proteases that modify diverse cellular substrates and play essential roles in eukaryots. The best studied are animal cytosolic calpains. Here, we focus on enigmatic membrane-anchored calpains, their structural and functional features as well as phylogenetic distribution. Based on domain composition, we identified four types of membrane-anchored calpains. Type 1 and 2 show broad phylogenetic distribution among unicellular protists and streptophytes suggesting their ancient evolutionary origin. Type 3 and 4 diversified early and are present in brown algae and oomycetes. The plant DEK1 protein is the only representative of membrane-anchored calpains that has been functionally studied. Here, we present up to date knowledge about its structural features, putative regulation, posttranslational modifications, and biological role. Finally, we discuss potential model organisms and available tools for functional studies of membrane-anchored calpains with yet unknown biological role. Mechanistic understanding of membrane-anchored calpains may provide important insights into fundamental principles of cell polarization, cell fate control, and morphogenesis beyond plants.

Low Trend for VZV-Associated Disease Patients to Visit Neurologists

Background

Herpes zoster (HZ) is a skin disease that can also cause virus-infectious peripheral neuropathies. Despite this, there is limited information on patient preferences for seeking medical attention for HZ and zoster-associated pain (ZAP). Our study aimed to evaluate how frequently patients with ZAP choose to visit neurologists for their symptoms.

Methods

This study conducted a retrospective review of electronic health records in three general hospitals from January 2017 to June 2022. Using association rule mining, the study analyzed referral behaviors.

Results

We identified 33,633 patients with 111,488 outpatient visits over 5.5 years. The study found that the majority of patients (74.77-91.22%) visited dermatologists during their first outpatient visit, while only a small percentage (0.86-1.47%) preferred to consult a neurologist. The proportion of patients referred to a specialist during their medical visit varied significantly between different specialties within the same hospital (p <0.05) and even within the same specialty (p<0.05). There was a weak association (Lift:1.00-1.17) of referral behaviors between dermatology and neurology. Across the three hospitals, the average number of visits to a neurologist for ZAP was 1.42-2.49, with an average electronic health record duration of 11-15 days per patient. After consulting with a neurologist, some patients were referred to other specialists.

Conclusion

It was observed that patients with HZ and ZAP tended to visit a variety of specialists, with only a small number seeking the assistance of neurologists. However, from the perspective of neuroprotection, it is the duty of neurologists to provide more means.

Capsaicin-induced depolymerization of axonal microtubules mediates analgesia for trigeminal neuropathic pain

ABSTRACT

Capsaicin is a specific agonist of transient receptor potential vanilloid 1 (TRPV1), which is enriched in nociceptors. Capsaicin not only produces acute pain but also leads to long-lasting analgesia in patients with chronic pain. Although capsaicin-induced TRPV1 and Ca 2+ /calpain-dependent ablation of axonal terminals is necessary for long-lasting analgesia, the mechanisms underlying capsaicin-induced ablation of axonal terminals and its association with analgesia are not fully understood. Microtubules are composed of tubulin polymers and serve as a main axonal cytoskeleton maintaining axonal integrity. In this study, we hypothesized that capsaicin would increase the depolymerization of microtubules and lead to axonal ablation and analgesia for trigeminal neuropathic pain. Paclitaxel, a microtubule stabilizer, decreased capsaicin-induced ablation of axonal terminals in time-lapsed imaging in vitro. Capsaicin increases free tubulin in dissociated sensory neurons, which was inhibited by paclitaxel. Consistently, subcutaneous injection of paclitaxel prevented capsaicin-induced axonal ablation in the hind paw skin. Capsaicin administration to the facial skin produced analgesia for mechanical hyperalgesia in mice with chronic constriction injury of the infraorbital nerve, which was prevented by the coadministration of paclitaxel and capsaicin. Whole-mount staining of facial skin showed that paclitaxel reduced capsaicin-induced ablation of peptidergic afferent terminals. Despite the suggested involvement of TRPV1 Ser801 phosphorylation on microtubule integrity, capsaicin-induced analgesia was not affected in TRPV1 S801A knock-in mice. In conclusion, capsaicin-induced depolymerization of axonal microtubules determined capsaicin-induced ablation of nociceptive terminals and the extent of analgesia. Further understanding of TRPV1/Ca 2+ -dependent mechanisms of capsaicin-induced ablation and analgesia may help to improve the management of chronic pain.

Mitochondrial Reactive Oxygen Species Mediate Activation of TRPV1 and Calcium Entry Following Peripheral Sensory Axotomy

Axons that are physically separated from their soma activate a series of signaling events that results in axonal self-destruction. A critical element of this signaling pathway is an intra-axonal calcium rise that occurs just prior to axonal fragmentation. Previous studies have shown that preventing this calcium rise delays the onset of axon fragmentation, yet the ion channels responsible for the influx, and the mechanisms by which they are activated, are largely unknown. Axonal injury can be modeled in vitro by transecting murine dorsal root ganglia (DRG) sensory axons. We coupled transections with intra-axonal calcium imaging and found that Ca²⁺ influx is sharply reduced in axons lacking trpv1 (for transient receptor potential cation channel vanilloid 1) and in axons treated with capsazepine (CPZ), a TRPV1 antagonist. Sensory neurons from trpv1^–/– mice were partially rescued from degeneration after transection, indicating that TRPV1 normally plays a pro-degenerative role after axonal injury. TRPV1 activity can be regulated by direct post-translational modification induced by reactive oxygen species (ROS). Here, we tested the hypothesis that mitochondrial ROS production induced by axotomy is required for TRPV1 activity and subsequent axonal degeneration. We found that reducing mitochondrial depolarization with NAD⁺ supplementation or scavenging ROS using NAC or MitoQ sharply attenuates TRPV1-dependent calcium influx induced by axotomy. This study shows that ROS-dependent TRPV1 activation is required for Ca²⁺ entry after axotomy.

Calpain Inhibitors as Potential Therapeutic Modulators in Neurodegenerative Diseases

It is considered a significant challenge to understand the neuronal cell death mechanisms with a suitable cure for neurodegenerative disorders in the coming years. Calpains are one of the best-considered "cysteine proteases activated" in brain disorders. Calpain is an important marker and mediator in the pathophysiology of neurodegeneration. Calpain activation being the essential neurodegenerative factor causing apoptotic machinery activation, it is crucial to develop reliable and effective approaches to prevent calpain-mediated apoptosis in degenerating neurons. It has been recently seen that the "inhibition of calpain activation" has appeared as a possible therapeutic target for managing neurodegenerative diseases. A systematic literature review of PubMed, Medline, Bentham, Scopus, and EMBASE (Elsevier) databases was conducted. The present article reviews the basic pathobiology and role of selective calpain inhibitors used in various neurodegenerative diseases as a therapeutic target.

Pharmacotherapy of Painful Diabetic Neuropathy: A Clinical Update

The rising prevalence of diabetes mellitus (DM) leads on to an increase in chronic diabetic complications. Diabetic peripheral neuropathies (DPNs) are common chronic complications of diabetes. Distal symmetric polyneuropathy is the most prevalent form. Most patients with DPN will remain pain-free; however, painful DPN (PDPN) occurs in 6-34% of all DM patients and is associated with reduced health-related-quality-of-life and substantial economic burden. Symptomatic treatment of PDPN and diabetic autonomic neuropathy is the key treatment goals. Using certain patient related characteristics, subjects with PDPN can be stratified and assigned targeted therapies to produce better pain outcomes. The aim of this review is to discuss the various pathogenetic mechanisms of DPN with special reference to the mechanisms leading to PDPN and the various pharmacological and non-pharmacological therapies available for its management. Recommended pharmacological therapies include anticonvulsants, antidepressants, opioid analgesics, and topical medications.

Enhanced Ocular Surface and Intraoral Nociception via a Transient Receptor Potential Vanilloid 1 Mechanism in a Rat Model of Obstructive Sleep Apnea

Obstructive sleep apnea (OSA), characterized by low arterial oxygen saturation during sleep, is associated with an increased risk of orofacial pain. In this study, we simulated chronic intermittent hypoxia (CIH) during the sleep/rest phase (light phase) to determine the role of transient receptor potential vanilloid 1 (TRPV1) in mediating enhanced orofacial nocifensive behavior and trigeminal spinal subnucleus caudalis (Vc) neuronal responses to capsaicin (a TRPV1 agonist) stimulation in a rat model of OSA. Rats were subjected to CIH (nadir O₂, 5%) during the light phase for 8 or 16 consecutive days. CIH yielded enhanced behavioral responses to capsaicin after application to the ocular surface and intraoral mucosa, which was reversed under normoxic conditions. The percentage of TRPV1-immunoreactive trigeminal ganglion neurons was greater in CIH rats than in normoxic rats and recovered under normoxic conditions after CIH. The ratio of large-sized TRPV1-immunoreactive trigeminal ganglion neurons increased in CIH rats. The density of TRPV1 positive primary afferent terminals in the superficial laminae of Vc was higher in CIH rats. Phosphorylated extracellular signal-regulated kinase (pERK)-immunoreactive cells intermingled with the central terminal of TRPV1-positive afferents in the Vc. The number of pERK-immunoreactive cells following low-dose capsaicin (0.33 µM) application to the tongue was significantly greater in the middle portion of the Vc of CIH rats than of normoxic rats and recovered under normoxic conditions after CIH. These data suggest that CIH during the sleep (light) phase is sufficient to transiently enhance pain on the ocular surface and intraoral mucosa via TRPV1-dependent mechanisms.

Modulating effects of capsaicin on glucose homeostasis and the underlying mechanism

Abnormal glucose homeostasis is linked to a variety of metabolic syndromes, such as insulin resistance, obesity, type-2 diabetes mellitus, hypertension and cardiovascular diseases. Maintenance of normal glucose homeostasis is important for the body to keep normal biological functions. As the major bioactive ingredient in chili peppers responsible for the pungent flavor, capsaicin has been reported to effectively improve glucose homeostasis with low cytotoxicity. In this review, the modulating effects of capsaicin on glucose homeostasis in cell models, animal models and human trials are summarized through both TRPV1 dependent and TRPV1 independent pathways. The relevant molecular mechanisms underlying its regulatory effects are also evaluated. Understanding the effects and mechanisms of capsaicin on glucose metabolism could provide theoretical evidence for its application in the food and pharmaceutical industries.

Resilience to capsaicin-induced mitochondrial damage in trigeminal ganglion neurons

Capsaicin is an agonist of transient receptor potential cation channel subfamily V member 1 (TRPV1). Strong TRPV1 stimulation with capsaicin causes mitochondrial damage in primary sensory neurons. However, the effect of repetitive and moderate exposure to capsaicin on the integrity of neuronal mitochondria remains largely unknown. Our electron microscopic analysis revealed that repetitive stimulation of the facial skin of mice with 10 mM capsaicin induced short-term damage to the mitochondria in small-sized trigeminal ganglion neurons. Further, capsaicin-treated mice exhibited decreased sensitivity to noxious heat stimulation, indicating TRPV1 dysfunction, in parallel with the mitochondrial damage in the trigeminal ganglion neurons. To analyze the capsaicin-induced mitochondrial damage and its relevant cellular events in detail, we performed cell-based assays using TRPV1-expressing PC12 cells. Dose-dependent capsaicin-mediated mitochondrial toxicity was observed. High doses of capsaicin caused rapid destruction of mitochondrial internal structure, while low doses induced mitochondrial swelling. Further, capsaicin induced a dose-dependent loss of mitochondria and autophagy-mediated degradation of mitochondria (mitophagy). Concomitantly, transcriptional upregulation of mitochondrial proteins, cytochrome c oxidase subunit IV, Mic60/Mitofilin, and voltage-dependent anion channel 1 was observed, which implied induction of mitochondrial biogenesis to compensate for the loss of mitochondria. Collectively, although trigeminal ganglion neurons transiently exhibit mitochondrial damage and TRPV1 dysfunction following moderate capsaicin exposure, they appear to be resilient to such a challenge. Our in vitro data show a dose-response relationship in capsaicin-mediated mitochondrial toxicity. We postulate that induction of mitophagy and mitochondrial biogenesis in response to capsaicin stimulation play important roles in repairing the damaged mitochondrial system.

17β-Estradiol Exacerbated Experimental Occlusal Interference-Induced Chronic Masseter Hyperalgesia by Increasing the Neuronal Excitability and TRPV1 Function of Trigeminal Ganglion in Ovariectomized Rats

Pain symptoms in temporomandibular disorders (TMD) predominantly affect reproductive women, suggesting that estrogen regulates pain perception. However, how estrogen contributes to chronic TMD pain remains largely unclear. In the present study, we performed behavioral tests, electrophysiology, Western blot and immunofluorescence to investigate the role and underlying mechanisms of estrogen in dental experimental occlusal interference (EOI)-induced chronic masseter mechanical hyperalgesia in rats. We found that long-term 17β-estradiol (E2) replacement exacerbated EOI-induced masseter hyperalgesia in a dose-dependent manner in ovariectomized (OVX) rats. Whole-cell patch-clamp recordings demonstrated that E2 (100 nM) treatment enhanced the excitability of isolated trigeminal ganglion (TG) neurons in OVX and OVX EOI rats, and EOI increased the functional expression of transient receptor potential vanilloid-1 (TRPV1). In addition, E2 replacement upregulated the protein expression of TRPV1 in EOI-treated OVX rats. Importantly, intraganglionic administration of the TRPV1 antagonist AMG-9810 strongly attenuated the facilitatory effect of E2 on EOI-induced masseter mechanical sensitivity. These results demonstrate that E2 exacerbated EOI-induced chronic masseter mechanical hyperalgesia by increasing TG neuronal excitability and TRPV1 function. Our study helps to elucidate the E2 actions in chronic myogenic TMD pain and may provide new therapeutic targets for relieving estrogen-sensitive pain.

Injectable Capsaicin for the Management of Pain Due to Osteoarthritis

Capsaicin is a potent agonist of the TRPV1 channel, a transduction channel that is highly expressed in nociceptive fibers (pain fibers) throughout the peripheral nervous system. Given the importance of TRPV1 as one of several transduction channels in nociceptive fibers, much research has been focused on the potential therapeutic benefits of using TRPV1 antagonists for the management of pain. However, an antagonist has two limitations. First, an antagonist in principle generally only affects one receptor. Secondly, most antagonists must have an ongoing presence on the receptor to have an effect. Capsaicin overcomes both liabilities by disrupting peripheral terminals of nociceptive fibers that express TRPV1, and thereby affects all of the potential means of activating that pain fiber (not just TRPV1 function). This disruptive effect is dependent on the dose and can occur within minutes. Thus, unlike a typical receptor antagonist, continued bioavailability at the level of the receptor is not necessary. By disrupting the entire terminal of the TRPV1-expressing nociceptive fiber, capsaicin blocks all the activation mechanisms within that fiber, and not just TRPV1 function. Topical capsaicin, an FDA approved treatment for neuropathic pain, addresses pain from abnormal nociceptor activity in the superficial layers of the skin. Effects after a single administration are evident over a period of weeks to months, but in time are fully reversible. This review focuses on the rationale for using capsaicin by injection for painful conditions such as osteoarthritis (OA) and provides an update on studies completed to date.

The Mysteries of Capsaicin-Sensitive Afferents

A fundamental subdivision of nociceptive sensory neurons is named after their unique sensitivity to capsaicin, the pungent ingredient in hot chili peppers: these are the capsaicin-sensitive afferents. The initial excitation by capsaicin of these neurons manifested as burning pain sensation is followed by a lasting refractory state, traditionally referred to as "capsaicin desensitization," during which the previously excited neurons are unresponsive not only to capsaicin but a variety of unrelated stimuli including noxious heat. The long sought-after capsaicin receptor, now known as TRPV1 (transient receptor potential cation channel, subfamily V member 1), was cloned more than two decades ago. The substantial reduction of the inflammatory phenotype of Trpv1 knockout mice has spurred extensive efforts in the pharmaceutical industry to develop small molecule TRPV1 antagonists. However, adverse effects, most importantly hyperthermia and burn injuries, have so far prevented any compounds from progressing beyond Phase 2. There is increasing evidence that these limitations can be at least partially overcome by approaches outside of the mainstream pharmaceutical development, providing novel therapeutic options through TRPV1. Although ablation of the whole TRPV1-expressing nerve population by high dose capsaicin, or more selectively by intersectional genetics, has allowed researchers to investigate the functions of capsaicin-sensitive afferents in health and disease, several "mysteries" remain unsolved to date, including the molecular underpinnings of "capsaicin desensitization," and the exact role these nerves play in thermoregulation and heat sensation. This review tries to shed some light on these capsaicin mechanisms.

Capsaicin Inhibits the Expression of Melanogenic Proteins in Melanocyte via Activation of TRPV1 Channel: Identifying an Inhibitor of Skin Melanogenesis

Chili pepper belongs to the genus Capsicum of Solanaceae family. Capsaicin is the primary capsaicinoid in placenta and flesh of chili pepper fruit, which has been shown to have various pharmacological functions, including gastric protection, anti-inflammation, and obesity treatment. Here, we revealed that capsaicin as well as chilli extract was able to inhibit synthesis of melanin in melanocytes. In cultured melanocytes, the melanin content was reduced to 54 ± 6.55% and 42 ± 7.41% with p < 0.001 under treatment of 50 μM capsaicin for 24 and 72 h, respectively. In parallel, the protein levels of tyrosinase and tyrosinase-related protein-1 were reduced to 62 ± 8.35% and 48 ± 8.92% with p < 0.001. Such an inhibitory effect of capsaicin was mediated by activation of transient receptor potential vanilloid 1-induced phosphorylation of extracellular signal-regulated kinase. This resulted in a degradation of microphthalmia-associated transcription factor, leading to reduction of melanogenic enzymes and melanin. These results revealed that capsaicin could be an effective inhibitor for skin melanogenesis. Hence, chili pepper, as our daily food, has potential in dermatological application, and capsaicin should be considered as a safe agent in treating hyperpigmentation problems.

Fight fire with fire: Neurobiology of capsaicin-induced analgesia for chronic pain

Capsaicin, the pungent ingredient in chili peppers, produces intense burning pain in humans. Capsaicin selectively activates the transient receptor potential vanilloid 1 (TRPV1), which is enriched in nociceptive primary afferents, and underpins the mechanism for capsaicin-induced burning pain. Paradoxically, capsaicin has long been used as an analgesic. The development of topical patches and injectable formulations containing capsaicin has led to application in clinical settings to treat chronic pain conditions, such as neuropathic pain and the potential to treat osteoarthritis. More detailed determination of the neurobiological mechanisms of capsaicin-induced analgesia should provide the logical rationale for capsaicin therapy and help to overcome the treatment's limitations, which include individual differences in treatment outcome and procedural discomfort. Low concentrations of capsaicin induce short-term defunctionalization of nociceptor terminals. This phenomenon is reversible within hours and, hence, likely does not account for the clinical benefit. By contrast, high concentrations of capsaicin lead to long-term defunctionalization mediated by the ablation of TRPV1-expressing afferent terminals, resulting in long-lasting analgesia persisting for several months. Recent studies have shown that capsaicin-induced Ca²⁺/calpain-mediated ablation of axonal terminals is necessary to produce long-lasting analgesia in a mouse model of neuropathic pain. In combination with calpain, axonal mitochondrial dysfunction and microtubule disorganization may also contribute to the longer-term effects of capsaicin. The analgesic effects subside over time in association with the regeneration of the ablated afferent terminals. Further determination of the neurobiological mechanisms of capsaicin-induced analgesia should lead to more efficacious non-opioidergic analgesic options with fewer adverse side effects.

TRPV1-Targeted Drugs in Development for Human Pain Conditions

The transient receptor potential vanilloid-1 (TRPV1) is a non-specific cation channel known for its sensitivity to pungent vanilloid compound (i.e. capsaicin) and noxious stimuli, including heat, low pH or inflammatory mediators. TRPV1 is found in the somatosensory system, particularly primary afferent neurons that respond to damaging or potentially damaging stimuli (nociceptors). Stimulation of TRPV1 evokes a burning sensation, reflecting a central role of the channel in pain. Pharmacological and genetic studies have validated TRPV1 as a therapeutic target in several preclinical models of chronic pain, including cancer, neuropathic, postoperative and musculoskeletal pain. While antagonists of TRPV1 were found to be a valuable addition to the pain therapeutic toolbox, their clinical use has been limited by detrimental side effects, such as hyperthermia. In contrast, capsaicin induces a prolonged defunctionalisation of nociceptors and thus opened the door to the development of a new class of therapeutics with long-lasting pain-relieving effects. Here we review the list of TRPV1 agonists undergoing clinical trials for chronic pain management, and discuss new indications, formulations or combination therapies being explored for capsaicin. While the analgesic pharmacopeia for chronic pain patients is ancient and poorly effective, modern TRPV1-targeted drugs could rapidly become available as the next generation of analgesics for a broad spectrum of pain conditions.

Transcriptome profiling of genes involved in nutrient uptake regulated by phosphate-solubilizing bacteria in pepper (Capsicum annuum L.)

Improving nutrient absorption in pepper has become a vital prerequisite for growth to produce a sustainable yield. In this study, transcriptome gene expression in pepper inoculated with two types of phosphate-solubilizing bacteria (PSB) and grown under low and high nutrient levels (LN and HN) was analyzed. Results showed that the root length increased when pepper was grown under LN; however, the root structure was intensively tight under HN. Our data revealed that the roots preferred horizontal growth than longitudinal growth under HN. PSB strains 'M01' and 'N3' significantly (P < 0.01) increased the P uptake by 70.44% and 98.20%, respectively, but decreased the Ca²⁺ content by 8.96% and 9.13%, respectively, compared with the control (L1). Although no remarkable difference was detected in the chlorophyll content, inoculation with the two PSB strains decreased the Fe³⁺ content in pepper under HN. The total clean sequenced data from samples ranged between 5,923,659,118 and 9,955,045,953 bp. Transcriptome profiling revealed 320 upregulated and 449 downregulated genes in L3 versus L1 and 468 upregulated and 532 downregulated genes in L4 versus L1. Gene ontology analysis revealed that the biological processes, including response to stress and secondary metabolic process, were involved. Several pathways were subordinate to glycosphingolipid biosynthesis and linoleic acid and nitrogen metabolisms. Analysis of the eukaryotic orthologous group function revealed that most differential genes were attributed to RNA processing and modification, transcription, and signal transduction. Our results provided new insights into the molecular mechanism related to nutrient uptake in pepper inoculated with PSBs.

Ursolic acid protects against cisplatin‑induced ototoxicity by inhibiting oxidative stress and TRPV1‑mediated Ca2+‑signaling

Cisplatin (CDDP) is widely used in clinical settings for the treatment of various cancers. However, ototoxicity is a major side effect of CDDP, and there is an associated risk of irreversible hearing loss. We previously demonstrated that CDDP could induce ototoxicity via activation of the transient receptor potential vanilloid receptor 1 (TRPV1) pathway and subsequent induction of oxidative stress. The present study investigated whether ursolic acid (UA) treatment could protect against CDDP‑induced ototoxicity. UA is a triterpenoid with strong antioxidant activity widely used in China for the treatment of liver diseases. This traditional Chinese medicine is mainly isolated from bearberry, a Chinese herb. The present results showed that CDDP increased auditory brainstem response threshold shifts in frequencies associated with observed damage to the outer hair cells. Moreover, CDDP increased the expression of TRPV1, calpain 2 and caspase‑3 in the cochlea, and the levels of Ca2+ and 4‑hydroxynonenal. UA co‑treatment significantly attenuated CDDP‑induced hearing loss and inhibited TRPV1 pathway activation. In addition, UA enhanced CDDP‑induced growth inhibition in the human ovarian cancer cell line SKOV3, suggesting that UA synergizes with CDDP in vitro. Collectively, the present data suggested that UA could effectively attenuate CDDP‑induced hearing loss by inhibiting the TRPV1/Ca²+/calpain‑oxidative stress pathway without impairing the antitumor effects of CDDP.

Ablation of TRPV1+ Afferent Terminals by Capsaicin Mediates Long-Lasting Analgesia for Trigeminal Neuropathic Pain

Trigeminal neuropathic pain (TNP) is often resistant to current pharmacotherapy, and there is a pressing need to develop more efficacious treatments. Capsaicin is a pungent ingredient of chili peppers and specifically activates transient receptor potential vanilloid subtype 1 (TRPV1), a Ca²⁺-permeable ion channel. Topical capsaicin invariably induces burning pain. Paradoxically, the transient pain is often followed by prolonged attenuation of the preexisting pathologic pain from the same region. However, the mechanisms underlying capsaicin-induced analgesia are not well understood. Although the reports of the involvement of TRPV1 and TRPV1+ afferents in neuropathic pain are controversial, we recently demonstrated that TRPV1 and TRPV1+ afferents are involved in mechanical hyperalgesia in mice with chronic constriction injury of the infraorbital nerve (ION-CCI). Consistently, chemogenetic inhibition of TRPV1-lineage (TRPV1-LN) afferents attenuated mechanical hyperalgesia and ongoing pain. In mice with ION-CCI, we found that a single focal injection of capsaicin into facial skin led to attenuation of mechanical hyperalgesia over two weeks. Capsaicin treatment also attenuated secondary hyperalgesia in extraterritorial mandibular skin. Furthermore, capsaicin treatment decreased ongoing pain. Longitudinal in vivo two-photon imaging of cutaneous nerve fibers showed that such capsaicin-induced analgesia is correlated with cutaneous nerve terminal density. Furthermore, preventing capsaicin-induced ablation of afferent terminals by co-administration of capsaicin with MDL28170, an inhibitor of calpain, abolished capsaicin-induced analgesia. These results suggest that a single focal injection of capsaicin induces long-lasting analgesia for neuropathic pain via selective ablation of TRPV1+ afferent terminals and that TRPV1+ afferents contribute to the maintenance of TNP.

Capsaicin-Induced Skin Desensitization Differentially Affects A-Delta and C-Fiber-Mediated Heat Sensitivity

Localized neuropathic pain can be relieved following the topical application of high-concentration capsaicin. This clinical effect is thought to be related to the temporary desensitization of capsaicin- and heat-sensitive epidermal nociceptors. The objective of the present study was to examine whether the changes in thermal sensitivity induced by high-concentration topical capsaicin can be explained entirely by desensitization of capsaicin-sensitive afferents. For this purpose, we characterized, in 20 healthy human volunteers, the time course and spatial extent of the changes in sensitivity to thermal stimuli preferentially activating heat-sensitive A-fiber nociceptors, heat-sensitive C-fiber afferents, and cool-sensitive A-fiber afferents. The volar forearm was treated with a high-concentration capsaicin patch for 1 h. Transient heat, warm and cold stimuli designed to activate Aδ- and C-fiber thermonociceptors, C-fiber warm receptors, and Aδ-fiber cold receptors were applied to the skin before and after treatment at days 1, 3, and 7. Reaction times, intensity ratings, and quality descriptors were collected. The stimuli were applied both within the capsaicin-treated skin and around the capsaicin-treated skin to map the changes in thermal sensitivity. We found that topical capsaicin selectively impairs heat sensitivity without any concomitant changes in cold sensitivity. Most interestingly, we observed a differential effect on the sensitivity to thermal inputs conveyed by Aδ- and C-fibers. Reduced sensitivity to Aδ-fiber-mediated heat was restricted to the capsaicin-treated skin, whereas reduced sensitivity to C-fiber-mediated heat extended well beyond the treated skin. Moreover, the time course of the reduced sensitivity to C-fiber-mediated input was more prolonged than the reduced sensitivity to Aδ-fiber-mediated input.

TRPV1 and TRPV1-Expressing Nociceptors Mediate Orofacial Pain Behaviors in a Mouse Model of Orthodontic Tooth Movement

Orthodontic force produces mechanical irritation and inflammation in the periodontium, which is inevitably accompanied by pain. Despite its prevalence, treatment of orthodontic pain is ineffective. Elucidating underlying neural mechanisms is critical to improving the management of orthodontic pain. We have assessed the contribution of transient receptor potential vanilloid subtype 1 (TRPV1) and the TRPV1-expressing subset of nociceptive afferents to pain behaviors induced by orthodontic force in mice. Microfocus X-ray computed tomography analysis showed that application of an orthodontic force of 10 g to the maxillary first molar produced reliable tooth movement in mice. Mouse grimace scale (MGS) was evaluated as an indication of non-evoked spontaneous pain and bite force (BF) was measured for assessing bite-evoked nocifensive behaviors. Orthodontic force increased MGS and decreased BF, both of which were interpreted as increased levels of pain. These behaviors peaked at 1d and returned near to the sham level at 7d. Retrograde labeling and immunohistochemical assays showed TRPV1-expressing peptidergic afferents are abundantly projected to the periodontium. Direct injection of resiniferatoxin into trigeminal ganglia (TG) decreased TRPV1-expressing afferents by half in the targeted region of TG. The chemical ablation of TRPV1-expressing afferents significantly attenuated orthodontic pain behaviors assessed by MGS and BF. Consistently, the knockout of TRPV1 also attenuated orthodontic force-induced changes in MGS and BF. These results suggest that TRPV1 and TRPV1-expressing trigeminal nociceptors constitute a primary pathway mediating orthodontic pain behaviors in mice. This model will be useful for mechanistic studies on orthodontic pain aimed at developing novel approaches for painless orthodontics.

Selective Sensory Axon Reinnervation and TRPV1 Activation

Current strategies to enhance regeneration of peripheral neurons involve broad activation of sensory, autonomic, and motor axons. Peripheral neuron regeneration is limited in persons with damage or disease of peripheral axons. Here, we provide evidence that subtoxic activation of TRPV1 channels in sensory neurons is associated with activation of growth and subtle changes in skin reinnervation. We identify a bidirectional, dose-related impact of capsaicin, a TRPV1 agonist, on sensory neurons and their axons with rises in their outgrowth plasticity at low doses and toxic neurodegeneration at high doses. Moreover, its impact on growth added to that of preconditioning. Neither outcome was observed in TRPV1 null neurons. We confirmed that low dose activation was associated with rises in neuronal calcium, as well as rises in TRPV1 mRNA transcripts. In mice with a sciatic nerve crush followed by a single application of capsaicin directly to the injury site, there was no impact on motor or myelinated axon recovery but there was evidence of better recovery of thermal sensation toward baseline with hyperalgesia. Moreover, skin reinnervation by epidermal axons approached contralateral levels. TRPV1 null mice displayed loss of thermal sensation during later recovery. In sensory axons innervating the pinna of the ear, local capsaicin rendered early axon loss followed by later hyperinnervation. Taken together, TRPV1 activation alters the regenerative behavior of adult neurons and their axons both in vitro and during epidermal reinnervation in vivo. The findings identify a selective manipulation that augments cutaneous innervation by thermosensitive axons.

TRPV1 Agonist, Capsaicin, Induces Axon Outgrowth after Injury via Ca2+/PKA Signaling

Preconditioning nerve injuries activate a pro-regenerative program that enhances axon regeneration for most classes of sensory neurons. However, nociceptive sensory neurons and central nervous system neurons regenerate poorly. In hopes of identifying novel mechanisms that promote regeneration, we screened for drugs that mimicked the preconditioning response and identified a nociceptive ligand that activates a preconditioning-like response to promote axon outgrowth. We show that activating the ion channel TRPV1 with capsaicin induces axon outgrowth of cultured dorsal root ganglion (DRG) sensory neurons, and that this effect is blocked in TRPV1 knockout neurons. Regeneration occurs only in NF200-negative nociceptive neurons, consistent with a cell-autonomous mechanism. Moreover, we identify a signaling pathway in which TRPV1 activation leads to calcium influx and protein kinase A (PKA) activation to induce a preconditioning-like response. Finally, capsaicin administration to the mouse sciatic nerve activates a similar preconditioning-like response and induces enhanced axonal outgrowth, indicating that this pathway can be induced in vivo. These findings highlight the use of local ligands to induce regeneration and suggest that it may be possible to target selective neuronal populations for repair, including cell types that often fail to regenerate.

Role of Transient Receptor Potential Vanilloid 1 in Electroacupuncture Analgesia on Chronic Inflammatory Pain in Mice

Chronic inflammatory pain may result from peripheral tissue injury or inflammation, increasing the release of protons, histamines, adenosine triphosphate, and several proinflammatory cytokines and chemokines. Transient receptor potential vanilloid 1 (TRPV1) is known to be involved in acute to subacute neuropathic and inflammatory pain; however, its exact mechanisms in chronic inflammatory pain are not elucidated. Our results showed that EA significantly reduced chronic mechanical and thermal hyperalgesia in the chronic inflammatory pain model. Chronic mechanical and thermal hyperalgesia were also abolished in TRPV1-/- mice. TRPV1 increased in the dorsal root ganglion (DRG) and spinal cord (SC) at 3 weeks after CFA injection. The expression levels of downstream molecules such as pPKA, pPI3K, and pPKC increased, as did those of pERK, pp38, and pJNK. Transcription factors (pCREB and pNFκB) and nociceptive ion channels (Nav1.7 and Nav1.8) were involved in this process. Inflammatory mediators such as GFAP, S100B, and RAGE were also involved. The expression levels of these molecules were reduced in EA and TRPV1-/- mice but not in the sham EA group. Our data provided evidence to support the clinical use of EA for treating chronic inflammatory pain.