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

Abstract

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.

Keywords: Analgesia; Calpain; Capsaicin; Microtubule; Neuropathic pain; Osteoarthritis; Resiniferatoxin; TRPV1.

Figure 1.. Potential contributors to capsaicin-induced analgesia.

A. Diagrammatic depiction of neurobiological effects following topical administration of capsaicin. In this review, topical capsaicin-induced neural effects are classified based on the dose of capsaicin, analgesic quality, duration and reversibility, and mechanistically classified as long-term defunctionalization via structural ablation, short-term defunctionalization via functional impairment and specific desensitization. Each phenomenon has apparent mechanistic linkages with the processes shown in B, C, or D. B. High concentrations of capsaicin or RTX activate TRPV1 and mediate influx of large amounts of Ca²⁺, leading to the activation of Ca²⁺-dependent protease calpain. Ablation of axonal terminals by calpain activation results in long-term defunctionalization of TRPV1+ afferents and mediates long-lasting analgesia for chronic pain. C. Low concentrations of capsaicin activate TRPV1 to induce Ca²⁺ influx and subsequent inactivation of voltage-dependent sodium channels (VDSC), voltage-dependent calcium channels (VDCC), and Piezo2, a mechanosensitive ion channel. Inactivation of these channels results in blockade of transduction of sensory stimuli, initiation and conduction of action potentials, which produces short-term defunctionalization via functional impairment, and leads to transient analgesia. D. Low concentrations of capsaicin produce TRPV1 activation and Ca²⁺ influx, which leads to receptor desensitization of TRPV1 ion channels. TRPV1 receptor desensitization decreases the response of TRPV1 to endogenous ligands or inflammatory mediators, and in turn reduces TRPV1-dependent hyperalgesia. The relative contribution and evidence supporting the contribution to analgesia for chronic pain is strongest in A and weakest in C.

Figure 2.. Hypothesis of capsaicin-induced analgesia for neuropathic pain.

A. Epidermal nerve fibers are composed of non-peptidergic and peptidergic TRPV1+ nociceptors. B. Partial nerve injury induces preferential degeneration of non-peptidergic afferents, which reduces epidermal nerve fiber density (ENFD). Wallerian degeneration and inflammation sensitize uninjured remaining peptidergic afferents. C. The remaining subpopulation of afferents is sensitive to capsaicin, and high concentrations of topical capsaicin begin to ablate afferent terminals within 1 day to produce analgesia for neuropathic pain. D. Capsaicin triggers regenerative processes in TRPV1+ afferents, and the ablated afferents are regenerated in 24 weeks (8 weeks in mice), at which point neuropathic pain returns. Retreatment with capsaicin induces analgesia in both humans and mice. As severity of neuropathy increases or is characterized by more chronic progression, degeneration of TRPV1+ epidermal fibers increases, and topical capsaicin is unlikely to produce analgesia.

Figure 3.. The effects of manipulating TRPV1+ afferents in contrast to blockage of the TRPV1 channel in a trigeminal neuropathic pain model.

In mice with chronic constriction injury of infraorbital nerve (ION-CCI), chemogenetic inhibition of peripheral or central terminals of TRPV1+ afferents reduces mechanical hyperalgesia. Chemogenetic inhibition was performed by the administration of clozapine-N oxide to TRPV1^Cre mice expressing inhibitory DREADD receptor. Pharmacological inhibition of TRPV1 channel function at central terminals within the trigeminal subnucleus caudalis attenuates mechanical hyperalgesia, whereas inhibition at peripheral terminals has no effect. Capsaicin administration at peripheral terminals activates TRPV1 and induces ablation of terminals of TRPV1+ afferents to produce analgesia for mechanical hyperalgesia. Summary of results from two publications (Kim et al., 2014; Wang et al., 2020).

Figure 4.. Ca²⁺-dependent calpain-mediated capsaicin-induced ablation.

A. A schematic of multicompartmental culture of DRG neurons in a microfluidic chamber (MFC). Primary afferent neurons were dissociated from TRPV1-GFP reporter mice and plated in one side of the MFC. Axonal terminals cross the barrier and reach the compartment on the other side (axonal compartment). Capsaicin application was restricted to the axonal compartment, where time-lapse imaging was performed. B-D. Representative time-lapse imaging of axonal terminals from MFC. Fluorescence signals from GFP were monitored before and after the axonal compartment was exposed to capsaicin (100 μM). Scale bar in B, 10 μm; arrow heads, ablated fibers; arrows, beaded fibers; asterisk, fibers not affected. E. TRPV1-mediated Ca²⁺ influx through TRPV1 pore is necessary for capsaicin-induced ablation of axonal terminals. Chelation of extracellular Ca²⁺ (EGTA), genetic knockout, or pore blocking (by ruthenium red) of TRPV1 prevents capsaicin-induced ablation of TRPV1-lineage axons. In addition, capsaicin-induced ablation is decreased by pharmacological inhibition of calpain, a Ca²⁺-dependent protease, by MDL28170, by siRNA knockdown of calpain, or by overexpression of calpastatin (cast), an endogenous inhibitor of calpain. Activation of calpain likely induces subsequent cytoskeletal degradation, which leads to ablation of axonal terminals. F. Factors/mechanisms that do not contribute to capsaicin-induced ablation: voltage-dependent Ca²⁺ channels (VDCC, inhibited by nifedipine); transient receptor potential subfamily M member 4 (TRPM4, a Ca²⁺-activated non-selective cationic channel, inhibited by 9-Phenanthrol); anoctamin 1 (ANO1, a Ca²⁺-activated Cl⁻ channel inhibited by A-01); mitochondrial permeabilization transition pore (mPTP, inhibited by cyclosporine A or TRO19622); Ca²⁺ from endoplasmic reticulum (ER, depleted by thapsigargin); or reactive oxygen species (ROS, scavenged by PBN or antioxidant supplement). A-D, reproduced from Wang et al., 2017 (Wang et al., 2017b). E and F, summary of results from Wang et al., 2017 (Wang et al., 2017b).

Figure 5.. Capsaicin-induced calpain-mediated ablation is necessary for capsaicin-induced analgesia of neuropathic pain.

A. Longitudinal in vivo two-photon imaging of GFP+ afferent terminals in intact hindpaw of a mouse expressing membrane-bound GFP from the TRPV1 locus. (Left) To assess changes in GFP+ afferent terminals, the same site was imaged repeatedly – before (Baseline) and 3 days after intraplantar injection of vehicle (Veh) or MDL28170 (MDL; a calpain inhibitor) followed by capsaicin. (Right) The proportion of GFP+ pixels/total pixels in the imaged area was calculated. N=7 hindpaws from 5 mice per group. **p<0.01 in Sidak posthoc assay following two-way ANOVA. B. Mechanical hyperalgesia in facial skin following chronic constriction injury of infraorbital nerve (ION-CCI) is attenuated by intradermal injection of capsaicin. Co-administration of capsaicin with MDL abolishes capsaicin-induced analgesia (compare CCI/Cap/MDL (red) with CCI/Cap/Veh (blue)). Two-way RM ANOVA followed by Bonferroni post-test (CCI/Cap/Veh vs CCI/Cap/MDL); ***p<0.0005, ****<0.0001. Reproduced from Wang et al., 2020 (Wang et al., 2020).