Research ArticlePain

TRPV1 promotes opioid analgesia during inflammation

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Sci. Signal.  02 Apr 2019:
Vol. 12, Issue 575, eaav0711
DOI: 10.1126/scisignal.aav0711

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Analgesic TRPV1

Opiates provide pain relief, but they are addictive, and patients often become desensitized to them and then require greater doses. Inflammation, which is common with painful injuries, enhances the analgesic efficacy of opioids. Basso et al. found that inflammation-induced activation of the channel TRPV1 promoted opioid sensitivity and analgesia in mice by relocalizing the protein β-arrestin2 within the cell such that it could not interact with the μ-opioid receptor and facilitate its desensitization. This finding potentially explains the analgesic effect of inflammation in patients taking opioids and may identify ways to therapeutically prevent the need for opiate dose escalation.

Abstract

Pain and inflammation are inherently linked responses to injury, infection, or chronic diseases. Given that acute inflammation in humans or mice enhances the analgesic properties of opioids, there is much interest in determining the inflammatory transducers that prime opioid receptor signaling in primary afferent nociceptors. Here, we found that activation of the transient receptor potential vanilloid type 1 (TRPV1) channel stimulated a mitogen-activated protein kinase (MAPK) signaling pathway that was accompanied by the shuttling of the scaffold protein β-arrestin2 to the nucleus. The nuclear translocation of β-arrestin2 in turn prevented its recruitment to the μ-opioid receptor (MOR), the subsequent internalization of agonist-bound MOR, and the suppression of MOR activity that occurs upon receptor desensitization. Using the complete Freund’s adjuvant (CFA) inflammatory pain model to examine the role of TRPV1 in regulating endogenous opioid analgesia in mice, we found that naloxone methiodide (Nal-M), a peripherally restricted, nonselective, and competitive opioid receptor antagonist, slowed the recovery from CFA-induced hypersensitivity in wild-type, but not TRPV1-deficient, mice. Furthermore, we showed that inflammation prolonged morphine-induced antinociception in a mouse model of opioid receptor desensitization, a process that depended on TRPV1. Together, our data reveal a TRPV1-mediated signaling pathway that serves as an endogenous pain-resolution mechanism by promoting the nuclear translocation of β-arrestin2 to minimize MOR desensitization. This previously uncharacterized mechanism may underlie the peripheral opioid control of inflammatory pain. Dysregulation of the TRPV1–β-arrestin2 axis may thus contribute to the transition from acute to chronic pain.

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