The sigma-1 receptor curtails endogenous opioid analgesia during sensitization of TRPV1 nociceptors

Abstract

Background and PurposePeripheral sensitization contributes to pathological pain. While prostaglandin E2 (PGE2) and nerve growth factor (NGF) sensitize peptidergic C-nociceptors (TRPV1+), glial cell line-derived neurotrophic factor (GDNF) sensitizes non-peptidergic C-neurons (IB4+). The sigma-1 receptor (sigma-1R) is a Ca2+-sensing chaperone known to modulate opoid analgesia. This receptor binds both to TRPV1 and the mu opioid receptor, although the functional repercussions of these physical interactions in peripheral sensitization are unknown.Experimental ApproachWe tested the effects of sigma-1 antagonism on PGE2-, NGF-, and GDNF-induced mechanical and heat hyperalgesia in mice. We used immunohistochemistry to determine the presence of endomorphin-2, an endogenous mu receptor agonist, on dorsal root ganglion (DRG) neurons. Recombinant proteins were used to study the interactions between sigma-1R, mu- receptor, and TRPV1. We used calcium imaging to study the effects of sigma-1 antagonism on PGE2-induced sensitization of TRPV1+ nociceptors.Key ResultsSigma1 antagonists reversed PGE2- and NGF-induced hyperalgesia but not GDNF-induced hyperalgesia. Endomorphin-2 was detected on TRPV1+ but not on IB4+ neurons. Peripheral opioid receptor antagonism by naloxone methiodide or administration of an anti-endomorphin-2 antibody to a sensitized paw reversed the antihyperalgesia induced by sigma-1 antagonists. Sigma-1 antagonism transfers sigma-1R from TRPV1 to mu receptors, suggesting that sigma-1R participate in TRPV1-mu receptor crosstalk. Moreover, sigma-1 antagonism reversed, in a naloxone-sensitive manner, PGE2-induced sensitization of DRG neurons to the calcium flux elicited by capsaicin, the prototypic TRPV1 agonist.Conclusion and ImplicationsSigma-1 antagonism harnesses endogenous opioids produced by TRPV1+ neurons to reduce hyperalgesia by increasing mu receptor activity.