Peroxynitrite (PN, ONOO(-)) is a potent
oxidant and nitrating agent that contributes to
pain through peripheral and spinal mechanisms, but its supraspinal role is unknown. We present evidence here that PN in the rostral ventromedial medulla (RVM) is essential for descending nociceptive modulation in rats during inflammatory and
neuropathic pain through PN-mediated suppression of
opioid signaling.
Carrageenan-induced
thermal hyperalgesia was associated with increased
3-nitrotyrosine (NT), a PN
biomarker, in the RVM. Furthermore, intra-RVM microinjections of the PN decomposition catalyst Fe(III)-5,10,15,20-tetrakis(N-methyl-pyridinium-4-yl)porphyrin (FeTMPyP(5+)) dose-dependently reversed this
thermal hyperalgesia. These effects of FeTMPyP(5+) were abrogated by intra-RVM
naloxone, implicating potential interplay between PN and
opioids. In support, we identified NT colocalization with the endogenous
opioid enkephalin (ENK) in the RVM during
thermal hyperalgesia, suggesting potential in situ interactions. To address the functional significance of such interactions, we exposed
methionine-enkephalin (MENK) to PN and identified the major metabolite, 3-nitrotyrosine-methionine-sulfoxide (NSO)-MENK, using liquid chromatography-mass spectrometry. Next, we isolated, purified, and tested NSO-MENK for
opioid receptor binding affinity and
analgesic effects. Compared to MENK, this NSO-MENK metabolite lacked appreciable binding affinity for δ, μ, and κ
opioid receptors.
Intrathecal injection of NSO-MENK in rats did not evoke antinociception, suggesting that PN-mediated chemical modifications of ENK suppress
opioid signaling. When extended to
chronic pain, intra-RVM FeTMPyP(5+) produced
naloxone-sensitive reversal of
mechanical allodynia in rats following chronic constriction injury of the sciatic nerve. Collectively, our data reveal the central role of PN in RVM descending facilitation during inflammatory and
neuropathic pain potentially through anti-
opioid activity.