Morphine paradoxically prolongs neuropathic pain in rats by amplifying spinal NLRP3 inflammasome activation
- aDepartment of Psychology and Neuroscience, University of Colorado, Boulder, CO 80309;
- bThe Center for Neuroscience, University of Colorado, Boulder, CO 80309;
- cDiscipline of Pharmacology, School of Medicine, University of Adelaide, Adelaide, SA 5005, Australia;
- dDepartment of Pharmacology, University of North Carolina, Chapel Hill, NC 27599;
- eChemical Biology Laboratory, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China;
- fDepartment of Chemistry and Biochemistry, University of Colorado, Boulder, CO 80309;
- gBioFrontiers Institute, University of Colorado, Boulder, CO 80309;
- hThe Center for Neuroscience, University of Colorado, Boulder, CO 80309;
- iChemical Biology Research Branch, National Institute on Drug Abuse and National Institute on Alcohol Abuse and Alcoholism, Bethesda, MD 20892;
- jCenter of Basic Molecular Science, Department of Chemistry, Tsinghua University, Beijing 100082, China
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Edited by David Julius, University of California, San Francisco, CA, and approved April 19, 2016 (received for review February 16, 2016)

Significance
Pain after disease/damage of the nervous system is predominantly treated with opioids, but without exploration of the long-term consequences. We demonstrate that a short course of morphine after nerve injury doubles the duration of neuropathic pain. Using genetic and pharmacological interventions, and innovative Designer Receptor Exclusively Activated by Designer Drugs disruption of microglia reactivity, we demonstrate that opioid-prolonged neuropathic pain arises from spinal microglia and NOD-like receptor protein 3 inflammasome formation/activation. Inhibiting these processes permanently resets amplified pain to basal levels, an effect not previously reported. These data support the “two-hit hypothesis” of amplification of microglial activation—nerve injury being the first “hit,” morphine the second. The implications of such potent microglial “priming” has fundamental clinical implications for pain and may extend to many chronic neurological disorders.
Abstract
Opioid use for pain management has dramatically increased, with little assessment of potential pathophysiological consequences for the primary pain condition. Here, a short course of morphine, starting 10 d after injury in male rats, paradoxically and remarkably doubled the duration of chronic constriction injury (CCI)-allodynia, months after morphine ceased. No such effect of opioids on neuropathic pain has previously been reported. Using pharmacologic and genetic approaches, we discovered that the initiation and maintenance of this multimonth prolongation of neuropathic pain was mediated by a previously unidentified mechanism for spinal cord and pain—namely, morphine-induced spinal NOD-like receptor protein 3 (NLRP3) inflammasomes and associated release of interleukin-1β (IL-1β). As spinal dorsal horn microglia expressed this signaling platform, these cells were selectively inhibited in vivo after transfection with a novel Designer Receptor Exclusively Activated by Designer Drugs (DREADD). Multiday treatment with the DREADD-specific ligand clozapine-N-oxide prevented and enduringly reversed morphine-induced persistent sensitization for weeks to months after cessation of clozapine-N-oxide. These data demonstrate both the critical importance of microglia and that maintenance of chronic pain created by early exposure to opioids can be disrupted, resetting pain to normal. These data also provide strong support for the recent “two-hit hypothesis” of microglial priming, leading to exaggerated reactivity after the second challenge, documented here in the context of nerve injury followed by morphine. This study predicts that prolonged pain is an unrealized and clinically concerning consequence of the abundant use of opioids in chronic pain.
Footnotes
- ↵1To whom correspondence should be addressed. Email: peter.grace{at}colorado.edu.
Author contributions: P.M.G., X.W., M.V.B., H.H.Y., S.F.M., and L.R.W. designed research; P.M.G., K.A.S., E.L.G., X.W., M.V.B., T.J.F., N.D.A., L.I.G., D.B., Y.Z., A.L.E., and S.C. performed research; D.J.U., K.C., S.C., K.C.R., and B.L.R. contributed new reagents/analytic tools; P.M.G. analyzed data; and P.M.G., X.W., S.C., S.F.M., and L.R.W. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1602070113/-/DCSupplemental.
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