• Vinod Tiwari, Fei Yang, Shao-Qiu He, Ronen Shechter, Chen Zhang, Bin Shu, Tong Zhang, Vineeta Tiwari, Yun Wang, Xinzhong Dong, Yun Guan, and Srinivasa N Raja.
• From the Department of Anesthesiology and Critical Care Medicine, Johns Hopkins University, School of Medicine, Baltimore, Maryland (Vinod Tiwari, F.Y., S.-Q.H., R.S., B.S., Vineeta Tiwari, Y.G., S.N.R.); Department of Anesthesiology, Beijing Chaoyang Hospital, Capital Medical University, Beijing, China (C.Z., Y.W.); Department of Anesthesiology, Tongji Hospital, Tongji Medical College, Wuhan, China (B.S.); Department of Neurology, Beijing Tiantan Hospital, Capital Medical University, Beijing, China (T.Z.); The Solomon H. Snyder Department of Neuroscience, Center for Sensory Biology, Johns Hopkins University, School of Medicine, Baltimore, Maryland (X.D.); and Howard Hughes Medical Institute, Johns Hopkins University, School of Medicine, Baltimore, Maryland (X.D.).
• Anesthesiology. 2016 Mar 1; 124 (3): 706-20.

BackgroundOpioids have long been regarded as the most effective drugs for the treatment of severe acute and chronic pain. Unfortunately, their therapeutic efficacy and clinical utility have been limited because of central and peripheral side effects.MethodsTo determine the therapeutic value of peripheral μ-opioid receptors as a target for neuropathic pain treatment, the authors examined the effects of dermorphin [D-Arg2, Lys4] (1-4) amide (DALDA), a hydrophilic, peripherally acting μ-opioid receptor agonist, in male and female rats with spinal nerve ligation-induced neuropathic pain. The authors also utilized behavioral, pharmacologic, electrophysiologic, and molecular biologic tools to characterize DALDA's possible mechanisms of action in male rats.ResultsDALDA, administered subcutaneously, had 70 times greater efficacy for inhibiting thermal (n = 8 to 11/group) than mechanical hypersensitivity (n = 6 to 8/group) in male rats. The pain inhibitory effects of DALDA on mechanical and heat hypersensitivity were abolished in animals pretreated with systemic methylnaltrexone (n = 7 to 9/group), a peripheral μ-opioid receptor antagonist. In the spinal wide-dynamic range neurons, systemic DALDA inhibited C-fiber-mediated, but not A-fiber-mediated, response in neuropathic male rats (n = 13). In primary sensory neurons, DALDA inhibited the capsaicin-induced [Ca2+] increase more than the β-alanine-induced [Ca] increase (n = 300); capsaicin and β-alanine activate subpopulations of neurons involved in the signaling of heat and mechanical pain, respectively. DALDA-treated rats (n = 5 to 8/group) did not exhibit motor deficits and locomotor impairment suggesting that it does not induce central side effects.ConclusionsThese findings suggest that DALDA may represent a potential alternative to current opioid therapy for the treatment of neuropathic pain and is likely to be associated with minimal adverse effects.

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