• Jin-Miao Lu, Nian Gong, Yan-Chao Wang, and Yong-Xiang Wang.
• King's Lab, School of Pharmacy, Shanghai Jiao Tong University, Shanghai, China.
• Br. J. Pharmacol. 2012 Mar 1;165(6):1941-55.

Background And PurposeSpinal reactive oxygen species (ROS) are critically involved in chronic pain. D-Amino acid oxidase (DAAO) oxidizes D-amino acids such as D-serine to form the byproduct hydrogen peroxide without producing other ROS. DAAO inhibitors are specifically analgesic in tonic pain, neuropathic pain and cancer pain. This study examined the role of spinal hydrogen peroxide in pain and the mechanism of the analgesic effects of DAAO inhibitors.Experimental ApproachFormalin-induced pain behaviours and spinal hydrogen peroxide levels were measured in rodents.Key ResultsFormalin injected into the paw increased spinal hydrogen peroxide synchronously with enhanced tonic pain; both were effectively prevented by i.t. fluorocitrate, a selective astrocyte metabolic inhibitor. Given systemically, the potent DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) blocked spinal DAAO enzymatic activity and specifically prevented formalin-induced tonic pain in a dose-dependent manner. Although CBIO maximally inhibited tonic pain by 62%, it completely prevented the increase in spinal hydrogen peroxide. I.t. catalase, an enzyme specific for decomposition of hydrogen peroxide, completely depleted spinal hydrogen peroxide and prevented formalin-induced tonic pain by 65%. Given systemically, the ROS scavenger PBN (phenyl-N-tert-butylnitrone) also inhibited formalin-induced tonic pain and increase in spinal hydrogen peroxide. Formalin-induced tonic pain was potentiated by i.t. exogenous hydrogen peroxide. CBIO did not increase spinal D-serine level, and i.t. D-serine did not alter either formalin-induced tonic pain or CBIO's analgesic effect.Conclusions And ImplicationsSpinal hydrogen peroxide is specifically and largely responsible for formalin-induced pain, and DAAO inhibitors produce analgesia by blocking spinal hydrogen peroxide production rather than interacting with spinal D-serine.© 2011 The Authors. British Journal of Pharmacology © 2011 The British Pharmacological Society.

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