Contributions of spinal d-amino acid oxidase to chronic morphine-induced hyperalgesia

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Spinal d-amino acid oxidase (DAAO) is an FAD-dependent peroxisomal flavoenzyme which mediates the conversion of neutral and polar d-amino acids (including d-serine) to the corresponding α-keto acids, and simultaneously produces hydrogen peroxide and ammonia. This study has aimed to explore the potential contributions of spinal DAAO and its mediated hydrogen peroxide/d-serine metabolism to the development of morphine-induced hyperalgesia. Bi-daily subcutaneous injections of morphine to mice over 7 days induced thermal hyperalgesia as measured by both the hot-plate and tail-immersion tests, and spinal astroglial activation with increased spinal gene expression of DAAO, glial fibrillary acidic protein (GFAP) and pro-inflammatory cytokines (interleukin-1β (IL-1β), interleukin-6 (IL-6) and tumor necrosis factor-α (TNF-α)). Subcutaneous injections of the potent DAAO inhibitor CBIO (5-chloro-benzo[d]isoxazol-3-ol) prevented and reversed the chronic morphine-induced hyperalgesia. CBIO also inhibited both astrocyte activation and the expression of pro-inflammatory cytokines. Intrathecal injection of the hydrogen peroxide scavenger PBN (phenyl-N-tert-butylnitrone) and of catalase completely reversed established morphine hyperalgesia, whereas subcutaneous injections of exogenous d-serine failed to alter chronic morphine-induced hyperalgesia. These results provided evidence that spinal DAAO and its subsequent production of hydrogen peroxide rather than the d-serine metabolism contributed to the development of morphine-induced hyperalgesia.

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