We have found that mutation of D-amino acid oxidase (DAO) diminished formalin-induced tonic pain. The present research further studied the analgesic effects of a series of DAO inhibitors in this model. 5-Chlorobenzo[d]isoxazol-3-ol (CBIO), 4H-thieno[3,2-b]pyrrole-5-carboxylic acid (compound 8), 5-methylpyrazole-3-carboxylic acid (AS057278), sodium benzoate, and 4-nitro-3-pyrazole carboxylic aci...