A series of experiments using technologies of gene mutation and silencing as well as chemical biology have demonstrated that spinal
D-amino acid oxidase (DAAO) contributes to the development of central sensitization-mediated
chronic pain and might be a potential molecular target for the treatment of
chronic pain. DAAO inhibitors are now under clinical investigations for the management of chronic
neuropathic pain. This study examined the interactions between
morphine and the DAAO inhibitor CBIO (5-chloro-benzo[d]
isoxazol-3-ol) in
pain and
analgesia tolerance mainly in the
formalin test. Given subcutaneously CBIO acutely interacted with
morphine in
analgesia in an additive manner both in the acute nociception settings (the
formalin acute phase nociception, hot-plate test and tail immersion test) and in
formalin-induced tonic
pain. Bi-daily exposure of CBIO given subcutaneously for 7 days did not produce self-tolerance to
analgesia or cross-tolerance to
morphine whereas 7-day subcutaneous
morphine induced self-tolerance to
analgesia but not cross-tolerance to CBIO. More importantly, subcutaneous co-administrations or even single dose of CBIO completely prevented or reversed
morphine tolerance to
analgesia (exhibited by a single dose or a dose-response curve of
morphine) in both
formalin-induced acute phase nociception and tonic phase
pain. These results, for the first time, identified DAAO as an efficacious molecule mediating
morphine tolerance, in addition to clarifying the complex interactions between
morphine and DAAO inhibitors probed by CBIO, and provided a pharmacological basis for DAAO inhibitors in combination with
morphine to clinically manage
pain.