Cathepsin S in the spinal microglia contributes to remifentanil-induced hyperalgesia in rats

Cysteine protease Cathepsin S (CatS) expressed by spinal microglia has been shown to play a critical role in nerve injury and inflammation-induced chronic pain. However, whether microglial CatS contributes to remifentanil-induced acute hyperalgesia remains unstudied. In the present study, intravenous remifentanil infusion induced a significant increase in the expression of premature and mature form of CatS in the activated microglia in the spinal cord. Spinal delivery of irreversible CatS inhibitor LHVS reduced hyperalgesia, attenuated activation of spinal microglia and blocked phosphorylation of NMDA receptor NR1 subunit induced by remifentanil. Furthermore, inhibition of microglia by minocycline effectively suppressed remifentanil-induced hyperalgesia, as well as CatS upregulation. In addition, remifentanil infusion also induced an increase in reactive oxygen species (ROS) levels in spinal neurons. Systemic administration of ROS scavenger PBN was sufficient to suppress remifentanil-induced painful hypersensitivity. Removal of ROS by PBN prevented upregulation of mature CatS in spinal microglia. However, increased protein level of premature form of CatS was not affected by PBN. Altogether, our findings demonstrate that neuronal ROS promote maturation of microglial CatS which facilitates activation of NMDA in the spinal dorsal horn. Therefore, such mechanism is involved in neuron-microglia positive feedback and contributes to remifentanil-induced hyperalgesia.