Deletion of the CCK2 receptor gene reduces mechanical sensitivity and abolishes the development of hyperalgesia in mononeuropathic mice

Previous studies suggest that cholecystokinin (CCK) is implicated in the modulation of pain sensitivity and the development of neuropathic pain. We used CCK2 receptor deficient (CCK2 (-/-)) mice and assessed their mechanical sensitivity using Von Frey filaments, as well as the development and time course of mechanical hyperalgesia in a model of neuropathic pain. We found that CCK2 (-/-) mice displayed mechanical hyposensitivity, which was reversed to the level of wild-type animals after administration of naloxone (0.1-10 mg/kg). On the other hand, injection of L-365260 (0.01-1 mg/kg), an antagonist of CCK2 receptors, decreased dose-dependently, mechanical sensitivity in wild-type mice. The mechanism of reduced mechanical sensitivity in CCK2 (-/-) mice may be explained by changes in interactions between CCK and opioid systems. Indeed, CCK2 (-/-) mice natively expressed higher levels of lumbar CCK1, opioid delta and kappa receptors. Next, we found that CCK2 (-/-) mice did not develop mechanical hyperalgesia in the Bennett's neuropathic pain model. Induction of neuropathy resulted in decrease of lumbar pro-opiomelanocortin (POMC) gene expression in wild-type mice, but increase of POMC expression in CCK2 (-/-) mice. In addition, induction of neuropathy resulted in further increase of opioid delta receptor in CCK2 (-/-) mice. Gene expression results indicate up-regulation of opioid system in CCK2 (-/-) mice, which apparently result in decreased neuropathy score. Our study suggests that not only pain sensitivity, but also mechanical sensitivity and the development of neuropathic pain are regulated by antagonistic interactions between CCK and opioid systems.