Spinally applied ketamine or morphine attenuate peripheral inflammation and hyperalgesia in acute and chronic phases of experimental arthritis.

Inflammation causes sensitization of peripheral and central nociceptive neurons. Pharmacological modulation of the latter has successfully been used for clinical pain relief. In particular, inhibitors of the NMDA glutamate receptor such as ketamine and agonists at the mu-opioid receptor such as morphine are broadly used. Besides driving the propagation of pain signals, spinal mechanisms are also discussed to modulate inflammation in the periphery. Here, we tested the hypothesis that intrathecally applied ketamine or morphine not only reduce pain-related behavior, but also attenuate induction and maintenance of the inflammatory response in a model of chronic antigen-induced arthritis (AIA). Ketamine, morphine or vehicle was applied to the spinal cords of anesthesized animals with AIA. Swelling and histopathological changes were assessed after 6h (acute phase). Intrathecal catheters were implanted in another set of animals with AIA and substances were applied continuously. During the observation period of 21 days, inflammation and pain-related behavior were assessed. Ketamine and morphine significantly reduced arthritis severity as indicated by reduced joint swelling, but even more intriguingly by reduced infiltration with inflammatory cells and joint destruction in the acute and the chronic phase of arthritis. Morphine showed strong antinociceptive effects in the acute phase only, while the newly established effective dose for ketamine in a continuous application design reduced hyperalgesia in the acute and the chronic stage. In conclusion, both compounds exhibit anti-inflammatory effects during induction and maintenance of arthritis when applied intrathecally. These data thus propose a role of spinal NMDA- and opioid-receptors in the neuronal control of immune-mediated inflammation.