Parenterally administered kainic acid induces a persistent hyperalgesia in the mouse and rat

Nociceptive primary afferent C-fibers express a subset of glutamate receptors that are sensitive to kainic acid. Thus, we tested the possibility that activation of these receptors alters nociception. Intraperitoneal (i.p.) injection of kainic acid induced a persistent thermal hyperalgesia, when tested using the hot plate (mice) and tail flick (mice and rats) assays, and mechanical hyperalgesia when tested using von Frey monofilaments (rats), but had no effect on acetic acid-induced chemical nociception (mice). When administered i. p., 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), an (R, S)-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid HBr/kainate (AMPA/KA) antagonist, completely blocked hyperalgesia. When injected intrathecally (i.t.), kainic acid itself failed to induce hyperalgesia and AMPA/KA antagonists given i.t. also failed to attenuate the hyperalgesic effect of kainic acid administered i.p. , indicating that the spinal cord is not the primary site of action. Kainic acid injected subcutaneously in the back of mice decreased response latencies in the hot plate and tail flick assays, indicating that hyperalgesia is achieved by a variety of parenteral routes of injection. Histological evaluation of rat spinal cord and dorsal root ganglia revealed no neurodegenerative changes 24 h after kainic acid. Together these data suggest that a persistent hyperalgesia results from the transient activation of AMPA/KA receptors that are located outside the spinal cord, perhaps on the distal projections of primary afferent fibers.