Actions of kainate and AMPA selective glutamate receptor ligands on nociceptive processing in the spinal cord

Kainate receptors expressing the GluR5 subunit of glutamate receptor are present at high levels on small diameter primary afferent neurones that are considered to mediate nociceptive inputs. This suggests that GluR5 selective ligands could be novel analgesic agents. The role of kainate receptors on C fibre primary afferents has therefore been probed using three compounds that are selective for homomeric GluR5 receptors. The agonist, ATPA, and the antagonists, LY294486 and LY382884, have been tested in four models of nociception: responses evoked by noxious stimulation of the periphery have been recorded electrophysiologically (1) from hemisected spinal cords from neonatal rats in vitro, (2) from single motor units in adult rats in vivo, (3) from dorsal horn neurones in adult rats in vivo, and (4) in hotplate tests with conscious mice. In some protocols comparisons were made with the AMPA selective antagonist GYKI 53655. The agonist ATPA reduced nociceptive reflexes in vitro, but failed to have effects in vivo. In all tests, the GluR5 antagonists reduced nociceptive responses but only at doses that also affected responses to exogenous AMPA. The AMPA antagonist reduced nociceptive responses at doses causing relatively greater reductions of responses to exogenous AMPA. The results indicate that GluR5 selective ligands do reduce spinal nociceptive responses, but they are not strongly analgesic under these conditions of acute nociception.