Antagonism of stimulation-produced analgesia by naloxone and N-methyl-D-aspartate: role of opioid and N-methyl-D-aspartate receptors.

The present study aims to investigate the influence of electrical stimulation of periaqueductal gray (PAG) following peripheral nerve injury and its modulation by naloxone and N-methyl-D-aspartate (NMDA). Chronic neuropathic pain was induced by chronic constriction injury of the sciatic nerve, and subsequently a cannula was implanted in the PAG area for the purpose of electrical stimulation and intra-PAG drug administration. Intra-PAG administration of morphine, ketamine, and their combination were found to elicit antinociceptive response on hot-plate test. Electrical stimulation of PAG was also observed to demonstrate decreased pain response on hot-plate test, and this effect was reversed by the administration of naloxone, NMDA, and their combination, when injected into the PAG area. These findings suggest that apart from the opioid receptors, probably NMDA receptors also have a role to play in stimulation-produced analgesia.