Opioidergic effects of nonopioid analgesics on the central nervous system.

1. The analgesic effect of nonsteroidal anti-inflammatory drugs (NSAIDs) is partly due to the fact that they act upon the periaqueductal gray matter (PAG) and the rostral ventromedial medulla of the brain stem and thus activate the descending pain-control system, which inhibits nociceptive transmission at the spinal dorsal horn. 2. The analgesic action of dipyrone (metamizol) and of lysine-acetylsalicylate (LASA), two well-known NSAIDs. whether microinjected into the PAG or given systemically, can be reverted by naloxone. Repeated administration of dipyrone or LASA induces tolerance to their antinociceptive effect, with cross-tolerance to morphine, and a withdrawal syndrome upon naloxone administration. Dipyrone tolerance can be reverted by proglumide, a cholecystokinin antagonist. 3. These findings reveal a close association between the central action of NSAIDs and endogenous opioids.