The antinociceptive effect of PAG-microinjected dipyrone in rats is mediated by endogenous opioids of the rostral ventromedial medulla

Microinjection of non-opioid analgesics, such as dipyrone (DIP), into the p eriaqueductal gray matter (PAG) in rats causes an inhibition of nociceptive circuits in the spinal cord. We have herein investigated whether this effe ct is mediated by opioidergic mechanisms in the rostral ventromedial medull a (RVM), which is an important relay between the PAG and the spinal cord. T he responses of spinal wide-dynamic-range neurons to noxious stimulation of their receptive field (RF) were inhibited by microinjection of DIP (100 mu g/0.5 mu l) into PAG. Subsequent microinjection of naloxone (NAL; 0.5 mu g /0.5 mu l) into RVM reversed this inhibition. The present and previous resu lts suggest that non-opioid analgesics, as well as opiates, inhibit nocicep tion by activating descending opioidergic mechanisms in PAG and RVM. (C) 20 00 Elsevier Science B.V. All rights reserved.