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

Microinjection of non-opioid analgesics, such as dipyrone (DIP), into the periaqueductal gray matter (PAG) in rats causes an inhibition of nociceptive circuits in the spinal cord. We have herein investigated whether this effect is mediated by opioidergic mechanisms in the rostral ventromedial medulla (RVM), which is an important relay between the PAG and the spinal cord. The responses of spinal wide-dynamic-range neurons to noxious stimulation of their receptive field (RF) were inhibited by microinjection of DIP (100 microg/0.5 microl) into PAG. Subsequent microinjection of naloxone (NAL; 0.5 microg/0.5 microl) into RVM reversed this inhibition. The present and previous results suggest that non-opioid analgesics, as well as opiates, inhibit nociception by activating descending opioidergic mechanisms in PAG and RVM.