Antinociception induced by PAG-microinjected dipyrone (metamizol) in rats: involvement of spinal endogenous opioids.

Dipyrone microinjection into the periaqueductal gray matter (PAG) elicits antinociception in rats by activating endogenous opioidergic circuits in PAG and the rostral ventromedial medulla. We have now found that endogenous opioids in the spinal cord are also involved. Responses of dorsal spinal neurons to noxious stimulation of a hindpaw were diminished (to 38-44%) by dipyrone microinjection (100 microg/0.5 microl) into the PAG. This was abolished by application of naloxone (50 microg/50 microl) to the spinal cord. The fact that dipyrone, a non-opioid analgesic, activates opioidergic circuits may be clinically important.