Electrophysiological evidence for a spinal antinociceptive action of dipyrone

Electrophysiological experiments in anesthetized cats and rats were performed in order to study the effects of dipyrone on single afferent fibers from the knee joint and on spinal cord neurons with knee joint input. The neurons were activated and/or rendered hyperexcitable by an acute inflammation in the knee joint. In the joint nerve in cats, intravenous dipyrone (25-100 mg/kg) reduced ongoing activity in 10/12 thinly myelinated afferents but only in 1/10 unmyelinated afferents; the responses to movements of the inflamed knee were reduced in 8/10 thinly myelinated but only in 3/10 unmyelinated units. The reduction of activity was significant 20-30 min after application and was maximal at 60-180 min. In the spinal cord of spinalized cats, intravenous dipyrone (25-100 mg/kg) reduced ongoing activity and/or responses to pressure onto the inflamed knee in 14/16 neurons and in non-spinalized rats similar effects were seen in 10/11 neurons. Effects on spinal cord neurons started 5-10 min after application and were maximal after 20-40 min. These data show pronounced suppression of inflammation-induced nociception by dipyrone and they suggest that the spinal cord is a major site of action of this compound.