Noxious stimulation increases glutamate and arginine in the periaqueductalgray matter in rats: a microdialysis study

The periaqueductal gray matter (PAG) is an important center in the modulati on of behavioral responses during nociception and stress. In the present ex periment, extracellular excitatory amino acid overflow in the PAG was measu red every :30 s during noxious stimulation. A combination of in vivo brain microdialysis in freely moving rats and capillary zone electrophoresis with laser induced-fluorescence detection allowed us to detect short lasting ch anges of excitatory amino acid in. dialysates. A formalin injection in the hindpaw of the rat increased glutamate, arginine and aspartate concentratio n in PAG dialysates. This increase was calcium and nerve impulse-dependent, suggesting neuronal and glial origin of glutamate and arginine, respective ly. Handling, pinching or saline injection in the hind paw did not increase glutamate showing that this neurochemical phenomenon is related to painful and persistent noxious stimulation. The results suggest that a rapid excit ation of the PAG occurs during noxious stimulation. The role of glutamate a nd arginine in analgesia is discussed. (C) 2000 International Association f or the Study of Pain. Published by Elsevier Science B.V. All rights reserve d.