Neurochemical changes after morphine, dizocilpine or riluzole in the ventral posterolateral thalamic nuclei of rats with hyperalgesia

A number of studies suggest the involvement of glutamate in central hyperal gesia through NMDA receptors in animal models of inflammation. Most studies analyze glutamate effects at the spinal cord level. In this work, the effe cts of morphine, dizocilpine and riluzole on the hyperalgesia induced by ca rrageenan administration in the rat paw model were investigated. The effect s of morphine and riluzole on the release of glutamate and aspartate and on the concentrations of citrulline and arginine in dialysates of the ventral posterolateral nucleus of the thalamus were also examined. All three drugs decreased hyperalgesia when administered prior to carrageenan injection. M orphine decreased the glutamate concentration in dialysates of the ventral posterolateral nucleus but did not affect the concentrations of the other a mino acids. The effect of morphine was observed in the absence of painful s timulation and when pressure applied to the rat paw induced a nociceptive r eaction. Riluzole decreased the concentrations of glutamate and aspartate a nd those of citrulline and arginine in the presence or absence of painful s timulation. These experiments suggest that morphine and riluzole attenuate the hyperalgesia induced by injection of carrageenan in the rat hind paw, a t least partly, by decreasing glutamate release in the ventral posterolater al thalamic nucleus. (C) 2000 Elsevier Science B.V. All rights reserved.