Neural changes in acute arthritis in monkeys. IV. Time-course of amino acid release into the lumbar dorsal horn

Abstract

Extracellular levels of amino acids were measured during the development of experimental arthritis in anesthetized monkeys. Levels of glutamate, aspartate, glycine, serine, glutamine, taurine, cysteic acid and asparagine were each measured in consecutive 30 min samples before, during and for several hours after injection of kaolin and carrageenan into the articular capsule of one knee. Samples were obtained via a microdialysis probe placed in the lumbar dorsal horn ipsilateral to the injected knee and assayed using HPLC with fluorescence detection. Glutamate, aspartate, glycine and serine increased transiently following intra-articular injection of inflammatory agents. During this period glutamine levels decreased. A second phase of release then occurred which included more prolonged changes in amino acid levels that were sometimes of greater magnitude than those immediately following the injection. In animals which were later observed to have depletion of SP in the dorsal horn of the inflamed side, taurine levels increased starting after the Glu, Asp and Gly had plateaued at near baseline concentrations. Thus during the first stages of joint inflammation EAAs are released into the dorsal horn, followed by increased levels of IAAs, possibly representing activation of the descending endogenous analgesia system. This phase is followed by a semiacute response consisting in part of increased extracellular levels of SP and Tau. While SP is presumably part of an ascending nociceptive transmission system, Tau could be part of a second system aimed at reducing excessive neural activity including neural transmission resulting in intense maintained pain.