SPINAL PHARMACOLOGY OF THERMAL HYPERESTHESIA INDUCED BY CONSTRICTION INJURY OF SCIATIC-NERVE - EXCITATORY AMINO-ACID ANTAGONISTS

This study evaluated the effects of spinally administered excitatory amino acid antagonists on the thermal hyperesthetic state induced by unilateral partial ligation of the sciatic nerve in the rat. The measured response was the latency to paw withdrawal of each hind paw after application of a focused heat lamp on the plantar surface of the paw through a glass plate upon which the animal stood. In this work, antagonists (MK801, DL-2-amino-5-phosphonovalerate, ketamine) of the N-methyl-D-aspartate receptor (NMDA), the glycine potentiation site at the NMDA receptor (5-chloro-indole-2-carboxylic acid) and non-NMDA receptor (kynurenic acid: g-D-glutamylaminomethyl sulphonate) were injected through chronically implanted lumbar intrathecal catheters in normal rats (no lesions) and in rats with unilateral constriction injury. In the normal rat study, NMDA and non-NMDA antagonists had little effect upon paw withdrawal latency at intrathecal doses which did not produce readily detectable motor weakness. In the hyperesthetic rat study, NMDA antagonists would temporarily eliminate the hyperesthetic state at doses below those which altered the response latency of the normal paw or which altered motor function. These results suggested that spinal NMDA receptors play an important role in the hyperesthetic state induced by peripheral nerve injury.