Effect of repeated cold stress on capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

Abstract

We investigated the effect of repeated cold stress (RCS) on the capsaicin-evoked release of glutamate from the primary afferent fibers of the rat, and compared this with the effect of inoculation of complete Freund's adjuvant (adjuvant inoculation). The release of glutamate was measured using a fluorometric on-line continuous monitoring system in which the immobilized glutamate dehydrogenase column was connected to an in vitro superfusion system. In the presence of 0.3 μM tetrodotoxin, the application of 1 μM capsaicin to spinal dorsal horn slices evoked glutamate release (18.6±1.2 pmol mg −1 protein, n=11). In rats subjected to RCS (RCS rats), the release of glutamate evoked by 1 μM capsaicin was markedly increased to 272% ( n=6, P<0.05) of the value for the control group, although the basal release was not significantly altered ( n=6, P>0.05). Adjuvant inoculation produced a significant increase in the basal and capsaicin (1 μM) evoked release of glutamate to 141 and 344% ( n=6, P<0.05) of the value for the control group, respectively. The present results suggest that the facilitated release of glutamate from capsaicin-sensitive primary afferent terminals in the spinal dorsal horn is, at least in part, involved in the hyperalgesia of RCS rats as well as the complete Freund's adjuvant-induced hyperalgesia.