EVIDENCE THAT GLUTAMATE IS RELEASED FROM CAPSAICIN-SENSITIVE PRIMARY AFFERENT-FIBERS IN RATS - STUDY WITH ONLINE CONTINUOUS MONITORING OF GLUTAMATE

The aim of this study is to elucidate whether the excitatory amino aci d glutamate is released from capsaicin-sensitive primary afferent fibe rs, and to compare the releasing effect of capsaicin on glutamate with that on substance P. The release of glutamate was measured using a fl uorometric on-line continuous monitoring system, in which the immobili zed glutamate dehydrogenase column was connected to an in vitro superf usion system. In the presence of 0.3 mu M tetrodotoxin, 2-min applicat ion of capsaicin produced an increased outflow of glutamate, as well a s an increase in the release of immunoreactive substance P from dorsal horn slices of the rat. The release of glutamate was concentration-de pendently increased by capsaicin at concentrations in the range of 0.1 -3 mu M, and the release evoked by 10 mu M capsaicin was not higher th an that evoked by 3 mu M. On the other hand, capsaicin at concentratio ns of 1-10 mu M produced a concentration-dependent increase in the rel ease of immunoreactive substance P, without effect at 0.1 mu M. The am ount of glutamate release evoked by 3 mu M capsaicin was about 42.8 pm ol.mg(-1) protein, and 290 times that of immunoreactive substance P. T he release of glutamate by 3 mu M capsaicin was suppressed by the depl etion of calcium from the superfusate. Capsaicin at 3 mu M failed to i ncrease the release of glutamate from the dorsal horn slices of the ra ts made an L(4)-L(6) dorsal rhizotomy. These results suggest that caps aicin evoked the release of glutamate from primary afferent fibers in the dorsal horn and that glutamate may play an important role in pain transmission between primary afferent fibers and dorsal horn neurons.