NITRIC-OXIDE CONTRIBUTES TO BOTH SPINAL NOCICEPTIVE TRANSMISSION AND ITS DESCENDING INHIBITION IN RATS - AN IMMUNOCYTOCHEMICAL STUDY

The present study was designed to determine if nitric oxide (NO) was i nvolved both in the dorsal horn responses to the primary nociceptive i nputs and the descending inhibitory action on these responses. The fir st part of the experiments showed that when formalin was injected into one hindpaw, the nociceptive c-fos expression in the lumbar dorsal ho rn ipsilateral to the injection was suppressed dose-dependently by int rathecal (i.t.) administration of N-omega-nitro-L-arginine (L-NNA), a nitric oxide synthase inhibitor. In the second part of the study, the formalin injection was carried out into two hindpaws of the rats with a sectioned dorsal quadrant at the thoracic spinal level, in these rat s, there was a significant suppression of c-fos expression in the dors al horn on the side with intact dorsal quadrant, reasonably owing to t he preservation of the spinally descending inhibitory fibers from the supraspinal level; furthermore, this suppression could be canceled fol lowing i.t. L-NNA administration. The results suggest that endogenous NO not only facilitates the perception of nociceptive inputs at the sp inal level but also enhances the descending inhibition upon the spinal nociception. (C) 1998 Elsevier Science Ireland Ltd.