Neurokinin-1 receptor mediated formalin-induced c-fos expression in the rat spinal cord

In the present work, the effects of tachykinin receptor antagonists on formalininduced c-fos expression in the rat spinal cord were studied by a combination of immunocytochemical and neuropharmacological methods. Formalin injected into a hindpaw of rat only resulted in c-fos expression in the ipsilateral dorsal horn neurons of the spinal cord. Fos-positive neurons were distributed mainly in medial parts of the lamina I and the outer lamina II . A moderate number of Fos-positive neurons were present in laminae IV-V , and a few in the inner lamina II and in laminae III, VI and X . Selective NK-1 antagonist L-668, 169 (0.1, 1 and 10 μg) administered intrathecally before injection of formalin into a hindpaw significantly reduced the number of Fos-labeled neurons in the dorsal horn in a dose-dependent manner (P < 0.01), while selective NK-2 antagonist L-659,877 (10 μg) and selective NK-3 antagonist SR-142,801 (10 μg) were ineffective (P > 0.05). Our results indicated that NK-1 receptor mediated formalin-induced c-fos expression in the spinal cord, suggesting NK-1 receptor might play an important role in the spinal transmission of nociceptive messages.