[Effect of morphine on pain threshold and C-fos expression induced by substance P]

Using pain measurement and immunohistochemistry, we have observed that the intrathecal injection (it) SP produced dose-related pain response. A larger dose of SP (5 micrograms, 10 micrograms) could reduce the pain threshold and evoke c-fos expression, but no obvious effect was produced by a smaller dose of SP (0.5 microgram, 1 microgram). Morphine (it) not only increased the pain threshold, but also inhibited pain response and c-fos expression of spinal cord induced by it SP, the effects could be reversed by naloxone.