Endomorphin-1 reduces carrageenan-induced fos expression in the rat spinal dorsal horn

Intraplantar injection of carrageenan induced significant Fos expression in the superficial and deep spinal dorsal horn at the L(4)-L(5)segments and extensive peripheral edema of the ipsilateral foot in rats. Intraplantar injection of endomorphin-1, endogenous ligand for mu opioid receptor, in the same region produced dose-dependent reduction of carrageenan-induced Fos expression and peripheral edema, which were completely blocked by co-administration of intraplantar injection of naloxone (20 microgram). The systemic injection of the highest dose of endomorphin-1 (50 microgram) had no significant reductory effect on Fos expression and peripheral edema. These results further provided a strong evidence for involvement of mu opioid receptor in peripheral analgesia, particularly in inflammation pain.