Local peripheral effects of μ-opioid receptor agonists in neuropathic pain in rats

Our Study was designed to demonstrate peripheral antinociception of the mu-opioid receptor agonists: morphine (MF), [D-Ala(2), N-Me-Phe(4). Gly(5)-ol]enkephalin (DAMGO). endomorphin-1 (EM-1) and endomorphin-2 (EM-2) in Bennett's rat model of neuropathic pain. All the agonists were effective in antagonizing allodynia after their intraplantar (i.pl.) but not subcutaneous (s.c.) administration. Opioid peptides: DAMGO, EM-1 and EM-2 were more effective compared with corresponding doses of morphine (opioid alkaloid) in alleviating chronic pain. Peripheral mu-opioid receptors mediated the observed effects, as was evidenced by the i.pl. treatment with naloxone methiodide (active only at the site of injection) and by cyprodime, a selective mu-opioid receptor antagonist. These results have shown that opioid peptides are effective also after local treatment. and that their peripheral use may be of therapeutic interest in long-term management of chronic pain. (C) 2004 Elsevier Ireland Ltd. All rights reserved.