Analgesic effects of endomorphin-1 and endomorphin-2 in the formalin test in mice

Two recently isolated peptides, endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and end omorphin-2 (Tyr-Pro-Phe-Phe-NH2), are highly selective mu-opioid receptor a gonists with analgesic actions in the tail-flick test. To further assess th e analgesic properties of these peptides, the effects of endomorphin-1. end omorphin-2, and morphine were examined in the formalin test. Male Swiss Web ster mice were injected i.c.v. with endomorphin-1, endomorphin-2, or morphi ne (0, 1, 3, 10 mu g) 5 min before injection of 20 mu l of 5% formalin s.c, into the plantar surface of one hind-paw The mice were observed for 60 min after formalin injection. Endomorphin-1 and endomorphin-2 produced dose-de pendent analgesia that was shorter in duration than for morphine. Increased locomotion was observed after morphine, but not after endomorphin-1 or end omorphin-2. These findings extend previous results and suggest that endomor phins may have therapeutic potential for the treatment of acute pain. (C) 2 000 Elsevier Science Inc. All rights reserved.