Analgesic effect of endomorphin-1

AIM: To study the analgesic effect of endomorphin-1 (EM-1). METHODS: The experiment was performed in rats and mice to study the analgesic effect of intraperitoneal ( ip) injection of EM-1 with tail stimulation-vocalization test, writhing test, adjuvant arthritis, and neuropathic pain model and to compare it with the analgesic effects produced by intracerebroventricular ( icv) and intrathecal (it) administrations. RESULTS: 1) EM-1 raised the pain threshold dose-dependently in tail stimulation-vocalization test in rats and inhibited the writhing responses induced by ip acetic acid in mice. EM-1 also decreased the hyperalgesia in both adjuvant arthritis and neuropathic pain model. 2) The analgesic effect induced by central ( icv and it) administration of EM-1 was faster and more powerful than that induced by peripheral ( ip) administration. 3) The analgesic effect of EM-1 was reversed by naloxone (opioid receptor antagonist), as well as by cyprodime (mu -opioid receptor selective antagonist). Repeated administrations of EM-1 induced tolerance. CONCLUSION: EM-1 had a definite analgesic effect and the analgesic effect of EM-1 was mediated by central mu -opioid receptor.