Intrathecal endomorphin-1 produces antinociceptive activities modulated by alpha 2-adrenoceptors in the rat tail flick, tail pressure and formalin tests

Shuanglin Hao, Osamu Takahata, Hiroshi Iwasaki

Research output: Contribution to journalArticle

46 Scopus citations

Abstract

It is known that spinal morphine produces antinociception that is modulated by alpha 2-adrenoceptors. Endomorphin-1, a newly-isolated endogenous opioid ligand, shows the greatest selectivity and affinity for the mu-opiate receptor of any endogenous substance found to date and may serve as a natural ligand for the mu-opiate receptor. We examined the antinociceptive effects of endomorphin-1 administered intrathecally (i.t.) in the rat tail flick, tail pressure and formalin tests. Intrathecal endomorphin-1 produced dose-dependent antinociceptive effects in the three tests. ED50 (CI95) values for antinociception of i.t. endomorphin-1 in the tail flick test and tail pressure test were 1.9 (0.96-3.76) nmol and 1.8 (0.84.2) nmol, respectively. ED50 (CI95) values for phase 1 and phase 2 in the formalin test were 12.5 (7.9-19.8) nmol and 17.5 (10.2-30) nmol, respectively. Pretreatment with i.t. beta-funaltrexamine (a mu-opioid receptor selective antagonist) significantly antagonized the antinociceptive effects of endomorphin-1 in the three tests. Beta-funaltrexamine alone had not effects on the three tests. The antinociceptive effects of endomorphin-1 were also antagonized by i.t. yohimbine (an alpha 2-adrenoceptor selective antagonist). The combination of ineffective doses of i.t. clonidine (an alpha 2- adrenoceptor agonist) and endomorphin-1 produced a significant antinociception in the three tests. The results showed that intrathecal endomorphin-1 produced antinociception in a dose-dependent manner in the rat tail flick, tail pressure and formalin tests, which was mediated by spinal mu-opioid receptors and modulated by alpha 2-adrenoceptors. (C) 2000 Elsevier Science Inc.

Original languageEnglish (US)
Pages (from-to)195-204
Number of pages10
JournalLife Sciences
Volume66
Issue number15
DOIs
StatePublished - Mar 3 2000
Externally publishedYes

Keywords

  • Alpha 2- adrenoceptor
  • Endomorphin-1
  • Formalin tests
  • Rat
  • Tail flick
  • Tail pressure

ASJC Scopus subject areas

  • Pharmacology

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