Activation and internalization of the μ-opioid receptor by the newly discovered endogenous agonists, endomorphin-1 and endomorphin-2

K. McConalogue, E. F. Grady, J. Minnis, B. Balestra, M. Tonini, N. C. Brecha, N. W. Bunnett, C. Sternini

Research output: Contribution to journalArticlepeer-review


The multiple effects of opiate alkaloids, important therapeutic drugs used for pain control, are mediated by the neuronal μ-opioid receptor. Among the side effects of these drugs is a profound impairment of gastrointestinal transit. Endomorphins are opioid peptides recently isolated from the nervous system, which have high affinity and selectivity for μ-opioid receptors. Since the μ-opioid receptor undergoes ligand-induced receptor endocytosis in an agonist-dependent manner, we compared the ability of endomorphin-1, endomorphin-2 and the μ-opioid receptor peptide agonist, [D- Ala2,MePhe4,Gly-ol5] enkephalin (DAMGO), to induce receptor endocytosis in cells transfected with epitope-tagged μ-opioid receptor complementary DNA, and in myenteric neurons of the guinea-pig ileum, which naturally express this receptor. Immunohistochemistry with antibodies to the FLAG epitope or to the native receptor showed that the μ-opioid receptor was mainly located at the plasma membrane of unstimulated cells. Endomorphins and DAMGO induced μ- opioid receptor endocytosis into early endosomes, a process that was inhibited by naloxone. Quantification of surface receptors by flow cytometry indicated that endomorphins' and DAMGO stimulated endocytosis with similar time-course and potency. They inhibited with similar potency electrically induced cholinergic contractions in the longitudinal muscle-myenteric plexus preparation through an action antagonized by naloxone. The apparent affinity estimate of naloxone (pA2 ~ 8.4) is consistent with antagonism at the μ- opioid receptor in myenteric neurons. These results indicate that endomorphins directly activate the μ-opioid receptor in neurons, thus supporting the hypothesis that they are ligands mediating opioid actions in the nervous system. Endomorphin-induced μ-opioid receptor activation can be visualized by receptor endocytosis.

Original languageEnglish (US)
Pages (from-to)1051-1059
Number of pages9
Issue number3
StatePublished - May 1999


  • Excitatory and inhibitory enteric neurons
  • Myenteric neurons
  • Neurogenic cholinergic contractions
  • Opiate alkaloids
  • Opioid peptides
  • Receptor endocytosis

ASJC Scopus subject areas

  • General Neuroscience


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