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

Neuroscience. 1999 Mar;90(3):1051-9. doi: 10.1016/s0306-4522(98)00514-4.


The multiple effects of opiate alkaloids, important therapeutic drugs used for pain control, are mediated by the neuronal miro-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 micro-opioid receptors. Since the miro-opioid receptor undergoes ligand-induced receptor endocytosis in an agonist-dependent manner, we compared the ability of endomorphin-1, endomorphin-2 and the micro-opioid receptor peptide agonist, [D-Ala2,MePhe4,Gly-ol5]-enkephalin (DAMGO), to induce receptor endocytosis in cells transfected with epitope-tagged micro-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 micro-opioid receptor was mainly located at the plasma membrane of unstimulated cells. Endomorphins and DAMGO induced micro-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 approximately 8.4) is consistent with antagonism at the micro-opioid receptor in myenteric neurons. These results indicate that endomorphins directly activate the micro-opioid receptor in neurons, thus supporting the hypothesis that they are ligands mediating opioid actions in the nervous system. Endomorphin-induced micro-opioid receptor activation can be visualized by receptor endocytosis.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • Analgesics, Opioid / pharmacology*
  • Animals
  • Cell Line / metabolism
  • Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
  • Enkephalins / pharmacology
  • Flow Cytometry
  • Guinea Pigs
  • Ileum / drug effects
  • Ileum / innervation
  • Ileum / physiology
  • In Vitro Techniques
  • Male
  • Muscle Contraction / drug effects
  • Muscle, Smooth / drug effects
  • Muscle, Smooth / physiology
  • Myenteric Plexus / cytology
  • Myenteric Plexus / drug effects
  • Myenteric Plexus / metabolism
  • Neurons / metabolism
  • Oligopeptides / pharmacology*
  • Rats
  • Receptors, Opioid, mu / drug effects*
  • Receptors, Opioid, mu / metabolism*
  • Tissue Distribution / physiology


  • Analgesics, Opioid
  • Enkephalins
  • Oligopeptides
  • Receptors, Opioid, mu
  • endomorphin 1
  • Enkephalin, Ala(2)-MePhe(4)-Gly(5)-
  • endomorphin 2