Morphine induces µ opioid receptor endocytosis in guinea pig enteric neurons following prolonged receptor activation.

BACKGROUND & AIMS: The µ opioid receptor (µOR) undergoes rapid endocytosis after acute stimulation with opioids and most opiates, but not with morphine. We investigated whether prolonged activation of µOR affects morphine's ability to induce receptor endocytosis in enteric neurons. METHODS: We compared the effects of morphine, a poor µOR-internalizing opiate, and (D-Ala2,MePhe4,Gly-ol5) enkephalin (DAMGO), a potent µOR-internalizing agonist, on µOR trafficking in enteric neurons and on the expression of dynamin and ß-arrestin immunoreactivity in the ileum of guinea pigs rendered tolerant by chronic administration of morphine. RESULTS: Morphine (100 µmol/L) strongly induced endocytosis of µOR in tolerant but not naive neurons (55.7% ± 9.3% vs 24.2% ± 7.3%; P < .001) whereas DAMGO (10 µmol/L) strongly induced internalization of µOR in neurons from tolerant and naive animals (63.6% ± 8.4% and 66.5% ± 3.6%). Morphine- or DAMGO-induced µOR endocytosis resulted from direct interactions between the ligand and the µOR because endocytosis was not affected by tetrodotoxin, a blocker of endogenous neurotransmitter release. Ligand-induced µOR internalization was inhibited by pretreatment with the dynamin inhibitor, dynasore. Chronic morphine administration resulted in a significant increase and translocation of dynamin immunoreactivity from the intracellular pool to the plasma membrane, but did not affect ß-arrestin immunoreactivity. CONCLUSIONS: Chronic activation of µORs increases the ability of morphine to induce µOR endocytosis in enteric neurons, which depends on the level and cellular localization of dynamin, a regulatory protein that has an important role in receptor-mediated signal transduction in cells.