Morphine induces μ opioid receptor endocytosis in guinea pig enteric neurons following prolonged receptor activation.
OBJECTIVE
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 β-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 β-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.
CURE Digestive Diseases Research Center, Veterans Administration Greater Los Angeles Healthcare System, Los Angeles, California 90073, USA.
Simona
S
Patierno
Laura
L
Anselmi
Ingrid
I
Jaramillo
David
D
Scott
Rachel
R
Garcia
Catia
C
Sternini
D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D019390:Arrestins / Q000032:analysis; D034281:Dynamins / Q000032:analysis / Q000037:antagonists & inhibitors; D004705:Endocytosis / Q000187:drug effects*; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)- / Q000494:pharmacology; D017615:Enteric Nervous System / Q000187:drug effects*; D006168:Guinea Pigs; D006835:Hydrazones / Q000494:pharmacology; D007082:Ileum / Q000737:chemistry / Q000187:drug effects; D008297:Male; D009020:Morphine / Q000494:pharmacology*; D017450:Receptors, Opioid, mu / Q000819:agonists*; D015398:Signal Transduction / Q000187:drug effects; D013779:Tetrodotoxin / Q000494:pharmacology; D000071557:beta-Arrestins
D000701:Analgesics, Opioid; D019390:Arrestins; D006835:Hydrazones; C511794:N'-(3,4-dihydroxybenzylidene)-3-hydroxy-2-naphthahydrazide; D017450:Receptors, Opioid, mu; D000071557:beta-Arrestins; D020875:Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; D013779:Tetrodotoxin; D009020:Morphine; D034281:Dynamins