Organotypic cultures and ileal neuromuscular preparations were used to dete
rmine (i) whether endogenous release of opioids by electrical stimulation i
nduces mu receptor endocytosis, and (ii) whether and under which conditions
Ligand-induced mu receptor endocytosis influences the responsiveness of ne
urons expressing native mu receptor. In longitudinal muscle-myenteric plexu
s preparations, electrical stimulation at 20 Hz induced a prominent endocyt
osis of mu receptors in enteric neurons, indicating endogenous release of o
pioids. A similar massive endocytosis was triggered by exogenous applicatio
n of the mu receptor agonist, [D-Ala(2),MePhe(4),Gly-ol(5)] enkephalin, whe
reas exogenous application of morphine was ineffective. [D-Ala(2),MePhe(4),
Gly-ol(5)] enkephalin and morphine induced a concentration-dependent inhibi
tion of neurogenic cholinergic twitch contractions to electrical stimulatio
n at 0.1 Hz. beta-Chlornaltrexamine shifted to the right the inhibitory cur
ve of both agonists with a concentration-dependent reduction of the maximum
agonist response, which is consistent with the existence of spare mu opioi
d receptors. Under these conditions, the induction of mu receptor endocytos
is by exogenously applied [D-Ala(2),MePhe(4),Gly-ol(5)] enkephalin diminish
ed the inhibitory effect of this agonist on twitch contractions and tritiat
ed acetylcholine release. In contrast, there was no reduction of the inhibi
tory effect of morphine, which failed to induce mu receptor endocytosis, on
neurogenic cholinergic response.
These results provide the first evidence for the occurrence of mu receptor
endocytosis in neurons by endogenously released opioids and show that agoni
st-dependent mu receptor endocytosis could serve as a mechanism to regulate
mu opioid receptor responsiveness to ligand stimulation when the opioid re
ceptor reserve is reduced. (C) 2000 IBRO. Published by Elsevier Science Ltd
. All rights reserved.