Role of agonist-dependent receptor internalization in the regulation of muopioid receptors

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.