Morphine Promotes Rapid, Arrestin-Dependent Endocytosis of μ-Opioid Receptors in Striatal Neurons

Abstract

Morphine activates μ-opioid receptors (MORs) without promoting their rapid endocytosis in a number of cell types. A previous study suggested that morphine can drive rapid redistribution of MORs in the nucleus accumbens, but it was not possible in thisin vivostudy to identify a specific membrane trafficking pathway affected by morphine, to exclude possible indirect actions of morphine via opiate-regulated neural circuitry, or to define the mechanism of this morphine-dependent regulation. In the present study, we addressed these questions using dissociated primary cultures of rat striatal neurons as a model system. Morphine promoted a rapid redistribution of both endogenous and recombinant MORs within 30 min after drug addition to the culture medium. This effect was mediated by rapid endocytosis and occurred in a cell-autonomous manner, as indicated by its detection in cells plated at low density and in cultures in which depolarization was blocked by tetrodotoxin. Morphine-induced endocytosis of MORs was quantitatively similar to that induced by the enkephalin analogd-Ala²-N-Me-Phe⁴-Glycol⁵-enkephalin, and endocytosis induced by both ligands was inhibited by a dominant-negative mutant version of arrestin-3 (β-arrestin-2). These results extend previousin vivoresults and indicate that morphine is indeed capable of driving rapid endocytosis of μ-opioid receptors in an important subset of opiate-responsive CNS neurons. They also suggest a cellular mechanism by which β-arrestins may modulate the physiological effects of morphinein vivo.