Regulation of Mu Opioid Receptor Trafficking, Signaling, and Recycling in Neurons

Mu opioid receptors (MOR) expressed in the central nervous system mediate a widerange of physiological effects ranging from analgesia to addiction. Much remainsunknown as to how these receptors are regulated by drugs at the neuronal level. This wasaddressed using both epitope-tagged and endogenous MORs expressed in multipleneuronal populations. In contrast to observations of MOR trafficking in heterologous celllines, morphine drove robust and rapid internalization of MORs in medium spiny neuronsof the striatum. Both trafficking and signaling of MORs can be significantly altered byco-activation of another G-protein coupled receptor (GPCR), the neurokinin1 receptor(NK1R). Co-activation of NK1Rs expressed on the same neuron significantly diminishedthe extent to which opioid drugs were able to drive MOR endocytosis. Both mutationaldisruption of high affinity βarrestin binding to NK1Rs and overexpression of βarrestinprevented this cell autonomous and non-reciprocal inhibition of MOR endocytosis,suggesting that competition for cytoplasmic βarrestins plays a major role in NK1Rmediatedinhibition of MOR endocytosis. Additionally, this interaction produced areduced opioid-induced desensitization of adenylyl cyclase signaling in striatal neurons.After internalization, rapid and efficient recycling of MORs in neurons was found todepend on a specific sorting sequence in the cytoplasmic tail of the receptor, which waspreviously identified to drive MOR recycling in non-neural cells. Although this sortingsequence is functionally interchangeable with the recycling sequence of another signalingreceptor, the β2 adrenergic receptor (β2AR), the kinetics of exocytic insertion into theplasma membrane during recycling differed between the two receptors. These resultsidentify a novel function of sequence-directed recycling in mediating rapid local deliveryof signaling receptors to the somatodendritic plasma membrane