UCSF

Mu opioid receptors (MOR) expressed in the central nervous system mediate a wide

range of physiological effects ranging from analgesia to addiction. Much remains

unknown as to how these receptors are regulated by drugs at the neuronal level. This was

addressed using both epitope-tagged and endogenous MORs expressed in multiple

neuronal populations. In contrast to observations of MOR trafficking in heterologous cell

lines, morphine drove robust and rapid internalization of MORs in medium spiny neurons

of the striatum. Both trafficking and signaling of MORs can be significantly altered by

co-activation of another G-protein coupled receptor (GPCR), the neurokinin1 receptor

(NK1R). Co-activation of NK1Rs expressed on the same neuron significantly diminished

the extent to which opioid drugs were able to drive MOR endocytosis. Both mutational

disruption of high affinity βarrestin binding to NK1Rs and overexpression of βarrestin

prevented this cell autonomous and non-reciprocal inhibition of MOR endocytosis,

suggesting that competition for cytoplasmic βarrestins plays a major role in NK1Rmediated

inhibition of MOR endocytosis. Additionally, this interaction produced a

reduced opioid-induced desensitization of adenylyl cyclase signaling in striatal neurons.

After internalization, rapid and efficient recycling of MORs in neurons was found to

depend on a specific sorting sequence in the cytoplasmic tail of the receptor, which was

previously identified to drive MOR recycling in non-neural cells. Although this sorting

sequence is functionally interchangeable with the recycling sequence of another signaling

receptor, the β2 adrenergic receptor (β2AR), the kinetics of exocytic insertion into the

plasma membrane during recycling differed between the two receptors. These results

identify a novel function of sequence-directed recycling in mediating rapid local delivery

of signaling receptors to the somatodendritic plasma membrane.