RGS14 prevents morphine from internalizing Mu-opioid receptors in periaqueductal gray neurons

Opioid agonists display different capacities to stimulate mu-opioid receptor (MOR) endocytosis, which is related to their ability to provoke the phosphorylation of specific cytosolic residues in the MORs. Generally, opioids that efficiently promote MOR endocytosis and recycling produce little tolerance, as is the case for [d-Ala2, N-MePhe4,Gly-ol5] encephalin (DAMGO). However, morphine produces rapid and profound antinociceptive desensitization in the adult mouse brain associated with little MOR internalization. The regulator of G-protein signaling, the RGS14 protein, associates with MORs in periaqueductal gray matter (PAG) neurons, and when RGS14 is silenced morphine increased the serine 375 phosphorylation in the C terminus of the MOR, a GRK substrate. Subsequently, these receptors were internalized and recycled back to the membrane where they accumulated on cessation of antinociception. These mice now exhibited a resensitized response to morphine and little tolerance developed. Thus, in morphine-activated MORs the RGS14 prevents GRKs from phosphorylating those residues required for β-arresting-mediated endocytosis. Moreover morphine but not DAMGO triggered a process involving calcium/calmodulin-dependent kinase II (CaMKII) in naïve mice, which contributes to MOR desensitization in the plasma membrane. In RGS14 knockdown mice morphine failed to activate this kinase. It therefore appears that phosphorylation and internalization of MORs disrupts the CaMKII-mediated negative regulation of these opioid receptors.