GPRASP1 deletion suppresses antinociceptive tolerance during chronic activation of delta opioid receptor in persistent pain

Delta-opioid receptor (DOR) agonists display antinociceptive properties with less adverse effects than morphine derivatives and other opioids targeting mu-opioid receptor. However, a loss of their antinociceptive effect (tolerance) rapidly develops upon repeated treatments and could be due to the fast degradation of DOR. The G Protein-coupled Receptor-Associated Sorting Protein 1 (GPRASP1) has been involved in DOR sorting for degradation in vitro. Here, using a novel mouse line combining a knock-out for GPRASP1 and a knock-in for a fluorescent DOR-eGFP, we show that GPRASP1 is mandatory for the development of antinociceptive tolerance to the DOR agonist SNC80 in models of neuropathic and inflammatory pain. Surprisingly, DOR down-regulation upon repeated activations, phosphorylation kinetics of SNC80-activated DOR and DOR coupling to Gi proteins were unaffected by the loss of GPRASP1. Instead, changes in the dynamics of DOR plasma membrane localization were observed in GPRASP1 knockout neurons. Affinity-Purification of DOR followed by Mass Spectrometry analyses revealed, for the first time in the brain, that SNC80 alters DOR's interactome. In the absence of GPRASP1, the association of activated DOR with newly identified partners, particularly Synapse differentiation-inducing gene protein 1 SYNG1, was reduced. These findings indicate that SNC80-induced antinociceptive tolerance is not driven by DOR down-regulation but rather by GPRASP1-mediated changes in DOR trafficking and downstream signaling.