G protein-coupled receptor desensitization is typically mediated by receptor phosphorylation by G proteincoupled
receptor kinase (GRK) and subsequent arrestin binding; morphine, however, was previously found to
activate a c-Jun N-terminal kinase (JNK)-dependent, GRK/arrestin-independent pathway to produce mu opioid
receptor (MOR) inactivation in spinally-mediated, acute anti-nociceptive responses [Melief et al.] [1]. In the current
study, we determined that JNK2 was also required for centrally-mediated analgesic tolerance to morphine
using the hotplate assay. We compared JNK activation by morphine and fentanyl in JNK1−/−, JNK2−/−, JNK3−/−,
and GRK3−/−mice and found that both compounds specifically activate JNK2 in vivo; however, fentanyl activation
of JNK2 was GRK3-dependent, whereasmorphine activation of JNK2 was GRK3-independent. InMOR-GFP expressing
HEK293 cells, treatment with either arrestin siRNA, the Src family kinase inhibitor PP2, or the protein kinase C
(PKC) inhibitor Gö6976 indicated that morphine activated JNK2 through an arrestin-independent Src- and PKCdependent
mechanism, whereas fentanyl activated JNK2 through a Src-GRK3/arrestin-2-dependent and PKCindependent
mechanism. This study resolves distinct ligand-directed mechanisms of JNK activation by mu opioid
agonists and understanding ligand-directed signaling at MOR may improve opioid therapeutic
