Prolonged morphine treatment targets delta opioid receptors to neuronal plasma membranes and enhances delta-mediated antinociception

Opioid receptors are known to undergo complex regulatory changes in respons e to ligand exposure. In the present study, we examined the effect of morph ine on the in vitro and in vivo density and trafficking of delta opioid rec eptors (delta ORs). Prolonged exposure (48 hr) of cortical neurons in cultu re to morphine (10 gm) resulted in a robust increase in the internalization of Fluo-deltorphin, a highly selective fluorescent SOR agonist. This effec t was mu -mediated because it was entirely blocked by the selective mu opio id receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 and was repr oduced using the selective mu agonist fentanyl citrate. Immunogold electron microscopy revealed a marked increase in the cell surface density of delta ORs in neurons exposed to morphine, indicating that the increase in Fluo-d eltorphin internalization was caused by increased receptor availability. Pr olonged morphine exposure had no effect on delta OR protein levels, as asse ssed by immunocytochemistry and Western blotting, suggesting that the incre ase in bioavailable delta ORs was caused by recruitment of reserve receptor s from intracellular stores and not from receptor neosynthesis. Complementa ry in vivo studies demonstrated that chronic treatment of adult rats with m orphine (5-15 mg/kg, s.c., every 12 hr) similarly augmented targeting of de lta ORs to neuronal plasma membranes in the dorsal horn of the spinal cord. Furthermore, this treatment markedly potentiated intrathecal D-[Ala(2)]del torphin II-induced antinociception. Taken together, these results demonstra te that prolonged stimulation of neurons with morphine markedly increases r ecruitment of intracellular delta ORs to the cell surface, both in vitro an d in vivo. We propose that this type of receptor subtype cross-mobilization may widen the transduction repertoire of G-protein-coupled receptors and o ffer new therapeutic strategies.