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

Opioid receptors are known to undergo complex regulatory changes in response to ligand exposure. In the present study, we examined the effect of morphine on the in vitro and in vivo density and trafficking of delta opioid receptors (delta ORs). Prolonged exposure (48 hr) of cortical neurons in culture to morphine (10 gm) resulted in a robust increase in the internalization of Fluo-deltorphin, a highly selective fluorescent SOR agonist. This effect was mu -mediated because it was entirely blocked by the selective mu opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 and was reproduced 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-deltorphin internalization was caused by increased receptor availability. Prolonged morphine exposure had no effect on delta OR protein levels, as assessed by immunocytochemistry and Western blotting, suggesting that the increase in bioavailable delta ORs was caused by recruitment of reserve receptors from intracellular stores and not from receptor neosynthesis. Complementary in vivo studies demonstrated that chronic treatment of adult rats with morphine (5-15 mg/kg, s.c., every 12 hr) similarly augmented targeting of delta ORs to neuronal plasma membranes in the dorsal horn of the spinal cord. Furthermore, this treatment markedly potentiated intrathecal D-[Ala(2)]deltorphin II-induced antinociception. Taken together, these results demonstrate that prolonged stimulation of neurons with morphine markedly increases recruitment of intracellular delta ORs to the cell surface, both in vitro and in vivo. We propose that this type of receptor subtype cross-mobilization may widen the transduction repertoire of G-protein-coupled receptors and offer new therapeutic strategies.