Internalization of μ-opioid receptors in rat spinal cord slices

CELLS immunoreactive for the μ-opioid receptor (MOR) in laminae I–II of the spinal cord were identified as small neurons with rostro-caudal dendrites. In spinal cord slices, [D-Ala,MePhe-Gly-ol]enkephalin (DAMGO) or etorphine (1 μM) caused naloxone-sensitive MOR endocytosis in 100% of these neurons, whereas the selective γ- and k-opioid agonists [D-Pen]enkephalin (DPDPE) and spiradoline mesylate (U-62,066), respectively, produced negligible internalization at 1 μM. The EC50 for DAMGO was 30 nM, similar to its potency to inhibit cAMP accumulation and to increase [γ-S]GTP binding. MOR internalization followed an exponential timecourse with a half-life of 1.7 min. MOR internalization in spinal cord slices was faster and occurred at lower agonist concentrations than in MOR-transfected cells, suggesting that spinal cord neurons have a more effective coupling of MORs to intracellular components mediating endocytosis.