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Internalization of mu-opioid receptors in rat spinal cord slices
J C Marvizón1, E F Grady, J Waszak-McGee
1Department of Medicine and Physiology, University of California, Los Angeles 90095, USA.
Abstract:
Cells immunoreactive for the mu-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-Ala2,MePhe4-Gly-ol5]enkephalin (DAMGO) or etorphine (1 microM) caused naloxone-sensitive MOR endocytosis in 100% of these neurons, whereas the selective delta- and kappa-opioid agonists [D-Pen2,5]enkephalin (DPDPE) and spiradoline mesylate (U-62,066), respectively, produced negligible internalization at 1 microM. The EC50 for DAMGO was 30 nM, similar to its potency to inhibit cAMP accumulation and to increase [gamma-35S]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.
Insights
Mu-opioid receptors (MOR) in spinal cord neurons undergo rapid endocytosis upon agonist stimulation. This process is specific to MOR agonists and occurs more efficiently in native spinal cord cells than in transfected cells.
Area of Science:
- Neuroscience
- Pharmacology
- Cell Biology
Background:
- Mu-opioid receptors (MOR) are crucial in pain modulation within the central nervous system.
- Understanding MOR trafficking, including endocytosis, is key to developing effective analgesics.
Purpose of the Study:
- To investigate the characteristics of MOR endocytosis in specific spinal cord neurons.
- To compare MOR internalization in native spinal cord cells versus heterologous expression systems.
Main Methods:
- Immunohistochemistry to identify MOR-expressing neurons in spinal cord slices.
- Measurement of MOR internalization using specific opioid agonists (DAMGO, etorphine, DPDPE, spiradoline).
- Quantification of agonist potency (EC50) and internalization kinetics (half-life).
Main Results:
- Mu-opioid receptor (MOR) agonists (DAMGO, etorphine) induced naloxone-sensitive endocytosis in 100% of identified spinal neurons.
- Selective delta- and kappa-opioid agonists showed negligible MOR internalization.
- MOR internalization in spinal cord slices was faster and occurred at lower concentrations than in MOR-transfected cells.
Conclusions:
- Spinal cord neurons exhibit efficient and specific MOR endocytosis mediated by MOR agonists.
- Native spinal cord neurons demonstrate a more effective coupling of MORs to endocytic machinery compared to transfected cells.