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Agonist-induced mu opioid receptor phosphorylation and functional desensitization in rat thalamus
1Department of Pharmaceutical Sciences, School of Pharmacy, University of Maryland, Baltimore, MD 21201, USA.
Abstract:
By metabolically labeling tissue slices from striatum and thalamus with [32P]orthophosphoric acid and immunoprecipitating the receptor with mu receptor-specific antiserum, we found that the endogenous mu receptor in the brain tissue did undergo phosphorylation. The phosphorylation occurred at basal level (no drug treatment) and was enhanced with DAMGO-treatment. The enhancement of the phosphorylation was blocked by naloxone. Morphine stimulation also increased the phosphorylation, but the amount of enhancement was less than that caused by DAMGO-treatment. Mu receptor phosphorylation in the thalamus was much greater than the striatum, while no phosphorylation of the mu receptor in the cerebellum was detected, even with DAMGO treatment. The extent of mu receptor phosphorylation identified in the thalamus, striatum and cerebellum is consistent with the previous studies of mu receptor distribution. The time course and dose-response studies demonstrated that mu receptor phosphorylation was a rapid event, exhibited a positive dose-dependent response, and was similar to that observed in the cloned mu receptor in CHO cells. Furthermore, we correlated the change of mu receptor phosphorylation with the desensitization of the mu receptor function, specifically, inhibition of adenylyl cyclase activity in the thalamus of morphine-tolerant rats. We found that in the thalamus of rats chronically treated with morphine, the enhancement of mu receptor phosphorylation in basal and DAMGO-treated samples paralleled the desensitization of DAMGO-mediated inhibition of adenylyl cyclase. Our results suggest that mu receptor phosphorylation in vivo may play an important role in the modulation of mu receptor function following both acute exposure to morphine and during the development of morphine tolerance.
Insights
Brain mu-opioid receptors undergo phosphorylation, a process enhanced by DAMGO and morphine, and linked to morphine tolerance. This phosphorylation is crucial for modulating receptor function.
Area of Science:
- Neuroscience
- Molecular Pharmacology
- Opioid Receptor Signaling
Background:
- The mu-opioid receptor (MOR) is a key target for opioid analgesics.
- Understanding MOR regulation is vital for managing pain and addiction.
- Phosphorylation is a known post-translational modification affecting receptor function.
Purpose of the Study:
- To investigate the endogenous mu-opioid receptor phosphorylation in brain tissue.
- To determine the role of MOR phosphorylation in acute opioid effects and tolerance.
Main Methods:
- Metabolic labeling of rat brain tissue slices (striatum, thalamus, cerebellum) with [32P]orthophosphoric acid.
- Immunoprecipitation of MOR using MOR-specific antiserum.
- Treatment with DAMGO (a MOR agonist), morphine, and naloxone (a MOR antagonist).
- Assessment of adenylyl cyclase activity in morphine-tolerant rats.
Main Results:
- Endogenous MOR undergoes basal and agonist-enhanced phosphorylation in striatum and thalamus, blocked by naloxone.
- DAMGO induced greater phosphorylation than morphine.
- MOR phosphorylation was significantly higher in the thalamus than the striatum; absent in the cerebellum.
- MOR phosphorylation is a rapid, dose-dependent event, mirroring cloned MOR behavior.
- Increased MOR phosphorylation in morphine-tolerant rats paralleled adenylyl cyclase desensitization.
Conclusions:
- Mu-opioid receptor phosphorylation in vivo is a significant regulatory mechanism.
- Phosphorylation plays a role in both acute opioid responses and the development of morphine tolerance.
- This finding provides insights into opioid receptor desensitization and tolerance mechanisms.