Related Experiment Videos
mu-Opioid receptor down-regulation and cAMP-dependent protein kinase phosphorylation in a mouse model of chronic
1Department of Pharmacology and Toxicology, Medical College of Virginia, Virginia Commonwealth University, Richmond, VA 23298, USA.
Abstract:
Results of radioligand binding and transfected receptor studies indicate that mu-receptor down-regulation and phosphorylation may be critical to the expression of morphine tolerance. In this study, an animal model of morphine tolerance was used to correlate antinociception with changes in receptor number and phosphorylation state. mu-Opioid receptor protein was quantitated by Western immunoassay of brainstem tissue from morphine-treated mice. Degree of receptor phosphorylation was assessed using immunoprecipitation (IP) of the receptor followed by back-phosphorylation. Acutely administered morphine produced no changes in mu-receptor quantity. Chronic morphine administration resulted in a 50% reduction in receptor protein quantity over placebo-treated samples. Back-phosphorylation experiments showed a drop in cAMP-dependent protein kinase A (PKA)-induced receptor phosphorylation shortly after acute morphine administration, followed by a naloxone-reversible increase in phosphorylation of the receptor that correlated with the onset of antinociception. Chronic morphine administration resulted in a decrease in PKA-induced phosphorylation of the mu-receptor. Since it has been shown that PKA activity is enhanced in the brains of morphine-tolerant mice, this decrease in mu-receptor phosphorylation suggests that the mu-receptor may be structurally or conformationally altered in the morphine-tolerant state.
Insights
Morphine tolerance involves changes in mu-opioid receptors. Chronic morphine reduces receptor levels and alters phosphorylation, suggesting structural changes contribute to tolerance.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Morphine tolerance is a significant clinical challenge.
- Mu-opioid receptor (MOR) down-regulation and phosphorylation are implicated in tolerance.
- Understanding these molecular changes is crucial for developing effective pain management strategies.
Purpose of the Study:
- To investigate the correlation between antinociception and changes in MOR quantity and phosphorylation state in an animal model of morphine tolerance.
- To quantify MOR protein levels and assess receptor phosphorylation following acute and chronic morphine administration.
Main Methods:
- Utilized a mouse model of morphine tolerance.
- Quantified mu-opioid receptor protein using Western immunoassay in brainstem tissue.
- Assessed receptor phosphorylation state via immunoprecipitation and back-phosphorylation assays.
Main Results:
- Acute morphine administration did not alter mu-receptor quantity.
- Chronic morphine administration led to a 50% reduction in mu-receptor protein levels compared to placebo.
- Receptor phosphorylation showed an initial decrease followed by a naloxone-reversible increase correlating with antinociception after acute morphine.
- Chronic morphine resulted in decreased cAMP-dependent protein kinase A (PKA)-induced mu-receptor phosphorylation.
Conclusions:
- Mu-opioid receptor down-regulation occurs with chronic morphine exposure.
- Altered mu-receptor phosphorylation, particularly reduced PKA-induced phosphorylation, is associated with morphine tolerance.
- These molecular alterations suggest a potential structural or conformational change in the mu-opioid receptor in the morphine-tolerant state.