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Acute and chronic morphine treatments and morphine withdrawal differentially regulate GRK2 and GRK5 gene expression
1National Laboratory of Medical Neurobiology, Fudan University Medical Center, 138 Yi Xue Yuan Road, Shanghai 200032, People's Republic of China.
Abstract:
Opioid agonist stimulates activation of G protein-coupled receptor kinase (GRK) and causes desensitization of opioid signaling, which plays an important role in opioid tolerance. The current study investigated the potential regulatory effects of acute and chronic morphine administration and withdrawal on GRK2 and GRK5 gene expression in rat brain. Our results showed that the initial morphine treatment (10 mg/kg) significantly increased GRK mRNA levels in cerebral cortex, hippocampus, and lateral thalamic nuclei. A significant decrease in GRK5 mRNA levels was observed in periaqueductal gray. In strong contrast, repeated administration of morphine for 9 days failed to cause any significant increase in GRK5 mRNA in any of these brain regions. Chronic morphine treatment resulted in 30-70% down-regulation of GRK2 expression in cerebral cortex, hippocampus, thalamus, and locus coeruleus, opposite to what observed with the single morphine administration. Moreover, spontaneous and naloxone-precipitated morphine withdrawal resulted in aberrant increases in GRK2 and GRK5 mRNA levels in these brain regions. Taken together, our study suggests that opioid not only induces rapid negative feedback regulation on opioid signals through activation of GRK but also exerts its impact, via controlling levels of GRK gene expression, on the regulatory machinery itself over a longer period of time in brain.
Insights
Opioid agonists affect G protein-coupled receptor kinase (GRK) gene expression. Morphine alters GRK2 and GRK5 levels acutely, chronically, and during withdrawal, impacting opioid signaling regulation in the brain.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Opioid agonists activate G protein-coupled receptor kinases (GRKs).
- GRK activation leads to opioid signaling desensitization and tolerance.
- Understanding GRK regulation is crucial for opioid-based therapies.
Purpose of the Study:
- To investigate the effects of acute and chronic morphine administration and withdrawal on GRK2 and GRK5 gene expression in rat brain.
- To elucidate the role of GRKs in opioid tolerance and signaling modulation.
Main Methods:
- Gene expression analysis of GRK2 and GRK5 mRNA in specific rat brain regions.
- Acute and chronic morphine administration protocols.
- Assessment of gene expression during spontaneous and naloxone-precipitated withdrawal.
Main Results:
- Acute morphine (10 mg/kg) increased GRK mRNA in the cerebral cortex, hippocampus, and lateral thalamic nuclei, but decreased GRK5 in the periaqueductal gray.
- Chronic morphine treatment (9 days) did not increase GRK5 mRNA but downregulated GRK2 expression (30-70%) in multiple brain regions.
- Morphine withdrawal (spontaneous and naloxone-induced) led to significant increases in GRK2 and GRK5 mRNA levels.
Conclusions:
- Opioid administration dynamically regulates GRK gene expression in the brain.
- Acute morphine induces rapid feedback, while chronic morphine alters the regulatory machinery.
- Withdrawal states exhibit aberrant GRK expression, suggesting a role in protracted opioid effects.