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Morphine-induced downregulation of mu-opioid receptors in neonatal rat brain
1Department of Neuroscience, Albert Einstein College of Medicine, Bronx, NY 10461.
Abstract:
Chronic administration of morphine to pre- and postnatal rats produced a marked decrease in brain mu-opioid receptor density with-out change in receptor affinity. No significant changes in delta- or kappa-receptors were observed. This downregulation was accompanied by tolerance to the analgesic actions of morphine. In neonates exposed to morphine from postnatal day one, mu-receptor number was significantly depressed until postnatal day 8, then increased gradually to control levels by day 14 of treatment. Longer treatment produced no further change in opioid receptors. These data represent the first demonstration of in vivo downregulation of brain mu-opioid receptors following morphine administration and provide evidence for a unique plasticity of the immature opioid receptor system.
Insights
Chronic morphine exposure in developing rats reduces brain mu-opioid receptors, leading to tolerance. This downregulation in young rats recovers by day 14, showing opioid receptor system plasticity.
Area of Science:
- Neuroscience
- Pharmacology
- Developmental Biology
Background:
- Opioid receptors are crucial for pain modulation.
- Chronic opioid use can lead to tolerance and dependence.
- The effects of opioids on developing brain systems are not fully understood.
Purpose of the Study:
- To investigate the impact of chronic morphine administration on brain opioid receptor density in pre- and postnatal rats.
- To examine the relationship between opioid receptor changes and morphine tolerance.
- To assess the plasticity of the immature opioid receptor system.
Main Methods:
- Rats were administered morphine chronically from pre- and postnatal stages.
- Brain mu-, delta-, and kappa-opioid receptor density and affinity were measured.
- Analgesic tolerance to morphine was assessed.
- Opioid receptor levels were monitored over time in neonates.
Main Results:
- Chronic morphine caused a significant decrease in brain mu-opioid receptor density without altering receptor affinity.
- No significant changes were observed in delta- or kappa-receptors.
- This mu-opioid receptor downregulation correlated with tolerance to morphine's analgesic effects.
- In neonates, mu-receptor levels decreased until postnatal day 8, then recovered by day 14.
- Extended morphine treatment did not cause further changes in opioid receptors.
Conclusions:
- This study demonstrates the in vivo downregulation of brain mu-opioid receptors following morphine administration.
- The immature opioid receptor system exhibits unique plasticity, with observed recovery from downregulation.
- These findings provide critical insights into the neurobiological adaptations to chronic opioid exposure during development.