Related Experiment Videos
Differential opioid agonist regulation of the mouse mu opioid receptor
A D Blake1, G Bot, J C Freeman
1Department of Pharmacology, University of Pennsylvania School of Medicine, Philadelphia 19104, USA.
Abstract:
Mu opioid receptors mediate the analgesia induced by morphine. Prolonged use of morphine causes tolerance development and dependence. To investigate the molecular basis of tolerance and dependence, the cloned mouse mu opioid receptor with an amino-terminal epitope tag was stably expressed in human embryonic kidney (HEK) 293 cells, and the effects of prolonged opioid agonist treatment on receptor regulation were examined. In HEK 293 cells the expressed mu receptor showed high affinity, specific, saturable binding of radioligands and a pertussis toxin-sensitive inhibition of adenylyl cyclase. Pretreatment (1 h, 3 h, or overnight) of cells with 1 microM morphine or [D-Ala2MePhe4,Gly(ol)5]enkephalin (DAMGO) resulted in no apparent receptor desensitization, as assessed by opioid inhibition of forskolin-stimulated cAMP levels. In contrast, the morphine and DAMGO pretreatments (3 h) resulted in a 3-4-fold compensatory increase in forskolin-stimulated cAMP accumulation. The opioid agonists methadone and buprenorphine are used in the treatment of addiction because of a markedly lower abuse potential. Pretreatment of mu receptor-expressing HEK 293 cells with methadone or buprenorphine abolished the ability of opioids to inhibit adenylyl cyclase. No compensatory increase in forskolin-stimulated cAMP accumulation was found with methadone or buprenorphine; these opioids blocked the compensatory effects observed with morphine and DAMGO. Taken together, these results indicate that methadone and buprenorphine interact differently with the mouse mu receptor than either morphine or DAMGO. The ability of methadone and buprenorphine to desensitize the mu receptor and block the compensatory rise in forskolin-stimulated cAMP accumulation may be an underlying mechanism by which these agents are effective in the treatment of morphine addiction.
Insights
Methadone and buprenorphine desensitize mu opioid receptors differently than morphine, blocking compensatory cAMP increases. This distinct interaction may explain their effectiveness in treating opioid addiction and dependence.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Mu opioid receptors are central to morphine's analgesic effects but prolonged use leads to tolerance and dependence.
- Understanding the molecular mechanisms of opioid tolerance and dependence is crucial for developing effective treatments.
- Human embryonic kidney (HEK) 293 cells expressing the mouse mu opioid receptor provide a model system to study receptor regulation.
Purpose of the Study:
- To investigate the molecular basis of mu opioid receptor tolerance and dependence.
- To examine the effects of prolonged opioid agonist treatment on mu opioid receptor regulation in HEK 293 cells.
- To compare the regulatory effects of morphine, DAMGO, methadone, and buprenorphine on mu opioid receptors.
Main Methods:
- Stable expression of a tagged mouse mu opioid receptor in HEK 293 cells.
- Assessment of receptor binding affinity, specificity, and adenylyl cyclase inhibition.
- Pretreatment of cells with various opioid agonists (morphine, DAMGO, methadone, buprenorphine) followed by measurement of forskolin-stimulated cAMP levels.
Main Results:
- Morphine and DAMGO pretreatment did not cause receptor desensitization but led to a compensatory increase in cAMP accumulation.
- Methadone and buprenorphine pretreatment abolished opioid inhibition of adenylyl cyclase and prevented the compensatory cAMP rise.
- These findings suggest methadone and buprenorphine interact distinctively with the mu opioid receptor compared to morphine and DAMGO.
Conclusions:
- Methadone and buprenorphine exhibit unique mu opioid receptor desensitization properties.
- Their ability to block compensatory cAMP increases may underlie their lower abuse potential and efficacy in addiction treatment.
- These distinct receptor interactions offer insights into the molecular mechanisms of opioid dependence and therapeutic strategies.