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Neurobiological mechanisms of opioid tolerance and dependence
1INSERM U 288, Faculté de Médecine Pitié-Salpêtrière, Paris, France.
Abstract:
The multiplicity of opioid receptors (mu, delta, kappa) and the limited knowledge of their coupling mechanisms explain why cellular and biochemical changes underlying opioid tolerance/dependence remain poorly understood. Following chronic exposure to opioids, both down- and up-regulation of opioid receptors can occur, depending on the receptor type and/or the central region examined. As these changes generally appear after the tolerance is installed, they are very likely not responsible for it. Instead, opioid tolerance seems to be associated with some uncoupling (probably functional rather than physical) of the opioid receptors from G proteins normally associated with them, therefore resulting in a loss of the capacity of these proteins to exchange GDP for GTP. However, considerable variations might exist in the mechanisms underlying tolerance from one opioid receptor type to another. With regard to dependence, an increase in adenylate cyclase activity, and therefore of cyclic AMP levels and certain protein kinase activities, have been claimed to be responsible for this phenomenon in some cell types. As highly selective opioid agonists and antagonists are now available, experiments with such compounds are expected to yield more informative data on the consequences of the chronic stimulation of a given receptor type. This should contribute to a better understanding of the biochemical and cellular events really responsible for the development of morphine tolerance and dependence.
Insights
Opioid tolerance may stem from receptors uncoupling from G proteins, not receptor number changes. Dependence might involve increased adenylate cyclase activity, impacting cellular signaling pathways.
Area of Science:
- Neuropharmacology
- Molecular Biology
- Cellular Signaling
Background:
- Opioid tolerance and dependence mechanisms are poorly understood due to receptor multiplicity (mu, delta, kappa) and complex coupling.
- Chronic opioid exposure can lead to receptor down- or up-regulation, but these changes often occur after tolerance develops.
Purpose of the Study:
- To elucidate the cellular and biochemical changes underlying opioid tolerance and dependence.
- To investigate the role of G protein coupling and adenylate cyclase activity in these phenomena.
Main Methods:
- Analysis of opioid receptor regulation (down- and up-regulation) following chronic opioid exposure.
- Investigation of opioid receptor-G protein uncoupling and its effect on GTP exchange.
- Examination of adenylate cyclase activity, cyclic AMP levels, and protein kinase activity in relation to dependence.
Main Results:
- Opioid tolerance appears linked to functional uncoupling of opioid receptors from G proteins, impairing GDP/GTP exchange, rather than receptor number changes.
- Mechanisms of tolerance may vary significantly between different opioid receptor types.
- Dependence in some cell types is associated with increased adenylate cyclase activity, leading to elevated cyclic AMP and protein kinase activity.
Conclusions:
- Functional uncoupling of opioid receptors from G proteins is a likely mechanism for opioid tolerance.
- Adenylate cyclase activation may play a role in opioid dependence.
- Selective opioid agonists/antagonists are crucial for future research to clarify receptor-specific consequences of chronic stimulation.