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Irreversible binding of [3H]beta-funaltrexamine to brain slices of morphine-tolerant and -dependent mice
M Takahashi1, P S Portoghese, A E Takemori
1Department of Pharmacology, Medical School, University of Minnesota, Minneapolis.
Abstract:
The specific, irreversible binding of [3H]beta-funaltrexamine (beta-FNA) to mu opioid receptors in slices from the corpus striatum and midbrain region containing the ventral tegmentum area (VTA) was accomplished by incubation with [3H]beta-FNA followed by a washing procedure with 1 microM unlabeled naltrexone in the washing medium. Regional distribution studies of the specific, irreversible binding of [3H]beta-FNA revealed that slices of striatum and midbrain were richest in mu opioid receptors. Slices of cerebral cortex and medulla-pons had much lower amounts and the cerebellum had virtually no mu opioid receptors. Examination of the kinetics of the specific, irreversible binding of [3H]beta-FNA revealed that although the maximum binding of beta-FNA to striatal and midbrain slices of control and morphine tolerant/dependent mice did not differ, the rate at which beta-FNA associated with the receptor was increased in the slices from morphine tolerant/dependent animals. The increase in association probably occurred at the initial recognition step before alkylation takes place and is attributable to an increase in affinity of mu opioid receptors for beta-FNA in both the striatal and midbrain regions of morphine tolerant/dependent mice. This finding supports and expands earlier conclusions that the affinity of mu opioid receptors for opioid antagonists is increased during the development of opiate tolerance and dependence in mice.
Insights
Morphine tolerance and dependence increase the affinity of mu opioid receptors for opioid antagonists like beta-funaltrexamine (beta-FNA). This enhanced binding occurs in brain regions rich in these receptors, such as the striatum and midbrain.
Area of Science:
- Neuroscience
- Pharmacology
- Molecular Biology
Background:
- Opioid receptors, particularly mu opioid receptors, are crucial targets for pain management and are implicated in addiction.
- Understanding the molecular changes in opioid receptors during tolerance and dependence is vital for developing effective treatments.
Purpose of the Study:
- To investigate the specific binding of [3H]beta-funaltrexamine (beta-FNA) to mu opioid receptors.
- To determine the regional distribution of mu opioid receptors in mouse brain slices.
- To examine the kinetics of beta-FNA binding in control versus morphine-tolerant/dependent mice.
Main Methods:
- Incubation of mouse brain slices (corpus striatum, midbrain, cerebral cortex, medulla-pons, cerebellum) with [3H]beta-funaltrexamine (beta-FNA).
- Washing procedure with unlabeled naltrexone to determine specific binding.
- Kinetic analysis of beta-FNA binding to assess association rates and receptor affinity.
Main Results:
- Striatum and midbrain showed the highest density of mu opioid receptors.
- Cerebral cortex and medulla-pons had lower amounts, while the cerebellum had negligible amounts.
- In morphine-tolerant/dependent mice, the rate of beta-FNA association with mu opioid receptors increased, suggesting higher affinity.
Conclusions:
- Mu opioid receptor density varies significantly across different brain regions.
- Opiate tolerance and dependence in mice are associated with an increased affinity of mu opioid receptors for opioid antagonists.
- This enhanced affinity may play a role in the neurobiological adaptations underlying tolerance and dependence.