Changes in [(3)H]glibenclamide binding to mouse forebrain membranes during morphine tolerance

Area of Science:

• Neuroscience
• Pharmacology
• Molecular Biology

Background:

• Opioid analgesics like morphine are widely used for pain management.
• Development of tolerance to morphine necessitates higher doses for equivalent pain relief.
• ATP-sensitive potassium (K(ATP)) channels are implicated in opioid action and tolerance.

Purpose of the Study:

• To investigate the binding characteristics of the K(ATP) channel blocker [3H]glibenclamide in the forebrain of morphine-naive and morphine-tolerant mice.
• To determine if morphine tolerance is associated with changes in the number or affinity of K(ATP) channel binding sites.

Main Methods:

• Radioligand binding assays using [3H]glibenclamide on forebrain membrane preparations (P(2) fraction).
• Induction of morphine tolerance via continuous subcutaneous infusion of morphine using osmotic minipumps.
• Analysis of binding kinetics (association, dissociation), competition, and saturation using Scatchard and Hofstee plots.

Main Results:

• In naive mice, [3H]glibenclamide binding was biphasic, indicating two distinct binding sites with different affinities (K(D) 0.13 nM and 3.17 nM).
• In tolerant mice, binding became monophasic, with a single population of sites (K(D) 0.87 nM) and a Hill coefficient near unity.
• Competition studies showed altered affinity for sulfonylureas in tolerant animals, and dissociation was also monophasic.

Conclusions:

• Morphine tolerance in mice is associated with a reduction in the number of [3H]glibenclamide binding sites or a shift in affinity.
• The data suggest that adaptive changes occur in K(ATP) channels during the development of morphine tolerance.
• These findings highlight the role of K(ATP) channels in the neurobiological mechanisms underlying opioid tolerance.