Guanine nucleotide-binding protein-coupled and -uncoupled states of opioid receptors and their relevance to the determination of subtypes

Opioid receptors are currently classified as μ, δ, and κ types, but various subtypes have also been proposed. We have investigated whether subtypes exist by using [ ³H]bremazocine. [ ³H]Bremazocine binds to twice as many naloxone-sensitive sites as other nonselective opioid agonists, as shown in four membrane types that have very different ratios of μ, δ, and κ receptor types. [ ³H]Bremazocine binding is completely inhibited by an excess (in unlabeled form) of other opioid ligands, with Hill coefficients of 0.8- 0.95. These paradoxes can be explained if there are high- and low-affinity states of the μ, δ, and κ receptors and bremazocine binds with similar affinities to both states. We propose that these states are the guanine nucleotide-binding protein (G-protein)-coupled form and the uncoupled form of each receptor. As evidence for this proposal, the [ ³H]bremazocine binding suffered little or no loss with G-protein-uncoupling treatments, whereas binding of other opioid agonists was fully sensitive. We conclude that [ ³H]bremazocine offers a tool for the measurement of the total pools of coupled and uncoupled opioid receptors and that much of the previous characterization of opioid receptor subtypes reflects, instead, a significant pool of G-protein-uncoupled opioid receptors.