Abstract

Morphine-6β-glucuronide (M6G) is a potent morphine metabolite. In an effort to further explore its mechanisms of action, we synthesized³H-M6G of high specific activity and examined its binding. Although its affinity toward traditional mureceptors is similar to morphine in binding assays in brain and in Chinese hamster ovary cells stably transfected with MOR-1, M6G is >100-fold more potent than morphine in analgesic assays. This apparent discrepancy cannot be explained by differing intrinsic activities of the two drugs because both agents are partial agonists with similar efficacies in adenylyl cyclase assays in the transfected cell lines. Behavioral studies have implied the possibility of a distinct M6G receptor. Detailed binding studies in brain tissue reveal evidence for heterogeneity. Nonlinear regression analysis of³H-M6G saturation studies reveals two components. The lower-affinity component (K_D = 1.93 ± 0.6 nM) corresponds to labeling of traditionalmu receptors. In addition, ³H-M6G labels another site of low abundance with very high affinity (K_D = 68 ± 7 pM). Competition studies indicate that both sites are relatively muselective. However, several compounds clearly distinguish between the two sites. These binding studies support the concept of a unique M6G receptor responsible for its analgesic activity.