Abstract
Equilibrium binding studies in sheep thalamic homogenates indicated that morphine-3-glucuronide (M3G) had an apparent affinity for μ 1-opioid binding sites (IC 50 = 178 ± 40 nM, K i = 116 ± 25 nM, mean ± s.e.m., n = 4) similar to that reported by Pasternak and co-workers (1). However, when the chemical purity of M3G was investigated using high-performance-liquid-chromatography (HPLC) with electrochemical detection, it was found to be contaminated with 0.5% (molar basis) of morphine. Reduction of the morphine contamination of M3G to 0.08% resulted in a 7.2-fold decrease in apparent binding affinity (IC 50 = 1279 ± 287 nM, k i = 766 ± 30 nM, mean ± s.e.m., n = 4), indicating that the small percentage of morphine present in the M3G raw material drug is the likely explanation for M3G's apparent binding to μ 1-opioid receptors.
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