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  • Title: In vivo receptor binding of opioid drugs at the mu site.
    Author: Rosenbaum JS, Holford NH, Sadée W.
    Journal: J Pharmacol Exp Ther; 1985 Jun; 233(3):735-40. PubMed ID: 2989495.
    Abstract:
    The in vivo receptor binding of a series of opioid drugs was investigated in intact rats after s.c. administration of [3H]etorphine tracer, which selectively binds to mu sites in vivo. Receptor binding was determined by a membrane filtration assay immediately after sacrifice of the animals and brain homogenization. Coadministration of unlabeled opioid drugs together with tracer led to a dose-dependent decrease of in vivo tracer binding. Estimates of the doses required to occupy 50% of the mu sites in vivo established the following potency rank order: diprenorphine, naloxone, buprenorphine, etorphine, levallorphan, cyclazocine, sufentanil, nalorphine, ethylketocyclazocine, ketocyclazocine, pentazocine, morphine. In vivo-in vitro differences among the relative receptor binding potencies were only partially accounted for by differences in their access to the brain and the regulatory effects of Na+ and GTP, which are expected to reduce agonist affinities in vivo. The relationship among mu receptor occupancy in vivo and pharmacological effects of the opioid drugs is described.
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