Effects of β-funaltrexamine on radiolabeled opioid binding

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Abstract

β-Funaltrexamine (β-FNA) potently competed with the binding of a series of radiolabeled opiates and opioid peptides in standard binding assays with IC50 values under 10 nM. In addition, higher concentrations of β-FNA produced an irreversible inhibition of binding which was relatively selective for μ receptors: δ binding was not affected much. The production of irreversible inhibition of [3H]dihydromorphine binding required concentrations of β-FNA over 10-fold higher than β-FNA concentrations needed in standard competition studies. Both μ1 and μ2 sites were irreversibly inhibited by β-FNA, but μ1 sites were more sensitive. The reversible and irreversible inhibition in these in vitro binding assays by β-FNA were quite similar to naloxonazine. However, the activity of β-FNA in the guinea-pig ileum suggests that it may not distinguish between μ1 and gm2 receptors as effectively as naloxonazine in bioassays and in vivo.

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