Opioid receptor binding characteristics of the non-equilibrium μ antagonist, β-funaltrexamine (β-FNA)

β-Funaltrexamine (β-FNA) bound to mouse brain membranes in a reversible and an irreversible (not removed by washing of the membrane) manner, and a portion of each type of binding was opioid-specific. Addition of 100 mM NaCl to the incubating medium enhanced the binding of β-FNA to membranes. Using membranes preincubated with β-FNA (1 μM) and then washed three times, the maximum number of binding sites available to [³H]morphine was markedly diminished whereas the affinity of morphine for binding sites was not significantly altered. The binding of [³H]naltrexone was also reduced markedly by β-FNA pretreatment. In similarly pretreated membranes, the binding of [³H]methionine enkephalin [³H][D-Ala²,D-Leu⁵]enkephalin (DADLE) or [³H]ethylketazocine was reduced to a smaller extent. Using brain membranes from mice pretreated with a single subcutaneous injection of β-FNA (100 mg/kg) 48 h prior to use, the binding of [³H]methionine enkephalin was unaffected whereas the number of binding sites available to [³H]morphine was significantly reduced. The inhibition by various ligands of the reversible binding of [³H]β-FNA resembled the relative ability of the same ligands to inhibit the binding of [³H]ethylketazocine. It was concluded that the irreversible portion of the binding of β-FNA demonstrates a selectivity for μ over δ binding sites, and that the reversible portion of the binding of β-FNA demonstrates a selectivity for κ binding sites over μ or δ binding sites. As such, the binding characteristics of β-FNA are consistent with its profile in vivo and in isolated tiussue studies in vitro.