Characterization of [³H]naltrindole binding to delta opioid receptors in mouse brain and mouse vas deferens: Evidence for delta opioid receptor heterogeneity

Naltrindole (NTI) is a potent and selective nonpeptide delta opioid receptor antagonist. This study reports on the binding characteristics of [³H]NTI (specific activity = 30.5 Cl/mmole) for mouse brain and vas deferens (MVD) tissues. In brain, [³H]NTI had unusually high specific binding to delta receptors (80% at its Kd concentration) relative to other selective delta receptor radioligands. Saturation Kd values with 95% confidence intervals for mouse brain and MVD tissue preparations were 56.2 (41.8-75.7) and 104 (25.8-420) pM, respectively. These Kd values were significantly different (P = .028) and [³H]NTI binding to both tissues was best fit by a one-site model. Receptor densities were 83.9 (66.8-106) fmol/mg of protein for mouse brain and 14.8 (7.03-31.2) fmol/mg of protein for the MVD. Binding inhibition studies showed that NTI and the delta opioid receptor agonists [4'-Cl-Phe⁴]DPDPE and [D-Ala², Glu⁴]deltorphin had high affinity for the sites labeled by [³H]NTI in both tissue preparations whereas mu [Tyr-Pro- ψ-MePhe-D-Pro-NH₂ (PL-17)] and kappa (U-69593) agonists had micromolar affinity. Both agonists recognized multiple sites in mouse brain under control (with 5 mM Mg⁺⁺) and treatment (with 50 μM guanylyl-5'- imidodiphosphate and 100 mM NaCl) conditions but only single-site binding was observed for MVD (only control condition tested) [D-Ala², Glu⁴]deltorphin showed about 6.5-fold selectivity for a portion (≃33%) of mouse brain sites (Ki = 130 pM) compared to sites labeled by [³H]NTI in MVD (Ki = 1200 pM) under control conditions. No significant difference was observed for [4'-Cl- Phe⁴]DPDPE binding affinity to both tissues (Ki = 450-680 pM) under control conditions. The affinity of opioid agonists, but not antagonists at [³H]NTI binding sites in mouse brain, was substantially reduced by the presence of guanylyl-5'-imidodiphosphate and sodium ions consistent with guanine nucleotide binding protein regulation of the delta receptors. The portions of high- and low-affinity sites recognized by [4'-Cl-Phe⁴]DPDPE and [D-Ala², Glu⁴]deltorphin in mouse brain labeled by [³H]NTI under treatment conditions were not significantly different (each subtype represented ≃50% of the total population) suggesting delta receptor heterogeneity in this tissue. It is concluded that [³H]NTI binds to delta opioid receptor affinity states and subtypes with equal affinity and can be used for their characterization in conjunction with different treatment conditions and ligands.