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

L. Fang, R. J. Knapp, R. Horvath, T. O. Matsunaga, R. C. Haaseth, V. J. Hruby, F. Porreca, H. I. Yamamura

Research output: Contribution to journalArticle

58 Scopus citations

Abstract

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.

Original languageEnglish (US)
Pages (from-to)836-846
Number of pages11
JournalJournal of Pharmacology and Experimental Therapeutics
Volume268
Issue number2
StatePublished - Jan 1 1994

ASJC Scopus subject areas

  • Molecular Medicine
  • Pharmacology

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