[3H]naltrindole: a potent and selective ligand for labeling delta-opioid receptors

Naltrindole (NTI) is a selective and potent delta-opioid antagonist which preferentially antagonizes a subset of selective delta-opioid agonists. The purpose of this study was to evaluate whether [3H]NTI, the first radiolabeled delta-opioid antagonist, could selectively label delta-opioid receptors in a synaptosomal preparation. Increasing temperature and protein concentration (0.1-1.6 mg protein) increased the specific binding of [3H]NTI. Monovalent and divalent cations (0.01-100 mM) had minimal effects on the binding properties of [3H]NTI, in contrast to their effects on binding of the delta agonists [3H]DPDPE and [3H]DSLET. Subfractionation of rat brain homogenates revealed that [3H]NTI and [3H]DSLET primarily labeled binding sites in synaptosomal and microsomal fractions, whereas [3H]DPDPE labelled half as many sites in synaptosomal fraction. The Bmax determined for [3H]NTI in crude synaptosomal fraction was 95 +/- 12 fmol/mg. The dissociation constant (Kd) was determined from three different methods to be 0.08 +/- 0.02 nM (Scatchard analysis), 0.07 +/- 0.02 nM (competition study) and 0.03 +/- 0.005 nM (kinetic analysis). [3H]NTI binding was not significantly inhibited by mu- or kappa-opioid ligands or by nonopioid compounds. These results demonstrate that [3H]NTI is a potent and selective radioligand for delta-opioid receptors in rat brain preparations.