CHARACTERIZATION OF [H-3] NALTRINDOLE BINDING TO DELTA OPIOID RECEPTORS IN RAT-BRAIN

[H-3]Naltrindole binding characteristics were determined using homogenized rat brain tissue. Saturation binding studies at 25-degrees-C measured an equilibrium dissociation constant (K(d)) value of 37.0 +/- 3.0 pM and a receptor density (B(max)) value of 63.4 +/- 2.0 fmol/mg protein. Association binding studies showed that equilibrium was reached within 90 min at a radioligand concentration of 30 pM. Naltrindole, as well as the ligands selective for delta (delta) opioid receptors, such as pCI-DPDPE and Deltorphin II inhibited [H-3]naltrindole binding with nanomolar IC50 values. Ligands selective for mu (mu) and kappa (kappa) opioid receptors were only effective in inhibiting [3H]naltrindole binding at micromolar concentrations. From these data, we conclude that [3H]naltrindole is a high affinity, selective radioligand for delta opioid receptors.