A high affinity, highly selective ligand for the delta opioid receptor: [³H]-[D-PEN², pCl-PHE⁴, D-PEN⁵]enkephalin

Binding characteristics of a new, conformationally constrained, halogenated enkephalin analogue, [³H]-[D-penicillamine², pCl-Phe⁴, D-penicillamine⁵]enkephalin ([³H]pCl-DPDPE), were determined using homogenized rat brain tissue. Saturation binding studies at 25°C determined a dissociation constant (K_d) of 328 ± 27 pM and a receptor density (B_max) of 87.2 ± 4.2 fmol/mg protein. Kinetic studies demonstrated biphasic association for [³H]pCl-DPDPE, with association rate constants of 5.05 × 10⁸ ± 2.5 × 10⁸ and 0.147 ± 10⁸ ± 0.014 × 10⁸ M^-1 min^-1. Dissociation was monophasic with a dissociation rate constant of 2.96 × 10^-3 ± 0.25 × 10^-1 min^-1. The average K_d values determined by these kinetic studies were 8.4 ± 2.7 pM and 201 ± 4 pM. Competitive inhibition studies demonstrated that [³H]pCl-DPDPE has excellent selectively for the delta opioid receptor. [³H]pCl-DPDPE binding was inhibited by low concentrations of ligands selective for delta opioid receptor relative to the concentrations required by ligands selective for mu and kappa sites. These data show that [³H]pCl-DPDPE is a highly selective, high affinity ligand which should be useful in characterizing the delta opioid receptor.