A novel type of rat brain β opioid receptors differenciated by cyclopropylphenylalanine-containing enkephalin analog

Abstract

The stabilization of bioactive conformation of peptides reinforces the ligandreceptor interaction, resulting in enhancement of biological activity. The incorporation of cyclopropylamino acids (∇AA) into a peptide chain is expected to constrain effectively its conformation due to the small φ and φ angles. In the opioid peptide-receptor system, the presence of δ and μ opioid receptors in brain was demonstrated by examining the binding affinities of radio-labeled ligands specific for each receptor subtype. When we assayed enkephalin analog containing E (2R,3S)-cyclopropylphenylalanine (∇Phe), this analog exhibited a high affinity for δ receptors and a very weak affinity for μ receptors in rat brain [1]. However, it was completely inactive not only in guinea pig ileum (μ -prototype) but also in mouse vas deferens (MVD, δ-prototype) with no antagonist activity. Extremely weak affinity to MVD receptors was also evidenced by the binding assay [2]. Although these data suggested that E -(2R,3S)-∇ Phe-containing enkephalin distinguishes the δ receptors in the central and peripheral nervous systems, it is not clear yet whether or not a novel type of δ receptors exist either in the central or peripheral tissue. Cyclopropylphenylalanine possesses four stereoisomers in the E -(2R,3S)-, E -(2S,3R)-, Z -(2R,3R)-, and Z -(2S,3S)-configurations (Figure 1). In the present study, using highly receptor specific radio-labeled ligands, we have evaluated all of ∇Phe-containing enkephalin analogs for their ability to bind to δ and μ opioid receptors in rat brain.