Delta and mu opiate receptor probes: fluorescent enkephalins with high receptor affinity and specificity.

The fluorescent amino acid, L-1-pyrenylalanine (Pya) was incorporated into [D-Ala2,Leu5]enkephalin and its methyl ester at position 4 or 5. Pya-enkephalins showed strong fluorescent intensity and displayed high binding affinity for opiate receptors. Pya4-enkephalins showed high specificity for the mu receptors, while Pya5-enkephalins showed high specificity and selectivity for the delta receptors. Particularly, [D-Ala2,Pya5]enkephalin was as potent as the most utilized delta-specific ligand of [D-Ala2,D-Leu5]enkephalin (DADLE), and yet its delta-selectivity was about 5-times greater than that of DADLE. Thus, Pya-enkephalins per se can be utilized as a fluorescent probe or tracer for the opiate receptor-binding assays.