BINDING CHARACTERISTICS OF THE NOVEL HIGHLY SELECTIVE DELTA-AGONIST, [H-3] ILE(5,6)DELTORPHIN-II

Following the description of the [H-3]deltorphin II, it has been repor ted that the modification of deltorphin II with the substitution of Va l(5,6) residues by the more hydrophobic Ile(5,6) residues leads to an increased affinity and selectivity. The Ile(5,6)deltorphin II (Tyr-D-A la-Phe-Gly-Ile-Ile-Gly-NH2) was tritiated by catalytic dehalogenation and labelled rat brain membrane sites with a K-d value of 0.40 nM and a B-max of 121 fmol/mg protein. Competition binding experiments with v arious unlabelled subtype specific opioid receptor ligands resulted in mu/delta and kappa/delta selectivity ratios of 2400 and 18 000 respec tively. Due to its high 6 receptor affinity, delta selectivity and ver y low non-specific binding (< 20%), [H-3]Ile(5.6)deltorphin II, is a v ery useful tool for the identification and characterisation of delta o pioid receptors.