Bivalent opioid peptides synthesized from μ selective monomers display preferential selectivity for δ receptors

A series of dimeric opioid peptides derived from μ selective compounds was synthesized to investigate whether μ and δ receptors coexist as distinct recognition sites on the same receptor complex. Some compounds were several times more potent than the corresponding monomers in the MVD (mouse vas deferens) smooth muscle preparation, which is rich in δ receptors, but yet retaining substantial activity in the GPI (guinea pig ileum). It is suggested that δ receptors might differ from μ receptors in that they possess an additional accessory recognition site, to which could bind a particular amino acid residue present in the second halves of these bivalent ligands. This residue could play the role of 'address' at δ opioid receptors.