There is burgeoning evidence that at least some of the pharmacological effects of opioids may be mediated via opioid receptors that are organized as dimers or oligomers. Moreover, it is now apparent that in some cases opioid receptors can associate with non-opioid receptors to form heterodimers. The implications of such findings are profound, as changes in the organizational structure of opioid receptors in the CNS may have a bearing on tolerance and physical dependence. For these reasons, the broad, long-term objective of this project is to develop potent, selective nonpeptide bivalent ligands as an approach to identify opioid receptor dimers/oligomers in vitro and in vivo. Bivalent ligands that are highly selective will be used as tools to investigate the pharmacology associated with receptor dimers. The specific aims of this project are to employ the bivalent ligand approach to investigate mu-delta heterodimeric opioid receptors because of the large body of evidence for interaction between these receptors. Such ligands will contain mu agonist and delta antagonist pharmacophores in the same molecule. Spacers of varying lengths will be employed to link the pharmacophores. Control ligands will also be synthesized to evaluate whether or not occupation of neighboring mu and delta recognition sites occurs. The ligands will be initially tested in the PI?s laboratory, and subsequently in the laboratory of Dr. Law. Binding and functional studies will be conducted in cultured cells that contain known ratios of co-expressed mu and delta receptors. Bivalent ligands whose binding and function are consistent with the "bridging" of neighboring opioid receptors will be employed as tools in chronic studies in mice to determine whether or not physical dependence and tolerance accompanies antinociception. The structure-activity data generated from the in vitro studies will be employed to model heterodimeric mu-delta opioid receptors in Dr. Ferguson?s lab. The potential clinical applications of bivalent ligands as analgesics devoid of abuse potential will be explored.