Ligands
Until comparatively recently, there were few pharmacological tools for the study of δ receptors. As a consequence, our understanding of their function is much more limited than those of the other opioid receptors for which selective ligands have long been available.
However there are now several selective delta opioid agonists available, including peptides such as DPDPE and deltorphin II, and non-peptide drugs such as SNC-80, the more potent (+)-BW373U86, a newer drug DPI-287, which does not produce the problems with convulsions seen with the earlier agents, and the mixed mu/delta agonist DPI-3290, which is a much more potent analgesic than the more highly selective delta agonists. Selective antagonists for the delta receptor are also available, with the best known being the opiate derivative naltrindole.
Read more about this topic: Delta Opioid Receptor