The mu, kappa and delta properties of various opioid agonists

Using the delta-enriched mouse vas deferens and the kappa-enriched guinea-pig ileal longitudinal muscle preparations, a large variety of so-called 'mu, kappa, delta and sigma' opioid agonists were tested for their mu, kappa and delta properties. Buprenorphine and U50,488H appeared to be the most highly selective agonists for mu and kappa opioid receptors, respectively. Most ligands identified as specific agonists are in fact only selective and appear to interact at more than one receptor type.