The direction of opiodid agonists towards mu-, delta- and epsilon-receptors in the vas deferens of the mouse and the rat

Chronic treatment of mice with specific opioids results in the development of tolerance of particular opiate receptors in the mouse vas deferens (MVD). Accordingly, the infusion of animals with the specific δ-receptor ligand D-Ala²,D-Leu⁵]-enkephalin (DADL) or the potent μ-agonist sufentanyl (SUF) produces MVD highly tolerant to δ- and μ-agonists, respectively. Investigating a series of opioids in these preparations provides unequivocal evidence for the simultaneous existence of δ- and μ-receptors in the MVD. Thus, the possibility exists to obtain vasa deferentia, which almost exclusively contain either μ- or δ-opiate receptors. In combination with the rat vas deferens (RVD), a supposedly selective ε-receptor preparation, useful tools are provided for the classification of opioids according to their preference for the μ-, δ- and ε-type of the opiate receptors.