Are there subtypes (isoreceptors) of multiple opiate receptors in the mouse vas deferens?

The hypothesis of heterogeneity of opiate receptors was tested with the electrically stimulated mouse vas deferens. The experimental approach was based on the phenomenon of tolerance development of selective opiate receptors. It was found that opioids classified as mu-receptor agonists, e.g. sufentanyl and normorphine, do not necessarily exhibit cross-tolerance to each other. The same holds true for a variety of presumed kappa-receptor agonists. On the other hand, the delta-receptor agonists under investigation displayed complete cross-tolerance. These findings give rise to an interpretation which may postulate the existence of subtypes (isoreceptors) of the already known opiate receptors.