Some aspects of opiate receptors.

There is accumulating evidence that the existence of various endorphins is parallelled by the existence of multiple opiate receptors for these different ligands. At present, it appears that opiate binding sites in the mouse vas deferens are specifically sensitive to enkephalins, while the receptors of the vas deferens of the rat are selectively sensitive to beta-endorphin. Although enkephalin is present in the guinea pig ileum, it is not yet known if there are other endogenous ligands that preferentially combine with the mu-receptors located in this tissue. We suggest that a preference of the particular opiate receptors for specific ligands with different properties (e.g., their different susceptibility to enzymatic degradation) could characterize these binding sites as receptors for either neurotransmitters or hormones.