Mu-, delta-, kappa-opioid receptors and their subtypes. A critical review with emphasis on radioligand binding experiments.

Since the early 1970's, when specific binding sites for opiates were first described, there has been a vast literature on opiate receptors, their subtypes, and even the multiplicity of these subtypes. In the present review, the signal transduction pathways, structure, and brain and spinal cord localization of the established subtypes (mu, delta, kappa) are reviewed. In addition, evidence suggesting heterogeneity of these subtypes, in particular from radioligand binding studies, is discussed critically.