Opioid Receptors and their Pharmacological Profiles

Abstract

Opioid receptors can be divided into three major classes, which are called μ, δ and k-receptors. The molecular basis of the receptors is discussed and a hypothesis of the binding of bivalent ligands to the receptor is presented. Furthermore the mechanism of action, the distribution and the probable function of these classes is overviewed. Increasing evidence is accumulating that the classical binding model cannot explain completely the interaction of opioids with their receptors. In addition to the μ-receptors, high affinity μ₁ binding sites have been demonstrated. Similarly, the delta receptors may be divided in δ₁ and δ₂. The significance of these subclasses is not yet fully understood. The high affinity μ₁-binding sites, however, represent probably an activated receptor complex, e.g. the complex between the receptor and the guanine-nucleotide-binding protein.