What are opioid receptors?

What are opioid receptors?

Opioid receptors are class A (rhodopsin-like) subclass γ GPCRs with endogenous and exogenous opioids as ligands. Opioids are psychoactive chemicals which mediate a variety of physiological responses mainly in the central and peripheral nervous system as well as in the gastrointestinal tract. There are four types of opioid receptors, kappa-, delta-, mu- and Nociceptin receptors (See Figure 1). The activation of the first three upon ligand binding is involved in stimulating analgesia, with δ and µ receptors being involved in the development of physical dependence on and withdrawal from drugs, such as the analgesic butorphanol. The first three receptors can be divided into subtypes, there are three subtypes of κ receptors (κ1κ2κ3), two of delta (δ1,δ2) and three of Mu (µ1µ2µ3).  The Mu receptor subclasses have very distinct physiological effects and functions. Only one subtype has been identified in Nociceptin receptors called ORL1. Although these four classes of opioid receptors share some common physiological effects on stimulation, they have characteristic functions which differ them from each other, for example, stimulation of the delta opioid receptor is associated with antidepressive effects, kappa-receptors with dysphoria, Mu receptors with euphoria and Nociceptin receptors with anxiety and depression. All of these receptors are found in the brain and all of them, except for delta opioid receptors, are found in the spinal chord, and all of them except for Nociceptin receptors are found in the peripheral nervous system. Only Mu opioid receptors are found in the intestinal tract. All of these receptors act through GABAergic (inhibitory) neurotransmission. 

             

 Figure 1. Human Opioid Receptor families^[1].