Endogenous opioid systems Current concepts and clinical correlations

The endogenous opioid system consists of 3 families of opioid peptides, beta-endorphin, enkephalins, and dynorphins, and 3 families of receptors, mu (MOR), delta (lambda, DOR), and kappa (KOR). Opioid peptides and their receptors have a widespread but selective distribution in the central and peripheral nervous systems, particularly in circuits involved in pain modulation, reward, responses to stress, and autonomic control. Whereas opioid receptor activation elicits presynaptic and postsynaptic inhibition, the differential expression and strategic localization of the different opioid receptor subtypes in projection neurons and local inhibitory interneurons provides for a wide range of opioid-induced behavioral effects. The opioid system has an important role in mechanisms of supraspinal, spinal, and peripheral analgesia; reward-mediating food intake and drug addiction; and modulation of emotion and stress responses. Endogenous opioids may participate in the pathophysiology of Parkinson disease (PD) and seizures, and may have an important role in mechanisms of neuroprotection. All these subjects have been recently reviewed.(1-11)