Endogenous Opioid System

The endogenous opioid system is a signaling system made up of opioid peptides produced by the body and the receptors they activate. Major peptide families include endorphins, enkephalins, and dynorphins, which are generated from larger precursor proteins. Their best-characterized receptors are the mu, delta, and kappa opioid receptors; the related nociceptin system is often discussed alongside them .^1,2

These signaling pathways are widely distributed in the brain, spinal cord, and peripheral tissues. They help regulate pain processing, reward and motivation, stress responses, and several autonomic and neuroendocrine functions. Different endogenous peptides can activate more than one opioid receptor, with differing affinities, so their biological effects depend on the peptide, receptor, tissue, and neural circuit involved .¹

The endogenous opioid system is not the same thing as opioid drugs. Morphine and many prescription or illicit opioids act on some of the same receptors, but the body's own peptides are released in tightly regulated spatial and temporal patterns and serve diverse physiological roles. Their normal function therefore cannot be reduced to producing pain relief or euphoria .^1,2