The opioid system consists of endogenous peptides (such as endorphins, enkephalins, and dynorphins) and their receptors (mu, delta, and kappa) in the brain and throughout the body. This system plays a vital role in modulating pain, reward, and addictive behaviors. Exogenous opioids, such as morphine, heroin, and synthetic opioids like fentanyl, interact with these receptors to produce their effects.
