The endogenous opioid system, also referred to as the endogenous opiate system, governs a broad range of behavioral and physiological functions. Although first identified in the context of its analgesic (i.e., pain-relieving) effects, it is well established that the endogenous opioid system regulates numerous biological processes, including learning and memory, sociality, mood and emotion, stress responses, ingestive behaviors, and reproduction. Accordingly, the endogenous opioid system has been implicated in numerous psychopathologies and other medical conditions, including substance abuse, depression, anxiety, and chronic pain, to name a few. Finally, endogenous opioids distribute widely outside the central nervous system and mediate functioning of the gastrointestinal, hepatic, renal, cardiovascular, respiratory, thermoregulatory, and immune systems.

The endogenous opioid system consists of several families of bioactive neuropeptides as well as their ...