Figure 1:
Action of endogenous opioid (Met-enkephalin, leu-enkephalin, β-endorphin, and dynorphin), exogenous opioid (morphine and codeine) and peripheral opioid antagonists. Schematic representation of the mechanisms of action of opioids and peripheral opioid antagonists. A) In this situation, a patient is treated with opioids only (blue circle). Opioids act on mu-receptors (red receptors). The opioids have a central analgesic action since they cross the blood-brain barrier, they also have a peripheral action by acting on the mu-receptors in the digestive tract and inhibit the motricity of the latter. B) Patient is treated with opioids (blue circle) and peripheral opioid antagonists (purple circle). Opioids have a central analgesic action since they cross the blood-brain barrier, whereas peripheral opioid antagonists do not entry into central nervous system and do not antagonize the analgesic action of opioids. Peripheral antagonists, on the other hand, bind on mu-receptors in the gastrointestinal tract thwarting the digestive motor slowing action of opiates.