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.