Final answer:
Opioid-induced constipation occurs due to opioids binding to mu-opioid receptors in the gastrointestinal tract, leading to decreased bowel motility and harder stools.
Step-by-step explanation:
The mechanism of action behind opioid-induced constipation is due to the activation of mu-opioid receptors within the gastrointestinal tract. When opioids bind to these receptors, they decrease the motility of the smooth muscle in the gastrointestinal tract, which leads to a reduced peristalsis and an increase in the absorption of fluids from the feces. This results in harder, drier stools that are more difficult to pass. Opioids also increase sphincter tone, further contributing to constipation.
Drugs like alvimopan are opioid antagonists, specifically targeting those peripheral mu-opioid receptors, hence they are used to alleviate opioid-induced constipation without affecting the analgesic effects in the central nervous system (CNS). Other opiate antagonists like naloxone and naltrexone also bind to the opioid receptors but are primarily used to reverse the effects of opioids, such as in cases of overdose or to aid in addiction treatment.