What defines the differences in the mechanism of action between Mu opioid receptor agonists and antagonists?

The distinction in the mechanisms of action between Mu opioid receptor agonists and antagonists is best captured by the fact that antagonists allow for bowel movement. Mu opioid receptor agonists, such as morphine, bind to the Mu receptors and activate them, leading to effects such as pain relief, euphoria, and sedation. However, one of the side effects associated with agonists is constipation, as they can slow down gastrointestinal motility.

On the other hand, Mu opioid receptor antagonists, like naloxone or naltrexone, bind to these receptors but do not activate them. Instead, they block the receptor's activation by agonists. This blocking action can help counteract the constipating effects of opioid agonists, thereby allowing for normal bowel movements to occur. This is particularly important in clinical scenarios where an opioid-induced constipation may be problematic, and the use of an antagonist can relieve this side effect effectively.

This understanding highlights how the interaction with the Mu opioid receptors can lead to different physiological outcomes, depending on whether an agonist or antagonist is present, thus demonstrating the specific roles they play in pain management and gastrointestinal function.