What mechanism contributes to constipation as a side effect of opioid agonists?

The mechanism contributing to constipation as a side effect of opioid agonists is primarily the inhibition of peristalsis. Opioids exert their analgesic effects by binding to mu-opioid receptors in the central nervous system and peripheral tissues, including the gastrointestinal (GI) tract. When opioids bind to these receptors in the gut, they reduce the motility of the intestines by inhibiting the peristaltic movement that is necessary for the normal progression of bowel contents. This slowed movement leads to increased water absorption from the intestinal contents, resulting in harder, drier stools and an overall slowing of bowel transit, which culminates in constipation.

Other options do not accurately reflect the effects of opioids on bowel function. Increased salivation, for instance, is not a typical effect of opioids and does not contribute to constipation. Similarly, an augmented defecation reflex would not lead to constipation; rather, it may suggest increased bowel activity, which is contrary to the effects of opioids. Lastly, increased intestinal motility would also not correlate with constipation as it would encourage the movement of fecal matter through the intestines instead. Thus, the inhibition of peristalsis stands as the primary mechanism through which opioid agonists induce constipation.