Figure 9.

Proposed mechanisms underlying OBD. Opioid receptor-dependent mechanism via enteric nervous system (ENS) leads to fecal retention in the distal colon, which induces expression of COX-2 and NGF in the colonic smooth muscle. The fecal retention-dependent expression of COX-2 and NGF may underlie motility dysfunction and visceral hyperalgesia in prolonged use of opioids. Our study shows that prevention of fecal retention by colon cleansing with clear liquid diet eliminates the effect of opioid receptor-dependent mechanism and blocks the initiation of fecal retention-dependent mechanism. COX-2, cyclo-oxygenase-2; NGF, nerve growth factor; OBD, opioid-induced bowel dysfunction.