The central and peripheral influences of opioids on gastrointestinal propulsion.

Abstract

The constipating effects of extracts of the poppy plant Papaver somniferum are among the oldest known pharmacological actions, and their application as antidiarrheal remedies preceded the use of opium preparations for analgesia. However, we are still far from a thorough understanding of how opioids' influence the gut in spite of considerable recent advances, including the discovery of specific binding sites and their endogenous ligands (65). While the mechanism of pain relief by morphine-like natural alkaloids and related syn­ thetic narcotic analgesics is currently assumed to involve exclusively the central nervous system (CNS) , both their direct effects on the bowel and their centrally elicited actions are believed to account for constipation (66). Opioid receptors and endorphins are widely distributed in the CNS and throughout the gastrointestinal (GI) tract, implying possible participation of the endogenous opiate system at either level in the regulation of gut functions, including motility. This article represents an effort to lay out a framework against which to assess the relative roles of central and peripheral opioid-specific mechanisms affecting gastrointestinal propulsion. Our sights have been kept on subjects of