The current status of opioid research on gastrointestinal motility

Apparently conflicting data on opioid effects on gastrointestinal motility have been reported in the literature. The current status is reviewed and an attempt is made to find a common denominator to discrepant results by suggesting functionally contrasting opioid systems modulating the same physiological functions. Upon superimposition, these contrasting systems might result in opposite opioid effects dependent on the actual functional balance between the systems at the time of drug administration. Inhibitory neuromodulation at multiple sites leading to either inhibition or disinhibition by opioids may serve as a common basis of their contrasting effects. This interpretation, though consistent with most of the currently available data, is still a working hypothesis.