Effects of morphine on visceral nociception evoked by colorectal distension in rats: comparative examinations of electrophysiological and behavioral responses.

The purpose of this study was to compare the effects of intravenously administered morphine on electrophysiological and behavioral responses to colorectal distension (CRD) and to examine the influence of noxious stimuli applied to another part of the body (a laminectomy) on the visceromotor response to CRD. The effects of morphine (0.1-6.4 mg·kg-1) were examined in rate anesthetized with pentobarbital. Electrophysiological (n=16) and behavioral experiments (n=47) were done. Electrophysiological experiments were conducted to examine the effects of morphine on the responses of visceral dorsal horn neurons to CRD; behavioral studies were conducted to compare the effects of morphine with and without a laminectomy (intact group:n=24; laminectomy group:n=23). Morphine suppressed the evoked activities of the visceral dorsal horn neurons in a dose-dependent manner. Similar suppression of the behavioral visceromotor response was observed. Visceromotor thresholds were significantly lower in the intact group than in the laminectomy group during the control study. When morphine was administered, the visceromotor thresholds in both groups increased to a similar level. Behavioral and neurophysiological responses to CRD were suppressed in a similar fashion by morphine. Although laminectomy affected the threshold values of CRD for visceromotor responses, the laminectomy per se plays an insignificant role when adequate morphine is administered.