Different targets for i.v. vs. i.c.v. administered morphine for its effect on colonic motility in dogs.

The effects of intracerebroventricular (i.c.v.) or intravenous (i.v.) administration of morphine on colonic motility were investigated in conscious dogs chronically fitted with a strain gauge transducer sutured to the serosa of the proximal colon. Morphine administered i.v. (100 micrograms/kg) or i.c.v. (10 micrograms/kg) induced similar increases in the motility index for about 3 h. The pattern of colonic contractions after i.v. morphine mainly consisted of an increase in the number of phases of contractile activity each lasting about 5 min. Morphine i.c.v. administered induced a peculiar pattern consisting of short (0.5-1.5 min) phases of contractile waves occurring at a high rate (10-15 per h). The effects of i.v. morphine were abolished after previous i.v. (1 microgram/kg) or i.c.v. (0.1 micrograms/kg) administration of naloxone or methyl-levallorphan, a narcotic antagonist with a high peripheral selectivity (100 micrograms/kg i.v., 10 micrograms/kg i.c.v.). The effects of i.c.v. morphine were not modified by previous i.v. or i.c.v. administration of naloxone (100 micrograms/kg). These results suggest that the stimulatory effect of i.v. morphine on colonic motility involves both central and peripheral components. The i.c.v. administration of morphine does not reproduce its central effect when given i.v. but acts on different central receptors.