The effects of intracerebroventricular bombesin on gastric emptying and small and large bowel transit were evaluated in female rats using radioactive marker techniques. Gastric emptying was studied by determining the rate of emptying of tritium-labeled polyethylene glycol from the stomach after intracerebroventricular saline or bombesin, while intestinal transit was determined after the direct instillation of radiochromium into the small and large bowels. Dose- and time-response studies were conducted. Bombesin (0.01-1.0 μg, intracerebroventricularly) slowed gastric emptying in a dose-related manner. Time-course studies of gastric emptying after administration of the peptide concurrently with, or 10 or 20 min before the marker, showed maximum effects 20 min after bombesin. This effect was blocked by subdiaphragmatic vagotomy. Small intestinal transit was also delayed by intracerebroventricular bombesin (0.01-3.0 μg); when studied by giving the peptide concurrently with, or 10 or 20 min before the marker, maximal effects were seen 20 min after bombesin. In contrast, transit in the large intestine was stimulated when bombesin (1 μg, intracerebroventricularly) and the marker were given concurrently, while a small (nonsignificant) delay of transit occurred at 20 min after doses of 0.01-1.0 μg. Fed rats showed a significant (p < 0.05, Student's t-test) increase in fecal boli within the first 30 min after intracerebroventricular bombesin (1 μg) (3.66 ± 0.49, bombesin; 1.16 ± 0.54, saline). Peripherally administered bombesin (20 μg/kg, introperitoneally) had no effect on gastric emptying. These results indicate that bombesin exerts profound effects on the mammalian gastrointestinal tract via the central nervous system, probably through a vagally mediated motor pathway. Furthermore, the qualitatively different effects of this peptide on the small and large bowels emphasize that these different organs respond to drugs in different manners and with different time-courses.
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