Evidence for both inhibitory and excitatory effects of morphine on gastric secretion in the rat

The relationship between a wide range of doses of i.p. morphine and the effects of gastric secretion in several rat experimental models was investigated. In unoperated rats, morphine 5-15 mg/kg dose-dependently decreased gastric acidity, but an excitatory effect was observed with 0.5-1.5 mg/kg. A dose-dependent inhibition was also obtained in conscious pylorus-ligated rats with morphine 5-15 mg/kg, whereas no significant effects were found at lower doses. By contrast, acid concentration was enhanced with morphine 0.5-15 mg/kg in anaesthetized pylorus-ligated rats and in stomach lumen perfused rats. The changes of pepsin concentration correlated with the changes in gastric secretory volume. An increase in the volume was associated with a decrease in pepsin concentration, while pepsin amount per stomach was dose-dependently increased in all the conditions under which morphine increased gastric acidity. All the effects of morphine on acid and pepsin secretion were prevented by naloxone 1 mg/kg i.p. Overall results indicate that morphine may induce both excitatory and inhibitory effects on gastric secretion in the rat: these effects depend on the dose of morphine and the experimental conditions.