ROLES OF CENTRAL AND PERIPHERAL MU-OPIOID, DELTA-OPIOID AND KAPPA-OPIOID RECEPTORS IN THE MEDIATION OF GASTRIC-ACID SECRETORY EFFECTS IN THE RAT

  • 1 February 1988
    • journal article
    • research article
    • Vol. 244  (2) , 456-462
Abstract
The opioid receptors involved in the mediation of gastric acid secretory effects were studied in the pylorus-ligated rat. The effects of i.c.v. and i.v. administration of morphine and mu ([D-Ala2, NMePhe4, Gly5-ol]enkephalin and Tyr-Pro-NMePhe D-ProNH2)-, delta ([D-Pen2, D-Pen5]enkephalin)- and kappa-selective [trans-3,4-dichloro-N-methyl-N-(2-91-pyrolidinyl)-cyclohexyl]-benzeneacetamide methanesulfonate (U-50, 488H), dynorphin (1-9), dynorphin-(1-17), nalorphine, .alpha.-neoendorphin and ethyl-ketocyclazocine opioid receptor agonists on gastric volume and acid output were examined. Morphine, [D-Ala2, NMePhe4, Gly5-ol]enkephalin and Tyr-Pro-NMePhe-D-Pro-NH2 decreased gastric acid secretion more potently after i.c.v. than after i.v. administration. The inhibitory effect of i.v. administered morphine on gastric acid secretion was not blocked by the quaternary opioid antagonist naltrexone methylbromide when given s.c. However, when naltrexone methylbromide was administered i.c.v., it blocked completely the effects of i.c.v. morphine and partially antagonized the effects of i.v. morphine, indicating a central site of action for morphine. The delta-selective agonist [D-Pen2, D-Pen5]enkephalin did not alter gastric acid secretion after i.c.v or i.v. administration. The kappa-selective opioid agonist U-50, 488H produced a dose-dependent increase in gastric acid secretion after i.v. but not i.c.v. administration. The other kappa-selective agonists tested did not produce a significant increase in gastric acid secretion after i.c.v. or i.v. administration. The increase in gastric acid secretion produced by U-50, 488H was blocked by pretreatment with the opioid receptor antagonist naloxone, the nonselective muscarinic receptor antagonist atropine and the M1 selective muscarinic receptor antagonist pirenzepine. The secretory effects of U-50, 488H were not blocked by pretreatment with the ganglionic blocker hexamethonium, the H2 receptor antagonist metiamide, reserpine, or the separate or combined action of the alpha and beta adrenergic receptor antagonists phentolamine and propranolol, respectively. The results of our studies suggest that in rats 1) stimulation of central and possibly peripheral mu opioid receptors decreases gastic acid secretion; 2) delta opioid receptors are not involved in the mediation of gastric acid secretion; and 3) U-50, 488H, a kappa-selective opioid agonist, increases gastric acid secretion by a mechanism which involves the local release of acetylcholine acting on M1 receptors on the parietal cell to secrete acid.