Vagovagal inhibition of motilin-induced phase III contractions by antral acidification in dog stomach

Motilin Stimulates Gastric Acid Secretion in Coordination with Ghrelin in Suncus murinus

Motilin and ghrelin constitute a peptide family, and these hormones are important for the regulation of gastrointestinal motility. In this study, we examined the effect of motilin and ghrelin on gastric acid secretion in anesthetized suncus (house musk shrew, Suncus murinus), a ghrelin- and motilin-producing mammal. We first established a gastric lumen-perfusion system in the suncus and confirmed that intravenous (i.v.) administration of histamine (1 mg/kg body weight) stimulated acid secretion. Motilin (0.1, 1.0, and 10 μg/kg BW) stimulated the acid output in a dose-dependent manner in suncus, whereas ghrelin (0.1, 1.0, and 10 μg/kg BW) alone did not induce acid output. Furthermore, in comparison with the vehicle administration, the co-administration of low-dose (1 μg/kg BW) motilin and ghrelin significantly stimulated gastric acid secretion, whereas either motilin (1 μg/kg BW) or ghrelin (1 μg/kg BW) alone did not significantly induce gastric acid secretion. This indicates an additive role of ghrelin in motilin-induced gastric acid secretion. We then investigated the pathways of motilin/motilin and ghrelin-stimulated acid secretion using receptor antagonists. Treatment with YM 022 (a CCK-B receptor antagonist) and atropine (a muscarinic acetylcholine receptor antagonist) had no effect on motilin or motilin-ghrelin co-administration-induced acid output. In contrast, famotidine (a histamine H2 receptor antagonist) completely inhibited motilin-stimulated acid secretion and co-administration of motilin and ghrelin induced gastric acid output. This is the first report demonstrating that motilin stimulates gastric secretion in mammals. Our results also suggest that motilin and co-administration of motilin and ghrelin stimulate gastric acid secretion via the histamine-mediated pathway in suncus.

Effects of stimulation of the insular cortex on execution of the antrofundal reflex in conscious dogs

Neuroanatomical studies demonstrated the existence of direct descending projections from the insular cortex to the area of the solitary tract nucleus responsible for mediating the vagovagal reflexes of the proximal part of the gastrointestinal tract. These studies provided grounds for suggesting that one of the mechanisms mediating the influences of the insular cortex on stomach function may be modulation of its vagovagal reflex reactions, one of which is the antrofundal inhibitory reflex. Experiments on conscious dogs were performed to study the effects of electrical stimulation of the insular cortex on execution of the antrofundal gastric reflex in conditions of intermittent gastrointestinal tract activity during fasting. Stretching of the walls of the antral segment of the stomach during the active period of intermittent gastric activity led to suppression of contractions in the fundal segment. Electrical stimulation of the insular cortex was found to prolong this reflex reaction. Thus, one result of activation of the insular area of the cortex is enhancement of the inhibitory vagovagal gastric reflexes, in all probability occurring as a result of modulation of neurotransmission in the vagovagal reflex arc system.

Ghrelin induces fasted motor activity of the gastrointestinal tract in conscious fed rats

Ghrelin is a newly discovered orexigenic peptide originating from the stomach. However, its action in regulating the fed and fasted motor activity of the digestive tract is not fully understood. In the present study, we examined the effects of intracerebroventricular (I.C.V.) and intravenous (I.V.) injection of ghrelin on the physiological fed and fasted motor activities in the stomach and duodenum of freely moving conscious rats. I.C.V. and I.V. injection of ghrelin induced fasted motor activity in the duodenum in normal fed rats, while I.V. injection of ghrelin induced fasted motor activity in both the stomach and duodenum in vagotomized rats. The effects of I.C.V. and I.V. injected ghrelin were blocked by growth hormone secretagogue receptor (GHS-R) antagonist given by the same route and also blocked by immunoneutralization of neuropeptide Y (NPY) in the brain. The effects of I.V. injected ghrelin were not altered by I.C.V. injection of GHS-R antagonist in vagotomized rats. Injection of GHS-R antagonist blocked the fasted motor activity in both the stomach and duodenum in vagotomized rats but did not affect the fasted motor activity in normal rats. Low intragastric pH inhibited the effect of ghrelin. The present results indicate that ghrelin is involved in regulation of fasted motor activity in the stomach and duodenum. Peripheral ghrelin may induce the fasted motor activity by activating the NPY neurons in the brain, probably through ghrelin receptors on vagal afferent neurons. Once the brain mechanism is eliminated by truncal vagotomy, ghrelin might be primarily involved in the regulation of fasted motor activity through ghrelin receptors on the stomach and duodenum. The action of ghrelin to induce fasted motor activity is strongly affected by intragastric pH; low pH inhibits the action.