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Motilin and ghrelin are the gastrointestinal (GI) hormones released in a fasting state to stimulate the GI motility of the migrating motor complex (MMC). We focused on coordination of the ghrelin/motilin family in gastric contraction in vivo and in vitro using the house musk shrew (Suncus murinus), a ghrelin- and motilin-producing mammal. To measure the contractile activity of the stomach in vivo, we recorded GI contractions either in the free-moving conscious or anesthetized S. murinus and examined the effects of administration of motilin and/or ghrelin on spontaneous MMC in the fasting state. In the in vitro study, we also studied the coordinative effect of these hormones on the isolated stomach using an organ bath. In the fasting state, phase I, II, and III contractions were clearly recorded in the gastric body (as observed in humans and dogs). Intravenous infusion of ghrelin stimulated gastric contraction in the latter half of phase I and in the phase II in a dose-dependent manner. Continuous intravenous infusion of ghrelin antagonist (d-Lys3-GHRP6) significantly suppressed spontaneous phase II contractions and prolonged the time of occurrence of the peak of phase III contractions. However, intravenous infusion of motilin antagonist (MA-2029) did not inhibit phase II contractions but delayed the occurrence of phase III contractions of the MMC. In the in vitro study, even though a high dose of ghrelin did not stimulate contraction of stomach preparations, ghrelin administration (10(-10)-10(-7) M) with pretreatment of a low dose of motilin (10(-10) M) induced gastric contraction in a dose-dependent manner. Pretreatment with 10(-8) M ghrelin enhanced motilin-stimulated gastric contractions by 10 times. The interrelation of these peptides was also demonstrated in the anesthetized S. murinus. The results suggest that ghrelin is important for the phase II contraction and that coordination of motilin and ghrelin are necessary to initiate phase III contraction of the MMC.

Although many studies have demonstrated the action of motilin on migrating motor complex by using human subjects and relatively large animals, the precise physiological mechanisms of motilin remain obscure. One reason for the lack of progress in this research field is that large animals are generally not suitable for molecular-level study. To overcome this problem, in this study, we focused on the house musk shrew (Suncus murinus, order: Insectivora, suncus named as laboratory strain) as a small model animal, and we present here the results of motilin gene cloning and its availability for motilin study. The motilin gene has a high homology sequence with that of other mammals, including humans. Suncus motilin is predicted to exist as a 117-residue prepropeptide that undergoes proteolytic cleavage to form a 22-amino-acid mature peptide. The results of RT-PCR showed that motilin mRNA is highly expressed in the upper small intestine, and low levels of expression were found in many tissues. Morphological analysis revealed that suncus motilin-producing cells were present in the upper small intestinal mucosal layer but not in the myenteric plexus. Administration of suncus motilin to prepared muscle strips of rabbit duodenum showed almost the same contractile effect as that of human motilin. Moreover, suncus stomach preparations clearly responded to suncus or human motilin stimulation. To our knowledge, this is the first report that physiological active motilin was determined in small laboratory animals, and the results of this study suggest that suncus is a suitable model animal for studying the motilin-ghrelin family.

Abstract: Motilin is a peptide hormone involved in gastrointestinal motility. GPR38, initially cloned as an orphan receptor, is now considered a specific receptor for motilin. Previously, molecular characterization of the motilin receptor had only been performed in mammalian and fish species. In this study, we cloned cDNA for chicken motilin receptor from the duodenum and characterized its primary structure, tissue distribution, and biological activity. The cDNA encoded 349 amino acids showing significant overall sequence identity to mammalian motilin receptors. Chicken motilin increased intracellular Ca2+ concentration in human embryonic kidney (HEK) 293 cells transiently expressing the recombinant chicken motilin receptor. Comparison of the cDNA sequence with the genomic sequence of chicken motilin receptor revealed that the chicken motilin receptor gene consists of two exons separated by an intron. Real-time PCR analysis showed that chicken motilin receptor mRNA is expressed in a wide range of tissues in 21-day-old chickens, with markedly high levels in the proventriculus, duodenum, and oviduct. The expression levels of the mRNA in the proventriculus and duodenum were highest just before hatching and rapidly decreased during post-hatch development. These results suggest that chicken motilin receptor is largely involved in gastrointestinal functions at pre- and post-hatch periods through an intracellular signaling pathway accompanied by an increase in Ca2+ levels. [Copyright &y& Elsevier]

Abstract: We previously identified ghrelin and motilin genes in Suncus murinus (suncus), and also revealed that motilin induces phase III-like strong contractions in the suncus stomach in vivo, as observed in humans and dogs. Moreover, repeated migrating motor complexes were found in the gastrointestinal tract of suncus at regular 120-min intervals. We therefore proposed suncus as a small laboratory animal model for the study of gastrointestinal motility. In the present study, we identified growth hormone secretagogue receptor (GHS-R) and motilin receptor (GPR38) genes in the suncus. We also examined their tissue distribution throughout the body. The amino acids of suncus GHS-R and GPR38 showed high homology with those of other mammals and shared 42% amino acid identity. RT-PCR showed that both the receptors were expressed in the hypothalamus, medulla oblongata, pituitary gland and the nodose ganglion in the central nervous system. In addition, GHS-R mRNA expressions were detected throughout the stomach and intestine, whereas GPR38 was expressed in the gastric muscle layer, lower intestine, lungs, heart, and pituitary gland. These results suggest that ghrelin and motilin affect gut motility and energy metabolism via specific receptors expressed in the gastrointestinal tract and/or in the central nervous system of suncus. [Copyright &y& Elsevier]