A verification study of gastrointestinal motility-stimulating action of guinea-pig motilin using isolated gastrointestinal strips from rabbits and guinea-pigs

Motilin, a Novel Orexigenic Factor, Involved in Feeding Regulation in Yangtze Sturgeon (Acipenser dabryanus)

Motilin is a gastrointestinal hormone that is mainly produced in the duodenum of mammals, and it is responsible for regulating appetite. However, the role and expression of motilin are poorly understood during starvation and the weaning stage, which is of great importance in the seeding cultivation of fish. In this study, the sequences of Yangtze sturgeon (Acipenser dabryanus Motilin (AdMotilin)) motilin receptor (AdMotilinR) were cloned and characterized. The results of tissue expression showed that by contrast with mammals, AdMotilin mRNA was richly expressed in the brain, whereas AdMotilinR was highly expressed in the stomach, duodenum, and brain. Weaning from a natural diet of T. Limnodrilus to commercial feed significantly promoted the expression of AdMotilin in the brain during the period from day 1 to day 10, and after re-feeding with T. Limnodrilus the change in expression of AdMotilin was partially reversed. Similarly, it was revealed that fasting increased the expression of AdMotilin in the brain (3 h, 6 h) and duodenum (3 h), and the expression of AdMotilinR in the brain (1 h) in a time-dependent manner. Furthermore, it was observed that peripheral injection of motilin-NH2 increased food intake and the filling index of the digestive tract in the Yangtze sturgeon, which was accompanied by the changes of AdMotilinR and appetite factors expression in the brain (POMC, CART, AGRP, NPY and CCK) and stomach (CCK). These results indicate that motilin acts as an indicator of nutritional status, and also serves as a novel orexigenic factor that stimulates food intake in Acipenser dabryanus. This study lays a strong foundation for the application of motilin as a biomarker in the estimation of hunger in juvenile Acipenser dabryanu during the weaning phase, and enhances the understanding of the role of motilin as a novel regulator of feeding in fish.

Motilin is a regulator of gastric contraction in Japanese fire belly newts (Cynops pyrrhogaster), in vitro studies using isolated gastrointestinal strips of newts, rabbits, and chickens

The effects of newt motilin on the contractility of the isolated gastrointestinal (GI) tract from Japanese fire belly newts (newt) were examined to clarify whether motilin regulates GI motility in urodele amphibians. In addition, contractile responsiveness to motilins from seven species of vertebrates (human, chicken, turtle, alligator, axolotol, newt and zebrafish) were compared in GI preparations from three different animals (rabbit duodenum, chicken ileum and newt stomach) to determine the species-specific action of motilin. Newt motilin (10^-10 M - 10^-6 M) caused a contraction of cognate gastric strips, while the upper, middle, and lower intestinal strips were insensitive. The rank order of motilins for contractile activity in newt gastric strips was newt > alligator > axolotol > chicken > turtle > human ≫ zebrafish. On the other hand, newt motilin caused a weak contraction in the rabbit duodenum (human > alligator = chicken > turtle > newt ≧ axolotol > zebrafish), and it was ineffective in the chicken ileum (chicken > turtle > alligator > human ≫ newt, axolotol and zebrafish). This study demonstrates that motilin induces contraction in the GI tract of a urodele amphibian, the newt, in a region (stomach)-specific manner and further indicates that a ligand-receptor interaction of the motilin system is a species-specific manner probably due to differences in the amino acid sequence of motilin.

Why is motilin active in some studies with mice, rats, and guinea pigs, but not in others? Implications for functional variability among rodents

The gastrointestinal (GI) hormone motilin helps control human stomach movements during hunger and promotes hunger. Although widely present among mammals, it is generally accepted that in rodents the genes for motilin and/or its receptor have undergone pseudonymization, so exogenous motilin cannot function. However, several publications describe functions of low concentrations of motilin, usually within the GI tract and CNS of mice, rats, and guinea pigs. These animals were from institute‐held stocks, simply described with stock names (e.g., “Sprague–Dawley”) or were inbred strains. It is speculated that variation in source/type of animal introduces genetic variations to promote motilin‐sensitive pathways. Perhaps, in some populations, motilin receptors exist, or a different functionally‐active receptor has a good affinity for motilin (indicating evolutionary pressures to retain motilin functions). The ghrelin receptor has the closest sequence homology, yet in non‐rodents the receptors have a poor affinity for each other's cognate ligand. In rodents, ghrelin may substitute for certain GI functions of motilin, but no good evidence suggests rodent ghrelin receptors are highly responsive to motilin. It remains unknown if motilin has functional relationships with additional bioactive molecules formed from the ghrelin and motilin genes, or if a 5‐TM motilin receptor has influence in rodents (e.g., to dimerize with GPCRs and create different pharmacological profiles). Is the absence/presence of responses to motilin in rodents’ characteristic for systems undergoing gene pseudonymization? What are the consequences of rodent supplier‐dependent variations in motilin sensitivity (or other ligands for receptors undergoing pseudonymization) on gross physiological functions? These are important questions for understanding animal variation.

Pheasant motilin, its distribution and gastrointestinal contractility-stimulating action in the pheasant

Previously, pheasant motilin was identified as a 22-amino acid peptide with a sequence of FVPFFTQSDI QKMQEKERIK GQ. In the present study, the distribution of pheasant motilin mRNA was determined and compared with that of ghrelin, a motilin-related peptide. The effects of pheasant motilin on the cognate gastrointestinal (GI) muscle strips were also examined in an in vitro contraction study. The expression of pheasant motilin mRNA was highest in the small intestine (duodenum, jejunum and ileum), moderate in the colon and very low in the brain, lung, heart, pancreas, esophagus, proventriculus, gizzard and caecum, and this distribution was in contrast with that of ghrelin mRNA. Pheasant motilin caused contraction of the cognate GI tract in a region-dependent manner, similar to chicken motilin. The contraction in the small intestine was large and was not affected by atropine. In contrast, contraction in the proventriculus was small and was decreased by atropine. The crop and colon were insensitive to pheasant motilin. Neither GM109 nor MA2029, mammalian motilin receptor antagonists inhibited the contractions of pheasant motilin. Erythromycin was ineffective in the pheasant ileum, although it caused contraction of the rabbit duodenum. These results indicate that pheasant motilin caused contraction through an action on smooth muscles in the small intestine and an action on enteric cholinergic nerves in the proventriculus. This high responsiveness of the small intestine suggests that motilin is a regulator of small intestinal motility in avians, and the characteristic of the motilin receptor in the pheasant might be different from that in mammals, as is that in chickens.

Motilin Comparative Study: Structure, Distribution, Receptors, and Gastrointestinal Motility

Motilin, produced in endocrine cells in the mucosa of the upper intestine, is an important regulator of gastrointestinal (GI) motility and mediates the phase III of interdigestive migrating motor complex (MMC) in the stomach of humans, dogs and house musk shrews through the specific motilin receptor (MLN-R). Motilin-induced MMC contributes to the maintenance of normal GI functions and transmits a hunger signal from the stomach to the brain. Motilin has been identified in various mammals, but the physiological roles of motilin in regulating GI motility in these mammals are well not understood due to inconsistencies between studies conducted on different species using a range of experimental conditions. Motilin orthologs have been identified in non-mammalian vertebrates, and the sequence of avian motilin is relatively close to that of mammals, but reptile, amphibian and fish motilins show distinctive different sequences. The MLN-R has also been identified in mammals and non-mammalian vertebrates, and can be divided into two main groups: mammal/bird/reptile/amphibian clade and fish clade. Almost 50 years have passed since discovery of motilin, here we reviewed the structure, distribution, receptor and the GI motility regulatory function of motilin in vertebrates from fish to mammals.

Motilin- and ghrelin-induced contractions in isolated gastrointestinal strips from three species of frogs

Ghrelin (GHRL) and motilin (MLN), gut peptides isolated from the mucosa of the stomach and duodenum, respectively, stimulate gastrointestinal (GI) motility in mammals and birds. However, the functions of MLN and GHRL in amphibian GI tracts have not been examined in detail. To clarify the regulation of GI motility by the two peptides, the effects of human MLN and rat GHRL on contractility of isolated GI strips from three species of frogs, the black-spotted pond frog (pond frog; Pelophylax nigromaculata), bullfrog (Lithobates catesbeiana) and Western clawed frog (Xenopus; Xenopus tropicalis), were examined in in vitro experiments. The GI tract of each frog was divided into the stomach, upper intestine, middle intestine and lower intestine. Human MLN caused contractions of the stomach in the pond frog and upper intestine in the bullfrog and Xenopus, but other GI regions were insensitive to human MLN. Erythromycin did not cause contraction of the upper intestine of the bullfrog and Xenopus. Rat GHRL did not cause contraction of the stomach and small intestines in the pond frog and bullfrog, but it caused a concentration-dependent contraction in the stomach and upper intestine of Xenopus, while des-acyl rat GHRL did not cause any contraction of them. In conclusion, human MLN caused the contraction of the stomach or upper intestine in the three species of frogs, but GHRL was effective only in the stomach and upper intestine of Xenopus. On the basis of these data, MLN but not GHRL causes the GI region-dependent contractions in the frogs.

Regulation of Gastrointestinal Motility by Motilin and Ghrelin in Vertebrates

The energy balance of vertebrates is regulated by the difference in energy input and energy expenditure. Generally, most vertebrates obtain their energy from nutrients of foods through the gastrointestinal (GI) tract. Therefore, food intake and following food digestion, including motility of the GI tract, secretion and absorption, are crucial physiological events for energy homeostasis. GI motility changes depending on feeding, and GI motility is divided into fasting (interdigestive) and postprandial (digestive) contraction patterns. GI motility is controlled by contractility of smooth muscles of the GI tract, extrinsic and intrinsic neurons (motor and sensory) and some hormones. In mammals, ghrelin (GHRL) and motilin (MLN) stimulate appetite and GI motility and contribute to the regulation of energy homeostasis. GHRL and MLN are produced in the mucosal layer of the stomach and upper small intestine, respectively. GHRL is a multifunctional peptide and is involved in glucose metabolism, endocrine/exocrine functions and cardiovascular and reproductive functions, in addition to feeding and GI motility in mammals. On the other hand, the action of MLN is restricted and species such as rodentia, including mice and rats, lack MLN peptide and its receptor. From a phylogenetic point of view, GHRL and its receptor GHS-R1a have been identified in various vertebrates, and their structural features and various physiological functions have been revealed. On the other hand, MLN or MLN-like peptide (MLN-LP) and its receptors have been found only in some fish, birds and mammals. Here, we review the actions of GHRL and MLN with a focus on contractility of the GI tract of species from fish to mammals.