Almost 50 years have passed since the discovery of motilin. However, actions of motilin on gastrointestinal (GI) motility are different from species and motilin does not cause GI contraction in rodents (rats, mice and guineapigs). Additionally, actions of motilin also differ from GI regions and experiment conditions (in vitro or in vivo) even in the same species. Due to these characteristics of motilin responses, number of papers for motilin research is small compared to that of a motilin-related peptide, ghrelin (discovered at 1999) and knowledge of motilin and its receptor have been limited and unsorted. Recently motilin and its receptor (MLN-R) have been also identified in non-mammalian vertebrates (birds, reptiles, amphibians and fish). This review summarized the distribution, structure, receptor expression and GI motility-stimulating action of motilin in a range of species including fish to mammals. A highly conserved N-terminal structure (1-10) commencing the amino acid indicated by phenylalanine was thought to be essential for GI motility stimulating action of motilin in mammalian/avian motilin lineage. Reptile motilin is considered to be in the transition stage to mammalian/avian type, i.e. alligator motilin has phenylalanine but other motilins (snake, turtle and lizard) have tyrosine at first position of N-terminal. On the other hand, the sequences of fish and amphibian motilins are quite different from those of mammalian/avian motilin. Therefore, in the molecular evolution of motilin, there may have been a major event at the time the reptiles emerged. The differences in motilin sequences are due to mutations in protein coding domains during species evolution which were probably motivated by adaptation. In contrast, the C-terminal sequence (11-22) is more conserved than that of the N-terminal, suggesting that the C-terminal may exert an as yet unknown function in addition to stimulation of GI motility as mediated via the N-terminal. Molecular biologically, MLN-R can be divided into two main groups: mammal/bird/reptile/amphibian clade (group A) and fish clade (group B). Group A can be divided into two clades: terrestrial type (mammals, birds and reptiles) and semi-aquatic type (amphibians). The clade of the terrestrial MLN-Rs can be further divided into three clades (mammals, birds/reptiles and reptiles (reptile-1)), and birds/reptiles clade is divided into birds and reptile-2 (alligator/crocodile MLN-Rs). Reptile-2 clade is included in the same umbrella with the bird clade, as in the case of motilin structure. Group B may have characteristics that match the aquatic inhabiting nature of fish. In mammals, motilin is an important regulator of the phase III of interdigestive migrating motor complex (MMC) in the stomach of humans, dogs, house musk shrews, monkeys and opossum through activation of smooth muscle cells, enteric neurons or vago-vagal reflex pathway. Gastric MMCs induced by motilin contribute to maintenance of normal GI functions and transmits a hunger signal from peripheral (stomach) to brain. Motilin has been identified in other mammals (rabbits, ruminants and pigs), but roles of motilin in these animals have not been understood well due to different physiological characteristics of MMC and different feeding behavior. In birds, motilin and MLN-Rs have been also identified and motilin caused contraction of small intestine and contributed to initiation of rhythmic oscillating complexes in the intestine. Motilin did not cause the contraction of GI strips in the fish but caused the contraction of urodelian amphibians (newts) and reptiles in a GI region-dependent manner as in the bird/mammals. Through these comparative studies in different vertebrates, it can be seen for the first time that the GI motility-stimulating action of motilin is not common in vertebrates because motilin stimulates GI contraction in mammals, birds, reptiles and amphibians but not in fish. This review, covering a wide range of motilin research including not only mammals but also non-mammals (comparative biology of motilin), will help to understand the contribution of the motilin system to animals, including evolution
