Does motilin primarily regulate gastrointestinal motility in fish?

Motilin (MLN) is a gut motility-stimulating peptide hormone that regulates the gastric migrating motor complex in humans, dogs, monkeys, and house musk shrews (Suncus). MLN and its receptors are found not only in mammals but also in other vertebrates (birds, reptiles, amphibians, and fish). This review focuses on the physiological roles of MLN in fish. Fish MLNs are produced in the mucosa of the upper small intestine. MLNs in amphibians, reptiles, birds, and mammals are 22-amino-acid peptides with phenylalanine at the first position of the N-terminus, except for reptile MLNs. In contrast, fish MLNs are shorter, and the N-terminal starts with histidine instead of phenylalanine. The function of MLN can be inferred from the distribution of MLN receptors. MLN receptors are commonly expressed in the central nervous system and gastrointestinal (GI) tract of fish, as in mammals. In the central nervous system, MLN is thought to be involved in the regulation of feeding and drinking. However, MLN receptors in the fish GI tract are limited to the intestinal mucosa, and MLN fails to induce contraction in the fish GI tract. In amphibians, birds, and mammals, the expression of MLN receptors extends to GI smooth muscles and enteric neurons and is involved in the regulation of GI motility. These findings suggest that MLN is not a regulator of GI motility in fish, but instead regulates the functions of intestinal mucosal cells and central neurons.