Molecular characterization and distribution of motilin and motilin receptor in the Japanese medaka <italic>Oryzias latipes</italic>.

Motilin (MLN) is a peptide hormone originally isolated from the mucosa of the porcine intestine. Its orthologs have been identified in various vertebrates. Although MLN regulates gastrointestinal motility in tetrapods from amphibians to mammals, recent studies indicate that MLN is not involved in the regulation of isolated intestinal motility in zebrafish, at least in vitro. To determine the unknown function of MLN in teleosts, we examined the expression of MLN and the MLN receptor (MLNR) at the cellular level in Japanese medaka (<italic>Oryzias latipes</italic>). Quantitative PCR revealed that <italic>mln</italic> mRNA was limitedly expressed in the gut, whereas <italic>mlnr</italic> mRNA was not detected in the gut but was expressed in the brain and kidney. By in situ hybridization and immunohistochemistry, <italic>mlnr</italic> mRNA was detected in the dopaminergic neurons of the area postrema in the brain and the noradrenaline-producing cells in the interrenal gland of the kidney. Furthermore, we observed efferent projections of <italic>mlnr</italic>-expressing dopaminergic neurons in the lobus vagi (XL) and nucleus motorius nervi vagi (NXm) of the medulla oblongata by establishing a transgenic medaka expressing the enhanced green fluorescence protein driven by the <italic>mlnr</italic> promoter. The expression of dopamine receptor mRNAs in the XL and cholinergic neurons in NXm was confirmed by in situ hybridization. These results indicate novel sites of MLN activity other than the gastrointestinal tract. MLN may exert central and peripheral actions through the regulation of catecholamine release in medaka. [ABSTRACT FROM AUTHOR]