Immuno-electron microscopic study of differential localization of motilin and serotonin in the rabbit duodenal epithelium.

A guinea pig antibody against rabbit motilin was generated to study the localization of motilin-containing cells in the rabbit small intestine with special reference to the co-localization of motilin and serotonin. A pre- and post-embedding technique for immuno-electron microscopy was used; duodenal sections were stained with either motilin or serotonin in the pre-embedding DAB-nickel reaction, followed by subsequent staining of ultrathin sections of positive cells with either motilin or serotonin in the post-embedding immunogold reaction. Samples were divided into four groups: 1) pre-motilin, post-motilin, 2) pre-motilin, post-serotonin, 3) pre-serotonin, post-serotonin, and 4) pre-serotonin, post-motilin. Motilin-containing cells in the rabbit duodenum were characterized by round granules (395.3 +/- 66.1 nm in diameter) with medium electron density, located basally or in the perinuclear cytoplasm. In contrast, serotonin-containing cells were characterized by round to pleomorphic secretory granules (344.5 +/- 90.5 nm in diameter with electron dense cores and prominent halos. In motilin-containing cells, massive aggregations of immunogold particles reacted to motilin occurred over secretory granules. A few immunogold particles scattered diffusely over the cytoplasm reacted to serotonin; however, this reaction appeared to be background staining because the density was not changed if the section was treated by preabsorption. In serotonin-containing cells, immunogold particles reacted to serotonin were aggregated over the secretory granules and a large number of gold particles were scattered diffusely at the extragranular cytoplasm; however, very few or no immunogold particles were observed within the cells which reacted to motilin. Results of the present study indicate that motilin and serotonin are not co-localized in the epithelial endocrine cells of the rabbit intestine.