Immunocytochemical detection of orexin A in endocrine cells of the developing mouse gut

Orexins are novel neuropeptides that were originally localized in neurons of the hypothalamus and neuronal fibers of the brain. Recently orexin A and its receptor have also been reported in neurons and endocrine cells of the gastrointestinal tract. Because no studies have been done at the embryonic period, we studied the appearance and distribution of orexin A during the development of mouse gastrointestinal tract using immunocytochemical methods. Immunoreactivity to orexin A was detected in neuroendocrine cells of the pyloric region of the stomach at gestational Day 14 and 1 day after in the small intestine. The numbers of immunoreactive cells progressively increased through development until the adult pattern was reached. Staining of reverse-face sections demonstrated that orexin A and serotonin co-localized in some endocrine cells of the mouse stomach and small intestine. These findings suggest that orexin A may be relevant in the growth and maturation of the gastrointestinal tract during intrauterine life.