Induction of Fos protein in neurons in the medulla oblongata after motion- and X-irradiation-induced emesis in musk shrews (Suncus murinus).

To clarify the anatomical location of medullary neurons associated with vomiting, the musk shrew (Suncus murinus), a small animal used as a model for emesis, was exposed to various emetic stimuli and patterns of neuronal excitation were investigated by Fos immunohistochemistry. In motion experiments, musk shrews were shaken for 30 min on a tabletop shaker (displacement=25 mm and frequency=1.2 Hz). Ten of fifteen animals vomited frequently (Mo-FV group); the other five animals did not vomit (Mo-NV group). In radiation experiments, X-ray irradiation (10 Gy) of the whole body caused frequent vomiting in all of seven experimental animals (Ra-FV group). In the Mo-FV group, many Fos-immunoreactive (Fos-ir) neurons were detected in the nucleus of the solitary tract (NTS) and the reticular formation. The distribution pattern of Fos-ir neurons in the Mo-NV group was similar to that in the Mo-FV group, but the Mo-NV group had significantly fewer positive neurons in the NTS and the reticular formation around the nucleus ambiguus. In the Ra-FV group, numerous Fos-ir neurons were observed in the area postrema, an area containing no positive neurons in the motion-stimulated animals. The number of Fos-ir neurons in the NTS of the Ra-FV group was not statistically different from that of the Mo-NV group. In the Mo-FV and Ra-FV groups, Fos-ir neurons were clustered in the reticular formation at the dorsal-dorsomedial edge of the nucleus ambiguus at the level of the rostral medulla, while few such clusters were observed in the Mo-NV group. These neurons may play a role in the regulation of the vomiting response.