Abdominal vagi mediate c-Fos expression induced by X-ray irradiation in the nucleus tractus solitarii of the rat.

The mechanism of induction of emesis by X-ray irradiation remains largely unknown. The purpose of the present research was to clarify the neuronal basis of the induction of nausea induced by X-ray irradiation analyzing c-Fos expression in the nucleus tractus solitarii (NTS) as a marker of cellular excitation. We confirmed that the dose of X-ray irradiation (4 Gy) used for the present research could actually induce nausea by preliminary measurement of kaolin intake. Induction of c-Fos immunoreactivity in the NTS was observed in the animals that received X-ray irradiation of the whole body. The mean number of c-Fos positive cells in the animals that received irradiation was significantly larger than that in the non-irradiated animals. Partial exposure of the abdomen to X-rays showed significantly greater c-Fos expression than that of the head. These results indicated the presence of a certain route for transmitting information from the periphery toward the central nervous system by X-ray irradiation. The number of c-Fos positive cells induced by X-ray irradiation in animals vagotomized at the subdiaphragmatic level was lower than that in sham-operated animals. Animals receiving a serotonin subtype three (5-HT3, 5-hydroxytryptamine) receptor antagonist (tropisetron, ICS 205-930, 3-tropanyl-indole-3-carboxylate) showed a significant reduction in c-Fos protein expression compared to animals receiving a vehicle. These results strongly suggested that X-ray irradiation activates 5-HT3 receptors on the terminals of the abdominal vagal nerves to excite the afferent pathway, thereby inducing emesis.