Laryngeal afferent-mediated c-fos expression in the dorsal motor nucleus of the vagus, which innervates the abdominal viscera.

Abstract

Relaxation of the stomach is achieved by inhibiting cholinergic myenteric neurons that connect with vagal preganglionic neurons. In addition, non-adrenergic non-cholinergic (NANC) myenteric neurons that were activated by the excitation of vagal preganglionic neurons induce relaxation. Our previous study demonstrated that electrical stimulation of the superior laryngeal nerve (SLN) evokes relaxation in the proximal stomach. The relaxation evoked by electrical stimulation of the SLN was still observed after injection of atropine, albeit reduced in magnitude. NANC myenteric neurons activated by the excitation of vagal preganglionic neurons may function in laryngeal afferent-mediated gastric relaxation. The present study aimed to identify whether excitation of the vagal preganglionic neurons plays a role in the laryngeal afferent-mediated relaxation of the stomach using histological techniques. Excitation of neurons in the dorsal motor nucleus of the vagus (DMV) was detected by c-fos immunoreactivity after electrical stimulation of the SLN. Abdominal-projecting neurons were identified by intraperitoneal injection of the retrograde tracer fluorogold (FG). Electrical stimulation of the left SLN induced c-fos expression in the dorsal vagal complex (DVC), which includes the area postrema (AP), the nucleus of the solitary tract (NTS) and the DMV, bilaterally. The paragigantocellular nucleus, the ventrolateral reticular nucleus and the area surrounding the nucleus ambiguus were also stained. In the DVC, FG-labeled cells were only observed in the DMV, which contains the cell bodies of the vagal preganglionic neurons. Cells labeled by both c-fos and FG were detected in the DMV. This suggested that the abdominal-projecting DMV neurons were excited in response to electrical stimulation of the SLN. To identify the gastric-projecting neurons, FG was injected into the anterior wall of the proximal stomach. Electrical stimulation of the SLN induced c-fos expression in FG-positive DMV neurons, but the number of FG-positive neurons was smaller than that after intraperitoneal injection. Most double-positive cells were located at the rostrocaudal middle level of the DMV. Additionally, immunofluorescence labelling of nitric oxide synthase (NOS), c-fos and FG was performed to clarify whether c-fos- and FG-double-positive cells had nitric oxide (NO). FG was injected into the anterior wall of the stomach. Triple-positive cells for FG, c-fos and NOS were observed in the caudal DMV. As the NOS-containing preganglionic neurons in the DMV are considered “command NANC neurons”, NANC myenteric neurons may function in the gastric relaxation induced by SLN- afferent neurons. Taken together, excitation of SLN afferent neurons may induce gastric relaxation via NANC myenteric neurons in addition to cholinergic myenteric neurons.