Functional effects and characteristics of cecum-projecting neurons in the dorsal motor nucleus of the vagus of rats.

Preganglionic neurons in the dorsal motor nucleus of the vagus (DMV) innervate most of the gastrointestinal tract; with the stomach and the cecum/proximal colon having a greater proportion of vagal input. Cecum-projecting neurons have been thought to be distinct from other preganglionic neurons due to their location within the DMV, but it is unknown whether these neurons innervate the cecum exclusively or what effect their activation has on cecal motor activity. Therefore, we investigated the extent of coinnervation of cecum and stomach by vagal neurons, their neurochemistry, and the effect of DMV stimulation on intracecal and intragastric volumes. Fluorescent retrograde tracers injected into the serosa of the cecum and stomach revealed that in the DMV 49+/-5% CTB-labeled cecum-projecting neurons also innervated the stomach. Immunocytochemical staining for nitric oxide (NO) synthase and tyrosine hydroxylase indicated that only 3+/-1% and 4+/-1% of cecum-projecting neurons contained these markers, respectively. In anesthetized rats gastric and cecal volumes were measured by prototypic miniaturized dual barostats that were developed for use in rodents. Microinjection of l-glutamate into the DMV increased gastric contractile activity and tone, and reduced on-going cecum contractile activity (2.6+/-0.7 contractions/2 min after injection versus 8.2+/-0.4 contractions/2 min before injection, N = 5). The barostat was able to detect decreases (-0.88+/-0.13 ml) and increases (0.25+/-0.05 ml) in cecum volume in response to carbachol and sodium nitroprusside, respectively. In summary, cecum-projecting neurons are not an entirely exclusive population within the DMV because a percentage of these also innervate the stomach. Central vagal stimulation can modulate both gastric and cecum contractile activity. Together, these data support a role of the vagus in neural reflexes involving gastric and large bowel motor function, such as the immediate phase of the gastrocolonic reflex.