Brunner's Glands in the Duodenal Bulb Are Indirectly Innervated by Vagal Preganglionics That Connect With Cholinergic Enteric Neurons in the Myenteric Plexus.

Canonical parasympathetic innervation of tissues/organs is accomplished by two in-series neurons, often called pre- and postganglionic neurons. Such two-neuron in-series organization has recently been challenged by claiming direct vagal preganglionic innervation of Brunner's glands (BGs) in the duodenal bulb, as well as the spleen, and the liver. To resolve this controversial issue, we labeled vagal preganglionic neurons in the dorsal motor nucleus of the vagus (DMV) by injecting a Cre-dependent AAV construct to express tdTomato in ChAT-IRES-Cre transgenic mice and analyzed the distribution of labeled fibers in the duodenal bulb. We found that labeled vagal preganglionic fibers terminate exclusively in myenteric ganglia but not in or between BGs. This lack of direct BG innervation is unlikely to result from incomplete labeling of vagal preganglionic neurons, as the entire extent of the DMV was well covered by bilateral injections, and there were abundant vagal axons and terminals in the gastric myenteric plexus and other vagal targets. BGs were innervated by a consistent supply of cholinergic (VAChT-positive) and sparsely by adrenergic (TH-positive) nerve fibers. There was also a moderate supply of vagal afferents anterogradely traced from the nodose ganglia and CGRP-positive fibers. We conclude that BGs in the mouse are not directly innervated by vagal preganglionic neurons of DMV origin but receive cholinergic innervation from myenteric plexus neurons serving as postganglionic neurons. Further experiments will be necessary to clarify the issue for other vagal targets.