Properties of cholinergic and non-cholinergic submucosal neurons along the mouse colon

Submucosal neurons are vital regulators of water and electrolyte secretion and local blood flow in the gut. Due to the availability of transgenic models for enteric neuropathies, the mouse has emerged as the research model of choice, but much is still unknown about the murine submucosal plexus. The progeny of choline acetyltransferase (ChAT)-Cre × ROSA26(YFP) reporter mice, ChAT-Cre;R26R-yellow fluorescent protein (YFP) mice, express YFP in every neuron that has ever expressed ChAT. With the aid of the robust YFP staining in these mice, we correlated the neurochemistry, morphology and electrophysiology of submucosal neurons in distal colon. We also examined whether there are differences in neurochemistry along the colon and in neurally mediated vectorial ion transport between the proximal and distal colon. All YFP(+) submucosal neurons also contained ChAT. Two main neurochemical but not electrophysiological groups of neurons were identified: cholinergic (containing ChAT) or non-cholinergic. The vast majority of neurons in the middle and distal colon were non-cholinergic but contained vasoactive intestinal peptide. In the distal colon, non-cholinergic neurons had one or two axons, whereas the cholinergic neurons examined had only one axon. All submucosal neurons exhibited S-type electrophysiology, shown by the lack of long after-hyperpolarizing potentials following their action potentials and fast excitatory postsynaptic potentials (EPSPs). Fast EPSPs were predominantly nicotinic, and somatic action potentials were mediated by tetrodotoxin-resistant voltage-gated channels. The size of submucosal ganglia decreased but the proportion of cholinergic neurons increased distally along the colon. The distal colon had a significantly larger nicotinic ion transport response than the proximal colon. This work shows that the properties of murine submucosal neurons and their control of epithelial ion transport differ between colonic regions. There are several key differences between the murine submucous plexus and that of other animals, including a lack of conventional intrinsic sensory neurons, which suggests there is an incomplete neuronal circuitry within the murine submucous plexus.