Electrophysiological and morphological properties of submucosal neurons in the mouse distal colon

The pathophysiology of intestinal secretion is increasingly studied using transgenic mouse models of colonic inflammation, but little is known about the properties of single submucosal neurons regulating epithelial secretion in the mouse. Therefore, these neurons were characterized in the mouse distal colon submucosal plexus using electrophysiological and morphological techniques. Action potentials and synaptic responses in single neurons were recorded in submucosal preparations in vitro using intracellular electrodes filled with neurobiotin, enabling subsequent morphological examination. Most neurons (similar to 90%) were classified as S-type neurons based on the presence of fast excitatory postsynaptic potentials (EPSPs), absence of prolonged after-hyperpolarizations, and uni-axonal morphology. These neurons rarely fired more than one action potential in response to intracellular depolarization but spontaneous fast EPSPs were observed. Fibre tract stimulation (20 Hz) elicited slow EPSPs in many neurons (37%) and slow inhibitory potentials were also observed in a subset of neurons (18%). None of these cells exhibited electrophysiological features suggestive of AH neurons. However, morphological studies demonstrated that 12% of neurons had two axons, and an immunohistochemical marker of mouse AH neurons, calcitonin gene-related peptide, was co-localized with the pan-neuronal marker HuD in 9% of neurons. Cell counts indicated that mouse distal colon ganglia were much smaller (> 80% contain one to five neurons) compared to guinea-pig distal colon ganglia (similar to 30% contain > 10 neurons). This study has shown that mouse distal colon submucosal ganglia are relatively small and, based on combined morphological and electrophysiological features, most neurons exhibit S-type characteristics.