Projections of enteric motor neurons in the mouse distal colon

This study examined the projections in vitro of excitatory and inhibitory colonic motor nerves in the unaffected siblings of mice with hereditary aganglionic colon--a strain of animals used as a model of Hirschsprung's disease. Microelectrode recordings from the circular muscle layer showed ongoing fluctuations in membrane potential (presumably junction potentials) that were sensitive to tetrodotoxin. Transmural electrical stimulation of the intrinsic nerves produced cholinergic excitatory and inhibitory junction potentials. The latter responses were more prolonged when recorded aboral to the stimulating electrodes. Apamin reduced, but did not block, these inhibitory potentials. Cholinergic excitatory junction potentials were abolished by d-tubocurarine when recording 2 mm from the stimulating electrodes; inhibitory junction potentials were recorded up to 10 mm in the presence of this drug. Thus, the intrinsic excitatory cholinergic motor neurons have relatively short projections to the circular smooth muscle compared with the projections of the inhibitory motor neurons.