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Acetylcholine Exerts Inhibitory and Excitatory Actions on Mouse Ileal Pacemaker Activity: The Role of Muscarinic versus Nicotinic Receptors.

The effect of acetylcholine (ACh) on pacemaking and spontaneous contractions in the gastrointestinal tract is not well-characterized. The current study aims to profile the effect of several muscarinic and nicotinic receptor agonists and antagonists on pacemaker potentials in the ICR mouse ileum. Pacemaker potentials of whole-thickness mouse ileal segments were recorded extracellularly using a 60-channel microelectrode array (MEA) platform. A spatiotemporal analysis integrated the frequency, amplitude and velocity measurements of pacemaker currents. Comparative data were obtained by recording spontaneous smooth muscle tone in a conventional organ bath. On the MEA, ACh (0.3-300 μM) and bethanechol (0.3-300 μM) reduced ileal pacemaker potentials, significantly. The inhibitory effect of ACh was mimicked by donepezil (300 μM), but not nicotine (0.3-7 mM). Atropine (300 μM), but not hexamethonium (300 μM), reversed the inhibitory actions of ACh and bethanechol and revealed excitatory properties manifested as increases in pacemaker frequency. A spatial analysis also revealed that atropine, but not hexamethonium, reversed the ACh-induced distortion of pacemaker propagation activity. Atropine (0.001-3 mM) and hexamethonium (0.3-7 mM) alone were inactive. In the organ bath, ACh (300 nM) and bethanechol (30 μM) induced ileal tonic contractions, while inhibiting basal spontaneous contractions at 300 μM. Atropine (1 μM), but not hexamethonium (1-300 μM), reversed both the tonic contractions and the inhibition of the spontaneous contractions of ACh and bethanechol, and revealed an excitatory effect manifested as an increasing in frequency of contractions. Muscarinic, but not nicotinic, receptors appear to mediate the inhibitory actions of ACh on mouse ileal pacemaker potentials.