Expression and function of 5-HT4 receptors in the mouse enteric nervous system

The aim of the current study was to identify enteric 5-HT4 splice variants, locate enteric 5-HT4 receptors, determine the relationship, if any, of the 5-HT4 receptor to 5-HT1P activity, and to ascertain the function of 5-HT4 receptors in enteric neurophysiology. 5-HT4a, 5-HT4b, 5-HT4e, and 5-HT4f isoforms were found in mouse brain and gut. The ratio of 5-HT4 expression to that of the neural marker, synaptophysin, was higher in gut than in brain but was similar in small and large intestines. Submucosal 5-HT4 expression was higher than myenteric. Although transcripts encoding 5-HT4a and 5-HT4b isoforms were more abundant, those encoding 5-HT4e and 5-HT4f were myenteric plexus specific. In situ hybridization revealed the presence of transcripts encoding 5-HT4 receptors in subsets of enteric neurons, interstitial cells of Cajal, and smooth muscle cells. IgY antibodies to mouse 5-HT4 receptors were raised, affinity purified, and characterized. Nerve fibers in the circular muscle and the neuropil in ganglia of both plexuses were highly 5-HT4 immunoreactive, although only a small subset of neurons contained 5-HT4 immunoreactivity. No 5-HT4-immunoreactive nerves were detected in the mucosa. 5-HT and 5-HT1P agonists evoked a G protein-mediated long-lasting inward current that was neither mimicked by 5-HT4 agonists nor blocked by 5-HT4 antagonists. In contrast, the 5-HT4 agonists renzapride and tegaserod increased the amplitudes of nicotinic evoked excitatory postsynaptic currents. Enteric neuronal 5-HT4 receptors thus are presynaptic and probably exert their prokinetic effects by strengthening excitatory neurotransmission.