Expression and function of 5-HT4receptors in the mouse enteric nervous system

Abstract

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-HT_1P activity, and to ascertain the function of 5-HT₄ receptors in enteric neurophysiology. 5-HT_4a, 5-HT_4b, 5-HT_4e, and 5-HT_4f 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-HT_4a and 5-HT_4b isoforms were more abundant, those encoding 5-HT_4e and 5-HT_4f 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-HT_1P 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.