5HT2A receptors:: location and functional analysis in intestines of wild-type and 5-HT2A knockout mice

The distribution and function of the 5-hydroxytryptamine (5-HT2A) receptor were investigated in the intestines of wild-type (5-HT2A +/+) and knockout (5-HT2A -/-) mice. In 5-HT2A +/+ mice, rats, and guinea pigs, 5-HT2A receptor immunoreactivity was found on circular and longitudinal smooth muscle cells, neurons, enterocytes, and Paneth cells. Muscular 5-HT2A receptors were concentrated in caveolae; neuronal 5-HT2A receptors were found intracellularly and on the plasma membranes of nerve cell bodies and axons. Neuronal 5-HT2A immunoreactivity was detected as early as E14 in ganglia, intravillus nerves, and the deep muscle plexus. The 5-HT2A -/- colon did not express 5-HT2A receptors and did not contract in response to exogenous 5-HT. 5-HT2A -/- enterocytes were smaller, Paneth cells fewer, and muscle layers thinner (and showed degeneration) compared with those of 5-HT2A +/+ littermates. The 5-HT2A receptor may thus be required for the maintenance and/or development of enteric neuroeffectors and other enteric functions, although gastrointestinal and colonic transit times in 5-HT2A -/- and +/+ mice did not differ significantly.