We tested the hypothesis that 5-HT promotes the differentiation of enteric neurons by stimulating a developmentally regulated receptor expressed by crest-derived neuronal progenitors. 5-HT and the 5-HT(2) agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine(.)HCl (DOI) enhanced in vitro differentiation of enteric neurons, both in dissociated cultures of mixed cells and in cultures of crest-derived cells isolated from the gut by immunoselection with antibodies to p75(NTR). The promotion of in vitro neuronal differentiation by 5-HT and DOI was blocked by the 5-HT(1/2) antagonist methysergide, the pan-5-HT(2) antagonist ritanserin, and the 5-HT(2B/2C)-selective antagonist SB206553. The 5-HT(2A)-selective antagonist ketanserin did not completely block the developmental effects of 5-HT. 5-HT induced the nuclear translocation of mitogen-activated protein kinase. This effect was blocked by ritanserin. mRNA encoding 5-HT(2A) and 5-HT(2B) receptors was detected in the fetal bowel (stomach and small and large intestine), but that encoding the 5-HT(2C) receptor was not. mRNA encoding the 5-HT(2B) receptor and 5-HT(2B) immunoreactivity were found to be abundant in primordial [embryonic day 15 (E15)-E16] but not in mature myenteric ganglia. 5-HT(2B)-immunoreactive cells were found to be a subset of cells that expressed the neuronal marker PGP9.5. These data demonstrate for the first time that the 5-HT(2B) receptor is expressed in the small intestine as well as the stomach and that it is expressed by enteric neurons as well as by muscle. It is possible that by stimulating 5-HT(2B) receptors, 5-HT affects the fate of the large subset of enteric neurons that arises after the development of endogenous sources of 5-HT.

. 5-HT2A receptors: location and functional analysis in intestines of wild-type and 5-HT2A knockout mice. Am J Physiol Gastrointest Liver Physiol 282: G877-G893, 2002. First published January 9, 2002 10.1152/ajpgi.00435.2001.-The distribution and function of the 5-hydroxytryptamine (5-HT2A) receptor were investigated in the intestines of wild-type (5-HT2A ϩ/ϩ) and knockout (5-HT2A Ϫ/Ϫ) mice. In 5-HT2A ϩ/ϩ 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 Ϫ/Ϫ colon did not express 5-HT2A receptors and did not contract in response to exogenous 5-HT. 5-HT2A Ϫ/Ϫ enterocytes were smaller, Paneth cells fewer, and muscle layers thinner (and showed degeneration) compared with those of 5-HT2A ϩ/ϩ 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 Ϫ/Ϫ and ϩ/ϩ mice did not differ significantly. serotonin receptors; intestinal motility; immunocytochemistry THE ENTERIC NERVOUS SYSTEM (ENS) is different from the autonomic innervation of other organs, because it can mediate coordinated behaviors of the gut without central nervous system input (28,32,53). The presence in the bowel of intrinsic primary afferent neurons (IPANs) enables the ENS to respond to luminal stimuli. Because no nerve fibers enter the lumen, Bü lbring and Crema (7) proposed that sensory transduction is transepithelial, involving the pressure-induced secretion of 5-HT from enterochromaffin (EC) cells to stimulate the mucosal processes of the submucosal IPANs that initiate reflexes. This hypothesis has since been confirmed, and EC cell-derived 5-HT is now thought to activate both peristaltic (21,36,37,47,50,63) and secretory reflexes (12,73).In addition to its role in the initiation of enteric reflexes, 5-hydroxytryptamine (5-HT) also functions in ganglionic neurotransmission within the ENS. A subset of myenteric interneurons is serotonergic (8,13,22,29,31,76,77); therefore, 5-HT antagonists can block peristaltic reflexes by inhibiting enteric serotonergic neurotransmission (46, 60, 82) as well as by interfering with the paracrine stimulation of IPANs. 5-HT from EC cells also plays a role in extrinsic sensation by stimulating extrinsic primary afferent nerves (2, 39, 40).The bowel contains an abundance of 5-HT receptor subtypes located on neurons, smooth muscle, and epithelial cells (24,25,30). Enteric neuronal 5-HT receptors include 5-HT 1A (26,48,49,62), 5-HT 1P (6, 59, 64), 5-HT 2B (18), 5-HT 3 (15,25,42,59), and 5-HT 4 (36,37,62,80). Enteric members of the 5-HT 2 family are associated with s...

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Serotonin (5-HT) is a mediator (through 5-HT1P receptors) of slow EPSPs in myenteric ganglia of the small intestine. The effect of 5-HT can be mimicked by elevating cAMP; therefore, we tested the hypothesis that the slow EPSP-like response to 5-HT is cAMP-mediated. Guinea pig gut was enzymatically dissociated; myenteric ganglia remained intact and were collected by filtration. Neurons in the isolated ganglia retained their ability to manifest the slow EPSP-like response to 5-HT. Exposure to 5-HT raised the ganglionic level of cAMP (ED50 0.3 microM). This effect was not antagonized by the 5-HT1P antagonist, N-acetyl-5-hydroxytryptophyl-5-hydroxytryptophan amide (100.0 microM), or mimicked by the 5-HT1P agonist, 5-hydroxyindalpine (10.0 microM). Increases in cAMP were also evoked by the 5-HT1 agonist, 5-carboxyamidotryptamine (10.0 microM), the 5-HT2 agonist, (+-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI; 1.0-10.0 microM), and by the 5-HT4 agonists, renzapride (1.0-10.0 microM) and 5-methoxytryptamine (1.0-10.0 microM); however, neither the 5-HT1/5-HT2 antagonists, spiperone, methysergide, and methiothepin, nor the 5-HT4 antagonist, tropisetron (ICS 205-930; 10.0 microM), were able to inhibit the rise in cAMP evoked by these compounds or by 5-HT (0.1-10.0 microM). The 5-HT-evoked elevation of cAMP was antagonized by ketanserin (10.0 microM), which also blocked the effects of 5-methoxytryptamine and DOI, but not those of renzapride. The effective concentration of DOI, however, was higher than that needed for activation of 5-HT2 receptors, and Northern analysis using a cDNA probe encoding the rat 5-HT2 receptor failed to reveal the presence of 5-HT2 mRNA in myenteric ganglia, although it hybridizes with mRNA of the right size in the guinea pig brain. Compounds that failed to change levels of cAMP or to antagonize the action of 5-HT included 8-hydroxy-di-n-propylamino tetralin, R58639, R88226, and sumatriptan. It is concluded that the receptor responsible for the 5-HT-induced rise in cAMP in ganglia isolated from the guinea pig myenteric plexus is not a known subtype of 5-HT receptor. Since the pharmacology of this novel receptor is different from that of the slow EPSP-like response to 5-HT, the receptor probably does not mediate the slow EPSP.

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