Serotonin‐induced increase in cAMP in ganglia isolated from the myenteric plexus of the guinea pig small intestine: Mediation by a novel 5‐HT receptor

Abstract

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.

Review article: roles played by 5‐hydroxytryptamine in the physiology of the bowel

“…The transductional properties of the 5‐HT 1P receptor have been studied more recently than its operational characteristics. Since the high affinity binding of tritiated 5‐HT to enteric ganglionic membranes is inhibited by GTPγS, 78 , 79 it was inferred that the 5‐HT 1P receptor would probably turn out to be G protein‐coupled. This inference has since been confirmed by the electrophysiological observations that intracellular injection of GTPγS potentiates, while GDPβS and pertussis toxin inhibit, slow responses of myenteric neurones to 5‐HT; 76 , 80 furthermore, 5‐HT stimulates the binding of 35 S‐GTPγS to Go protein in isolated membranes prepared from myenteric ganglia 76 .…”

“…The likely transductional pathway from the 5‐HT 1P receptor to the calcium‐dependent potassium channel that is closed by the receptor appears to involve Go, a phosphatidyl choline‐phospholipase C, diacylglycerol, and both protein kinases A (PKA) and C (PKC) 80 . PKA is not directly activated by a Gs protein coupled to the 5‐HT 1P receptor, 76 although 5‐HT does cause an increase in cAMP to occur within myenteric neurones 79 . 5‐HT thus does not stimulate the binding of 35 S‐GTPγS to Gs protein in myenteric ganglionic membranes 76 .…”

5-HT2Areceptors: location and functional analysis in intestines of wild-type and 5-HT2Aknockout mice

“…There have been suggestions that a 5-HT 2 receptor might be involved in the modulation of enteric neuronal activity (19,35,43,68). The current study was thus undertaken to test the hypothesis that 5-HT 2A receptors are present on enteric neurons as well as on smooth muscle.…”

“…The 5-HT 2B receptor, which was originally known as the rat fundus receptor because of its location on the smooth muscle of gastric rumen (10,11,20,54,56), has now been shown also to be expressed on intestinal neurons, to be developmentally regulated, and to promote the development of enteric neurons (18). The 5-HT 2C receptor is not expressed in the gut (18).There have been suggestions that a 5-HT 2 receptor might be involved in the modulation of enteric neuronal activity (19,35,43,68). The current study was thus undertaken to test the hypothesis that 5-HT 2A receptors are present on enteric neurons as well as on smooth muscle.…”

Adenosine A2 receptor‐mediated excitation of a subset of AH/Type 2 neurons and elevation of cAMP levels in myenteric ganglia of guinea‐pig ileum

“…This apparent discrepancy between electrophysiological and biochemical data could be due to a reduced sensitivity of acutely dissociated ganglia (used in this study) to agonist-induced elevation of cAMP levels. In fact, a recent report by Fiorica-Howells et al 44 showed that, when acutely dissociated myenteric ganglia are given time to recover overnight in culture, they respond more vigorously to the agonist serotonin with an elevation of cAMP in the absence of forskolin.…”