Further characterization of 5‐hydroxytryptamine receptors (putative 5‐HT_2B) in rat stomach fundus longitudinal muscle

Abstract

1 The present study was undertaken to isolate and characterize pharmacologically homogeneous populations of 5-hydroxytryptamine (5-HT) receptors from a possible mixed receptor population mediating contraction of the longitudinal muscle of rat stomach fundus. Our aim was to extend the pharmacological characterization of the 5-HT2B receptor which is reported to be expressed in this preparation. 2 To minimize spontaneous activity and any influence of circular muscle on the contractile response, narrow (-l1.5 x 20 mm) segments of mucosa-denuded longitudinal muscle were used. Under these conditions, blockade of monoamine oxidase with pargyline (1001M for 15 min) caused a leftward displacement of concentration-effect curves for both 5-methoxytryptamine (5-MeO-T) and tryptamine. Neither pargyline nor a number of uptake inhibitors affected responses to 5-HT. 3 In pargyline pretreated preparations, the order of potency of a number of tryptamine analogues was as follows: 5-MeO-T > a-Me-5-HT >1 5-HT> 5-carboxamidotryptamine (5-CT)> tryptamine> 2-Me-5-HT. In addition several ligands known to act as agonists at either 5-HT2A or 5-HT2c receptors including 1-m-chlorophenylpiperazine (m-CPP), Ru 24969, MK 212 and SCH 23390 were also agonists in rat fundus whilst sumatriptan, renzapride and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were very weak or inactive. With the exception of 2-Me-5-HT and m-CPP, most agonists produced monophasic concentration-effect curves consistent with an interaction at a single site. High concentrations of 2-Me-5-HT evoked relaxations which were blocked by phentolamine (1 pLM) suggesting an interaction with a-adrenoceptors. m-CPP often evoked biphasic concentration-effect curves with a second contractile phase which was insensitive to yohimbine at concentrations higher than required for antagonism of responses to 5-HT. 4 LY 53857, methiothepin, methysergide, ritanserin and ICI 170809 were potent but non-surmountable antagonists of 5-HT in rat fundus. In contrast, several ligands behaved as surmountable antagonists with the following order of potency: rauwolscine > yohimbine = mesulergine > mianserin = SB 204070 > WY 26703 > SB 200646> pirenpirone> renzapride. DAU 6285, granisetron, spiperone, ketanserin, phentolamine and GR 127935 did not affect responses to 5-HT at concentrations up to 1 pM. The agonist and concentration independent profile of antagonism supported a single site interaction for both agonists and antagonists. 5 We conclude that despite small differences concerning the enantiomeric selectivity and affinity of rauwolscine and yohimbine, the close pharmacological identity of 5-HT receptors in rat stomach fundus and the recently cloned 5-HT2B receptor is maintained. SB 200646, which demonstrates some selectivity for 5-HT receptors in rat stomach fundus, should provide a useful ligand for confirmation of this view and allow discrimination of 5-HT2B function both in vitro and in vivo.

Immunohistochemical localisation of the serotonin 5‐HT2B receptor in mouse gut, cardiovascular system, and brain

“…Only recently, the new subtype of 5-HT2 receptors, 5-HT2B, has been cloned from human, mouse and rat species [9,[13][14][15]. It has been shown to mediate the contraction of the rat stomach fundus [26] and human intestine [27]. Therefore, this report confirms the 5-HT2B localisation and the probable function of the 5-HT2B receptor in serotonin-mediated gut contractions.…”

5‐HT receptor types in the rat ileum longitudinal muscle: focus on 5‐HT2 receptors mediating contraction

“…3). This observation suggests that a population of 5-HT 2B receptors is present not only in the rat stomach fundus, 20 but also in the rat ileum. Indeed, mRNA encoding 5-HT 2B receptors has been extracted from rat small intestine.…”

“…On the smooth muscle of the stomach fundus, high-affinity 5-HT 2B receptors mediate contraction. 20 Recently McLean & Coupar found smooth muscle 5-HT 7 -like receptors and neuronal 5-HT 3 receptors to mediate the contraction of the rat jejunum. 19 Our aim was to characterize the 5-HT receptor mediating the main component of the motor response to 5-HT of the rat ileum (longitudinal orientation).…”

The human serotonin 5‐HT2B receptor: Pharmacological link between 5‐HT2 and 5‐HT1D receptors

“…The correlations are even stronger if we separate the antagonists from the agonists (Table 2): the human 5-HT2B pharmacology correlates with the pig 5-HT2C [20] (r = 0.489, P < 0.01, n = 35) or with the antagonist values (r = 0.708, P < 0.01, n = 23) but are not significant for the agonist values ( Table 2). The correlation with functional values of the rat fundus contractions [21] is significant (r = 0.723, P < 0.01, n = 14) as well as with those of agonists (r = 0.714, P < 0.05, n = 8) which indicates that the functionality of the 5-HT2B receptor is conserved. In addition, the recently reported human 5-HT2B receptor [22] has an identical sequence and its IP3 coupling values correlate strongly with our binding data (r = 0.964, P < 0.01, n = 7).…”