Comparison of 5-HT4 receptors in guinea-pig colon and rat oesophagus: effects of novel agonists and antagonists

Abstract

5-HT4 receptors in isolated distal colon myenteric plexus of guinea-pig, mediating contraction of longitudinal smooth muscle, have been further characterized by selective agonists and antagonists. The indole agonists, 5-HT and 5-methoxytryptamine (5-MeOT), were full agonists (relative to 5-HT) with potency values (pEC50) of 8.0 +/- 0.1 (n = 50) and 7.8 +/- 0.1 (n = 12), respectively. 5-HT4 receptor agonists of other structural classes, including benzimidazolones (BIMU 1 and BIMU 8), and benzamides ((S)-zacopride, (R)-zacopride, renzapride, SC 49518) were partial agonists with intrinsic activities less than that of 5-HT. In general, the potencies for these compounds at 5-HT4 receptors in guinea-pig colon were similar to the potencies seen in the rat isolated oesophagus, where 5-HT4 receptors mediate relaxation. GR 113808 ¿[1-[2-[(methylsulfonyl)amino]ethyl]-4-piperidinyl] methyl1-methyl-1H-indole-3-carboxylate¿, RS 39604 ¿1-[4-amino-5-chloro-2-(3, 5-dimethoxybenzyloxy)phenyl]-3[1-[2-[(methylsulfonyl)amino] ethyl]-4-piperidinyl]-1-propanone hydrochloride and SB 204070 ¿(1-n-butyl-4-piperidinyl)methyl 8-amino-7-chloro-1, 4-benzodioxane-5-carboxylate¿ antagonized 5-HT responses with pA2 values of 9.1 +/- 0.1, 9.0 +/- 0.2 and 11.0 +/- 0.1, respectively. These affinity values were similar to those obtained at 5-HT4 receptors in isolated rat oesophagus (9.0+/- 0.4, 9.3 +/- 0.1 and 10.6 +/- 0.1 respectively). Despite these operational similarities between 5-HT4 receptors in guinea-pig colon and rat oesophagus, several novel compounds have revealed important differences between 5-HT4 receptors in the two tissues. For example, the substituted benzoate, RS 23597 ¿3-(piperidine-1-yl) propyl-4-amino-5-chloro-2-methoxybenzoate hydrochloride, acted as a partial agonist (intrinsic activity 0.5) in guinea-pig colon with a potency of 7.6 +/-0.1 (n = 16). In isolated rat oesophagus, however, this compound was a surmountable antagonist (pA2 = 7.8 +/- 0.1) with no intrinsic activity. In contrast, the substituted naphthalimide (S)RS 56532 ¿(S)-6-amino-5-chloro-2-(1-azabicyclo[2, 2, 2]octan-3-yl) 2,3-dihydro-1H-benz[de] isoquinoline-1,3-dione hydrochloride¿, was a potent (pEC50 = 7.9 +/- 0.1), efficacious partial agonist (intrinsic activity = 0.8) in the rat oesophagus. However, in guinea-pig colon, it was a surmountable antagonist with an affinity (pKB) of 9.4 +/- 0.1. Furthermore, several novel, selective, 5-HT4 compounds also showed opposing patterns of intrinsic activities similar to those described for RS 23597 and (S)RS 56532. It is concluded that these differences are inconsistent with differences in 5-HT4 receptor reserves, and may suggest that 5-HT4 receptors in the guinea-pig colon and the rat oesophagus can be operationally distinguished.

TRPV1 Function in Mouse Colon Sensory Neurons Is Enhanced by Metabotropic 5-Hydroxytryptamine Receptor Activation

“…Whereas 5-HT 2 receptor agonists mimicked the facilitating action of 5-HT on TRPV1 function in response to acid, the HT 4 receptor agonists cisapride and 5-MeOT had potencies lower than that of 5-HT, thus behaving only as partial agonists. The structures of these compounds and/or their receptor coupling efficiencies might explain the different potencies (Leung et al, 1996). In addition, these agonists may affect other 5-HT receptors (Hensley and Cohen, 1992;Sebben et al, 1994;Tsou et al, 1994).…”

Cloning and Expression of a Human Serotonin 5‐HT4 Receptor cDNA

“…Its moderate potency in binding tests on both membrane preparations (pK1 of 6.7 for the human and guinea pig reeepior) is in good agreement with the first reports about the inhibition of 5-HT-stimulated AC on mouse eollieular neurons [pK, = 6.15 (Ansanay et al, 1992)] and binding data (pK1 of 6.7) on human brain membranes (Waeber et a]., 1993). The tested agonists, including the full agonists 5-HT and 5-MeOT and the partial agonist eisapride, exhibited lower affinities on both membrane preparations in competition with a radiolabeled antagonist than reported in functional tests on the guinea pig colon, where the pEC50 values were 8.0, 7.8, and 7.5, respectively (Schuurkes et al, 1985;Leung et al, 1996). Agonists bind with high affinity to the G protein-coupled state of the receptor and have lower affinity for the uncoupled receptor.…”

Differences in Signal Transduction of Two 5-HT4Receptor Splice Variants: Compound Specificity and Dual Coupling with Gαs- and Gαi/o-Proteins

“…5-HT 4 receptors have also been shown to be widely distributed in peripheral organs such as the gastrointestinal tract, the myocardium, the urinary bladder, and the adrenal glands (for review, see Ford and Clarke, 1993). Furthermore, pharmacological differences have been identified in various tissues and species, suggesting a heterogeneity of 5-HT 4 receptors (Ford and Clarke, 1993;Leung et al, 1996). Indeed, Gerald et al (1995) have initially cloned two alternative splice variants coding for short (5-HT 4S ) and long (5-HT 4L ) isoforms of the receptor, which could be the structural basis for functional diversity.…”