5-HT4 receptor agonists enhance both cholinergic and nitrergic activities in human isolated colon circular muscle

Previous studies have demonstrated mixed inhibitory and facilitatory effects of 5-hydroxytryptamine-4 (5-HT(4)) receptor agonists on electrical field stimulation (EFS)-induced responses in human isolated colon. Here we report three types of responses to EFS in human isolated colon circular muscle: monophasic cholinergic contraction during EFS, biphasic response (nitrergic relaxation during EFS followed by cholinergic contraction after termination of EFS) and triphasic response (cholinergic contraction followed by nitrergic relaxation during EFS and a tachykininergic contraction after EFS). The effects of two 5-HT(4) receptor agonists, prucalopride and tegaserod were then investigated on monophasic responses only. Each compound inhibited contractions during EFS in a concentration-dependent manner. In the presence of N(omega)-nitro-l-arginine methyl ester (l-NAME) however, prucalopride and tegaserod enhanced the contractions in a concentration-dependent manner. In strips where the tone was elevated with substance-P and treated with scopolamine, EFS-induced relaxations were enhanced by the two agonists. The above observed effects by the two agonists were abolished by 5-HT(4) receptor antagonist SB-204070. The two agonists did not alter the tone raised by substance-P in the presence of scopolamine and l-NAME and did not affect carbachol-induced contractions in the presence of tetrodotoxin. These results suggest that in the circular muscle of human colon, 5-HT(4) receptor agonists simultaneously facilitate the activity of neurones which release the inhibitory and excitatory neurotransmitters, nitric oxide and acetylcholine respectively.     

Synergy between 5-HT4 receptor activation and acetylcholinesterase inhibition in human colon and rat forestomach

Abstract 5‐Hydroxytryptamine (5‐HT₄) receptor agonists increase gastrointestinal (GI) motility by enhancing enteric acetylcholine release which is then metabolized by acetylcholinesterase (AChE) to inactive metabolites. As both AChE inhibitors and, more usually, 5‐HT₄ receptor agonists are used to increase GI motility, an understanding of how these two different types of drugs might interact becomes of great importance. Our aim was to investigate the hypothesis that the effect of AChE inhibition will synergise with the ability of 5‐HT₄ receptor agonism to increase cholinergic activity, leading to an effect greater than that evoked by each action alone. We tested the activity of the 5‐HT₄ receptor agonist, prucalopride (10 nmol L^?1–30 μmol L^?1) and an AChE inhibitor, neostigmine (1 nmol L^?1–10 μmol L^?1) on cholinergically mediated contractions elicited by electrical field stimulation of human isolated colon circular muscle and rat isolated forestomach longitudinal strips. The experiments with human colon were performed in the presence of an inhibitor of nitric oxide synthase (N^ω‐nitro‐l ‐arginine methyl ester, 300 μmol L^?1). Prucalopride and neostigmine both enhanced cholinergic contractions in both tissues. The effect of prucalopride was inhibited in both tissues by SB‐204070, a 5‐HT₄ receptor antagonist. In the presence of a minimum effective concentration of neostigmine (30 nmol L^?1) and a submaximum concentration of prucalopride (3 μmol L^?1) the enhancement of contractions was greater than either compound alone in both tissues. These data demonstrate that the combination of prucalopride and neostigmine potentiate cholinergic contractions more than their arithmetic sum of their individual values. The results suggest that a synergy between 5‐HT₄ receptor agonism and AChE inhibition could be established pharmacologically which could be utilized as a novel prokinetic approach to functional GI disorders.     

GSK962040: a small molecule, selective motilin receptor agonist, effective as a stimulant of human and rabbit gastrointestinal motility

Abstract There is an urgent clinical need for a safe, efficacious stimulant of gastric emptying; current therapies include erythromycin (an antibiotic with additional properties which preclude chronic use) and metoclopramide (a 5‐hydroxytryptamine type 4 receptor agonist and an antagonist at brain D₂ receptors, associated with movement disorders). To move away from the complex motilide structure of erythromycin, a small molecule motilin receptor agonist, GSK962040, was identified and characterized. The compound was evaluated using recombinant human receptors, rabbit and human isolated stomach preparations known to respond to motilin and in vivo, by measuring its ability to increase defecation in conscious rabbits. At the human motilin receptor, the pEC₅₀ (the negative logarithm to base 10 of the EC₅₀ value, the concentration of agonist that produces 50% of the maximal response) values for GSK962040 and erythromycin as agonists were, respectively, 7.9 and 7.3; GSK962040 had no significant activity at a range of other receptors (including ghrelin), ion channels and enzymes. In rabbit gastric antrum, GSK962040 300 nmol L^?1–10 μmol L^?1 caused a prolonged facilitation of the amplitude of cholinergically mediated contractions, to a maximum of 248 ± 47% at 3 μmol L^?1. In human‐isolated stomach, GSK962040 10 μmol L^?1, erythromycin 10 μmol L^?1 and [Nle¹³]‐motilin 100 nmol L^?1, each caused muscle contraction of similar amplitude. In conscious rabbits, intravenous doses of 5 mg kg^?1 GSK962040 or 10 mg kg^?1 erythromycin significantly increased faecal output over a 2‐h period. Together, these data show that GSK962040, a non‐motilide structure, selectively activates the motilin receptor. Simplification of the structural requirements to activate this receptor greatly facilitates the design of potentially new medicines for gastroparesis.