Increased colonic transit in rats produced by a combination of a cholinesterase inhibitor with a 5-HT4 receptor agonist

Increased cholinergic stimulation and accelerated gastrointestinal (GI) transit may be produced by direct stimulation of the acetylcholine (ACh) receptor with an appropriate agonist by increased release of ACh from cholinergic nerve terminals or by a decreased removal or breakdown of ACh within cholinergic synapses. The acetylcholinesterase inhibitor, neostigmine, and the 5-HT(4) receptor partial agonist tegaserod, are two agents with known prokinetic activity which work by different mechanisms that result in increased levels of ACh at cholinergic synapses innervating intestinal smooth muscle. Here, we aimed to investigate the potential synergistic effect on colonic transit that may occur with concomitant use of these two agents. Colonic transit was indirectly assessed in rats via measurements of fecal pellet output every 30 min for 2.5 h following administration of neostigmine (0.003-0.1 mg kg(-1), i.p.), tegaserod (0.01-1.0 mg kg(-1), i.p.), or a combination of both compounds. When administered alone, neostigmine or tegaserod caused a dose-dependent increase in fecal pellet output. In combination, low doses of the two agents which did not produce statistically significant effects alone, compared to the vehicle, caused a significant increase in fecal pellet output. Combinations of higher doses of neostigmine and tegaserod did not display synergy. In summary, when combined at low doses, neostigmine and tegaserod produce synergistic effects manifested by a statistically significant increase in the expulsion of fecal pellets. A combination of an anticholinesterase agent with a 5-HT(4) receptor agonist may prove to be a useful therapeutic approach to treat conditions associated with slow GI transit.