Ascending neural pathways in the isolated guinea-pig ileum: effect of muscarinic M1, M2 and M3 cholinergic antagonists.

The effect of muscarinic cholinoceptor antagonists was investigated on the ascending neural pathways activated by electrical stimulation in the guinea-pig ileum. For comparison, prejunctional and postjunctional effects of muscarinic cholinoceptor antagonists were also studied on circular smooth muscle. A two-compartment (oral and anal compartments) bath was used to study the ascending neural pathways. These were activated by electrical field stimulation in the anal compartment and the resulting contraction of the intestinal circular muscle in the oral compartment was recorded isotonically. Pirenzepine (10-300 nM), a muscarinic M1 cholinoceptor antagonist, reduced the ascending neural contractions in a concentration-dependent fashion when applied either to the oral or anal compartments (11-52% and 13-55% inhibition, respectively, P < 0.05). Pirenzepine inhibited (31+/-7%, P < 0.05) the acetylcholine (100 nM)-induced contractions at a higher non-selective concentration (300 nM), while its effect on the electrically-induced contractions was biphasic (10 and 30nM: 8-15% increase, P<0.05; 100 and 300 nM: 16-28% inhibition, P<0.05). The muscarinic M2 cholinoceptor antagonist methoctramine (3-100 nM) did not modify the contractions produced by 100 nM acetylcholine, electrically-induced contractions and the ascending neural contractions (when applied to either compartment). Parafluorohexahydrosiladifenidol (3-100 nM), a muscarinic M3 cholinoceptor antagonist, inhibited the contractions produced by 100 nM acetylcholine (19-81% and 15-69%), electrically-induced contractions (11-71% and 12-72%) and the ascending neural contractions (13-76% and 866%) when applied to the oral compartment, but it was without effect when applied to the anal compartment. These studies suggest that in the enteric ascending neural pathway, muscarinic M1 receptors are involved in neuroneuronal transmission, muscle contraction is mediated by muscarinic M3 cholinergic receptors, whereas muscarinic M2 receptors do not seem to participate.