1,3-Di(2-tolyl)guanidine blocks nicotinic response in guinea pig myenteric neurons

Ditolylguanidine (DTG) is a ligand which binds with high affinity to neuronal sigma receptors. Activation of sigma receptors inhibits the release of acetylcholine (ACh) from guinea pig ileum myenteric plexus preparations. A study was therefore undertaken to investigate the action of sigma receptor ligands on single neurons. Nicotinic responses to locally applied ACh onto single neurons of the guinea pig ileum myenteric plexus were studied using intracellular recording techniques. DTG and (+)-SKF10047 (N-allylnormetazocine) produced a concentration-dependent suppression of the depolarization of enteric neurons evoked by ionophoresis of ACh. The EC₅₀ values for DTG and (+)-SKF10047 were 4.7 and 3.8 μM, respectively, and were similar to that for hexamethonium (3.2 μM). The inhibition of the ACh-depolarization was not mediated at sigma receptors because (-)-SKF10047 and Bridge-DPG (2-imino-1,3H-dibenzo[d,f]-[1,3]-diazepine), which are inactive at sigma receptors, were as potent as DTG and (+)-SKF10047. DTG and hexamethonium (each at 1 μM) were more effective blockers of ACh-induced inward currents at a holding potential of -100 mV than at -40 mV. This voltage dependence is consistent with a channel blocking mechanism. DTG (10 μM) did not affect the depolarization (mediated by 5-HT₃ receptors) induced by pressure application of 5-HT onto single neurons. DTG and Bridge-DPG inhibited contractures of the longitudinal muscle-myenteric plexus preparation elicited by dimethylphenylpiperazinium noncompetitively (EC₅₀ values were 8.0 and 12.3 μM, respectively) whereas DTG but not Bridge DPG inhibited 5-HT-induced contractions of the longitudinal muscle-myenteric plexus noncompetitively. The results indicate that DTG causes a voltage-dependent block of nicotinic-ion channels at low micromolar concentrations. It is concluded that this is unrelated to an action at sigma receptors.