A physiologically-evoked M1-muscarinic depolarization in guinea-pig inferior mesenteric ganglion neurons.

The possibility of physiologically-evoked muscarinic excitatory synaptic potentials was examined in the inferior mesenteric ganglion (IMG) with intracellular microelectrodes in vitro. Three types of depolarizing responses were evoked concurrently by colonic distension: (1) fast nicotinic excitatory postsynaptic potentials (EPSPs); (2) an 'intermediate' time course depolarization and (3) a long time course potential that persisted throughout the period of distension. After hexamethonium was superfused over the ganglia the long time course potential was observed in 91% of IMG neurons. Intermediate time course of depolarizations were observed in 100% of IMG neurons and correlated with each propulsive contraction of the distal colon. The intermediate depolarizations had an average amplitude of 1.8 +/- 0.1 mV (n = 175 individual events; 27 preparations) with an average duration of 11.9 +/- 0.8 sec (n = 28 individual events). The intermediate time course synaptic potentials were accompanied by an increase in input resistance of 15% (n = 6). Superfusion of atropine (1 microM; n = 6) on the IMG or the M1-selective antagonist pirenzepine (1 microM; n = 5) abolished the intermediate time course synaptic potentials during distension. Superfusion of the M2-selective antagonist AF-DX 116 (1 microM; n = 4) had no effect. In all preparations examined, distension-induced intermediate time course depolarizations were blocked by tetrodotoxin (TTX) (3 microM). Pressure ejection of carbachol (1 mM, 60-100 ms pulses) evoked depolarizations of similar amplitude (6.5 +/- 0.7 mV; n = 18) and duration to the intermediate depolarizations observed during propulsive contractions.(ABSTRACT TRUNCATED AT 250 WORDS)