Muscarinic synaptic potentials in guinea-pig myenteric plexus neurones

1. Intracellular recordings were made from neurones in the myenteric plexus of the guinea-pig ileum in vitro. 2. A single pulse stimulus to the presynaptic fibres entering a myenteric ganglion elicited a fast excitatory post-synaptic potential (e.p.s.p.) in type 1 (S) cells: in about one-quarter of cells this was followed by a slow e.p.s.p. 3. The slow e.p.s.p. had a latency of 122 ms and a total duration of 3-15 s. It was reversibly abolished by hyoscine (1 microM). 4. Both the fast e.p.s.p. and the slow e.p.s.p. could be mimicked by ionophoretic application of a single brief pulse of acetylcholine (ACh) to the neurone soma. The depolarization which mimicked the fast e.p.s.p. was reversibly blocked by hexamethonium. The depolarization which mimicked the slow e.p.s.p. was reversibly blocked by hyoscine. 5. Type 2 (AH) neurones did not show fast or slow e.p.s.p.s in response to a single pulse stimulus. Stimuli comprising many pulses (typically 10 Hz for 3 s) evoked a slow e.p.s.p. in which the initial portion was partly blocked by hyoscine, but which was predominantly non-cholinergic.