Synaptic inhibition of the M‐current: slow excitatory post‐synaptic potential mechanism in bullfrog sympathetic neurones.

1. Slow muscarinic excitatory post‐synaptic currents (slow e.p.s.c.s) generated by preganglionic nerve stimuli were recorded in voltage‐clamped bullfrog sympathetic neurones. 2. IM‐‐an outward, voltage‐dependent, K+‐current‐‐was inhibited during the slow e.p.s.c., and membrane conductance was reduced in a voltage‐dependent manner. 3. The slow e.p.s.c. was associated with reduced outward rectification in the steady‐state current‐‐voltage (I/V) curve at membrane potentials more positive than‐‐60 m V, with no change in the shape of the non‐rectifying part of the I/V curve at more negative potential. 4. The amplitude of the slow e.p.s.c. was reduced by membrane hyperpolarization, to zero at membrane potentials equal to, or more negative than, ‐60 m V. The voltage sensitivity of the slow e.p.s.c. accorded with that of IM. 5. It is concluded that the slow e.p.s.c. results from a selective inhibition of IM.