Progression of M-current inhibition after agonist removal

1. With the use of a fast-flow system, brief applications (40 ms) of acetylcholine on whole cell voltage-clamped dissociated bullfrog sympathetic neurons caused an inward current due to the opening of channels associated with nicotinic acetylcholine receptors. In cells clamped at -39 mV, a slower inward current developed after a latency of approximately 200 ms, owing to M-current inhibition. 2. The second phase of the response was mediated by the activation of muscarinic receptors and evoked by muscarine. 3. The time elapsed from the termination of maximal agonist application (10 microM muscarine) to the initiation of recovery was 4.5 s. This value was independent of the duration of the stimulus between 0.5 and 10 s. In contrast, applications > 4 s were necessary to reach 95% of the maximal inhibition during agonist application. 4. These results have three important implications. 1) The M-current modulation time course does not reflect either the time course of muscarinic receptor activation, or the time course of second-messenger production. 2) The latency of M-current inhibition is not due to the distance between muscarinic acetylcholine receptor (AChR) and nicotinic AChR. 3) Saturating concentrations of the second messenger are produced in < 500 ms in response to maximal concentrations of agonist.