Using single-channel recording, we have been able to obtain some insight into the molecular mechanism of a modulatory transmitter action in Aplysia sensory neurones. Our results show that serotonin produces a slow EPSP and increases action potential duration in the sensory neurones by producing prolonged closures of the S potassium channel. Such closures appear to be mediated by cyclic AMP-dependent phosphorylation of a membrane protein which may be the channel. Modulation of S channels by serotonin also occurs in sensory neurone growth cones. This provides the first direct evidence that channel modulation occurs in nerve processes and increases the likelihood of channel modulation at the nerve terminal.

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