The second messenger cyclic AMP has been variously reported to affect the electrical activity of different neurones by decreasing outward potassium current1-3, increasing outward current4 and increasing inward current5-8. The recently developed patch clamp method of recording single ionic channels9 allows direct measurement of the action of cyclic AMP on membrane conductances. Using the patch clamp, the closure of potassium channels by cyclic AMP has previously been documented on the single channel level10. We report here that in a bursting molluscan neurone, intracellular iontophoresis of cyclic AMP under voltage clamp elicits an inward current of maximal amplitude in the pacemaker voltage region. Patch-clamp analysis reveals inward channels whose opening frequency is augmented by cyclic AMP stimulation and whose activity accompanies burst episodes. Channel opening frequency is significantly increased by depolarization of the whole soma, but not by focal depolarization of the patch; this may reflect the action of another second messenger that acts in concert with cyclic AMP to confer voltage sensitivity.
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