Flickering of a nicotinic ion channel to a subconductance state

Nicotinic acetylcholine channels show bursts of activity where open channel currents are separated from each other by short closed periods called flickers. These flickers presumably represent transitions from the open state to the state preceding the first opening of a burst (doubly liganded, closed state). Using tissue cultured chick pectoral muscle, we have examined the amplitude distribution of flickers. Of those events sufficiently long to permit accurate measurement of the amplitude (approximately 25% of all flickers), approximately two-thirds had a mean current equal to 10% of the fully open channel. The remaining one-third did appear to close completely. The subconducting flicker state is not a requisite step preceding channel opening. We conclude that there are three types of flicker events: a short event (time constant approximately 0.1 ms) whose current distribution is uncertain and two longer events (time constant approximately 1 ms), one of which has a current approximately 10% of the main open state and the other of which has a current indistinguishable from zero. In contrast, the amplitude of flickers induced by the local anesthetic QX-222 is indistinguishable from zero.