The ionic channel of neural nicotinic acetylcholine receptors is funnel-shaped

Membrane currents evoked by iontophoretically applied acetylcholine were recorded from non-dissociated neurons of rat superior cervical ganglion using the whole-cell patch-clamp recording method. Blocking effects produced by a series of specially synthesized organic compounds, the blockers of the open channel of ganglionic nicotinic acetylcholine receptors, used as tools, were studied, and dimensions of the channel were deduced from correlation between the sizes of the blocking molecules and their blocking activities. Two channel cross-profiles were found, small and large, as approximated by the rectangles with the most probable dimensions 5.8 x 8.0 A and 7.0 x (8.4 - 9.0) A, at intra- and extracellular sides of the channel, correspondingly, in addition to the 6.1 x 8.3 A (medium) cross-profile found in our earlier work and localized between the two above. These findings indicate that the channel is funnel-shaped in the area where the open channel blockers are bound. The binding site is localized at the level of the medium cross-profile or between it and small cross-profile. The small cross-profile is probably a selectivity filter.