Molecular mechanisms of open-channel blockade in nicotinic acetylcholine receptors of autonomic ganglia neurons

Tetraethylammonium ions block the nicotinic cholinergic receptors of cochlear outer hair cells

Cochlear outer hair cells (OHCs) express nicotinic acetylcholine receptors (nAChRs) whose calcium permeability allow the activation of co-localized Ca(2+)-sensitive K+ channels (SK-type). The large organic cation tetraethylammonium (TEA) is known to block at millimolar concentration voltage-gated and Ca(2+)-activated K+ currents in OHCs. In the present study, we show that extracellular TEA blocked much more efficiently, at micromolar concentrations, ACh-evoked K+ currents in isolated guinea pig OHCs. The dose-inhibition curve indicated an IC(50) of 60 microM, a value two orders of magnitude lower than the one reported on SK or BK channels. The site of the blocking action was on the extracellular side of the plasma membrane since 10 mM intracellular TEA did not prevent or change the characteristics of the ACh-evoked K+ current. The block of this K+ current in OHCs was mainly explained by a direct action of TEA at the nAChRs. Indeed, we demonstrated that extracellular TEA inhibited directly the ionotropic cation current flowing through the nAChRs (IC(50)=30 microM). This study demonstrated for the first time that extracellular TEA is an effective blocker of the OHCs' nAChRs.

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.