Channel opening duration in adult muscle nAChRs determined by activated external ACh binding site

We recorded currents through the cell membrane at single nAChR molecules, held at ACh or Epibatidine (Ebd) concentrations of 0.01, 0.1, 1, 10 or 100 M. The measured current amplitudes had an absolutely fixed value of 15 pA. This was valid for different agonists at all concentrations. Binding an agonist at one or both sites in the ring of subunits allowed to open the channel, the site that initiated the opening determined the duration of the final opening of the channel. In addition, the current flow was continuously interrupted by < 3 s shut times. The resolution of our records was optimized to reach 5 s, but was insufficient to resolve an unknown proportion of shorter shut times. Therefore, measured durations of openings are overestimated, and cited in brackets: {tau}o1 (3 s) elicited by agonist-binding at the {delta}-site, {tau}o2 and {tau}o3 (40 and 183 s) by binding at the {epsilon}-site, and {tau}o4 (752 s) by binding at the {delta}- and {epsilon}-site. Mono-liganded nAChRs trigger short bursts of 0.6 ms duration. Bi-liganded nAChRs generate long bursts that at low agonist concentrations last 12 ms. Above 10 M ACh, long bursts are shortened, with 100 M ACh, to 5 ms, and further at higher concentrations. While ACh was the main agonist, Ebd bound more effectively than ACh to the {epsilon}-site.