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

Abstract

ABSTRACTWe 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 µM. 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 µs shut times. The resolution of our records was optimized to reach 5 µs, but was insufficient to resolve an unknown proportion of shorter shut times. Therefore, measured durations of openings are overestimated, and cited in brackets: τo1 (3 µs) elicited by agonist-binding at the δ-site, τo2 and τo3 (40 and 183 µs) by binding at the ε-site, and τo4 (752 µs) by binding at the ε- and δ-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 µM ACh, long bursts are shortened, with 100 µM ACh, to 5 ms, and further at higher concentrations. While ACh was the main agonist, Ebd bound more effectively than ACh to the ε-site.SIGNIFICANCETransmembrane currents were recorded from single nAChR molecules. Binding ACh at one of the external receptor sites opens the central current channel repeatedly for a period selected by the activated receptor site, and transmembrane current can flow with fixed 15 pA amplitude (same at various agonists and range of concentrations, unaffected by desensitization). Current flow is interrupted by fixed < 3 µs blocks of the central channel that border each opening. Sets of current block and opening can be repeated for receptor-determined periods of time and form bursts of channel openings. The results may help to interpret the structural reactions in the center of the molecule that are addressed in the introduction.