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SUMMARY1. Post-synaptic currents and responses to ionophoretically applied acetylcholine (ACh) were recorded at 34-37 'C from rabbit superior cervical ganglion neurones clamped at -80 mV membrane potential. Atropine (1 #mU) was used to block muscarinic receptors.2. The fast excitatory post-synaptic current (e.p.s.c.) reversed at -9-6+ 17 mV and decayed with a single exponential time course. The e.p.s.c. decay time constant, Td, was 4 5+0 3 msec and increased as the membrane was hyperpolarized (e-fold increase in Td corresponded to 140 mV hyperpolarization). Miniature e.p.s.c.s (m.e.p.s.c.s) decayed with time constants similar to those of the e.p.s.c.3. The decay of the e.p.s.c. was slowed by lowering temperature but remained a single exponential; the changes Of Td with temperature followed the Arrhenius equation (Q10 = 3 7).4. In most of the neurones studied the analysis of ACh noise spectra revealed two kinetic components with mean time constants TN, = 11+04 msec and TN, = 5 0 + 0-Q msec. In a few neurones only the TN1 component was found. Similar two-component ACh noise spectra were observed in the neurones not treated with atropine. TN1 and TN, components revealed temperature dependences similar to each other and close to that of Td.5. The values of TN, and TN1 and the ratio between the contributions of the TN, and TN, components to the ACh noise spectrum did not depend on the dose of ACh.6. The single channel conductance is 36 + 3 pS. A single ACh quantum opens about 150 ionic channels and the e.p.s.c. consists of 4-243 quanta.7. It is suggested that in mammalian sympathetic ganglion neurones there are two types of nicotinic ACh receptor channels, with short and long lifetimes, and that the kinetics of e.p.s.c. and m.e.p.s.c. are determined by the activity of the longer lifetime channel type.

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SUMMARY1. The elementary currents flowing through single channels opened by acetylcholine were recorded in rat superior cervical ganglion neurones using patch-clamp methods. Acetylcholine (30 ,zM) was included in the patch electrode (cell-attached recordings) or applied by ionophoresis (outside-out configuration). All measurements were made at 23-25°C and mostly at -110 mV.2. Channel openings appeared both as single events and as bursts of events. One population of the currents observed had a conductance of 20-0 + 0-2 pS (mean+ s.E. of mean, n = 4). A second population had a conductance of about 50 pS, occurred more rarely, and was not included in further analysis. 3. Four channel closed time periods and two channel open time periods were found from the distributions of closed and open times. It was found that shorter channel openings (about 0-2 ms) appeared in isolation, whereas longer openings (duration 1-3 + 0-2 ms, n = 4) appeared as bursts of openings separated by the shortest channel closed time periods (about 0-15 ms). The next shortest closed time (about 2 ms) apparently corresponds to the lifetime of the channel not activated by acetylcholine. The two longer closed times (about 80 ms and 1 s) may reflect desensitization. The mean burst duration was 8-5 + 1-2 ms (n = 4), giving about six openings per burst.4. Because the time constant of decay of the excitatory post-synaptic current is more similar to the burst duration than to the duration of individual single openings, it is suggested that acetylcholine released from presynaptic nerves may result in a burst of openings rather than a single opening.5. On the basis of the above assumption, the rate constants were calculated for a sequential model in which acetylcholine binds to the receptor (forward rate k+1 = 2-3 x 107 M-1 S-1; reverse rate k11 = 1235 s-1) which then undergoes a conformational change to an open state (forward rate /? =6293 s-1; reverse rate = 894 s-1).6. When heptamethonium (30 ,sM) was added to the solution in the patch electrode, the burst duration was markedly shortened, but there was no change in the closed time between two openings within the burst. This effect was voltage-dependent,