Acetylcholine-induced current fluctuations and fast excitatory post-synaptic currents in rabbit sympathetic neurones

Post-synaptic currents and responses to ionophoretically applied acetylcholine (ACh) were recorded at 34-37 degrees C from rabbit superior cervical ganglion neurones clamped at -80 mV membrane potential. Atropine (1 microM) was used to block muscarinic receptors. The fast excitatory post-synaptic current (e.p.s.c.) reversed at -9.6 +/- 1.7 mV and decayed with a single exponential time course. The e.p.s.c. decay time constant, tau d, was 4.5 +/- 0.3 msec and increased as the membrane was hyperpolarized (e-fold increase in tau d 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. The decay of the e.p.s.c. was slowed by lowering temperature but remained a single exponential; the changes of tau d with temperature followed the Arrhenius equation (Q10 = 3.7). In most of the neurones studied the analysis of ACh noise spectra revealed two kinetic components with mean time constants tau N1 = 1.1 +/- 0.1 msec and tau N2 = 5.0 +/- 0.5 msec. In a few neurones only the tau N1 component was found. Similar two-component ACh noise spectra were observed in the neurones not treated with atropine. tau N1 and tau N2 components revealed temperature dependences similar to each other and close to that of tau d. The values of tau N1 and tau N2 and the ratio between the contributions of the tau N1 and tau N2 components to the ACh noise spectrum did not depend on the dose of ACh. 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. 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.