Different synaptic channel kinetics in sympathetic B and C neurons of the bullfrog.

Sympathetic ganglia of the frog contain two types of principal neurons, B and C cells, which are innervated by two distinct classes of cholinergic preganglionic axons, B and C fibers. This study examined and compared the kinetics of nicotinic ACh-gated channels that produce the fast excitatory postsynaptic currents (EPSCs) in B and C cells. Neurons were identified and voltage-clamped at -50 mV, 22 degrees C. The EPSC decayed as a single exponential for both cell types, but the decay time constants differed considerably; 10.2 +/- 2.3 msec (mean +/- SD) for C cells and 5.5 +/- 1.1 msec for B cells. Analysis of ACh-induced membrane current fluctuations revealed spectra that fit single-Lorentzian functions and gave estimates of the mean channel open time of 9.8 +/- 2.0 msec and 5.2 +/- 0.9 msec for C and B cells, respectively. Thus, the clear difference in EPSC decay rate for these two types of ganglion cells is most likely due to the differences in the mean lifetimes of their synaptic channels.