Rectification of synaptic and acetylcholine currents in the mouse submandibular ganglion cells.

1. Synaptic currents and responses to acetylcholine (ACh) were recorded from mouse submandibular ganglion (SMG) cells under whole-cell voltage clamp. 2. The peak amplitude of excitatory synaptic currents (ESCs) as well as the currents evoked by the ionophoretic application of ACh followed a unique non-linear current-voltage (I-V) relation. The chord conductance of the whole-cell currents decreased with depolarization of the membrane potential and became virtually 0 at 50 mV. 3. The decay of ESCs was described by two exponential functions. Both the fast (tau f) and slow (tau s) time constants were sharply decreased at depolarizing potentials beyond -40 mV, being insensitive to hyperpolarizing potentials more than -50 mV. 4. Single ACh receptor channels were characterized by the whole-cell current noise analysis. The single-channel currents followed Ohm's law at negative membrane potentials but tended to reach a plateau at positive membrane potentials. The mean slope conductance measured between -40 and -20 mV was 28.5 pS. 5. The product of the number of functional channels (N) and the probability of a channel being open (p) showed a steep voltage dependence. The value of Np at 20 mV was only 31% of that at -20 mV. 6. The noise power spectrum was best fitted by a double-Lorentzian function. Both the fast (tau f) and slow (tau s) time constants were sharply decreased by depolarizations beyond -20 mV. being less sensitive to membrane potentials more negative than -30 mV. 7. The non-linear I-V relation of ESCs was attributed in part to the voltage dependence of p and in part to the voltage dependence of the single-channel conductance (gamma) of ACh receptor channels.