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.
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