The characteristics of synaptic currents and responses to acetylcholine of rat submandibular ganglion cells

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Abstract
Synaptic currents and responses to acetylcholine (ACh) were recorded at 20.degree. C from rat submandibular ganglion cells by a 2 microelectrode voltage-clamp technique. The peak amplitude (ap) of excitatory synaptic currents (e.s.c.) was linearly related to membrane potential (Em), with a reversal potential close to -10 mV. E.s.c. decayed with a bi-exponential time course, the fast phase comprising just over half the total amplitude. The time constant (.tau.f) of the fast phase was 5-9 ms, while that of the slow phase (.tau.s) was 27-45 ms. The relative amplitudes of the 2 components remained constant at different membrane potentials, showing that the reversal potential was the same for both. Both .tau.f and .tau.S increased as the cell was hyperpolarized, the ratio .tau.(-80)/.tau.(-40) being about 1.6 for both fast and slow components. Increasing the Ca concentration from 2.5-7.5 mM increased the amplitude of both components by .apprx. 40% and also prolonged the synaptic currents 30-50%, its effect being slightly greater on .tau.s than on .tau.f. In contrast to e.s.c., spontaneous or K-evoked miniature synaptic currents (m.s.c.s) showed a simple exponential decay with a time constant (.tau.m.s.c.) very similar to .tau.f.cntdot. .tau.m.s.c. showed the same sensitivity to membrane potential and calcium concentration as .tau.f. In the presence of neostigmine (10 .mu.M) e.s.c. were prolonged, .tau.f .apprx. 3.5-fold and .tau.s .apprx. 2.5-fold. The decay remained bi-exponential, with little change in the relative amplitude or voltage-dependence of the 2 components. M.s.c. were prolonged to a lesser extent (1.5- to 2-fold) and the voltage dependence of .tau.m.s.c. was unaffected by neostigmine. Reduction of the quantal content of the e.s.c. by low Ca-high Mg solution did not affect the time course. The relative amplitudes, and the time constants of the 2 components were unchanged even with a 90% reduction of ap. Voltage-jump studies, in which the cell was abruptly hyperpolarized by 20-40 mV during a response to ionophoretically applied ACh, showed a relaxation pattern consisting of 2 distinct exponential components, whose relative amplitudes varied considerably in different cells. The 2 rate constants .tau.f.rel and .tau.S.rel were somewhat shorter than .tau.f and .tau.s for e.s.c.s, the difference being genrally less than 2-fold. Measurements of ACh noise revealed 2 kinetic components, the time constants of which corresponded closely to .tau.f and .tau.s for e.s.c. On the assumption that the 2 components represent channels of equal conductance, the single channel conductance, .gamma., was calculated to be 31 .+-. 3 pS, similar to that of endplate channels. Two kinetic components of e.s.c. and ACh responses probably represent 2 distinct classes of ACh-operated ionic channels, whose mean lifetime differs .apprx. 5-fold. The 2 types of channel show the same ionic selectivity and their mean lifetime varies in the same way with the membrane potential. The absence of a slow component in m.s.c. suggests that the 2 types of channels are spatially separate in the membrane.