Voltage- and activity-dependence of a novel chloride conductance dynamically controls the resting status of the intact rat sympathetic neuron

We discovered a novel voltage-dependent, non-inactivating chloride conductance in the intact, mature rat sympathetic neuron, by means of the two-electrode voltage-clamp technique. When the neuron is hyperpolarized, inward transient currents ensue; thereafter they decay over tens of seconds, accompanied by internal chloride ion redistribution (which however remains more concentrated than at equilibrium) and a parallel decrease in cell input conductance, suggesting that the current is sustained by channels initially open and then slowly closed by hyperpolarization. Here we report a remarkable activity-dependence of this conductance. Both direct and synaptic neuron tetanization (15 Hz, 10 s duration to saturate the response) result in a long-lasting (not less than 15 min) increase of cell input conductance (+70-150% 10 min after tetanus) and an inward current with the same time course. Following synaptic stimulation, both processes display similar properties under current- or voltage-clamp conditions, and upon direct stimulation they are unaffected by external calcium. The posttetanic effects are sustained by gCl increase, since both conductance and current modifications are blocked by 0.5 mM 9AC (a chloride channel blocker) but unaffected by TEACl or cesium chloride treatments. The chloride channel kinetic properties are modified by stimulation: their voltage sensitivity and rate of closure in response to hyperpolarization strongly increase. When the voltage dependence of the three major conductances governing the cell subthreshold status (gCl, gK and gL) is evaluated over the -40/-110 mV membrane potential range in resting or stimulated neurons, voltage-conductance profiles drastically change, due exclusively to gCl increase. This previously neglected, active chloride conductance moulds neuronal excitability below threshold, and appears to host an intrinsic mechanism, a memory of previous neuron activity, which makes the chloride current a likely candidate for natural controller of the balance between opposite resting currents, and thus of membrane potential level.