Correction of conductance measurements in non-space-clamped structures:: 1.: Voltage-gated K+ channels

To understand functions of a single neuron, such as propagation and generation of synaptic or action potentials, a detailed description of the kinetics and distribution of the underlying ionic conductances is essential. In voltage-clamp experiments, incomplete space clamp distorts the recorded currents, rendering accurate analysis impossible. Here, we present a simple numerical algorithm that corrects such distortions. The method performs a stepwise approximation of the conductance density at the site of a local voltage clamp. This is achieved by estimating membrane conductances in a simulation that yields simulated clamp currents, which are then fitted to the distorted recordings from the non-space-clamped structure, relying on accurately reconstructed cell morphology and experimentally determined passive properties. The method enabled accurate retrieval of the local densities, kinetics, and density gradients of somatic and dendritic channels. Neither the addition of noise nor variation of passive parameters significantly reduced the performance of the correction algorithm. The correction method was applied to two-electrode voltage-clamp recordings of K+ currents from the apical dendrite of layer 5 neocortical pyramidal neurons. The generality and robustness of the algorithm make it a useful tool for voltage-clamp analysis of voltage-gated currents in structures of any morphology that is amenable to the voltage-clamp technique.