Low frequency voltage clamp: recording of voltage transients at constant average command voltage

We implemented a simple feedback system that modifies the conventional curr ent clamp mode of a patch clamp amplifier so that transient potentials, suc h as action potentials and synaptic potentials, can be measured as in the u sual current clamp, while the average membrane potential is kept constant a t a value chosen by the user. The circuit thus works like the current clamp for high frequency signals and like a voltage clamp for low frequency sign als. We delineate its transfer properties and give application examples. (C ) 2000 Elsevier Science B.V. All rights reserved.