AMPLIFICATION OF HIGH-FREQUENCY SYNAPTIC INPUTS BY ACTIVE DENDRITIC MEMBRANE PROCESSES

ACTIVE membrane mechanisms have been found in the dendrites of many ne rve cells(1-7). Their contribution to dendritic signal processing, how ever, remains unclear(8) because few experimental preparations allow f or detailed characterization of dendritic physiology under normal cond itions, To investigate the functional implications of active dendritic processes in an in vivo preparation, we compared the response propert ies of different types of non-spiking, motion-sensitive interneurons o f the fly visual system, only one of which is equipped with a fast sod ium inward current in its axon as well as in its dendrite. We report h ere that cells with fast activating sodium currents can respond to tem poral changes in their synaptic input signals up to much higher freque ncies than can those which lack such currents. Thus fast sodium curren ts lead to a frequency-dependent amplification of synaptic signals, en hancing cellular responses specifically to transient inputs which othe rwise would be attenuated because of the passive properties of the den dritic tree.