Amplification of high-frequency synaptic inputs by active dendritic membrane processes

ACTIVE membrane mechanisms have been found in the dendrites of many nerve cells(1-7). Their contribution to dendritic signal processing, however, remains unclear(8) because few experimental preparations allow for detailed characterization of dendritic physiology under normal conditions, To investigate the functional implications of active dendritic processes in an in vivo preparation, we compared the response properties of different types of non-spiking, motion-sensitive interneurons of the fly visual system, only one of which is equipped with a fast sodium inward current in its axon as well as in its dendrite. We report here that cells with fast activating sodium currents can respond to temporal changes in their synaptic input signals up to much higher frequencies than can those which lack such currents. Thus fast sodium currents lead to a frequency-dependent amplification of synaptic signals, enhancing cellular responses specifically to transient inputs which otherwise would be attenuated because of the passive properties of the dendritic tree.