Somatic EPSP amplitude is independent of synapse location in hippocampal pyramidal neurons

Most neurons receive thousands of synaptic inputs onto widely spread dendri tes. Because of dendritic filtering, distant synapses should have less effi cacy than proximal ones. To investigate this, we characterized the amplitud e and kinetics of excitatory synaptic input across the apical dendrites of CA1 pyramidal neurons using dual whole-cell recordings. We found that dendr itic EPSP amplitude increases with distance from the soma, counterbalancing the filtering effects of the dendrites and reducing the location dependenc e of somatic EPSP amplitude. Dendritic current injections and a multi-compa rtmental computer model demonstrated that dendritic membrane properties hav e only a minor role in elevating the local EPSP. Instead a progressive incr ease in synaptic conductance seems to be primarily responsible for normaliz ing the amplitudes of individual inputs.