ABSTRACT
Balanced excitation and inhibition contributes to clamping excitability, input gating, and dynamic range expansion in many brain circuits. However, it is unknown if the balancing mechanism operates at the level of networks, ensembles or individual projections. We optogenetically stimulated hippocampal CA3 neurons in hundreds of different combinations, and monitored CA1 neuron responses in mouse brain slices. We observed that all arbitrary input combinations from CA3, from tens of synapses to the order of single synapses, elicited excitation followed by tightly proportional inhibition. CA1 neurons summed these complementary inputs and exhibited gain control in the form of subthreshold divisive normalization (SDN). Biophysically, SDN emerged because inhibitory onset advanced toward excitatory onset with increasing input strength. This caused clipping of peak amplitudes and faster peak times, resulting in shared input information coding between the two. Our results suggest that SDN may be a general gain and timing control mechanism in balanced feedforward networks.