Excitation is balanced by inhibition to cortical neurons across a wide range of conditions. To understand how this relationship is maintained, we broadly suppressed the activity of parvalbumin-expressing (PV + ) inhibitory neurons and asked how this affected the balance of excitation and inhibition throughout auditory cortex. Activating archaerhodopsin in PV + neurons effectively suppressed them in layer 4. However, the resulting increase in excitation outweighed Arch suppression and produced a net increase in PV + activity in downstream layers. Consequently, suppressing PV + neurons did not reduce inhibition to principal neurons (PNs) but instead resulted in a tightly coordinated increase in both excitation and inhibition. The increase in inhibition constrained the magnitude of PN spiking responses to the increase in excitation and produced nonlinear changes in spike tuning. Excitatory-inhibitory rebalancing is mediated by strong PN-PV + connectivity within and between layers and is likely engaged during normal cortical operation to ensure balance in downstream neurons. Here, Moore et al. show how feedforward cortical circuitry rapidly rebalances excitation and inhibition. This explains how network-level optogenetic suppression of cortical inhibitory cells can paradoxically increase their spiking activity.
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