Synaptic input-selective homeostasis safeguards developing cortical neurons toward setpoint activity

Homeostatic plasticity stabilizes brain function by maintaining neuronal activity around a setpoint. Although homeostatic mechanisms are well characterized in mature circuits, it remains unknown how developing neurons, which progress from near silence at birth to sustained activity, establish and safeguard their setpoint activity levels against perturbations. Here we show that activity setpoints are intrinsic properties of neuronal identity actively achieved by cell-autonomous, targeted synaptic remodeling during development. Cortical layer 2/3 pyramidal cells were sparsely silenced from their birth by overexpressing the inward-rectifier potassium channel Kir2.1 to chronically suppress their excitability. These Kir2.1-expressing neurons, however, progressively overcame this perturbation during postnatal development and ultimately reached the activity levels comparable to neighboring control neurons. This recovery was accompanied by enhanced synaptic excitation and reduced inhibition that did not follow the classical global multiplicative synaptic scaling. Instead, excitation from infragranular layers, rather than layer 4 and layer 2/3, was selectively strengthened through increasing quantal amplitudes and unsilencing silent synapses that required the insertion of GluA2-containing AMPA receptors. Inhibition recruited by excitatory inputs was selectively reduced at specific pathways. These results identify a synaptic input-selective homeostatic program enabling cortical neurons to reach activity setpoints, revealing a new mechanism contributing to neurodevelopmental robustness.