Sparsification of AP firing in adult-born hippocampal granule cells via voltage-dependent α5-GABAA receptors

GABA can depolarize immature neurons close to the action potential (AP) threshold in development and adult neurogenesis. Nevertheless, GABAergic synapses effectively inhibit AP firing in newborn granule cells of the adult hippocampus as early as 2 weeks post mitosis. The underlying mechanisms are largely unclear. Here we analyzed GABAergic inputs in newborn 2- to 4-week-old hippocampal granule cells mediated by soma-targeting parvalbumin (PV) and dendrite-targeting somatostatin (SOM) interneurons. Surprisingly, both interneuron subtypes activate 5-subunit containing GABAA receptors (5-GABAARs) in young neurons, showing a nonlinear voltage dependence with increasing conductance around the AP threshold. By contrast, in mature cells, PV interneurons mediate linear GABAergic synaptic currents lacking 5-subunits, while SOM-interneurons continue to target nonlinear 5-GABAARs. Computational modelling shows that the voltage-dependent amplification of 5-GABAAR opening in young neurons is crucial for inhibition of AP firing to generate balanced and sparse firing activity, even with depolarized GABA reversal potentials.