Association cortical areas in the mouse contain a large population of fast-spiking GABAergic neurons that do not express parvalbumin

GABAergic neurons represent 10-15% of the neuronal population of the cortex but exert a powerful control over information flow in cortical circuits. The largest GABAergic class in the neocortex is represented by the parvalbumin-expressing (PV-INs) fast-spiking neurons, which provide powerful somatic inhibition to their postsynaptic targets. Recently, the density of PV-INs has been shown to be lower in associative areas of the mouse cortex as compared to sensory and motor areas. Modeling work based on these quantifications linked the low-density of PV-INs with specific computations of associative cortices. However, it is still unknown whether the total GABAergic population of association cortices is smaller or whether another GABAergic type can compensate for the low density of PV-INs. In the present study we investigated these hypotheses using a combination of neuroanatomy, mouse genetics and neurophysiology. We found that the GABAergic population of association areas is comparable to that of primary sensory areas, and it is enriched of fast-spiking neurons that do not express PV and were not accounted for by previous quantifications. We developed an intersectional viral strategy to demonstrate that the synaptic output of fast-spiking neurons is comparable across cortical regions. Our results provide quantifications of the density and output strength of fast-spiking GABAergic neurons and offers new biological constrains to refine current models of cortical computations.