  GABAergic interneurons can be subdivided into three subclasses: parvalbumin
positive (PV), somatostatin positive (SOM) and serotonin positive neurons. With
principal cells (PCs) they form complex networks. We examine PCs and PV
responses in mouse anterior lateral motor cortex (ALM) and barrel cortex (S1)
upon PV photostimulation in vivo. In layer 5, the PV response is paradoxical:
photoexcitation reduces their activity. This is not the case in ALM layer 2/3.
We combine analytical calculations and numerical simulations to investigate how
these results constrain the architecture. Two-population models cannot account
for the results. Networks with three inhibitory populations and V1-like
architecture account for the data in ALM layer 2/3. Our data in layer 5 can be
accounted for if SOM neurons receive inputs only from PCs and PV neurons. In
both four-population models, the paradoxical effect implies not too strong
recurrent excitation. It is not evidence for stabilization by inhibition.
