Distinct Roles of SOM and VIP Interneurons during Cortical Up States

During cortical network activity, recurrent synaptic excitation among pyramidal neurons is approximately balanced by synaptic inhibition, which is provided by a vast diversity of inhibitory interneurons. The relative contributions of different interneuron subtypes to inhibitory tone during cortical network activity is not well-understood. We previously showed that many of the major interneuron subtypes in mouse barrel cortex are highly active during Upstates (Neske et al., 2015); while fast-spiking (FS), parvalbumin (PV) positive cells were the most active interneuron subtype, many non-fast-spiking (NFS), PV-negative interneurons were as active or more active than neighboring pyramidal cells. This suggests that the NFS cells could play a role in maintaining or modulating Upstates. Here, using optogenetic techniques, we further dissected the function al roles during Up states of two major NFS, PV-negative interneuron subtypes: somatostatin (SOM) positive cells and vasoactive intestinal peptide (VIP)-positive cells. We found that while pyramidal cell excitability during Upstates significantly increased when SOM cells were optogenetically silenced, VIP cells did not influence pyramidal cell excitability either upon optogenetic silencing or activation. VIP cells failed to contribute to Upstates despite their ability to inhibit SOM cells strongly. We suggest that the contribution of VIP cells to the excitability of pyramidal cells may vary with cortical state.