Functional networks of inhibitory neurons orchestrate synchrony in the hippocampus

Inhibitory interneurons are a critical component of cortical circuits. Beyond providing inhibition, they have been proposed to coordinate the firing of excitatory neurons within cell assemblies. While many studies have dissected the function of specific interneuron subtypes, the relationship between interneurons and pyramidal cell synchrony in vivo remains unclear. We use an all-optical approach to simultaneously record hippocampal interneurons and pyramidal cells, and test the network influence of single interneurons. We show that CA1 interneurons form a functionally interconnected network that promotes synchrony through disinhibition during awake immobility without altering endogenous cell assemblies. A network model indicates that cell assemblies and dense but unspecific connectivity between interneurons are necessary ingredients to explain our experimental results. Thus, interneurons may not only operate via division of labor, but also through concerted activity. Our data challenge the idea of a purely decorrelating or segregating function of inhibition.