Cell-type specific contributions to theta-gamma coupled rhythms in the hippocampus

The distinct inhibitory cell types that participate in cognitively relevant nested brain rhythms require that model circuits reflect this diversity. The co-expression of theta and gamma rhythms may represent a general coding scheme and particular changes in these coupled rhythms occur in disease states. We develop a population rate model of the CA1 hippocampus that encompasses circuits of three inhibitory cell types (bistratified cells, parvalbumin (PV)-expressing and cholecystokinin (CCK)-expressing basket cells) and pyramidal cells to examine coupled rhythms. We constrain parameters and perform theoretical and numerical analyses of the model. We find that CCK-expressing basket cells initiate the coupled rhythms and regularize theta, and PV-expressing basket cells enhance both theta and gamma rhythms. Pyramidal and bistratified cells govern the generation of theta rhythms, PV-expressing basket and pyramidal cells play dominant roles in controlling theta frequencies. We predict circuit motifs for theta-gamma coupled rhythms which may be generally applicable.