Contribution of interneuron types to the emergence of fast network activity patterns in the mouse dentate gyrus

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Rhythmic activities of neuronal networks occur at various frequencies ranging from 0.5 up to 300 Hz. In this broad frequency range, particularly theta- (6-10 Hz) modulated gamma (30-130 Hz) oscillations attracted highest attention because their functional relationship to coding, storing and recall of information by synchronously active cell assemblies is most evident. In the dentate gyrus, as the input gate of the hippocampus, theta-modulated gamma activity is essential for spatial navigation and acquisition of new conscious memories in humans and rodents. Here, gamma activity patterns emerge in two spatially, temporally and spectrally distinct regimes, as low gamma at 30-75 Hz, occurring globally in the entire dentate gyrus, and as focal more spatially restricted high gamma activities at 75-150 Hz (Strüber et al., 2017; Lastoci & Klausberger, 2017). In contrast, high gamma in the dentate gyrus is not phase correlated to the remaining hippocampal subfields indicating that it is locally restricted to subnetworks of the dentate gyrus. Such local high-frequency gamma activities may allow local processing of incoming information and the segregation of information streams into non-overlapping memories. Thus, local high gamma activity may increase storage capacity of the dentate gyrus. The circuit mechanisms that may support the generation of gamma oscillations in the dentate gyrus remain, however, largely unknown. Here we aim to address this fundamental problem by first, recording action potentials from glutamatergic granule cells (GCs), the principal cells of the dentate gyrus, and GABAergic inhibitory interneurons by performing juxtacellular recordings from individual cells in the dentate gyrus of head-fixed non-anaesthetized mice exposed to a virtual reality. Second, we will fill cells during recording with a dye for subsequent morphological and immunohistochemical identification. Third, by analyzing the temporal relationship of single action potentials to the phases of the extracellularly recorded rhythmic field potentials, we aim to examine differences in the contribution of the various neuron types to theta as well as slow and fast gamma activity patterns. We believe that the results of this interdisciplinary research will provide new information on the cellular mechanisms underlying the generation of theta-modulated gamma oscillations in the rodent dentate gyrus and thereby will improve our understanding on the role of interneurons in cognitive functions.