Slow gamma rhythms in CA3 are entrained by slow gamma activity in the dentate gyrus

In hippocampal area CA1, slow (~25-55 Hz) and fast (~60-100 Hz) gamma rhythms are coupled with different CA1 afferents. CA1 slow gamma is coupled to inputs from CA3, and CA1 fast gamma is coupled to inputs from the medial entorhinal cortex (Colgin et al. 2009). CA3 gives rise to highly divergent associational projections, and it is possible that reverberating activity in these connections generates slow gamma rhythms in the hippocampus. However, hippocampal gamma is maximal upstream of CA3, in the dentate gyrus (DG) region (Bragin et al. 1995a). Thus, it is possible that slow gamma in CA3 is driven by inputs from DG. Yet, few studies have examined slow and fast gamma rhythms in DG recordings. Here, we investigated slow and fast gamma rhythms in paired recordings from DG and CA3 in freely moving rats in order to determine whether slow and fast gamma rhythms in CA3 are entrained by DG. We found that slow gamma rhythms, as opposed to fast gamma rhythms, were particularly prominent in DG. We investigated directional causal influences between DG and CA3 using Granger causality analysis and found that DG slow gamma influences CA3 slow gamma. Moreover, DG place cell spikes were strongly phase-locked to CA3 slow gamma rhythms, suggesting that DG excitatory projections to CA3 may underlie this directional influence. These results indicate that slow gamma rhythms do not originate in CA3 but rather that slow gamma activity upstream in DG entrains slow gamma rhythms in CA3.

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