Temporal organization of GABAergic interneurons in the intermediate CA1 hippocampus during network oscillations.

Travelling theta oscillations and sharp wave-associated ripples (SWRs) provide temporal structures to neural activity in the CA1 hippocampus. The contribution of rhythm-generating GABAergic interneurons to network timing across the septotemporal CA1 axis remains unknown. We recorded the spike-timing of identified parvalbumin (PV)-expressing basket, axo-axonic, oriens-lacunosum moleculare (O-LM) interneurons, and pyramidal cells in the intermediate CA1 (iCA1) of anesthetized rats in relation to simultaneously detected network oscillations in iCA1 and dorsal CA1 (dCA1). Distinct interneuron types were coupled differentially to SWR, and the majority of iCA1 SWR events occurred simultaneously with dCA1 SWR events. In contrast, iCA1 theta oscillations were shifted in time relative to dCA1 theta oscillations. During theta cycles, the highest firing of iCA1 axo-axonic cells was followed by PV-expressing basket cells and subsequently by O-LM together with pyramidal cells, similar to the firing sequence of dCA1 cell types reported previously. However, we observed that this temporal organization of cell types is shifted in time between dCA1 and iCA1, together with the respective shift in theta oscillations. We show that GABAergic activity can be synchronized during SWR but is shifted in time from dCA1 to iCA1 during theta oscillations, highlighting the flexible inhibitory control of excitatory activity across a brain structure.