We studied hippocampal sharp-wave ripples and theta and CA1 pyramidal cell activity during trace eyeblink conditioning in rabbits. Conditioning trials suppressed ripples while increasing theta for a period of several seconds. A quarter of the cells increased firing in response to the conditioned stimulus and fired extensively during endogenous theta as well as ripples. The role of endogenous theta epochs in off-line memory consolidation should be studied further.
The hippocampal dentate gyrus is known to be crucial for cognitive tasks involving so-called pattern separation and completion. The exact neuronal mechanisms behind these phenomena are still unclear, but it is hypothesized that sparse firing of dentate gyrus principal cells during exploration corresponds to separate patterns of contextual stimuli. Less is known of the activity of dentate principal cells when the cognitive task does not require the active involvement of external stimuli such as during immobility. Therefore, we recorded the activity of dentate gyrus principal cells, granule and mossy cells, and interneurons while the animal was immobile during a fixed interval task and a subsequent rest period. While interneurons were constantly active, both mossy and granule cells preferentially fired during the immobility period. However, all cell types fired during NREM sleep, especially during dentate spikes. Therefore, we were able to show that both principal cells and interneurons are active during the fixed interval tasks and subsequent sleep in distinct phases. This will allow future studies focusing on the role of different single cell activity of dentate gyrus during immobility.
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