Abstract
Sharp wave-ripple complexes (SW-Rs), a transient form of high-frequency field oscillations observed in the hippocampus, are thought to mediate memory consolidation. They are initiated mainly in hippocampal CA3 area and propagate to the entorhinal cortex through the subiculum; however, little is known about how SW-Rs are initiated and propagate. Here, we used functional multineuronal calcium imaging to monitor SW-R-relevant neuronal activity from the subiculum at single-cell resolution. An unexpected finding was that a subset of subicular neurons was activated immediately before hippocampal SW-Rs. The SW-R-preceding activity was not abolished by surgical lesion of the CA1-to-subiculum projection, and thus, it probably arose from entorhinal inputs. Therefore, SW-Rs are likely to be triggered by entorhinal-to-CA3/CA1 inputs. Moreover, the subiculum is not merely a passive intermediate region that SW-Rs pass through, but rather, it seems to contribute to an active modification of neural information related to SW-Rs.
Highlights
Sharp wave-ripple complexes (SW-Rs), a transient form of high-frequency field oscillations observed in the hippocampus, are thought to mediate memory consolidation
Spiking activity was optically captured from subicular neurons using functional multineuronal calcium imaging (fMCI), while local field potential (LFP) were recorded from CA1 (Fig. 2A). fMCI detects action potentials through action potential-evoked transient calcium elevations in the cell bodies of individual neurons[13]; simultaneous recording of cell-attached recordings and fMCI from the same cells revealed that spikes were tightly associated with individual somatic DF/F transients (Fig. 2B)
Some calcium transients were time-locked to SW-Rs; 10.0 6 2.8% of the calcium spikes occurred within 200 ms relative to the SW-R peak time
Summary
Sharp wave-ripple complexes (SW-Rs), a transient form of high-frequency field oscillations observed in the hippocampus, are thought to mediate memory consolidation They are initiated mainly in hippocampal CA3 area and propagate to the entorhinal cortex through the subiculum; little is known about how SW-Rs are initiated and propagate. The subiculum, which is located between the hippocampal CA1 region and the presubiculum, is the major target of CA1 output[6] and receives direct synaptic inputs from entorhinal cortical and subcortical regions[7,8,9] It may integrate information of the hippocampus and the entorhinal cortex; physiological evidence for this notion is still sparse, and the spatiotemporal neuronal activity in the subiculum during SW-Rs remains unclear. We unexpectedly found that a fraction of subicular activation occurred before hippocampal SW-R events
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