Hippocampal ripple oscillations and inhibition-first network models: frequency dynamics and response to GABA modulators

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Item Type:	Article
Title:	Hippocampal ripple oscillations and inhibition-first network models: frequency dynamics and response to GABA modulators
Creators Name:	Donoso, J.R., Schmitz, D., Maier, N. and Kempter, R.
Abstract:	Hippocampal ripples are involved in memory consolidation, but the mechanisms underlying their generation remain unclear. Models relying on interneuron networks in the CA1 region disagree on the predominant source of excitation to interneurons: either ‘direct', via the Schaffer collaterals that provide feedforward input from CA3 to CA1, or ‘indirect', via the local pyramidal cells in CA1, which are embedded in a recurrent excitatory-inhibitory network. Here, we used physiologically constrained computational models of basket-cell networks to investigate how they respond to different conditions of transient, noisy excitation. We found that direct excitation of interneurons could evoke ripples (140—220 Hz) that exhibited intra-ripple frequency accommodation (IFA) and were frequency-insensitive to GABA modulators, as previously shown in in-vitro experiments. In addition, the indirect excitation of the basket-cell network enabled the expression of IFA in the fast-gamma range (90—140 Hz), as in vivo. In our model, IFA results from a hysteresis phenomenon in which the frequency responds differentially to the rising and descending phases of the transient excitation. Such a phenomenon predicts a maximum oscillation frequency occurring several milliseconds before the peak of excitation. We confirmed this prediction for ripples in brain slices from male mice. These results suggest that ripple and fast-gamma episodes are produced by the same interneuron network that is recruited via different excitatory input pathways, which could be supported by the previously reported intralaminar connectivity bias between basket cells and functionally distinct subpopulations of pyramidal cells in CA1. Taken together, our findings unify competing inhibition-first models of rhythm generation in the hippocampus.
Keywords:	Basket Cells, CA1, Fast Gamma, Memory Consolidation, Network Oscillations, Sharp Wave/Ripple Complexes, Animals, Mice, Rats
Source:	Journal of Neuroscience
ISSN:	0270-6474
Publisher:	Society for Neuroscience
Volume:	38
Number:	12
Page Range:	3124-3146
Date:	21 March 2018
Official Publication:	https://doi.org/10.1523/JNEUROSCI.0188-17.2018
External Fulltext:	View full text on PubMed Central
PubMed:	View item in PubMed

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