SUMMARY Alzheimer's disease (AD) results in cognitive decline and altered network activity, but the mechanisms are unknown. To identify such mechanisms, we studied human amyloid precursor protein (hAPP) transgenic mice, which simulate key aspects of AD. Electroencephalographic recordings in hAPP mice revealed spontaneous epileptiform discharges, indicating network hypersynchrony, primarily during reduced gamma oscillatory activity. Because this oscillatory rhythm is generated by inhibitory parvalbumin (PV) cells, network dysfunction in hAPP mice might arise from impaired PV cells. Supporting this hypothesis, hAPP mice and AD patients had decreased levels of the interneuron-specific and PV cell–predominant voltage-gated sodium channel subunit Nav1.1. Restoring Nav1.1 levels in hAPP mice by Nav1.1-BAC expression increased inhibitory synaptic activity and gamma oscillations and reduced hypersynchrony, memory deficits, and premature mortality. We conclude that reduced Nav1.1 levels and PV cell dysfunction critically contribute to abnormalities in oscillatory rhythms, network synchrony, and memory in hAPP mice and possibly in AD.

GABA A receptors (GABA A Rs) assembled of different subunits mediate tonic and phasic inhibition in hippocampal neurons. CA1/CA3 pyramidal cells (PCs) predominantly express ␣5 subunits whereas dentate gyrus granule cells (DGGCs) and molecular layer (ML) interneurons predominantly express ␦ subunits. Both ␣5-and ␦-containing GABA A Rs mediate tonic inhibition. We have shown previously that mice lacking ␣5 subunits (Gabra5 Ϫ/Ϫ ) have a residual tonic current in CA1/CA3 PCs because of an upregulation of ␦ subunits, but the basis of the residual tonic current in DGGCs and ML interneurons of mice lacking the ␦ subunit (Gabrd Ϫ/Ϫ ) is still unknown. We now show that wild-type DGGCs have a small tonic current mediated by ␣5 subunit-containing GABA A Rs responsible for ϳ29% of the total tonic current. To better identify the GABA A Rs mediating tonic inhibition in hippocampal neurons, we generated mice lacking both ␣5 and ␦ subunits (Gabra5/Gabrd Ϫ/Ϫ ). Recordings from CA1/CA3 PCs, DGGCs, and ML interneurons in these mice show an absence of tonic currents without compensatory changes in spontaneous IPSCs (sIPSCs), sEPSCs, and membrane resistance. The absence of tonic inhibition results in spontaneous gamma oscillations recordable in vitro in the CA3 pyramidal layer of these mice, which can be mimicked in wild-type mice by blocking ␣5 subunit-containing GABA A Rs with 50 nM L-655,708. In conclusion, depending on the cell type, the ␣5 and ␦ subunits are the principal GABA A R subunits responsible for mediating the lion's share of tonic inhibition in hippocampal neurons.

Gamma frequency (30 -100 Hz) network oscillations occur in the intact hippocampus during awake, attentive behavior. Here, we explored the underlying cellular mechanisms in an in vitro model of persistent gamma-frequency oscillations, induced by bath application of 20 M carbachol in submerged hippocampal slices at 30 Ϯ 1°C. Current-source density analysis of the field oscillation revealed a prominent alternating sink-source pair in the perisomatic and apical dendritic regions of CA3. To elucidate the active events generating these extracellular dipoles, we examined the firing properties of distinct neuron types. Visually guided unit recordings were obtained from individual CA3 neurons followed by intracellular labeling for anatomical identification. Pyramidal cells fired at 2.82 Ϯ 0.7 Hz, close to the negative peak of the oscillation (0.03 Ϯ 0.65 msec), and often in conjunction with a negative spike-like component of the field potential. In contrast, all phase-coupled interneurons fired after this negative peak. Perisomatic inhibitory interneurons fired at high frequency (18.1 Ϯ 2.7 Hz), shortly after the negative peak (1.97 Ϯ 0.95 msec) and were strongly phase-coupled. Dendritic inhibitory interneurons fired at lower frequency (8.4 Ϯ 2.4 Hz) and with less fidelity and a longer delay after the negative peak (4.3 Ϯ 1.1 msec), whereas interneurons with cell body in the stratum radiatum often showed no phase relationship with the field oscillation. The phase and spike time data of individual neurons, together with the current-source density analysis, support a synaptic feedback model of gamma oscillations primarily involving pyramidal cells and inhibitory cells targeting their perisomatic region.

Gamma frequency network oscillations are assumed to be important in cognitive processes, including hippocampal memory operations, but the precise functions of these oscillations remain unknown. Here, we examine the cellular and network mechanisms underlying carbachol-induced fast network oscillations in the hippocampus in vitro, which closely resemble hippocampal gamma oscillations in the behaving rat. Using a combination of planar multielectrode array recordings, imaging with voltage-sensitive dyes, and recordings from single hippocampal neurons within the CA3 gamma generator, active current sinks and sources were localized to the stratum pyramidale. These proximal currents were driven by phase-locked rhythmic inhibitory inputs to pyramidal cells from identified perisomatic-targeting interneurons. AMPA receptor-mediated recurrent excitation was necessary for the synchronization of interneuronal discharge, which strongly supports a synaptic feedback model for the generation of hippocampal gamma oscillations.

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