Phase-coupled oscillator models can predict hippocampal inhibitory synaptic connections

Linking minimal and detailed models of CA1 microcircuits reveals how theta rhythms emerge and their frequencies controlled

The wide variety of cell types and their biophysical complexities pose a challenge in our ability to understand oscillatory activities produced by cellular-based computational network models. This challenge stems from their high-dimensional and multiparametric natures. To overcome this, we implement a solution by linking minimal and detailed models of CA1 microcircuits that generate intrahippocampal (3-12 Hz) theta rhythms. We leverage insights from minimal models to guide explorations of more detailed models and obtain a cellular perspective of theta generation. Our findings distinguish the pyramidal cells as the theta rhythm initiators and reveal that their activity is regularized by the inhibitory cell populations, supporting a proposed hypothesis of an "inhibition-based tuning" mechanism. We find a strong correlation between input current to the pyramidal cells and the resulting local field potential theta frequency, indicating that intrinsic pyramidal cell properties underpin network frequency characteristics. This work provides a cellular-based foundation from which in vivo theta activities can be explored.

Spatial coherence and stationarity of local field potentials in an isolated whole hippocampal preparation in vitro

Local field potential (LFP) multielectrode recordings of spontaneous rhythms in an isolated whole hippocampal preparation are characterized with respect to their spatial variability within the hippocampus, and their frequency properties. Using simulated data, we categorize potential relationships between frequency variation over time in LFP recordings and spatial variability between electrodes. We then use data recorded from the intact preparation to distinguish between our theoretical categories. We find that the LFP recordings have a close to spatially invariant frequency distribution (not phase) across the hippocampus, and differ in frequency only in a component that may be seen as physiological noise. From these facts, we conclude that the isolated hippocampal LFP recordings represent a single signal and may be regarded as a unitary circuitry. We additionally examine phase differences across our recording sites. We use our characterization of the hippocampal isolate's properties to predict its spatial coherence in response to high frequency stimulation. We find that there is a finely tuned inverse relationship between temporal variability in the hippocampal isolate's LFP recordings and their spatial coherence.

Bidirectional multisite seizure propagation in the intact isolated hippocampus: the multifocality of the seizure "focus"

Localizing the seizure focus is difficult and frequently, multiple sites are found. This reflects our poor understanding of the fundamental mechanisms of seizure generation and propagation. We used multisite electrophysiological recordings in two seizure models and voltage-sensitive dye imaging, to spatiotemporally characterize the initiation and propagation of seizures in an intact epileptogenic brain region, the isolated hippocampus. In low-magnesium perfusate, seizures always originated in the temporal region, and propagated along the septotemporal axis to the septal region. After the seizure spread across the hippocampus, the bursts within a seizure became bidirectional, with different propagation patterns at different frequencies. When the intact hippocampus was separated along the septotemporal axis, independent bidirectional activity was observed in the two halves, and region-specific cuts to the tissue reveal that the CA3 region is critical for seizure generation and propagation. In a second seizure model, using focal tetanic stimulation of the septal and temporal CA3 region, seizures always originated at the stimulated site with bidirectionality later developing at different frequencies, as noted in the low magnesium model, behavior compatible with coupled neuronal network oscillators. These data provide novel insights into the dynamic multifocality of seizure onset and propagation, revealing that the current concept of a single seizure "focus" is complex.

A fundamental oscillatory state of isolated rodent hippocampus

Population neuronal rhythms of various frequencies are observed in the rodent hippocampus during distinct behavioural states. However, the question of whether the hippocampus exhibits properties of spontaneous rhythms and population synchrony in isolation has not been definitively answered. To address this, we developed a novel preparation for studying neuronal rhythms in a relatively large hippocampal tissue in vitro. We isolated the whole hippocampus from mice up to 28 days postnatal age, removing the dentate gyrus while preserving the functional CA3-to-CA1 connections. Placing the hippocampal isolate in a perfusion chamber for electrophysiological assessment extracellular recordings from the CA1 revealed rhythmic field potential of 0.5 to </= 4 Hz that occurred spontaneously and propagated along the ventro-dorsal hippocampal axis. We provide convergent evidence, via measurements of extracellular pH and K(+), recordings of synaptic and intracellular activities and morphological assessments, verifying that these rhythms were not the consequence of hypoxia. Data obtained via simultaneous extracellular and patch clamp recordings suggest that the spontaneous rhythms represent a summation of GABAergic IPSPs originating from pyramidal neurons, which result from synchronous discharges of GABAergic inhibitory interneurons. Similar spontaneous field rhythms were also observed in the hippocampal isolate prepared from young gerbils and rats. Based on these data, we postulate that the spontaneous rhythms represent a fundamental oscillatory state of the hippocampal circuitry isolated from extra-hippocampal inputs.