Elsevier

Neurocomputing

Volumes 38–40, June 2001, Pages 1043-1049
Neurocomputing

A model for neuronal gamma and theta oscillations by two networks of hippocampal interneurons

https://doi.org/10.1016/S0925-2312(01)00396-4Get rights and content

Abstract

We hypothesized that fast and slow GABAA synapses underlie the hippocampal gamma and theta oscillations. A model network comprising two populations of interneurons was constructed. One population corresponds to the slow GABAA synapse, and the other to the fast GABAA synapses. Simulation results showed that the two populations of the interneurons could induce theta modulated gamma oscillation in the pyramidal membrane potential, and this theta modulation was most apparent with interaction between the two populations. These results indicate that inhibition through the GABAA synapses plays important roles in neuronal membrane oscillations.

Introduction

The hippocampal theta oscillation is considered to play an important role in construction of memory, because synaptic plasticity, a model of elemental process of learning, is easily induced during this oscillation [8]. It is hypothesized that neuronal oscillations are caused by repetitive synaptic inhibitions by inhibitory interneurons [5]. As for the gamma oscillation, inhibition through fast GABAA receptor channels (GABAA R/C) is the most possible candidate for the oscillation generation [10]. For the theta oscillation, however, the mechanisms underlying the oscillation is unclear so far. Recently, a new type of GABAAR/C, which induces almost five-fold slower inhibition than dose the conventional fast GABAAR/C, was discovered in the stratum lacunosum-moleculare (SLM) of hippocampal CA1 region [9], [1]. In the present study, we hypothesized that this slow GABAAR/C is underlying the theta oscillation, and investigated whether the fast and slow GABAAR/Cs can generate gamma and theta oscillations in the pyramidal membrane potential using a model network of hippocampal CA1 region.

Section snippets

Model

We constructed a model network comprising a pyramidal cell, interneurons in the str. pyramidale (SP interneurons) for the gamma oscillation, and SLM interneurons for the theta oscillation (Fig. 1a). In our model network, the SLM interneurons (n=10) and SP interneurons (n=10) are connected in all-to-all fashion within and between the populations. Although there is no evidence for the connections within the SLM interneurons, we assumed these connections by the slow GABAA synapses. We also assumed

Results and discussion

To activate the network, we injected steady current to the pyramidal cell and SP interneurons corresponding to the depolarizing effect of the cholinergic projections from the medial septal region [7]. To the SLM interneurons, we injected oscillatory currents considering the periodical GABAergic input from the medial septal region [11].

Fig. 2 shows activities of each cell types in the model network. In Fig. 2a, the SLM interneuron fired at 5Hz due to the input frequency at 5Hz. Oscillations

Isao Yamaguchi is working as a researcher at Corporate Research Center, Fuji Xerox Co., Ltd. His research theme is to create “knowledge technology” inspired by neuroscience. He is interested in computational theories of neuronal systems, especially in the memory functions in the hippocampal circuit.

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Isao Yamaguchi is working as a researcher at Corporate Research Center, Fuji Xerox Co., Ltd. His research theme is to create “knowledge technology” inspired by neuroscience. He is interested in computational theories of neuronal systems, especially in the memory functions in the hippocampal circuit.

Kazuhisa Ichikawa is working for Corporate Research Center Fuji Xerox Co., Ltd. He received Ph.D. from Kyoto University. The goal of the research is to elucidate mechanisms of information processing especially for memory and learning in the brain, and thereby to construct knowledge technology. He is an invited researcher of Waseda University.

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