Hippocampal rhythm generation: Gamma-related theta-frequency resonance in CA3 interneurons |
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Authors: | G Orbán T Kiss M Lengyel P Érdi |
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Institution: | (1) Department of Biophysics, KFKI Research Institute for Particle and Nuclear Physics, Hungarian Academy of Sciences, P.O. Box 49, 1525 Budapest, Hungary, HU |
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Abstract: | During different behavioral states different population activities are present in the hippocampal formation. These activities
are not independent: sharp waves often occur together with high-frequency ripples, and gamma-frequency activity is usually
superimposed on theta oscillations. There is both experimental and theoretical evidence supporting the notion that gamma oscillation
is generated intrahippocampally, but there is no generally accepted view about the origin of theta waves. Precise timing of
population bursts of pyramidal cells may be due to a synchronized external drive. Membrane potential oscillations recorded
in the septum are unlikely to fulfill this purpose because they are not coherent enough. We investigated the prospects of
an intrahippocampal mechanism supplying pyramidal cells with theta frequency periodic inhibition, by studying a model of a
network of hippocampal inhibitory interneurons. As shown previously, interneurons are capable of generating synchronized gamma-frequency
action potential oscillations. Exciting the neurons by periodic current injection, the system could either be entrained in
an oscillation with the frequency of the inducing current or exhibit in-phase periodic changes at the frequency of single
cell (and network) activity. Simulations that used spatially inhomogeneous stimulus currents showed anti-phase frequency changes
across cells, which resulted in a periodic decrease in the synchrony of the network. As this periodic change in synchrony
occurred in the theta frequency range, our network should be able to exhibit the theta-frequency weakening of inhibition of
pyramidal cells, thus offering a possible mechanism for intrahippocampal theta generation.
Received: 23 February 2000 / Accepted in revised form: 30 June 2000 |
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