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Abstract
The hippocampus and specifically the CA3-CA1 areas exhibit a variety of intrinsic
rhythms that span frequencies from the slow theta range (4-8 Hz) up to fast ripples
(150-200 Hz). Various computational models of different complexities have been developed
in an effort to simulate such population oscillations and uncover their underlying
mechanisms. Nevertheless most studies focus on specific rhythms generated in localized
areas. They do not address more complicated phenomena such as the so called Sharp
Wave-Ripple complex observed in CA1 EEG recordings, which is believed to be generated
through the combination of large pyramidal dendritic depolarizations and fast synchronous
interneuronal firing, induced by CA3 population bursts.
Here we present the combination of two simple but realistic models of CA3 and CA1
respectively, connected together in an effort to examine the effects of such CA3 bursts
on CA1. Both network models are computationally simple one dimensional arrays of pyramidal
and interneuron populations interacting only via fast AMPA and GABAA synapses. They
were constructed using the 2-compartment Pinsky-Rinzel model for single pyramidal
cells and the one-compartment Wang-Buzsaki model for interneurons, reproducing the
basic firing properties of hippocampal cells. Connectivity schemes and postsynaptic
potentials are based on biological data, making the network topology as realistic
as possible.
Our CA3 model is a highly recurrent network that reproduces a number of different
features observed in real recordings or simulations of more sophisticated models (e.g.
activity propagation in the disinhibited array, Carbachol-induced oscillations (5-6
Hz) and others). Most importantly it exhibits semi-synchronised population bursts
that have been observed in CA3 slices and in vivo.
Recurrent pyramidal cell connections are almost absent in the CA1 network which is
based mostly on the interaction between pyramidal cells with a strongly connected
interneuronal network. The main rhythm reproduced here is the gamma frequency of the
pure interneuron population and its long range synchronization through the interaction
with pyramidal cells. The two models are coupled together in a feedforward CA3-to-CA1
scheme that mimics Shaffer collaterals.
The effects of CA3 bursts in the intrinsic CA1 activity are examined along with the
role of various parameters values and stimuli. Our final goal is to simulate Sharp
Wave-Ripple complexes and uncover the hippocampal features that generate them. We
believe that our model is ideally suited to shedding further light on the potential
mechanisms of such synchronous hippocampal events.