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The Journal of Neurophysiology Vol. 82 No. 1 July 1999, pp. 202-208
Copyright ©1999 by the American Physiological Society
1Instituto de Bioingeniería,
Prida, Liset Menendez de la and
Juan V. Sanchez-Andres.
Nonlinear Frequency-Dependent Synchronization in the Developing
Hippocampus. J. Neurophysiol. 82: 202-208, 1999.
Synchronous population activity is present both in
normal and pathological conditions such as epilepsy. In the immature
hippocampus, synchronous bursting is an electrophysiological
conspicuous event. These bursts, known as giant depolarizing potentials
(GDPs), are generated by the synchronized activation of interneurons
and pyramidal cells via GABAA,
N-methyl-D-aspartate, and AMPA receptors.
Nevertheless the mechanism leading to this synchronization is still
controversial. We have investigated the conditions under which
synchronization arises in developing hippocampal networks. By means of
simultaneous intracellular recordings, we show that GDPs result from
local cooperation of active cells within an integration period prior to
their onset. During this time interval, an increase in the number of
excitatory postsynaptic potentials (EPSPs) takes place building up full
synchronization between cells. These EPSPs are correlated with
individual action potentials simultaneously occurring in neighboring
cells. We have used EPSP frequency as an indicator of the neuronal
activity underlying GDP generation. By comparing EPSP frequency with
the occurrence of synchronized GDPs between CA3 and the fascia dentata
(FD), we found that GDPs are fired in an all-or-none manner, which is
characterized by a specific threshold of EPSP frequency from which
synchronous GDPs emerge. In FD, the EPSP frequency-threshold for GDP
onset is 17 Hz. GDPs are triggered similarly in CA3 by appropriate
periodic stimulation of mossy fibers. The frequency threshold for CA3
GDP onset is 12 Hz. These findings clarify the local mechanism of
synchronization underlying bursting in the developing hippocampus,
indicating that GDPs are fired when background levels of EPSPs or
action potentials have built up full synchronization by firing at
specific frequencies (>12 Hz). Our results also demonstrate that
spontaneous EPSPs and action potentials are important for the
initiation of synchronous bursts in the developing hippocampus.
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