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The Journal of Neurophysiology Vol. 83 No. 1 January 2000, pp. 359-366
Copyright ©2000 by the American Physiological Society
Department of Biosciences, Division of Animal Physiology, University of Helsinki, 00014, Finland
Lamsa, Karri,
J. Matias Palva,
Eva Ruusuvuori,
Kai Kaila, and
Tomi Taira.
Synaptic GABAA Activation Inhibits AMPA-Kainate
Receptor-Mediated Bursting in the Newborn
(P0-P2) Rat Hippocampus. J. Neurophysiol. 83: 359-366, 2000. The mechanisms of synaptic transmission in the rat
hippocampus at birth are assumed to be fundamentally different from
those found in the adult. It has been reported that in the CA3-CA1
pyramidal cells a conversion of "silent" glutamatergic synapses to
conductive
-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid
(AMPA) synapses starts gradually after P2. Further, GABA via its
depolarizing action seems to give rise to grossly synchronous yet slow
calcium oscillations. Therefore, GABA is generally thought to have a
purely excitatory rather than an inhibitory role during the first
postnatal week. In the present study field potential recordings and
gramicidin perforated and whole cell clamp techniques as well as
K+-selective microelectrodes were used to examine the
relative contributions of AMPA and GABAA receptors to
network activity of CA3-CA1 pyramidal cells in the newborn rat
hippocampus. As early as postnatal day (P0-P2), highly coherent spontaneous
firing of CA3 pyramidal cells was seen in vitro. Negative-going
extracellular spikes confined to periodic bursts (interval 16 ± 3 s) consisting of 2.9 ± 0.1 spikes were observed in stratum
pyramidale. The spikes were accompanied by AMPA-R-mediated
postsynaptic currents (PSCs) in simultaneously recorded pyramidal
neurons (7.6 ± 3.0 unitary currents per burst). In CA1 pyramidal
cells synchronous discharging of CA3 circuitry produced a barrage of
AMPA currents at >20 Hz frequencies, thus demonstrating a transfer of
the fast CA3 network activity to CA1 area. Despite its depolarizing
action, GABAA-R-mediated transmission appeared to exert
inhibition in the CA3 pyramidal cell population. The
GABAA-R antagonist bicuculline hypersynchronized the output of glutamatergic CA3 circuitry and increased the network-driven excitatory input to the pyramidal neurons, whereas the
GABAA-R agonist muscimol (100 nM) did the opposite.
However, the occurrence of unitary GABAA-R currents was
increased after muscimol application from 0.66 ± 0.16 s
1 to 1.43 ± 0.29 s
1. It was
concluded that AMPA synapses are critical in the generation of
spontaneous high-frequency bursts in CA3 as well as in CA3-CA1 transmission as early as P0-P2 in rat hippocampus.
Concurrently, although GABAA-R-mediated depolarization may
excite hippocampal interneurons, in CA3 pyramidal neurons it can
restrain excitatory inputs and limit the size of the activated neuronal population.
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