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Journal of Neuroscience, Vol 15, 6946-6956, Copyright © 1995 by Society for Neuroscience
Coupling potentials in CA1 neurons during calcium-free-induced field burst activity
TA Valiante, JL Perez Velazquez, SS Jahromi and PL Carlen
Playfair Neuroscience Unit, Toronto Hospital Research Institute, Ontario, Canada.
Small amplitude depolarizations (fast prepotentials, spikelets) recorded in
mammalian neurons are thought to represent either dendritic action
potentials or presynaptic action potentials attenuated by gap junctions. We
have used whole-cell recordings in an in vitro calcium- free model of
epilepsy to record spikelets from CA1 neurons of the rat hippocampus. It
was found that spikelet appearance was closely correlated with the
occurrence of dye coupling between pyramidal neurons, indicating that both
phenomena share a common substrate. Spikelets were characterized according
to waveform (amplitude and shape) and temporal occurrence. Spikelet
amplitudes were found to be invariant with neuronal membrane potential, and
their pattern of occurrence was indistinguishable from patterns of action
potential firing in these cells. Voltage and current recordings revealed a
spikelet waveform that was usually biphasic, comprised of a rapid
depolarization followed by a slower hyperpolarization. Numerical
differentiation of spike bursts resulted in waveforms similar to recorded
spikelet sequences, while numerical integration of spikelets yielded
waveforms that were indistinguishable from action potentials. Modification
of spikelet waveforms by the potassium channel blocker tetraethylammonium
chloride suggests that spikelets may arise from both resistive and
capacitive transmission of presynaptic action potentials. Intracellular
alkalinization and acidification brought about by perfusion with NH4Cl
caused changes in spikelet frequency, consistent with reported alterations
of field burst activity in this model of epilepsy. These results suggest
that spikelets result from gap junctional communication, and may be
important determinants of neuronal activity during seizure-like activity.
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