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Abstract:
Theta rhythm oscillations appear in rat hippocampal EEG
during active exploration, and are accompanied by rhythmic firing
of hippocampal inhibitory interneurons. This could cause phasic
changes in presynaptic inhibition of synaptic transmission.
Modeling demonstrates that such phasic presynaptic inhibition could
enhance encoding and retrieval of weak sequences (Sohal and
Hasselmo, 1998a,b). Physiological experiments demonstrate that
evoked synaptic potentials in stratum radiatum of region CA1 of the
hippocampus change during different phases of theta rhythm
oscillations, with an average 6% change in rising slope of evoked
potentials across 10 rats (F(9,7200)=15.1, p<.0001). Separate
experiments demonstrate that the time course of presynaptic
inhibition should be sufficiently rapid for modulation to change
within each theta cycle. Stimulation of one pathway (ipsilateral
CA3 to CA1) causes heterosynaptic depression of evoked potentials
on a separate pathway (commissural connections from contralateral
CA1). This heterosynaptic depression has a decay time constant of
approximately 200 msec, which is sufficiently rapid for phasic
changes during theta.
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