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A mechanism for hippocampal place cell firing during the
hyperpolarizing phases of the theta rhythm
Victoria Booth and Gina R. Poe
University of Michigan
During phase precession, hippocampal place cells fire over a range of
phases of the population theta rhythm. The majority of spikes occur,
however, around the peaks of the theta rhythm when the somatic
membrane of the cell is presumably most depolarized. During REM sleep,
on the other hand, place cells associated with familiar environments
fire preferentially at the troughs of the theta rhythm when the
somatic membrane of the cell is most hyperpolarized (Poe et al, Brain
Research, 2000). A locus of similar place cell firing in the theta
trough has been identified in a recent analysis of phase precession
data from awake animals (Yamaguchi et al, J Neurophysiol, 2002). We
propose that such preferential firing in the theta trough could result
from a gating of synaptic inputs by the 180 degree phase differential
in the theta rhythm at the distal dendritic and cellular layers. In
particular, distal dendritic synaptic inputs (from layer III of the
entorhinal cortex) arriving at the peak of the dendritic theta rhythm,
equivalently the trough of the somatic theta rhythm, would be
successfully propagated to generate an action potential while those
arriving at the trough of dendritic theta, equivalently the peak of
somatic theta, would generate only subthreshold responses. We use a
realistic, multicompartment CA1 pyramidal cell model (Migliore, J
Comput Neurosci, 2002) to investigate this proposed mechanism.