The Journal of Neuroscience, April 15, 1999, 19(8):3223-3237
Mechanisms of Neural Synchrony in the Septohippocampal Pathways
Underlying Hippocampal Theta Generation
Brian H.
Bland,
Scott D.
Oddie, and
Luis V.
Colom
Department of Psychology, Behavioral Neuroscience Research Group,
University of Calgary, Calgary, Alberta, Canada T2N 1N4
Using urethane-anesthetized rats, 18 simultaneously recorded
septohippocampal cell pairs (36 individual cells), each classified as
theta-related according to the criteria of Colom and Bland (1987), were
studied during four spontaneously occurring hippocampal field
conditions: (1) large amplitude irregular activity (LIA) only; (2) the
transition from LIA to theta; (3) theta only; and (4) the transition
from theta to LIA. The main objective was to study the temporal
relationships and degree of neural synchrony between the discharges of
the cell pairs, using both time-averaged and time-dependent joint
peristimulus time histogram correlation techniques, during the four
conditions, to determine their contribution to the control of
oscillation and synchrony (theta) in the hippocampus. The study
demonstrated that the transition from the LIA state to the theta field
state in the hippocampus required a temporal sequence of changes in
theta-related cellular activity occurring on average 500 msec preceding
the transition: (1) the medial septum inhibits hippocampal theta-OFF
cells; (2) medial septal tonic theta-ON cells provide tonic
depolarizing inputs to initiate membrane potential oscillations (MPOs)
in hippocampal phasic theta-ON cells, whereas medial septal phasic
theta-ON cells synchronize the MPOs of hippocampal phasic theta-ON
cells and the discharges of hippocampal tonic theta-ON cells. Much of
the time preceding the LIA to theta transition is accounted for by
recruitment of these theta-related cell populations. Conversely,
"turning off" the theta state occurs abruptly and involves the
medial septal disinhibition of hippocampal theta-OFF cells.
Key words:
mechanisms; neural synchrony; septohippocampal; theta
generation; membrane potentials; joint peristimulus time
histograms
Copyright © 1999 Society for Neuroscience 0270-6474/99/1983223-15$05.00/0
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