Action potentials and relations to the theta rhythm of medial septal neurons in vivo |
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Authors: | E S Brazhnik S E Fox |
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Institution: | (1) Department of Physiology, Box 31, State University of New York, Health Science Center at Brooklyn, 450 Clarkson Avenue, Brooklyn, NY 11203, USA, US |
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Abstract: | The influence of the medial septal nucleus and the nucleus of the diagonal band of Broca (MS-DB) on the hippocampal theta
rhythm includes both cholinergic and γ-aminobutyric acid (GABAergic) components. To understand the intrinsic septal interactions
and the separate contributions of the cholinergic and GABAergic septohippocampal neurons to the theta rhythm in behaving animals,
it is essential to be able to identify these two classes from extracellular recordings. Here the durations of extracellularly
recorded action potentials are compared with the other characteristics of the neurons. Extracellular recordings were taken
from neurons of the MS-DB both in freely moving rats (114 cells) and in urethane-anesthetized rats (112 cells). These were
compared with intracellular recordings taken from MS-DB neurons in urethane-anesthetized rats (58 cells). Hippocampal EEG
was recorded from above the CA1 pyramidal cell layer (CA1 theta) and near the hippocampal fissure (dentate theta) to compare
the firing phase across cells. Here it is shown that two major types of rhythmically bursting cells in the MS-DB that had
been distinguished previously in intracellular recordings in vivo are also separable in extracellular recordings in vivo on
the basis of the durations of their action potentials. In both awake and anesthetized rats the main properties of the two
cell types were found to differ: firing rate, phase-relation to the hippocampal theta rhythm and sensitivity of their rhythmicity
to blockade of muscarinic transmission. As was previously shown for intracellular recordings in anesthetized rats, it is shown
here that in awake rats, too, the more rapidly firing brief-spike (putative GABAergic) cells fired with highest probability
on the negative phase of the dentate theta, whereas the more slowly firing long-spike (putative cholinergic) cells fired mostly
on the positive phase. Previous work showed that in intracellular recordings from anesthetized rats the rhythmic firing of
most brief-spike cells was still retained even during muscarinic blockade, but that of most long-spike cells was lost. Here
we also report a recategorization according to spike duration of existing extracellular recordings taken from anesthetized
rats, confirming the above observation with much larger numbers of cells. Three additional major new findings are also reported
here. (1) In awake rats, muscarinic blockade has relatively little effect on either cell type. (2) Under anesthesia, the firing
rates of both cell types are lower than in awake rats, but the effect is greater on the long-spike cells, where the anesthesia
also reduces the rhythmicity of the cell firing. (3) Rhythmicity of the putative GABAergic cells is also retained after local
injection of GABA-A antagonist, whereas that of the putative cholinergic cells is eliminated. We conclude that either systemic
muscarinic blockade or urethane anesthesia alone have relatively little effect on neurons in the defined above MS-DB, but
a combination of the two has profound effects on the rhythmicity of the cholinergic cells, largely sparing the GABAergic cells.
Taken together, the results suggest that generation of theta rhythm requires a background of excitatory influences on the
hippocampus (that can be maintained by either muscarinic or glutamatergic inputs) in combination with the phasic disinhibitory
action mediated by the GABAergic MS-DB projection. They also provide additional support for the notion that the phasic activity
in local collaterals of GABAergic MS-DB cells contributes to the phasic modulation of the firing of cholinergic septohippocampal
neurons.
Received: 13 October 1998 / Accepted: 15 March 1999 |
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Keywords: | Hippocampal theta rhythm Medial septal neurons Acetylcholine GABA Freely moving rats |
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