THE LOW-FREQUENCY RHYTHMS IN THE THALAMUS OF INTACT-CORTEX AND DECORTICATED
Timofeev, I. and M. Steriade.
Laboratoire de Neurophysiologie, Facult[acute]e de M[acute]edecine,
Universit[acute]e Laval, Quebec, Canada G1K 7P4.
APStracts 3:0179N, 1996.
SUMMARY AND CONCLUSIONS
1. The patterns and synchronization of low-frequency, sleep-like rhythms
(slow, spindle and delta oscillations) were compared in the intact-cortex and
decorticated hemispheres of cats under ketamine-xylazine anesthesia.
Intracellular recordings were performed in intact and decorticated hemispheres
from 58 rostrolateral thalamic reticular (RE) neurons and from 164
thalamocortical (TC) neurons in the ventrolateral (VL) nucleus. In the
decorticated hemisphere, dual intracellular recordings were performed from 5
RE-VL cell couples and from 12 TC cell couples within the VL nucleus. In
addition, field potentials were simultaneously recorded from the neocortex
(electroencephalogram, EEG) and ipsilateral thalamus (electrothalamogram,
EThG) of the intact (right) hemisphere, while EThG was recorded from the VL
nucleus of the decorticated (left) hemisphere. 2. The slow oscillation (less
than 1 Hz) was absent in all 72 VL cells and in 23 out of 25 RE cells from the
decorticated hemisphere, as well as in the EThG recorded from the VL nucleus
in the decorticated hemisphere, whereas it was simultaneously present in the
cortex and thalamus of the intact hemisphere. The remaining 2 RE neurons (8%)
in the decorticated hemisphere oscillated in close time relation with the slow
oscillation in the cortex and thalamus of the opposite hemisphere; averaged
activities showed that the onset of depolarization in RE cell followed 12 ms
after the sharp depth-negative (depolarizing) component in the contralateral
cortex. We view this result as the electrophysiological correlate of a
disynaptic excitatory pathway consisting of crossed cortical projections,
first relayed in contralateral dorsal thalamic nuclei. 3. The patterns of
thalamic spindles (7-14 Hz) differed between the two hemispheres. Whereas the
decorticated hemisphere displayed prolonged, waxing-and-waning spindles, the
spindles in the intact-cortex hemisphere were short, exclusively waning and
followed the depth-negative component of cortical slow oscillation. This
result indicates that the synchronized corticothalamic drive associated with
the slow oscillation fully entrains thalamic circuits from the onset of
spindles, thus preventing further waxing. Similar differences between waxing-
and-waning and waning spindles were obtained by stimulating with different
intensities the thalamus in the decorticated hemisphere. 4. Simultaneous
intracellular recordings from two VL cells or from RE and VL cells showed
nearly simultaneous spindle sequences in the decorticated hemisphere. 5. The
hyperpolarization-activated, intrinsic delta oscillation (1-4 Hz) of TC cells
was asynchronous in the decorticated hemisphere. 6 . These results strengthen
the idea that the slow oscillation is cortical in origin; demonstrate a full,
short-range, intrathalamic synchrony of spindles in the absence of cortex; and
indicate that the pattern of spindles, a sleep rhythm that is conventionally
regarded as purely thalamic, is shaped by the corticothalamic feedback.
Received 21 June 1996; accepted in final form 21 August 1996.
APS Manuscript Number J499-6.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 19 September 1996