LOW-FREQUENCY RHYTHMS IN THE THALAMUS OF INTACT-CORTEX AND DECORTICATED CATS

1. The patterns and synchronization of low-frequency, sleeplike rhythm s (slow, spindle and delta oscillations) were compared in the intact-c ortex and decorticated hemispheres of cats under ketamine-xylazine ane sthesia. Intracellular recordings were performed in intact and decorti cated hemispheres from 58 rostrolateral thalamic reticular (RE) neuron s and from 164 thalamocortical(TC) neurons in the ventrolateral (VL) n ucleus. In the decorticated hemisphere, dual intracellular recordings were performed from five RE-VL cell couples and from 12 TC cell couple s within the VL nucleus. In addition, field potentials were simultaneo usly recorded from the neocortex (electroencephalogram) and ipsilatera l thalamus [electrothalamogram (EThG)] of the intact (right) hemispher e, while EThG was recorded from the VL nucleus of the decorticated (le ft) hemisphere. 2. The slow oscillation (<1 Hz) was absent in all 72 V L cells and in 23 of 25 RE cells from the decorticated hemisphere, as well as in the EThG recorded from the VL nucleus in the decorticated h emisphere, whereas it was simultaneously present in the cortex and tha lamus of the intact hemisphere. The remaining two RE neurons (8%) in t he 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 ce ll followed 12 ms after the sharp depth-negative (depolarizing) compon ent in the contralateral cortex. We view this result as the electrophy siological correlate of a disynaptic excitatory pathway consisting of crossed cortical projections, first relayed in contralateral dorsal th alamic nuclei. 3. The patterns of thalamic spindles (7-14 Hz) differed between the two hemispheres. Whereas the decorticated hemisphere disp layed prolonged, waxing and waning spindles, the spindles in the intac t-cortex hemisphere were short and exclusively waning and followed the depth-negative component of cortical slow oscillation. This result in dicates that the synchronized corticothalamic drive associated with th e 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 wit h different intensities the thalamus in the decorticated hemisphere. 4 . Simultaneous intracellular recordings from two VL cells or from RE a nd VL cells showed nearly simultaneous spindle sequences in the decort icated 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 oscilla tion is cortical in origin; demonstrate a full, short-range, intrathal amic synchrony of spindles in the absence of cortex; and indicate that the pattern of spindles, a sleep rhythm that is conventionally regard ed as purely thalamic, is shaped by the corticothalamic feedback.