Characteristic changes in the slow cortical waves after a 6-h sleep deprivation in rat

Sleep deprivation is followed by an increase in EEG delta power during recovery sleep indicating an increased sleep propensity. Delta waves mostly reflect the rhythmic recurring of generalized hyperpolarizations (DOWN state) in cortical neurons causing large, deep-positive waves in the LFP. Enhancement of delta power in recovery sleep can be the consequence of either the more frequent occurrence, or the higher amplitude caused by higher synchrony, or the longer duration of these DOWN states. In the present experiments, we examined these possibilities and found the strongest increase in the incidence of slow deep positive LFP waves (slow waves) following sleep deprivation indicating enhancement of DOWN state inducing and/or weakening of UP state maintaining processes. The strong decrease in multiunit activity during slow waves was preceded by a gradual buildup of activity. The significant correlation between these changes both in control and recovery recordings indicate that excitation might determine the subsequent drop in activity. Increased sensitivity of cortical neurons to excitation might offer an explanation for this observation. Current-source-density analysis indicated in our experiments that the first sources during DOWN states appeared in layer III-IV. Activation was then displaced to layer V. In the motor cortex, both corticocortical and thalamocortical fibers terminate in layer III that provides a strong feedforward excitation to layer V. As propagation of facilitatory signals through cortical layers is downwardly biased, disfacilitation might also follow this pattern. Sleep deprivation caused only quantitative differences in the sink-source patterns, indicating that existing processes were enhanced by sleep deprivation.