Anatomically heterogeneous pyramidal cells in supragranular layers of the dorsal cortex show the surface-to-deep firing frequency increase during natural sleep

To explore how supragranular cortical pyramidal cells (Pyrs) are recruited during natural sleep, we applied a method of juxtacellular recording/labeling in freely moving rats. We asked how different Pyrs activate during sleep events such as down-to-up-state transition (DUt), spindle (SO), and REM sleep-related theta oscillations (ThO). Based on the polarity of the down-states (DS) recorded along with the cell firing, we divided Pyrs as superficial (negative DS, layer 2 – L2, n=35) or deep (positive DS, layer 3 – L3, n=35). Most of the Pyrs showed very sporadic firing behavior, with a lower average frequency for L2 than for L3 (0.32+/-0.32Hz vs 0.57+/-0.54Hz). In addition, L2 showed a lower probability of spikes and lower firing frequency within all measured events (DUt: 4+/-3.9% vs 7.5+/-6.9%; 0.39+/-0.39Hz vs 0.67+/-0.64Hz; SO: 14.9+/-11.8% vs 25.9+/-18.2%; 0.53+/-0.55Hz vs 0.83+/-0.71Hz; and ThO: 0.22+/-0.17Hz vs 0.82+/-1.25Hz). The number of bursting events and the number of spikes within the bursts was lower for L2 than for L3. Interestingly, the L2 didn't show firing preference within the spindle or theta cycle, while 40% of the L3 showed strong phase-related activity, preferring the descending phase or the trough of the spindle cycle. The average firing frequency during slow wave sleep was correlated with the cortical depth of the cells (R=0.424, p=0.007). We conclude that during natural sleep the Pyrs in supragranular layers of the dorsal cortex show a surface-to-deep firing frequency increase. Application of dimensionality reduction methods for the identification of subclasses of Pyrs with different anatomical and electrophysiological characteristics is underway.