EEG related neuronal activity in the pedunculopontine tegmental nucleus of urethane anaesthetized rats

Cholinergic pathways ascending from the brainstem are considered as a decisive part of the reticular activating system. We recorded unit activity from the cholinergic pedunculopontine tegmental nucleus with extracellular microelectrodes in urethane-anesthetized rats and monitored cortical electroencephalogram (EEG) to examine the possible role of the nucleus in cortical activation. We found two types of cells showing EEG-correlated firing patterns. In one group, firing rate increased during cortical activation (F cell), while in another, higher rate was accompanied by cortical slow waves (S cell). Phasic changes in the firing rate of pedunculopontine neurons and in the cortical EEG were also analyzed. Changes of single unit activity in F cells always occurred before short periods of low-voltage fast activity appeared in the cortical EEG. The S cells were more variable with respect to the temporal relation. In some of the S cells, changes in firing rate preceded changes in the EEG patterns, while in others they occurred after a certain delay. Our results indicate that F-cells in the PPT might be involved in the initiation of tonic and phasic changes in cortical activation.