Subcortical waking and sleep during lateral hypothalamic 'somnolence' in rats

Following extensive bilateral lateral hypothalamic damage, rats appear “somnolent.” Cortical EEG shows persistent high voltage delta, reinforcing the impression of sleep. Preoperatively and postoperatively, we simultaneously measured cortical and subcortical (hippocampal and pontine) EEG, muscular events (neck muscle EMG and eye movement EOG), and behavior, which, as aggregates, differentially define quiet sleep, active sleep, and waking. Postoperatively, though cortical activity was persistently slow, subcortical EEG, muscular events, and behavior, as aggregates, revealed quiet sleep, active sleep, and waking, organized subcortically, intact and alternating, but disconnected from the persistent slow cortical activity. For example, preoperatively, active sleep included cortical low voltage fast activity, hippocampal theta, episodic pontine spike bursts, flat EMG, and rapid eye movements, without any organized behavior. Postoperatively, the same aggregate of subcortical and muscular events indicated the presence of active sleep. Similarly so, for subcortically organized quiet sleep and spontaneous waking. Such waking, termed “drowsy-wakefulness,” is a low-arousal form, perhaps related to drowsiness in other species, and to human hypersomnia.