West Roxbury Veterans Administration Medical Center and Harvard Medical School, West Roxbury, Massachusetts 02132, USA.
Narcolepsy is a human sleep disorder resulting from the loss of neurons containing the neuropeptide orexin, also known as hypocretin. Cataplexy, which is a sudden loss of muscle tone during waking, is an important diagnostic symptom of narcolepsy. In humans and canines with narcolepsy, cataplexy is considered to be a separate and distinct behavioral state. However, in the mouse model of the disease this issue is not resolved. The present study monitored the activity of forty four neurons in the rostral pons in hypocretin knock-out mice. Majority of the neurons were active during wake and REM sleep, while four neurons were selectively active during REM sleep. All of these neurons were less active during cataplexy compared with REM sleep. Thus, although cataplexy and REM sleep share many common features, including the muscle atonia, cataplexy is a distinct state in mice.