Sleep-waking discharge of neurons in the posterior lateral hypothalamus ofthe albino rat

The sleep-waking discharge patterns of neurons in the posterior lateral hyp othalamus (PLH) were investigated in the rat. Previous studies in the cat d emonstrated that this region contained neurons that fired tonically at low rates (2-4 Hz) during waking, decreased firing in non-rapid eye-movement (N REM) sleep and nearly ceased firing during rapid eye-movement (REM) sleep. These "REM-off" neurons were proposed to be histaminergic neurons of the tu beromammillary nucleus (TM). Since many anatomical and physiological studie s ate performed in the rat, we sought to examine the sleep-waking discharge of these neurons in this animal. We found three main types of discharge pa tterns among PLH neurons. Waking-related neurons decreased their discharge in NREM sleep, and remained at low rates during REM sleep. A subpopulation of these neurons discharged very little during REM sleep (< 0.2 Hz) (REM-of f neurons), Waking/REM-related neurons decreased their discharge in NREM sl eep and returned to waking rates in REM sleep. REM-related neurons decrease d their discharge in NREM sleep and increased their discharge during REM sl eep higher than waking rates. No NREM-related discharge patterns were recor ded. Waking-related and waking/REM-related neurons were similar in location within the PLH and action potential duration. Some REM-off and other wakin g-related neurons were recorded within the boundaries of the histaminergic TM, however, not all waking-related and REM-off neurons were found within t his region. Furthermore, neurons with waking/REM-related and state-indiffer ent discharge patterns were localized within the TM. These results suggest that waking-related and/or REM-off neurons may not be exclusively histamine rgic in rats. (C) 1999 Elsevier Science B.V. All rights reserved.