Single Cell Activity Patterns of Pedunculopontine Tegmentum Neurons Across the Sleep-Wake Cycle in the Freely Moving Rats Subimal Datta * and Donald F. Siwek Sleep Research Laboratory, Program in Behavioral Neuroscience and Department of Psychiatry, Boston University School of Medicine, Boston, Massachusetts Microinjections of the excitatory amino acid, L-glutamate into the cholinergic cell compartment of the pedunculo- pontine tegmentum (PPT) of the rat induces both wake- fulness and/or rapid eye movement (REM) sleep depend- ing on the glutamate dosage. However, no studies have systematically recorded the electrical activity of these cells in the freely moving rat across the sleep-wake cycle. In this study, we have recorded the spontaneous activity patterns of single PPT cells (n = 70) in the freely moving rat across the sleep-wake cycle. PPT neurons were clas- sified into three groups based on patterns in their spon- taneous activity. The first group of cells (12.86%) was more active during REM sleep than they were during wakefulness or slow-wave sleep (SWS). The second group of cells (60.0%) was more active during REM and wakefulness than during SWS. The firing rate of the third group of cells (27.14%) did not change as a function of behavioral state. This study also demonstrated that the level of activity within the cholinergic cell compartment of the PPT during SWS drops to 7.4% of that observed during wakefulness and that during REM sleep it changes to 65.5% of wakefulness levels. These findings indicate that in the freely moving rat, the discharging of PPT neurons correlates with wakefulness and REM sleep. Additionally, these neurons may be an integral part of the brainstem wakefulness and REM sleep-generating mechanisms in the rat. © 2002 Wiley-Liss, Inc. Key words: pedunculopontine tegmentum nucleus; cho- linergic cell compartment; sleep-wake cycle; single cell activity; REM-on; Wake-REM-on; cortical activation The pedunculopontine tegmentum nucleus (PPT) in the brainstem is one of the major sources of acetylcholine and nitric oxide critical to the induction and maintenance of rapid eye movement (REM) sleep (Steriade and Mc- Carley, 1990; Datta, 1995; Datta et al., 1997). Some of the most compelling observations supporting this statement are from single cell recording studies in the PPT of be- having cats. Such studies have identified several different patterns in the electrical activity of these cells (Saito et al., 1977; El-Mansari et al., 1989, 1990; Sakai et al., 1990; Steriade et al., 1990a,b; Datta, 1995). The firing rates of two types of neurons correlate well with the initiation and maintenance of REM sleep. The firing rate of the first type of neuron called REM-on cells, increased as the animal passed from wakefulness to slow-wave sleep (SWS) and then on to REM sleep. The second group of neurons called Wake-REM-on cells, were tonically active both during wakefulness and REM sleep. Since the beginning of modern sleep research, the domestic cat has been the animal of choice for studies of the experimental neurophysiology of sleep (for reviews see Datta, 1995, 1997). The use of rats in sleep research has increased sharply in the last decade for several reasons, including smaller size, greater ease of acquisition and care, increased practicality for studies of the cellular and molec- ular mechanisms of sleep, despite the fact that most re- searchers have assumed it was not possible to obtain good quality recordings of electrooculogram (EOG) and ponto- geniculo-occipital (PGO) wave activities from this species (reviewed in Datta and Hobson, 2000). We have recently shown that the rat can be instrumented, as in the cat, making it a suitable animal model for neurophysiologic sleep studies in the brainstem (Datta and Hobson, 2000; Datta et al., 2001a). Although the use of rats vs. cats as the species of choice for sleep researchers has increased, basic information describing rat single cell activity in the REM sleep generating network in relation to sleep-wake pat- terns has not been forthcoming. Recent local microinjection studies demonstrate that glutamate microinjection into the PPT increases the du- ration of REM sleep in the rat (Datta et al., 2001a; Datta, Contract Grant Sponsor: National Institutes of Health; Contract grant numbers: MH59839, NS34004, AG09525. *Correspondence to: Subimal Datta, Sleep Research Laboratory, Depart- ment of Psychiatry, Boston University School of Medicine, M-913, 715 Albany Street, Boston, MA 02118. E-mail: subimal@bu.edu Received 10 May 2002; Revised 24 June 2002; Accepted 25 June 2002 Published online 27 August 2002 in Wiley InterScience (www. interscience.wiley.com). DOI: 10.1002/jnr.10405 Journal of Neuroscience Research 70:611– 621 (2002) © 2002 Wiley-Liss, Inc.