Behavioural Brain Research 243 (2013) 231–238 Contents lists available at SciVerse ScienceDirect Behavioural Brain Research j ourna l ho mepage: www.elsevier.com/locate/bbr Research report Sleep–wake characterization of double MT 1 /MT 2 receptor knockout mice and comparison with MT 1 and MT 2 receptor knockout mice Stefano Comai 1 , Rafael Ochoa-Sanchez 1 , Gabriella Gobbi ∗ Neurobiological Psychiatry Unit, Department of Psychiatry, McGill University Health Center, McGill University, Montreal, QC, Canada h i g h l i g h t s ◮ Lack of MT 1 receptors decreases REMS time and EEG REMS theta power. ◮ Lack of MT 2 receptors decreases NREMS time and EEG NREMS delta power. ◮ Lack of both MT 1 and MT 2 receptors increases 24-h wakefulness time. ◮ MT 1 /MT 2 receptors knockout mice display increased EEG wake and REMS theta power. ◮ MT 1 and MT 2 receptors have distinct effects in modulating sleep stages. a r t i c l e i n f o Article history: Received 5 October 2012 Received in revised form 12 December 2012 Accepted 8 January 2013 Available online xxx Keywords: Sleep–wake cycle Melatonin MT1 receptor MT2 receptor Knockout mice a b s t r a c t The neurohormone melatonin activates two G-protein coupled receptors, MT 1 and MT 2 . Melatonin is implicated in circadian rhythms and sleep regulation, but the role of its receptors remains to be defined. We have therefore characterized the spontaneous vigilance states in wild-type (WT) mice and in three different types of transgenic mice: mice with genetic inactivation of MT 1 (MT 1 -/- ), MT 2 (MT 2 -/- ) and both MT 1 /MT 2 (MT 1 -/- /MT 2 -/- ) receptors. Electroencephalographic (EEG) and electromyographic sleep–wake patterns were recorded across the 24-h light–dark cycle. MT 1 -/- mice displayed a decrease (-37.3%) of the 24-h rapid eye movement sleep (REMS) time whereas MT 2 -/- mice showed a decrease (-17.3%) of the 24-h non rapid eye movement sleep (NREMS) time and an increase in wakefulness time (14.8%). These differences were the result of changes occurring in particular during the light/inactive phase. Surprisingly, MT 1 -/- /MT 2 -/- mice showed only an increase (8.9%) of the time spent awake during the 24-h. These changes were correlated to a decrease of the REMS EEG theta power in MT 1 -/- mice, of the NREMS EEG delta power in MT 2 -/- mice, and an increase of the REMS and wakefulness EEG theta power in MT 1 -/- /MT 2 -/- mice. Our results show that the genetic inactivation of both MT 1 and MT 2 receptors produces an increase of wakefulness, likely as a result of reduced NREMS due to the lack of MT 2 receptors, and reduced REMS induced by the lack of MT 1 receptors. Therefore, each melatonin receptor subtype differently regulates the vigilance states: MT 2 receptors mainly NREMS, whereas MT 1 receptors REMS. © 2013 Elsevier B.V. All rights reserved. 1. Introduction The neurohormone melatonin (MLT), synthesized by the pineal gland during the dark period of the light/dark cycle, acts mostly through two G protein-coupled receptors, MT 1 and MT 2 , with high affinity for MLT (Ki ≈ 0.1 nM). It can also act through MT 3 receptors, which are the homologue of the human quinone reductase, but with ∗ Corresponding author at: Neurobiological Psychiatry Unit, Dept. of Psychiatry, McGill University, 1033 Pine Avenue West, Montreal, QC, Canada H3A 1A1. Tel.: +1 514 398 1290; fax: +1 514 398 4866. E-mail address: gabriella.gobbi@mcgill.ca (G. Gobbi). 1 These authors contributed equally to this work lower affinity (Kd = 3–9 nM)[1], and with receptor-independent mechanisms [2]. MLT acts at both central and peripheral lev- els affecting circadian rhythms, sleep, mood, cardiovascular and immune systems, metabolism, cell proliferation, and pain [3]. MLT, by acting through MT 1 and MT 2 receptors, controls the activity of the suprachiasmatic nucleus (SCN) [4–6], the “master clock”, which drives circadian manifestations of behavior and body physiology [7,8]. Sleep is regulated by circadian and homeostatic processes [8]; however, it is still a matter of debate whether MLT indirectly regulates sleep through the control of circadian pro- cesses such as body temperature or modulates the activity of brain nuclei directly involved in the regulation of sleep. Several human [9–11] and animal [12,13] studies have shown hypnotic proper- ties of exogenous MLT, but others have failed to demonstrate these 0166-4328/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.bbr.2013.01.008