Orexin-A acts on the paraventricular nucleus of the midline thalamus to inhibit locomotor activity in rats Yonghui Li a,b , Sa Li b , Nan Sui a , Gilbert J. Kirouac b, ⁎ a Key Laboratory of Mental Health, Institute of Psychology, Chinese Academy of Sciences, Beijing,100101, China b Department of Oral Biology, Faculty of Dentistry, 780 Bannatyne Avenue, University of Manitoba, Winnipeg, Manitoba, Canada R3E 0W2 abstract article info Article history: Received 1 April 2009 Received in revised form 11 June 2009 Accepted 29 June 2009 Available online 12 July 2009 Keywords: Orexin Hypocretin Midline thalamic nuclei Paraventricular nucleus of thalamus Locomotion Morphine Arousal Drug addiction Sensitization Orexins (hypocretins) are novel peptides that have been shown to play a role in control of behavioral arousal. The paraventricular nucleus of the midline thalamus (PVT) is one area of the brain that is the most densely innervated by orexin fibers. In addition, the PVT sends a dense projection to the nucleus accumbens, an area of the striatum involved in the regulation of locomotion. This study was done to determine the effect of microinjections of orexin-A (OXA) or the orexin receptor antagonist SB334867 in the PVT on locomotor activity (LA) in morphine-naïve and morphine-sensitized rats. Microinjections of OXA (3 μg/500 nl) in or near the PVT inhibited LA in rats tested in a novel and familiar environment as well as in rats expressing behavioral sensitization to repeated injections of morphine. In contrast, microinjections of SB334867 had no effect on LA in any of the test situations. Using an approach involving experimenter based analysis of ethological behaviors; we found that microinjections of OXA in the midline thalamus decreased LA while at the same time increasing the expression of grooming and freezing. These results suggest that OXA can act on the PVT and the midline thalamus to produce arousal independent of LA. © 2009 Elsevier Inc. All rights reserved. 1. Introduction Orexin-A (OXA) and orexin-B (OXB) are novel peptides that are exclusively found in the lateral and perifornical areas of the posterior hypothalamus (de Lecea et al.,1998; Sakurai et al., 1998). While having a restricted distribution within the hypothalamus, orexin neurons provide extensive and widespread projections to many regions of the brain where orexins have been shown to regulate a number of physiological, endocrine, and homeostatic functions (Ferguson and Samson, 2003; Harris and Aston-Jones, 2006; Kukkonen et al., 2002; Sakurai, 2006). Orexins act at two different G-protein coupled receptors: the orexin-1 receptor (OX1R) which is selective for OXA and the orexin-2 receptor (OX2R) which is a nonselective for both OXA and OXB (Sakurai et al., 1998). Although different functions have been attributed to orexins, a large body of evidence suggests that orexins play a key role in brain arousal (Carter et al., 2009; Sakurai, 2007). In fact, narcolepsy, a sleep disorder associated with a difficulty in maintaining wakefulness in humans, appears to be due to a deficiency in orexin transmission in the brain (Sakurai, 2007; Taheri et al., 2002). An arousal role for orexins is supported by studies showing that administrations of OXA in the lateral ventricles in- creased locomotor activity (LA), food intake, and grooming in rodents (Espana et al., 2002; Jones et al., 2001; Nakamura et al., 2000; Rodgers et al., 2000). More recent research evidence also indicates that the orexins may produce arousal of the brain reward system and may be involved in the mechanisms of drug addiction (Boutrel and de Lecea, 2008; de Lecea et al., 2006; Harris and Aston-Jones, 2006; Harris et al., 2005). Although most nuclei of the thalamus are not innervated by orexin fibers, a group of midline and intralaminar thalamic nuclei that are associated with mechanisms of arousal and attention (Groenewegen and Berendse, 1994; Smith et al., 2004; Van der Werf et al., 2002) receive prominent innervation from orexin neurons (Kirouac et al., 2005; Peyron et al., 1998). Of the midline and intralaminar nuclei, the paraventricular nucleus of the thalamus (PVT) receives an especially dense input from orexin neurons (Kirouac et al., 2005). The PVT also contains a high density of the OX1R and OX2R mRNA (Marcus et al., 2001) and electrophysiological studies show that most PVT neurons are depolarized in the presence of orexins through both OX1R and OX2R mediated mechanisms (Huang et al., 2006; Ishibashi et al., 2005; Kolaj et al., 2007). The PVT is also notable for providing a very dense projection to the nucleus accumbens (Berendse and Groenewe- gen, 1990; Li and Kirouac, 2008; Moga et al., 1995; Vertes and Hoover, 2008), a part of the ventral striatum strongly associated with the regulation of LA and complex behaviors (Mogenson et al., 1980; Nicola, 2007; Pennartz et al., 1994). In addition to the direct Pharmacology, Biochemistry and Behavior 93 (2009) 506–514 ⁎ Corresponding author. Tel.: +1 204 977 5696; fax: +1 204 789 3913. E-mail address: kirouac@cc.umanitoba.ca (G.J. Kirouac). 0091-3057/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.pbb.2009.06.017 Contents lists available at ScienceDirect Pharmacology, Biochemistry and Behavior journal homepage: www.elsevier.com/locate/pharmbiochembeh