Blockade of orexin receptor 1 attenuates the development of morphine tolerance and physical dependence in rats Elaheh Erami a , Hassan Azhdari-Zarmehri b, c, ⁎, Abolfazl Rahmani b, c , Elmira Ghasemi-Dashkhasan b, c , Saeed Semnanian d , Abbas Haghparast e a Physiology- Pharmacology Research Center, Rafsanjan University of Medical Sciences, Rafsanjan, Iran b Cellular and Molecular Research Center, Qazvin University of Medical Sciences, Qazvin, Iran c Department of Physiology, Qazvin University of Medical Sciences, Qazvin, Iran d Department of Physiology, School of Medical Sciences, Tarbiat Modares University, Tehran, Iran e Neuroscience Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran abstract article info Article history: Received 7 February 2012 Received in revised form 12 August 2012 Accepted 15 August 2012 Available online 27 August 2012 Keywords: Morphine Orexin receptor 1 Tolerance Dependence Withdrawal syndrome Rat The goals of this study were to evaluate the effects of pretreatment by orexin receptor-1 antagonist on the development of morphine tolerance and physical dependence in rat. Animals were rendered dependent on morphine by subcutaneous (SC) injection of morphine sulfate (10 mg/kg) at set intervals of 12 h for 10 days. Just before the morphine administration, the animals received SB-334867, a selective orexin recep- tor 1 (OXR1) antagonist. To assess morphine tolerance, the antinociceptive responses of morphine were mea- sured using the warm-water tail immersion test before and after its administration. On day 11, naloxone was injected 2 h after morphine administration and the physical dependence evaluated by quantifying/scoring naloxone-precipitated withdrawal signs for 30 min. The effect of chronic SB-334867 on locomotion was carried out by calculating the number of grid crossings as a measure of locomotor activity. Our findings dem- onstrated that although morphine-tolerance tended to develop in response to repeated injections of mor- phine, pre-treatment of OXR1 antagonist prevented this effect, causing a delay in the development of morphine-tolerance. Moreover, co-administration of orexin receptor 1 antagonist with morphine significant- ly decreased the somatic signs of withdrawal including diarrhea, teeth chattering, jumping, and defecation. Administration of SB-334867 alone or in a chronic co-administration with morphine failed to change locomotor activity. These results suggest that the activation of OXR1 might be involved in the development of morphine tolerance and dependence. © 2012 Elsevier Inc. All rights reserved. 1. Introduction Morphine is one of the strongest analgesics widely used for the re- lief of severe pain, however; the drawback on repeated administra- tion of morphine that induces a physical dependence is highly limited to the use of opium. Development of morphine tolerance and physical dependence start with adaptive mechanisms which result in short-term as well as long-term alterations in the opioid-sensitive neural functions and networks (Williams et al., 2001). It has been reported that orexins (hypocretins) are important neuropeptides with a wide variety of functions such as the regulation of appetite (Sakurai et al., 1998), neuroendocrine functions (Date et al., 1999; Russell et al., 2001), involvement in arousal (Chemelli et al., 1999; Lin et al., 1999), pain modulation (Bingham et al., 2001; Azhdari Zarmehri et al., 2008, 2011) and that OX1R is also involved in stress-induced analgesia (Watanabe et al., 2005; Sofi-Abadi et al., 2011). Moreover, orexin cells were demonstrated to be involved in motivated behaviors for rewards and addiction to morphine (Georgescu et al., 2003; Narita et al., 2006; Sharf et al., 2008; Li et al., 2010). Orexin-A (ORXA, a cyclic 33-amino acid peptide, also known as hypocretin-1) and orexin-B (ORXB, or hypocretin-2, a linear 28-amino acid peptide) are produced from the precursor protein (prepro-orexin) in neurons which are restricted to a few regions of the lateral hypothalamus (LH) (Peyron et al., 1998; Sakurai et al., 1998). ORXA and ORXB activate two G-protein coupled receptors, the orexin receptor 1 (OX1R) and orexin receptor 2 (OX2R) (Sakurai et al., 1998). Orexin receptors and orexinergic projections are localized in the regions previously shown to play a role in mor- phine dependence and tolerance (Peyron et al., 1998; Mondal et al., 1999). Although the orexinergic system plays an important role in the expression of withdrawal signs in animals dependent on mor- phine (Georgescu et al., 2003; Sharf et al., 2008; Azizi et al., 2010), the involvement of orexins and their receptors in the development Pharmacology, Biochemistry and Behavior 103 (2012) 212–219 ⁎ Corresponding author at: Cellular and Molecular Research Center, Department of Physiology, Qazvin University of Medical Sciences, Iran. Tel./fax: +98 281 3336005. E-mail address: hasan.azhdari@gmail.com (H. Azhdari-Zarmehri). 0091-3057/$ – see front matter © 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.pbb.2012.08.010 Contents lists available at SciVerse ScienceDirect Pharmacology, Biochemistry and Behavior journal homepage: www.elsevier.com/locate/pharmbiochembeh