Increased hypocretin-1 (orexin-a) levels in cerebrospinal fluid of rats after short-term forced activity Paulo J.F. Martins a , Va ˆnia D’Almeida a,b, * , Ma ´rio Pedrazzoli a , Ling Lin c , Emmanuel Mignot c , Sergio Tufik a a Department of Psychobiology/Sleep Institute, Universidade Federal de Sa ˜o Paulo, Sa ˜o Paulo, Brazil b Department of Pediatrics, Universidade Federal de Sa ˜o Paulo, Rua Napolea ˜o de Barros 925, Sa ˜o Paulo, 04024-002, Brazil c Department of Psychiatry and Behavioral Sciences, Center for Narcolepsy, Stanford University School of Medicine, 701B Welch Road, Rm. 146, Palo Alto, CA 94304-5742, USA Received 20 May 2003; received in revised form 25 September 2003; accepted 1 October 2003 Abstract The hypocretins (orexins) are recently discovered neuropeptides initially associated with feeding behavior and sleep regulation. However, the normal function of these peptides is unclear and a number of studies have reported a role in energy homeostasis and locomotor activity. Exercise (or physical activity) is the most powerful way of challenging the internal homeostatic process. This study examines the circadian differences in response to forced activity and homeostatic challenges on hypocretin-1 (Hcrt-1) levels in the cerebrospinal fluid (CSF) of rats. Hcrt-1 levels were decreased after long-term immobilization at the end of active phase (zeigeber time-0, ZT-0) and increased after short-term forced swimming in the rest phase (ZT-8). Nevertheless, no effects were observed after short-term immobilization, total sleep deprivation or cold exposure. We concluded that despite the relation between hypocretins, stress and sleep regulation reported in the literature, short-term total sleep deprivation, immobilization and cold exposure did not induce increases in CSF Hcrt-1 levels at ZT-0 and ZT-8. On the other hand, the relationship between hypocretinergic system activation and motor activation is reinforced by decrease in Hcr-1 levels after long-term immobilization at ZT-0 and its increased levels after short-term forced swimming at ZT-8 in CSF of rats. D 2003 Elsevier B.V. All rights reserved. Keywords: Sleep deprivation; Cold; Immobilization; Swimming; Stress; Exercise 1. Introduction Hypocretins-1 and -2 (orexin A and B) are hypothalamic neuropeptides [1] initially associated with feeding behavior [2]. Later, these peptides were related to sleep–wake regulation and the pathophysiology of narcolepsy [3]. How- ever, their normal function is not fully understood, despite a number of studies suggesting a role in energy homeostasis and promoting or maintaining wakefulness [4]. Exogenous hypocretin-1 administration in rats produced a significant increase in wakefulness, running-wheel activity and spontaneous physical activity [5,6]. These findings were related to increases in cell firing of locus coeruleus [7], ventral tegmental area of dopaminergic system [8] and serotonergic system activation [9] induced by intracerebro- ventricular (ICV) hypocretin-1 administration. Moreover, hypocretin-1 injection in arcuate nucleus of rats increased oxygen consumption, heart rate and colonic temperature [10]. According to Sakurai [4], hypocretin neurons may pro- vide an integrative link between peripheral metabolism and central regulation of behaviors required for an adaptive response to homeostatic challenges. Exercise (or physical activity) is the most powerful way to challenge the internal homeostatic process [11]. The pulmonary flow and cardiac debit may increase by factors of 15 and 3.6, respectively, during exercise, and may boost oxygen consumption to 10 times the ‘at rest’ level [12]. Although ICV administration is by far the most frequent- ly investigated aspect of the functioning of hypocretins, 0167-0115/$ - see front matter D 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.regpep.2003.10.003 * Corresponding author. Rua Napolea ˜o de Barros, 925- 3rd floor Sa ˜o Paulo-, 04024-002, Brazil. Tel.: +55-11-5539-0155ext.153; fax: +55-11- 5572-5092. E-mail address: valmeida.dped@epm.br (V. D’Almeida). www.elsevier.com/locate/regpep Regulatory Peptides 117 (2004) 155 – 158