Research reportOrexin-A (hypocretin-1) is possibly involved in generation of anxiety-like behavior
Introduction
Orexin-A (hypocretin-1) and orexin-B (hypocretin-2) were initially identified as endogenous peptide ligands for two orphan G protein-coupled receptors [20]. Mammalian orexin-A is a 33-amino acid peptide with two sets of intrachain disulfide bonds and mammalian orexin-B is a 28-amino acid peptide. Two orexin receptor subtypes named orexin receptor-1 (OX1R) and orexin receptor-2 (OX2R) have been identified in mammals [20]. OX1R has almost 50 times greater affinity for orexin-A as compared to orexin-B, while OX2R has comparable affinities for both orexin-A and orexin-B [20].
Orexin-producing neurons are localized to the lateral hypothalamic area and posterior hypothalamus [4], [11], [15], [17], [20]. These neurons project widely to various brain regions; among them cerebral cortex, olfactory bulb, hippocampus, amygdala, septum, diagonal band of Broca, bed nucleus of the stria terminals, thalamus, anterior and posterior hypothalamus, midbrain, brainstem and spinal cord have been described [4], [11], [15], [17], [20]. This wide distribution of orexin projections suggests that orexin peptides play a substantial role in various physiological functions. Indeed, it has been reported that at least orexin-A is involved in the control of vigilance and feeding behavior [2], [6], [20].
Recently, reports indicating an involvement of orexin in response to stress are emerging. For example, intracerebroventricular (icv) injection of orexin-A increased stress-like behaviors such as face washing, grooming, searching and burrowing [8], [9]. However, until now there have been only speculations about an effect of orexin on anxiety. The purpose of the present study was to investigate the central effect of orexin-A on the anxiety level of rodents by using light–dark exploration and elevated plus-maze paradigms.
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Materials
All animal experiments were approved by the Animal Care and Use Committee of Eisai. Male C57BL/6NCrj mice (25–30 g, Charles River, Japan) were housed 10 per cage, and used for light–dark exploration test, elevated plus-maze test and measurement of spontaneous locomotor activity. Male WISTAR rats (300–400 g, Charles River, Japan) were housed two per cage and used for elevated plus-maze test and measurement of spontaneous locomotor activity. All animals were maintained under a 12:12-h light–dark
Light–dark exploration test (mouse)
Time spent in the light compartment was significantly reduced by icv injection of orexin-A (Fig. 1A). There was significant difference among four groups (F(3,38) = 4.11, P < 0.05, ANOVA). There were also significant differences between aCSF-injected group and orexin-A (0.1 nmol)-injected group (P < 0.05, t test), and between aCSF-injected group and orexin-A (1.0 nmol)-injected group (P < 0.01, t test).
By centrally applied CRF, an anxiety-inducing neuropeptide [1], [22], we could elicit a
Discussion
Present findings indicate that icv injection of orexin-A has an anxiogenic-like effect in the mouse light–dark exploration test and in the mouse elevated plus-maze test and is likely to have one in the rat elevated plus-maze test. In the light–dark exploration test, orexin-A-injected mice displayed less exploration in the aversive light compartment than aCSF-injected ones. In the elevated plus-maze test, orexin-A-injected mice displayed less exploration in the aversive open arms than
Acknowledgments
We gratefully acknowledge Risa Ryu for expert technical assistance and Dennis F. Fiorino for helpful discussions.
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