Orexin-A (hypocretin-1) is possibly involved in generation of anxiety-like behavior

doi:10.1016/j.brainres.2005.03.002

Brain Research

Volume 1044, Issue 1, 17 May 2005, Pages 116-121

https://doi.org/10.1016/j.brainres.2005.03.002 Get rights and content

Abstract

Orexins (hypocretins) are neuropeptides expressed specifically in neurons in the lateral hypothalamic area and are known to be involved in the regulation of vigilance and feeding behavior. However, the relationship between orexin and emotional behaviors like anxiety is still poorly understood. Therefore, in this report we evaluated the effect of intracerebroventricular injection of orexin-A in two major anxiety tests, the light–dark exploration test (mouse) and the elevated plus-maze test (mouse, rat). Orexin increased time spent in the dark compartment in the light–dark test and time spent on the closed arms in the elevated plus-maze test. These results were not caused by a hypothetical sedative or activity-inducing effect of orexin-A because spontaneous locomotor activity did not alter upon orexin-A application under novel conditions. We therefore suggest an anxiogenic effect of orexin-A. To our knowledge, this is the first report about a relationship between orexin-A and anxiety.

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.

Section snippets

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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Research reportOrexin-A (hypocretin-1) is possibly involved in generation of anxiety-like behavior

Abstract

Introduction

Section snippets

Materials

Light–dark exploration test (mouse)

Discussion

Acknowledgments

Cell

Eur. J. Pharmacol.

Eur. J. Pharmacol.

Brain Res.

Biochem. Biophys. Res. Commun.

Brain Res.

J. Neurosci. Methods

Eur. J. Pharmacol.

Neuropharmacology

Cell

Neuropharmacology

Research report
Orexin-A (hypocretin-1) is possibly involved in generation of anxiety-like behavior