Elsevier

Neuropharmacology

Volume 62, Issue 2, February 2012, Pages 775-783
Neuropharmacology

Arousal effects of orexin A on acute alcohol intoxication-induced coma in rats

https://doi.org/10.1016/j.neuropharm.2011.08.047Get rights and content

Abstract

The key role of the hypothalamic neuropeptides orexins in maintenance and promotion of arousal has been well established in normal mammalian animals, but whether orexins exert arousal effects under pathological condition such as coma was little studied. In this study, a model of unconscious rats induced by acute alcohol intoxication was used to examine the effects of orexins through intracerebroventricular injection. The results revealed that either orexin A or orexin B induced decrease of duration of loss of right reflex in alcohol-induced unconscious rats. In the presence of the selective orexin receptor 1 antagonist SB 334867 and orexin receptor 2 antagonist TCS OX2 29, the excitatory action of orexin A was completely blocked. Our data further presented that orexin A also induced reduction of delta power in EEG in these rats. Single-unit recording experiment in vivo demonstrated that orexin A could evoke increase of firing activity of prefrontal cortex neurons in unconscious rats. This excitation was completely inhibited by an H1 receptor antagonist, pyrilamine, whereas application of α1-adrenoreceptor antagonist prazosin or 5-HT2 selective receptor antagonist ritanserin partially attenuated the excitatory effects of orexin A on these neurons. Consistently, the results of EEG recordings showed that microinjection of pyrilamine, prazosin, or ritanserin suppressed reduction of delta power in EEG induced by orexin A on unconscious rats. Thus, these data suggest that orexins exert arousal effects on alcohol-induced unconscious rats by the promotion of cortical activity through activation of histaminergic, noradrenergic and serotonergic systems.

This article is part of a Special Issue entitled ‘Post-Traumatic Stress Disorder’.

Highlights

► Alcohol-induced comatose rats were used to examine the excitatory effect by orexins. ► Orexins exert a promoting-arousal effect on these rats by activation of two receptors. ► The excitatory effects by orexins depend on activation of subcortical arousal systems.

Introduction

Orexin A and B (hypocretin-1 and -2, respectively) are hypothalamic neuropeptides cleaved from the same preprotein, preprohypocretin/preproorexin (de Lecea et al., 1998, Sakurai et al., 1998). Several lines of evidence have confirmed the key role of the orexin system in maintaining state of wakefulness (Saper et al., 2005, Sutcliffe and de Lecea, 2002), which mainly depends on excitatory actions on several subcortical arousal systems, including noradrenergic locus coeruleus (LC), histaminergic tuberomammillary nuclei (TMN), serotonergic dorsal raphe (DR), and the nonspecific thalamocortical projection system (Bayer et al., 2001, Bourgin et al., 2000, Brown et al., 2002, Burlet et al., 2002, Chen et al., 2008, Eriksson et al., 2001, Hagan et al., 1999, Huang et al., 2001, Jones, 2003, Trivedi et al., 1998), as well as the cerebral cortex (Bayer et al., 2004, Li et al., 2010, Song et al., 2006, Song et al., 2005, Xia et al., 2005a, Xia et al., 2009). Nonetheless, little is known about whether orexins exert an arousal-promoting effect under pathological condition such as coma. Clinical investigations have shown that the level of orexin A in cerebrospinal fluid (CSF) is undetectable in “Hashimoto’s encephalopathy” associated with coma (Castillo et al., 2004). Furthermore, Rejdak et al. describe a decreased level of orexin A in the CSF of patients after acute brain injury caused by haemorrhagic stroke (Rejdak et al., 2005). These results are in line with the previous observation in patients with traumatic brain injury (Ripley et al., 2001). Most importantly, orexins have been found to play a pivotal role in the emergence from anesthesia (Kelz et al., 2008, Shirasaka et al., 2011) by significantly shortening anesthesia time (Kushikata et al., 2003) and inducing electroencephalogram (EEG) arousal in these anesthetized rats (Dong et al., 2009, Dong et al., 2006). Together these findings have led to speculation that pharmacological manipulation of orexin system may open new avenue for the treatment of coma.

Coma is a state of continuous ‘eyes-closed’ unconsciousness characterized by the total absence of arousal and awareness, which is often caused by a variety of factors such as chemical intoxications, serious diseases, and physical injuries (Schiff and Plum, 2000). Among these, acute alcohol intoxication has been one of the most common reasons of coma because alcohol is an available, legal, and frequently used drug and intoxicant worldwide (Tokuda et al., 2003). At high concentrations, alcohol acts as an anesthetic and respiratory depressant on the central nervous system, which impairs cognitive processing and even results in coma (Givens, 1997). Prefrontal cortex (PFC), a brain region with higher EEG frequency during waking and critical for cognitive function (Lambe and Aghajanian, 2003, Yamasaki et al., 2002), is more vulnerable to the effects of alcoholism than other brain regions/systems (Oscar-Berman and Marinkovic, 2003). Expressions of orexin receptors have been observed in deeper layers of PFC (van den Pol, 1999), and our previous electrophysiological experiments have demonstrated orexin A exerts an excitatory effect on the firing activities of PFC neurons in rats in vitro (Li et al., 2010, Song et al., 2006, Song et al., 2005, Xia et al., 2005a). Whether orexins exert a promoting-arousal effect in alcohol-induced unconscious rats and the effect of orexins on PFC neurons in unconscious rats in vivo is still unclear.

In the present study, we examined the effects of intracerebroventricular (i.c.v.) injection of orexin A on the behavior and EEG of unconscious rats induced by acute alcohol intoxication as well as the spontaneous activities of PFC neurons by single-unit recording in vivo. We then further explored the possible mechanism involved in the excitatory effects of orexin A on alcohol-induced unconscious rats by using orexin receptor 1 (OX1R) antagonist SB 334867 and orexin receptor 2 (OX2R) antagonist TCS OX2 29.

Section snippets

Animals

Adult female Sprague-Dawley rats weighing 230–280 g were used in this study. Animals were individually housed on a 12-h light/12-h dark cycle. Experiments were performed during the light phase. Animal use and all experimental protocols were approved by the Third Military Medical University Animal Care Committee.

Loss of righting reflex test

According to an earlier report (El Yacoubi et al., 2003), alcohol was given i.p. at 4.48 g/kg in a v/v solution of 32% to induce coma. An animal was considered to have lost its righting

The inhibitory effects of alcohol on the duration of LORR and EEG of rats

The changes in behavior and EEG of unconscious rats were judged by the duration of LORR and the comparison of the percentage of delta wave in EEG before and after application of alcohol. LORR was observed shortly after injection (i.p.) of 32% ethanol solution and lasted for 3.07 ± 0.94 h (n = 30). Concurrently, the delta activity in EEG, a marker of slow wave sleep homeostasis (Borbely and Achermann, 1999), was assessed. As shown in Fig. 1A, no delta activities were observed in the waking rats,

Discussion

The results of the current study mainly demonstrate that i.c.v. microinjection of orexin A may exert an arousal-promoting effect in coma induced by acute alcohol intoxication, because orexin A reduces the duration of LORR as well as the delta power in EEG in the unconscious rats. The arousal effects of orexin A on these unconscious rats may be realized by the activation of histaminergic, noradrenergic and serotonergic systems, which lead to the increase of excitability in cerebral cortex, such

Acknowledgments

We acknowledge the support of the National Natural Science Foundation of China (No. 30870817), Program of Science and Technology of PLA (No. 06MB237) and the Major State Basic Research Development of China (No. 2011CB503700).

References (59)

  • B. Ripley et al.

    Hypocretin levels in sporadic and familial cases of canine narcolepsy

    Neurobiol. Dis.

    (2001)
  • T. Sakurai et al.

    Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior

    Cell

    (1998)
  • Y. Shigemoto et al.

    Participation of histaminergic H1 and noradrenergic alpha 1 receptors in orexin A-induced wakefulness in rats

    Brain Res.

    (2004)
  • C.H. Song et al.

    Modulatory effects of hypocretin-1/orexin-A with glutamate and gamma-aminobutyric acid on freshly isolated pyramidal neurons from the rat prefrontal cortex

    Neurosci. Lett.

    (2006)
  • D.R. Stevens et al.

    Alpha 2-adrenergic receptor-mediated presynaptic inhibition of GABAergic IPSPs in rat histaminergic neurons

    Neuropharmacology

    (2004)
  • P. Trivedi et al.

    Distribution of orexin receptor mRNA in the rat brain

    FEBS. Lett.

    (1998)
  • A. Yamanaka et al.

    Orexins activate histaminergic neurons via the orexin 2 receptor

    Biochem. Biophys. Res. Commun.

    (2002)
  • M. Amargos-Bosch et al.

    Co-expression and in vivo interaction of serotonin1A and serotonin2A receptors in pyramidal neurons of prefrontal cortex

    Cereb. Cortex

    (2004)
  • A.F. Arnsten

    Adrenergic targets for the treatment of cognitive deficits in schizophrenia

    Psychopharmacology (Berl.)

    (2004)
  • L. Bayer et al.

    Orexins (hypocretins) directly excite tuberomammillary neurons

    Eur. J. Neurosci.

    (2001)
  • L. Bayer et al.

    Exclusive postsynaptic action of hypocretin-orexin on sublayer 6b cortical neurons

    J. Neurosci.

    (2004)
  • A.A. Borbely et al.

    Sleep homeostasis and models of sleep regulation

    J. Biol. Rhythms

    (1999)
  • P. Bourgin et al.

    Hypocretin-1 modulates rapid eye movement sleep through activation of locus coeruleus neurons

    J. Neurosci.

    (2000)
  • R.E. Brown et al.

    Convergent excitation of dorsal raphe serotonin neurons by multiple arousal systems (orexin/hypocretin, histamine and noradrenaline)

    J. Neurosci.

    (2002)
  • S. Burlet et al.

    Direct and indirect excitation of laterodorsal tegmental neurons by hypocretin/orexin peptides: implications for wakefulness and narcolepsy

    J. Neurosci.

    (2002)
  • P.R. Castillo et al.

    Undetectable CSF hypocretin-1 in “Hashimoto’s encephalopathy” associated with coma

    Neurology

    (2004)
  • X.W. Chen et al.

    Hypocretin-1 potentiates NMDA receptor-mediated somatodendritic secretion from locus ceruleus neurons

    J. Neurosci.

    (2008)
  • L. de Lecea et al.

    The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity

    Proc. Natl. Acad. Sci. U.S.A.

    (1998)
  • T. Deboer et al.

    Sleep regulation in the Djungarian hamster: comparison of the dynamics leading to the slow-wave activity increase after sleep deprivation and daily torpor

    Sleep

    (2003)
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