Arousal effects of orexin A on acute alcohol intoxication-induced coma in rats
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)
- et al.
Activation of orexin signal in basal forebrain facilitates the emergence from sevoflurane anesthesia in rat
Neuropeptides
(2009) - et al.
Caffeine reduces hypnotic effects of alcohol through adenosine A2A receptor blockade
Neuropharmacology
(2003) - et al.
Wake-promoting and sleep-suppressing actions of hypocretin (orexin): basal forebrain sites of action
Neuroscience
(2001) - et al.
The orexin receptor antagonist SB-334867 dissociates the motivational properties of alcohol and sucrose in rats
Brain Res.
(2011) - et al.
Orexinergic neurons and barbiturate anesthesia
Neuroscience
(2003) - et al.
Hypocretin (orexin) induces calcium transients in single spines postsynaptic to identified thalamocortical boutons in prefrontal slice
Neuron
(2003) - et al.
Design and validation of a computer-based sleep-scoring algorithm
J. Neurosci. Methods
(2004) - et al.
Involvement of the serotonergic system in orexin-induced behavioral alterations in rats
Regul. Pept.
(2002) - et al.
Decreased brain histamine content in hypocretin/orexin receptor-2 mutated narcoleptic dogs
Neurosci. Lett.
(2001) - et al.
Functional role of alpha1-adrenoceptors in the locus coeruleus: a microdialysis study
Brain Res.
(2005)
Hypocretin levels in sporadic and familial cases of canine narcolepsy
Neurobiol. Dis.
Orexins and orexin receptors: a family of hypothalamic neuropeptides and G protein-coupled receptors that regulate feeding behavior
Cell
Participation of histaminergic H1 and noradrenergic alpha 1 receptors in orexin A-induced wakefulness in rats
Brain Res.
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.
Alpha 2-adrenergic receptor-mediated presynaptic inhibition of GABAergic IPSPs in rat histaminergic neurons
Neuropharmacology
Distribution of orexin receptor mRNA in the rat brain
FEBS. Lett.
Orexins activate histaminergic neurons via the orexin 2 receptor
Biochem. Biophys. Res. Commun.
Co-expression and in vivo interaction of serotonin1A and serotonin2A receptors in pyramidal neurons of prefrontal cortex
Cereb. Cortex
Adrenergic targets for the treatment of cognitive deficits in schizophrenia
Psychopharmacology (Berl.)
Orexins (hypocretins) directly excite tuberomammillary neurons
Eur. J. Neurosci.
Exclusive postsynaptic action of hypocretin-orexin on sublayer 6b cortical neurons
J. Neurosci.
Sleep homeostasis and models of sleep regulation
J. Biol. Rhythms
Hypocretin-1 modulates rapid eye movement sleep through activation of locus coeruleus neurons
J. Neurosci.
Convergent excitation of dorsal raphe serotonin neurons by multiple arousal systems (orexin/hypocretin, histamine and noradrenaline)
J. Neurosci.
Direct and indirect excitation of laterodorsal tegmental neurons by hypocretin/orexin peptides: implications for wakefulness and narcolepsy
J. Neurosci.
Undetectable CSF hypocretin-1 in “Hashimoto’s encephalopathy” associated with coma
Neurology
Hypocretin-1 potentiates NMDA receptor-mediated somatodendritic secretion from locus ceruleus neurons
J. Neurosci.
The hypocretins: hypothalamus-specific peptides with neuroexcitatory activity
Proc. Natl. Acad. Sci. U.S.A.
Sleep regulation in the Djungarian hamster: comparison of the dynamics leading to the slow-wave activity increase after sleep deprivation and daily torpor
Sleep
Cited by (23)
Trigeminal nerve electrical stimulation: An effective arousal treatment for loss of consciousness
2021, Brain Research BulletinCitation Excerpt :Here we found that TNS treatment can obviously reverse the TBI-induced decreases of entropy (Fig. 2D). Alterations in the levels of endogenous peptides, depending on the brain region and injury severity, were frequently reported in humans and animal models in LOC state (Jia et al., 2012; Kang et al., 2017). To determine if consciousness behavior was mediated by changes in neuropeptides profiles, we quantified the expression of five neuropeptides in CSF during the first 4 h following TBI.
Orexin-A promotes EEG changes but fails to induce anxiety in rats
2019, Behavioural Brain ResearchTransposable elements, placental development, and oocyte activation: Cellular stress and AMPK links jumping genes with the creation of human life
2018, Medical HypothesesCitation Excerpt :Several neurotransmitters that are critical for promoting wakefulness and maintaining arousal activate AMPK, including acetylcholine, norepinephrine, orexin, serotonin, histamine, and dopamine [180,192–197]. Furthermore, orexin-A exerts arousal effects on alcohol-induced unconscious rats, microinjections of histaminergic agonists into the nucleus basalis magnocellularis (NBM) facilitates emergence from isoflurane anesthesia, and the anticholinesterase physostigmine (decreases the breakdown of acetylcholine) reverses propofol-induced loss of consciousness in human volunteers [198–200]. Additionally, methylphenidate (an inhibitor of dopamine and norepinephrine transporters) induces emergence during continuous propofol anesthesia in rats and activation of D1 dopamine receptors promotes AMPK activation and induces emergence in isoflurane-anesthetized rats, providing further evidence that AMPK activation (e.g. via metformin, etc.) may represent a central node that links consciousness and accelerated emergence from anesthesia with placental development, TE activation/transposition, and oocyte maturation and activation [195,201,202].