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Sleep and Vigilance

, Volume 2, Issue 1, pp 33–38 | Cite as

Neurobiological Role of Hypocretin in Regulation of Psychiatric Disorders

  • Suzana Monteiro
  • Barbara Monteiro
  • Flavia Paes
  • Antônio Egídio Nardi
  • Nuno Rocha
  • Eric Murillo-Rodriguez
  • Sergio Machado
Review
  • 332 Downloads

Abstract

Hypocretins are hypothalamic neuropeptides acting on the regulation of several physiological functions, the most important being the control of arousal. Hypocretin 1 and hypocretin 2 are derived from the same precursor and both bind to the orexin receptors. The hypocretinergic system has been a target of several studies that try to understand its function on the regulation of mood and behavior. The hypocretinergic system has a direct relationship with the pathways related to emotions and reward system, besides the interaction with the stress circuit. This article aims to analyze the relationship of hypocretins with anxiety, stress and depression, through a review of the existing literature.

Keywords

Hypocretin Anxiety Depression Stress 

1 Introduction

Hypocretins, also known as orexins, are neuropeptides synthesized in the posterior lateral hypothalamus, with involvement in various physiological functions and pathological conditions [1]. Hypocretin 1 (named as orexin A) and hypocretin 2 (named as orexin B) are derived from the same precursor peptide containing 33 and 28 amino acids, respectively, and which bind to the orexin receptors [2]. Orexin 2 receptors (OXR2) bind to both forms with the same affinity, whereas the orexin type 1 receptor (OXR1) displays a higher affinity for hypocretin 1 [2]. Although hypocretinergic neurons are present in the posterior lateral hypothalamus, they project pathways to the central nervous system (spinal cord, brainstem, hypothalamus, thalamus, some cortical regions and limbic system) and also peripheral system, including the vagus nerve [3]. In addition, orexin receptors have an extensive distribution suggesting a relevant role in adaptive functions and functions regulated by the limbic system (Fig. 1). The distribution of these receptors is extensive and most often the subtypes overlap. However, some brain regions preferentially express a subtype, suggesting a certain degree of selectivity [3].
Fig. 1

The distribution of the wake-modulating system in the brain. The drawing shows elements that participate in the control of waking including: tuberomammillary nucleus (TMN; histamine [HA]), basal forebrain (BF; acetylcholine (ACh)/GABA), orexin, locus coeruleus (LC; noradrenaline [NE]), raphe nuclei (serotonin, 5-HT), substantia nigra (SN)/ventral tegmental area (VTA) and ventral periaqueductal gray (vPGAM; dopamine, [DA])

OXR1 expression is observed in various brain regions, including the prefrontal and infralimbic cortex, hippocampus, amygdala, dorsal striatum nucleus, paraventricular thalamic nucleus, anterior hypothalamus, dorsal raphe and locus coeruleus [4, 5]. The OXR2 subtype is found in the amygdala, the nucleus of the dorsal striatum, paraventricular thalamic nucleus, dorsal raphe and peduncle–pontine nuclei and accumbens [4, 5]. Recent studies suggest that OXR1 is involved in various functions, especially regulation of emotion, reward system and autonomic function [6], while OXR2 is mainly involved in the regulation of wakefulness [7]. This divergence in receptor functions is also observed in the regulation of mood and affect, having an important role in depression, anxiety and stress [8].

The hypocretinergic system is modulated by multiple endocrine signals and neural inputs from other areas, suggesting that this system is influenced by internal and external stimuli and is involved in several physiological functions, including the sleep–wake cycle, energetic metabolism, behavioral and neuroendocrinological responses to stress and reward system [9, 10, 11]. Diseases such as depression and anxiety, as well as conditions such as stress would affect this system, which will have the potential to modulate mood through its neuroanatomical projections and the expression of its receptors in certain brain regions [8].

Dysregulation of sleep/wake cycle predisposes to metabolic and psychiatric disorders [4]. The evidences of the role of the hypocretinergic system on the modulation of several physiological functions and mental diseases emerged after the discovery of this system, 15 years ago [10]. But the exact mechanism how this modulation occurs is not well known. In the last decade, a significant advance was made on the complex interactions between brain systems that control the transition of sleep and wake states [4]. Recently, a lot of research has been done with OXR1 and OXR2 antagonists, promising therapeutic target and encouraging investments of the pharmaceutic industry on basics researches. The database of these pre-clinical studies is limited and the involvement of the hypocretinergic system is still questionable [10]. Thus, the objective of the present study was to critically discuss the relationship between orexin and psychiatric disorders. Here, a literature search was conducted using the databases PubMed, ISI Web of Knowledge and PsycInfo combining the following terms: “hypocretin”, “depression”, “stress” and “schizophrenia”. All articles were published in English and without temporal restriction. Additional references were identified through hand search of the selected articles.

2 Hypocretin as Regulator of Waking State

The waking state is controlled by several neurobiological networks, including circadian, homeostatic influences as well as by genetic, molecular, neuroanatomical and neurochemical elements [12, 13, 14]. For example, waking is modulated by the activity of diverse neurotransmitter systems such as noradrenaline, dopamine, serotonin, acetylcholine, histamine and hypocretin [3, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24] (Fig. 2). A small number of hypocretinergic neurons are exclusively localized into the lateral hypothalamic area and send their projections throughout the brain as well as the spinal cord [25, 26, 27]. One might think that as a result of this projection pathway, the hypocretinergic system might have multiple physiological functions apart from controlling wakefulness, including motor control, drug reward and mental health [28, 29, 30]. In the following sections, we highlight the experimental evidence published in the literature regarding hypocretin as a mood modulator.
Fig. 2

Schematic representation of the hypocretinergic system that modulates the sleep–wake cycle. Hypocretins/orexins placed in the hypothalamus send inhibitory rostral projections to the thalamus and basal forebrain containing cholinergic and GABAergic neurons. Moreover, hypocretin/orexin cluster of neurons in the hypothalamus sends stimulatory caudal projections to noradrenergic (NE, locus coeruleus), cholinergic (ACh, PPT/LDT), histaminergic (HA, tuberomammillary nucleus) and dopaminergic (DA) as well as serotoninergic (5-HT, raphe nuclei) nuclei

3 Psychiatric Disorders Regulated by Hypocretins

The hypocretinergic system has been related with narcolepsy, a disorder characterized by hypersomnolence during normal wakefulness [3, 31, 32, 33, 34, 35]. However, recent evidence has suggested that the hypocretinergic system is linked with other health issues, such as obesity, mood and other psychiatric disorders [28, 29, 36, 37, 38, 39]. This system has a close relationship (functional and anatomical) with pathways that regulate the autonomic system, mood, emotions and the reward system [5].

3.1 Mood Regulated by Hypocretins

The relationship between hypocretin and mood disorders, specifically depression, has received special attention in recent years with divergent findings among various authors. There has been an association between OXR1 gene polymorphism and mood disorders and high hypocretin levels correlated with positive content emotions and social interaction. In addition, non-genetic factors such as chronic stress also cause hypocretin to be involved in the pathogenesis of depression [4]. The projection of hypocretinergic neurons to the hippocampus explains the involvement of this system in the learning and memory-related process of depression, known as learned helplessness [1, 2, 4, 40].

It is possible that dysregulation of the hypothalamic–pituitary–adrenal axis (HPA), which is often associated with major depression, is in part an inability of the paraventricular thalamic nucleus to adapt to chronic stress. More recent studies demonstrate that exposure to a stressor stimulus over a long period of time would lead to a downregulation of hypocretinergic activity. This early exposure to stress would increase the levels of hypocretin 1 in the hypothalamic regions and the expression of OXR1 in the frontal cortex; thus, there would be a consequent reduction in the synaptic availability of hypocretin (hypocretinergic dysfunction) [41]. In contrast, in the case of patients with depression and their reduction after treatment with anti-depressant sertraline, Salomon et al. [42] demonstrated the opposite effect, reporting high levels of hypocretin 1 in CSF, unlike Schmidt et al. [43], who did not show any association between depression and cerebrospinal fluid levels of hypocretin.

Two hypotheses explain these divergent findings. First, we must consider that depression is a naturally heterogenic disease, influenced by environmental, genetic and comorbid factors [4, 8]. Second, recent studies report divergent roles for hypocretin receptors, and both exert functions that counterbalanced brain regions involved in mood regulation, such as the hippocampus, frontal tegmental area and prefrontal cortex [4, 8]. In the study by Scott et al. [44], knockout mice for OXR1 presented decrease in depressive symptoms, whereas knockout mice for OXR2 showed an increase of these symptoms.

3.2 Anxiety Regulated by Hypocretins

The subnuclei of the basolateral and central amygdala, the prefrontal cortex and the paraventricular thalamic nucleus are regions known to be involved in anxiety and all have hypocretinergic connections [2]. The application of hypocretin 1 or 2 in the central sub-nucleus of the amygdala, rich in hypocretinergic projections, excites the neurons of this region producing an increase in anxious behavior [2, 11]. On the other hand, inhibition of glutamate and the action of OXR2 in the basolateral region would lead to a relief in anxiety. Studies have shown an increase in the release of hypocretin in the amygdala and in the cerebrospinal fluid of anxiety patients, suggesting a possible hyperactivity state of the hypocretinergic system in these patients [2, 11]. In these studies, the levels of hypocretin increased during wakefulness and fell during sleep, but the highest peak was during an acute emotional state, whether positive or negative.

The pre-limbic region of the prefrontal cortex would be, for anatomical and functional reasons, a potential area for the action of hypocretin in the regulation of anxiety, but no study has demonstrated this relationship. Heydendael et al. [45] have demonstrated that the stimulation of hypocretinergic receptors in the paraventricular thalamic nucleus produces fear and anxiety-like behavior and the blockade of these receptors in the same area has anxiolytic effects.

3.3 Stress Regulated by Hypocretins

The role of hypocretin in chronic stress occurs through the depolarization of neurons present in the paraventricular thalamic nucleus (PVN), a site rich in type 1 and 2 receptors. This nucleus plays an important role in the regulation of the neuroendocrine system and in the behavioral adaptation after a severe stressor stimulus. In chronic stress, it is necessary to reduce the circadian rhythm of body temperature and the activation of the hypothalamic–pituitary–adrenal (HPA) axis, and may also be the link between circadian cycle disorders, chronic stress and depression, mainly through the hypocretinergic pathways [4, 6].

Hypocretin would activate the HPA axis, including corticotrophin releasing hormone (CRH), adrenocorticotropic hormone (ACTH) and corticosterone, stimulating stress behavior [9, 46]. The hypocretinergic system, in addition to receiving inputs from CRH-producing pathways, also sends projections that stimulate the brain regions responsible for the production of this hormone [10]. This evidence suggests that these two systems are involved in stress management, although acting in different brain areas. While the CRH acts on the motor cortex, the prefrontal cortex, the dorsal portions of the caudate and putamen nuclei, the cingulate and amygdala, the hypocretin act on the nucleus accumbens, dorsal thalamus, amygdala, ventral hippocampus and frontal cortex [10]. The region that has the largest neural network of CRH stimulation to the hypocretinergic neurons is the nucleus of the dorsal striatum (BNST), belonging to the limbic system [41].

3.4 Other Psychiatric Disorders Regulated by Hypocretins

Regarding psychiatric disorders, schizophrenia affects approximately 1% of the world’s population. This disease has been managed by using pharmacological means including compounds that interact with monoaminergic transmission [47, 48, 49, 50]. Recent data have suggested the putative role of hypocretin in modulation of schizophrenia [36]. For example, Sansa and coworkers [39] reported that patients with schizophrenia showed positive correlation with human leukocyte antigen (HLA) DQB1*06:02. Importantly, narcolepsy with cataplexy has been strongly associated with the same HLA [51]. Thus, it is likely that hypocretin might be present in patients with schizophrenia. Further studies have confirmed this observation. For instance, the plasma levels of hypocretin 1 in 127 patients with schizophrenia were determined. In patients, the clinical symptoms on the Positive and Negative Syndrome Scale for schizophrenia as well as executive function by the Wisconsin Card Sorting test (WCST) were assessed. Basically, it was found that patients with schizophrenia had higher levels of hypocretin 1 compared to healthy controls and schizophrenic patients tended to have lower perseverative errors and higher failures to maintain set in the WCST [52]. These findings corroborated with previous studies [53] and the results are in concordance with posterior observations [37, 54]. Beyond the neurobiological role of hypocretin on the modulation of sleep–wake cycle, an accumulative body of evidence has suggested the putative influence of this peptidergic system in schizophrenia. Thus, a novel and interesting approach for targeting hypocretin for the development of novel antipsychotic medications remains to be elucidated.

4 Orexin Receptor Antagonists

New drugs involving the hypocretinergic system are being studied. Several different chemical structures can bind to one or both hypocretin receptors; when the antagonist binds to a single receptor, it is called SORAs, and dual antagonists are called DORAs. Four DORAs have undergone clinical trials: almorexant, suvorexant, filorexant and SB-649868. Only suvorexant went through phase 3 and it was filed in the USA and Japan as a new treatment for insomnia in 2013. It selectively blocks neuropeptides hypocretin 1 and 2 from binding to the orexin receptor types 1 and 2, suppressing wakefulness [10, 55]. The tight regulation of the sleep/wake cycle is critical for mental well-being and drugs that acts in this cycle could have a place in psychiatry. Significant progress has been made in the knowledge of the contribution of the orexin system to mental health; for example, several studies indicate that orexin antagonists, in particular selective OXR1 antagonist, can reduce drug seeking [4, 55]. There is data indicating that depression may be associated with decreased orexin system function, and this knowledge raises concerns for the long-term use of orexin antagonists. Therefore, a greater understanding of the changes to orexin receptor expression in the brain areas will be necessary to predict the outcomes of therapeutic manipulation of orexin signaling [55].

5 Conclusions

Hypocretin is involved in the modulation of several neurobiological systems, both central and peripheral, but the exact mechanism by which this modulation occurs is not well known. Growing interest has been seen toward this system and its role in modulating anxious and depressive behavior. Advances in orexin antagonists may lead to the development of new molecules for the treatment of sleep disorders and psychiatric conditions. New researches need to be done to understand all mechanisms involved in psychiatric disorders and the hypocretinergic system.

Notes

Compliance with Ethical Standards

Ethical Standards

All data reported in this paper are from public repositories.

Conflict of interest

The authors declare no conflict of interest.

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Copyright information

© Springer Nature Singapore Pte Ltd. 2018

Authors and Affiliations

  1. 1.Laboratory of Panic and RespirationInstitute of Psychiatry of Federal University of Rio de JaneiroRio De JaneiroBrazil
  2. 2.Health SchoolPolytechnic Institute of PortoPortoPortugal
  3. 3.Intercontinental Neuroscience Research GroupPortoPortugal
  4. 4.Laboratorio de Neurociencias Moleculares e Integrativas, Escuela de Medicina, División Ciencias de la SaludUniversidad Anáhuac MayabMéridaMexico
  5. 5.Grupo de Investigación en Envejecimiento, División Ciencias de la SaludUniversidad Anáhuac MayabMéridaMexico

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