The Orexin/Hypocretin System: Functional Roles and Therapeutic Potential

The Orexins/Hypocretins System: Functional Roles and Therapeutic Potential summarizes research on both the physiological functioning of orexins, their impact on homeostatic processes, and related disorders. The book encompasses the effects on appetite, sleep, substance abuse, cognition, and anxiety. Additionally, it examines new therapeutic approaches utilizing orexins, including utilization of orexin receptors for drug development. It is essential reading for neuroscience researchers interested in brain-behavior relationships, as well as psychiatrists, endocrinologists and pharmacologists.

• Provides an overview of new research on orexins/hypocretins
• Includes an overview of intracellular signaling and orexin physiology
• Discusses the effects on arousal, appetite, cognition, addiction and anxiety
• Examines orexin based therapies and their potential use in disorders
• Explores orexin receptors for drug development

• Language: English
• Publisher: Academic Press
• Release date: Jan 3, 2019
• ISBN: 9780128137529

Book preview

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Preface

Joshua A. Burk; Jim R. Fadel

In 1998, two research groups independently reported the discovery of novel neuropeptides—termed hypocretins or orexins—expressed in a restricted part of the brain encompassing the lateral hypothalamus, perifornical area, and dorsomedial hypothalamus. In the ensuing two decades since this discovery, there has been steady and growing progress to understand the variety of functions involving this neuropeptide system. Indeed, a PubMed search in 2018 using the term hypocretin OR orexin yields nearly 5000 hits—an astonishing number that reflects both the importance of these neuropeptides and the diversity of their physiological functions. The goal of this book is to provide an update about this progress after over 20 years of research in this area. To achieve this goal, we are fortunate to include chapters from many of the leading scientists who have studied orexins/hypocretins.

De Lecea played a lead role in the discovery of the hypocretins. In the first chapter, he and Jennings provide additional background on the orexin/hypocretin system and provide a broad overview of some of the relevant neuroanatomy and major research directions that have been spurred by their discovery. Kukkonen provides a comprehensive overview of the orexin/hypocretin receptor pharmacology, including endogenous signaling pathways and the growing array of drugs that act on either of the described orexin/hypocretin receptors. Collectively, these chapters provide the foundation in understanding the neural architecture of the orexin/hypocretin system. The book then moves to chapters focusing on the functions mediated by the orexin/hypocretin system.

Chapters by Kotz and colleagues and a chapter by Petrovich describe the role of orexins/hypocretins in feeding behavior and energy expenditure. These chapters describe the critical role of orexins/hypocretins in mediating feeding behavior—consistent with the localization of these neurons in the lateral hypothalamus—but also place the orexin system within a larger context of contributing to metabolism rates and body weight.

The role of the orexin/hypocretin system in motivated behavior is not restricted to feeding. More recently, orexins/hypocretins are recognized to have an important role in drug abuse mediated in part by extensive interactions between these neuropeptides and neural pathways classically implicated in reinforcement and reward, such as the mesocorticolimbic dopamine system. This topic is addressed by chapters contributed by España, focusing on dopaminergic signaling and by Matzeu & Martin-Fardon, focusing on cocaine addiction.

Another more recently recognized role for orexins is aspects of cognition and learning. The present authors include a chapter about orexins/hypocretins in cognition and Berrendero contributes a chapter about orexin/hypocretins regarding fear learning. Finally, we include some future directions for research regarding orexins/hypocretins, especially the development of compounds that may be useful for treating clinical conditions. Other topics, such as sleep, are discussed in multiple chapters in this book, as the changes in arousal associated with the orexin system affect a number of functions, from energy expenditure to cognition. Together, we hope that this book provides a useful, broad overview of many of the research areas and exciting developments that have followed the original description of orexins/hypocretins along with some ideas to stimulate future research.

Chapter 1

Hypocretins (Orexins): Twenty Years of Dissecting Arousal Circuits

Kimberly J. Jennings; Luis de Lecea    Department of Psychiatry and Behavioral Sciences, Stanford University, Stanford, CA, United States

Abstract

Twenty years ago, the hypocretins/orexins were independently identified by de Lecea et al. and Sakurai et al. as novel neuropeptides expressed exclusively in the lateral hypothalamus (de Lecea et al., 1998; Sakurai et al., 1998). Although neither paper mentioned sleep, an explosion of research over the intervening twenty years has revealed a critical role for Hcrt in promoting arousal and regulating sleep/wake stability. Moreover, the Hcrt system has proven especially amenable to genetically and anatomically targeted manipulations, facilitating rapid and systematic description of the circuit mechanisms mediating arousal. This chapter will review the evidence revealing Hcrt's role in arousal and the insights gained by using the Hcrt system as a foothold to gain access to arousal circuits.

Keywords

Hypocretin; Orexin; Arousal; Wake; Sleep

Abbreviations

5-HT 

serotonin

BF 

basal forebrain

Ca² + 

calcium

CSF 

cerebrospinal fluid

DA 

dopamine

DREADDs 

designer receptors exclusively activated by designer drugs

DRN 

dorsal raphe nucleus

GABA 

γ amino butyric acid

GPCR 

G-protein-coupled receptor

GRIN 

gradient refractive index

HA 

histamine

Hcrt 

hypocretin

HcrtR 

hypocretin receptor

HPA 

hypothalamic-pituitary-adrenal

Hz 

Hertz

I.C.V. 

interocerebroventricular

LC 

locus coeruleus

LDT 

laterodorsal tegmental nucleus

LH 

lateral hypothalamus

MPN 

median preoptic nucleus

mRNA 

messenger ribonucleic acid

NAc 

nucleus accumbens

NE 

norepinephrine

NREM 

nonrapid eye movement

POA 

preoptic area

PPT 

pedunculopontine tegmental nucleus

REM 

rapid eye movement

TMN 

tuberomammillary nucleus

VLPO 

ventrolateral preoptic area

VTA 

ventral tegmental area

Acknowledgments

We thank Dr. Susan M. Tyree for her helpful comments during the preparation of this manuscript. This work was supported by grants from NIH (R01 MH087592, R01 MH102638) to LdL.

Conflicts of Interest

The authors declare no conflicts of interest.

The Hypocretins: A Brief Introduction

The hypocretins (Hcrt, also known as orexins) were discovered by two independent research groups using complementary approaches (de Lecea et al., 1998; Sakurai et al., 1998). De Lecea and colleagues were seeking to identify specific genes enriched in the hypothalamus that might play a role in the wide-ranging functions performed by this heterogeneous brain region (de Lecea et al., 1998; Gautvik et al., 1996). They identified an mRNA whose expression was restricted to the perifornical region of the lateral hypothalamus (LH), and which was predicted to encode the precursor for two secreted peptides (Hcrt1 and Hcrt2, also known as orexin-A and orexin-B). These cells were observed to project widely throughout the brain and one of these peptides stimulated cultured hypothalamic neurons, consistent with a role in neurotransmission. These neuropeptides were named hypocretins due to their selective expression in the hypothalamus and sequence similarity to the incretin family of peptides (de Lecea et al., 1998).

Sakurai and colleagues were seeking to identify the natural ligands for orphaned GPCRs. They identified two peptides that each activated two related orphaned GPCRs (hypocretin/orexin receptor 1 and 2, or HcrtR1 and HcrtR2). They further demonstrated that I.C.V. infusion of either peptide in the rat increased food intake whereas fasting increased expression of these peptides. Thus they named these peptides orexins, after the Greek orexis for appetite (Sakurai et al., 1998). However, both hypocretin and orexin refer to the same neuropeptides, and we now know that the function of Hcrt extends far beyond appetite regulation.

Hcrt neurons number approximately 5000 in rodents and 20,000–50,000 in humans (Date et al., 1999; Elias et al., 1998; Nambu et al., 1999; Peyron et al., 1998; Thannickal et al., 2000). Although Hcrt neurons are restricted to a small subregion of the LH, they project widely throughout the brain. Outside of the hypothalamus, the most robust projections are seen in the locus coeruleus (LC), thalamic paraventricular nuclei, periaqueductal gray, septum, bed nucleus of the stria terminalis, raphe peribrachial pontine region, medullary reticular formation, and the nucleus of the solitary tract (Chen, Dun, Kwok, Dun, & Chang, 1999; Date et al., 1999; Peyron et al., 1998). Many of these target regions are associated with arousal, hinting early on that Hcrt may play a role in sleep/wake regulation.

Hcrt neurons were quickly identified to be glutamatergic (Rosin, Weston, Sevigny, Stornetta, & Guyenet, 2003; Torrealba, Yanagisawa, & Saper, 2003). Hcrt neurons also coexpress a number of other substances, including dynorphin, galanin, neurotensin, proenkephalin, netafin, cocaine and amphetamine related transcript (CART), neuronal activity related-pentraxin (NARP), and protein delta-like 1 homolog (DLK-1) (Chou et al., 2001; Furutani et al., 2013; Hakansson, de Lecea, Sutcliffe, Yanagisawa, & Meister, 1999; Meister, Perez-Manso, & Daraio, 2013; Mickelsen et al., 2017; Reti, Reddy, Worley, & Baraban, 2002). Dynorphin is observed packaged with Hcrt in dense core vesicles within the VTA (Muschamp et al., 2014), but the potential corelease of other substances with Hcrt is unclear. Hcrt neurons are nonoverlapping with other sleep-regulating lateral hypothalamic neuropeptidergic populations, including melanin-concentrating hormone (MCH) and neuropeptide VF-expressing neurons (Peyron et al., 1998; Yelin-Bekerman et al., 2015).

Hcrt Signaling Regulates Arousal

Hcrt Signaling Is Disrupted in Narcolepsy-Cataplexy

Soon after Hcrt's initial discovery, it was reported that knocking out the Hcrt gene in mice caused fragmented sleep/wake and behavioral arrests resembling cataplexy—a behavioral phenotype closely resembling narcolepsy in humans (Chemelli et al., 1999; Mochizuki et al., 2004). Mice with Hcrt neuronal degeneration show similarly disrupted sleep/wake and cataplexy-like attacks (Hara et al., 2001). Dogs with a heritable form of narcolepsy were also identified to have a mutation in the gene encoding HcrtR2 (Lin et al., 1999). These data suggested that disrupted Hcrt signaling may underlie narcolepsy in humans. Indeed, patients with narcolepsy have reduced or absent Hcrt1 in CSF (Nishino, Ripley, Overeem, Lammers, & Mignot, 2000) and an 85%–95% reduction in Hcrt neurons (Peyron et al., 2000; Thannickal et al., 2000). This was a landmark discovery in associating a restricted and nonredundant peptidergic system with disease, and low CSF Hcrt1 is now a diagnostic criterion for narcolepsy-cataplexy.

Researchers have evaluated the potential for viral gene transfer to rescue Hcrt signaling and attenuate narcoleptic symptoms in animal models. Widespread overexpression of Hcrt with an ectopically expressed transgene prevented cataplexy-like arrests and improved REM sleep abnormalities in Hcrt neuron-ablated mice (Mieda et al., 2004a), although widespread ectopic overexpression disrupts sleep/wake patterning in Hcrt-intact mice (Willie et al., 2011). Targeted transfer of the Hcrt gene to the LH (Liu et al., 2008), zona incerta (Liu et al., 2011), dorsolateral pons (Blanco-Centurion, Liu, Konadhode, Pelluru, & Shiromani, 2013), mediobasal hypothalamus (Kantor et al., 2013), and amygdala (Liu, Blanco-Centurion, Konadhode, Luan, & Shiromani, 2016) also improve symptoms of sleep fragmentation and cataplexy in Hcrt-deficient mice. Although application of such gene therapies in clinical settings remains a distant goal, these results suggest that Hcrt gene transfer may be a viable avenue for treatment to be considered as the technology matures.

Hcrt Promotes Wakefulness

Pharmacological studies in animal models have established a wake-promoting function for Hcrt. Central infusions of Hcrt increase wakefulness and decrease REM and NREM sleep (Hagan et al., 1999; Piper, Upton, Smith, & Hunter, 2000; Yamanaka et al., 2002), including local infusions into the LC (Bourgin et al., 2000; Hagan et al., 1999), basal forebrain (Espana, Baldo, Kelley, & Berridge, 2001; Methippara, Alam, Szymusiak, & McGinty, 2000; Thakkar, Ramesh, Strecker, & McCarley, 2001), tuberomammillary nucleus (TMN) (Huang et al., 2001), and pons (Xi, Fung, Yamuy, Morales, & Chase, 2002; Xi, Morales, & Chase, 2001). On the other hand, administration of HcrtR antagonists decrease wakefulness and increase both REM and NREM sleep (Brisbare-Roch et al., 2007; Winrow et al., 2011). Patterns of Hcrt neural activity and Hcrt release are consistent with a role for Hcrt in promoting arousal. Hcrt neuronal activation, CSF Hcrt1 concentration, and extracellular Hcrt1 concentrations are elevated during the natural active phase of both nocturnal and diurnal species (Estabrooke et al., 2001; Fujiki et al., 2001; Kiyashchenko et al., 2002; Modirrousta, Mainville, & Jones, 2005; Yoshida et al., 2001; Zeitzer et al., 2003). A higher temporal resolution description of Hcrt activity emerged in 2005, when Lee et al. and Mileykovskiy et al. independently described that Hcrt neurons in rats are phasically active during active waking, less active during quiet waking, and silent during sleep (Lee, Hassani, & Jones, 2005; Mileykovskiy, Kiyashchenko, & Siegel, 2005). It was further reported that Hcrt neurons ramp up activity shortly before wake in mice (Takahashi, Lin, & Sakai, 2008). In humans, Hcrt release within the amygdala was found to be elevated around wake onset relative to sleep (Blouin et al., 2013).

The development of optogenetic tools enabled manipulation of Hcrt neurons with ms precision to better understand the role of phasic Hcrt neural activity in promoting wake (Tyree & de Lecea, 2017a). Acute optogenetic stimulation of Hcrt neurons at frequencies above 5 Hz evoked wake from both REM and NREM sleep (Adamantidis, Zhang, Aravanis, Deisseroth, & de Lecea, 2007). Hcrt peptides are essential for the wake-promoting effects of optogenetic stimulation, as Hcrt-deficient animals fail to show increased wake probability (Adamantidis et al., 2007). Furthermore, high sleep pressure blocks the wake-promoting effects of Hcrt stimulation (Carter, Adamantidis, Ohtsu, Deisseroth, & de Lecea, 2009), suggesting that sleep homeostasis gates the wake-promoting effects of Hcrt neuronal activity. In complementary studies, optogenetic inhibition of Hcrt neurons during the inactive phase using a halorhodopsin construct increased NREM sleep (Tsunematsu et al., 2011), whereas inhibition with an archaerhodopsin construct during the active phase increased total sleep and decreased wake (Tsunematsu et al., 2013). Consistent with these findings, chemogenetic manipulation of Hcrt neuronal activity using DREADDs, which act on a min-hr timescale, bidirectionally modulated time spent in wake and sleep (Sasaki et al., 2011). Thus Hcrt neural activity promotes sleep➔wake transitions.

Differential Contributions of HcrtR1 and HcrtR2

Hcrt acts through two GPCRs to exert neuroexcitatory effects. HcrtR1 (also known as orexin receptor 1) has a higher affinity for Hcrt1 over Hcrt2 by an order of magnitude, whereas HcrtR2 (also known as orexin receptor 2) displays equal affinity for Hcrt1 and Hcrt2 (Sakurai et al., 1998). Consistent with Hcrt projections, HcrtRs are expressed widely throughout the brain. However, HcrtR1 and HcrtR2 have distinct if overlapping distributions, with some wake-promoting target regions primarily expressing Hcrt1 (e.g., LC, cholinergic neurons of the LDT and PPT), some expressing HcrtR2 (e.g., TMN), and some expressing both (e.g., DRN) (Lu, Bagnol, Burke, Akil, & Watson, 2000; Marcus et al., 2001; Trivedi, Yu, MacNeil, Van der Ploeg, & Guan,