Behavioral and sleep/wake characteristics of mice lacking norepinephrine and hypocretin

Hypocretin Mediates Sleep and Wake Disturbances in a Mouse Model of Traumatic Brain Injury

Traumatic brain injury (TBI) is a major cause of disability worldwide. Post-TBI sleep and wake disturbances are extremely common and difficult for patients to manage. Sleep and wake disturbances contribute to poor functional and emotional outcomes from TBI, yet effective therapies remain elusive. A more comprehensive understanding of mechanisms underlying post-TBI sleep and wake disturbance will facilitate development of effective pharmacotherapies. Previous research in human patients and animal models indicates that altered hypocretinergic function may be a major contributor to sleep-wake disturbance after TBI. In this study, we further elucidate the role of hypocretin by determining the impact of TBI on sleep-wake behavior of hypocretin knockout (HCRT KO) mice. Adult male C57BL/6J and HCRT KO mice were implanted with electroencephalography recording electrodes, and pre-injury baseline recordings were obtained. Mice were then subjected to either moderate TBI or sham surgery. Additional recordings were obtained and sleep-wake behavior determined at 3, 7, 15, and 30 days after TBI or sham procedures. At baseline, HCRT KO mice had a significantly different sleep-wake phenotype than control C57BL/6J mice. Post-TBI sleep-wake behavior was altered in a genotype-dependent manner: sleep of HCRT KO mice was not altered by TBI, whereas C57BL/6J mice had more non-rapid eye movement sleep, less wakefulness, and more short wake bouts and fewer long wake bouts. Numbers of hypocretin-positive cells were reduced in C57BL/6J mice by TBI. Collectively, these data indicate that the hypocretinergic system is involved in the alterations in sleep-wake behavior that develop after TBI in this model, and suggest potential therapeutic interventions.

Sleep and Sedative States Induced by Targeting the Histamine and Noradrenergic Systems

Sedatives target just a handful of receptors and ion channels. But we have no satisfying explanation for how activating these receptors produces sedation. In particular, do sedatives act at restricted brain locations and circuitries or more widely? Two prominent sedative drugs in clinical use are zolpidem, a GABA_A receptor positive allosteric modulator, and dexmedetomidine (DEX), a selective α2 adrenergic receptor agonist. By targeting hypothalamic neuromodulatory systems both drugs induce a sleep-like state, but in different ways: zolpidem primarily reduces the latency to NREM sleep, and is a controlled substance taken by many people to help them sleep; DEX produces prominent slow wave activity in the electroencephalogram (EEG) resembling stage 2 NREM sleep, but with complications of hypothermia and lowered blood pressure—it is used for long term sedation in hospital intensive care units—under DEX-induced sedation patients are arousable and responsive, and this drug reduces the risk of delirium. DEX, and another α2 adrenergic agonist xylazine, are also widely used in veterinary clinics to sedate animals. Here we review how these two different classes of sedatives, zolpidem and dexmedetomideine, can selectively interact with some nodal points of the circuitry that promote wakefulness allowing the transition to NREM sleep. Zolpidem enhances GABAergic transmission onto histamine neurons in the hypothalamic tuberomammillary nucleus (TMN) to hasten the transition to NREM sleep, and DEX interacts with neurons in the preoptic hypothalamic area that induce sleep and body cooling. This knowledge may aid the design of more precise acting sedatives, and at the same time, reveal more about the natural sleep-wake circuitry.

Optogenetic control of hypocretin (orexin) neurons and arousal circuits

In 1998, our group discovered a cDNA that encoded the precursor of two putative neuropeptides that we called hypocretins for their hypothalamic expression and their similarity to the secretin family of neuropeptides. In the last 16 years, numerous studies have placed the hypocretin system as an integrator of homeostatic functions with a crucial, non-redundant function as arousal stabilizer. We recently applied optogenetic methods to interrogate the role of individual neuronal circuits in sleep-to-wake transitions. The neuronal connections between the hypocretin system and the locus coeruleus (LC) seem to be crucial in establishing the appropriate dynamic of spontaneous awakenings.

Functional wiring of hypocretin and LC-NE neurons: implications for arousal

To survive in a rapidly changing environment, animals must sense their external world and internal physiological state and properly regulate levels of arousal. Levels of arousal that are abnormally high may result in inefficient use of internal energy stores and unfocused attention to salient environmental stimuli. Alternatively, levels of arousal that are abnormally low may result in the inability to properly seek food, water, sexual partners, and other factors necessary for life. In the brain, neurons that express hypocretin neuropeptides may be uniquely posed to sense the external and internal state of the animal and tune arousal state according to behavioral needs. In recent years, we have applied temporally precise optogenetic techniques to study the role of these neurons and their downstream connections in regulating arousal. In particular, we have found that noradrenergic neurons in the brainstem locus coeruleus (LC) are particularly important for mediating the effects of hypocretin neurons on arousal. Here, we discuss our recent results and consider the implications of the anatomical connectivity of these neurons in regulating the arousal state of an organism across various states of sleep and wakefulness.

β-adrenergic signaling regulates evolutionarily derived sleep loss in the Mexican cavefish

Sleep is a fundamental behavior exhibited almost universally throughout the animal kingdom. The required amount and circadian timing of sleep differs greatly between species in accordance with habitats and evolutionary history. The Mexican blind cavefish, Astyanax mexicanus, is a model organism for the study of adaptive morphological and behavioral traits. In addition to loss of eyes and pigmentation, cave populations of A. mexicanus exhibit evolutionarily derived sleep loss and increased vibration attraction behavior, presumably to cope with a nutrient-poor environment. Understanding the neural mechanisms of evolutionarily derived sleep loss in this system may reveal critical insights into the regulation of sleep in vertebrates. Here we report that blockade of β-adrenergic receptors with propranolol rescues the decreased-sleep phenotype of cavefish. This effect was not seen with α-adrenergic antagonists. Treatment with selective β1-, β2-, and β3-antagonists revealed that the increased sleep observed with propranolol could partially be explained via the β1-adrenergic system. Morphological analysis of catecholamine circuitry revealed conservation of gross catecholaminergic neuroanatomy between surface and cave morphs. Taken together, these findings suggest that evolutionarily derived changes in adrenergic signaling underlie the reduced sleep of cave populations.

Association between the activation of MCH and orexin immunorective neurons and REM sleep architecture during REM rebound after a three day long REM deprivation

Rapid eye movement (REM) sleep rebound following REM deprivation using the platform-on-water method is characterized by increased time spent in REM sleep and activation of melanin-concentrating hormone (MCH) expressing neurons. Orexinergic neurons discharge reciprocally to MCH-ergic neurons across the sleep-wake cycle. However, the relation between REM architecture and the aforementioned neuropeptides remained unclear. MCH-ergic neurons can be divided into two subpopulations regarding their cocaine- and amphetamine-regulated transcript (CART) immunoreactivity, and among them the activation of CART-immunoreactive subpopulation is higher during the REM rebound. However, the possible role of stress in this association has not been elucidated. Our aims were to analyze the relationship between the architecture of REM rebound and the activation of hypothalamic MCH-ergic and orexinergic neurons. We also intended to separate the effect of stress and REM deprivation on the subsequent activation of subpopulations of MCH-ergic neurons. In order to detect neuronal activity, we performed MCH/cFos and orexin/cFos double immunohistochemistry on home cage, sleep deprived and sleep-rebound rats using the platform-on-water method with small and large (stress control) platforms. Furthermore, REM architecture was analyzed and a triple MCH/CART/cFos immunohistochemistry was also performed on the rebound groups in the same animals. We found that the activity of MCH- and orexin-immunoreactive neurons during REM rebound was positively and negatively correlated with the number of REM bouts, respectively. A negative reciprocal correlation was also found between the activation of MCH- and orexin-immunoreactive neurons during REM rebound. Furthermore, difference between the activation of CART-immunoreactive (CART-IR) and non-CART-immunoreactive MCH-ergic neuron subpopulations was found only after selective REM deprivation, it was absent in the large platform (stress control) rebound group. These results support the role of CART-IR subpopulation of MCH-ergic neurons and the inverse relationship of MCH and orexin in the regulation of REM sleep after REM sleep deprivation.

Genetic analysis of sleep

Almost 20 years ago, the gene underlying fatal familial insomnia was discovered, and first suggested the concept that a single gene can regulate sleep. In the two decades since, there have been many advances in the field of behavioral genetics, but it is only in the past 10 years that the genetic analysis of sleep has emerged as an important discipline. Major findings include the discovery of a single gene underlying the sleep disorder narcolepsy, and identification of loci that make quantitative contributions to sleep characteristics. The sleep field has also expanded its focus from mammalian model organisms to Drosophila, zebrafish, and worms, which is allowing the application of novel genetic approaches. Researchers have undertaken large-scale screens to identify new genes that regulate sleep, and are also probing questions of sleep circuitry and sleep function on a molecular level. As genetic tools continue to be refined in each model organism, the genes that support a specific function in sleep will become more apparent. Thus, while our understanding of sleep still remains rudimentary, rapid progress is expected from these recently initiated studies.

Endogenous ouabain-like compounds in locus coeruleus modulate rapid eye movement sleep in rats

Although the detailed mechanism of spontaneous generation and regulation of rapid eye movement sleep (REMS) is yet unknown, it has been reported that noradrenergic REM-OFF neurons in the locus coeruleus (LC) cease firing during REMS and, if they are kept active, REMS is significantly reduced. On the other hand, the activity as well as expression of Na-K ATPase has been shown to increase in the LC following REMS deprivation. Ouabain is a specific inhibitor of Na-K ATPase, and endogenous ouabain-like compounds are present in the brain. These findings led us to propose that a decrease in the level of ouabain-like compounds spontaneously available in and around the LC would stimulate and increase the REM-OFF neuronal activities in this region and thus would reduce REMS. To test this hypothesis, we generated anti-ouabain antibodies and then microinjected it bilaterally into the LC in freely moving chronically prepared rats and recorded electrophysiological signals for evaluation of sleep-wakefulness states; suitable control experiments were also conducted. Injection of anti-ouabain antibodies into the LC, but not into adjacent brain areas, significantly reduced percent REMS (mean +/- SEM) from 7.12 (+/-0.74) to 3.63 (+/-0.65). The decrease in REMS was due to reduction in the mean frequency of REMS episode, which is likely due to increased excitation of the LC REM-OFF neurons. Control microinjections of normal IgG did not elicit this effect. These results support our hypothesis that interactions of naturally available endogenous ouabain-like compounds with the Na-K ATPase in the LC modulate spontaneous REMS.

Optogenetic deconstruction of sleep-wake circuitry in the brain

How does the brain regulate the sleep–wake cycle? What are the temporal codes of sleep and wake-promoting neural circuits? How do these circuits interact with each other across the light/dark cycle? Over the past few decades, many studies from a variety of disciplines have made substantial progress in answering these fundamental questions. For example, neurobiologists have identified multiple, redundant wake-promoting circuits in the brainstem, hypothalamus, and basal forebrain. Sleep-promoting circuits have been found in the preoptic area and hypothalamus. One of the greatest challenges in recent years has been to selectively record and manipulate these sleep–wake centers in vivo with high spatial and temporal resolution. Recent developments in microbial opsin-based neuromodulation tools, collectively referred to as “optogenetics,” have provided a novel method to demonstrate causal links between neural activity and specific behaviors. Here, we propose to use optogenetics as a fundamental tool to probe the necessity, sufficiency, and connectivity of defined neural circuits in the regulation of sleep and wakefulness.

Phospholipase C-beta4 is essential for the progression of the normal sleep sequence and ultradian body temperature rhythms in mice

Background

The sleep sequence: i) non-REM sleep, ii) REM sleep, and iii) wakefulness, is stable and widely preserved in mammals, but the underlying mechanisms are unknown. It has been shown that this sequence is disrupted by sudden REM sleep onset during active wakefulness (i.e., narcolepsy) in orexin-deficient mutant animals. Phospholipase C (PLC) mediates the signaling of numerous metabotropic receptors, including orexin receptors. Among the several PLC subtypes, the β4 subtype is uniquely localized in the geniculate nucleus of thalamus which is hypothesized to have a critical role in the transition and maintenance of sleep stages. In fact, we have reported irregular theta wave frequency during REM sleep in PLC-β4-deficient mutant (PLC-β4−/−) mice. Daily behavioral phenotypes and metabotropic receptors involved have not been analyzed in detail in PLC-β4−/− mice, however.

Methodology/Principal Findings

Therefore, we analyzed 24-h sleep electroencephalogram in PLC-β4−/− mice. PLC-β4−/− mice exhibited normal non-REM sleep both during the day and nighttime. PLC-β4−/− mice, however, exhibited increased REM sleep during the night, their active period. Also, their sleep was fragmented with unusual wake-to-REM sleep transitions, both during the day and nighttime. In addition, PLC-β4−/− mice reduced ultradian body temperature rhythms and elevated body temperatures during the daytime, but had normal homeothermal response to acute shifts in ambient temperatures (22°C–4°C). Within the most likely brain areas to produce these behavioral phenotypes, we found that, not orexin, but group-1 metabotropic glutamate receptor (mGluR)-mediated Ca²⁺ mobilization was significantly reduced in the dorsal lateral geniculate nucleus (LGNd) of PLC-β4−/− mice. Voltage clamp recordings revealed that group-1 mGluR-mediated currents in LGNd relay neurons (inward in wild-type mice) were outward in PLC-β4−/− mice.

Conclusions/Significance

These lines of evidence indicate that impaired LGNd relay, possibly mediated via group-1 mGluR, may underlie irregular sleep sequences and ultradian body temperature rhythms in PLC-β4−/− mice.

Hypocretin-2 saporin lesions of the ventrolateral periaquaductal gray (vlPAG) increase REM sleep in hypocretin knockout mice

Ten years ago the sleep disorder narcolepsy was linked to the neuropeptide hypocretin (HCRT), also known as orexin. This disorder is characterized by excessive day time sleepiness, inappropriate triggering of rapid-eye movement (REM) sleep and cataplexy, which is a sudden loss of muscle tone during waking. It is still not known how HCRT regulates REM sleep or muscle tone since HCRT neurons are localized only in the lateral hypothalamus while REM sleep and muscle atonia are generated from the brainstem. To identify a potential neuronal circuit, the neurotoxin hypocretin-2-saporin (HCRT2-SAP) was used to lesion neurons in the ventral lateral periaquaductal gray (vlPAG). The first experiment utilized hypocretin knock-out (HCRT-ko) mice with the expectation that deletion of both HCRT and its target neurons would exacerbate narcoleptic symptoms. Indeed, HCRT-ko mice (n = 8) given the neurotoxin HCRT2-SAP (16.5 ng/23nl/sec each side) in the vlPAG had levels of REM sleep and sleep fragmentation that were considerably higher compared to HCRT-ko given saline (+39%; n = 7) or wildtype mice (+177%; n = 9). However, cataplexy attacks did not increase, nor were levels of wake or non-REM sleep changed. Experiment 2 determined the effects in mice where HCRT was present but the downstream target neurons in the vlPAG were deleted by the neurotoxin. This experiment utilized an FVB-transgenic strain of mice where eGFP identifies GABA neurons. We verified this and also determined that eGFP neurons were immunopositive for the HCRT-2 receptor. vlPAG lesions in these mice increased REM sleep (+79% versus saline controls) and it was significantly correlated (r = 0.89) with loss of eGFP neurons. These results identify the vlPAG as one site that loses its inhibitory control over REM sleep, but does not cause cataplexy, as a result of hypocretin deficiency.

Effects of saporin-induced lesions of three arousal populations on daily levels of sleep and wake

The hypocretin (HCRT) neurons are located only in the perifornical area of the lateral hypothalamus and heavily innervate the cholinergic neurons in the basal forebrain (BF), histamine neurons in the tuberomammillary nucleus (TMN), and the noradrenergic locus ceruleus (LC) neurons, three neuronal populations that have traditionally been implicated in regulating arousal. Based on the innervation, HCRT neurons may regulate arousal by driving these downstream arousal neurons. Here, we directly test this hypothesis by a simultaneous triple lesion of these neurons using three saporin-conjugated neurotoxins. Three weeks after lesion, the daily levels of wake were not changed in rats with double or triple lesions, although rats with triple lesions were asleep more during the light-to-dark transition period. The double- and triple-lesioned rats also had more stable sleep architecture compared with nonlesioned saline rats. These results suggest that the cholinergic BF, TMN, and LC neurons jointly modulate arousal at a specific circadian time, but they are not essential links in the circuitry responsible for daily levels of wake, as traditionally hypothesized.

Vulnerability to insomnia: the role of familial aggregation

BACKGROUND

The goal of this study was to determine the degree of familial aggregation in vulnerability to stress-related sleep disturbance among siblings. One approach to investigating a potential "familial" predisposition to sleep disturbance is to examine the relationship between siblings on a standard measure of vulnerability to stress-related sleep disturbance.

DESIGN

Cross-sectional data on insomnia, vulnerability to stress-related sleep disturbance, sleepiness, habitual sleep, and additional demographic variables was collected separately from pairs of biological siblings. Data were collected during a 15-20min phone assessment.

PARTICIPANTS

Interviews on a total of 62 individuals (31 sibling pairs) were completed. A total of 8 individuals and their respective siblings were excluded after meeting conservative criteria for Diagnostic and Statistical Manual of Mental Disorders, Fourth edition (DSM-IV)-based insomnia. The mean age of the sample was 51.1+/-12.1 years (range 18-70) and habitual nightly total sleep time averaged 6.91+/-1.42h/night.

RESULTS

Individuals completed the Ford Insomnia Response to Stress Test (FIRST), a standardized measure of individual vulnerability to stress-induced sleep disturbance. The intraclass correlation coefficient (ICC) was r =0.61, df=23, p =0.001 for the relationship between siblings in FIRST scores. This indicated that 37.2% of the variance in vulnerability to stress-related sleep disturbance can be accounted for by familial aggregation. This relationship remained after controlling for potential confounds including age, gender, shift schedule, and psychiatric history.

CONCLUSIONS

Our data support the notion that vulnerability to stress-related sleep disturbance has a strong familial aggregation. Additional studies are needed to determine the genetic or environmental origins of this relationship and its underlying biological substrates.

Sympathetic and hyperthermic reactions by orexin A: role of cerebral catecholaminergic neurons

This experiment tested the effect of a lesion of cerebral catecholaminergic neurons on the sympathetic and thermogenic effects induced by an intracerebroventicular (icv) injection of orexin A. The firing rates of the sympathetic nerves to the interscapular brown adipose tissue (IBAT), along with IBAT, colonic temperatures and heart rate were monitored in urethane-anesthetized male Sprague-Dawley rats before an injection of orexin A (1.5 nmol) into the lateral cerebral ventricle and over a period of 150 min after the injection. Three days before the experiment, the rats were pre-treated with an icv injection of 6-hydroxydopamine (6-OHDA) or 6-OHDA plus desipramine or saline. The results show that orexin A increases the sympathetic firing rate, IBAT, colonic temperatures and heart rate in the rats pre-treated with saline. This increase is blocked by the pre-treatment with 6-OHDA alone or 6-OHDA plus desipramine. These findings indicate that cerebral catecholaminergic neurons (particularly the dopaminergic pathway) play a fundamental role in the complex reactions related to activation of the orexinergic system.