The effect of age on prepro-orexin gene expression and contents of orexin A and B in the rat brain

Age-dependent down-regulation of orexin receptors in trigeminal nucleus caudalis correlated with attenuation of orexinergic analgesia in rats

Aging is related to a variety of physiological organ changes, including central and peripheral nervous systems. It has been reported that the orexin signaling has a potential analgesic effect in different models of pain, especially inflammatory pulpal pain. However, the age-induced alteration in dental pain perception and orexin analgesia has not yet been fully elucidated. Here, we tested that how aging may change the effect of orexin-A on nociceptive behaviors in a rat dental pulp pain model. The expression levels of orexin receptors and the nociceptive neuropeptides substance P (SP) and calcitonin-related gene peptide (CGRP) were also assessed in the trigeminal nucleus caudalis (TNC) of young and aged rats. Dental pulp pain was induced by intradental application of capsaicin (100 μg). The immunofluorescence technique was used to evaluate the expression levels. The results show less efficiency of orexin-A to ameliorate pain perception in aged rats as compared to young rats. In addition, a significant decrease in the number of orexin 1 and 2 receptors was observed in the TNC of aged as compared to young rats. Dental pain-induced SP and CGRP overexpression was also significantly inhibited by orexin-A injection into the TNC of young animals. In contrast, orexin-A could not produce such effects in the aged animals. In conclusion, the older age-related reduction of the antinociceptive effect of orexin may be due to the downregulation of its receptors and inability of orexin signaling to inhibit the expression of nociceptive neuropeptides such as SP and CGRP in aged rats.

Orexin antagonism and substance-P: Effects and interactions on polycystic ovary syndrome in the wistar rats

BACKGROUND

Polycystic ovary syndrome (PCOS) is a prevalent endocrine disorder without definitive treatments. Orexin and Substance-P (SP) neuropeptides can affect the ovarian steroidogenesis. Moreover, there are limited studies about the role of these neuropeptides in PCOS. We aimed here to clarify the effects of orexins and SP in PCOS as well as any possible interactions between them.

METHODS

For this purpose, the animals (n = five rats per group) received intraperitoneally a single dose of SB-334,867-A (orexin-1 receptor antagonist; OX1Ra), JNJ-10,397,049 (orexin-2 receptor antagonist; OX2Ra), and CP-96,345 (neurokinin-1 receptor antagonist; NK1Ra), alone or in combination with each other after two months of PCOS induction. The blocking of orexin and SP receptors was studied in terms of ovarian histology, hormonal changes, and gene expression of ovarian steroidogenic enzymes.

RESULTS

The antagonists' treatment did not significantly affect the formation of ovarian cysts. In the PCOS groups, the co-administration of OX1Ra and OX2Ra as well as their simultaneous injections with NK1Ra significantly reversed testosterone levels and Cyp19a1 gene expression when compared to the PCOS control group. There were no significant interactions between the PCOS groups that received NK1Ra together with one or both OX1R- and OX2R-antagonists.

CONCLUSION

The blocking of the orexin receptors modulates abnormal ovarian steroidogenesis in the PCOS model of rats. This suggests that the binding of orexin-A and -B to their receptors reduces Cyp19a1 gene expression while increasing testosterone levels.

To sleep or not to sleep - Effects on memory in normal aging and disease

Sleep behavior undergoes significant changes across the lifespan, and aging is associated with marked alterations in sleep amounts and quality. The primary sleep changes in healthy older adults include a shift in sleep timing, reduced slow-wave sleep, and impaired sleep maintenance. However, neurodegenerative and psychiatric disorders are more common among the elderly, which further worsen their sleep health. Irrespective of the cause, insufficient sleep adversely affects various bodily functions including energy metabolism, mood, and cognition. In this review, we will focus on the cognitive changes associated with inadequate sleep during normal aging and the underlying neural mechanisms.

Physiological Role of Orexinergic System for Health

Orexins, or hypocretins, are excitatory neuropeptides involved in the regulation of feeding behavior and the sleep and wakefulness states. Since their discovery, several lines of evidence have highlighted that orexin neurons regulate a great range of physiological functions, giving it the definition of a multitasking system. In the present review, we firstly describe the mechanisms underlining the orexin system and their interactions with the central nervous system (CNS). Then, the system's involvement in goal-directed behaviors, sleep/wakefulness state regulation, feeding behavior and energy homeostasis, reward system, and aging and neurodegenerative diseases are described. Advanced evidence suggests that the orexin system is crucial for regulating many physiological functions and could represent a promising target for therapeutical approaches to obesity, drug addiction, and emotional stress.

An overview of the orexinergic system in different animal species

Orexin (hypocretin), is a neuropeptide produced by a subset of neurons in the lateral hypothalamus. From the lateral hypothalamus, the orexin-containing neurons project their fibres extensively to other brain structures, and the spinal cord constituting the central orexinergic system. Generally, the term ''orexinergic system'' usually refers to the orexin peptides and their receptors, as well as to the orexin neurons and their projections to different parts of the central nervous system. The extensive networks of orexin axonal fibres and their terminals allow these neuropeptidergic neurons to exert great influence on their target regions. The hypothalamic neurons containing the orexin neuropeptides have been implicated in diverse functions, especially related to the control of a variety of homeostatic functions including feeding behaviour, arousal, wakefulness stability and energy expenditure. The broad range of functions regulated by the orexinergic system has led to its description as ''physiological integrator''. In the last two decades, the orexinergic system has been a topic of great interest to the scientific community with many reports in the public domain. From the documentations, variations exist in the neuroanatomical profile of the orexinergic neuron soma, fibres and their receptors from animal to animal. Hence, this review highlights the distinct variabilities in the morphophysiological aspects of the orexinergic system in the vertebrate animals, mammals and non-mammals, its presence in other brain-related structures, including its involvement in ageing and neurodegenerative diseases. The presence of the neuropeptide in the cerebrospinal fluid and peripheral tissues, as well as its alteration in different animal models and conditions are also reviewed.

Orexin (hypocretin) mediates light-dependent fluctuation of hippocampal function in a diurnal rodent

Environmental lighting conditions play a central role in cognitive function, but the underlying mechanisms remain unclear. Utilizing a diurnal rodent model, the Nile grass rat (Arvicanthis niloticus), we previously found that daytime light intensity affects hippocampal function in this species in a manner similar to its effects in humans. Compared to animals housed in a 12:12 h bright light-dark (brLD) cycle, grass rats kept in a 12:12 h dim light-dark (dimLD) cycle showed impaired spatial memory in the Morris water maze (MWM) and reduced CA1 apical dendritic spine density. The present study explored the neural substrates mediating the effects of daylight intensity on hippocampal function focusing on the hypothalamic orexin (hypocretin) system. First, animals housed in dimLD were treated with daily intranasal administration of orexin A peptide over five training days of the MWM task. Compared to vehicle controls, this treatment led to superior spatial memory accompanied by increased phosphorylation of Ca²⁺ /calmodulin-dependent protein kinase II α and glutamate receptor 1 within the CA1. To assess the role of hippocampal orexinergic signaling, an adeno-associated viral vector (AAV) expressing an orexin receptor 1 (OX1R) shRNA was injected into the dorsal hippocampus targeting the CA1 of animals housed in brLD. AAV-mediated knockdown of OX1R within the hippocampus resulted in deficits in MWM performance and reduced CA1 apical dendritic spine density. These results are consistent with the view that the hypothalamic orexinergic system underlies the modulatory role of daytime illumination on hippocampal function in diurnal mammals.

Intranasal insulin and orexins to treat age-related cognitive decline

The intranasal (IN) administration of neuropeptides, such as insulin and orexins, has been suggested as a treatment strategy for age-related cognitive decline (ARCD). Because dysfunctional neuropeptide signaling is an observed characteristic of ARCD, it has been suggested that IN delivery of insulin and/or orexins may restore endogenous peptide signaling and thereby preserve cognition. IN administration is particularly alluring as it is a relatively non-invasive method that directly targets peptides to the brain. Several laboratories have examined the behavioral effects of IN insulin in young, aged, and cognitively impaired rodents and humans. These studies demonstrated improved performance on various cognitive tasks following IN insulin administration. Fewer laboratories have assessed the effects of IN orexins; however, this peptide also holds promise as an effective treatment for ARCD through the activation of the cholinergic system and/or the reduction of neuroinflammation. Here, we provide a brief overview of the advantages of IN administration and the delivery pathway, then summarize the current literature on IN insulin and orexins. Additional preclinical studies will be useful to ultimately uncover the mechanisms underlying the pro-cognitive effects of IN insulin and orexins, whereas future clinical studies will aid in the determination of the most efficacious dose and dosing paradigm. Eventually, IN insulin and/or orexin administration may be a widely used treatment strategy in the clinic for ARCD.

Intranasal administration of orexin peptides: Mechanisms and therapeutic potential for age-related cognitive dysfunction

Cognitive impairment is a core feature of several neuropsychiatric and neurological disorders, including narcolepsy and age-related dementias. Current pharmacotherapeutic approaches to cognitive enhancement are few in number and limited in efficacy. Thus, novel treatment strategies are needed. The hypothalamic orexin (hypocretin) system, a central integrator of physiological function, plays an important role in modulating cognition. Several single- and dual-orexin receptor antagonists are available for various clinical and preclinical applications, but the paucity of orexin agonists has limited the ability to research their therapeutic potential. To circumvent this hurdle, direct intranasal administration of orexin peptides is being investigated as a prospective treatment for cognitive dysfunction, narcolepsy or other disorders in which deficient orexin signaling has been implicated. Here, we describe the possible mechanisms and therapeutic potential of intranasal orexin delivery. Combined with the behavioral evidence that intranasal orexin-A administration improves cognitive function in narcoleptic and sleep-deprived subjects, our neurochemical studies in young and aged animals highlights the capacity for intranasal orexin administration to improve age-related deficits in neurotransmission. In summary, we highlight prior and original work from our lab and from others that provides a framework for the use of intranasal orexin peptides in treating cognitive dysfunction, especially as it relates to age-related cognitive disorders.

Effects of Intranasal Orexin-A (Hypocretin-1) Administration on Neuronal Activation, Neurochemistry, and Attention in Aged Rats

Cognitive function represents a key determinative factor for independent functioning among the elderly, especially among those with age-related cognitive disorders. However; existing pharmacotherapeutic tactics for treating these disorders provide only modest benefits on cognition. The hypothalamic orexin (hypocretin) system is uniquely positioned, anatomically and functionally, to integrate physiological functions that support proper cognition. The ongoing paucity of orexin receptor agonists has mired the ability to study their potential as cognitive enhancers. Fortunately, intranasal administration of native orexin peptides circumvents this issue and others concerning peptide transport into the central nervous system (CNS). To investigate the ability of intranasal orexin-A (OxA) administration to improve the anatomical, neurochemical, and behavioral substrates of age-related cognitive dysfunction, these studies utilized a rodent model of aging combined with acute intranasal administration of saline or OxA. Here, intranasal OxA increases c-Fos expression in several telencephalic brain regions that mediate important cognitive functions, increases prefrontal cortical acetylcholine efflux, and alters set-shifting-mediated attentional function in rats. Ultimately, these studies provide a framework for the possible mechanisms and therapeutic potential of intranasal OxA in treating age-related cognitive dysfunction.

Increased acetylcholine and glutamate efflux in the prefrontal cortex following intranasal orexin-A (hypocretin-1)

Orexin/hypocretin neurons of the lateral hypothalamus and perifornical area are integrators of physiological function. Previous work from our laboratory and others has shown the importance of orexin transmission in cognition. Age-related reductions in markers of orexin function further suggest that this neuropeptide may be a useful target for the treatment of age-related cognitive dysfunction. Intranasal administration of orexin-A (OxA) has shown promise as a therapeutic option for cognitive dysfunction. However, the neurochemical mechanisms of intranasal OxA administration are not fully understood. Here, we use immunohistochemistry and in vivo microdialysis to define the effects of acute intranasal OxA administration on: (i) activation of neuronal populations in the cortex, basal forebrain, and brainstem and (ii) acetylcholine (ACh) and glutamate efflux in the prefrontal cortex (PFC) of Fischer 344/Brown Norway F1 rats. Acute intranasal administration of OxA significantly increased c-Fos expression, a marker for neuronal activation, in the PFC and in subpopulations of basal forebrain cholinergic neurons. Subsequently, we investigated the effects of acute intranasal OxA on neurotransmitter efflux in the PFC and found that intranasal OxA significantly increased both ACh and glutamate efflux in this region. These findings were independent from any changes in c-Fos expression in orexin neurons, suggesting that these effects are not resultant from direct activation of orexin neurons. In total, these data indicate that intranasal OxA may enhance cognition through activation of distinct neuronal populations in the cortex and basal forebrain and through increased neurotransmission of ACh and glutamate in the PFC.

Effects of Aging on Cortical Neural Dynamics and Local Sleep Homeostasis in Mice

Healthy aging is associated with marked effects on sleep, including its daily amount and architecture, as well as the specific EEG oscillations. Neither the neurophysiological underpinnings nor the biological significance of these changes are understood, and crucially the question remains whether aging is associated with reduced sleep need or a diminished capacity to generate sufficient sleep. Here we tested the hypothesis that aging may affect local cortical networks, disrupting the capacity to generate and sustain sleep oscillations, and with it the local homeostatic response to sleep loss. We performed chronic recordings of cortical neural activity and local field potentials from the motor cortex in young and older male C57BL/6J mice, during spontaneous waking and sleep, as well as during sleep after sleep deprivation. In older animals, we observed an increase in the incidence of non-rapid eye movement sleep local field potential slow waves and their associated neuronal silent (OFF) periods, whereas the overall pattern of state-dependent cortical neuronal firing was generally similar between ages. Furthermore, we observed that the response to sleep deprivation at the level of local cortical network activity was not affected by aging. Our data thus suggest that the local cortical neural dynamics and local sleep homeostatic mechanisms, at least in the motor cortex, are not impaired during healthy senescence in mice. This indicates that powerful protective or compensatory mechanisms may exist to maintain neuronal function stable across the life span, counteracting global changes in sleep amount and architecture.

SIGNIFICANCE STATEMENT The biological significance of age-dependent changes in sleep is unknown but may reflect either a diminished sleep need or a reduced capacity to generate deep sleep stages. As aging has been linked to profound disruptions in cortical sleep oscillations and because sleep need is reflected in specific patterns of cortical activity, we performed chronic electrophysiological recordings of cortical neural activity during waking, sleep, and after sleep deprivation from young and older mice. We found that all main hallmarks of cortical activity during spontaneous sleep and recovery sleep after sleep deprivation were largely intact in older mice, suggesting that the well-described age-related changes in global sleep are unlikely to arise from a disruption of local network dynamics within the neocortex.

Recovery from Coma Post-Cardiac Arrest Is Dependent on the Orexin Pathway

Cardiac arrest (CA) affects >550,000 people annually in the United States whereas 80-90% of survivors suffer from a comatose state. Arousal from coma is critical for recovery, but mechanisms of arousal are undefined. Orexin-A, a hypothalamic excitatory neuropeptide, has been linked to arousal deficits in various brain injuries. We investigated the orexinergic system's role in recovery from CA-related neurological impairments, including arousal deficits. Using an asphyxial CA and resuscitation model in rats, we examine neurological recovery post-resuscitation in conjunction with changes in orexin-A levels in cerebrospinal fluid (CSF) and orexin-expressing neurons. We also conduct pharmacological inhibition of orexin post-resuscitation. We show that recovery from neurological deficits begins between 4 and 24 h post-resuscitation, with additional recovery by 72 h post-resuscitation. Orexin-A levels in the CSF are lowest during periods of poorest arousal post-resuscitation (4 h) and recover to control levels by 24 h. Immunostaining revealed that the number of orexin-A immunoreactive neurons declined at 4 h post-resuscitation, but increased to near normal levels by 24 h. There were no significant changes in the number of neurons expressing melanin-concentrating hormone, another neuropeptide localized in similar hypothalamus regions. Last, administration of the dual orexin receptor antagonist, suvorexant, during the initial 24 h post-resuscitation, led to sustained neurological deficits. The orexin pathway is critical during early phases of neurological recovery post-CA. Blocking this early action leads to persistent neurological deficits. This is of considerable clinical interest given that suvorexant recently received U.S. Food and Drug Administration approval for insomnia treatment.

Upregulation of orexin/hypocretin expression in aged rats: Effects on feeding latency and neurotransmission in the insular cortex

Aging is associated with changes in numerous homeostatic functions, such as food intake, that are thought to be mediated by the hypothalamus. Orexin/hypocretin neurons of the hypothalamus regulate several physiological functions, including feeding, sleep and wakefulness. Evidence from both clinical and animal studies supports the notion that aging is associated with loss or dysregulation of the orexin system. Here, we used virus-mediated gene transfer to manipulate expression of orexin peptides in young and aged rats and examined behavioral and neurochemical correlates of food intake in these animals. Aged rats showed slower feeding latencies when presented with palatable food compared to young control rats, and these deficits were ameliorated by upregulation of orexin expression. Similarly, young animals treated with a virus designed to decrease preproorexin expression showed longer feeding latencies reminiscent of aged control rats. Feeding was also associated with increased acetylcholine, glutamate and GABA efflux in insular cortex of young control animals. Orexin upregulation did not restore deficits in feeding-elicited release of these neurotransmitters in aged rats, but did enhance basal neurotransmitter levels which may have contributed to the behavioral correlates of these genetic manipulations. These studies demonstrate that age-related deficits in behavioral and neurochemical measures of feeding are likely to be mediated, in part, by the orexin system. Because these same neurotransmitter systems have been shown to underlie orexin effects on cognition, treatments which increase orexin function may have potential for improving both physiological and cognitive manifestations of certain age-related disorders.

Dexamethasone Chemotherapy Does Not Disrupt Orexin Signaling

BACKGROUND

Steroid-induced sleep disturbance is a common and highly distressing morbidity for children receiving steroid chemotherapy for the treatment of pediatric acute lymphoblastic leukemia (ALL). Sleep disturbance can negatively impact overall quality of life, neurodevelopment, memory consolidation, and wound healing. Hypothalamic orexin neurons are influential wake-promoting neurons, and disturbances in orexin signaling leads to abnormal sleep behavior. A new class of drug, the orexin receptor antagonists, could be an intriguing option for sleep disorders caused by increased orexinergic output. Our aim was to examine the impact of ALL treatment doses of corticosteroids on the orexin system in rodents and in children undergoing treatment for childhood ALL.

METHODS

We administered repeated injections of dexamethasone to rodents and measured responsive orexin neural activity compared to controls. In children with newly diagnosed standard risk B-cell ALL receiving dexamethasone therapy per Children's Oncology Group (COG) induction therapy from 2014-2016, we collected pre- and during-steroids matched CSF samples and measured the impact of steroids on CSF orexin concentration.

RESULTS

In both rodents, all markers orexin signaling, including orexin neural output and orexin receptor expression, were preserved in the setting of dexamethasone. Additionally, we did not detect a difference in pre- and during-dexamethasone CSF orexin concentrations in children receiving dexamethasone.

CONCLUSIONS

Our results demonstrate that rodent and human orexin physiology is largely preserved in the setting of high dose dexamethasone. The data obtained in our experimental model fail to demonstrate a causative role for disruption of the orexin pathway in steroid-induced sleep disturbance.

Orexin receptors exert a neuroprotective effect in Alzheimer's disease (AD) via heterodimerization with GPR103

Orexins are neuropeptides that regulate the sleep-wake cycle and feeding behaviour. QRFP is a newly discovered neuropeptide which exerts similar orexigenic activity, thus playing an important role in energy homeostasis and regulation of appetite. The exact expression and signalling characteristics and physiological actions of QRFP and its receptor GPR103 are poorly understood. Alzheimer's disease (AD) patients experience increased nocturnal activity, excessive daytime sleepiness, and weight loss. We hypothesised therefore that orexins and QRFP might be implicated in the pathophysiology of AD. We report that the down-regulation of hippocampal orexin receptors (OXRs) and GPR103 particularly in the cornu ammonis (CA) subfield from AD patients suffering from early onset familial AD (EOFAD) and late onset familial AD (LOAD). Using an in vitro model we demonstrate that this downregulation is due to to Aβ-plaque formation and tau hyper-phosphorylation. Transcriptomics revealed a neuroprotective role for both orexins and QRFP. Finally we provide conclusive evidence using BRET and FRET that OXRs and GPR103 form functional hetero-dimers to exert their effects involving activation of ERK1/2. Pharmacological intervention directed at the orexigenic system may prove to be an attractive avenue towards the discovery of novel therapeutics for diseases such as AD and improving neuroprotective signalling pathways.

Sleep disorders, obesity, and aging: the role of orexin

The hypothalamic neuropeptides orexin A and B (hypocretin 1 and 2) are important homeostatic mediators of central control of energy metabolism and maintenance of sleep/wake states. Dysregulation or loss of orexin signaling has been linked to narcolepsy, obesity, and age-related disorders. In this review, we present an overview of our current understanding of orexin function, focusing on sleep disorders, energy balance, and aging, in both rodents and humans. We first discuss animal models used in studies of obesity and sleep, including loss of function using transgenic or viral-mediated approaches, gain of function models using exogenous delivery of orexin receptor agonist, and naturally-occurring models in which orexin responsiveness varies by individual. We next explore rodent models of orexin in aging, presenting evidence that orexin loss contributes to age-related changes in sleep and energy balance. In the next section, we focus on clinical importance of orexin in human obesity, sleep, and aging. We include discussion of orexin loss in narcolepsy and potential importance of orexin in insomnia, correlations between animal and human studies of age-related decline, and evidence for orexin involvement in age-related changes in cognitive performance. Finally, we present a summary of recent studies of orexin in neurodegenerative disease. We conclude that orexin acts as an integrative homeostatic signal influencing numerous brain regions, and that this pivotal role results in potential dysregulation of multiple physiological processes when orexin signaling is disrupted or lost.

Stability of CART peptide expression in the nucleus accumbens in aging

Aging is accompanied by changes of several anorexigenic and orexigenic neuropeptides expressed in various brain areas that control food intake and these changes correlate with senescent anorexia. During aging expression of cocaine- and amphetamine-regulated transcript (CART) peptide was reported to be reduced in the hypothalamic nuclei related to food intake. Although CART peptide is abundant in the nucleus accumbens that also plays a crucial role in the food intake regulation, no data is available about the CART peptide expression in this region through aging. In the present study, CART peptide immunoreactivity was compared in the nucleus accumbens of young adult (4- and 7-month-old) middle-aged (15-month-old) and aging (25-32-month-old) Long-Evans rats. The density of CART-immunoreactive cells and axon terminals in the nucleus accumbens was measured with computer-aided densitometry. CART-immunodensity was similar in the old rats and in the younger animals without significant difference between age groups. In addition, no gender-difference was observed when CART-immunoreactivities in the nucleus accumbens of male and female animals were compared. Our results indicate that CART peptide expression in the nucleus accumbens is stable in adults and does not change with age.

N-Acetylmannosamine improves sleep-wake quality in middle-aged mice: relevance to autonomic nervous function

Aging is associated with a variety of physiological changes originating peripherally and centrally, including within the autonomic nervous system. Sleep-wake disturbances constitute reliable hallmarks of aging in several animal species and humans. Recent studies have been interested in N-acetylmannosamine (ManNAc) a potential therapeutic agent for improving quality of life, as well as preventing age-related cognitive decline. In this study, ManNAc (5.0 mg/ml) was administered in the drinking water of middle-aged male C57BL/6J mice (55 weeks old) for 7 days. Mice were housed under a 12:12 h light:dark cycle at 23-24 °C. We evaluated bio-behavioral activity using electrocardiogram, body temperature and locomotor activity recorded by an implanted telemetry transmitter. To estimate sleep-wake profile, surface electroencephalogram and electromyogram leads connected to a telemetry transmitter were also implanted in mice. Autonomic nervous activity was evaluated using power spectral analysis of heart rate variability. ManNAc-treated mice spent more time in a wakeful state and less time in slow wave sleep during the dark phase. Parasympathetic nervous activity was increased following ManNAc treatment, then the sympatho-vagal balance was shifted predominance of parasympathetic nervous system. Furthermore, improvement in sleep-wake pattern was associated with increased parasympathetic nervous activity. These results suggest that ManNAc treatment can improve bio-behavioral activity and sleep-wake quality in middle-aged mice. This may have implications for improving sleep patterns in elderly humans.

Early expression of hypocretin/orexin in the chick embryo brain

Hypocretin/Orexin (H/O) neuropeptides are released by a discrete group of neurons in the vertebrate hypothalamus which play a pivotal role in the maintenance of waking behavior and brain state control. Previous studies have indicated that the H/O neuronal development differs between mammals and fish; H/O peptide-expressing cells are detectable during the earliest stages of brain morphogenesis in fish, but only towards the end of brain morphogenesis (by ∼85% of embryonic development) in rats. The developmental emergence of H/O neurons has never been previously described in birds. With the goal of determining whether the chick developmental pattern was more similar to that of mammals or of fish, we investigated the emergence of H/O-expressing cells in the brain of chick embryos of different ages using immunohistochemistry. Post-natal chick brains were included in order to compare the spatial distribution of H/O cells with that of other vertebrates. We found that H/O-expressing cells appear to originate from two separate places in the region of the diencephalic proliferative zone. These developing cells express the H/O neuropeptide at a comparatively early age relative to rodents (already visible at 14% of the way through fetal development), thus bearing a closer resemblance to fish. The H/O-expressing cell population proliferates to a large number of cells by a relatively early embryonic age. As previously suggested, the distribution of H/O neurons is intermediate between that of mammalian and non-mammalian vertebrates. This work suggests that, in addition to its roles in developed brains, the H/O peptide may play an important role in the early embryonic development of non-mammalian vertebrates.

Orexin/hypocretin based pharmacotherapies for the treatment of addiction: DORA or SORA?

Addiction is a chronic relapsing disorder which presents a significant global health burden and unmet medical need. The orexin/hypocretin system is an attractive potential therapeutic target as demonstrated by the successful clinical trials of antagonist medications like Suvorexant for insomnia. It is composed of two neuropeptides, orexin-A and orexin-B and two excitatory and promiscuous G-protein coupled receptors, OX1 and OX2. Orexins are known to have a variety of functions, most notably in regulating arousal, appetite and reward. The orexins have been shown to have a role in mediating the effects of several drugs of abuse, such as cocaine, morphine and alcohol via projections to key brain regions such as the ventral tegmental area, nucleus accumbens and prefrontal cortex. However, it has not yet been demonstrated whether the dual orexin receptor antagonists (DORAs) under development for insomnia are ideal drugs for the treatment of addiction. The question of whether to use a DORA or single orexin receptor antagonist (SORA) for the treatment of addiction is a key question that will need to be answered in order to maximize the clinical utility of orexin receptor antagonists. This review will examine the role of the orexin/hypocretin system in addiction, orexin-based pharmacotherapies under development and factors affecting the selection of one or both orexin receptors as drug targets for the treatment of addiction.

Sleep, its regulation and possible mechanisms of sleep disturbances

The state of sleep consists of different phases that proceed in successive, tightly regulated order through the night forming a physiological program, which for each individual is different but stabile from one night to another. Failure to accomplish this program results in feeling of unrefreshing sleep and tiredness in the morning. The program core is constructed by genetic factors but regulated by circadian rhythm and duration and intensity of day time brain activity. Many environmental factors modulate sleep, including stress, health status and ingestion of vigilance-affecting nutrients or medicines (e.g. caffeine). Acute sleep loss results in compromised cognitive performance, memory deficits, depressive mood and involuntary sleep episodes during the day. Moreover, prolonged sleep curtailment has many adverse health effects, as evidenced by both epidemiological and experimental studies. These effects include increased risk for depression, type II diabetes, obesity and cardiovascular diseases. In addition to voluntary restriction of sleep, shift work, irregular working hours, jet lag and stress are important factors that induce curtailed or bad quality sleep and/or insomnia. This review covers the current theories on the function of normal sleep and describes current knowledge on the physiologic effects of sleep loss. It provides insights into the basic mechanisms of the regulation of wakefulness and sleep creating a theoretical background for understanding different disturbances of sleep.

Effects of changes in energy homeostasis and exposure of noxious insults on the expression of orexin (hypocretin) and its receptors in the brain

This review summarizes data regarding the brain expression of the orexin (hypocretin) system including: prepro-orexin (PPO), orexin A (OxA), orexin B (OxB) and the two orexin receptors 1 and 2 (OxR1, OxR2). Clinical data is limited to OxA and OxB in cerebral spinal fluid and serum/plasma, thus necessitating the development of animal models to undertake mechanistic studies. We focus on changes in animal models that were either exposed to a regime of altered sleep, metabolic energy homeostasis, exposed to drugs and noxious insults. Many more expressional studies are available for PPO, OxA and OxB levels, compared to studies of the receptors. Interestingly, the direction and pattern of change for PPO, OxA and OxB is inconsistent amongst studies, whereas for the receptors, there tends to be increased expression for both OxR1 and OxR2 after alterations in energy homeostasis, and an increased expression after noxious insults or exposure to some drugs. The clinical implications of these results from animal models are discussed in light of the findings from human studies, and future research directions are suggested to fill knowledge gaps with regard to the orexin system, particularly during early brain development.

Food for thought: the role of appetitive peptides in age-related cognitive decline

Through their well described actions in the hypothalamus, appetitive peptides such as insulin, orexin and leptin are recognized as important regulators of food intake, body weight and body composition. Beyond these metabolic activities, these peptides also are critically involved in a wide variety of activities ranging from modulation of immune and neuroendocrine function to addictive behaviors and reproduction. The neurological activities of insulin, orexin and leptin also include facilitation of hippocampal synaptic plasticity and enhancement of cognitive performance. While patients with metabolic disorders such as obesity and diabetes have greater risk of developing cognitive deficits, dementia and Alzheimer's disease (AD), the underlying mechanisms that are responsible for, or contribute to, age-related cognitive decline are poorly understood. In view of the importance of these peptides in metabolic disorders, it is not surprising that there is a greater focus on their potential role in cognitive deficits associated with aging. The goal of this review is to describe the evidence from clinical and pre-clinical studies implicating insulin, orexin and leptin in the etiology and progression of age-related cognitive decline. Collectively, these studies support the hypothesis that leptin and insulin resistance, concepts normally associated with the hypothalamus, are also applicable to the hippocampus.

Orexin: a potential role in the process of obstructive sleep apnea

Obstructive sleep apnea (OSA) is a complicated disease with an unrecognized mechanism. Obesity, sex, age, and smoking have been found to be independent correlates of OSA. Orexin (also named hypocretin) mainly secreted by lateral hypothalamus neurons has a wide array of biological functions like regulating sleep, energy levels and breathing. Several clinical studies found ties between orexin and OSA. Because of the close correlation between orexin and obesity, sex, age and smoking (which are the key risk factors for OSA patients), we hypothesize that orexin may play a key role in the pathogenesis of OSA.

Effects of orexins A and B on expression of orexin receptors and progesterone release in luteal and granulosa ovarian cells

Orexin-A and orexin-B are neuropeptides controlling sleep-wakefulness, feeding and neuroendocrine functions via their G protein-coupled receptors, orexin-1R and orexin-2R. They are synthesized in the lateral hypothalamus and project throughout the brain. Orexins and orexin receptors have also been described outside the brain. Previously we demonstrated the presence of both receptors in the ovary, their increased expression during proestrous afternoon and the dependence on the gonadotropins. Here we studied the effects of orexins on the mRNA expression of both receptors, by quantitative real-time PCR, on luteal cells from superovulated rat ovaries and granulosa cells from diethylstilbestrol-treated rat ovaries. Effects on progesterone secretion were also measured. In luteal cells, 1 nM of either orexin-A or orexin-B decreased progesterone secretion. Orexin-A treatment increased expression of both orexin-1R and orexin-2R mRNA. The effect on orexin-1R mRNA expression was abolished by an orexin-1R selective receptor antagonist SB-334867 and the effect on orexin-2R mRNA expression was abolished by a selective orexin-2R antagonist JNJ-10397049. Orexin-B did not modify orexin-1R mRNA expression, but increased orexin-2R mRNA expression. The effect of orexin-B on orexin-2R was abolished by a selective orexin-2R antagonist. Neither the expression of orexin receptors nor progesterone secretions by granulosa cells were affected by orexins. FSH, as positive control, increased both steroid hormones secretion, but did not induce the expression of OX receptors in granulosa cells isolated from late preantral/early antral follicles. Finally in ovaries obtained immediately after sacrifice, the expression of orexin-1R and orexin-2R was higher in superovulated rat ovaries compared to control or diethylstilbestrol treated rat ovaries. A selective presence and function of both orexinergic receptors in luteal and granulosa cells is described, suggesting that the orexinergic system may have a functional role in the ovary.

Aging-related deficits in orexin/hypocretin modulation of the septohippocampal cholinergic system

The medial septum (MS) of the basal forebrain contains cholinergic neurons that project to the hippocampus, support cognitive function, and are implicated in age-related cognitive decline. Hypothalamic orexin/hypocretin neurons innervate and modulate basal forebrain cholinergic neurons and provide direct inputs to the hippocampus. However, the precise role of orexin in modulating hippocampal cholinergic transmission--and how these interactions are altered in aging--is unknown. Here, orexin A was administered to CA1 and the MS of young (3-4 months) and aged (27-29 months) Fisher 344/Brown Norway rats, and hippocampal acetylcholine efflux was analyzed by in vivo microdialysis. At both infusion sites, orexin A dose-dependently increased hippocampal acetylcholine in young, but not aged rats. Moreover, immunohistochemical characterization of the MS revealed no change in cholinergic cell bodies in aged animals, but a significant decrease in orexin fiber innervation to cholinergic cells. These findings indicate that: (1) Orexin A modulates hippocampal cholinergic neurotransmission directly and transsynaptically in young animals, (2) Aged animals are unresponsive to orexin A, and (3) Aged animals undergo an intrinsic reduction in orexin innervation to cholinergic cells within the MS. Alterations in orexin regulation of septohippocampal cholinergic activity may contribute to age-related dysfunctions in arousal, learning, and memory.

Aging-related alterations in orexin/hypocretin modulation of septo-hippocampal amino acid neurotransmission

GABAergic neurons of the medial septum of the basal forebrain make up a substantial portion of the septo-hippocampal pathway fibers, and are known to modulate hippocampal amino acid neurotransmission and support cognitive function. Importantly, these neurons are also implicated in age-related cognitive decline. Hypothalamic orexin/hypocretin neurons innervate and modulate the activity of these basal forebrain neurons and also provide direct inputs to the hippocampus. However, the precise role of orexin inputs in modulating hippocampal amino acid neurotransmission--as well as how these interactions are altered in aging--has not been defined. Here, orexin A (OxA) was administered to CA1 and the medial septum of young (3-4 months) and aged (27-29 months) Fisher 344 Brown Norway rats, and hippocampal GABA and glutamate efflux was analyzed by in vivo microdialysis. Following CA1 infusion of OxA, extracellular GABA and glutamate efflux was increased, but the magnitude of orexin-mediated efflux was not altered as a function of age. However, medial septum infusion of OxA did not impact hippocampal efflux in young rats, while aged rats exhibited a significant enhancement in GABA and glutamate efflux compared to young counterparts. Furthermore, immunohistochemical characterization of the medial septum revealed a significant decrease in parvalbumin (PV)-positive cell bodies in aged animals, and a significant reduction in orexin fiber innervation to the remaining GABAergic cells within the septum, while orexin innervation to the hippocampus was unaltered by the aging process. These findings indicate that: (1) OxA directly modulates hippocampal amino acid neurotransmission in young animals, (2) Aged animals show enhanced responsivity to exogenous OxA activation of the septo-hippocampal pathway, and (3) Aged animals undergo an intrinsic reduction in medial septum PV-immunoreactivity and a decrease in orexin innervation to remaining septal PV neurons. Alterations in orexin regulation of septo-hippocampal activity may contribute to age-related dysfunctions in arousal, learning, and memory.

Disease and Degeneration of Aging Neural Systems that Integrate Sleep Drive and Circadian Oscillations

Sleep/wake and circadian rest-activity rhythms become irregular with age. Typical outcomes include fragmented sleep during the night, advanced sleep phase syndrome and increased daytime sleepiness. These changes lead to a reduction in the quality of life due to cognitive impairments and emotional stress. More importantly, severely disrupted sleep and circadian rhythms have been associated with an increase in disease susceptibility. Additionally, many of the same brain areas affected by neurodegenerative diseases include the sleep and wake promoting systems. Any advances in our knowledge of these sleep/wake and circadian networks are necessary to target neural areas or connections for therapy. This review will discuss research that uses molecular, behavioral, genetic and anatomical methods to further our understanding of the interaction of these systems.

Endoplasmic reticulum stress in wake-active neurons progresses with aging

Fragmentation of wakefulness and sleep are expected outcomes of advanced aging. We hypothesize that wake neurons develop endoplasmic reticulum dyshomeostasis with aging, in parallel with impaired wakefulness. In this series of experiments, we sought to more fully characterize age-related changes in wakefulness and then, in relevant wake neuronal populations, explore functionality and endoplasmic reticulum homeostasis. We report that old mice show greater sleep/wake transitions in the active period with markedly shortened wake periods, shortened latencies to sleep, and less wake time in the subjective day in response to a novel social encounter. Consistent with sleep/wake instability and reduced social encounter wakefulness, orexinergic and noradrenergic wake neurons in aged mice show reduced c-fos response to wakefulness and endoplasmic reticulum dyshomeostasis with increased nuclear translocation of CHOP and GADD34. We have identified an age-related unfolded protein response injury to and dysfunction of wake neurons. It is anticipated that these changes contribute to sleep/wake fragmentation and cognitive impairment in aging.

Age-related loss of orexin/hypocretin neurons

Aging is associated with many physiological alterations-such as changes in sleep patterns, metabolism and food intake-suggestive of hypothalamic dysfunction, but the effects of senescence on specific hypothalamic nuclei and neuronal groups that mediate these alterations is unclear. The lateral hypothalamus and contiguous perifornical area (LH/PFA) contains several populations of neurons, including those that express the neuropeptides orexin (hypocretin) or melanin-concentrating hormone (MCH). Collectively, orexin and MCH neurons influence many integrative homeostatic processes related to wakefulness and energy balance. Here, we determined the effect of aging on numbers of orexin and MCH neurons in young adult (3-4 months) and old (26-28 months) Fisher 344/Brown Norway F1 hybrid rats. Aged rats exhibited a loss of greater than 40% of orexin-immunoreactive neurons in both the medial and lateral (relative to the fornix) sectors of the LH/PFA. MCH-immunoreactive neurons were also lost in aged rats, primarily in the medial LH/PFA. Neuronal loss in this area was not global as no change in cells immunoreactive for the pan-neuronal marker, NeuN, was observed in aged rats. Combined with other reports of altered receptor expression or behavioral responses to exogenously-administered neuropeptide, these data suggest that compromised orexin (and, perhaps, MCH) function is an important mediator of age-related homeostatic disturbances of hypothalamic origin. The orexin system may represent a crucial substrate linking homeostatic and cognitive dysfunction in aging, as well as a novel therapeutic target for pharmacological or genetic restoration approaches to preventing or ameliorating these disturbances.

Age-associated changes of appetite-regulating peptides

Aging is associated with a progressive decrease in appetite and food intake. The reasons for the decline in food intake are multifactorial, and relate to both peripheral and central mechanisms. Current studies about the regulation of food intake suggest that there are many central mediators that control the appetite. To determine the mechanism of age-associated decrease in appetite and food intake, we focused on the age-associated changes of the suppressing and stimulatory effect of some appetite-regulating peptides. At first, we examined cholecystokinin (CCK), one of the typical appetite-suppressing factors. Although sensitivity to CCK is enhanced in old animals, the mechanism underlying this effect has not been elucidated. Next, we focused on the appetite-stimulating peptides, orexin-A, neuropeptide Y (NPY) and ghrelin, which are known to play a critical role in food intake. To determine the age-associated decrease in appetite and food intake, we compared the stimulatory effect of intracerebroventricular administration of orexin-A, NPY and ghrelin. We report the studies of the age-associated changes of appetite-regulating peptides in this review.

Elevated sleep quality and orexin receptor mRNA in obesity-resistant rats

Objective

To determine if resistance to weight gain is associated with alterations in sleep/wake states and orexin receptor gene expression.

Design

Three-month old obesity susceptible Sprague-Dawley (SD) and obesity resistant (OR) rats were fed standard rodent chow. Sleep/wake cycle was measured by radiotelemetry and orexin receptor profiles in sleep/wake regulatory areas of the brain were quantified by quantitative RT-PCR.

Subjects

Adult male obesity susceptible SD and selectively-bred OR rats.

Measurements

Body weight, food intake, energy efficiency, percent time spent in active wake, quiet wake, slow-wave sleep (SWS), rapid eye movement (REM) sleep, number and mean duration of sleep/wake episodes, number of stage transitions, SWS sleep delta power and orexin receptor mRNA levels were measured.

Results

Obesity resistant rats weighed significantly less and had lower energy efficiency than SD rats. Food intake was not different between SD and OR rats. Time spent in quiet wake was similar between groups, and therefore active wake and quiet wake were combined and are referred to as ‘wakefulness’. Obesity resistant rats spent significantly more time in wakefulness and less time in SWS compared to SD rats during the 24 h recording period. Relative to SD rats, OR rats had significantly fewer sleep/wake episodes and the duration of the episodes were prolonged, indicating less fragmented sleep. Further, OR rats had fewer transitions between sleep stages, which indicates that OR rats were behaviorally more stable and had more consolidated sleep than obesity susceptible SD rats. Obesity resistant rats exhibited lower delta power during SWS sleep, indicating a lower sleep drive. Our results demonstrated greater orexin receptor gene expression in sleep regulatory brain areas in OR rats.

Conclusion

These results demonstrate that prolonged wakefulness, better sleep quality, lower sleep drive and greater orexin signaling may confer protection against obesity.

Age- and gender-specific changes of hypocretin immunopositive neurons in C57Bl/6 mice

Loss of neurons or neuronal functions over time has been hypothesized to contribute to the dysregulation of autonomic functions observed in aging. In this study, we evaluated the total number of the hypothalamic hypocretin (orexin) immunopositive neurons in 100, 400, 800 and 1000-day-old male and female C57Bl/6 mice that are commonly used in aging studies in vertebrates. Males had 15-20% more hypocretin immunopositive neurons (HIN) than females at all ages examined. Neuronal number for both sexes was stable in the first 400 days of life, but started declining between 400 and 800 days with rates of approximately 1 neuron/day. The rate of loss doubled in males between 800 and 1000 days of age. The total average number of HIN for males was 2251+/-139 at 100 days, 2235+/-112 at 400 days, 1914+/-81 at 800 days, and 1596+/-301 at 1000 days. The total average number of HIN for females was 1805+/-76 at 100 days, 1887+/-118 at 400 days, and 1521+/-181 at 800 days. Evaluation of the time-dependent decline in the number of hypocretin immunopositive neurons may help to explain the physiological changes in sleep or energy homeostasis regulation during aging.

Age- and gender-specific changes of hypocretin immunopositive neurons in C57Bl/6 mice

Loss of neurons or neuronal functions over time has been hypothesized to contribute to the dysregulation of autonomic functions observed in aging. In this study, we evaluated the total number of the hypothalamic hypocretin (orexin) immunopositive neurons in 100, 400, 800 and 1000-day-old male and female C57Bl/6 mice that are commonly used in aging studies in vertebrates. Males had 15-20% more hypocretin immunopositive neurons (HIN) than females at all ages examined. Neuronal number for both sexes was stable in the first 400 days of life, but started declining between 400 and 800 days with rates of approximately 1 neuron/day. The rate of loss doubled in males between 800 and 1000 days of age. The total average number of HIN for males was 2251+/-139 at 100 days, 2235+/-112 at 400 days, 1914+/-81 at 800 days, and 1596+/-301 at 1000 days. The total average number of HIN for females was 1805+/-76 at 100 days, 1887+/-118 at 400 days, and 1521+/-181 at 800 days. Evaluation of the time-dependent decline in the number of hypocretin immunopositive neurons may help to explain the physiological changes in sleep or energy homeostasis regulation during aging.

Orexin activation precedes increased NPY expression, hyperphagia, and metabolic changes in response to sleep deprivation

Several pieces of evidence support that sleep duration plays a role in body weight control. Nevertheless, it has been assumed that, after the identification of orexins (hypocretins), the molecular basis of the interaction between sleep and energy homeostasis has been provided. However, no study has verified the relationship between neuropeptide Y (NPY) and orexin changes during hyperphagia induced by sleep deprivation. In the current study we aimed to establish the time course of changes in metabolite, endocrine, and hypothalamic neuropeptide expression of Wistar rats sleep deprived by the platform method for a distinct period (from 24 to 96 h) or sleep restricted for 21 days (SR-21d). Despite changes in the stress hormones, we found no changes in food intake and body weight in the SR-21d group. However, sleep-deprived rats had a 25-35% increase in their food intake from 72 h accompanied by slight weight loss. Such changes were associated with increased hypothalamus mRNA levels of prepro-orexin (PPO) at 24 h followed by NPY at 48 h of sleep deprivation. Conversely, sleep recovery reduced the expression of both PPO and NPY, which rapidly brought the animals to a hypophagic condition. Our data also support that sleep deprivation rapidly increases energy expenditure and therefore leads to a negative energy balance and a reduction in liver glycogen and serum triacylglycerol levels despite the hyperphagia. Interestingly, such changes were associated with increased serum levels of glucagon, corticosterone, and norepinephrine, but no effects on leptin, insulin, or ghrelin were observed. In conclusion, orexin activation accounts for the myriad changes induced by sleep deprivation, especially the hyperphagia induced under stress and a negative energy balance.

Developmental and aging change of orexin-A and -B immunoreactive neurons in the male rat hypothalamus

Orexin/hypocretin is indicated to affect various physiological functions and behaviors, such as energy balance, feeding, wake-sleep cycle, stress response, and reproduction. This study investigated postnatal development and aging changes of the orexin neuron in the male rat hypothalamus. The brain tissue of rats from 1 week to 24 months old was analyzed by immunohistochemistry for two forms of orexin peptides, orexin-A and -B. The number of immunoreactive cells for each age group was counted and the immunoreactive intensity was also analyzed in order to reveal the changes in the number of expressing cells and the relative amount of the peptides. The number of orexin immunoreactive cells increased from postnatal 2 weeks to maturation, then slightly decreased and stabilized until the age of 8 months old, but it was significantly decreased by 24 months old. The intensity of the immunoreaction followed almost the same pattern. Our findings demonstrate that orexin neurons are increased during maturation and then are significantly decreased during the period from 8 to 24 months old, indicating an involvement of orexin in the physiological changes in rat aging such as energy balance, sleep, stress response, and reproduction.

Systemic and intrabasalis administration of the orexin-1 receptor antagonist, SB-334867, disrupts attentional performance in rats

RATIONALE

Orexin neurons project to a number of brain regions, including onto basal forebrain cholinergic neurons. Basal forebrain corticopetal cholinergic neurons are known to be necessary for normal attentional performance. Thus, the orexin system may contribute to attentional processing.

OBJECTIVES

We tested whether blockade of orexin-1 receptors would disrupt attentional performance.

METHODS

Rats were trained in a two-lever sustained attention task that required discrimination of a visual signal (500, 100, 25 ms) from trials with no signal presentation. Rats received systemic or intrabasalis administration of the orexin-1 receptor antagonist, SB-334867, prior to task performance.

RESULTS

Systemic administration of the orexin-1 receptor antagonist, SB-334867 (5.0 mg/kg), decreased detection of the longest duration signal. Intrabasalis SB-334867 (0.60 microg) decreased overall accuracy on trials with longer signal durations.

CONCLUSIONS

These findings suggest that orexins contribute to attentional processing, although neural circuits outside of basal forebrain corticopetal cholinergic neurons may mediate some of these effects.

The wake-promoting effects of hypocretin-1 are attenuated in old rats

Disruption of sleep is a frequent complaint among elderly humans and is also evident in aged laboratory rodents. The neurobiological bases of age-related sleep/wake disruption are unknown. Given the critical role of the hypocretins in sleep/wake regulation, we sought to determine whether the wake-promoting effect of hypocretin changes with age in Wistar rats, a strain in which age-related changes in both sleep and hypocretin signaling have been reported. Intracerebroventricular infusions of hypocretin-1 (10 and 30 μg) significantly increased wake time relative to vehicle in both young (3 mos) and old (25 mos) Wistar rats. However, the magnitude and duration of the wake-promoting effects were attenuated with age. An increase of parameters associated with homeostatic sleep recovery after sleep deprivation, including non-rapid eye movement (NR) sleep time, NR delta power, the ratio of NR to rapid eye movement (REM) sleep, and NR consolidation, occurred subsequent to Hcrt-induced waking in young but not old rats. ICV infusions of hypocretin-2 (10 and 30 μg) produced fewer effects in both young and old rats. These data demonstrate that activation of a major sleep/wake regulatory pathway is attenuated in old rats.

Orexin/hypocretin modulation of the basal forebrain cholinergic system: Role in attention

The basal forebrain cholinergic system (BFCS) plays a role in several aspects of attentional function. Activation of this system by different afferent inputs is likely to influence how attentional resources are allocated. While it has been recognized for some time that the hypothalamus is a significant source of projections to the basal forebrain, the phenotype(s) of these inputs and the conditions under which their regulation of the BFCS becomes functionally relevant are still unclear. The cell bodies of neurons expressing orexin/hypocretin neuropeptides are restricted to the lateral hypothalamus and contiguous perifornical area but have widespread projections, including to the basal forebrain. Orexin fibers and both orexin receptor subtypes are distributed in cholinergic parts of the basal forebrain, where application of orexin peptides increases cell activity and cortical acetylcholine release. Furthermore, disruption of orexin signaling in the basal forebrain impairs the cholinergic response to an appetitive stimulus. In this review, we propose that orexin inputs to the BFCS form an anatomical substrate for links between arousal and attention, and that these interactions might be particularly important as a means by which interoceptive cues bias allocation of attentional resources toward related exteroceptive stimuli. Dysfunction in orexin-acetylcholine interactions may play a role in the arousal and attentional deficits that accompany neurodegenerative conditions as diverse as drug addiction and age-related cognitive decline.

Orexin/hypocretin modulation of the basal forebrain cholinergic system: insights from in vivo microdialysis studies

Since its discovery less than a decade ago, interest in the hypothalamic orexin/hypocretin system has blossomed due to the diversity and importance of the roles played by these neuropeptides. Orexin neurons have widespread projections throughout the central nervous system and intense research has focused on elucidating the pathways and mechanisms by which orexins exert their diverse array of functions. Our group has recently focused on orexin inputs to the basal forebrain cholinergic system, which plays a crucial role in cognitive--particularly attentional--function. Orexin cells provide a robust input to cholinergic neurons in the basal forebrain and act here to modulate cortical acetylcholine release. Orexin A also increases local glutamate release within the basal forebrain, suggesting an additional, indirect effect of orexins on basal forebrain cholinergic activity. Orexin activation of the basal forebrain cholinergic system appears to be especially relevant in the context of homeostatic challenges, such as food deprivation. Thus, orexins can stimulate cortical cholinergic transmission which, in turn, may promote the detection and selection of stimuli related to physiological needs. In this manner, orexin interactions with the basal forebrain cholinergic system are likely to form a link between arousal and attention in support of the cognitive components of motivated behavior.

Orexin neuronal changes in the locus coeruleus of the aging rhesus macaque

Orexin neuropeptides regulate arousal state and excite the noradrenergic locus coeruleus (LC), so it is plausible that an age-related loss of orexin neurons and projections to the LC contributes to poor sleep quality in elderly humans and nonhuman primates. To test this hypothesis we examined orexin B-immunoreactivity in the lateral hypothalamic area (LHA) and the LC of male rhesus macaques (Macaca mulatta) throughout the life span. Orexin perikarya, localized predominantly in the LHA, showed identical distribution patterns irrespective of age. Similarly, orexin neuron number and serum orexin B concentrations did not differ with age. In contrast, orexin B-immunoreactive axon density in the LC of old animals was significantly lower than that observed in the young or adult animals. Furthermore, the age-related decline was associated with a significant decrease in tyrosine hydroxylase (TH) mRNA in the LC, despite no change in TH-immunoreactive neuron number. Taken together, these data suggest that age-related decreases in excitatory orexin innervation to the noradrenergic LC may contribute to the etiology of poor sleep quality in the elderly.

Age-related changes of hypocretin in basal forebrain of guinea pig

Hypocretin-1 (hcrt-1) and hypocretin-2 (hcrt-2) have been implicated in a wide variety of functions including sleep and wakefulness as well as related behaviors. Many of these functions of the hypocretins involve the activation of cholinergic neurons in the basal forebrain (BF). These neurons have been shown to exhibit age-related changes in a variety of species. In the present experiment, in adult and aged guinea pigs, we compared hypocretin immunoreactivity in regions of the BF that include the medial septal nucleus (MS), the vertical and horizontal limbs of the diagonal band of Broca (VDB and HDB) and the magocellular preoptic nucleus (MCPO). In adult guinea pigs (3-5 months of age), all of the preceding BF regions contained dense hypocretin fibers with varicosities. On the contrary, in old guinea pigs (27-28 months), although the MS exhibited a similar intensity of hypocretin immunoreactivity compared with the adult guinea pig, there was a significant decrease in the intensity of immunoreactivity of hypocretinergic fibers in the VDB, HDB and MCPO. These data indicate that the hypocretinergic innervation of specific nuclei of the BF is compromised during the aging process. We suggest that the reduction in hypocretinergic innervation of the BF nuclei may contribute to the age-related changes in the states of sleep and wakefulness as well as deficits in related systems that occur in old age.

The hypocretins and sleep

The hypocretins (also called the orexins) are two neuropeptides derived from the same precursor whose expression is restricted to a few thousand neurons of the lateral hypothalamus. Two G-protein coupled receptors for the hypocretins have been identified, and these show different distributions within the central nervous system and differential affinities for the two hypocretins. Hypocretin fibers project throughout the brain, including several areas implicated in regulation of the sleep/wakefulness cycle. Central administration of synthetic hypocretin-1 affects blood pressure, hormone secretion and locomotor activity, and increases wakefulness while suppressing rapid eye movement sleep. Most human patients with narcolepsy have greatly reduced levels of hypocretin peptides in their cerebral spinal fluid and no or barely detectable hypocretin-containing neurons in their hypothalamus. Multiple lines of evidence suggest that the hypocretinergic system integrates homeostatic, metabolic and limbic information and provides a coherent output that results in stability of the states of vigilance.

Opposite effects of sleep rebound on orexin OX1 and OX2 receptor expression in rat brain

Orexins (hypocretins) have been implicated in the regulation of the normal sleep-wake cycle, in sensorimotor programming, and in other homeostatic and neuroregulatory processes. The present study examined the effects of sleep deprivation (SD) and sleep recovery on the expression of orexin 1 receptors (OX1R) and orexin 2 receptors (OX2R) throughout the brain. Rats were sacrificed either immediately after 96 h of sleep deprivation (SD group) or after SD followed by 24 h of sleep recovery (Rebound group). Prepro-orexin mRNA showed a non-significant increase in the SD group relative to controls, but a pronounced and significant increase in the Rebound group (+88%, P < 0.007). Similarly, sleep deprivation produced no effect on OX1R or OX2R mRNA levels. However, in the Rebound group, OX1R mRNA levels increased significantly, compared to either control or SD groups, in 37 of 92 brain regions analyzed, with particularly strong effects in the amygdala and hypothalamus. Changes in OX2R mRNA levels were also seen only in the sleep Rebound group, but they were fewer in number (10 out of 86 regions), were in the direction of decreased rather than increased expression, and were predominantly confined to cerebral cortical areas. These observations indicate that some factor associated with sleep recovery, possibly the compensatory increase in REM sleep, has strong effects on the orexin system at the mRNA level. They further indicate that,pOX1 and OX2 receptors are affected in opposite way and that the former are more vulnerable to these effects than the latter.