Activation of the basal forebrain by the orexin/hypocretin neurones

Cerebrospinal fluid melanin-concentrating hormone (MCH) and hypocretin-1 (HCRT-1, orexin-A) in Alzheimer's disease

Ancillary to decline in cognitive abilities, patients with Alzheimer’s disease (AD) frequently suffer from behavioural and psychological symptoms of dementia (BPSD). Hypothalamic polypeptides such as melanin-concentrating hormone (MCH) and hypocretin-1 (HCRT-1, orexin-A) are promoters of sleep-wake regulation and energy homeostasis and are found to impact on cognitive performance. To investigate the role of MCH and HCRT-1 in AD, cerebrospinal fluid (CSF) levels were measured in 33 patients with AD and 33 healthy subjects (HS) using a fluorescence immunoassay (FIA). A significant main effect of diagnosis (F(1,62) = 8.490, p<0.01) on MCH levels was found between AD (93.76±13.47 pg/mL) and HS (84.65±11.40 pg/mL). MCH correlated with T-tau (r = 0.47; p<0.01) and P-tau (r = 0.404; p<0.05) in the AD but not in the HS. CSF-MCH correlated negatively with MMSE scores in the AD (r = −0.362, p<0.05) and was increased in more severely affected patients (MMSE_≤20) compared to HS (p<0.001) and BPSD-positive patients compared to HS (p<0.05). In CSF-HCRT-1, a significant main effect of sex (F(1,31) = 4.400, p<0.05) with elevated levels in females (90.93±17.37 pg/mL vs. 82.73±15.39 pg/mL) was found whereas diagnosis and the sex*diagnosis interaction were not significant. Elevated levels of MCH in patients suffering from AD and correlation with Tau and severity of cognitive impairment point towards an impact of MCH in AD. Gender differences of CSF-HCRT-1 controversially portend a previously reported gender dependence of HCRT-1-regulation. Histochemical and actigraphic explorations are warranted to further elucidate alterations of hypothalamic transmitter regulation in AD.

Physiology of the orexinergic/hypocretinergic system: a revisit in 2012

The neuropeptides orexins and their G protein-coupled receptors, OX(1) and OX(2), were discovered in 1998, and since then, their role has been investigated in many functions mediated by the central nervous system, including sleep and wakefulness, appetite/metabolism, stress response, reward/addiction, and analgesia. Orexins also have peripheral actions of less clear physiological significance still. Cellular responses to the orexin receptor activity are highly diverse. The receptors couple to at least three families of heterotrimeric G proteins and other proteins that ultimately regulate entities such as phospholipases and kinases, which impact on neuronal excitation, synaptic plasticity, and cell death. This article is a 10-year update of my previous review on the physiology of the orexinergic/hypocretinergic system. I seek to provide a comprehensive update of orexin physiology that spans from the molecular players in orexin receptor signaling to the systemic responses yet emphasizing the cellular physiological aspects of this system.

Development of SCN connectivity and the circadian control of arousal: a diminishing role for humoral factors?

The suprachiasmatic nucleus (SCN) is part of a wake-promoting circuit comprising the dorsomedial hypothalamus (DMH) and locus coeruleus (LC). Although widely considered a "master clock," the SCN of adult rats is also sensitive to feedback regarding an animal's behavioral state. Interestingly, in rats at postnatal day (P)2, repeated arousing stimulation does not increase neural activation in the SCN, despite doing so in the LC and DMH. Here we show that, by P8, the SCN is activated by arousing stimulation and that selective destruction of LC terminals with DSP-4 blocks this activational effect. We next show that bidirectional projections among the SCN, DMH, and LC are nearly absent at P2 but present at P8. Despite the relative lack of SCN connectivity with downstream structures at P2, day-night differences in sleep-wake activity are observed, suggesting that the SCN modulates behavior at this age via humoral factors. To test this hypothesis, we lesioned the SCN at P1 and recorded sleep-wake behavior at P2: Day-night differences in sleep and wake were eliminated. We next performed precollicular transections at P2 and P8 that isolate the SCN and DMH from the brainstem and found that day-night differences in sleep-wake behavior were retained at P2 but eliminated at P8. Finally, the SCN or DMH was lesioned at P8: When recorded at P21, rats with either lesion exhibited similarly fragmented wake bouts and no evidence of circadian modulation of wakefulness. These results suggest an age-related decline in the SCN's humoral influence on sleep-wake behavior that coincides with the emergence of bidirectional connectivity among the SCN, DMH, and LC.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Shining light on wakefulness and arousal

Alterations in arousal states are associated with multiple neuropsychiatric disorders, including generalized anxiety disorders, addiction, schizophrenia, and depression. Therefore, elucidating the neurobiological mechanisms controlling the boundaries between arousal, hyperarousal, and hypoarousal is a crucial endeavor in biological psychiatry. Substantial research over several decades has identified distinct arousal-promoting neural populations in the brain; however, how these nuclei act individually and collectively to promote and maintain wakefulness and various arousal states is unknown. We have recently applied optogenetic technology to the repertoire of techniques used to study arousal. Here, we discuss the recent results of these experiments and propose future use of this approach as a way to understand the complex dynamics of neural circuits controlling arousal and arousal-related behaviors.

Interfaces of sleep and anesthesia

In the past decades there has been an increasing focus on the relationship of sleep and anesthesia. This relationship bears on the fundamental scientific questions in anesthesiology, such as the mechanism of anesthetic-induced unconsciousness. However, given the increasing prevalence of sleep disorders in surgical patients, the interfaces of sleep and anesthesia are now a pressing clinical concern. This article discusses sleep and anesthesia from the perspective of phenotype, mechanism and function, with some concluding thoughts on the relevance to neuroanesthesiology.

Pareidolias: complex visual illusions in dementia with Lewy bodies

Patients rarely experience visual hallucinations while being observed by clinicians. Therefore, instruments to detect visual hallucinations directly from patients are needed. Pareidolias, which are complex visual illusions involving ambiguous forms that are perceived as meaningful objects, are analogous to visual hallucinations and have the potential to be a surrogate indicator of visual hallucinations. In this study, we explored the clinical utility of a newly developed instrument for evoking pareidolic illusions, the Pareidolia test, in patients with dementia with Lewy bodies—one of the most common causes of visual hallucinations in the elderly. Thirty-four patients with dementia with Lewy bodies, 34 patients with Alzheimer’s disease and 26 healthy controls were given the Pareidolia test. Patients with dementia with Lewy bodies produced a much greater number of pareidolic illusions compared with those with Alzheimer’s disease or controls. A receiver operating characteristic analysis demonstrated that the number of pareidolias differentiated dementia with Lewy bodies from Alzheimer’s disease with a sensitivity of 100% and a specificity of 88%. Full-length figures and faces of people and animals accounted for >80% of the contents of pareidolias. Pareidolias were observed in patients with dementia with Lewy bodies who had visual hallucinations as well as those who did not have visual hallucinations, suggesting that pareidolias do not reflect visual hallucinations themselves but may reflect susceptibility to visual hallucinations. A sub-analysis of patients with dementia with Lewy bodies who were or were not treated with donepzil demonstrated that the numbers of pareidolias were correlated with visuoperceptual abilities in the former and with indices of hallucinations and delusional misidentifications in the latter. Arousal and attentional deficits mediated by abnormal cholinergic mechanisms and visuoperceptual dysfunctions are likely to contribute to the development of visual hallucinations and pareidolias in dementia with Lewy bodies.

OX1 orexin/hypocretin receptor activation of phospholipase D

BACKGROUND AND PURPOSE

Orexin receptors potently signal to lipid messenger systems, and our previous studies have suggested that PLD would be one of these. We thus wanted to verify this by direct measurements and clarify the molecular mechanism of the coupling.

EXPERIMENTAL APPROACH

Orexin receptor-mediated PLD activation was investigated in CHO cells stably expressing human OX(1) orexin receptors using [(14) C]-oleic acid-prelabelling and the transphosphatidylation assay.

KEY RESULTS

Orexin stimulation strongly increased PLD activity - even more so than the phorbol ester TPA (12-O-tetradecanoyl-phorbol-13-acetate), a highly potent activator of PLD. Both orexin and TPA responses were mediated by PLD1. Orexin-A and -B showed approximately 10-fold difference in potency, and the concentration-response curves were biphasic. Using pharmacological inhibitors and activators, both orexin and TPA were shown to signal to PLD1 via the novel PKC isoform, PKCδ. In contrast, pharmacological or molecular biological inhibitors of Rho family proteins RhoA/B/C, cdc42 and Rac did not inhibit the orexin (or the TPA) response, nor did the molecular biological inhibitors of PKD. In addition, neither cAMP elevation, Gα(i/o) nor Gβγ seemed to play an important role in the orexin response.

CONCLUSIONS AND IMPLICATIONS

Stimulation of OX(1) receptors potently activates PLD (probably PLD1) in CHO cells and this is mediated by PKCδ but not other PKC isoforms, PKDs or Rho family G-proteins. At present, the physiological significance of orexin-induced PLD activation is unknown, but this is not the first time we have identified PKCδ in orexin signalling, and thus some specific signalling cascade may exist between orexin receptors and PKCδ.

Adenosine inhibits glutamatergic input to basal forebrain cholinergic neurons

Adenosine has been proposed as an endogenous homeostatic sleep factor that accumulates during waking and inhibits wake-active neurons to promote sleep. It has been specifically hypothesized that adenosine decreases wakefulness and promotes sleep recovery by directly inhibiting wake-active neurons of the basal forebrain (BF), particularly BF cholinergic neurons. We previously showed that adenosine directly inhibits BF cholinergic neurons. Here, we investigated 1) how adenosine modulates glutamatergic input to BF cholinergic neurons and 2) how adenosine uptake and adenosine metabolism are involved in regulating extracellular levels of adenosine. Our experiments were conducted using whole cell patch-clamp recordings in mouse brain slices. We found that in BF cholinergic neurons, adenosine reduced the amplitude of AMPA-mediated evoked glutamatergic excitatory postsynaptic currents (EPSCs) and decreased the frequency of spontaneous and miniature EPSCs through presynaptic A(1) receptors. Thus we have demonstrated that in addition to directly inhibiting BF cholinergic neurons, adenosine depresses excitatory inputs to these neurons. It is therefore possible that both direct and indirect inhibition may synergistically contribute to the sleep-promoting effects of adenosine in the BF. We also found that blocking the influx of adenosine through the equilibrative nucleoside transporters or inhibiting adenosine kinase and adenosine deaminase increased endogenous adenosine inhibitory tone, suggesting a possible mechanism through which adenosine extracellular levels in the basal forebrain are regulated.

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.

Orexin receptors as therapeutic drug targets

Orexin (hypocretin) receptor antagonists stand as a model for the development of targeted CNS small-molecule therapeutics. The identification of mutations in the gene for the orexin 2 receptor responsible for canine narcolepsy, the demonstration of a hypersomnolence phenotype in hypocretin knockout mice and the disruption in orexin signaling in narcoleptic patients provides clear genetic proof of concept for targeting orexin-induced arousal for the treatment of insomnia. The full characterization of the genes encoding orexin and its two cognate receptors enabled the rapid development of in vitro and ex vivo assays with which to identify lead compound structures and to optimize potency and pharmacokinetic properties. Polysomnographic measures with cross-species translatability capable of measuring the sleep-promoting effects of orexin receptor antagonists from mice to man, and the existence of knockout models not only allow efficacy assessment but also the demonstration of mechanism of action. Focused efforts by a number of groups have identified potent compounds of diverse chemical structure with differential orexin receptor selectivity for either the orexin 1 receptor (OX₁R) or the orexin 2 receptor (OX₂R), or both. This work has yielded tool compounds that, along with genetic models, have been used to specifically define the role these receptors in mediating orexin-induced arousal and vigilance state control. Optimized dual receptor antagonists with favorable pharmacokinetic and safety profiles have now demonstrated efficacy in clinical development and represent a distinct mechanism of action for the treatment of insomnia relative to current standard of care.

Orexin neurons and emotional stress

Stress increases cardiac function, ventilation, and body temperature and induces analgesia. These changes, which result in an increase in metabolic rate, oxygen supply, and the conduction velocity of nerve impulses, prepare the body for a fight-or-flight response. A part of the hypothalamus called the defense area has long been known to play a key role in these responses, but the precise mechanisms are largely unknown. Our recent findings suggest that orexin (hypocretin) neurons act as a master switch of the fight-or-flight response. In addition, our results, as well as those from other researchers, suggest that orexin neurons do not modulate specific behaviors such as the fight-or-flight responses but rather integrate the autonomic functions and behaviors in a broad sense or in a vigilance state-dependent manner. The orexin system seems to be a pivotal link between the subconscious and the conscious brain functions.

Endogenous opiates and behavior: 2010

This paper is the thirty-third consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2010 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration (Section 16); and immunological responses (Section 17).

Galanthamine plus estradiol treatment enhances cognitive performance in aged ovariectomized rats

We hypothesize that beneficial effects of estradiol on cognitive performance diminish with age and time following menopause due to a progressive decline in basal forebrain cholinergic function. This study tested whether galanthamine, a cholinesterase inhibitor used to treat memory impairment associated with Alzheimer's disease, could enhance or restore estradiol effects on cognitive performance in aged rats that had been ovariectomized in middle-age. Rats were ovariectomized at 16-17 months of age. At 21-22 months of age rats began receiving daily injections of galanthamine (5mg/day) or vehicle. After one week, half of each group also received 17ß-estradiol administered subcutaneously. Rats were then trained on a delayed matching to position (DMP) T-maze task, followed by an operant stimulus discrimination/reversal learning task. Treatment with galanthamine+estradiol significantly enhanced the rate of DMP acquisition and improved short-term delay-dependent spatial memory performance. Treatment with galanthamine or estradiol alone was without significant effect. Effects were task-specific in that galanthamine+estradiol treatment did not significantly improve performance on the stimulus discrimination/reversal learning task. In fact, estradiol was associated with a significant increase in incorrect responses on this task after reversal of the stimulus contingency. In addition, treatments did not significantly affect hippocampal choline acetyltransferase activity or acetylcholine release. This may be an effect of age, or possibly is related to compensatory changes associated with long-term cholinesterase inhibitor treatment. The data suggest that treating with a cholinesterase inhibitor can enhance the effects of estradiol on acquisition of a DMP task by old rats following a long period of hormone deprivation. This could be of particular benefit to older women who have not used hormone therapy for many years and are beginning to show signs of mild cognitive impairment. Potential mechanisms for these effects are discussed.

Hypocretin and its emerging role as a target for treatment of sleep disorders

The neuropeptides hypocretin-1 and -2 (orexin A and B) are critical in the regulation of arousal and maintenance of wakefulness. Understanding the role of the hypocretin system in sleep/wake regulation has come from narcolepsy-cataplexy research. Deficiency of hypocretin results in loss of sleep/wake control with consequent unstable transitions from wakefulness into non-rapid eye movement (REM) and REM sleep, and clinical manifestations including daytime hypersomnolence, sleep attacks, and cataplexy. The hypocretin system regulates sleep/wake control through complex interactions between monoaminergic/cholinergic wake-promoting and GABAergic sleep-promoting neuronal systems. Research for the hypocretin agonist and the hypocretin antagonist for the treatment of sleep disorders has vigorously increased over the past 10 years. This review will focus on the origin, functions, and mechanisms in which the hypocretin system regulates sleep and wakefulness, and discuss its emerging role as a target for the treatment of sleep disorders.

Orexin receptors: pharmacology and therapeutic opportunities

Orexin-A and -B (also known as hypocretin-1 and -2) are neuropeptides produced in the lateral hypothalamus that promote many aspects of arousal through the OX1 and OX2 receptors. In fact, they are necessary for normal wakefulness, as loss of the orexin-producing neurons causes narcolepsy in humans and rodents. This has generated considerable interest in developing small-molecule orexin receptor antagonists as a novel therapy for the treatment of insomnia. Orexin antagonists, especially those that block OX2 or both OX1 and OX2 receptors, clearly promote sleep in animals, and clinical results are encouraging: Several compounds are in Phase III trials. As the orexin system mainly promotes arousal, these new compounds will likely improve insomnia without incurring many of the side effects encountered with current medications.

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.

Role of adenosine and wake-promoting basal forebrain in insomnia and associated sleep disruptions caused by ethanol dependence

Insomnia is a severe symptom of alcohol withdrawal; however, the underlying neuronal mechanism is yet unknown. We hypothesized that chronic ethanol exposure will impair basal forebrain (BF) adenosinergic mechanism resulting in insomnia-like symptoms. We performed a series of experiments in Sprague-Dawley rats to test our hypothesis. We used Majchrowicz's chronic binge ethanol protocol to induce ethanol dependency. Our first experiment verified the effects of ethanol withdrawal on sleep-wakefulness. Significant increase in wakefulness was observed during ethanol withdrawal. Next, we examined c-Fos expression (marker of neuronal activation) in BF wake-promoting neurons during ethanol withdrawal. There was a significant increase in the number of BF wake-promoting neurons with c-Fos immunoreactivity. Our third experiment examined the effects of ethanol withdrawal on sleep deprivation induced increase in BF adenosine levels. Sleep deprivation did not increase BF adenosine levels in ethanol dependent rats. Our last experiment examined the effects of ethanol withdrawal on equilibrative nucleoside transporter 1 and A1 receptor expression in the BF. There was a significant reduction in A1 receptor and equilibrative nucleoside transporter 1 expression in the BF of ethanol dependent rats. Based on these results, we suggest that insomnia observed during ethanol withdrawal is caused because of impaired adenosinergic mechanism in the BF.

Orexin neurons are indispensable for stress-induced thermogenesis in mice

Orexin neurons contribute to cardiovascular, respiratory and analgesic components of the fight-or-flight response against stressors. Here, we examined whether the same is true for stress-induced hyperthermia. We used prepro-orexin knockout mice (ORX-KO) and orexin neuron-ablated mice (ORX-AB) in which the latter lack not only orexin, but also other putative neurotransmitter/modulators contained in the orexin neurons. In response to repetitive insertion of a temperature probe into their rectum (handling stress), ORX-KO mice showed a normal temperature change as compared to that of wild-type littermates (WT) while ORX-AB showed an attenuated response. Stress-induced expression of uncoupling protein-1, a key molecule in non-shivering thermogenesis in the brown adipose tissue (BAT), was also blunted in ORX-AB but not in ORX-KO. When the BAT was directly activated by a β3 adrenergic agonist, there was no difference in the resultant BAT temperature among the groups, indicating that BAT per se was normal in ORX-AB. In WT and ORX-KO, handling stress activated orexin neurons (as revealed by increased expression of c-Fos) and the resultant hyperthermia was largely blunted by pre-treatment with a β3 antagonist. This observation further supports the notion that attenuated stress-induced hyperthermia in ORX-AB mice was caused by a loss of orexin neurons and abnormal BAT regulation. This study pointed out, for the first time, the possible importance of co-existent neurotransmitter/modulators in the orexin neurons for stress-induced hyperthermia and the importance of integrity of the orexin neurons for full expression of multiple facets of the fight-or-flight response.