Measurement of hypocretin/orexin content in the mouse brain using an enzyme immunoassay: the effect of circadian time, age and genetic background

The Roles of Histamine Receptor 1 (hrh1) in Neurotransmitter System Regulation, Behavior, and Neurogenesis in Zebrafish

Histamine receptors mediate important physiological processes and take part in the pathophysiology of different brain disorders. Histamine receptor 1 (HRH1) is involved in the development of neurotransmitter systems, and its role in neurogenesis has been proposed. Altered HRH1 binding and expression have been detected in the brains of patients with schizophrenia, depression, and autism. Our goal was to assess the role of hrh1 in zebrafish development and neurotransmitter system regulation through the characterization of hrh1-/- fish generated by the CRISPR/Cas9 system. Quantitative PCR, in situ hybridization, and immunocytochemistry were used to study neurotransmitter systems and genes essential for brain development. Additionally, we wanted to reveal the role of this histamine receptor in larval and adult fish behavior using several quantitative behavioral methods including locomotion, thigmotaxis, dark flash and startle response, novel tank diving, and shoaling behavior. Hrh1-/- larvae displayed normal behavior in comparison with hrh1+/+ siblings. Interestingly, a transient abnormal expression of important neurodevelopmental markers was evident in these larvae, as well as a reduction in the number of tyrosine hydroxylase 1 (Th1)-positive cells, th1 mRNA, and hypocretin (hcrt)-positive cells. These abnormalities were not detected in adulthood. In summary, we verified that zebrafish lacking hrh1 present deficits in the dopaminergic and hypocretin systems during early development, but those are compensated by the time fish reach adulthood. However, impaired sociability and anxious-like behavior, along with downregulation of choline O-acetyltransferase a and LIM homeodomain transcription factor Islet1, were displayed by adult fish.

Misdiagnosis of narcolepsy caused by a false positive orexin-A/hypocretin-1 enzyme immune assay

The diagnosis of narcolepsy is based on clinical history, sleep studies, and, in some cases, cerebrospinal fluid orexin-A/hypocretin-1 measurement. The gold standard for orexin measurement is the radioimmune assay (RIA), but other commercial kits are also available, such as the enzyme immune assay (EIA). The specificity of orexin EIA in humans is unknown. We report four cases where orexin levels were measured by EIA and resulted in false positives and the misdiagnosis of narcolepsy. Therefore, orexin EIA measurement should be strongly discouraged in a clinical setting.

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 and Histamine Cross-Talk on Hypothalamic Neuron Counts in Mice

The loss of hypothalamic neurons that produce wake-promoting orexin (hypocretin) neuropeptides is responsible for narcolepsy type 1 (NT1). While the number of histamine neurons is increased in patients with NT1, results on orexin-deficient mouse models of NT1 are inconsistent. On the other hand, the effect of histamine deficiency on orexin neuron number has never been tested on mammals, even though histamine has been reported to be essential for the development of a functional orexin system in zebrafish. The aim of this study was to test whether histamine neurons are increased in number in orexin-deficient mice and whether orexin neurons are decreased in number in histamine-deficient mice. The hypothalamic neurons expressing L-histidine decarboxylase (HDC), the histamine synthesis enzyme, and those expressing orexin A were counted in four orexin knock-out mice, four histamine-deficient HDC knock-out mice, and four wild-type C57BL/6J mice. The number of HDC-positive neurons was significantly higher in orexin knock-out than in wild-type mice (2,502 ± 77 vs. 1,800 ± 213, respectively, one-tailed t-test, P = 0.011). Conversely, the number of orexin neurons was not significantly lower in HDC knock-out than in wild-type mice (2,306 ± 56 vs. 2,320 ± 120, respectively, one-tailed t-test, P = 0.459). These data support the view that orexin peptide deficiency is sufficient to increase histamine neuron number, supporting the involvement of the histamine waking system in the pathophysiology of NT1. Conversely, these data do not support a significant role of histamine in orexin neuron development in mammals.

Impact of daytime light intensity on the central orexin (hypocretin) system of a diurnal rodent (Arvicanthis niloticus)

The neuropeptide orexin/hypocretin is implicated in sleep and arousal, energy expenditure, reward, affective state and cognition. Our previous work using diurnal Nile grass rats (Arvicanthis niloticus) found that orexin mediates the effects of environmental light, particularly daytime light intensity, on affective and cognitive behaviours. The present study further investigated how daytime light intensity affects the central orexin system in male and female grass rats. Subjects were housed for 4 weeks in 12:12 hr dim light:dark (50 lux, dimLD) or in 12:12 hr bright light:dark cycle (1000 lux, brightLD). Day/night fluctuations in some orexin measures were also assessed. Despite similar hypothalamic prepro-orexin mRNA expression across all conditions, there were significantly more orexin-immunoreactive neurons, larger somata, greater optical density or higher orexin A content at night (ZT14) than during the day (ZT2), and/or in animals housed in brightLD compared to dimLD. Grass rats in brightLD also had higher cisternal CSF levels of orexin A. Furthermore, orexin receptor OX1R and OX2R proteins in the medial prefrontal cortex were higher in brightLD than dimLD males, but lower in brightLD than dimLD females. In the CA1 and dorsal raphe nucleus, females had higher OX1R than males without any significant effects of light condition, and OX2R levels were unaffected by sex or light. These results reveal that daytime light intensity alters the central orexin system of both male and female diurnal grass rats, sometimes sex-specifically, and provides insight into the mechanisms underlying how daytime light intensity impacts orexin-regulated functions.

Pitolisant to Treat Excessive Daytime Sleepiness and Cataplexy in Adults with Narcolepsy: Rationale and Clinical Utility

Narcolepsy is a sleep disorder marked by chronic, debilitating excessive daytime sleepiness and can be associated with cataplexy, sleep paralysis and sleep-related hallucinations. Pharmacological therapy for narcolepsy primarily aims to increase wakefulness and reduce cataplexy attacks. Pitolisant is a first-in-class agent utilizing histamine to improve wakefulness by acting as an antagonist/inverse agonist of the presynaptic histamine 3 receptor. This review summarizes the clinical efficacy, safety and tolerability of pitolisant in treating the symptoms of narcolepsy. Randomized and observational studies demonstrate pitolisant to be effective in treating both hypersomnolence and cataplexy while generally being well tolerated at prescribed doses. The most common adverse reactions include headache, insomnia and nausea.

Activation of lateral hypothalamic group III metabotropic glutamate receptors suppresses cocaine-seeking following abstinence and normalizes drug-associated increases in excitatory drive to orexin/hypocretin cells

The perifornical/lateral hypothalamic area (LHA) orexin (hypocretin) system is involved in drug-seeking behavior elicited by drug-associated stimuli. Cocaine exposure is associated with presynaptic plasticity at LHA orexin cells such that excitatory input to orexin cells is enhanced acutely and into withdrawal. These changes may augment orexin cell reactivity to drug-related cues during abstinence and contribute to relapse-like behavior. Studies in hypothalamic slices from drug-naïve animals indicate that agonism of group III metabotropic glutamate receptors (mGluRs) reduces presynaptic glutamate release onto orexin cells. Therefore, we examined the group III mGluR system as a potential target to reduce orexin cell excitability in-vivo, including in animals with cocaine experience. First, we verified that group III mGluRs regulate orexin cell activity in behaving animals by showing that intra-LHA infusions of the selective agonist L-(+)-2-Amino-4-phosphonobutyric acid (L-AP4) reduces c-fos expression in orexin cells following 24 h food deprivation. Next, we extended these findings to show that intra-LHA L-AP4 infusions reduced discriminative stimulus-driven cocaine-seeking following withdrawal. Importantly, L-AP4 had no effect on lever pressing for sucrose pellets or general motoric behavior. Finally, using whole-cell patch-clamp recordings from identified orexin cells in orexin-GFP transgenic mice, we show enhanced presynaptic drive to orexin cells following 14d withdrawal and that this plasticity can be normalized by L-AP4. Together, these data indicate that activation of group III mGluRs in LHA reduces orexin cell activity in vivo and may be an effective strategy to suppress cocaine-seeking behavior following withdrawal. These effects are likely mediated, at least in part, by normalization of presynaptic plasticity at orexin cells that occurs as a result of cocaine exposure. This article is part of the Special Issue entitled 'Hypothalamic Control of Homeostasis'.

Brain histamine modulates recognition memory: possible implications in major cognitive disorders

Several behavioural tests have been developed to study and measure emotionally charged or emotionally neutral memories and how these may be affected by pharmacological, dietary or environmental manipulations. In this review, we describe the experimental paradigms used in preclinical studies to unravel the brain circuits involved in the recognition and memorization of environmentally salient stimuli devoid of strong emotional value. In particular, we focus on the modulatory role of the brain histaminergic system in the elaboration of recognition memory that is based on the judgement of the prior occurrence of an event, and it is believed to be a critical component of human declarative memory. The review also addresses questions that may help improve the treatment of impaired declarative memory described in several affective and neuropsychiatric disorders such as ADHD, Alzheimer's disease and major neurocognitive disorder. LINKED ARTICLES: This article is part of a themed section on New Uses for 21st Century. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v177.3/issuetoc.

Interactions of the histamine and hypocretin systems in CNS disorders

Histamine and hypocretin neurons are localized to the hypothalamus, a brain area critical to autonomic function and sleep. Narcolepsy type 1, also known as narcolepsy with cataplexy, is a neurological disorder characterized by excessive daytime sleepiness, impaired night-time sleep, cataplexy, sleep paralysis and short latency to rapid eye movement (REM) sleep after sleep onset. In narcolepsy, 90% of hypocretin neurons are lost; in addition, two groups reported in 2014 that the number of histamine neurons is increased by 64% or more in human patients with narcolepsy, suggesting involvement of histamine in the aetiology of this disorder. Here, we review the role of the histamine and hypocretin systems in sleep-wake modulation. Furthermore, we summarize the neuropathological changes to these two systems in narcolepsy and discuss the possibility that narcolepsy-associated histamine abnormalities could mediate or result from the same processes that cause the hypocretin cell loss. We also review the changes in the hypocretin and histamine systems, and the associated sleep disruptions, in Parkinson disease, Alzheimer disease, Huntington disease and Tourette syndrome. Finally, we discuss novel therapeutic approaches for manipulation of the histamine system.

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.

Interactions of the orexin/hypocretin neurones and the histaminergic system

Histaminergic and orexin/hypocretin systems are components in the brain wake-promoting system. Both are affected in the sleep disorder narcolepsy, but the role of histamine in narcolepsy is unclear. The histaminergic neurones are activated by the orexin/hypocretin system in rodents, and the development of the orexin/hypocretin neurones is bidirectionally regulated by the histaminergic system in zebrafish. This review summarizes the current knowledge of the interactions of these two systems in normal and pathological conditions in humans and different animal models.

Physiological and psychosocial age-related changes associated with reduced food intake in older persons

Dietary intake changes during the course of aging. Normally an increase in food intake is observed around 55 years of age, which is followed by a reduction in food intake in individuals over 65 years of age. This reduction in dietary intake results in lowered levels of body fat and body weight, a phenomenon known as anorexia of aging. Anorexia of aging has a variety of consequences, including a decline in functional status, impaired muscle function, decreased bone mass, micronutrient deficiencies, reduced cognitive functions, increased hospital admission and even premature death. Several changes during lifetime have been implicated to play a role in the reduction in food intake and the development of anorexia of aging. These changes are both physiological, involving peripheral hormones, senses and central brain regulation and non-physiological, with differences in psychological and social factors. In the present review, we will focus on age-related changes in physiological and especially non-physiological factors, that play a role in the age-related changes in food intake and in the etiology of anorexia of aging. At the end we conclude with suggestions for future nutritional research to gain greater understanding of the development of anorexia of aging which could lead to earlier detection and better prevention.

Involvement of spinal orexin A in the electroacupuncture analgesia in a rat model of post-laparotomy pain

BACKGROUND

Orexin A (OXA, hypocretin/hcrt 1) is a newly discovered potential analgesic substance. However, whether OXA is involved in acupuncture analgesia remains unknown. The present study was designed to investigate the involvement of spinal OXA in electroacupuncture (EA) analgesia.

METHODS

A modified rat model of post-laparotomy pain was adopted and evaluated. Von Frey filaments were used to measure mechanical allodynia of the hind paw and abdomen. EA at 2/15 Hz or 2/100 Hz was performed once on the bilateral ST36 and SP6 for 30 min perioperatively. SB-334867, a selective orexin 1 receptor (OX1R) antagonist with a higher affinity for OXA than OXB, was intrathecally injected to observe its effect on EA analgesia.

RESULTS

OXA at 0.3 nmol and EA at 2/15 Hz produced respective analgesic effects on the model (P<0.05). Pre-surgical intrathecal administered of SB-334867 30 nmol antagonized OXA analgesia and attenuated the analgesic effect of EA (P<0.05). However, SB-334867 did not block fentanyl-induced analgesia (P>0.05). In addition, naloxone, a selective opioid receptor antagonist, failed to antagonize OXA-induced analgesia (P>0.05).

CONCLUSIONS

The results of the present study indicate the involvement of OXA in EA analgesia via OX1R in an opioid-independent way.

The histaminergic system regulates wakefulness and orexin/hypocretin neuron development via histamine receptor H1 in zebrafish

The histaminergic and hypocretin/orexin (hcrt) neurotransmitter systems play crucial roles in alertness/wakefulness in rodents. We elucidated the role of histamine in wakefulness and the interaction of the histamine and hcrt systems in larval zebrafish. Translation inhibition of histidine decarboxylase (hdc) with morpholino oligonucleotides (MOs) led to a behaviorally measurable decline in light-associated activity, which was partially rescued by hdc mRNA injections and mimicked by histamine receptor H1 (Hrh1) antagonist pyrilamine treatment. Histamine-immunoreactive fibers targeted the dorsal telencephalon, an area that expresses histamine receptors hrh1 and hrh3 and contains predominantly glutamatergic neurons. Tract tracing with DiI revealed that projections from dorsal telencephalon innervate the hcrt and histaminergic neurons. Translation inhibition of hdc decreased the number of hcrt neurons in a Hrh1-dependent manner. The reduction was rescued by overexpression of hdc mRNA. hdc mRNA injection alone led to an up-regulation of hcrt neuron numbers. These results suggest that histamine is essential for the development of a functional and intact hcrt system and that histamine has a bidirectional effect on the development of the hcrt neurons. In summary, our findings provide evidence that these two systems are linked both functionally and developmentally, which may have important implications in sleep disorders and narcolepsy. development via histamine receptor H1 in zebrafish.

The histaminergic tuberomammillary nucleus is critical for motivated arousal

Obtaining food, shelter or water, or finding a mating partner are examples of motivated behaviors, which are essential to preserve the species. The full expression of such behaviors requires a high but optimal arousal state. We tested the idea that tuberomammillary nucleus (TMN) histamine neurons are crucial to generate such motivated arousal, using a model of the appetitive phase of feeding behavior. Hungry rats enticed with food within a wire mesh box showed intense goal-directed motor activity aimed at opening the box, an increase in core temperature, a fast histamine release in the hypothalamus and an early increase in Fos immunoreactivity in TMN and cortical neurons. Enticing with stronger-tasting food induced stronger motor, temperature and Fos immunoreactivity brain responses than ordinary food pellets. TMN lesion greatly decreased all of those responses. We conclude that histamine neurons increase arousal and vegetative activity, allowing the normal unfolding of voluntary, goal-directed behavior such as obtaining food.

Orexins/hypocretins and aminergic systems

Orexin/hypocretin neurones in the posterior hypothalamus are mutually connected with noradrenergic, serotonergic, dopaminergic, histaminergic, and cholinergic neurone systems. They activate these targets by direct post-synaptic and indirect pre-synaptic mechanisms and in turn receive inhibitory feedback and excitatory feed forward control. With respect to behavioural state control, orexin/hypocretin neurones are conducting the orchestra of biogenic amines. This review highlights the role of these players in the control of energy administration, sleep-wake architecture, cortical activation, plasticity, and memory functions in health and disease.

Orexin/hypocretin and histamine: distinct roles in the control of wakefulness demonstrated using knock-out mouse models

To determine the respective role played by orexin/hypocretin and histamine (HA) neurons in maintaining wakefulness (W), we characterized the behavioral and sleep-wake phenotypes of orexin (Ox) knock-out (-/-) mice and compared them with those of histidine-decarboxylase (HDC, HA-synthesizing enzyme)-/- mice. While both mouse strains displayed sleep fragmentation and increased paradoxical sleep (PS), they presented a number of marked differences: (1) the PS increase in HDC(-/-) mice was seen during lightness, whereas that in Ox(-/-) mice occurred during darkness; (2) contrary to HDC(-/-), Ox(-/-) mice had no W deficiency around lights-off, nor an abnormal EEG and responded to a new environment with increased W; (3) only Ox(-/-), but not HDC(-/-) mice, displayed narcolepsy and deficient W when faced with motor challenge. Thus, when placed on a wheel, wild-type (WT), but not littermate Ox(-/-) mice, voluntarily spent their time in turning it and as a result, remained highly awake; this was accompanied by dense c-fos expression in many areas of their brains, including Ox neurons in the dorsolateral hypothalamus. The W and motor deficiency of Ox(-/-) mice was due to the absence of Ox because intraventricular dosing of orexin-A restored their W amount and motor performance whereas SB-334867 (Ox1-receptor antagonist, i.p.) impaired W and locomotion of WT mice during the test. These data indicate that Ox, but not HA, promotes W through enhanced locomotion and suggest that HA and Ox neurons exert a distinct, but complementary and synergistic control of W: the neuropeptide being more involved in its behavioral aspects, whereas the amine is mainly responsible for its qualitative cognitive aspects and cortical EEG activation.

Orexin (hypocretin) gene transfer diminishes narcoleptic sleep behavior in mice

Gene transfer has proven to be an effective neurobiological tool in a number of neurodegenerative diseases, but it is not known if it can correct a sleep disorder. Narcolepsy is a neurodegenerative sleep disorder linked to the loss of neurons containing the neuropeptide orexin, also known as hypocretin. Here, a replication-defective herpes simplex virus-1 amplicon-based vector was constructed to transfer the gene for mouse prepro-orexin into mice with a genetic deletion of the orexin gene. After in vitro tests confirmed successful gene transfer into cells, the gene vector was delivered to the lateral hypothalamus of orexin knockout (KO) mice where the orexin peptide was robustly expressed in the somata and processes of numerous neurons, and the peptide product was detected in the cerebrospinal fluid. During the 4-day life-span of the vector the incidence of cataplexy declined by 60%, and the levels of rapid eye movement sleep during the second half of the night were similar to levels in wild-type mice, indicating that narcoleptic sleep-wake behavior in orexin KO mice can be improved by targeted gene transfer.

IGFBP3 colocalizes with and regulates hypocretin (orexin)

Background

The sleep disorder narcolepsy is caused by a vast reduction in neurons producing the hypocretin (orexin) neuropeptides. Based on the tight association with HLA, narcolepsy is believed to result from an autoimmune attack, but the cause of hypocretin cell loss is still unknown. We performed gene expression profiling in the hypothalamus to identify novel genes dysregulated in narcolepsy, as these may be the target of autoimmune attack or modulate hypocretin gene expression.

Methodology/Principal Findings

We used microarrays to compare the transcriptome in the posterior hypothalamus of (1) narcoleptic versus control postmortem human brains and (2) transgenic mice lacking hypocretin neurons versus wild type mice. Hypocretin was the most downregulated gene in human narcolepsy brains. Among many additional candidates, only one, insulin-like growth factor binding protein 3 (IGFBP3), was downregulated in both human and mouse models and co-expressed in hypocretin neurons. Functional analysis indicated decreased hypocretin messenger RNA and peptide content, and increased sleep in transgenic mice overexpressing human IGFBP3, an effect possibly mediated through decreased hypocretin promotor activity in the presence of excessive IGFBP3. Although we found no IGFBP3 autoantibodies nor a genetic association with IGFBP3 polymorphisms in human narcolepsy, we found that an IGFBP3 polymorphism known to increase serum IGFBP3 levels was associated with lower CSF hypocretin-1 in normal individuals.

Conclusions/Significance

Comparison of the transcriptome in narcolepsy and narcolepsy model mouse brains revealed a novel dysregulated gene which colocalized in hypocretin cells. Functional analysis indicated that the identified IGFBP3 is a new regulator of hypocretin cell physiology that may be involved not only in the pathophysiology of narcolepsy, but also in the regulation of sleep in normal individuals, most notably during adolescence. Further studies are required to address the hypothesis that excessive IGFBP3 expression may initiate hypocretin cell death and cause narcolepsy.

Changes in brain orexin levels in a rat model of depression induced by neonatal administration of clomipramine

Depression is associated with a deficiency of serotonergic neurons that have been found to suppress orexinergic neurons, which in turn activate these neurons in a feedback loop. This evidence suggests that orexins may be involved in the pathology of depression. Long Evans rats were treated with clomipramine (CLI) and saline (SAL) from postnatal days 8 through 21. One set of rats from both groups was sacrificed at 35 days of age for quantification of orexins in multiple brain regions. At 3-4 months of age a second set of rats was tested for immobility in a forced swim procedure, a common test for depressive signs in rats, and a third set was sacrificed for the quantification of orexins. Compared with the control rats, adult rats with neonatal CLI treatment had (1) increased forced swim immobility and (2) increased orexins A and B in the hypothalamus. However, both orexins A and B levels were decreased in multiple brain regions in the juvenile CLI rats compared with same-age controls. We concluded that although orexin levels were decreased in juvenile CLI rats, adult CLI rats with features of depression had significantly higher levels of hypothalamic orexins compared with adult controls. These results imply that orexins are likely to be involved in the pathological regulation of depression.

Histamine in the nervous system

Histamine is a transmitter in the nervous system and a signaling molecule in the gut, the skin, and the immune system. Histaminergic neurons in mammalian brain are located exclusively in the tuberomamillary nucleus of the posterior hypothalamus and send their axons all over the central nervous system. Active solely during waking, they maintain wakefulness and attention. Three of the four known histamine receptors and binding to glutamate NMDA receptors serve multiple functions in the brain, particularly control of excitability and plasticity. H1 and H2 receptor-mediated actions are mostly excitatory; H3 receptors act as inhibitory auto- and heteroreceptors. Mutual interactions with other transmitter systems form a network that links basic homeostatic and higher brain functions, including sleep-wake regulation, circadian and feeding rhythms, immunity, learning, and memory in health and disease.

Sleep is increased by weight gain and decreased by weight loss in mice

OBJECTIVE

To determine whether weight loss could reverse excessive sleep in high-fat diet-induced obesity.

DESIGN

Three groups of mice participated in the study. A weight gain/loss group was fed with high-fat food for 6 weeks (weight gain), and regular food again for 4 weeks (weight loss). A control group and a weight gain only group were fed with regular food and high-fat food, respectively, for 10 weeks after the baseline.

PARTICIPANTS

Adult male C57BL/6 mice.

MEASUREMENTS

The amounts of wake, rapid eye movement sleep (REMS) and non-REM sleep (NREMS) were determined at week 0 (baseline), week 6, and week 10.

RESULTS

The weight gain/loss group displayed a significant decrease in wakefulness and increases in NREMS and episodes of NREMS during 6 weeks of weight gain, which were reversed during subsequent 4 weeks of weight loss. The weight gain only group displayed significant decrease in wakefulness and increase of NREMS and REMS at both week 6 and week 10. The control group did not show significant sleep alterations during the experiment.

CONCLUSION

These observations indicate that sleep alterations induced by weight gain are reversed by weight loss in obese animals. These data may shed light on the mechanisms underlying the well-established association between obesity and sleepiness in humans and may lead to new therapeutic strategies for these 2 increasingly prevalent problems in the modern societies.

Role of orexin input in the diurnal rhythm of locus coeruleus impulse activity

Activation of noradrenergic locus coeruleus (LC) neurons promotes wakefulness and behavioral arousal. In rats, LC neurons receive circadian inputs via a circuit that originates in the suprachiasmatic nucleus (SCN) and relays through the dorsomedial hypothalamus (DMH) to LC; this circuit input increases LC activity during the active period. DMH neurons expressing the peptide neurotransmitter orexin/hypocretin are ideally situated to act as a relay between SCN and LC due to their synaptic inputs from SCN and innervation of LC. Here, we examined the hypothesis that orexin is involved in transmitting circadian signals to LC using single-unit recordings of LC neurons in anesthetized rats maintained in 12:12 light-dark housing. We replicated earlier findings from this lab that LC neurons fire significantly faster on average during the active compared to rest periods. Local microinjection of an orexin antagonist, SB-334867-A attenuated the impulse activities of the fastest firing population of LC neurons during the active period. We also found that DMH orexin neurons project preferentially to LC and express a diurnal rhythm of activation that correlates with LC neuronal firing frequency. Therefore, we propose that DMH orexin neurons play a role in modulating the day-night differences of LC impulse activity.

Histamine and Schizophrenia

With the availability of an increased number of experimental tools, for example potent and brain-penetrating H1-, H2-, and H3-receptor ligands and mutant mice lacking the histamine synthesis enzyme or the histamine receptors, the functional roles of histaminergic neurons in the brain have been considerably clarified during the recent years, particularly their major role in the control of arousal, cognition, and energy balance. Various approaches tend to establish the implication of histaminergic neurons in schizophrenia. A strong hyperactivity of histamine neurons is induced in rodent brain by administration of methamphetamine or NMDA-receptor antagonists. Histamine neuron activity is modulated by typical and atypical neuroleptics. H3-receptor antagonists/inverse agonists display antipsychotic-like properties in animal models of the disease. Because of the limited predictability value of most animal models and the paucity of drugs affecting histaminergic transmission that were tried so far in human, the evidence remains therefore largely indirect, but supports a role of histamine neurons in schizophrenia.

Defective carbohydrate metabolism in mice homozygous for the tubby mutation

Tub is a member of a small gene family, the tubby-like proteins (TULPs), with predominant expression in neurons. Mice carrying a mutation in Tub develop retinal and cochlear degeneration as well as late-onset obesity with insulin resistance. During behavioral and metabolic testing, we found that homozygous C57BL/6J-Tub(tub) mice have a lower respiratory quotient than C57BL/6J controls before the onset of obesity, indicating that tubby homozygotes fail to activate carbohydrate metabolism and instead rely on fat metabolism for energy needs. In concordance with this, tubby mice show higher excretion of ketone bodies and accumulation of glycogen in the liver. Quantitation of liver mRNA levels shows that, during the transition from light to dark period, tubby mice fail to induce glucose-6-phosphate dehydrogenase (G6pdh), the rate-limiting enzyme in the pentose phosphate pathway that normally supplies NADPH for de novo fatty acid synthesis and glutathione reduction. Reduced G6PDH protein levels and enzymatic activity in tubby mice lead accordingly to lower levels of NADPH and reduced glutathione (GSH), respectively. mRNA levels for the lipolytic enzymes acetyl-CoA synthetase and carnitine palmitoyltransferase are increased during the dark cycle and decreased during the light period, and several citric acid cycle genes are dysregulated in tubby mice. Examination of hypothalamic gene expression showed high levels of preproorexin mRNA leading to accumulation of orexin peptide in the lateral hypothalamus. We hypothesize that abnormal hypothalamic orexin expression leads to changes in liver carbohydrate metabolism and may contribute to the moderate obesity observed in tubby mice.

Correlation of psycho-neuroendocrine-immune (PNI) gene expression with symptoms of acute infectious mononucleosis

Acute infection is known to perturb psycho-neuroendocrine-immune (PNI) gene expression. Oligonucleotide microarrays were used to examine PNI gene expression in the peripheral blood of 13 subjects with infectious mononucleosis (IM). Novel peripheral blood gene expression activity was correlated with central-nervous-system-mediated symptoms including fatigue and sleep disturbance. Of note, expression of the MADS box transcription enhancer factor 2 polypeptide C (MEF2C) gene, previously implicated in skeletal muscle myogenesis, correlated with symptoms of musculo-skeletal pain and fatigue. Expression of the hypocretin/orexin receptor HCRTR2, which has been implicated in narcolepsy, correlated with sleep disturbance. And, VACHT, the vesicular acetylcholine transporter, was highly correlated with neurocognitive disturbance. The expression of both HCRTR2 and MEF2C in the peripheral blood was validated by reverse transcription PCR. Thus, investigation of the PNI response in peripheral blood may provide novel insights into the complex pathophysiology of centrally mediated disease states.

Enhanced antinociception by intracerebroventricularly administered orexin A in histamine H1 or H2 receptor gene knockout mice

Orexins are neuropeptides that are mostly expressed in the posterior and lateral hypothalamus, and related to the central control of appetite, arousal, and antinociception. Orexin neurons projected to the tuberomammillary nucleus and orexins may release histamine from the histamine neurons in this nucleus. Histamine is known to cause hypernociception. The roles of histamine H1 and H2 receptors in the orexin A-induced antinociception, however, have not been clarified yet. Here we studied the effects of histamine H1 and H2 receptors on orexin A-produced antinociception using histamine receptor knockout mice in four assays of nociception; the hot-plate, the tail-flick, the tail-pressure and the capsaicin tests. Furthermore we studied effects of histamine H1 and H2 receptor antagonists on orexin A-produced antinociception in C57BL/6 mice. The antinociceptive effects of i.c.v. orexin A were greater in histamine H1 receptor or H2 receptor knockout mice than in the wild-type mice in all four assays of pain. Furthermore, treatment of C57BL/6 mice with a combination of i.c.v. orexin A and d-chlorpheniramine (a histamine H1 receptor antagonist) or cimetidine (a histamine H2 receptor antagonist) showed a greater antinociception than i.c.v. orexin A alone in all four assays. These findings suggest the possibility that orexin A may activate H1 and H2 receptors in the supraspinal levels through the release of histamine from neurons, which might attenuate the antinociceptive effects of orexin A. Thus, the blocking of the histamine H1 or H2 receptor may produce antinociception and enhance the orexin A-induced antinociception.

Orexin A promotes histamine, but not norepinephrine or serotonin, release in frontal cortex of mice

AIM

To investigate the effects of orexin A on release of histamine, norepinephrine, and serotonin in the frontal cortex of mice.

METHODS

Samples for measuring histamine, norepinephrine, and serotonin contents were collected by in vivo microdialysis of the frontal cortex of anesthetized mice. The histamine, noradrenaline, and serotonin content in dialysates were measured by HPLC techniques.

RESULTS

Intracrebroventricular injection of orexin A at doses of 12.5, 50, and 200 pmol per mouse promoted histamine release from the frontal cortex in a dose-dependent manner. At the highest dose given, 200 pmol, orexin A significantly induced histamine release, with the maximal magnitude being 230% over the mean basal release. The enhanced histamine release was sustained for 140 min, and then gradually returned to the basal level. However, no change in norepinephrine or serotonin release was observed under application of the same dose of orexin A.

CONCLUSION

These results suggest that the arousal effect of orexin A is mainly mediated by histamine, not by norepinephrine or serotonin.

The diurnal rhythm of hypocretin in young and old F344 rats

STUDY OBJECTIVES

Hypocretins (HCRT-1 and HCRT-2), also known as orexins, are neuropeptides localized in neurons surrounding the perifornical region of the posterior hypothalamus. These neurons project to major arousal centers in the brain and are implicated in regulating wakefulness. In young rats and monkeys, levels of HCRT-1 are highest at the end of the wake-active period and lowest toward the end of the sleep period. However, the effects of age on the diurnal rhythm of HCRT-1 are not known.

DESIGN

To provide such data, cerebrospinal fluid (CSF) was collected from the cisterna magna of young (2-month-old, n = 9), middle-aged (12 months, n = 10), and old (24 months, n = 10) F344 rats at 4-hour intervals, (beginning at zeitgeber [ZT]0, lights on). CSF was collected once from each rat every 4 days at 1 ZT point. After collecting the CSF at all of the time points, the rats were kept awake by gentle handling for 8 hours (ZT 0-ZT8), and the CSF was collected again at the end of the sleep-deprivation procedure. HCRT-1 levels in the CSF were determined by radioimmunoassay

SETTINGS

Basic neuroscience research lab.

MEASUREMENTS AND RESULTS

Old rats had significantly less HCRT-1 in the CSF versus young and middle-aged rats (P < .002) during the lights-on and lights-off periods and over the 24-hour period. In old rats, significantly low levels of HCRT-1 were evident at the end of the lights-off period (predominantly wake-active period). The old rats continued to have less HCRT-1 even after 8 hours of prolonged waking. Northern blot analysis did not show a difference in pre-proHCRT mRNA between age groups.

CONCLUSIONS

In old rats there is a 10% decline in CSF HCRT-1 over the 24-hour period. Functionally, if there is less HCRT-1, which our findings indicated, and there is also a decline in HCRT receptor mRNA, as has been previously found, then the overall consequence would be diminished action of HCRT at target sites. This would diminish the waking drive, which in the elderly could contribute to the increased tendency to fall asleep during the normal wake period.

Endocrinology of anorexia of ageing

Appetite and food intake decrease with normal ageing, predisposing to the development of under-nutrition. Under-nutrition is common in older people and has been implicated in the development and progression of chronic diseases commonly affecting the elderly, as well as in increasing mortality. An understanding of the factors that contribute to the physiological and pathological declines in food intake in older people is likely to aid in the development of effective forms of prevention and treatment. Ageing affects many of the endocrine factors involved in the control of appetite and feeding but few studies have been performed in humans to clarify these changes. Possible hormonal causes of the anorexia of ageing include increased activity of cholecystokinin, leptin and various cytokines and reduced activity of ghrelin and testosterone.