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

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.

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.

Elevated HB-EGF expression in neural stem cells causes middle age obesity by suppressing Hypocretin/Orexin expression

Obesity is common in the middle aged population and it increases the risks of diabetes, cardiovascular diseases, certain cancers, and dementia. Yet, its etiology remains incompletely understood. Here, we show that ectopic expression of HB-EGF, an important regulator of neurogenesis, in Nestin⁺ neuroepithelial progenitors with the Cre-LoxP system leads to development of spontaneous middle age obesity in male mice accompanied by hyperglycemia and insulin resistance. The Nestin-HB-EGF mice show decreases in food uptake, energy expenditure, and physical activity, suggesting that reduced energy expenditure underlies the pathogenesis of this obesity model. However, HB-EGF expression in appetite-controlling POMC or AgRP neurons or adipocytes fails to induce obesity. Mechanistically, HB-EGF suppresses expression of Hypocretin/Orexin, an orexigenic neuropeptide hormone, in the hypothalamus of middle aged Nestin-HB-EGF mice. Hypothalamus Orexin administration alleviates the obese and hyperglycemic phenotypes in Nestin-HB-EGF mice. This study uncovers an important role for HB-EGF in regulating Orexin expression and energy expenditure and establishes a midlife obesity model whose pathogenesis involves age-dependent changes in hypothalamus neurons.

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.

Age-specific treatment effects of orexin/hypocretin-receptor antagonism on methamphetamine-seeking behavior

BACKGROUND

Worldwide methamphetamine (METH) use has increased significantly over the last 10 years, and in the US, METH dependence has sky-rocketed among individuals with opioid use disorder. Of significant concern, METH use is gaining popularity among groups with susceptibility to developing severe substance use disorders, such as women and adolescents. Nevertheless, there is no established pharmacotherapy for METH addiction. Emerging evidence has identified the orexin/hypocretin system as an important modulator of reward-driven behavior and a potential target for the treatment of drug addiction and relapse. However, to date, there have been no investigations into the therapeutic efficacy of orexin/hypocretin receptor antagonists for METH-motivated behavior in adolescents or adults. In the present study, we examined the effects of selective antagonists of the orexin-1 (SB-334867, 20 mg/kg) and orexin-2 (TCS-OX2-29, 20 mg/kg) receptors on the reinstatement of METH seeking in both adolescent and adult male and female rats.

METHODS

Rats were trained to self-administer METH (0.05 mg/kg/inf, iv) during two 2-h sessions/day for 5 days. Following 20 sessions of extinction over 10 days, a within-subjects design was used to test for METH seeking precipitated by METH (1 mg/kg, ip) or METH cues after systemic pretreatment with SB-334867 or TCS-OX2-29.

RESULTS

SB-334867 reduced cue-induced reinstatement in males and females, regardless of age. Additionally, METH-induced METH seeking was attenuated by SB-334867 in adolescents and by TCS-OX2-29 in adults.

CONCLUSION

Selective orexin/hypocretin receptor antagonists have significant therapeutic potential for diminishing METH-seeking behavior, although their treatment efficacy may be influenced by age.

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.

Changes in sleep EEG with aging in humans and rodents

Sleep is one of the most ubiquitous but also complex animal behaviors. It is regulated at the global, systems level scale by circadian and homeostatic processes. Across the 24-h day, distribution of sleep/wake activity differs between species, with global sleep states characterized by defined patterns of brain electric activity and electromyography. Sleep patterns have been most intensely investigated in mammalian species. The present review begins with a brief overview on current understandings on the regulation of sleep, and its interaction with aging. An overview on age-related variations in the sleep states and associated electrophysiology and oscillatory events in humans as well as in the most common laboratory rodents follows. We present findings observed in different studies and meta-analyses, indicating links to putative physiological changes in the aged brain. Concepts requiring a more integrative view on the role of circadian and homeostatic sleep regulatory mechanisms to explain aging in sleep are emerging.

Hypocretin (Orexin) Replacement Therapies

Twenty-two years after their discovery, the hypocretins (Hcrts), also known as orexins, are two of the most studied peptidergic systems, involved in myriad physiological systems that range from sleep, arousal, motivation, homeostatic regulation, fear, anxiety and learning. A causal relationship between activity of Hcrt and arousal stability was established shortly after their discovery and have led to the development of a new class of drugs to treat insomnia. In this review we discuss the many faces of the Hcrt system and examine recent findings that implicate decreased Hcrt function in the pathogenesis of a number of neuropsychiatric conditions. We also discuss future therapeutic strategies to replace or enhance Hcrt function as a treatment option for these neuropsychiatric conditions.

(Neuro) Peptides, Physical Activity, and Cognition

Regular physical activity (PA) improves cognitive functions, prevents brain atrophy, and delays the onset of cognitive decline, dementia, and Alzheimer’s disease. Presently, there are no specific recommendations for PA producing positive effects on brain health and little is known on its mediators. PA affects production and release of several peptides secreted from peripheral and central tissues, targeting receptors located in the central nervous system (CNS). This review will provide a summary of the current knowledge on the association between PA and cognition with a focus on the role of (neuro)peptides. For the review we define peptides as molecules with less than 100 amino acids and exclude myokines. Tachykinins, somatostatin, and opioid peptides were excluded from this review since they were not affected by PA. There is evidence suggesting that PA increases peripheral insulin growth factor 1 (IGF-1) levels and elevated serum IGF-1 levels are associated with improved cognitive performance. It is therefore likely that IGF-1 plays a role in PA induced improvement of cognition. Other neuropeptides such as neuropeptide Y (NPY), ghrelin, galanin, and vasoactive intestinal peptide (VIP) could mediate the beneficial effects of PA on cognition, but the current literature regarding these (neuro)peptides is limited.

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.

Involvement of orexin/hypocretin in the expression of social play behaviour in juvenile rats

Social play is a highly rewarding and motivated behaviour displayed by juveniles of many mammalian species. We hypothesized that the orexin/hypocretin (ORX) system is involved in the expression of juvenile social play behaviour because this system is interconnected with brain regions that comprise the social behaviour and mesocorticolimbic reward networks. We found that exposure to social play increased recruitment of ORX-A neurons in juvenile rats. Furthermore, central administration of ORX-A decreased social play duration, while central blockade of ORX-1 receptors differentially altered social play duration in juvenile rats with low versus high baseline levels of social play (increasing social play in low baseline social play individuals and decreasing social play in high baseline social play individuals). Together, our results provided the first evidence of a role for the ORX system in the modulation of juvenile social play behaviour.

Sex- and Age-dependent Differences in Sleep-wake Characteristics of Fisher-344 Rats

Aging is a well-recognized risk factor for sleep disruption. The characteristics of sleep in aging include its disruption by frequent awakenings, a decline in both non-rapid eye movement (nonREM) and REM sleep amounts, and a weaker homeostatic response to sleep loss. Evidence also suggests that sleep in females is more sensitive to changes in the ovarian steroidal milieu. The Fischer-344 rats are commonly used experimental subjects in behavioral and physiological studies, including sleep and aging. Most sleep studies in Fischer-344 rats have used male subjects to avoid interactions between the estrus and sleep-waking cycles. The changes in the sleep-wake organization of female Fischer-344 rats, especially with advancing age, are not well-characterized. We determined sleep-waking features of cycling females across estrus stages. We also compared spontaneous and homeostatic sleep response profiles of young (3-4 months) and old (24-25 months) male and female Fischer-344 rats. The results suggest that: i) sleep-wake architectures across stages of estrus cycle in young females were largely comparable except for a significant suppression of REM sleep at proestrus night and an increase in REM sleep the following day; ii) despite hormonal differences, sleep-wake architecture in male and female rats of corresponding ages were comparable except for the suppression of REM sleep at proestrus night and higher nonREM delta power in recovery sleep; and iii) aging significantly affected sleep-wake amounts, sleep-wake stability, and homeostatic response to sleep loss in both male and female rats and that the adverse effects of aging were largely comparable in both sexes.

Orexin A as a modulator of dorsal lateral geniculate neuronal activity: a comprehensive electrophysiological study on adult rats

Orexins (OXA, OXB) are hypothalamic peptides playing crucial roles in arousal, feeding, social and reward-related behaviours. A recent study on juvenile rats suggested their involvement in vision modulation due to their direct action on dorsal lateral geniculate (dLGN) neurons. The present study aimed to verify whether a similar action of OXA can be observed in adulthood. Thus, in vivo and in vitro electrophysiological recordings on adult Wistar rats across light-dark and cortical cycles were conducted under urethane anaesthesia. OXA influenced ~28% of dLGN neurons recorded in vivo by either excitation or suppression of neuronal firing. OXA-responsive neurons did not show any spatial distribution nor represent a coherent group of dLGN cells, and responded to OXA similarly across the light-dark cycle. Interestingly, some OXA-responsive neurons worked in a cortical state-dependent manner, especially during the dark phase, and 'preferred' cortical activation over slow-wave activity induced by urethane. The corresponding patch clamp study confirmed these results by showing that < 20% of dLGN neurons were excited by OXA under both light regimes. The results suggest that OXA is involved in the development of the visual system rather than in visual processes and further implicate OXA in the mediation of circadian and arousal-related activity.

Effects of fasting on the central expression of appetite-regulating and reproductive hormones in wild-type and Casper zebrafish (Danio rerio)

Appetite and reproduction are closely related functions that are both regulated by brain hormones. Appetite stimulators include orexin and neuropeptide Y (NPY), and reproductive hormones include gonadotropin-releasing hormone (GnRH), gonadotropin-inhibitory hormone (GnIH), kisspeptin, and neurokinin B (NKB). GnRH stimulates the secretion of pituitary gonadotropes, and kisspeptin and GnIH modulate this action. Kisspeptin secretion is further controlled by neurokinin B (NKB) and dynorphin A (Dyn). To better understand the mechanisms regulating appetite and reproduction in fish, we examined the effects of fasting, reproductive stage, gender, and strain on the brain mRNA expression of appetite (orexin and NPY) and reproductive (GnRH, kisspeptin, GnIH, and NKB) hormones in zebrafish. In order to compare strains, we used both wild-type and transparent Casper zebrafish. In female wild-type zebrafish, fasting increased the expression of all hormones investigated, with the exception of Kiss2. Only NPY and Kiss2 were increased in male wild-type zebrafish during fasting. In Casper zebrafish, only GnIH and NKB in males were affected by fasting, suggesting that Casper fish may be more resistant to fasting than wild fish. Fasting increased expressions of orexin, GnRH2, Kiss1, GnIH and NKB in wild-type females with more eggs or larger eggs relative to body weight, compared to those with fewer or smaller eggs, suggesting that more mature females are more affected by fasting. No significant interactions of fasting and reproductive stage were noted in female Casper fish. To investigate whether differences between Casper and wild-type fish were due to genes involved in pigmentation, we compared the brain mRNA expressions of enzymes involved in melanin synthesis (tyrosinase and tyrosine hydroxylase - TH), melanocortin receptors (MC3R and MC4R), and the melanocortin precursor (proopiomelanocortin - POMC) between the two strains. Casper zebrafish had lower levels of MC3R, tyrosinase, TH1, TH2, and POMC than wild-type fish. Overall, our results suggest the existence of gender- and reproductive stage-specific, as well as strain-specific variations in the mechanisms regulating feeding and reproduction in zebrafish, and that the melanocortin system and melanin pathways may be in part responsible for these differences between strains.

Chemogenetic activation of orexin/hypocretin neurons ameliorates aging-induced changes in behavior and energy expenditure

Aging affects numerous physiological processes, as well as behavior. A large number of these processes are regulated, at least partially, by hypothalamic orexin neurons, and orexin tone may decrease with normal aging. In this study, we hypothesized that designer receptors exclusively activated by designer drugs (DREADD) stimulation of orexin neuronal activity will ameliorate the effect of aging on behavioral and metabolic alterations in young and middle-aged mice. DREADD targeting was achieved by stereotaxic injection of AAV vectors (AAV2-hSyn-DIO-hM3D(Gq)-mCherry) into the lateral hypothalamus of 5- and 12-mo old orexin-cre female mice and was confirmed by immunohistochemistry (IHC) analysis of orexin A and mCherry expression. After recovery, animals were subjected to a behavioral test battery consisting of the elevated plus maze (EPM), open field (OFT), and novel object recognition tests (NORT) to assess effects of aging on anxiety-like behavior, general locomotion, and working memory. A comprehensive laboratory animal monitoring system (CLAMS) was used to measure spontaneous physical activity (SPA) and energy expenditure (EE). The results indicate that activation of orexin neurons mitigates aging-induced reductions in anxiety-like behavior in middle-aged mice (P < 0.005) and increases locomotion in both young and middle-aged mice (P < 0.05). Activation of orexin neurons increases SPA (P < 0.01) and EE (P < 0.005) in middle-aged mice, restoring the levels to that observed in young animals. Results from this study identify orexin neurons as potential therapeutic targets for age-related impairments in cognitive and anxiety-related behavior, and energy balance.

Age-related central regulation of orexin and NPY in the short-lived African killifish Nothobranchius furzeri

Orexin A (OXA) and neuropeptide Y (NPY) are two hypothalamic neuropeptides involved in the regulation of feeding behavior and food intake in all vertebrates. Accumulating evidences document that they undergo age-related modifications, with consequences on metabolism, sleep/wake disorders and progression of neurodegenerations. The present study addressed the age related changes in expression and distribution of orexin A (its precursor is also known as hypocretin-HCRT) and NPY, and their regulation by food intake in the short-lived vertebrate model Nothobranchius furzeri. Our experiments, conducted on male specimens, show that: (a) HCRT and OXA and NPY mRNA and protein are localized in neurons of diencephalon and optic tectum, as well as in numerous fibers projecting through the entire neuroaxis, and are colocalized in specific nuclei; (b) in course of aging, HCRT and NPY expressing neurons are localized also in telencephalon and rhombencephalon; (c) HCRT expressing neurons increased slightly in the diencephalic area of old animals and in fasted animals, whereas NPY increased sharply; (d) central HCRT levels are not regulated neither in course of aging nor by food intake; and (e) central NPY levels are augmented in course of aging, and regulated by food intake only in young. These findings represent a great novelty in the study of central orexinergic and NPY-ergic systems in vertebrates', demonstrating an uncommon and unprecedented described regulation of these two orexigenic neuropeptides.

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.

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.

Reduction of the molecular chaperone binding immunoglobulin protein (BiP) accentuates the effect of aging on sleep-wake behavior

Sleep and wake quality, quantity, and architecture become modified with aging. Sleep and wake quality decline coinciding with increased fragmentation of both states across aging. We have previously shown that this age-related decline in sleep-wake quality is associated with increased endoplasmic reticular (ER) stress and decreased expression of the major ER chaperone binding immunoglobulin protein (BiP). BiP, also known as glucose-regulated protein 78, plays a key role in controlling the cellular response to ER stress, acting as a regulator of a protein homeostatic signaling pathway known as the unfolded protein response. Induction of BiP during cellular stress is part of an adaptive prosurvival mechanism. Here, using mice heterozygous for BiP, we investigated the effect of reduced BiP expression on sleep-wake behavior across aging; complete knockdown of BiP is embryonic lethal. We report that BiP heterozygosity accentuates the aging sleep-wake phenotype. Sleep and wake fragmentation was more pronounced in the BiP heterozygotes across the 3 ages examined. In mice lacking 1 functional copy of BiP, we observed an age-related significant reduction in wake bout duration and increase in wake bout numbers during the active period, as well as an increase in non rapid eye movement and rapid eye movement bout numbers accompanied by reduced bout durations of both non rapid eye movement and rapid eye movement during the sleep period. In addition, we observed increased ER stress in orexin neurons and occurrence of aggregates immunopositive for orexin at the terminals and projections of orexin neurons in the middle-aged BiP heterozygotes. Taken together, our data indicate that a reduction in the molecular chaperone BiP impacts sleep architecture across aging and that orexin processing is likely to be affected.

Electrophysiological Properties of Melanin-Concentrating Hormone and Orexin Neurons in Adolescent Rats

Orexin and melanin-concentrating hormone (MCH) neurons have complementary roles in various physiological functions including energy balance and the sleep/wake cycle. in vitro electrophysiological studies investigating these cells typically use post-weaning rodents, corresponding to adolescence. However, it is unclear whether these neurons are functionally mature at this period and whether these studies can be generalized to adult cells. Therefore, we examined the electrophysiological properties of orexin and MCH neurons in brain slices from post-weaning rats and found that MCH neurons undergo an age-dependent reduction in excitability, but not orexin neurons. Specifically, MCH neurons displayed an age-dependent hyperpolarization of the resting membrane potential (RMP), depolarizing shift of the threshold, and decrease in excitatory transmission, which reach the adult level by 7 weeks of age. In contrast, basic properties of orexin neurons were stable from 4 weeks to 14 weeks of age. Furthermore, a robust short-term facilitation of excitatory synapses was found in MCH neurons, which showed age-dependent changes during the post-weaning period. On the other hand, a strong short-term depression was observed in orexin neurons, which was similar throughout the same period. These differences in synaptic responses and age dependence likely differentially affect the network activity within the lateral hypothalamus where these cells co-exist. In summary, our study suggests that orexin neurons are electrophysiologically mature before adolescence whereas MCH neurons continue to develop until late adolescence. These changes in MCH neurons may contribute to growth spurts or consolidation of adult sleep patterns associated with adolescence. Furthermore, these results highlight the importance of considering the age of animals in studies involving MCH neurons.

Increased oxytocin-monomeric red fluorescent protein 1 fluorescent intensity with urocortin-like immunoreactivity in the hypothalamo-neurohypophysial system of aged transgenic rats

To visualize oxytocin in the hypothalamo-neurohypophysial system, we generated a transgenic rat that expresses the oxytocin-monomeric red fluorescent protein 1 (mRFP1) fusion gene. In the present study, we examined the age-related changes of oxytocin-mRFP1 fluorescent intensity in the posterior pituitary (PP), the supraoptic nucleus (SON) and the paraventricular nucleus (PVN) of transgenic rats. The mRFP1 fluorescent intensities were significantly increased in the PP, the SON and the PVN of 12-, 18- and 24-month-old transgenic rats in comparison with 3-month-old transgenic rats. Immunohistochemical staining for urocortin, which belongs to the family of corticotropin-releasing factor family, revealed that the numbers of urocortin-like immunoreactive (LI) cells in the SON and the PVN were significantly increased in 12-, 18- and 24-month-old transgenic rats in comparison with 3-month-old transgenic rats. Almost all of urocortin-LI cells co-exist mRFP1-expressing cells in the SON and the PVN of aged transgenic rats. These results suggest that oxytocin content of the hypothalamo-neurohypophysial system may be modulated by age-related regulation. The physiological role of the co-existence of oxytocin and urocortin in the SON and PVN of aged rats remains unclear.

Sex differences in age-related changes in the sleep-wake cycle

Age-related changes in sleep and circadian regulation occur as early as the middle years of life. Research also suggests that sleep and circadian rhythms are regulated differently between women and men. However, does sleep and circadian rhythms regulation age similarly in men and women? In this review, we present the mechanisms underlying age-related differences in sleep and the current state of knowledge on how they interact with sex. We also address how testosterone, estrogens, and progesterone fluctuations across adulthood interact with sleep and circadian regulation. Finally, we will propose research avenues to unravel the mechanisms underlying sex differences in age-related effects on sleep.

Spontaneous physical activity and mediators of energy homeostasis in the hypothalamus of mice from 4 to 10 months of age

NEW FINDINGS

What is the central question of this study? Is the initial decline of spontaneous physical activity (SPA) in mice related to impaired insulin and leptin signalling or brain-derived neurotrophic factor expression in the hypothalamus? What is the main finding and its importance? We showed that SPA started to decline at an early stage, concomitantly with an impairment of hypothalamic leptin signalling. Consequently, energy expenditure decreased and glucose tolerance worsened. Our results demonstrate the need to counteract the initial decline in SPA to avoid metabolic impairments and indicate the possible involvement of central leptin in the reduction in SPA with age. The biological control of physical activity is poorly understood. Age decreases insulin, leptin and brain-derived neurotrophic factor (BDNF) signalling in the hypothalamus, and all have been shown to modulate spontaneous physical activity (SPA). We investigated the age at which SPA starts to decline and whether this is associated with the emergence of hypothalamic insulin and leptin resistance and reduced BDNF expression. Spontaneous physical activity (and other parameters of locomotion) and energy expenditure were determined monthly in mice from the 4th to the 10th month of age. Metabolic and hypothalamic analyses were performed in 4-, 6- and 10-month-old mice. Spontaneous physical activity, distance travelled and speed of locomotion started to decrease in 6-month-old mice. The reduction in SPA became more evident from 8 months of age. Energy expenditure decreased from the 8th month. Hypothalamic BDNF protein expression and insulin signalling did not change throughout the time span studied. Leptin signalling decreased at 6 and 10 months compared with 4 months. Also, compared with 4 months, 6- and 10-month-old mice were glucose intolerant. In conclusion, SPA begins to decline in parallel with reduced hypothalamic leptin signalling. Metabolic impairment also manifests as SPA decreases, highlighting the need to understand the regulation of SPA in order to combat its decline.

Anatomical and electrophysiological development of the hypothalamic orexin neurons from embryos to neonates

The amount, quality, and diurnal pattern of sleep change greatly during development. Developmental changes of sleep/wake architecture are in a close relationship to brain development. The fragmentation of wake episodes is one of the salient features in the neonatal period, which is also observed in mature animals and human individuals lacking neuropeptide orexin/hypocretin signaling. This raises the possibility that developmental changes of lateral hypothalamic orexin neurons are relevant to the development of sleep/wake architecture. However, little information is available on morphological and physiological features of developing orexin neurons. To address the cellular basis for maturation of the sleep/wake regulatory system, we investigated the functional development of orexin neurons in the lateral hypothalamus. The anatomical development as well as the changes in the electrophysiological characteristics of orexin neurons was examined from embryonic to postnatal stages in orexin-EGFP mice. Prepro-orexin promoter activity was detectable at embryonic day (E) 12.0, followed by expression of orexin A after E14.0. The number of orexin neurons and their membrane capacitance reached similar levels to adults by postnatal day (P) 7, while their membrane potentials, firing rates, and action potential waveforms were developed by P21. The hyperpolarizing effect of serotonin, which is a major inhibitory signal for adult orexin neurons, was detected after E18.0 and matured at P1. These results suggest that the expression of orexin peptides precedes the maturation of electrophysiological activity of orexin neurons. The function of orexin neurons gradually matures by 3 weeks after birth, coinciding with maturation of sleep/wake architecture.

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.

Orexin and Central Modulation of Cardiovascular and Respiratory Function

Orexin makes an important contribution to the regulation of cardiorespiratory function. When injected centrally under anesthesia, orexin increases blood pressure, heart rate, sympathetic nerve activity, and the amplitude and frequency of respiration. This is consistent with the location of orexin neurons in the hypothalamus and the distribution of orexin terminals at all levels of the central autonomic and respiratory network. These cardiorespiratory responses are components of arousal and are necessary to allow the expression of motivated behaviors. Thus, orexin contributes to the cardiorespiratory response to acute stressors, especially those of a psychogenic nature. Consequently, upregulation of orexin signaling, whether it is spontaneous or environmentally induced, can increase blood pressure and lead to hypertension, as is the case for the spontaneously hypertensive rat and the hypertensive BPH/2J Schlager mouse. Blockade of orexin receptors will reduce blood pressure in these animals, which could be a new pharmacological approach for the treatment of some forms of hypertension. Orexin can also magnify the respiratory reflex to hypercapnia in order to maintain respiratory homeostasis, and this may be in part why it is upregulated during obstructive sleep apnea. In this pathological condition, blockade of orexin receptors would make the apnea worse. To summarize, orexin is an important modulator of cardiorespiratory function. Acting on orexin signaling may help in the treatment of some cardiovascular and respiratory disorders.

Orexin, Stress and Central Cardiovascular Control. A Link with Hypertension?

Orexin, the arousal peptide, originates from neurons located in an area of the dorsal hypothalamus well known for integrating defense responses and their cardiovascular component. Orexin neurons, which are driven in large part by the limbic forebrain, send projections to many regions in the brain, including regions involved in cardiovascular control, as far down as sympathetic preganglionic neurons in the spinal cord. Central injections of orexin evoke sympathetically mediated cardiovascular responses. Conversely, blockade of orexin receptors reduce the cardiovascular responses to acute stressors, preferentially of a psychological nature. More importantly, lasting upregulation of orexin signaling can lead to a hypertensive state. This can be observed in rats exposed to chronic stress as well as in strains known to display spontaneous hypertension such as the spontaneously hypertensive rat (SHR) or the hypertensive BPH/2J Schlager mouse. Thus, there is a link between orexin, stress and hypertension, and orexin upregulation could be a factor in the development of essential hypertension. Orexin receptor antagonists have anti-hypertensive effects that could be of clinical use.

Circadian Rhythms, Sleep, and Disorders of Aging

Sleep-wake cycles are known to be disrupted in people with neurodegenerative disorders. These findings are now supported by data from animal models for some of these disorders, raising the question of whether the disrupted sleep/circadian regulation contributes to the loss of neural function. As circadian rhythms and sleep consolidation also break down with normal aging, changes in these may be part of what makes aging a risk factor for disorders like Alzheimer's disease (AD). Mechanisms underlying the connection between circadian/sleep dysregulation and neurodegeneration remain unclear, but several recent studies provide interesting possibilities. While mechanistic analysis is under way, it is worth considering treatment of circadian/sleep disruption as a means to alleviate symptoms of neurodegenerative disorders.

Developmental changes in the hypothalamic mRNA levels of prepro-orexin and orexin receptors and their sensitivity to fasting in male and female rats

Orexin, which is also called as hypocretin (Hcrt), a product of the prepro-orexin (pp-orexin//Hcrt) gene, affects various physiological and behavioral functions, such as the sleep-wake cycle and appetite. The developmental changes in the hypothalamic mRNA levels of pp-prexin and the orexin receptors OX1R and OX2R and their sensitivity to fasting were evaluated in both male and female rats. During development, hypothalamic pp-orexin/Hcrt mRNA expression increased in both male and female rats, whereas hypothalamic OX1R mRNA expression decreased in both sexes. In addition, hypothalamic OX2R mRNA expression increased in male rats, but did not change in female rats. Fasting did not affect hypothalamic pp-orexin/Hcrt mRNA expression in either sex. Hypothalamic OX1R mRNA expression was increased by fasting in the prepubertal period (postnatal days 20 and 30) in female rats, but was not affected by fasting in males. In male rats, hypothalamic OX2R mRNA expression was decreased by fasting during the neonatal period (postnatal day 10), but not the prepubertal period (postnatal days 20 and 30). In females, hypothalamic OX2R mRNA expression was also decreased by fasting; however, the fasting-induced downregulation of hypothalamic OX2R expression persisted until postnatal day 20. These results indicate that the developmental patterns of components of the orexin system and their sensitivity to fasting during the neonatal and prepubertal periods only differ slightly between the sexes. These differences might be involved in the development of some physiological and behavioral functions.

Decreased orexin (hypocretin) immunoreactivity in the hypothalamus and pontine nuclei in sudden infant death syndrome

Infants at risk of sudden infant death syndrome (SIDS) have been shown to have dysfunctional sleep and poor arousal thresholds. In animal studies, both these attributes have been linked to impaired signalling of the neuropeptide orexin. This study examined the immunoreactivity of orexin (OxA and OxB) in the tuberal hypothalamus (n = 27) and the pons (n = 15) of infants (1-10 months) who died from SIDS compared to age-matched non-SIDS infants. The percentage of orexin immunoreactive neurons and the total number of neurons were quantified in the dorsomedial, perifornical and lateral hypothalamus at three levels of the tuberal hypothalamus. In the pons, the area of orexin immunoreactive fibres were quantified in the locus coeruleus (LC), dorsal raphe (DR), laterodorsal tegmental (LDT), medial parabrachial, dorsal tegmental (DTg) and pontine nuclei (Pn) using automated methods. OxA and OxB were co-expressed in all hypothalamic and pontine nuclei examined. In SIDS infants, orexin immunoreactivity was decreased by up to 21 % within each of the three levels of the hypothalamus compared to non-SIDS (p ≤ 0.050). In the pons, a 40-50 % decrease in OxA occurred in the all pontine nuclei, while a similar decrease in OxB immunoreactivity was observed in the LC, LDT, DTg and Pn (p ≤ 0.025). No correlations were found between the decreased orexin immunoreactivity and previously identified risk factors for SIDS, including prone sleeping position and cigarette smoke exposure. This finding of reduced orexin immunoreactivity in SIDS infants may be associated with sleep dysfunction and impaired arousal.

Spontaneously hypertensive rats have more orexin neurons in the hypothalamus and enhanced orexinergic input and orexin 2 receptor-associated nitric oxide signalling in the rostral ventrolateral medulla

NEW FINDINGS

What is the central question of this study? Our previous study demonstrates that elevated orexin 2 receptor (OX2R) activity within the rostral ventrolateral medulla (RVLM) contributes to hypertension in spontaneously hypertensive rats (SHRs), and a lower OX2R protein level was detected in their RVLM. The present study aims to explore the mechanisms underlying elevated orexinergic activity in the RVLM of SHRs, compared with their normotensive counterparts, Wistar-Kyoto rats. What is the main finding and its importance? Increased orexinergic input into the RVLM and enhanced OX2R responsiveness in the RVLM, which was mainly mediated by augmented OX2R-neuronal nitric oxide synthase signalling, may underlie the elevated OX2R activity within the RVLM of SHRs. Our previous study showed that elevated orexin 2 receptor (OX2R) activity within the rostral ventrolateral medulla (RVLM) contributes to hypertension in spontaneously hypertensive rats (SHRs). Herein, we investigated the mechanism(s) underlying the elevated OX2R activity. The following results were found. (i) More hypothalamic orexin A-immunoreactive (OXA-IR) cells existed in SHRs than in Wistar-Kyoto (WKY) rats at either 4 (2217 ± 43 versus 1809 ± 69) or 16 weeks of age (1829 ± 59 versus 1230 ± 84). The number of OXA-IR cells that project to the RVLM was higher in 16-week-old SHRs than in WKY rats (91 ± 11 versus 52 ± 11). (ii) Higher numbers of OXA-IR and RVLM-projecting OXA-IR cells were found in the dorsomedial and perifornical hypothalamus of 16-week-old SHRs. (iii) Spontaneously hypertensive rats had higher levels of orexin A and B in the hypothalamus and higher levels of orexin A in the RVLM than did WKY rats. (iv) Unilateral intra-RVLM application of OX2R agonist, orexin A or [Ala(11), d-Leu(15)]-orexin B (50 pmol) induced a larger pressor response in SHRs than in WKY rats. (v) Intra-RVLM pretreatment with a neuronal nitric oxide synthase (NOS) inhibitor, 7-nitro-indazole (2.5 pmol), or a soluble guanylate cyclase inhibitor, methylene blue (250 pmol), reduced the intra-RVLM [Ala(11), d-Leu(15) ]-orexin B-induced pressor response in both WKY rats and SHRs. In contrast, an inducible NOS inhibitor, aminoguanidine (100 pmol), was ineffective. (vi) Neuronal NOS was co-expressed with OX2R in RVLM neurons. In conclusion, increased orexinergic input and enhanced OX2R-neuronal NOS signalling may underlie elevated OX2R activity in the RVLM and contribute to the pathophysiology of hypertension in SHRs.

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.

Evolutionarily conserved regulation of hypocretin neuron specification by Lhx9

Loss of neurons that express the neuropeptide hypocretin (Hcrt) has been implicated in narcolepsy, a debilitating disorder characterized by excessive daytime sleepiness and cataplexy. Cell replacement therapy, using Hcrt-expressing neurons generated in vitro, is a potentially useful therapeutic approach, but factors sufficient to specify Hcrt neurons are unknown. Using zebrafish as a high-throughput system to screen for factors that can specify Hcrt neurons in vivo, we identified the LIM homeobox transcription factor Lhx9 as necessary and sufficient to specify Hcrt neurons. We found that Lhx9 can directly induce hcrt expression and we identified two potential Lhx9 binding sites in the zebrafish hcrt promoter. Akin to its function in zebrafish, we found that Lhx9 is sufficient to specify Hcrt-expressing neurons in the developing mouse hypothalamus. Our results elucidate an evolutionarily conserved role for Lhx9 in Hcrt neuron specification that improves our understanding of Hcrt neuron development.

Spontaneously hypertensive rats have more orexin neurons in their medial hypothalamus than normotensive rats

NEW FINDINGS

What is the central question of this study? Blockade of orexin receptors reduces blood pressure in spontaneously hypertensive rats (SHRs) but not in normotensive Wistar-Kyoto (WKY) rats, suggesting that upregulation of orexin signalling underlies the hypertensive phenotype of the SHR. However, it is not known what causes this upregulation. What is the main finding and its importance? Using orexin immunolabelling, we show that SHRs have 20% more orexin neurons than normotensive WKY and Wistar rats in the medial hypothalamus, which is a good match to their blood pressure phenotype. In contrast, there is no such match for the orexin neurons of the lateral hypothalamus. Essential hypertension may be linked to an increase in orexin neurons in the medial hypothalamus. The neuropeptide orexin contributes to the regulation of blood pressure as part of its role in the control of arousal during wakefulness and motivated behaviour (including responses to psychological stress). Recent work shows that pharmacological blockade of orexin receptors reduces blood pressure in spontaneously hypertensive rats (SHRs) but not in normotensive Wistar-Kyoto (WKY) rats. It is not clear why orexin signalling is upregulated in the SHR, but one possibility is that these animals have more orexin neurons than their normotensive WKY and Wistar relatives. To test this possibility, SHRs, WKY and Wistar male rats (6-16 weeks old) were killed, perfused and their brains sectioned and immunolabelled for orexin A. Labelled neurons were plotted and counted in the six best labelled hemisections (120 μm apart) of each brain. There were significantly more orexin neurons (+20%) in the medial hypothalamus (medial to fornix) of SHRs compared with WKY and Wistar rats (126 ± 4 versus 106 ± 5 and 104 ± 5 per hemisection, respectively, P < 0.05), which matches well the blood pressure phenotypes. In contrast, counts in the lateral hypothalamus did not match the blood pressure phenotypes (69 ± 2 versus 50 ± 3 and 76 ± 4, respectively). The results support the idea that orexin signalling is upregulated in the SHR and suggest that this is due, at least in part, to a greater number of orexin neurons in the medial hypothalamus. These medial orexin neurons, which are also involved in hyperarousal and stress responses, may contribute to the development of essential hypertension.

The orexin neuropeptide system: physical activity and hypothalamic function throughout the aging process

There is a rising medical need for novel therapeutic targets of physical activity. Physical activity spans from spontaneous, low intensity movements to voluntary, high-intensity exercise. Regulation of spontaneous and voluntary movement is distributed over many brain areas and neural substrates, but the specific cellular and molecular mechanisms responsible for mediating overall activity levels are not well understood. The hypothalamus plays a central role in the control of physical activity, which is executed through coordination of multiple signaling systems, including the orexin neuropeptides. Orexin producing neurons integrate physiological and metabolic information to coordinate multiple behavioral states and modulate physical activity in response to the environment. This review is organized around three questions: (1) How do orexin peptides modulate physical activity? (2) What are the effects of aging and lifestyle choices on physical activity? (3) What are the effects of aging on hypothalamic function and the orexin peptides? Discussion of these questions will provide a summary of the current state of knowledge regarding hypothalamic orexin regulation of physical activity during aging and provide a platform on which to develop improved clinical outcomes in age-associated obesity and metabolic syndromes.

Orexin receptor activity in the basal forebrain alters performance on an olfactory discrimination task

Cholinergic innervation of the prefrontal cortex is critical for various forms of cognition, although the efferent modulators contributing to acetylcholine (ACh) release are not well understood. The main source of cortical ACh, the basal forebrain, receives projections from lateral and perifornical hypothalamic neurons releasing the peptides orexin (orexin A; OxA, and orexin B; OxB), of which OxA is hypothesized to play a role in various cognitive functions. We sought to assess one such function known to be susceptible to basal forebrain cholinergic manipulation, olfactory discrimination acquisition, and reversal learning, in rats following intra-basal forebrain infusion of OxA or the orexin 1 receptor (OxR1) antagonist SB-334867. OxA administration facilitated, while OxR1 antagonism impaired performance on both the acquisition and reversal portions of the task. These data suggest that orexin acting in the basal forebrain may be important for cortical-dependant executive functions, possibly through the stimulation of cortical ACh release.

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.

Systemic regulation of mammalian ageing and longevity by brain sirtuins

Sirtuins regulate numerous important biological processes in mammals, including various age-associated pathophysiologies. However, whether sirtuins are critical to control ageing and longevity in mammals has been controversial. Recent studies have demonstrated critical roles of sirtuins in the brain, especially the hypothalamus, in governing multiple physiological functions. These data provide strong evidence that brain sirtuins regulate mammalian ageing and longevity at the organismal level.

Orexin restores aging-related brown adipose tissue dysfunction in male mice

The aging process causes an increase in percent body fat, but the mechanism remains unclear. In the present study we examined the impact of aging on brown adipose tissue (BAT) thermogenic activity as potential cause for the increase in adiposity. We show that aging is associated with interscapular BAT morphologic abnormalities and thermogenic dysfunction. In vitro experiments revealed that brown adipocyte differentiation is defective in aged mice. Interscapular brown tissue in aged mice is progressively populated by adipocytes bearing white morphologic characteristics. Aged mice fail to mobilize intracellular fuel reserves from brown adipocytes and exhibit deficiency in homeothermy. Our results suggest a role for orexin (OX) signaling in the regulation of thermogenesis during aging. Brown fat dysfunction and age-related assimilation of fat mass were accelerated in mice in which OX-producing neurons were ablated. Conversely, OX injections in old mice increased multilocular morphology, increased core body temperature, improved cold tolerance, and reduced adiposity. These results argue that BAT can be targeted for interventions to reverse age-associated increase in fat mass.

Intravenous prenatal nicotine exposure increases orexin expression in the lateral hypothalamus and orexin innervation of the ventral tegmental area in adult male rats

BACKGROUND

Approximately 18% of pregnant women continue to smoke tobacco cigarettes throughout pregnancy. Offspring exposed to tobacco smoke in utero exhibit a higher incidence of drug use in later stages of development relative to non-exposed children. Animal models indicate that prenatal nicotine (PN) exposure alone alters the development of the mesocorticolimbic dopamine (DA) system, which, in part, organizes motivated behavior and reward. The orexin/hypocretin neuropeptide system, which originates in the lateral hypothalamus (LH), projects to key areas of the mesocorticolimbic DA pathway. Previous research suggests that orexin exerts a major influence on motivation and reward.

METHODS

The present experiments determined if intravenous (IV) PN exposure alters (1) the expression of orexin neurons and melanin-concentrating hormone (MCH; positive control) in the LH; and (2) orexin projections from the LH onto DA neurons in the ventral tegmental area (VTA). Dams were injected with IV nicotine (0.05 mg/kg/injection) or saline 3×/day during gestational days 8-21. Tissues from adult male offspring (∼130 days) were examined using immunohistochemistry.

RESULTS

Relative to controls, offspring of IV PN exposure showed (1) increased numbers of orexin neurons in the LH, and no changes in the expression of MCH; and (2) increased orexin appositions on DA cells in the VTA.

CONCLUSION

The findings indicate that the influence of PN exposure is enduring, and suggests that the PN-induced modification of orexin expression on mesolimbic circuitry may contribute to the reported changes in motivated behaviors related to food and drug reward observed in offspring prenatally exposed to nicotine.

Orexins (hypocretins) contribute to fear and avoidance in rats exposed to a single episode of footshocks

Orexins (hypocretins) are peptides that have been shown to regulate behavioral arousal and wakefulness. Recent evidence indicates that orexin neurons are activated by stress and that orexins play a role in anxiety. The present paper describes a series of experiments that examined whether orexins are involved in the anxiety that resulted from exposing rats to an acute episode of footshocks (5 × 2 s of 1.5 mA shocks). We found that prepro-orexin (ppOX) mRNA was elevated in rats at 6 and 14 days after exposure to footshock and that ppOX mRNA levels were correlated with fear at 14 days post-shock. Systemic injections of the non-selective dual orexin receptor antagonist TCS-1102 (10 and 20 mg/kg, i.p.) were found to decrease fear and anxiety in rats 14 days after exposure to footshock. We also found that rats that exhibited a high level of immobility to a novel tone the day after the footshock episode (high responders, HR) showed significantly elevated levels of ppOX mRNA at 14 days post-shock compared to control rats. Furthermore, TCS-1102 (10 mg/kg, i.p.) was found to have anxiolytic effects that were specific for HR when tested in the elevated T-maze. This study provides evidence linking the orexin system to the anxiety produced by exposure of rats to a single episode of footshocks. It also provides preclinical evidence in support of the use of orexin antagonists for the treatment of anxiety in response to an acute episode of stress.

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.

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.

Adolescent gain in positive valence of a socially relevant stimulus: engagement of the mesocorticolimbic reward circuitry

A successful transition from childhood to adulthood requires adolescent maturation of social information processing. The neurobiological underpinnings of this maturational process remain elusive. This research employed the male Syrian hamster as a tractable animal model for investigating the neural circuitry involved in this critical transition. In this species, adult and juvenile males display different behavioral and neural responses to vaginal secretions, which contain pheromones essential for expression of sexual behavior in adulthood. These studies tested the hypothesis that vaginal secretions acquire positive valence over adolescent development via remodeling of neural circuits underlying sexual reward. Sexually naïve adult, but not juvenile, hamsters showed a conditioned place preference for vaginal secretions. Differences in behavioral response to vaginal secretions between juveniles and adults correlated with a difference in the vaginal secretion-induced neural activation pattern in mesocorticolimbic reward circuitry. Fos immunoreactivity increased in response to vaginal secretions in the medial amygdala and ventral tegmental dopaminergic cells of both juvenile and adult males. However, only in adults was there a Fos response to vaginal secretions in non-dopaminergic cells in interfascicular ventral tegmental area, nucleus accumbens core and infralimbic medial prefrontal cortex. These results demonstrate that a socially relevant chemosensory stimulus acquires the status of an unconditioned reward during adolescence, and that this adolescent gain in social reward is correlated with experience-independent engagement of specific cell groups in reward circuitry.