Effects of sleep on the cardiovascular and thermoregulatory systems: a possible role for hypocretins

Quest for Biomarkers of Positive Health: A Review

The positive health of a person can be defined as the ability to live long in good health, possibly with no activity limitation. No method is yet available for its objective assessment in individuals, and we propose a framework in this communication that can operationalize this concept. Instead of distal factors, such as diet and lifestyle because these are subjective and difficult to measure, we concentrate on the objectively measurable biomarkers such as immunity level, endorphins, and handgrip strength. The focus is on the major parameters that may protect from diseases and infirmity and can be assessed by noninvasive methods. A combination of such parameters may signify positive health. This may be a novel way to measure positive health at the individual level. In this communication, we briefly review the literature and identify a few major biomarkers that provide a protective shield and could determine the status of positive health at the individual level. This exercise demonstrates that the assessment of the positive health of a person is feasible. A scale based on these and other relevant parameters can be developed later that could quantitatively measure the exact level of positive health. As the exact combination of the parameters that protects from ailments is not fully known yet, a framework such as this may help in identifying the data gaps that require attention in this context. The proposed framework may initiate a discussion on indicators of positive health and characterize the parameters for intervention that could increase a healthy life.

Diurnal changes in blood pressure and heart rate in children with narcolepsy with cataplexy

The hypocretin neurons in the lateral hypothalamus are connected not only to brain alertness systems but also to brainstem nuclei that regulate blood pressure and heart rate. The premise is that regulation of blood pressure and heart rate is altered and affected by methylphenidate, a stimulant drug in children with narcolepsy with cataplexy. The changes in 24-hr ambulatory systolic and diastolic blood pressure and heart rate were compared among pre-treated narcolepsy with cataplexy patients (40 males, 10 females), with mean age 10.4 ± 3.5 years (M ± SD, range 5-17 years) with values from 100 archival age-sex-body mass index matched controls. Patients had a lower diurnal systolic blood pressure (-6.5 mmHg; p = 0.000) but higher heart rate (+11.0 bpm; p = 0.000), particularly evident in the waketime, while diastolic blood pressure was comparable. With methylphenidate (18 mg sustained release at 08:00 hours), patients with narcolepsy with cataplexy had higher systolic blood pressure (+4.6 mmHg, p = 0.015), diastolic blood pressure (+3.3 mmHg, p = 0.005) and heart rate (+7.1 bpm, p = 0.028) during wake time, but nighttime cardiovascular values were unchanged from pre-treated values; amplitude variation in cardiovascular values was unchanged over 24 hr. In conclusion, children with narcolepsy with cataplexy had downregulation blood pressure profile but a higher heart rate, and lesser non-dipping profiles. Daytime methylphenidate treatment increases only waketime blood pressure and further elevated heart rate values.

Potential of Polyphenols for Improving Sleep: A Preliminary Results from Review of Human Clinical Trials and Mechanistic Insights

Global epidemiologic evidence supports an interrelationship between sleep disorders and fruits and vegetable ingestion. Polyphenols, a broad group of plant substances, are associated with several biologic processes, including oxidative stress and signaling pathways that regulate the expression of genes promoting an anti-inflammatory environment. Understanding whether and how polyphenol intake is related to sleep may provide avenues to improve sleep and contribute to delaying or preventing the development of chronic disease. This review aims to assess the public health implications of the association between polyphenol intake and sleep and to inform future research. The effects of polyphenol intake, including chlorogenic acid, resveratrol, rosmarinic acid, and catechins, on sleep quality and quantity are discussed to identify polyphenol molecules that may improve sleep. Although some animal studies have investigated the mechanisms underlying the effects of polyphenols on sleep, the paucity of trials, especially randomized controlled trials, does not allow for conducting a meta-analysis to reach clear conclusions about the relationships among these studies to support the sleep-improving effects of polyphenols.

Progress of autonomic disturbances in narcolepsy type 1

Narcolepsy type 1 is a kind of sleep disorder characterized by a specific loss of hypocretin neurons in the lateral hypothalamus and reduced levels of hypocretin-1 in the cerebrospinal fluid. Hypocretin deficiency is associated with autonomic disorders. This article summarizes the autonomic disorders and possible mechanisms associated with narcolepsy type 1. Patients with narcolepsy type 1 often have various systemic autonomic symptoms, including non-dipping blood pressure, reduced heart rate variability, dynamic cerebral autoregulation impairment, reduced gastric motility and emptying, sleep-related erectile dysfunction, skin temperature abnormalities, and blunted pupillary light reflex. Similar findings should strengthen the recognition and intervention of these disturbances in clinical practice. In addition to hypocretin deficiency, current evidence also indicates that pharmacological therapy (including psychostimulants and anti-cataplectic drugs) and comorbidities may contribute to the alterations of autonomic system observed in narcolepsy type 1.

Body Weight and Metabolic Rate Changes in Narcolepsy: Current Knowledge and Future Directions

Narcolepsy is a known auto-immune disease that presents mainly in the teenage years with irresistible sleep attacks. Patients with narcolepsy, especially NT1, have been found to have a high prevalence of obesity and other metabolic derangements. This narrative review aimed to address the relationship between narcolepsy and changes in weight and metabolic rate, and discuss potential mechanisms for weight gain and metabolic changes and future research agendas on this topic. This article will provide a balanced, up-to-date critical review of the current literature, and delineate areas for future research, in order to understand the pathophysiological metabolic changes in narcolepsy. Articles using predefined keywords were searched for in PubMed and Google Scholar databases, with predefined inclusion and exclusion criteria. Compared to controls, patients with narcolepsy are more likely to be obese and have higher BMIs and waist circumferences. According to recent research, weight gain in narcolepsy patients may be higher during the disease's outset. The precise mechanisms causing this weight gain remains unknown. The available information, albeit limited, does not support differences in basal or resting metabolic rates between patients with narcolepsy and controls, other than during the time of disease onset. The evidence supporting the role of orexin in weight gain in humans with narcolepsy is still controversial, in the literature. Furthermore, the available data did not show any appreciable alterations in the levels of CSF melanin-concentrating hormone, plasma and CSF leptin, or serum growth hormone, in relation to weight gain. Other mechanisms have been proposed, including a reduction in sympathetic tone, hormonal changes, changes in eating behavior and physical activity, and genetic predisposition. The association between increased body mass index and narcolepsy is well-recognized; however, the relationship between narcolepsy and other metabolic measures, such as body fat/muscle distribution and metabolic rate independent of BMI, is not well documented, and the available evidence is inconsistent. Future longitudinal studies with larger sample sizes are needed to assess BMR in patients with narcolepsy under a standard protocol at the outset of narcolepsy, with regular follow-up.

Turn it off and on again: characteristics and control of torpor

Torpor is a hypothermic, hypoactive, hypometabolic state entered into by a wide range of animals in response to environmental challenge. This review summarises the current understanding of torpor. We start by describing the characteristics of the wide-ranging physiological adaptations associated with torpor. Next follows a discussion of thermoregulation, control of food intake and energy expenditure, and the interactions of sleep and thermoregulation, with particular emphasis on how those processes pertain to torpor. We move on to review the evidence for the systems that control torpor entry, including both the efferent circulating factors that signal the need for torpor, and the central processes that orchestrate it. Finally, we consider how the putative circuits responsible for torpor induction integrate with the established understanding of thermoregulation under non-torpid conditions and highlight important areas of uncertainty for future studies.

Turn it off and on again: characteristics and control of torpor

Torpor is a hypothermic, hypoactive, hypometabolic state entered into by a wide range of animals in response to environmental challenge. This review summarises the current understanding of torpor. We start by describing the characteristics of the wide-ranging physiological adaptations associated with torpor. Next follows a discussion of thermoregulation, control of food intake and energy expenditure, and the interactions of sleep and thermoregulation, with particular emphasis on how those processes pertain to torpor. We move on to take a critical view of the evidence for the systems that control torpor entry, including both the efferent circulating factors that signal the need for torpor, and the central processes that orchestrate it. Finally, we consider how the putative circuits responsible for torpor induction integrate with the established understanding of thermoregulation under non-torpid conditions and highlight important areas of uncertainty for future studies.

The mediation of central cyclooxygenase and lipoxygenase pathways in orexin-induced cardiovascular effects

Previously it was reported that central orexin (OX) and arachidonic acid (AA) signaling pathways played an active role in the control of the cardiovascular system. It was also reported that they have exhibited their cardiovascular control role by using similar central or peripheral mechanisms. However, there has been no study demonstrating the interaction between OX and AA signaling pathways in terms of cardiovascular control. The current study was designed to investigate the possible mediation of the central cyclooxygenase (COX) and lipoxygenase (LOX) pathways in OX-induced cardiovascular effects in the rats. Intracerebroventricular injection of OX increased blood pressure and heart rate in a dose-dependent manner in normotensive male Sprague Dawley rats. Moreover, the microdialysis study revealed that intracerebroventricular injected OX caused a time-dependent increase in the extracellular total prostaglandin concentrations in the posterior hypothalamus. Interestingly, central pretreatment with a non-selective COX inhibitor, ibuprofen, or a non-selective LOX inhibitor, nordihydroguaiaretic acid, partially reversed pressor and tachycardic cardiovascular responses evoked by central administration of OX. In summary, our findings show that the central treatment with OX causes pressor and tachycardic cardiovascular responses along with an increase in posterior hypothalamic extracellular total prostaglandin concentrations. Furthermore, our results also demonstrate that central COX and LOX pathways mediate, at least in part, centrally administered OX-evoked pressor and tachycardic responses, as well.

Activation of Orexin 1 Receptors in the Paraventricular Nucleus Contributes to the Development of Deoxycorticosterone Acetate-Salt Hypertension Through Regulation of Vasopressin

Salt-sensitivity is a major factor in the development of hypertension. The brain orexin system has been observed to play a role in numerous hypertensive animal models. However, orexin’s role in the pathology of salt-sensitive hypertension (SSH) remains to be adequately explored. We assessed the impact of orexin hyperactivity in the pathogenesis of the deoxycorticosterone acetate (DOCA) – salt rat model, specifically through modulation of Arginine Vasopressin (AVP). Adult male rats were separated into three groups: vehicle control, DOCA-salt, and DOCA-salt+OX1R-shRNA. DOCA-salt rats received subcutaneous implantation of a 21-day release, 75 mg DOCA pellet in addition to saline drinking water (1% NaCl and 0.2% KCl). DOCA-salt+OX1R-shRNA rats received bilateral microinjection of AAV2-OX1R-shRNA into the paraventricular nucleus (PVN) to knockdown function of the Orexin 1-Receptor (OX1R) within that area. Following 2-week to allow full transgene expression, a DOCA pellet was administered in addition to saline drinking solution. Vehicle controls received sham DOCA implantation but were given normal water. During the 3-week DOCA-salt or sham treatment period, mean arterial pressure (MAP) and heart rate (HR) were monitored utilizing tail-cuff plethysmography. Following the 3-week period, rat brains were collected for either PCR mRNA analysis, as well as immunostaining. Plasma samples were collected and subjected to ELISA analysis. In line with our hypothesis, OX1R expression was elevated in the PVN of DOCA-salt treated rats when compared to controls. Furthermore, following chronic knockdown of OX1R, the hypertension development normally induced by DOCA-salt treatment was significantly diminished in the DOCA-salt+OX1R-shRNA group. A concurrent reduction in PVN OX1R and AVP mRNA was observed in concert with the reduced blood pressure following AAV2-OX1R-shRNA treatment. Similarly, plasma AVP concentrations appeared to be reduced in the DOCA-salt+OX1R-shRNA group when compared to DOCA-salt rats. These results indicate that orexin signaling, specifically through the OX1R in the PVN are critical for the onset and maintenance of hypertension in the DOCA-salt model. This relationship is mediated, at least in part, through orexin activation of AVP producing neurons, and the subsequent release of AVP into the periphery. Our results outline a promising mechanism underlying the development of SSH through interactions with the brain orexin system.

A common neuronal mechanism of hypertension and sleep disturbances in spontaneously hypertensive rats: Role of orexinergic neurons

Epidemiologic studies have shown that sleep disorders are associated with the development of hypertension. The present study investigated dynamic changes in sleep patterns during the development of hypertension across the lifespan in spontaneously hypertensive rats (SHRs) and the neural mechanism that underlies these comorbidities, with a focus on the orexinergic system. Blood pressure in rats was measured using a noninvasive blood pressure tail cuff. Sleep was monitored by electroencephalographic and electromyographic recordings. Immunohistochemistry was used to detect the density and activity of orexinergic neurons in the perifornical nucleus. Hcrt2-SAP (400 or 800 ng) was microinjected in the lateral hypothalamus to lesion orexinergic neurons. Compared with Wistar-Kyoto rats, SHRs exhibited various patterns of sleep disturbances. In SHRs, dynamic changes in hypersomnia in the rats' active phase was not synchronized with the development of hypertension, but hyperarousal in the inactive phase and difficulties in falling asleep were observed concurrently with the development of hypertension. Furthermore, the density and activity of orexinergic neurons in the perifornical nucleus were significantly higher in SHRs than in age-matched Wistar-Kyoto rats. The reduction of orexinergic neurons in the lateral hypothalamus partially ameliorated the development of hypertension and prevented difficulties in falling asleep in SHRs. These results indicate that although the correlation between sleep disturbances and hypertension is very complex, common mechanisms may underlie these comorbidities in SHRs. Overactivity of the orexin system may be one such common mechanism.

Downregulation of Orexin Receptor in Hypothalamic Paraventricular Nucleus Decreases Blood Pressure in Obese Zucker Rats

Background

Orexin and its receptors are critical regulating sympathetic vasomotor tone under physiological and pathophysiological conditions. Orexin receptor 1 (OXR1) is upregulated in the paraventricular nucleus (PVN) in the hypothalamus and contributes to increased sympathetic outflow in obese Zucker rats (OZRs). We hypothesized that silencing OXR1 expression in the PVN decreases heightened blood pressure and elevated sympathetic outflow in OZRs.

Methods and Results

An adeno‐associated virus (AAV) vector containing a short hairpin RNA (shRNA) targeting rat OXR1 was designed to silence OXR1 expression in the PVN. The AAV‐OXR1‐shRNA or scrambled shRNA was injected into the PVN in OZRs. The arterial blood pressure in free‐moving OZRs was continuously monitored by using a telemetry approach. The firing activity of spinally projecting PVN neurons in rat brain slices was recorded 3 to 4 weeks after injection of viral vectors. The free‐moving OZRs treated with AAV‐OXR1‐shRNA had markedly lower OXR1 expression and lower mean arterial blood pressure, heart rate, and ratio of low‐ to high‐frequency components of heart rate variability compared with OZRs treated with scrambled shRNA. Furthermore, AAV‐OXR1‐shRNA treatment markedly reduced renal sympathetic nerve activity and attenuated sympathoexcitatory response induced by microinjection of orexin A into the PVN. In addition, treatment with AAV‐OXR1‐shRNA substantially decreased the basal firing activity of spinally projecting PVN neurons in OZRs and attenuated the excitatory effect of orexin A on the firing activity of these neurons.

Conclusions

These data suggest that chronic downregulation of OXR1 expression in the PVN reduces sympathetic vasomotor tone in obesity‐related hypertension.

Sleep-Wake Cycling and Energy Conservation: Role of Hypocretin and the Lateral Hypothalamus in Dynamic State-Dependent Resource Optimization

The hypocretin (Hcrt) system has been implicated in a wide range of physiological functions from sleep-wake regulation to cardiovascular, behavioral, metabolic, and thermoregulagtory control. These wide-ranging physiological effects have challenged the identification of a parsimonious function for Hcrt. A compelling hypothesis suggests that Hcrt plays a role in the integration of sleep-wake neurophysiology with energy metabolism. For example, Hcrt neurons promote waking and feeding, but are also sensors of energy balance. Loss of Hcrt function leads to an increase in REM sleep propensity, but a potential role for Hcrt linking energy balance with REM sleep expression has not been addressed. Here we examine a potential role for Hcrt and the lateral hypothalamus (LH) in state-dependent resource allocation as a means of optimizing resource utilization and, as a result, energy conservation. We review the energy allocation hypothesis of sleep and how state-dependent metabolic partitioning may contribute toward energy conservation, but with additional examination of how the loss of thermoregulatory function during REM sleep may impact resource optimization. Optimization of energy expenditures at the whole organism level necessitates a top-down network responsible for coordinating metabolic operations in a state-dependent manner across organ systems. In this context, we then specifically examine the potential role of the LH in regulating this output control, including the contribution from both Hcrt and melanin concentrating hormone (MCH) neurons among a diverse LH cell population. We propose that this hypothalamic integration system is responsible for global shifts in state-dependent resource allocations, ultimately promoting resource optimization and an energy conservation function of sleep-wake cycling.

The relationship between orexin levels and blood pressure changes in patients with narcolepsy

STUDY OBJECTIVE

Narcolepsy type 1 (NT1) is caused by a deficiency or absence of the neurotransmitter orexin. NT1 is also associated with a reduced nocturnal "dipping" of blood pressure (BP). The study objective was to analyze whether nocturnal BP values differed in patients depleted of orexin, versus those in whom production was preserved.

METHODS

We performed a retrospective analysis of the polysomnographic recordings, orexin levels, and BP values of patients with NT1. Data was collected from a total of 21 patients, divided into two groups as follows: those with a complete depletion of orexin (n = 11) (Group1), and those with a remaining, limited presence of orexin (n = 10) (Group 2).

RESULTS

The groups did not differ in terms of the clinical features of NT1 or sleep characteristics, with an exception of increased number of cataplexy episodes and increased percentage of sleep stage 2 in the Group 1. Daytime and nocturnal BP did not differ between the groups. Most patients, regardless of group, had a non-dipping blood pressure pattern, and no difference in dipping prevalence was observed between groups. The amplitude of the daytime to nighttime change in BP did not differ between the groups.

CONCLUSIONS

Non-dipping BP patterns are frequent among patients with narcolepsy type 1, but we saw no evidence that they depended on whether orexin levels were above or below the assay detection threshold. Therefore, our results do not support the hypothesis that in patients with narcolepsy type 1 residual orexin levels play a role in the control of nocturnal BP dipping.

Exercise increases the level of plasma orexin A in humans

BACKGROUND

The purpose of this research was to study the effects of exercise on the concentration of plasma orexin A, a peptide regulating several physiological functions.

METHODS

Blood samples were collected from participants (men, n=10; age: 24.4±2.93 years) 15, 0 min before the start of exercise, and 30, 45, 60 min after a cycle ergometer exercise at 75 W for 15 min. Also heart rate (HR), galvanic skin response (GSR), and rectal temperature were monitored.

RESULTS

The exercise induced a significant increase (p<0.01) in plasmatic orexin A with a peak at 30 min after the exercise bout, in association with an increase of the other three monitored variables: HR (p<0.01), GSR (p<0.05), and rectal temperature (p<0.01).

CONCLUSIONS

Our findings indicate that plasmatic orexin A is involved in the reaction to physical activity.

Orexins and the cardiovascular events of awakening

This brief review aims to provide an updated account of the cardiovascular events of awakening, proposing a testable conceptual framework that links these events with the neural control of sleep and the autonomic nervous system, with focus on the hypothalamic orexin (hypocretin) neurons. Awakening from non-rapid-eye-movement sleep entails coordinated changes in brain and cardiovascular activity: the neural "flip-flop" switch that governs state transitions becomes biased toward the ascending arousal systems, arterial blood pressure and heart rate rise toward waking values, and distal skin temperature falls. Arterial blood pressure and skin temperature are sensed by baroreceptors and thermoreceptors and may positively feedback on the brain wake-sleep switch, thus contributing to sharpen, coordinate, and stabilize awakening. These effects may be enhanced by the hypothalamic orexin neurons, which may modulate the changes in blood pressure, heart rate, and skin temperature upon awakening, while biasing the wake-sleep switch toward wakefulness through direct neural projections. A deeper understanding of the cardiovascular events of awakening and of their links with skin temperature and the wake-sleep neural switch may lead to better treatments options for patients with narcolepsy type 1, who lack the orexin neurons.

Attenuated cold defense responses in orexin neuron-ablated rats

Recent reports of the use of transgenic mice targeting orexin neurons show that the ablation of orexin neurons in the hypothalamus causes hypothermia during cold exposure. This suggests the importance of orexin neurons for cold-induced autonomic and physiological defense responses, including brown adipose tissue (BAT) thermogenesis and vasoconstriction in thermoregulatory cutaneous vascular bed. The present study investigated whether the ablation of orexin neurons attenuated cold-elicited BAT thermogenesis and cutaneous vasoconstriction. The study took advantage of our established conscious rat experimental model of direct measurement of BAT and body temperature and tail cutaneous blood flow. The study used transgenic orexin neurons-ablated (ORX-AB) rats and wild type (WT) rats. BAT temperature and tail artery blood flow with pre-implanted probes were measured, as well as behavioral locomotor activity under conscious free-moving condition. Gradually, the ambient temperature was decreased to below 5°C. ORX-AB rats showed an attenuated cold-induced BAT thermogenesis and behavioral activity, and delayed tail vasoconstriction. An ambient temperature that initiated BAT thermogenesis and established full cutaneous vasoconstriction was 14.1 ± 1.9 °C, which was significantly lower than 20.5 ± 1.9 °C, the corresponding value in WT rats (n = 10, P < 0.01). The results from this study suggest that the integrity of orexin-synthesising neurons in thermoregulatory networks is important for full expression of the cold defense responses.

"Non-dipping" is equally frequent in narcoleptic patients and in patients with insomnia

Narcolepsy with cataplexy (NC) is a neurological sleep disorder characterized by very low or undetectable concentration of hypocretin-1 in the cerebrospinal fluid. It has been recently found that patients with NC have disturbed circadian pattern of blood pressure, with more frequent non-dipping, compared to healthy controls. It has been hypothesized that lack of hypocretin may lead to increase in nocturnal blood pressure. This increase may result also from disturbed sleep architecture regardless of the deficiency of hypocretin. The aim of this study was to compare changes in values of daytime and nighttime blood pressure in NC patients and in patients with disturbed nocturnal sleep due to other sleep disorders. We have retrospectively compared polysomnographic and clinical data of 8 NC patients and 7 age- and sex controls suffering from insomnia. We have compared sleep architecture, mean blood pressure values and dipping pattern in both groups. The groups did not differ in terms of disturbances of sleep architecture. We have not found any statistical differences in values of daytime and nocturnal blood pressure. Non-dipping was equally frequent in both groups (87.5 and 85.7 %). Our results suggest that observed abnormalities in circadian changes of blood pressure values result from disturbed sleep architecture than from deficiency of hypocretin. Patients with sleep disorders should be carefully observed for the presence of increased blood pressure and other vascular risk factors.

Thermoregulation under pressure: a role for orexin neurons

In the past, studies on stress responses and sleep/wake regulation were performed separately. The discovery of orexin (hypocretin) in 1998, however, dramatically changed the course of research and new findings regarding its role in these complex processes provided a better insight into their interactions and intricacies. Orexin-containing neuronal activity has been found to be minimal during sleep. It increases during the waking period and further increases during the active waking period, which includes stress responses and exploratory behaviors. Autonomic regulation of the body, which includes body temperature, blood flow, and ventilation, is also activated along with the change in vigilance states. Our recent findings suggest that orexin neurons act as a conductor of orchestration for vigilance states, behaviors, and autonomic functions. Body temperature regulation by orexin neurons seems to be mediated by one of its cotransmitters while cardiovascular and respiratory regulation are mediated by orexin itself.

Estimation of the core temperature control during ambient temperature changes and the influence of circadian rhythm and metabolic conditions in mice

It has been speculated that the control of core temperature is modulated by physiological demands. We could not prove the modulation because we did not have a good method to evaluate the control. In the present study, the control of core temperature in mice was assessed by exposing them to various ambient temperatures (Ta), and the influence of circadian rhythm and feeding condition was evaluated. Male ICR mice (n=20) were placed in a box where Ta was increased or decreased from 27°C to 40°C or to -4°C (0.15°C/min) at 0800 and 2000 (daytime and nighttime, respectively). Intra-abdominal temperature (Tcore) was monitored by telemetry. The relationship between Tcore and Ta was assessed. The range of Ta where Tcore was relatively stable (range of normothermia, RNT) and Tcore corresponding to the RNT median (regulated Tcore) were estimated by model analysis. In fed mice, the regression slope within the RNT was smaller in the nighttime than in the daytime (0.02 and 0.06, respectively), and the regulated Tcore was higher in the nighttime than in the daytime (37.5°C and 36.0°C, respectively). In the fasted mice, the slope remained unchanged, and the regulated Tcore decreased in the nighttime (0.05 and 35.9°C, respectively), while the slopes in the daytime became greater (0.13). Without the estimating individual thermoregulatory response such as metabolic heat production and skin vasodilation, the analysis of the Ta-Tcore relationship could describe the character of the core temperature control. The present results show that the character of the system changes depending on time of day and feeding conditions.

Reduced brown adipose tissue thermogenesis during environmental interactions in transgenic rats with ataxin-3-mediated ablation of hypothalamic orexin neurons

Thermogenesis in brown adipose tissue (BAT) contributes to substantial increases in body temperature evoked by threatening or emotional stimuli. BAT thermogenesis also contributes to increases in body temperature that occur during active phases of the basic rest-activity cycle (BRAC), as part of normal daily life. Hypothalamic orexin-synthesizing neurons influence many physiological and behavioral variables, including BAT and body temperature. In conscious unrestrained animals maintained for 3 days in a quiet environment (24-26°C) with ad libitum food and water, we compared temperatures in transgenic rats with ablation of orexin neurons induced by expression of ataxin-3 (Orx_Ab) with wild-type (WT) rats. Both baseline BAT temperature and baseline body temperature, measured at the onset of BRAC episodes, were similar in Orx_Ab and WT rats. The time interval between BRAC episodes was also similar in the two groups. However, the initial slopes and amplitudes of BRAC-related increases in BAT and body temperature were reduced in Orx_Ab rats. Similarly, the initial slopes and amplitudes of the increases in BAT temperatures induced by sudden exposure to an intruder rat (freely moving or confined to a small cage) or by sudden exposure to live cockroaches were reduced in resident Orx_Ab rats. Constriction of the tail artery induced by salient alerting stimuli was also reduced in Orx_Ab rats. Our results suggest that orexin-synthesizing neurons contribute to the intensity with which rats interact with the external environment, both when the interaction is "spontaneous" and when the interaction is provoked by threatening or salient environmental events.

Why does sleep stop migraine?

The relationship between sleep and migraine headaches is complex. Changes in sleep patterns can trigger migraine attacks, and sleep disorders may be associated with increased migraine frequency. Furthermore, migraine patients and their doctors very consistently report that sleep relieves already established migraine attacks. Herein we will try to answer the question, "Why does sleep stop migraine?" Since evidence for this relationship is largely based on empirical clinical observation, we will not provide a clinical review of the association. Instead, we will focus on the pathophysiology of migraine attacks and its intersections with sleep biology.

Orexin, cardio-respiratory function, and hypertension

In this review we focus on the role of orexin in cardio-respiratory functions and its potential link to hypertension. (1) Orexin, cardiovascular function, and hypertension. In normal rats, central administration of orexin can induce significant increases in arterial blood pressure (ABP) and sympathetic nerve activity (SNA), which can be blocked by orexin receptor antagonists. In spontaneously hypertensive rats (SHRs), antagonizing orexin receptors can significantly lower blood pressure under anesthetized or conscious conditions. (2) Orexin, respiratory function, and central chemoreception. The prepro-orexin knockout mouse has a significantly attenuated ventilatory CO2 chemoreflex, and in normal rats, central application of orexin stimulates breathing while blocking orexin receptors decreases the ventilatory CO2 chemoreflex. Interestingly, SHRs have a significantly increased ventilatory CO2 chemoreflex relative to normotensive WKY rats and blocking both orexin receptors can normalize this exaggerated response. (3) Orexin, central chemoreception, and hypertension. SHRs have higher ABP and SNA along with an enhanced ventilatory CO2 chemoreflex. Treating SHRs by blocking both orexin receptors with oral administration of an antagonist, almorexant (Almxt), can normalize the CO2 chemoreflex and significantly lower ABP and SNA. We interpret these results to suggest that the orexin system participates in the pathogenesis and maintenance of high blood pressure in SHRs, and the central chemoreflex may be a causal link to the increased SNA and ABP in SHRs. Modulation of the orexin system could be a potential target in treating some forms of hypertension.

Orexin, cardio-respiratory function, and hypertension

In this review we focus on the role of orexin in cardio-respiratory functions and its potential link to hypertension. (1) Orexin, cardiovascular function, and hypertension. In normal rats, central administration of orexin can induce significant increases in arterial blood pressure (ABP) and sympathetic nerve activity (SNA), which can be blocked by orexin receptor antagonists. In spontaneously hypertensive rats (SHRs), antagonizing orexin receptors can significantly lower blood pressure under anesthetized or conscious conditions. (2) Orexin, respiratory function, and central chemoreception. The prepro-orexin knockout mouse has a significantly attenuated ventilatory CO₂ chemoreflex, and in normal rats, central application of orexin stimulates breathing while blocking orexin receptors decreases the ventilatory CO₂ chemoreflex. Interestingly, SHRs have a significantly increased ventilatory CO₂ chemoreflex relative to normotensive WKY rats and blocking both orexin receptors can normalize this exaggerated response. (3) Orexin, central chemoreception, and hypertension. SHRs have higher ABP and SNA along with an enhanced ventilatory CO₂ chemoreflex. Treating SHRs by blocking both orexin receptors with oral administration of an antagonist, almorexant (Almxt), can normalize the CO₂ chemoreflex and significantly lower ABP and SNA. We interpret these results to suggest that the orexin system participates in the pathogenesis and maintenance of high blood pressure in SHRs, and the central chemoreflex may be a causal link to the increased SNA and ABP in SHRs. Modulation of the orexin system could be a potential target in treating some forms of hypertension.

Antagonism of orexin receptors significantly lowers blood pressure in spontaneously hypertensive rats

In normal rats, central administration of orexin or exposure to certain forms of stress can induce significant increases in blood pressure and sympathetic nerve activity, which can be blocked by orexin receptor antagonists. The resting blood pressure is, however, unaffected by such antagonists, but is significantly lower in rodents with total loss of orexin, such as prepro-orexin knockout mice and orexin neuron-ablated orexin/ataxin-3 transgenic rats. We hypothesize that orexin is involved in the pathophysiology and maintenance of high blood pressure in the spontaneously hypertensive rat (SHR), a model of primary hypertension. To test this hypothesis, we measured orexin-A mRNA expression in the rostral ventrolateral medulla and antagonized both orexin receptors using an orally administered potent dual orexin receptor antagonist, almorexant, in SHRs and normotensive Wistar-Kyoto rats. In SHRs, there was a strong trend towards an increased orexin-A mRNA expression in the rostral ventrolateral medulla, and blocking orexin receptors markedly lowered blood pressure (from 182/152 ± 5/6 to 149/119 ± 9/8 mmHg; P < 0.001), heart rate (P < 0.001), sympathetic vasomotor tone (P < 0.001) and the noradrenaline levels in cerebrospinal fluid and plasma (P < 0.002). The significant antihypertensive effects of almorexant were observed in wakefulness and non-rapid eye movement sleep during both dark and light phases of the diurnal cycle only in SHRs. Blocking orexin receptors had no effect on blood pressure and sympathetic tone in normotensive Wistar-Kyoto rats. Our study links the orexin system to the pathogenesis of high blood pressure in SHRs and suggests that modulation of the orexin system could be a potential target in treating some forms of hypertension.

A unified survival theory of the functioning of the hypocretinergic system

This article advances the theory that the hypocretinergic (orexinergic) system initiates, coordinates, and maintains survival behaviors and survival-related processes (i.e., the Unified Survival Theory of the Functioning of the Hypocretinergic System or "Unified Hypocretinergic Survival Theory"). A priori presumptive support for the Unified Hypocretinergic Survival Theory emanates from the fact that neurons that contain hypocretin are located in the key executive central nervous system (CNS) site, the lateral hypothalamus, that for decades has been well-documented to govern core survival behaviors such as fight, flight, and food consumption. In addition, the hypocretinergic system exhibits the requisite morphological and electrophysiological capabilities to control survival behaviors and related processes. Complementary behavioral data demonstrate that all facets of "survival" are coordinated by the hypocretinergic system and that hypocretinergic directives are not promulgated except during survival behaviors. Importantly, it has been shown that survival behaviors are selectively impacted when the hypocretinergic system is impaired or rendered nonfunctional, whereas other behaviors are relatively unaffected. The Unified Hypocretinergic Survival Theory resolves the disparate, perplexing, and often paradoxical-appearing results of previous studies; it also provides a foundation for future hypothesis-driven basic science and clinical explorations of the hypocretinergic system.

Chemoreceptors, baroreceptors, and autonomic deregulation in children with obstructive sleep apnea

Obstructive sleep apnea (OSA) is highly prevalent sleep disorder of breathing in both adults and children that is fraught with substantial cardiovascular morbidities, the latter being attributable to a complex interplay between intermittent hypoxia (IH), episodic hypercapnia, recurrent large intra-thoracic pressure swings, and sleep disruption. Alterations in autonomic nervous system function could underlie the perturbations in cardiovascular, neurocognitive, immune, endocrine and metabolic functions that affect many of the patients suffering from OSA. Although these issues have received substantial attention in adults, the same has thus far failed to occur in children, creating a quasi misperception that children are protected. Here, we provide a critical overview of the evidence supporting the presence of autonomic nervous system (ANS) perturbations in children with OSA, draw some parallel assessments to known mechanisms in rodents and adult humans, particularly, peripheral and central chemoreceptor and baroreceptor pathways, and suggest future research directions.

Abnormal sleep-cardiovascular system interaction in narcolepsy with cataplexy: effects of hypocretin deficiency in humans

STUDY OBJECTIVE

Narcolepsy with cataplexy (NC) is associated with loss of hypocretin neurons in the lateral hypothalamus involved in the circadian timing of sleep and wakefulness, and many biologic functions including autonomic control. The authors investigated whether chronic lack of hypocretin signaling alters cardiovascular control during sleep in humans.

DESIGN

Comparison of 24-hr circadian rhythms, day-night, time- and state-dependent changes of blood pressure (BP) and heart rate (HR) in drug-free patients with NC and control subjects.

SETTING

University hospital.

PATIENTS OR PARTICIPANTS

Ten drug-free patients with NC (9 men, 1 woman) and 12 control subjects (9 men, 3 women).

INTERVENTIONS

N/A.

MEASUREMENTS AND RESULTS

Daytime BP was comparable in patients with NC and controls, but patients with NC displayed a nighttime nondipping BP pattern. The 24-hr circadian rhythmicity of BP and HR was normal in both groups. Systolic BP during nighttime rapid eye movement sleep was significantly increased in the NC group. The 24-hr HR was significantly higher in the NC group but the day-night and state-dependent HR modulations were intact. The nighttime BP pattern coupled in the NC group with increased sleep fragmentation and a higher prevalence of arousals, periodic limb movements in sleep (PLMS), and PLMS arousals. In an analysis of the sleep/cardiovascular interaction in the periods after sleep onset and preceding morning awakening, only PLMS were consistently associated with the blunted nighttime decrease in BP in the NC group.

CONCLUSIONS

Hypocretin deficiency in humans may couple with an altered nighttime BP regulation that can be associated with an increased cardiovascular risk. This finding may be the result not only of the hypocretinergic deficiency per se but also of the altered sleep/wake regulation characterizing NC.

Control of cardiovascular variability during undisturbed wake-sleep behavior in hypocretin-deficient mice

The central neural mechanisms underlying differences in cardiovascular variability between wakefulness, non-rapid-eye-movement sleep (NREMS), and rapid-eye-movement sleep (REMS) remain poorly understood. These mechanisms may involve hypocretin (HCRT)/orexin signaling. HCRT signaling is linked to wake-sleep states, involved in central autonomic control, and impaired in narcoleptic patients. Thus, we investigated whether HCRT signaling plays a role in controlling cardiovascular variability during spontaneous behavior in HCRT-deficient mice. HCRT-ataxin3 transgenic mice lacking HCRT neurons (TG), knockout mice lacking HCRT peptides (KO), and wild-type controls (WT) were instrumented with electrodes for sleep recordings and a telemetric blood pressure transducer. Fluctuations of systolic blood pressure (SBP) and heart period (HP) during undisturbed wake-sleep behavior were analyzed with the sequence technique, cross-correlation functions, and coherent averaging of SBP surges. During NREMS, all mice had lower SBP variability, greater baroreflex contribution to HP control at low frequencies, and greater amplitude of the central autonomic and baroreflex changes in HP associated with SBP surges than during wakefulness. During REMS, all mice had higher SBP variability and depressed central autonomic and baroreflex HP controls relative to NREMS. HP variability during REMS was higher than during NREMS in WT only. TG and KO also had lower amplitude of the cardiac baroreflex response to SBP surges during REMS than WT. These results indicate that chronic lack of HCRT signaling may cause subtle alterations in the control of HP during spontaneous behavior. Conversely, the integrity of HCRT signaling is not necessary for the occurrence of physiological sleep-dependent changes in SBP variability.

Sympathetic and catecholaminergic alterations in sleep apnea with particular emphasis on children

Sleep is involved in the regulation of major organ functions in the human body, and disruption of sleep potentially can elicit organ dysfunction. Obstructive sleep apnea (OSA) is the most prevalent sleep disorder of breathing in adults and children, and its manifestations reflect the interactions between intermittent hypoxia, intermittent hypercapnia, increased intra-thoracic pressure swings, and sleep fragmentation, as elicited by the episodic changes in upper airway resistance during sleep. The sympathetic nervous system is an important modulator of the cardiovascular, immune, endocrine and metabolic systems, and alterations in autonomic activity may lead to metabolic imbalance and organ dysfunction. Here we review how OSA and its constitutive components can lead to perturbation of the autonomic nervous system in general, and to altered regulation of catecholamines, both of which then playing an important role in some of the mechanisms underlying OSA-induced morbidities.

Orexins, feeding, and energy balance

In this chapter, we give an overview of the current status of the role of orexins in feeding and energy homeostasis. Orexins, also known as hypocretins, initially were discovered in 1998 as hypothalamic regulators of food intake. A little later, their far more important function as regulators of sleep and arousal came to light. Despite their restricted distribution, orexin neurons have projections throughout the entire brain, with dense projections especially to the paraventricular nucleus of the thalamus, the arcuate nucleus of the hypothalamus, and the locus coeruleus and tuberomammillary nucleus. Its two receptors are orexin receptor 1 and orexin receptor 2. These receptors show a specific and localized distribution in a number of brain regions, and a variety of different actions has been demonstrated upon their binding. Our group showed that through the autonomic nervous system, the orexin system plays a key role in the control of glucose metabolism, but it has also been shown to stimulate sympathetic outflow, to increase body temperature, heart rate, blood pressure, and renal sympathetic nerve activity. The well-known effects of orexin on the control of food intake, arousal, and wakefulness appear to be more extensive than originally thought, with additional effects on the autonomic nervous system, that is, to increase body temperature and energy metabolism.

Respiration and autonomic regulation and orexin

Orexin, a small neuropeptide released from neurons in the hypothalamus with widespread projections throughout the central nervous system, has broad biological roles including the modulation of breathing and autonomic function. That orexin activity is fundamentally dependent on sleep-wake state, and circadian cycle requires consideration of orexin function in physiological control systems in respect to these two state-related activity patterns. Both transgenic mouse studies and focal orexin receptor antagonism support a role for orexins in respiratory chemosensitivity to CO₂ predominantly in wakefulness, with further observations limiting this role to the dark period. In addition, orexin neurons participate in the regulation of sympathetic activity, including effects on blood pressure and thermoregulation. Orexin is also essential in physiological responses to stress. Orexin-mediated processes may operate at two levels: (1) in sleep-wake and circadian states and (2) in stress, for example, the defense or "fight-or-flight" response and panic-anxiety syndrome.

Orexin neurons and emotional stress

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

Cerebrospinal fluid hypocretin (orexin) levels are elevated by play but are not raised by exercise and its associated heart rate, blood pressure, respiration or body temperature changes

Hypocretin (Hcrt) has been implicated in the control of motor activity and in respiration and cardiovascular changes. Loss of Hcrt in narcolepsy is linked to sleepiness and to cataplexy, a sudden loss of muscle tone which is triggered by sudden strong emotions. In the current study we have compared the effects of treadmill running, to yard play on cerebrospinal fluid (CSF) Hcrt level in normal dogs. We find that treadmill locomotion, at a wide range of speeds, does not increase Hcrt level beyond baseline, whereas yard play produces a substantial increase in Hcrt, even though both activities produce comparable increases in heart rate, respiration and body temperature. We conclude that motor and cardiovascular changes are not sufficient to elevate CSF levels of Hcrt and we hypothesize that the emotional aspects of yard play account for the observed increase in Hcrt.