Orexin, cardio-respiratory function, and hypertension

Increased Expression of Orexin-A in Patients Affected by Polycystic Kidney Disease

Orexin-A is a neuropeptide product of the lateral hypothalamus that acts on two receptors, OX1R and OX2R. The orexinergic system is involved in feeding, sleep, and pressure regulation. Recently, orexin-A levels have been found to be negatively correlated with renal function. Here, we analyzed orexin-A levels as well as the incidence of SNPs in the hypocretin neuropeptide precursor (HCRT) and its receptors, HCRTR1 and HCRTR2, in 64 patients affected by autosomal dominant polycystic kidney disease (ADPKD) bearing truncating mutations in the PKD1 or PKD2 genes. Twenty-four healthy volunteers constituted the control group. Serum orexin-A was assessed by ELISA, while the SNPs were investigated through Sanger sequencing. Correlations with the main clinical features of PKD patients were assessed. PKD patients showed impaired renal function (mean eGFR 67.8 ± 34.53) and a statistically higher systolic blood pressure compared with the control group (p < 0.001). Additionally, orexin-A levels in PKD patients were statistically higher than those in healthy controls (477.07 ± 69.42 pg/mL vs. 321.49 ± 78.01 pg/mL; p < 0.001). Furthermore, orexin-A inversely correlated with blood pressure (p = 0.0085), while a direct correlation with eGFR in PKD patients was found. None of the analyzed SNPs showed any association with orexin-A levels in PKD. In conclusion, our data highlights the emerging role of orexin-A in renal physiology and its potential relevance to PKD. Further research is essential to elucidate the intricate mechanisms underlying orexin-A signaling in renal function and its therapeutic implications for PKD and associated cardiovascular complications.

Systematic characterization of regulatory variants of blood pressure genes

High blood pressure (BP) is the major risk factor for cardiovascular disease. Genome-wide association studies have identified genetic variants for BP, but functional insights into causality and related molecular mechanisms lag behind. We functionally characterize 4,608 genetic variants in linkage with 135 BP loci in vascular smooth muscle cells and cardiomyocytes by massively parallel reporter assays. High densities of regulatory variants at BP loci (i.e., ULK4, MAP4, CFDP1, PDE5A) indicate that multiple variants drive genetic association. Regulatory variants are enriched in repeats, alter cardiovascular-related transcription factor motifs, and spatially converge with genes controlling specific cardiovascular pathways. Using heuristic scoring, we define likely causal variants, and CRISPR prime editing finally determines causal variants for KCNK9, SFXN2, and PCGF6, which are candidates for developing high BP. Our systems-level approach provides a catalog of functionally relevant variants and their genomic architecture in two trait-relevant cell lines for a better understanding of BP gene regulation.

Estrogens, age, and, neonatal stress: panic disorders and novel views on the contribution of non-medullary structures to respiratory control and CO2 responses

CO₂ is a fundamental component of living matter. This chemical signal requires close monitoring to ensure proper match between metabolic production and elimination by lung ventilation. Besides ventilatory adjustments, CO₂ can also trigger innate behavioral and physiological responses associated with fear and escape but the changes in brain CO₂/pH required to induce ventilatory adjustments are generally lower than those evoking fear and escape. However, for patients suffering from panic disorder (PD), the thresholds for CO₂-evoked hyperventilation, fear and escape are reduced and the magnitude of those reactions are excessive. To explain these clinical observations, Klein proposed the false suffocation alarm hypothesis which states that many spontaneous panics occur when the brain's suffocation monitor erroneously signals a lack of useful air, thereby maladaptively triggering an evolved suffocation alarm system. After 30 years of basic and clinical research, it is now well established that anomalies in respiratory control (including the CO₂ sensing system) are key to PD. Here, we explore how a stress-related affective disorder such as PD can disrupt respiratory control. We discuss rodent models of PD as the concepts emerging from this research has influenced our comprehension of the CO₂ chemosensitivity network, especially structure that are not located in the medulla, and how factors such as stress and biological sex modulate its functionality. Thus, elucidating why hormonal fluctuations can lead to excessive responsiveness to CO₂ offers a unique opportunity to gain insights into the neuroendocrine mechanisms regulating this key aspect of respiratory control and the pathophysiology of respiratory manifestations of PD.

The effect of orexin on the hypoxic ventilatory response of female rats is greatest in the active phase during diestrus

We recently showed that in male rats, orexin contributes to the hypoxic ventilatory response (HVR), with a stronger effect in the active phase. The effect of orexin on the HVR in females has not been investigated. As estrogen can inhibit orexin neurons, here we hypothesized that orexin neurons are activated by hypoxia and facilitate the HVR only in diestrus, when estrogen is low. We exposed female rats (n = 10) to near-isocapnic hypoxia ([Formula: see text] from 0.21 to 0.09) over ∼5 min, after vehicle and again after suvorexant (a dual OxR antagonist; 20 mg/kg ip), with ventilation measured using whole body plethysmography. Each rat was tested in proestrus or estrus (p/estrus), and again in diestrus, during both inactive and active phases. We also performed immunohistochemistry (IHC) to determine the proportion of orexin neurons activated by acute hypoxia during diestrus (n = 6) or proestrus/estrus (n = 6) in the active phase. In the inactive phase, the HVR was unaffected by OxR blockade, irrespective of estrus stage. In the active phase, the effect of OxR blockade depended on stage: the slope of the HVR was significantly reduced by OxR blockade only during diestrus. IHC revealed that hypoxia activated more orexin neurons during diestrus compared with p/estrus. We conclude that in females, orexin neurons are activated by hypoxia and contribute to the HVR only in diestrus when estrogen levels are low. Stage of the estrus cycle should be considered when examining the physiological function of orexin neurons in females.NEW & NOTEWORTHY We previously showed that orexin facilitates the hypoxic ventilatory response (HVR) of adult male rats during the active phase. Others have shown that estrogen inhibits orexin neurons. Here we show that orexin neurons are activated by hypoxia and facilitate the HVR of adult female rats during the active phase, but only in diestrus. These data suggest that orexin neurons facilitate the HVR in females when they are free from the inhibitory effects of estrogen.

Sympathetic neural responses to sleep disorders and insufficiencies

Short sleep duration and poor sleep quality are associated with cardiovascular risk, and sympathetic nervous system (SNS) dysfunction appears to be a key contributor. The present review will characterize sympathetic function across several sleep disorders and insufficiencies in humans, including sleep deprivation, insomnia, narcolepsy, and obstructive sleep apnea (OSA). We will focus on direct assessments of sympathetic activation, e.g., plasma norepinephrine and muscle sympathetic nerve activity, but include heart rate variability (HRV) when direct assessments are lacking. The review also highlights sex as a key biological variable. Experimental models of total sleep deprivation and sleep restriction are converging to support several epidemiological studies reporting an association between short sleep duration and hypertension, especially in women. A systemic increase of SNS activity via plasma norepinephrine is present with insomnia and has also been confirmed with direct, regionally specific evidence from microneurographic studies. Narcolepsy is characterized by autonomic dysfunction via both HRV and microneurographic studies but with opposing conclusions regarding SNS activation. Robust sympathoexcitation is well documented in OSA and is related to baroreflex and chemoreflex dysfunction. Treatment of OSA with continuous positive airway pressure results in sympathoinhibition. In summary, sleep disorders and insufficiencies are often characterized by sympathoexcitation and/or sympathetic/baroreflex dysfunction, with several studies suggesting women may be at heightened risk.

Orexin/Hypocretin System Dysfunction in ESSENCE (Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations)

Early Symptomatic Syndromes Eliciting Neurodevelopmental Clinical Examinations (ESSENCE) is an umbrella term covering a wide range of neurodevelopmental difficulties and disorders. Thus, ESSENCE includes attention-deficit/hyperactivity disorder (ADHD), autism spectrum disorder (ASD), and other neurodevelopmental disorders (NDDs) and difficulties, with a variety of symptoms in cognitive, motor, sensory, social, arousal, regulatory, emotional, and behavioral developmental domains, frequently co-occurring and likely having partly common neurobiological substrates. The ESSENCE concept is a clinical paradigm that promotes organizing NDDs in everyday clinical practice according to their coexistence, symptom dimensions overlapping, and treatment possibilities. Despite increased knowledge regarding NDDs, the neurobiological mechanisms that underlie them and other ESSENCE-related problems, are not well understood. With its wide range of neural circuits and interactions with numerous neurotransmitters, the orexin/hypocretin system (Orx-S) is possibly associated with a variety of neurocognitive, psychobiological, neuroendocrine, and physiological functions and behaviors. Dysfunction of Orx-S has been implicated in various psychiatric and neurological disorders. This article provides an overview of Orx-S dysfunctions' possible involvement in the development, presentation, and maintenance of ESSENCE. We provide a focused review of current research evidence linking orexin neuropeptides with specific clinical NDDs symptoms, mostly in ADHD and ASD, within the Research Domain Criteria (RDoC) framework. We propose that Orx-S dysfunction might have an important role in some of these neurodevelopmental symptom domains, such as arousal, wakefulness, sleep, motor and sensory processing, mood and emotional regulation, fear processing, reward, feeding, attention, executive functions, and sociability. Our perspective is presented from a clinical point of view. Further, more thorough systematic reviews are needed as well as planning of extensive new research into the Orx-S's role in ESSENCE, especially considering RDoC elements.

Central respiratory chemoreception

Brain PCO2 is sensed primarily via changes in [H+]. Small pH changes are detected in the medulla oblongata and trigger breathing adjustments that help maintain arterial PCO2 constant. Larger perturbations of brain CO2/H+, possibly also sensed elsewhere in the CNS, elicit arousal, dyspnea, and stress, and cause additional breathing modifications. The retrotrapezoid nucleus (RTN), a rostral medullary cluster of glutamatergic neurons identified by coexpression of Phoxb and Nmb transcripts, is the lynchpin of the central respiratory chemoreflex. RTN regulates breathing frequency, inspiratory amplitude, and active expiration. It is exquisitely responsive to acidosis in vivo and maintains breathing autorhythmicity during quiet waking, slow-wave sleep, and anesthesia. The RTN response to [H+] is partly an intrinsic neuronal property mediated by proton sensors TASK-2 and GPR4 and partly a paracrine effect mediated by astrocytes and the vasculature. The RTN also receives myriad excitatory or inhibitory synaptic inputs including from [H+]-responsive neurons (e.g., serotonergic). RTN is silenced by moderate hypoxia. RTN inactivity (periodic or sustained) contributes to periodic breathing and, likely, to central sleep apnea. RTN development relies on transcription factors Egr2, Phox2b, Lbx1, and Atoh1. PHOX2B mutations cause congenital central hypoventilation syndrome; they impair RTN development and consequently the central respiratory chemoreflex.

Activation of Orexin System Stimulates CaMKII Expression

Hyperactivity of the orexin system within the paraventricular nucleus (PVN) has been shown to contribute to increased sympathetic nerve activity (SNA) and blood pressure (BP) in rodent animals. However, the underlying molecular mechanisms remain unclear. Here, we test the hypothesis that orexin system activation stimulates calcium/calmodulin-dependent kinase II (CaMKII) expression and activation, and stimulation of CaMKII expressing PVN neurons increases SNA and BP. Real-time PCR and/or western blot were carried out to test the effect of orexin-A administration on CaMKII expression in the PVN of normal Sprague Dawley (SD) rats and orexin receptor 1 (OX1R) expressing PC12 cells. Immunostaining was performed to assess OX1R cellular localization in the PVN of SD rats as well as orexin-A treatment on CaMKII activation in cultured hypothalamic neurons. In vivo sympathetic nerve recordings were employed to test the impact of optogenetic stimulation of CaMKII-expressing PVN neurons on the renal SNA (RSNA) and BP. The results showed that intracerebroventricular injection of orexin-A into the SD rat increases mRNA expression of CaMKII subunits in the PVN. In addition, Orexin-A treatment increases CaMKII expression and its phosphorylation in OX1R-expressing PC12 cells. Furthermore, Orexin-A treatment increases CaMKII activation in cultured hypothalamic neurons from neonatal SD rats. Finally, optogenetic excitation of PVN CaMKII-expressing neurons results in robust increases in RSNA and BP in SD rats. Our results suggest that increased orexin system activity activates CaMKII expression in cardiovascular relevant regions, and this may be relevant to the downstream cardiovascular effects of CaMKII.

Effect of the new dual orexin receptor antagonist daridorexant on nighttime respiratory function and sleep in patients with mild and moderate obstructive sleep apnea

In this randomized, double-blind, placebo-controlled, two-period crossover study, the effect of the dual orexin receptor antagonist daridorexant was evaluated on nighttime respiratory function and sleep in 28 patients with mild and moderate obstructive sleep apnea (OSA). In each period, 50 mg daridorexant or placebo was administered every evening for 5 days. The primary endpoint was apnea/hypopnea index (AHI) during total sleep time (TST) after the last dosing. Other endpoints included peripheral oxygen saturation (SpO2), sleep duration, latency to persistent sleep (LPS), wake after sleep onset (WASO), and sleep efficiency index (SEI). Pharmacokinetics, safety, and tolerability were also assessed. The mean treatment difference for AHI during TST (i.e. daridorexant - placebo) after the last dosing was 0.74 events/hour (90% confidence interval [CI]: -1.43, 2.92). The corresponding treatment difference for SpO2 during TST was 0.16% [90% CI: -0.21, 0.53]. Overall, there was no clinically relevant effect of daridorexant on AHI or SpO2-related data after single and repeated dosing irrespective of sleep phase (i.e. rapid eye movement [REM] vs non-REM). Moreover, after single and repeated dosing, daridorexant prolonged TST by 39.6 minutes (90% CI: 16.9, 62.3) and 38.8 minutes (19.7, 57.9), respectively, compared with placebo and favorably modulated other sleep-related endpoints (i.e. increased SEI, decreased WASO, and shortened LPS). It attained expected plasma concentrations and was well tolerated in patients with mild and moderate OSA. These results indicate that single and repeated doses of 50 mg daridorexant do not impair nighttime respiratory function and improve sleep in patients with mild and moderate OSA. Clinical Trial Registration: ClinicalTrials.gov NCT03765294. A study to investigate the effects of ACT-541468 on nighttime respiratory function in patients with mild to moderate obstructive sleep apnea. https://clinicaltrials.gov/ct2/show/ NCT03765294.

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

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.

Sleep disorders in Prader-Willi syndrome, evidence from animal models and humans

Prader-Willi Syndrome (PWS) is a complex genetic disorder with multiple cognitive, behavioral and endocrine dysfunctions. Sleep alterations and sleep disorders such as Sleep-disordered breathing and Central disorders of hypersomnolence are frequently recognized (either isolated or in comorbidity). The aim of the review is to highlight the pathophysiology and the clinical features of sleep disorders in PWS, providing the basis for early diagnosis and management. We reviewed the genetic features of the syndrome and the possible relationship with sleep alterations in animal models, and we described sleep phenotypes, diagnostic tools and therapeutic approaches in humans. Moreover, we performed a meta-analysis of cerebrospinal fluid orexin levels in patients with PWS; significantly lower levels of orexin were detected in PWS with respect to control subjects (although significantly higher than the ones of narcoleptic patients). Sleep disorders in humans with PWS are multifaceted and are often the result of different mechanisms. Since hypothalamic dysfunction seems to partially influence metabolic, respiratory and sleep/wake characteristics of this syndrome, additional studies are required in this framework.

Effect of the novel dual orexin receptor antagonist daridorexant on night-time respiratory function and sleep in patients with moderate chronic obstructive pulmonary disease

In patients with chronic obstructive pulmonary disease (COPD), sleep is often fragmented while, conversely, the use of sleep medications is of concern in these patients due to potential impairment of nocturnal breathing. This randomised, double-blind, placebo-controlled, two-period crossover study was conducted to evaluate the effect of the new dual orexin receptor antagonist daridorexant on night-time respiratory function and sleep in patients with moderate COPD. In each period, the highest Phase-III dose of 50 mg daridorexant or placebo was administered once daily in the evening for 5 consecutive days. The primary endpoint was peripheral oxygen saturation (SpO₂ ) during total sleep time (TST) after last dosing. Night-time respiratory function and sleep were further evaluated based on the apnea-hypopnea index (AHI), sleep duration, and objective sleep parameters. Pharmacokinetics, safety, and tolerability were also assessed. Primary endpoint analysis revealed no significant mean treatment difference (i.e. daridorexant - placebo) for SpO₂ during TST as it was 0.18% (90% confidence interval: -0.21 to 0.57). There was also no difference from placebo for SpO₂ during non-rapid eye movement (REM) and REM sleep at Night 5 and after first dosing. The AHI was slightly increased compared to placebo, but not to a clinically meaningful extent. In addition, daridorexant improved objective sleep parameters (i.e. prolonged TST, increased sleep efficiency, and decreased wake after sleep onset), reached expected plasma concentrations, and was safe and well tolerated. In conclusion, single and multiple doses of 50 mg daridorexant do not impair night-time respiratory function and improves sleep in patients with moderate COPD.

Effects of intracerebroventricular leptin and orexin-A on the baroreflex control of renal sympathetic nerve activity in conscious rats fed a normal or high-fat diet

This study examined the effect of leptin and orexin-A on autonomic baroreflex control in conscious Wistar rats exposed to high-fat (45% fat) or normal (3.4%) diet for 4 weeks. Renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) were monitored during the generation of baroreflex gain curves and acute volume expansion (VEP). Intracerebroventricular (ICV) leptin (1 μg/min) increased RSNA in the normal diet group (0.31 ± 0.04 vs 0.23 ± 0.03 mV/s) and MAP in the high-fat diet group (115 ± 5 vs 105 ± 5 mm Hg, P < .05). Orexin-A (50 ng/min) increased RSNA, HR and MAP in the high-fat diet group (0.26 ± 0.03 vs 0.22 ± 0.02 mV/s, 454 ± 8 vs 417 ± 12 beats/min, 117 ± 1 vs 108 ± 1 mm Hg) and the normal diet group (0.18 ± 0.05 vs 0.17 ± 0.05 mV/s, 465 ± 10 vs 426 ± 6 beats/min, 116 ± 2 vs 104 ± 3 mm Hg). Baroreflex sensitivity for RSNA was increased during ICV leptin by 50% in the normal diet group, compared to 14% in the high-fat diet group (P < .05). Similarly, orexin-A increased baroreflex sensitivity by 56% and 50% in the high-fat and normal diet groups, respectively (all P < .05). During ICV saline, VEP decreased RSNA by 31 ± 5% (P < .05) after 10 minutes and the magnitude of this response was blunted during ICV infusion of leptin (17 ± 2%, P < .05) but not orexin-A in the normal diet group. RSNA response to VEP was not changed during ICV leptin or orexin-A in the high-fat diet group. These findings indicate possible central roles for leptin and orexin-A in modulating the baroreflexes under normal or increased fat intake in conscious rats and potential therapeutic approaches for obesity associated hypertension.

Contribution of retrotrapezoid nucleus neurons to CO2 -amplified cardiorespiratory activity in spontaneously hypertensive rats

KEY POINTS

This study demonstrates that both CO₂ -induced respiratory and cardiovascular responses are augmented in spontaneously hypertensive rats (SHRs). Genetic ablation of the retrotrapezoid nucleus (RTN) neurons depresses enhanced hypercapnic ventilatory response and eliminates CO₂ -stimulated increase in arterial pressure and heart rate in SHRs. SHRs have a high protein level of pH-sensitive channels in the RTN, including the TASK-2 channel, Kv12.1 channel and acid-sensing ion channel 3. The inhibition of putative TASK-2 channel activity by clofilium diminishes amplified hypercapnic ventilatory and cardiovascular responses, and reduces the number of CO₂ -activated RTN neurons in SHRs. These results indicate that RTN neurons contribute to enhanced CO₂ -stimulated respiratory and cardiovascular responses in SHRs.

ABSTRACT

The respiratory regulation of cardiovascular activity is essential for maintaining an efficient ventilation and perfusion ratio. Activation of central respiratory chemoreceptors not only elicits a ventilatory response but also regulates sympathetic nerve activity and arterial blood pressure (ABP). The retrotrapezoid nucleus (RTN) is the most completely characterized cluster of central respiratory chemoreceptors. We hypothesize that RTN neurons contribute to augmented CO₂ -stimulated respiratory and cardiovascular responses in adult spontaneously hypertensive rats (SHRs). Our findings indicate that SHRs exhibit more enhanced hypercapnic cardiorespiratory responses than age-matched normotensive Wistar-Kyoto rats. Genetic ablation of RTN neurons notably depresses an enhanced hypercapnic ventilatory response (HCVR) and eliminates a CO₂ -stimulated greater increase in ABP and heart rate in SHRs. In addition, SHRs have a higher protein level of pH-sensitive channels in the RTN, including TASK-2 channels, Kv12.1 channels and acid-sensing ion channel 3. Administration of clofilium (i.p.), an unselective inhibitor of TASK-2 channels, not only significantly reduces the enhanced HCVR but also inhibits CO₂ -amplified increases in ABP and heart rate in SHRs. Moreover, clofilium significantly decreases the number of CO₂ -activated RTN neurons in SHRs. Taken together, we suggest that RTN neurons play an important role in enhanced hypercapnic ventilatory and cardiovascular responses in SHRs and the putative mechanism involved is associated with TASK-2 channel activity in the RTN.

Disruption of estradiol regulation of orexin neurons: a novel mechanism in excessive ventilatory response to CO2 inhalation in a female rat model of panic disorder

Panic disorder (PD) is ~2 times more frequent in women. An excessive ventilatory response to CO2 inhalation is more likely during the premenstrual phase. While ovarian hormones appear important in the pathophysiology of PD, their role remains poorly understood as female animals are rarely used in pre-clinical studies. Using neonatal maternal separation (NMS) to induce a "PD-like" respiratory phenotype, we tested the hypothesis that NMS disrupts hormonal regulation of the ventilatory response to CO2 in female rats. We then determined whether NMS attenuates the inhibitory actions of 17-β estradiol (E2) on orexin neurons (ORX). Pups were exposed to NMS (3 h/day; postnatal day 3-12). The ventilatory response to CO2-inhalation was tested before puberty, across the estrus cycle, and following ovariectomy. Plasma E2 and hypothalamic ORXA were measured. The effect of an ORX1 antagonist (SB334867; 15 mg/kg) on the CO2 response was tested. Excitatory postsynaptic currents (EPSCs) were recorded from ORX neurons using whole-cell patch-clamp. NMS-related increase in the CO2 response was observed only when ovaries were functional; the largest ventilation was observed during proestrus. SB334867 blocked this effect. NMS augmented levels of ORXA in hypothalamus extracts. EPSC frequency varied according to basal plasma E2 levels across the estrus cycle in controls but not NMS. NMS reproduces developmental and cyclic changes of respiratory manifestations of PD. NMS disrupts the inhibitory actions of E2 on the respiratory network. Impaired E2-related inhibition of ORX neurons during proestrus is a novel mechanism in respiratory manifestations of PD in females.

Interaction of Orexin A and Vasopressin in the Brain Plays a Role in Blood Pressure Regulation in WKY and SHR Rats

Background

Orexin A (OXA) and vasopressin (AVP) exert a central hypertensive effect due to an increase in sympathetic nerve activity. To date, little is known about the interaction of these 2 neuropeptides in the central regulation of blood pressure. The present study compared the consequences of infusion into the left cerebral ventricle (ICV) of OXA on mean arterial blood pressure (MABP) in normotensive (WKY) and spontaneously hypertensive (SHR) rats, and explored whether the central pressor action of OXA in these 2 strains depends on activation of brain AVP V1a receptors (V1aR).

Material/Methods

Ten groups of experiments were performed on 12-week-old WKY and SHR rats implanted with ICV cannulas for infusion of OXA (3 nmol) and V1aR antagonist (V1aRANT, 500 ng), administered separately and together. Levels of V1aR and OXR in the medulla oblongata of WKY and SHR rats were compared in separate series.

Results

We found that: 1) OXA significantly increased MABP only in WKY rats, 2) V1aRANT prevented an increase in MABP induced by OXA in WKY rats and decreased MABP in SHR rats, 3) OXA abolished the hypotensive action of V1aRANT in SHR rats, and 4) SHR rats had significantly higher levels of OX1R and V1aR proteins and OX1R mRNA in the brain medulla.

Conclusions

The present study shows that OXA and AVP can interact in the brain to affect blood pressure regulation, and that this interaction differs in normotension and hypertension.

Neuronal Networks in Hypertension: Recent Advances

Neurogenic hypertension is associated with excessive sympathetic nerve activity to the kidneys and portions of the cardiovascular system. Here we examine the brain regions that cause heightened sympathetic nerve activity in animal models of neurogenic hypertension, and we discuss the triggers responsible for the changes in neuronal activity within these regions. We highlight the limitations of the evidence and, whenever possible, we briefly address the pertinence of the findings to human hypertension. The arterial baroreflex reduces arterial blood pressure variability and contributes to the arterial blood pressure set point. This set point can also be elevated by a newly described cerebral blood flow-dependent and astrocyte-mediated sympathetic reflex. Both reflexes converge on the presympathetic neurons of the rostral medulla oblongata, and both are plausible causes of neurogenic hypertension. Sensory afferent dysfunction (reduced baroreceptor activity, increased renal, or carotid body afferent) contributes to many forms of neurogenic hypertension. Neurogenic hypertension can also result from activation of brain nuclei or sensory afferents by excess circulating hormones (leptin, insulin, Ang II [angiotensin II]) or sodium. Leptin raises blood vessel sympathetic nerve activity by activating the carotid bodies and subsets of arcuate neurons. Ang II works in the lamina terminalis and probably throughout the brain stem and hypothalamus. Sodium is sensed primarily in the lamina terminalis. Regardless of its cause, the excess sympathetic nerve activity is mediated to some extent by activation of presympathetic neurons located in the rostral ventrolateral medulla or the paraventricular nucleus of the hypothalamus. Increased activity of the orexinergic neurons also contributes to hypertension in selected models.

Progressive cardiorespiratory dysfunction in Kv1.1 knockout mice may provide temporal biomarkers of pending sudden unexpected death in epilepsy (SUDEP): The contribution of orexin

OBJECTIVE

Immediately preceding sudden unexpected death in epilepsy (SUDEP), patients experienced a final generalized tonic-clonic seizure (GTCS), rapid ventilation, apnea, bradycardia, terminal apnea, and asystole. Whether a progressive pathophysiology develops and increases risk of SUDEP remains unknown. Here, we determined (a) heart rate, respiratory rate, and blood oxygen saturation (SaO₂ ) in low-risk and high-risk knockout (KO) mice; and (b) whether blocking receptors for orexin, a cardiorespiratory neuromodulator, influences cardiorespiratory function mice or longevity in high-risk KO mice.

METHODS

Heart rate and SaO₂ were determined noninvasively with ECGenie and pulse oximetry. Respiration was determined with noninvasive airway mechanics technology. The role of orexin was determined within subject following acute treatment with a dual orexin receptor antagonist (DORA, 100 mg/kg). The number of orexin neurons in the lateral hypothalamus was determined with immunohistochemistry.

RESULTS

Intermittent bradycardia was more prevalent in high-risk KO mice, an effect that may be the result of increased parasympathetic drive. High-risk KO mice had more orexin neurons in the lateral hypothalamus. Blocking of orexin receptors differentially influenced heart rate in KO, but not wild-type (WT) mice. When DORA administration increased heart rate, it also decreased heart rate variability, breathing frequency, and/or hypopnea-apnea. Blocking orexin receptors prevented the methacholine (MCh)-induced increase in breathing frequency in KO mice and reduced MCh-induced seizures, via a direct or indirect mechanism. DORA improved oxygen saturation in KO mice with intermittent hypoxia. Daily administration of DORA to high-risk KO mice increased longevity.

SIGNIFICANCE

High-risk KO mice have a unique cardiorespiratory phenotype that is characterized by progressive changes in five interdependent endpoints. Blocking of orexin receptors attenuates some of these endpoints and increases longevity, supporting the notion that windows of opportunity for intervention exist in this preclinical SUDEP model.

Bariatric Surgery-Induced Resolution of Hypertension and Obstructive Sleep Apnea: Impact of Modulation of Body Fat, Ectopic Fat, Autonomic Nervous Activity, Inflammatory and Adipokine Profiles

BACKGROUND

Obesity-associated systemic hypertension (HTN) and obstructive sleep apnea (OSA) have multiple pathophysiological pathways including ectopic fat deposition, inflammation, altered adipokine profile, and increased sympathetic nervous activity. We characterized these potential mechanisms in severely obese patients with or without HTN and OSA. We also compared changes of these mechanisms at 12 months following biliopancreatic diversion with duodenal switch (BPD-DS) surgery according to HTN and OSA resolution.

METHODS

Sixty-two severely obese patients were evaluated at baseline and 12 months; 40 patients underwent BPD-DS. Blood samples, bioelectrical impedance analysis, computed tomography scan, and 24-h heart rate monitoring were performed. OSA have been determined with polysomnography and HTN with blood pressure measurement and medical file.

RESULTS

Patients with HTN (n = 35) and OSA (n = 32) were older men with higher ectopic fat deposition and lower parasympathetic nervous activity without difference in adipokines and inflammatory markers. Lower reduction in weight was observed in patients with unresolved HTN (-40.9 ± 3.3 kg vs. -55.6 ± 3.8 kg; p = 0.001) and OSA (-41.4 ± 10.7 kg vs. -51.0 ± 15.2 kg; p = 0.006). Visceral adipose tissue reduction was lower in patients with unresolved HTN (-171.0 ± 25.7 cm² vs. -274.5 ± 29.0 cm²; p = 0.001) in contrast to a trend for lower abdominal subcutaneous adipose tissue reduction in patients with unresolved OSA (-247.7 ± 91.5 cm² vs. -390.5 ± 109.1 cm²; p = 0.08). At 12 months, parasympathetic activity was lowest in unresolved HTN and OSA patients, without difference in adipokines and inflammatory biomarkers.

CONCLUSION

Lower ectopic fat mobilization, lower level of parasympathetic nervous activity, and lower subcutaneous adiposity mobilization may play a role in the pathophysiology of unresolved HTN and OSA following BPD-DS surgery.

Electroacupuncture reduces hypothalamic and medullary expression of orexins and their receptors in a rat model of chronic obstructive pulmonary disease

OBJECTIVES

Decreased lung function in chronic obstructive pulmonary disease (COPD) is correlated with abnormal excitability of the respiratory centre where orexin neuropeptides from the hypothalamus are responsible for regulating respiration. We hypothesised that improvements in pulmonary function with electroacupuncture (EA) may be related to orexins in a rat model of COPD.

METHODS

The COPD model was established by cigarette smoke exposure and lipopolysaccharide injection. Modelled rats received EA at BL13 and ST36 for two weeks, after which lung function was tested. Orexin levels in the hypothalamus and medulla were detected by ELISA, while mRNA/protein expression and localisation of orexins and their receptors were investigated using real time PCR, Western blotting and immunohistochemistry, respectively.

RESULTS

The decrease in lung function observed in COPD rats was improved after EA treatment. Orexin levels in the hypothalamus and medulla were significantly higher in COPD rats than in normal rats, but were significantly reduced in the EA-treated group. There was a negative correlation between orexin content and lung function. In the hypothalamus, mRNA and protein expression and immunoreactivity of orexins were significantly higher in the COPD group than in the normal group, but a significant decrease was observed after EA. In the medulla, the expression and immunoreactivity of orexin receptors were significantly higher in the COPD group than in the normal group, but a significant decrease was observed after EA.

CONCLUSIONS

The positive effect of EA on pulmonary function in COPD rats may be related to downregulation of orexins and their receptors in the medulla.

The Orexin System and Hypertension

In this review, we focus on the role of orexin signaling in blood pressure control and its potential link to hypertension by summarizing evidence from several experimental animal models of hypertension. Studies using the spontaneously hypertensive rat (SHR) animal model of human essential hypertension show that pharmacological blockade of orexin receptors reduces blood pressure in SHRs but not in Wistar-Kyoto rats. In addition, increased activity of the orexin system contributes to elevated blood pressure and sympathetic nerve activity (SNA) in dark-active period Schlager hypertensive (BPH/2J) mice, another genetic model of neurogenic hypertension. Similar to these two models, Sprague-Dawley rats with stress-induced hypertension display an overactive central orexin system. Furthermore, upregulation of the orexin receptor 1 increases firing of hypothalamic paraventricular nucleus neurons, augments SNA, and contributes to hypertension in the obese Zucker rat, an animal model of obesity-related hypertension. Finally, we propose a hypothesis for the implication of the orexin system in salt-sensitive hypertension. All of this evidence, coupled with the important role of elevated SNA in increasing blood pressure, strongly suggests that hyperactivity of the orexin system contributes to hypertension.

Optogenetic identification of hypothalamic orexin neuron projections to paraventricular spinally projecting neurons

Orexin neurons, and activation of orexin receptors, are generally thought to be sympathoexcitatory; however, the functional connectivity between orexin neurons and a likely sympathetic target, the hypothalamic spinally projecting neurons (SPNs) in the paraventricular nucleus of the hypothalamus (PVN) has not been established. To test the hypothesis that orexin neurons project directly to SPNs in the PVN, channelrhodopsin-2 (ChR2) was selectively expressed in orexin neurons to enable photoactivation of ChR2-expressing fibers while examining evoked postsynaptic currents in SPNs in rat hypothalamic slices. Selective photoactivation of orexin fibers elicited short-latency postsynaptic currents in all SPNs tested (n = 34). These light-triggered responses were heterogeneous, with a majority being excitatory glutamatergic responses (59%) and a minority of inhibitory GABAergic (35%) and mixed glutamatergic and GABAergic currents (6%). Both glutamatergic and GABAergic responses were present in the presence of tetrodotoxin and 4-aminopyridine, suggesting a monosynaptic connection between orexin neurons and SPNs. In addition to generating postsynaptic responses, photostimulation facilitated action potential firing in SPNs (current clamp configuration). Glutamatergic, but not GABAergic, postsynaptic currents were diminished by application of the orexin receptor antagonist almorexant, indicating orexin release facilitates glutamatergic neurotransmission in this pathway. This work identifies a neuronal circuit by which orexin neurons likely exert sympathoexcitatory control of cardiovascular function.NEW & NOTEWORTHY This is the first study to establish, using innovative optogenetic approaches in a transgenic rat model, that there are robust heterogeneous projections from orexin neurons to paraventricular spinally projecting neurons, including excitatory glutamatergic and inhibitory GABAergic neurotransmission. Endogenous orexin release modulates glutamatergic, but not GABAergic, neurotransmission in these pathways.

Orexin/Hypocretin and Organizing Principles for a Diversity of Wake-Promoting Neurons in the Brain

An enigmatic feature of behavioural state control is the rich diversity of wake-promoting neural systems. This diversity has been rationalized as 'robustness via redundancy', wherein wakefulness control is not critically dependent on one type of neuron or molecule. Studies of the brain orexin/hypocretin system challenge this view by demonstrating that wakefulness control fails upon loss of this neurotransmitter system. Since orexin neurons signal arousal need, and excite other wake-promoting neurons, their actions illuminate nonredundant principles of arousal control. Here, we suggest such principles by reviewing the orexin system from a collective viewpoint of biology, physics and engineering. Orexin peptides excite other arousal-promoting neurons (noradrenaline, histamine, serotonin, acetylcholine neurons), either by activating mixed-cation conductances or by inhibiting potassium conductances. Ohm's law predicts that these opposite conductance changes will produce opposite effects on sensitivity of neuronal excitability to current inputs, thus enabling orexin to differentially control input-output gain of its target networks. Orexin neurons also produce other transmitters, including glutamate. When orexin cells fire, glutamate-mediated downstream excitation displays temporal decay, but orexin-mediated excitation escalates, as if orexin transmission enabled arousal controllers to compute a time integral of arousal need. Since the anatomical and functional architecture of the orexin system contains negative feedback loops (e.g. orexin ➔ histamine ➔ noradrenaline/serotonin-orexin), such computations may stabilize wakefulness via integral feedback, a basic engineering strategy for set point control in uncertain environments. Such dynamic behavioural control requires several distinct wake-promoting modules, which perform nonredundant transformations of arousal signals and are connected in feedback loops.

Blockade of orexin receptors attenuates the cardiovascular response to air-jet stress in spontaneously hypertensive rats

This study tested the hypothesis that orexin plays a role in the elevated pressor response to acute stress in the spontaneously hypertensive rat (SHR). The pressor response to air jet stress (AJS) (n=11/group) was 2.5 times greater in vehicle treated SHR versus Wistar (WIS) rats. Systemic delivery of 30mg/kg of the dual orexin receptor antagonist almorexant did not significantly change resting mean arterial pressure (MAP) but did attenuate the pressor response elicited by AJS to a greater extent in the SHR compared to the Wistar rats (~65% versus ~33% reduction respectively; n=6/group). Alternatively 100mg/kg almorexant reduced resting MAP in the SHR (~25mm Hg drop) and attenuated both the heart rate (HR; ~50% reduction) and MAP (~62% reduction) response to AJS in both strains (n=6/group). Systemic application of SB-334867 (3mg/kg), an orexin receptor type 1 antagonist (n=5/group), selectively reduced resting MAP and attenuated the HR response to AJS in the SHR but had no effect on the pressor response in either strain. The potential role of endogenous orexin release in cardiovascular control in the SHR was linked to a significant increase in brain-derived neurotrophic factor mRNA expression in the hypothalamus and elevated orexin receptor expression (type 2 only) in the dorsal pons when compared to WIS (n=4/group). These results demonstrate that the exaggerated pressor response in the SHR to stress is linked to increased orexin receptor activation and possibly altered orexin receptor expression in the dorsal pons and BDNF expression in the hypothalamus.

Anatomical and functional connections between the locus coeruleus and the nucleus tractus solitarius in neonatal rats

This study was designed to investigate brain connections among chemosensitive areas in newborn rats. Rhodamine beads were injected unilaterally into the locus coeruleus (LC) or into the caudal part of the nucleus tractus solitarius (cNTS) in Sprague-Dawley rat pups (P7-P10). Rhodamine-labeled neurons were patched in brainstem slices to study their electrophysiological responses to hypercapnia and to determine if chemosensitive neurons are communicating between LC and cNTS regions. After 7-10 days, retrograde labeling was observed in numerous areas of the brainstem, including many chemosensitive regions, such as the contralateral LC, cNTS and medullary raphe. Whole-cell patch clamp was done in cNTS. In 4 of 5 retrogradely labeled cNTS neurons that projected to the LC, firing rate increased in response to hypercapnic acidosis (15% CO2), even in synaptic blockade medium (SNB) (high Mg(2+)/low Ca(2+)). In contrast, 2 of 3 retrogradely labeled LC neurons that projected to cNTS had reduced firing rate in response to hypercapnic acidosis, both in the presence and absence of SNB. Extensive anatomical connections among chemosensitive brainstem regions in newborn rats were found and at least for the LC and cNTS, the connections involve some CO2-sensitive neurons. Such anatomical and functional coupling suggests a complex central respiratory control network, such as seen in adult rats, is already largely present in neonatal rats by at least day P7-P10. Since the NTS and the LC play a major role in memory consolidation, our results may also contribute to the understanding of the development of memory consolidation.

An augmented CO2 chemoreflex and overactive orexin system are linked with hypertension in young and adult spontaneously hypertensive rats

KEY POINTS

Activation of central chemoreceptors by CO2 increases sympathetic nerve activity (SNA), arterial blood pressure (ABP) and breathing. These effects are exaggerated in spontaneously hypertensive rats (SHRs), resulting in an augmented CO2 chemoreflex that affects both breathing and ABP. The augmented CO2 chemoreflex and the high ABP are measureable in young SHRs (postnatal day 30-58) and become greater in adult SHRs. Blockade of orexin receptors can normalize the augmented CO2 chemoreflex and the high ABP in young SHRs and normalize the augmented CO2 chemoreflex and significantly lower the high ABP in adult SHRs. In the hypothalamus, SHRs have more orexin neurons, and a greater proportion of them increase their activity with CO2 . The orexin system is overactive in SHRs and contributes to the augmented CO2 chemoreflex and hypertension. Modulation of the orexin system may be beneficial in the treatment of neurogenic hypertension.

ABSTRACT

Activation of central chemoreceptors by CO2 increases arterial blood pressure (ABP), sympathetic nerve activity and breathing. In spontaneously hypertensive rats (SHRs), high ABP is associated with enhanced sympathetic nerve activity and peripheral chemoreflexes. We hypothesized that an augmented CO2 chemoreflex and overactive orexin system are linked with high ABP in both young (postnatal day 30-58) and adult SHRs (4-6 months). Our main findings are as follows. (i) An augmented CO2 chemoreflex and higher ABP in SHRs are measureable at a young age and increase in adulthood. In wakefulness, the ventilatory response to normoxic hypercapnia is higher in young SHRs (mean ± SEM: 179 ± 11% increase) than in age-matched normotensive Wistar-Kyoto rats (114 ± 9% increase), but lower than in adult SHRs (226 ± 10% increase; P < 0.05). The resting ABP is higher in young SHRs (122 ± 5 mmHg) than in age-matched Wistar-Kyoto rats (99 ± 5 mmHg), but lower than in adult SHRs (152 ± 4 mmHg; P < 0.05). (ii) Spontaneously hypertensive rats have more orexin neurons and more CO2 -activated orexin neurons in the hypothalamus. (iii) Antagonism of orexin receptors with a dual orexin receptor antagonist, almorexant, normalizes the augmented CO2 chemoreflex in young and adult SHRs and the high ABP in young SHRs and significantly lowers ABP in adult SHRs. (iv) Attenuation of peripheral chemoreflexes by hyperoxia does not abolish the augmented CO2 chemoreflex (breathing and ABP) in SHRs, which indicates an important role for the central chemoreflex. We suggest that an overactive orexin system may play an important role in the augmented central CO2 chemoreflex and in the development of hypertension in SHRs.

Effects of one's sex and sex hormones on sympathetic responses to chemoreflex activation

NEW FINDINGS

What is the topic of this review? This review summarizes sex-dependent differences in the sympathetic responses to chemoreflex activation, with a focus on the role of circulating sex hormones on the sympathetic outcomes. What advances does it highlight? The importance of circulating sex hormones for the regulation of sympathetic nerve activity in humans has only recently begun to be elucidated, and few studies have examined this effect during chemoreflex regulation. We review recent studies indicating that changes in circulating sex hormones are associated with alterations to chemoreflex-driven increases in sympathetic activity and highlight those areas which require further study. Sex-dependent differences in baseline sympathetic nerve activity are established, but little information exists on the influence of sex on sympathetic activation during chemoreflex stimulation. In this article, we review the evidence for the effect of sex on chemoreflex-driven increases in sympathetic nerve activity. We also review recent studies which indicate that changes in circulating sex hormones, as initiated by the menstrual cycle and hormonal contraceptive use, elicit notable changes in the muscle sympathetic activation during chemoreflex stimulation.

Heat and oxidative stress alter the expression of orexin and its related receptors in avian liver cells

Orexins (A and B) or hypocretins (1 and 2) are hypothalamic orexigenic neuropeptides that are involved in the regulation of several physiological processes in mammals. Recently, orexin has been shown to activate the hypothalamic-pituitary-adrenal (HPA) stress axis and emerging evidences identify it as a stress modulator in mammals. However, the regulation of orexin system by stress itself remains unclear. Here, we investigate the effects of heat, 4-Hydroxynonenal (4-HNE) and hydrogen peroxide (H2O2) stress on the hepatic expression of orexin (ORX) and its related receptors (ORXR1/2) in avian species. Using in vivo and in vitro models, we found that heat stress significantly down-regulated ORX and ORXR1/2 mRNA and protein abundances in quail liver and LMH cells. H2O2, however, decreased ORX protein and increased ORX mRNA levels in a dose dependent manner (P<0.05). The absence of correlation between orexin mRNA and protein levels suggests that H2O2 treatment modulates post-transcriptional mechanisms. 4-HNE had a biphasic effect on orexin system expression, with a significant up-regulation at low doses (10 and 20μM) and a significant down-regulation at a high dose (30μM). Taken together, our data indicated that hepatic orexin system could be a molecular signature in the heat and oxidative stress response.

Chronic heart failure alters orexin and melanin concentrating hormone but not corticotrophin releasing hormone-related gene expression in the brain of male Lewis rats

OBJECTIVE

The aim of this study was to investigate the effect of chronic heart failure (HF; 16 weeks post left coronary artery ligation) on the brain's orexin (ORX) and related neuropeptide systems.

METHODS

Indicators of cardiac function, including the percent fractional shortening (%FS) left ventricular posterior wall shortening velocity (LVPWSV) were assessed via echocardiography at 16 weeks post myocardial infarction or sham treatment in male Lewis rats (n=5/group). Changes in gene expression in HF versus control (CON) groups were quantified by real-time PCR in the hypothalamus, amygdala and dorsal pons.

RESULTS

HF significantly reduced both the %FS and LVPWSV when compared to CON animals (P<0.02). In the hypothalamus ORX gene expression was significantly reduced in HF and correlated with changes in cardiac function when compared to CON (P<0.02). No significant changes in hypothalamic ORX receptor (type 1 or type 2) gene expression were identified. Alternatively hypothalamic melanin concentrating hormone (MCH) gene expression was significantly upregulated in HF animals and negatively correlated with LVPWSV (P<0.006). In both the amygdala and dorsal pons ORX type 2 receptor expression was significantly down-regulated in HF compared to CON. ORX receptor type 1, CRH and CRH type 1 and type 2 receptor expressions were unchanged by HF in all brain regions analyzed.

CONCLUSION

These observations support previous work demonstrating that cardiovascular disease modulates the ORX system and identify that in the case of chronic HF the ORX system is altered in parallel with changes in MCH expression but independent of any significant changes in the central CRH system. This raises the new possibility that ORX and MCH systems may play an important role in the pathophysiology of HF.

The role of orexin in post-stroke inflammation, cognitive decline, and depression

Ischemic stroke results in diverse pathophysiologies, including cerebral inflammation, neuronal loss, cognitive dysfunction, and depression. Studies aimed at identifying therapeutic solutions to alleviate these outcomes are important due to the increase in the number of stroke patients annually. Recently, many studies have reported that orexin, commonly known as a neuropeptide regulator of sleep/wakefulness and appetite, is associated with neuronal cell apoptosis, memory function, and depressive symptoms. Here, we briefly summarize recent studies regarding the role and future perspectives of orexin in post-ischemic stroke. This review advances our understanding of the role of orexin in post-stroke pathologies, focusing on its possible function as a therapeutic regulator in the post-ischemic brain. Ultimately, we suggest the clinical potential of orexin to regulate post-stroke pathologies.