Functional roles of orexin in obstructive sleep apnea: From clinical observation to mechanistic insights

Detection of reduced orexin-A/hypocretin-1 and its fragments by orexin-A "gold-standard" radioimmunoassay

BACKGROUND

Orexin-A/hypocretin-1 level is determined in the cerebrospinal fluid samples as a part of clinical narcolepsy diagnostics utilizing a specific commercial radioimmunoassay (RIA); this assay is also widely used in research of many other conditions. The specificity of RIAs is in general variable, and little has been firmly disclosed about the specificity of this RIA assay. Thus, the validity of many research results obtained using the kit is unclear. At least metabolites of orexin-A have been proposed as potential interfering substances.

METHODS

Since this issue has not been systematically assessed, we decided to investigate it using synthetic variants of orexin-A and -B (intact peptides and peptide fragments as well as reduced orexin-A).

RESULTS

Our synthetic orexin-A bound correspondingly to the orexin-A standard included in the kit while orexin-B did not bind even at 10000-fold higher concentrations. Reduction of the disulfide bridges in orexin-A (giving orexin-A-SS) decreased its binding 25-fold. C-terminal truncation of orexin-A-SS was well tolerated - some of the fragments actually bound better than orexin-A-SS - while N-terminal truncation was not allowed.

CONCLUSIONS

The results demonstrate that the RIA kit is fairly selective for intact orexin-A among the peptides tested. However, this does not as such prove that it measures intact orexin-A in the physiological samples, and further studies including identification of physiological orexin-A metabolites are thus required. We also suggest that the redox milieu of cerebrospinal fluid - that has been suggested to vary in different diseases - may have an impact on what is measured with the kit.

Comorbidity of obstructive sleep apnea and narcolepsy: A challenging diagnosis and complex management

Introduction

Narcolepsy and obstructive sleep apnea syndrome (OSA) are relevant causes of excessive daytime sleepiness (EDS); although different for etiopathogenesis and symptoms, differential diagnosis is sometimes difficult, and guidelines are lacking concerning their management when coexisting in a same patient.

Methods

A narrative review of the literature was realized including PubMed, Scopus and Embase, aimed to regroup studies and case reports evaluating epidemiology, clinical and instrumental features and treatment of patients presenting comorbid NT1 and OSA. Moreover, a snowball search on the pathophysiology underpinnings of the association of the two disorder was realized.

Results

For adults, the prevalence of OSA in NT1 ranged from 24.8 % to 51.4 %. No studies were found concerning the treatment of EDS in double-diagnosis patients, but only case reports; these latter and the experience on patients with either NT or OSA suggest that modafinil, methylphenidate, pitolisant and solriamfetol are effective.

Discussion

Adults with NT1 showed a higher prevalence of OSA compared to the general population, but the reach of the results reviewed here is limited by the retrospective design of most of the studies and by the inhomogeneous utilization of diagnostic criteria. The association with OSA is likely to be explained by the involvement of orexin in hypercapnic-hypoxic responses: a deficit of orexin may promote obstructive events during sleep. Open questions warrant further investigation, especially orexin's involvement in other sleep disorders associated with EDS, and the more appropriate treatment for the OSA-narcolepsy comorbidity.

Potential Role of Sleep Disturbance in the Development of Early Puberty: Past Clinical Evidence for Future Management

The incidence of early puberty in children has been increasing. It has been suspected that both genetic and various environmental factors such as nutrition and hormonal exposure could influence the mechanisms underlying the earlier onset of puberty. Interestingly, several previous studies have reported a strong connection between sleep and puberty. Specifically, it was discovered that luteinizing hormone (LH), a potential marker for the onset of puberty, was increased during the deep sleep period. Furthermore, a high prevalence of early puberty was observed in patients with sleep disorders, especially in those experiencing narcolepsy. In this review article, findings related to the association between sleep disturbance and early puberty have been comprehensively summarized. Any contrary findings are also included and discussed. Advances in the knowledge surrounding sleep disturbance have led to a greater understanding of a correlation between early puberty and sleep disorder and provide alternative therapeutic options for the treatment of central precocious puberty in the future.

Estimating the prevalence and clinical causality of obstructive sleep apnea in paediatric narcolepsy patients

BACKGROUND

Numerous risk factors in paediatric narcolepsy may predispose them to obstructive sleep apnea (OSA). The concurrent presence of OSA in these patients might lead to underdiagnosing narcolepsy. This research investigates the prevalence and potential causality between OSA and paediatric narcolepsy.

METHODS

A case-control study coupled with a two-sample Mendelian randomization (MR) analysis was employed to explore the prevalence and causal link between paediatric narcolepsy and OSA risk.

RESULTS

The case-control study revealed that paediatric narcolepsy patients are at an increased risk of OSA, with an Odds ratio (OR) of 4.87 (95% CI: 2.20-10.71; P < 0.001). The inverse-variance weighted (IVW) model further suggests a potential causal link between narcolepsy and OSA (IVW OR: 4.671, 95% CI: 1.925-11.290; P < 0.001). Additionally, sensitivity analysis confirmed these findings' reliability.

CONCLUSION

The findings highlight an elevated prevalence and genetic susceptibility to OSA among paediatric narcolepsy patients, underscoring the necessity for clinical screening of OSA. Continued research is essential to clarify the pathogenic mechanisms and develop potential treatments.

Orexin increases the neuronal excitability of several brain areas associated with maintaining of arousal

Orexin is exclusively produced in neurons localized within the lateral hypothalamic area (LHA) and perifornical area (PFA). Orexin has been identified as a key promotor of arousal. The selective loss of orexinergic neurons results in narcolepsy. It is known that the intrinsic electrophysiological properties are critical for neurons to perform their functions in corresponding brain regions. In addition to hypothalamic orexin, other brain nuclei are involved in the regulation of sleep and wakefulness. Quite a lot of studies focus on elucidating orexin-induced regulation of sleep-wake states and modulation of neuronal electrophysiological properties in several brain regions. Here, we summarize that the orexinergic neurons exhibit spontaneous firing activity which is associated with the states of sleep-wake cycle. Orexin mainly exerts postsynaptic excitatory effects on multiple brain nuclei associated with the process of sleep and wakefulness. This review may provide a background to guide future research about the cellular mechanisms of orexin-induced maintaining of arousal.

Dissolving microneedles loaded with nimodipine for prevention of sleep disorders at a high altitude

Sleep disorders are one of the most common acute reactions on the plateau, which can cause serious complications. However, there is no effective and safe treatment currently available. Nimodipine (NMD) is a dihydropyridine calcium channel blocker with neuroprotective and vasodilating activity, mainly used for the treatment of ischemic brain injury. Commercial oral or injectable NMD formulations are not a good option for central neuron diseases due to their poor brain delivery. In this study, nimodipine dissolving microneedles (NDMNs) were prepared for the prevention of sleep disorders caused by hypoxia. NDMNs were composed of NMD and polyvinyl pyrrolidone (PVP) K90 with a conical morphology and high rigidity. After administration of NDMNs on the back neck of mice, the concentration of NMD in the brain was significantly higher than that of oral medication as was confirmed by the fluorescent imaging on mouse models. NDMNs enhanced cognitive function, alleviated oxidative stress, and improved the sleep quality of mice with high-altitude sleep disorders. The blockage of calcium ion overloading may be an important modulation mechanism. NDMNs are a promising and user-friendly formulation for the prevention of high-altitude sleep disorders.

Control of breathing by orexinergic signaling in the nucleus tractus solitarii

Orexin signaling plays a facilitatory role in respiration. Abnormalities in orexin levels correlate with disordered breathing patterns and impaired central respiratory chemoreception. Nucleus tractus solitarii (NTS) neurons expressing the transcription factor Phox2b contribute to the chemoreceptive regulation of respiration. However, the extent to which orexinergic signaling modulates respiratory activity in these Phox2b-expressing NTS neurons remains unclear. In the present study, the injection of orexin A into the NTS significantly increased the firing rate of the phrenic nerve. Further analysis using fluorescence in situ hybridization and immunohistochemistry revealed that orexin 1 receptors (OX1Rs) were primarily located in the ventrolateral subdivision of the NTS and expressed in 25% of Phox2b-expressing neurons. Additionally, electrophysiological recordings showed that exposure to orexin A increased the spontaneous firing rate of Phox2b-expressing neurons. Immunostaining experiments with cFos revealed that the OX1R-residing Phox2b-expressing neurons were activated by an 8% CO2 stimulus. Crucially, OX1R knockdown in these NTS neurons notably blunted the ventilatory response to 8% CO2, alongside an increase in sigh-related apneas. In conclusion, orexinergic signaling in the NTS facilitates breathing through the activation of OX1Rs, which induces the depolarization of Phox2b-expressing neurons. OX1Rs are essential for the involvement of Phox2b-expressing NTS neurons in the hypercapnic ventilatory response.

Sulfonamide Compounds as Orexin Receptor Agonists for Treating Sleep Disorders, Namely, Narcolepsy and Hypersomnia

Provided herein are novel sulfonamide compounds as orexin receptor agonists, their pharmaceutical compositions, the use of such compounds in treating sleep disorders, namely, narcolepsy and hypersomnia, and processes for preparing such compounds.

Traumatic brain injury and sleep in military and veteran populations: A literature review

BACKGROUND

Traumatic brain injury (TBI) is a hallmark of wartime injury and is related to numerous sleep wake disorders (SWD), which persist long term in veterans. Current knowledge gaps in pathophysiology have hindered advances in diagnosis and treatment.

OBJECTIVE

We reviewed TBI SWD pathophysiology, comorbidities, diagnosis and treatment that have emerged over the past two decades.

METHODS

We conducted a literature review of English language publications evaluating sleep disorders (obstructive sleep apnea, insomnia, hypersomnia, parasomnias, restless legs syndrome and periodic limb movement disorder) and TBI published since 2000. We excluded studies that were not specifically evaluating TBI populations.

RESULTS

Highlighted areas of interest and knowledge gaps were identified in TBI pathophysiology and mechanisms of sleep disruption, a comparison of TBI SWD and post-traumatic stress disorder SWD. The role of TBI and glymphatic biomarkers and management strategies for TBI SWD will also be discussed.

CONCLUSION

Our understanding of the pathophysiologic underpinnings of TBI and sleep health, particularly at the basic science level, is limited. Developing an understanding of biomarkers, neuroimaging, and mixed-methods research in comorbid TBI SWD holds the greatest promise to advance our ability to diagnose and monitor response to therapy in this vulnerable population.

Long-term intermittent hypoxia induces anxiety-like behavior and affects expression of orexin and its receptors differently in the mouse brain

Studies have revealed a possible connection between orexin, narcolepsy, and obstructive sleep apnea (OSA). Orexin has an important role in the maintenance of arousal and wakefulness/sleeping states. To better understand the pathophysiological mechanism of OSA, we used a chronic intermittent hypoxia (CIH) model in mice to mimic OSA. In this way, we explored the effect of CIH on the locomotor activity and orexin system in the hypothalamus, cerebral cortex, and brainstem of mice. Male C57BL/6 J mice (8 weeks) in the CIH group were exposed in a hypoxia chamber for 8 h/day for 28 weeks. The re-oxygenation groups comprised the W2 group and W4 group, which were exposed to 28 weeks of CIH followed by 2 weeks and 4 weeks of re-oxygenation, respectively. The open field test was undertaken to observe locomotor activity. mRNA expression of orexin, orexin receptor type 1 (OX1R), and OX2R mRNA was evaluated by real-time reverse transcription-quantitative polymerase chain reaction. Mice subjected to long-term CIH exhibited significant anxiety-like behavior during the light period, and this behavior lasted until 4 weeks of re-oxygenation. mRNA expression of orexin was upregulated in the hypothalamus. mRNA expression of OX1R mRNA in the cerebral cortex and brainstem was downregulated by CIH. Two weeks and 4 weeks of re-oxygenation could not reverse these alternations. Long-term CIH may induce anxiety-like behavior and re-oxygenation cannot reverse these behavior. Moreover, OX1R has a significant role in the anxiety-related symptoms observed in long-term CIH.

Implications of sleep disorders for periodontitis

PURPOSE

Periodontitis is a chronic inflammatory disease caused by multi-factors. Sleep is a natural physiologic process, and the sleep duration, quality, and patterns might be associated with periodontitis. Meanwhile, periodontitis might in turn induce systemic inflammation and thus impact sleep in different ways as well.

METHODS

To investigate the bidirectional relationship between sleep disorder and periodontitis, a literature search was conducted to reveal the interaction and possible mechanism between these two diseases.

RESULTS

The results show that sleep disorders can affect the progression of periodontitis via some pathomechanisms, and periodontitis also has a reverse impact on sleep.

CONCLUSION

Although the epidemiologic and clinical trials found the possible associations between sleep disorder and periodontitis, their relationship is still not that explicit. Further studies are warranted to shed light on them, to improve preventive health care.