Orexin A activates hypoglossal motoneurons and enhances genioglossus muscle activity in rats

Orexin receptor 2 agonist activates diaphragm and genioglossus muscle through stimulating inspiratory neurons in the pre-Bötzinger complex, and phrenic and hypoglossal motoneurons in rodents

Orexin-mediated stimulation of orexin receptors 1/2 (OX[1/2]R) may stimulate the diaphragm and genioglossus muscle via activation of inspiratory neurons in the pre-Bötzinger complex, which are critical for the generation of inspiratory rhythm, and phrenic and hypoglossal motoneurons. Herein, we assessed the effects of OX2R-selective agonists TAK-925 (danavorexton) and OX-201 on respiratory function. In in vitro electrophysiologic analyses using rat medullary slices, danavorexton and OX-201 showed tendency and significant effect, respectively, in increasing the frequency of inspiratory synaptic currents of inspiratory neurons in the pre-Bötzinger complex. In rat medullary slices, both danavorexton and OX-201 significantly increased the frequency of inspiratory synaptic currents of hypoglossal motoneurons. Danavorexton and OX-201 also showed significant effect and tendency, respectively, in increasing the frequency of burst activity recorded from the cervical (C3-C5) ventral root, which contains axons of phrenic motoneurons, in in vitro electrophysiologic analyses from rat isolated brainstem-spinal cord preparations. Electromyogram recordings revealed that intravenous administration of OX-201 increased burst frequency of the diaphragm and burst amplitude of the genioglossus muscle in isoflurane- and urethane-anesthetized rats, respectively. In whole-body plethysmography analyses, oral administration of OX-201 increased respiratory activity in free-moving mice. Overall, these results suggest that OX2R-selective agonists enhance respiratory function via activation of the diaphragm and genioglossus muscle through stimulation of inspiratory neurons in the pre-Bötzinger complex, and phrenic and hypoglossal motoneurons. OX2R-selective agonists could be promising drugs for various conditions with respiratory dysfunction.

Effects of the orexin receptor 2 agonist danavorexton on emergence from general anaesthesia and opioid-induced sedation, respiratory depression, and analgesia in rats and monkeys

BACKGROUND

Delayed emergence from general anaesthesia, opioid-induced sedation, and opioid-induced respiratory depression is associated with perioperative complications. We characterised the preclinical effects of the orexin receptor 2 (OX2R)-selective agonist danavorexton (TAK-925) on emergence from anaesthesia and reversal of fentanyl-induced sedation, respiratory depression, and analgesia.

METHODS

Emergence from isoflurane- or propofol-induced anaesthesia and fentanyl-induced sedation were investigated by righting reflex, rotarod, and electroencephalography in rats or monkeys. Fentanyl-induced respiratory depression was assessed by arterial blood gas analysis and whole-body plethysmography in rats and monkeys. Analgesia was evaluated using formalin- and skin incision-induced pain models in rats.

RESULTS

Danavorexton shortened emergence from isoflurane- or propofol-induced anaesthesia and from fentanyl-induced sedation at 1 (P=0.005), 3 (P=0.006), and 3 mg kg-1 s.c. (P=0.022), respectively, by righting reflex in rats. Danavorexton (10 mg kg-1 s.c.) accelerated recovery from isoflurane-, propofol- and fentanyl-induced motor impairment in separate rotarod tests in rats (P=0.008, P=0.007, P=0.017, respectively), and reversed anaesthesia and fentanyl-induced delta-power increases. Danavorexton shortened emergence (return of righting reflex) from isoflurane- or propofol-induced anaesthesia at 1 (P=0.002) and 3 mg kg-1 (P=0.004), respectively, in cynomolgus monkeys. Danavorexton (10 mg kg-1 s.c.) reversed fentanyl-induced increase in Pco2 (P=0.006), and decrease in Po2 (P=0.015) and pH (P<0.001) in rats, and at 3 mg kg-1 s.c. reversed fentanyl-induced increase in Pco2 (P=0.007), and decrease in Po2 (P=0.013) and SO2 (P=0.036) in monkeys. Danavorexton increased minute volume and tidal volume in fentanyl-treated animals. Danavorexton at ≤10 mg kg-1 s.c. did not compromise fentanyl analgesia in rat formalin- and skin incision-induced pain models.

CONCLUSIONS

Danavorexton promoted recovery from anaesthesia and fentanyl-induced sedation, and antagonised fentanyl-induced respiratory depression without compromising fentanyl analgesia.

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

Obstructive sleep apnea is the most common sleep-related breathing disorder. Repetitive episodes of the obstructive respiratory events lead to arousal, sleep fragmentation, and excessive daytime sleepiness. Orexin, also known as hypocretin, is one of the most important neurotransmitters responsible for sleep and arousal regulation. Deficiency of orexin has been shown to be involved in the pathogenesis of narcolepsy, which shares cardinal symptoms of sleep apnea and excessive daytime sleep with obstructive sleep apnea. However, the relationship between orexin and obstructive sleep apnea is not well defined. In this review, we summarize the current evidence, from in vitro, in vivo, and clinical data, regarding the association between orexin and obstructive sleep apnea. The effects of orexin on sleep apnea, as well as how the consequences of obstructive sleep apnea affect the orexin system function are also discussed. Additionally, the contrary findings are also included and discussed.

Obstructive sleep apnea

Obstructive sleep apnea (OSA) is a disease that results from loss of upper airway muscle tone leading to upper airway collapse during sleep in anatomically susceptible persons, leading to recurrent periods of hypoventilation, hypoxia, and arousals from sleep. Significant clinical consequences of the disorder cover a wide spectrum and include daytime hypersomnolence, neurocognitive dysfunction, cardiovascular disease, metabolic dysfunction, respiratory failure, and pulmonary hypertension. With escalating rates of obesity a major risk factor for OSA, the public health burden from OSA and its sequalae are expected to increase, as well. In this chapter, we review the mechanisms responsible for the development of OSA and associated neurocognitive and cardiometabolic comorbidities. Emphasis is placed on the neural control of the striated muscles that control the pharyngeal passages, especially regulation of hypoglossal motoneuron activity throughout the sleep/wake cycle, the neurocognitive complications of OSA, and the therapeutic options available to treat OSA including recent pharmacotherapeutic developments.

TASK1 and TASK3 in orexin neuron of lateral hypothalamus contribute to respiratory chemoreflex by projecting to nucleus tractus solitarius

TWIK-related acid-sensitive potassium channels (TASKs)-like current was recorded in orexin neurons in the lateral hypothalamus (LH), which are essential in respiratory chemoreflex. However, the specific mechanism responsible for the pH-sensitivity remains elusive. Thus, we hypothesized that TASKs contribute to respiratory chemoreflex. In the present study, we found that TASK1 and TASK3 were expressed in orexin neurons. Blocking TASKs or microinjecting acid artificial cerebrospinal fluid (ACSF) in the LH stimulated breathing. In contrast, alkaline ACSF inhibited breathing, which was attenuated by blocking TASK1. Damage of orexin neurons attenuated the stimulatory effect on respiration caused by microinjection of acid ACSF (at a pH of 6.5) or TASKs antagonists. The orexinA-positive fiber and orexin type 1 receptor (OX1R) neurons were located in the nucleus tractus solitarius (NTS). The exciting effect of acidosis in the LH on respiration was inhibited by blocking OX1R of the NTS. Taken together, we conclude that orexin neurons sense the extracellular pH change through TASKs and regulate respiration by projecting to the NTS.

Orexin A improves the cognitive impairment induced by chronic intermittent hypoxia in mice

The orexin neuron in lateral hypothalamus (LH) was involved in the regulation of sleep-wake cycle. However, the effect of orexin A (OXA) on cognitive impairment resulting from diverse diseases remains controversial. In this study, we investigated the effect of OXA on cognitive impairment induced by chronic intermittent hypoxia (CIH) in mice. Adult (10 weeks old) male C57BL/6 mice were randomly divided into the following four groups: normoxia control (NC)+normal saline (NS), NC + OXA, CIH + NS and CIH + OXA group. Following the CIH mice models establishment, OXA was injected into the right lateral ventricles of mice by a micro-injection system. Water maze test was used to assess spatial memory abilities of the mice. The expression of OXA and c-Fos in LH were analyzed by immunofluorescence staining. Apoptotic cell death and oxidative stress in hippocampus were evaluated using multiple methods including TUNEL, western blot and biochemical analysis. Behavioral tests revealed that CIH significantly increased the escape latency and time of arriving platform, of which were markedly decreased by OXA treatment. Similarly, the CIH + NS group was worse than NC + NS group in terms of the number of platform crossing and time in the target quadrant, of which were also significantly improved by OXA treatment. The number of OXA + neuron in LH was decreased, but the percentage of c-Fos+/OXA + neuron in LH was remarkably increased by CIH. Furthermore, we found that micro-injection of OXA attenuated CIH-induced apoptotic cell death and oxidative stress in the hippocampus. Our results suggested that OXA might improve cognitive impairment induced by CIH through inhibiting hippocampal apoptosis and oxidative stress.

The combination of intermittent electrical stimulation with acute intermittent hypoxia strengthens genioglossus muscle discharge in chronic intermittent hypoxia-pretreated rats

OBJECTIVE

Exploring whether the genioglossus discharge in chronic intermittent hypoxia(CIH) - pretreated rats could be enhanced by intermittent electrical stimulation combined with acute intermittent hypoxia(AIH).

METHODS

Rats were pretreated with CIH for 4 weeks and then were randomly divided into 6 groups: time control, intermittent electric stimulation, AIH, intermittent electric stimulation + AIH, continuous electric stimulation and continuous hypoxia exposure. The genioglossus discharges were recorded and compared before and after stimulation. Normoxic-treated rats were grouped and treated with the same stimulation protocols.

RESULTS

Intermittent electrical stimulation or AIH temporarily increased the activity of the genioglossus discharge, in which the degree of the increase was significantly higher in CIH-pretreated rats than in normoxic rats.After intermittent electrical stimulation, AIH evoked a sustained elevation of genioglossus discharge activities in CIH-pretreated rats, in which the degree of the increase was significantly higher than in rats induced by a single intermittent electric stimulation.

CONCLUSION

Intermittent electrical stimulation combined with AIH strengthens the genioglossus plasticity in CIH-pretreated rats.

Orexin (hypocretin) participates in central autonomic regulation during fight-or-flight response

Our daily life does not only involve a calm resting state but is rather full of perturbations that induce active states such as moving, eating, and communicating. During such active conditions, cardiorespiratory regulation should be adjusted according to bodily demand, which differs from that during the resting state, by modulating or resetting the operating point. To explore neural mechanisms in the state-dependent adjustment of central autonomic regulation, my research group has recently focused on the fight-or-flight response because the stressor induces not only cognitive, emotional, and behavioral changes but also autonomic changes. In this brief review, I will summarize our discovery using orexin knockout mice and orexin neuron-ablated mice for the possible contribution of orexin, a hypothalamic neuropeptide, to the state-dependent adjustment of the central autonomic regulation. In addition, I will introduce some recent discovery using optogenetic manipulation of the orexin and related systems. The diversity of synaptic control of the cardiovascular and respiratory neurons appears necessary for animals to adapt themselves to ever-changing life circumstances and behavioral states. The orexin system is likely to function as one of the essential modulators for coordinating the circuits controlling autonomic functions and behaviors.

Orexin-A differentially modulates inhibitory and excitatory synaptic transmission in rat inner retina

In this work, modulation by orexin-A of the release of glutamate and GABA from bipolar and amacrine cells respectively was studied by examining the effects of the neuropeptide on miniature excitatory postsynaptic currents (mEPSCs) and miniature inhibitory postsynaptic currents (mIPSCs) of rat retinal ganglion cells (GCs). Using RNAscope in situ hybridization in combination with immunohistochemistry, we showed positive signals for orexin receptor-1 (OX₁R) mRNA in the bipolar cell terminals and those for orexin receptor-2 (OX₂R) mRNA in the amacrine cell terminals. With whole-cell patch-clamp recordings in rat retinal slices, we demonstrated that application of orexin-A reduced the interevent interval of mEPSCs of GCs through OX₁R. However, it increased the interevent interval of mIPSCs, mediated by GABA_A receptors, through OX₂R. Furthermore, orexin-A-induced reduction of mEPSC interevent interval was abolished by the application of PI-PLC inhibitors or PKC inhibitors. In contrast, orexin-A-induced increase of GABAergic mIPSC interevent interval was mimicked by 8-Br-cAMP or an adenylyl cyclase activator, but was eliminated by PKA antagonists. Finally, application of nimodipine, an L-type Ca²⁺ channel blocker, increased both mEPSC and mIPSC interevent interval, and co-application of orexin-A no longer changed the mEPSCs and mIPSCs. We conclude that orexin-A increases presynaptic glutamate release onto GCs by activating L-type Ca²⁺ channels in bipolar cells, a process that is mediated by an OX₁R/PI-PLC/PKC signaling pathway. However, orexin-A decreases presynaptic GABA release onto GCs by inhibiting L-type Ca²⁺ channels in amacrine cells, a process that is mediated by an OX₂R/cAMP-PKA signaling pathway.

Whole-Brain Monosynaptic Inputs to Hypoglossal Motor Neurons in Mice

Hypoglossal motor neurons (HMNs) innervate tongue muscles and play key roles in a variety of physiological functions, including swallowing, mastication, suckling, vocalization, and respiration. Dysfunction of HMNs is associated with several diseases, such as obstructive sleep apnea (OSA) and sudden infant death syndrome. OSA is a serious breathing disorder associated with the activity of HMNs during different sleep–wake states. Identifying the neural mechanisms by which the state-dependent activities of HMNs are controlled may be helpful in providing a theoretical basis for effective therapy for OSA. However, the presynaptic partners governing the activity of HMNs remain to be elucidated. In the present study, we used a cell-type-specific retrograde tracing system based on a modified rabies virus along with a Cre/loxP gene-expression strategy to map the whole-brain monosynaptic inputs to HMNs in mice. We identified 53 nuclei targeting HMNs from six brain regions: the amygdala, hypothalamus, midbrain, pons, medulla, and cerebellum. We discovered that GABAergic neurons in the central amygdaloid nucleus, as well as calretinin neurons in the parasubthalamic nucleus, sent monosynaptic projections to HMNs. In addition, HMNs received direct inputs from several regions associated with respiration, such as the pre-Botzinger complex, parabrachial nucleus, nucleus of the solitary tract, and hypothalamus. Some regions engaged in sleep–wake regulation (the parafacial zone, parabrachial nucleus, ventral medulla, sublaterodorsal tegmental nucleus, dorsal raphe nucleus, periaqueductal gray, and hypothalamus) also provided primary inputs to HMNs. These results contribute to further elucidating the neural circuits underlying disorders caused by the dysfunction of HMNs.

Prevalence of Rapid Eye Movement-related Obstructive Sleep Apnea in Adult Narcolepsy

Objective The association between narcolepsy and rapid eye movement (REM)-related obstructive sleep apnea (OSA) has not been reported. This study aimed to examine the prevalence of REM-related OSA in narcolepsy patients. Methods From January 2013 to April 2018, 141 adult patients were diagnosed with narcolepsy using nocturnal polysomnography and the multiple sleep latency test. The prevalence of REM-related OSA in narcolepsy patients was retrospectively reviewed. Three criteria were used to determine REM-related OSA: Definition #1, an overall apnea-hypopnea index (AHI) ≥5 and AHI during REM (AHIREM)/AHI during non-rapid eye movement (NREM) (AHINREM) ≥2; Definition #2, an overall AHI ≥5 and AHI_REM/AHI_NREM≥2 and AHI_NREM <15; and Definition #3, an overall AHI ≥5 and AHI_REM/AHI_NREM≥2 and AHI_NREM <8 plus an REM sleep duration >10.5 minutes. Results Of the 141 narcolepsy patients, 26 were diagnosed with narcolepsy with cataplexy (NA-CA) and 115 with narcolepsy without cataplexy (NA w/o CA). Seventeen patients with NA-CA and 39 with NA w/o CA had OSA. According to Definition #1, the prevalence of REM-related OSA was 47.1% and 41.0%, respectively, in OSA patients with NA-CA and NA w/o CA; according to Definition #2, the respective prevalence was 47.1% and 38.5%, while that according to Definition #3 was 41.2% and 25.6%. No significant differences were found in the prevalence of REM-related OSA for each definition. Conclusion A high prevalence of REM-related OSA was confirmed in adult narcolepsy patients with OSA. Compared to previous reports, we noted a high frequency of REM-related OSA satisfying the relatively strict Definition #3. These results might reflect the pathophysiological characteristics of narcolepsy.

Neuroanatomical Basis of State-Dependent Activity of Upper Airway Muscles

Obstructive Sleep Apnea (OSA) is a common sleep-related respiratory disorder that is associated with cognitive, cardiovascular, and metabolic morbidities. The major cause of OSA is the sleep-related reduction of upper airway muscle tone that leads to airway obstructions in individuals with anatomically narrow upper airway. This reduction is mainly due to the suppressant effect of sleep on hypoglossal motoneurons that innervate upper airway muscles. The hypoglossal motoneurons have state-dependent activity, which is decreased during the transition from wakefulness to non-rapid eye movement sleep and is further suppressed during rapid eye movement sleep. Multiple neurotransmitters and their receptors have been implicated in the control of hypoglossal motoneuron activity across the sleep-wake states. However, to date, the results of the rigorous testing show that withdrawal of noradrenergic excitation and cholinergic inhibition essentially contribute to the depression of hypoglossal motoneuron activity during sleep. The present review will focus on origins of noradrenergic and cholinergic innervation of hypoglossal motoneurons and the functional role of these neurons in the state-dependent activity of hypoglossal motoneurons.

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.

Orexin and Central Modulation of Cardiovascular and Respiratory Function

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

Neural Control of the Upper Airway: Respiratory and State-Dependent Mechanisms

Upper airway muscles subserve many essential for survival orofacial behaviors, including their important role as accessory respiratory muscles. In the face of certain predisposition of craniofacial anatomy, both tonic and phasic inspiratory activation of upper airway muscles is necessary to protect the upper airway against collapse. This protective action is adequate during wakefulness, but fails during sleep which results in recurrent episodes of hypopneas and apneas, a condition known as the obstructive sleep apnea syndrome (OSA). Although OSA is almost exclusively a human disorder, animal models help unveil the basic principles governing the impact of sleep on breathing and upper airway muscle activity. This article discusses the neuroanatomy, neurochemistry, and neurophysiology of the different neuronal systems whose activity changes with sleep-wake states, such as the noradrenergic, serotonergic, cholinergic, orexinergic, histaminergic, GABAergic and glycinergic, and their impact on central respiratory neurons and upper airway motoneurons. Observations of the interactions between sleep-wake states and upper airway muscles in healthy humans and OSA patients are related to findings from animal models with normal upper airway, and various animal models of OSA, including the chronic-intermittent hypoxia model. Using a framework of upper airway motoneurons being under concurrent influence of central respiratory, reflex and state-dependent inputs, different neurotransmitters, and neuropeptides are considered as either causing a sleep-dependent withdrawal of excitation from motoneurons or mediating an active, sleep-related inhibition of motoneurons. Information about the neurochemistry of state-dependent control of upper airway muscles accumulated to date reveals fundamental principles and may help understand and treat OSA. © 2016 American Physiological Society. Compr Physiol 6:1801-1850, 2016.

Early improvement in obstructive sleep apnea and increase in orexin levels after bariatric surgery in adolescents and young adults

BACKGROUND

Obstructive sleep apnea (OSA) associated with obesity is known to improve after bariatric surgery, but little is known about early changes in this condition after surgery.

OBJECTIVES

To study the clinical course of OSA after bariatric surgery SETTING: Children's hospital in the United States METHODS: Adolescents and young adults with obstructive sleep apnea undergoing vertical sleeve gastrectomy (n = 6) or gastric bypass (n = 1) were enrolled in this prospective study. Participants underwent formal polysomnography before and at 3 and 5 weeks after bariatric surgery. Anthropometric measurements and assay for orexin and leptin were also performed at study visits. Thirty-one adolescents who underwent 2 polysomnography studies that were 4 weeks apart served as control patients.

RESULTS

Baseline mean (range) age of participants was 17.8 (15.4-20.7) years, 71% were male, with body mass index of 55.2 (41.3-61.6) kg/m² and had a median apnea hypopnea index (AHI) of 15.8 (7.1-23.8) events/hour. Differences in least-square means from longitudinal analysis did not show significant differences in AHI in the control group but showed significant postoperative decline in AHI relative to baseline. AHI declined postoperatively from baseline by 9.2 events/hour (95% confidence interval: 3.8 to 14.5) at 3 weeks (P = .002) and 9.1 events/hour (95% confidence interval: 3.8 to 14.5) at 5 weeks (P = .002); there was no significant change from 3 to 5 weeks in AHI. Leptin decreased and orexin levels increased significantly by 3 weeks postoperatively.

CONCLUSIONS

These observations suggest that OSA responds early and out of proportion to weight loss after metabolic and or bariatric surgery, thus weight independent factors may at least in part be responsible for early improvement in OSA postoperatively.

Signaling mechanism underlying the histamine-modulated action of hypoglossal motoneurons

Histamine, an important modulator of the arousal states of the central nervous system, has been reported to contribute an excitatory drive at the hypoglossal motor nucleus to the genioglossus (GG) muscle, which is involved in the pathogenesis of obstructive sleep apnea. However, the effect of histamine on hypoglossal motoneurons (HMNs) and the underlying signaling mechanisms have remained elusive. Here, whole-cell patch-clamp recordings were conducted using neonatal rat brain sections, which showed that histamine excited HMNs with an inward current under voltage-clamp and a depolarization membrane potential under current-clamp via histamine H1 receptors (H1Rs). The phospholipase C inhibitor U-73122 blocked H1Rs-mediated excitatory effects, but protein kinase A inhibitor and protein kinase C inhibitor did not, indicating that the signal transduction cascades underlying the excitatory action of histamine on HMNs were H1R/Gq/11 /phospholipase C/inositol-1,4,5-trisphosphate (IP3). The effects of histamine were also dependent on extracellular Na(+) and intracellular Ca(2+), which took place via activation of Na(+)-Ca(2+) exchangers. These results identify the signaling molecules associated with the regulatory effect of histamine on HMNs. The findings of this study may provide new insights into therapeutic approaches in obstructive sleep apnea. We proposed the post-synaptic mechanisms underlying the modulation effect of histamine on hypoglossal motoneuron. Histamine activates the H1Rs via PLC and IP3, increases Ca(2+) releases from intracellular stores, promotes Na(+) influx and Ca(2+) efflux via the NCXs, and then produces an inward current and depolarizes the neurons. Histamine modulates the excitability of HMNs with other neuromodulators, such as noradrenaline, serotonin and orexin. We think that these findings should provide an important new direction for drug development for the treatment of obstructive sleep apnea.

The effect of leptin replacement on sleep-disordered breathing in the leptin-deficient ob/ob mouse

Obese leptin-deficient (ob/ob) mice demonstrate defects in upper airway structural and neuromuscular control. We hypothesized that these defects predispose to upper airway obstruction during sleep, and improve with leptin administration. High-fidelity polysomnographic recordings were conducted to characterize sleep and breathing patterns in conscious, unrestrained ob/ob mice (23 wk, 67.2 ± 4.1 g, n = 13). In a parallel-arm crossover study, we compared responses to subcutaneous leptin (1 μg/h) vs. vehicle on respiratory parameters during NREM and REM sleep. Upper airway obstruction was defined by the presence of inspiratory airflow limitation (IFL), as characterized by an early inspiratory plateau in airflow at a maximum level (V̇Imax) with increasing effort. The severity of upper airway obstruction (V̇Imax) was assessed along with minute ventilation (V̇E), tidal volume (VT), respiratory rate (RR), inspiratory duty cycle, and mean inspiratory flow at each time point. IFL occurred more frequently in REM sleep (37.6 ± 0.2% vs. 1.1 ± 0.0% in NREM sleep, P < 0.001), and leptin did not alter its frequency. V̇Imax (3.7 ± 1.1 vs. 2.7 ± 0.8 ml/s, P < 0.001) and V̇E increased (55.4 ± 22.0 vs. 39.8 ± 16.4 ml/min, P < 0.001) with leptin vs. vehicle administration. The increase in V̇E was due to a significant increase in VT (0.20 ± 0.06 vs. 0.16 ± 0.05 ml, P < 0.01) rather than RR. Increases in V̇E were attributable to increases in mean inspiratory flow (2.5 ± 0.8 vs. 1.8 ± 0.6 ml/s, P < 0.001) rather than inspiratory duty cycle. Similar increases in V̇E and its components were observed in non-flow-limited breaths during NREM and REM sleep. These responses suggest that leptin stabilized pharyngeal patency and increased drive to both the upper airway and diaphragm during sleep.

Orexin-A differentially modulates AMPA-preferring responses of ganglion cells and amacrine cells in rat retina

By activating their receptors (OX1R and OX2R) orexin-A/B regulate wake/sleeping states, feeding behaviors, but the function of these peptides in the retina remains unknown. Using patch-clamp recordings and calcium imaging in rat isolated retinal cells, we demonstrated that orexin-A suppressed α-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)-preferring receptor-mediated currents (AMPA-preferring currents) in ganglion cells (GCs) through OX1R, but potentiated those in amacrine cells (ACs) through OX2R. Consistently, in rat retinal slices orexin-A suppressed light-evoked AMPA-preferring receptor-mediated excitatory postsynaptic currents in GCs, but potentiated those in ACs. Intracellular dialysis of GDP-β-S or preincubation with the Gi/o inhibitor pertussis toxin (PTX) abolished both the effects. Either cAMP/the protein kinase A (PKA) inhibitor Rp-cAMP or cGMP/the PKG blocker KT5823 failed to alter the orexin-A effects. Whilst both of them involved activation of protein kinase C (PKC), the effects on GCs and ACs were respectively eliminated by the phosphatidylinositol (PI)-phospholipase C (PLC) inhibitor and phosphatidylcholine (PC)-PLC inhibitor. Moreover, in GCs orexin-A increased [Ca(2+)]i and the orexin-A effect was blocked by intracellular Ca(2+)-free solution and by inositol 1,4,5-trisphosphate (IP3) receptor antagonists. In contrast, orexin-A did not change [Ca(2+)]i in ACs and the orexin-A effect remained in intracellular or extracellular Ca(2+)-free solution. We conclude that a distinct Gi/o/PI-PLC/IP3/Ca(2+)-dependent PKC signaling pathway, following the activation of OX1R, is likely responsible for the orexin-A effect on GCs, whereas a Gi/o/PC-PLC/Ca(2+)-independent PKC signaling pathway, following the activation of OX2R, mediates the orexin-A effect on ACs. These two actions of orexin-A, while working in concert, provide a characteristic way for modulating information processing in the inner retina.