Serotonin and NMDA receptors in respiratory long-term facilitation

Control of Breathing

Substantial advances have been made recently into the discovery of fundamental mechanisms underlying the neural control of breathing and even some inroads into translating these findings to treating breathing disorders. Here, we review several of these advances, starting with an appreciation of the importance of V̇A:V̇CO2:PaCO2 relationships, then summarizing our current understanding of the mechanisms and neural pathways for central rhythm generation, chemoreception, exercise hyperpnea, plasticity, and sleep-state effects on ventilatory control. We apply these fundamental principles to consider the pathophysiology of ventilatory control attending hypersensitized chemoreception in select cardiorespiratory diseases, the pathogenesis of sleep-disordered breathing, and the exertional hyperventilation and dyspnea associated with aging and chronic diseases. These examples underscore the critical importance that many ventilatory control issues play in disease pathogenesis, diagnosis, and treatment.

Increased spinal adenosine impairs phrenic long-term facilitation in aging rats

Moderate acute intermittent hypoxia (mAIH) elicits a form of spinal, respiratory motor plasticity known as phrenic long-term facilitation (pLTF). In middle-aged male and geriatric female rats, mAIH-induced pLTF is attenuated through unknown mechanisms. In young adults, mAIH activates competing intracellular signaling cascades, initiated by serotonin 2 and adenosine 2A (A2A) receptors, respectively. Spinal A2A receptor inhibition enhances mAIH-induced pLTF, meaning, serotonin dominates, and adenosine constrains mAIH-induced plasticity in the daily rest phase. Thus, we hypothesized elevated basal adenosine levels in the ventral cervical spinal cord of aged rats shifts this balance, undermining mAIH-induced pLTF. A selective A2A receptor antagonist (MSX-3) or vehicle was delivered intrathecally at C4 in anesthetized young (3-6 mo) and aged (20-22 mo) Sprague-Dawley rats before mAIH (3,5-min episodes; arterial Po2 = 45-55 mmHg). In young males, spinal A2A receptor inhibition enhanced pLTF (119 ± 5%) vs. vehicle (55 ± 9%), consistent with prior reports. In old males, pLTF was reduced to 25 ± 11%, but A2A receptor inhibition increased pLTF to levels greater than in young males (186 ± 19%). Basal adenosine levels in ventral C3-C5 homogenates are elevated two- to threefold in old vs. young males. These findings advance our understanding of age as a biological variable in phrenic motor plasticity and will help guide translation of mAIH as a therapeutic modality to restore respiratory and nonrespiratory movements in older populations afflicted with clinical disorders that compromise movement.NEW & NOTEWORTHY Advanced age undermines respiratory motor plasticity, specifically phrenic long-term facilitation (pLTF) following moderate acute intermittent hypoxia (mAIH). We report that spinal adenosine increases in aged male rats, undermining mAIH-induced pLTF via adenosine 2A (A2A) receptor activation, an effect reversed by selective spinal adenosine 2A receptor inhibition. These findings advance our understanding of mechanisms that impair neuroplasticity, and the ability to compensate for the onset of lung or neural injury with age, and may guide efforts to harness mAIH as a treatment for clinical disorders that compromise breathing and other movements.

Acute intermittent hypercapnic-hypoxia elicits central neural respiratory motor plasticity in humans

KEY POINTS

The occurrence of respiratory long-term facilitation following acute exposure to intermittent hypoxia is believed to be dependent upon CO₂ regulation - mechanisms governing the critical role of CO₂ have seldom been explored. We tested the hypothesis that acute intermittent hypercapnic-hypoxia (AIHH) enhances cortico-phrenic neurotransmission in awake healthy humans. The amplitude of diaphragmatic motor-evoked potentials induced by transcranial magnetic stimulation was increased after AIHH, but not the amplitude of compound muscle action potentials evoked by cervical magnetic stimulation. Mouth occlusion pressure (P_0.1 , indicator of neural respiratory drive) was also increased after AIHH, but not tidal volume or minute ventilation. Thus, moderate AIHH elicits central neural mechanisms of respiratory motor plasticity, without measurable ventilatory long-term facilitation in awake humans.

ABSTRACT

Acute intermittent hypoxia (AIH) elicits long-term facilitation (LTF) of respiration. Although LTF is observed when CO₂ is elevated during AIH in awake humans, the influence of CO₂ on corticospinal respiratory motor plasticity is unknown. Thus, we tested the hypotheses that acute intermittent hypercapnic-hypoxia (AIHH): 1) enhances cortico-phrenic neurotransmission (reflecting volitional respiratory control); and 2) elicits ventilatory LTF (reflecting automatic respiratory control). Eighteen healthy adults completed four study visits. Day 1 consisted of anthropometry and pulmonary function testing. On Days 2, 3 and 4, in a balanced alternating sequence, participants received: AIHH, poikilocapnic AIH, and normocapnic-normoxia (Sham). Protocols consisted of 15, 60-s exposures with 90-s normoxic intervals. Transcranial (TMS) and cervical (CMS) magnetic stimulation were used to induce diaphragmatic motor-evoked potentials and compound muscle action potentials, respectively. Respiratory drive was assessed via mouth occlusion pressure (P_0.1 ), and minute ventilation measured at rest. Dependent variables were assessed at baseline and 30-60 min post-exposures. Increases in TMS-evoked diaphragm potential amplitudes were observed following AIHH versus Sham (+28 ± 41%, p = 0.003), but not after AIH. No changes were observed in CMS-evoked diaphragm potential amplitudes. Mouth occlusion pressure also increased after AIHH (+21 ± 34%, p = 0.033), but not after AIH. Ventilatory LTF was not observed after any treatment. We demonstrate that AIHH elicits central neural mechanisms of respiratory motor plasticity and increases resting respiratory drive in awake humans. These findings may have important implications for neurorehabilitation after spinal cord injury and other neuromuscular disorders compromising respiratory motor function. Abstract Figure Legend In a single-blind, cross-over, sham-controlled trial, 18 healthy adults received in a balanced alternating sequence: normocapnic-normoxia (Sham), poikilocapnic acute intermittent hypoxia (AIH), and acute intermittent hypercapnic-hypoxia (AIHH). The study tested the hypothesis that AIHH enhances cortico-phrenic neurotransmission and elicits ventilatory long-term facilitation. Note the increase in the mean amplitude of diaphragmatic motor-evoked potentials (MEP) induced by transcranial magnetic stimulation 60 min after AIHH only, whereas the amplitude of diaphragmatic compound muscle action potentials evoked by cervical (phrenic nerve) stimulation were unchanged after AIHH, AIH and Sham. Traces are composite averages of all participants. Mouth occlusion pressure (P_0.1 ), an indicator of resting respiratory drive, was increased after AIHH, but not after AIH or Sham (see yellow shaded area). Traces are mouth pressure at the onset of an occluded inspiration during resting breathing. Finally, tidal volume (V_T ) was unchanged 30-60 min after AIHH, AIH and Sham. Our results indicate that moderate AIHH elicits a central neural mechanism of respiratory motor plasticity and increases resting respiratory drive in awake humans, without measurable ventilatory long-term facilitation. This article is protected by copyright. All rights reserved.

Therapeutic acute intermittent hypoxia: A translational roadmap for spinal cord injury and neuromuscular disease

We review progress towards greater mechanistic understanding and clinical translation of a strategy to improve respiratory and non-respiratory motor function in people with neuromuscular disorders, therapeutic acute intermittent hypoxia (tAIH). In 2016 and 2020, workshops to create and update a "road map to clinical translation" were held to help guide future research and development of tAIH to restore movement in people living with chronic, incomplete spinal cord injuries. After briefly discussing the pioneering, non-targeted basic research inspiring this novel therapeutic approach, we then summarize workshop recommendations, emphasizing critical knowledge gaps, priorities for future research effort, and steps needed to accelerate progress as we evaluate the potential of tAIH for routine clinical use. Highlighted areas include: 1) greater mechanistic understanding, particularly in non-respiratory motor systems; 2) optimization of tAIH protocols to maximize benefits; 3) identification of combinatorial treatments that amplify plasticity or remove plasticity constraints, including task-specific training; 4) identification of biomarkers for individuals most/least likely to benefit from tAIH; 5) assessment of long-term tAIH safety; and 6) development of a simple, safe and effective device to administer tAIH in clinical and home settings. Finally, we update ongoing clinical trials and recent investigations of tAIH in SCI and other clinical disorders that compromise motor function, including ALS, multiple sclerosis, and stroke.

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

Widespread appreciation that neuroplasticity is an essential feature of the neural system controlling breathing has emerged only in recent years. In this chapter, we focus on respiratory motor plasticity, with emphasis on the phrenic motor system. First, we define related but distinct concepts: neuromodulation and neuroplasticity. We then focus on mechanisms underlying two well-studied models of phrenic motor plasticity: (1) phrenic long-term facilitation following brief exposure to acute intermittent hypoxia; and (2) phrenic motor facilitation after prolonged or recurrent bouts of diminished respiratory neural activity. Advances in our understanding of these novel and important forms of plasticity have been rapid and have already inspired translation in multiple respects: (1) development of novel therapeutic strategies to preserve/restore breathing function in humans with severe neurological disorders, such as spinal cord injury and amyotrophic lateral sclerosis; and (2) the discovery that similar plasticity also occurs in nonrespiratory motor systems. Indeed, the realization that similar plasticity occurs in respiratory and nonrespiratory motor neurons inspired clinical trials to restore leg/walking and hand/arm function in people living with chronic, incomplete spinal cord injury. Similar application may be possible to other clinical disorders that compromise respiratory and non-respiratory movements.

Acute intermittent hypoxia evokes ventilatory long-term facilitation and active expiration in unanesthetized rats

Acute intermittent hypoxia (AIH) modifies the functioning of the respiratory network, causing respiratory motor facilitation in anesthetized animals and a compensatory increase in pulmonary ventilation in freely behaving animals. However, it is still unclear whether the ventilatory facilitation induced by AIH in unanesthetized animals is associated with changes in the respiratory pattern. We found that Holtzman male rats (80-150 g) exposed to AIH (10 × 6% O₂ for 30-40 s every 5 min, n = 9) exhibited a prolonged (30 min) increase in baseline minute ventilation (P < 0.05) compared to control animals (n = 13), combined with the occurrence of late expiratory peak flow events, suggesting the presence of active expiration. The increase in ventilation after AIH was also accompanied by reductions in arterial CO₂ and body temperature (n = 5-6, P < 0.05). The systemic treatment with ketanserin (a 5-HT₂ receptor antagonist) before AIH prevented the changes in ventilation and active expiration (n = 11) but potentiated the hypothermic response (n = 5, P < 0.05) when compared to appropriate control rats (n = 13). Our findings indicate that the ventilatory long-term facilitation elicited by AIH exposure in unanesthetized rats is linked to the generation of active expiration by mechanisms that may depend on the activation of serotonin receptors. In contrast, the decrease in body temperature induced by AIH may not require 5-HT₂ receptor activation.

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.

Effect of acute intermittent hypoxia on cortico-diaphragmatic conduction in healthy humans

Acute intermittent hypoxia (AIH) is a strategy to improve motor output in humans with neuromotor impairment. A single AIH session increases the amplitude of motor evoked potentials (MEP) in a finger muscle (first dorsal interosseous), demonstrating enhanced corticospinal neurotransmission. Since AIH elicits phrenic/diaphragm long-term facilitation (LTF) in rodent models, we tested the hypothesis that AIH augments diaphragm MEPs in humans. Eleven healthy adults (7 males, age = 29 ± 6 years) were tested. Transcranial and cervical magnetic stimulation were used to induce diaphragm MEPs and compound muscle action potentials (CMAP) recorded by surface EMG, respectively. Stimulus-response curves were generated prior to and 30-60 min after AIH. Diaphragm LTF was assessed by measurement of integrated EMG burst amplitude and frequency during eupnoeic breathing before and after AIH. Following baseline measurements, AIH was delivered from an oxygen generator connected to a facemask under poikilocapnic conditions (15 one minute episodes of 9% inspired oxygen with one minute room air intervals). There were no detectable changes in MEP (-1.5 ± 12.1%, p = 0.96) or CMAP (+0.1 ± 7.8%, p = 0.97) amplitudes across the stimulus-response curve. At stimulation intensities approximating 50% of the difference between minimum and maximum baseline amplitudes, MEP and CMAP amplitudes were also unchanged (p > 0.05). Further, no AIH effect was observed on diaphragm EMG activity during eupnoea post-AIH (p > 0.05). We conclude that unlike hand muscles, poikilocapnic AIH does not enhance diaphragm MEPs or produce diaphragm LTF in healthy humans.

Prolonged acute intermittent hypoxia improves forelimb reach-to-grasp function in a rat model of chronic cervical spinal cord injury

Repetitive acute intermittent hypoxia (AIH - brief, episodes of low inspired oxygen) elicits spinal motor plasticity, resulting in sustained improvements of respiratory and non-respiratory motor function in both animal models and humans with chronic spinal cord injury (SCI). We previously demonstrated that 7 days of AIH combined with task-specific training improves performance on a skilled locomotor task for at least 3 weeks post-treatment in rats with incomplete SCI. Here we investigated the effect of repetitive AIH administered for 12 wks on a forelimb reach-to-grasp task in a rat model of chronic, incomplete cervical SCI. In a replicated, sham-controlled, randomized and blinded study, male Spraque-Dawley rats were subject to partial hemisection at the 3rd cervical spinal segment, and exposed to daily AIH (10, 5 min episodes of 11% inspired O₂; 5 min intervals of 21% O₂) or sham normoxia (continuous 21% O₂) for 7 days beginning 8 weeks post-injury. Treatments were then reduced to 4 daily treatments per week, and continued for 11 weeks. Performance on 2 pre-conditioned motor tasks, single pellet reaching and horizontal ladder walking, was recorded each week for up to 12 weeks after initiating treatment; performance on spontaneous adhesive removal was also tested. SCI significantly impaired reach-to-grasp task performance 8 weeks post-injury (pre-treatment). Daily AIH improved reaching success by the first week of treatment versus sham controls, and this difference was maintained at 12 weeks (p < 0.0001). Daily AIH did not affect step asymmetry or stride length during ladder walking or adhesive removal time. Thus, prolonged AIH combined with task-specific training improved forelimb reach-to-grasp function in rats with a chronic cervical hemisection, but not off-target motor tasks. This study further supports the idea that daily AIH improves limb function when combined with task-specific training.

Carotid Bodies and the Integrated Cardiorespiratory Response to Hypoxia

Advances in our understanding of brain mechanisms for the hypoxic ventilatory response, coordinated changes in blood pressure, and the long-term consequences of chronic intermittent hypoxia as in sleep apnea, such as hypertension and heart failure, are giving impetus to the search for therapies to "erase" dysfunctional memories distributed in the carotid bodies and central nervous system. We review current network models, open questions, sex differences, and implications for translational research.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

Ventilatory responses to hypoxia vary widely depending on the pattern and length of hypoxic exposure. Acute, prolonged, or intermittent hypoxic episodes can increase or decrease breathing for seconds to years, both during the hypoxic stimulus, and also after its removal. These myriad effects are the result of a complicated web of molecular interactions that underlie plasticity in the respiratory control reflex circuits and ultimately control the physiology of breathing in hypoxia. Since the time domains of the physiological hypoxic ventilatory response (HVR) were identified, considerable research effort has gone toward elucidating the underlying molecular mechanisms that mediate these varied responses. This research has begun to describe complicated and plastic interactions in the relay circuits between the peripheral chemoreceptors and the ventilatory control circuits within the central nervous system. Intriguingly, many of these molecular pathways seem to share key components between the different time domains, suggesting that varied physiological HVRs are the result of specific modifications to overlapping pathways. This review highlights what has been discovered regarding the cell and molecular level control of the time domains of the HVR, and highlights key areas where further research is required. Understanding the molecular control of ventilation in hypoxia has important implications for basic physiology and is emerging as an important component of several clinical fields. © 2016 American Physiological Society. Compr Physiol 6:1345-1385, 2016.

Neither serotonin nor adenosine-dependent mechanisms preserve ventilatory capacity in ALS rats

In rats over-expressing SOD1G93A, ventilation is preserved despite significant loss of respiratory motor neurons. Thus, unknown forms of compensatory respiratory plasticity may offset respiratory motor neuron cell death. Although mechanisms of such compensation are unknown, other models of respiratory motor plasticity may provide a conceptual guide. Multiple cellular mechanisms give rise to phrenic motor facilitation; one mechanism requires spinal serotonin receptor and NADPH oxidase activity whereas another requires spinal adenosine receptor activation. Here, we studied whether these mechanisms contribute to compensatory respiratory plasticity in SOD1G93A rats. Using plethysmography, we assessed ventilation in end-stage SOD1G93A rats after: (1) serotonin depletion with parachlorophenylalanine (PCPA), (2) serotonin (methysergide) and A2A (MSX-3) receptor inhibition, (3) NADPH oxidase inhibition (apocynin), and (4) combined treatments. The ability to increase ventilation was not decreased by individual or combined treatments; thus, these mechanisms do not maintain breathing capacity at end-stage motor neuron disease. Possible mechanisms giving rise to enhanced breathing capacity with combined treatment in end-stage SOD1G93A rats are discussed.

Spinal nNOS regulates phrenic motor facilitation by a 5-HT2B receptor- and NADPH oxidase-dependent mechanism

Acute intermittent hypoxia (AIH) induces phrenic long-term facilitation (pLTF) by a mechanism that requires spinal serotonin (5-HT) receptor activation and NADPH oxidase (NOX) activity. Here, we investigated whether: (1) spinal nitric oxide synthase (NOS) activity is necessary for AIH-induced pLTF; (2) episodic exogenous nitric oxide (NO) is sufficient to elicit phrenic motor facilitation (pMF) without AIH (i.e. pharmacologically); and (3) NO-induced pMF requires spinal 5-HT2B receptor and NOX activation. In anesthetized, mechanically ventilated adult male rats, AIH (3 × 5-min episodes; 10% O2; 5 min) elicited a progressive increase in the amplitude of integrated phrenic nerve bursts (i.e. pLTF), which lasted 60 min post-AIH (45.1 ± 8.6% baseline). Pre-treatment with intrathecal (i.t.) injections of a neuronal NOS inhibitor (nNOS-inhibitor-1) near the phrenic motor nucleus attenuated pLTF (14.7 ± 2.5%), whereas an inducible NOS (iNOS) inhibitor (1400 W) had no effect (56.3 ± 8.0%). Episodic i.t. injections (3 × 5μl volume; 5 min) of a NO donor (sodium nitroprusside; SNP) elicited pMF similar in time-course and magnitude (40.4 ± 6.0%, 60 min post-injection) to AIH-induced pLTF. SNP-induced pMF was blocked by a 5-HT2B receptor antagonist (SB206553), a superoxide dismutase mimetic (MnTMPyP), and two NOX inhibitors (apocynin and DPI). Neither pLTF nor pMF was affected by pre-treatment with a protein kinase G (PKG) inhibitor (KT-5823). Thus, spinal nNOS activity is necessary for AIH-induced pLTF, and episodic spinal NO is sufficient to elicit pMF by a mechanism that requires 5-HT2B receptor activation and NOX-derived ROS formation, which indicates AIH (and NO) elicits spinal respiratory plasticity by a nitrergic-serotonergic mechanism.

The lateral habenular nucleus mediates signal transduction from the insular cortex in OSA rats

PURPOSE

The insular cortex (Ic) and habenular nuclei (Hb) of the limbic system are associated with human and animal dyspnea by regulating 5-hydroxytryptamine (5-HT) release in the raphe nuclei (RN). However, the Hb are composed of the medial habenular nucleus (MHb) and the lateral habenular nucleus (LHb). Therefore, it is still unclear whether the Ic signal is conducted through the MHb or LHb. This study aimed to investigate the role of the Hb and Ic in obstructive sleep apnea syndrome (OSA) and the functional relationship between these two structures.

METHODS

We monitored multiple indicators, including the respiration movement curve, neuronal activity in the MHb and LHb, and arterial blood gas, when stimulating the anterior Ic of Wistar rats. We compared the results with the control group (stimulating the surrounding cortex).

RESULTS

Electrical stimulation of the Ic in the rat brain caused respiratory disturbances, apnea, reduced blood pH, and aggravated base deficit (more negative base excess value) compared to control animals (p < 0.05). It also reduced the spontaneous firing of the MHb neurons but increased that of the LHb neurons. Electrical stimulation of the Ic induces apnea in rats in a similar manner to human OSA. The Ic and the Hb are functionally linked. Stimulation of the Ic inhibits the MHb, but activates the LHb. It induces OSA-like symptoms by enhancing LHb-mediated inhibition of the RN.

CONCLUSIONS

This study illustrates the mechanism by which an animal model of OSA is created by stimulating the Ic and promotes understanding of OSA pathogenesis.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

The impact of arousal state, sex, and sleep apnea on the magnitude of progressive augmentation and ventilatory long-term facilitation

We examined the impact of arousal state, sex, and obstructive sleep apnea (OSA) on the magnitude of progressive augmentation of the hypoxic ventilatory response and ventilatory long-term facilitation (vLTF). We also examined whether exposure to intermittent hypoxia during sleep has an impact on the apnea-hypopnea index (AHI) in individuals with OSA. Ten men and seven women with OSA, along with ten healthy men and ten healthy women, were exposed to twelve 2-min episodes of hypoxia (end-tidal PO(2): 50 Torr) in the presence of sustained hypercapnia (end-tidal PCO(2): 3 Torr above baseline), followed by a 30-min recovery period during wakefulness and sleep. The OSA participants completed an additional sham study during sleep. The AHI during the first hour of sleep following the intermittent hypoxia and sham protocols were compared. Progressive augmentation was only evident during wakefulness and was enhanced in the OSA participants. vLTF was evident during wakefulness and sleep. When standardized to baseline, vLTF was greater during wakefulness and was enhanced in the OSA group (men: wakefulness 1.39 ± 0.08 vs. sleep 1.14 ± 0.03; women: wakefulness 1.35 ± 0.03 vs. sleep 1.16 ± 0.05 fraction of baseline; P ≤ 0.001) compared with control (men: wakefulness 1.19 ± 0.03 vs. sleep 1.09 ± 0.03; women: wakefulness 1.26 ± 0.05 vs. sleep 1.08 ± 0.04 fraction of baseline; P ≤ 0.001). The AHI following exposure to intermittent hypoxia was increased (intermittent hypoxia 72.8 ± 7.3 vs. sham 56.5 ± 7.0 events/h; P ≤ 0.01). Sex-related differences were not observed for the primary measures. We conclude that progressive augmentation is not evident, and the magnitude of vLTF is diminished during sleep compared with wakefulness in men and women. However, when present, the phenomena are enhanced in individuals with OSA. The AHI data indicate that, under the prevailing experimental conditions, vLTF did not serve to mitigate apnea severity.

Similarities and differences in mechanisms of phrenic and hypoglossal motor facilitation

Intermittent hypoxia-induced long-term facilitation (LTF) is variably expressed in the motor output of several inspiratory nerves, such as the phrenic and hypoglossal. Compared to phrenic LTF (pLTF), less is known about hypoglossal LTF (hLTF), although it is often assumed that cellular mechanisms are the same. While fundamental mechanisms appear to be similar, potentially important differences exist in the modulation of pLTF and hLTF. The primary objectives of this paper are to: (1) review similarities and differences in pLTF and hLTF, pointing out knowledge gaps and (2) present new data suggesting that reduced respiratory neural activity elicits differential plasticity in phrenic and hypoglossal output (inactivity-induced phrenic and hypoglossal motor facilitation, iPMF and iHMF), suggesting that these motor pool-specific differences are not unique to LTF. Differences in fundamental mechanisms or modulation of plasticity among motor pools may confer the capacity to mount a complex ventilatory response to specific challenges, particularly in motor pools with different "jobs" in the control of breathing.

Prospective trial of efficacy and safety of ondansetron and fluoxetine in patients with obstructive sleep apnea syndrome

STUDY OBJECTIVE

Incremental withdrawal of serotonin during wake to sleep transition is postulated as a key mechanism that renders the pharyngeal airway collapsible. While serotonin promotion with reuptake inhibitors have demonstrated modest beneficial effects during NREM sleep on obstructive sleep apnea (OSA), animal studies suggest a potential therapeutic role for selective serotonin receptor antagonists (5-HT3) in REM sleep. We aimed to test the hypothesis that a combination of ondansetron (Ond) and fluoxetine (Fl) may effectively reduce expression of disordered breathing during REM and NREM sleep in patients with OSA.

DESIGN AND SETTING

A prospective, parallel-groups, single-center trial in patients with OSA.

PARTICIPANTS

35 adults with apnea hypopnea index (AHI) > 10; range 10-98.

INTERVENTION

Subjects were randomized to placebo, n = 7; Ond (24 mg QD), n = 9; Fl (5 mg QD) + Ond (12 mg QD), n = 9; and Fl (10 mg QD) + Ond (24 mg QD), n = 10.

MEASUREMENTS AND RESULTS

AHI was measured by in-lab polysomnography after a 7-day no-treatment period (Baseline) and on days 14 and 28 of treatment. The primary endpoint was AHI reduction at days 14 and 28. OND+FL resulted in approximately 40% reduction of baseline AHI at days 14 and 28 (unadjusted P < 0.03 for each) and improved oximetry trends. This treatment-associated relative reduction in AHI was also observed in REM and supine sleep.

CONCLUSIONS

Combined treatment with OND+FL is well-tolerated and reduces AHI, yielding a potentially therapeutic response in some subjects with OSA.

Urethane inhibits genioglossal long-term facilitation in un-paralyzed anesthetized rats

For approximately 3 decades, urethane has been (partially or solely) used as a successful anesthetic in numerous respiratory long-term facilitation (LTF) studies, which were performed on anesthetized, paralyzed, vagotomized and artificially ventilated animals of several different species. However, things become complicated when LTF of muscle activity is studied in un-paralyzed animals. For example, a commonly used acute intermittent hypoxia (AIH) protocol failed to induce muscle LTF in anesthetized, spontaneously breathing rats. But muscle LTF could be induced when hypoxic episode number was increased and/or anesthetics other than urethane were used. In these studies however, neither anesthetic nor paralysis was mentioned as a potential factor influencing AIH-induced muscle LTF. This study tested whether urethane inhibits AIH-induced genioglossal LTF (gLTF) in un-paralyzed ventilated rats, and if so, determined whether reducing urethane dose reverses this inhibition. Three groups of adult male Sprague-Dawley rats were anesthetized (Group 1: approximately 1.6 g kg(-1) urethane; Group 2: 50 mg kg(-1) alpha-chloralose +0.9-1.2 g kg(-1) urethane; Group 3: 0.9 g kg(-1) urethane +200-400 microg kg(-1) min(-1) alphaxalone), vagotomized and mechanically ventilated. Integrated genioglossus activity was measured before, during and after AIH (5 episodes of 3-min isocapnic 12% O(2), separated by 3-min hyperoxic intervals). The AIH-induced gLTF was absent in Group 1 rats (success rate was only approximately 1/7), but was present in Group 2 (in 10/12 rats) and Group 3 (in 11/11 rats) rats. The genioglossal response to hypoxia was not significantly different among the 3 groups. Collectively, these data suggest that urethane dose-dependently inhibits gLTF in un-paralyzed anesthetized rats.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

The role of the pontine respiratory complex in the response to intermittent hypoxia

These experiments were designed to determine the effects of EEG state on the response of rats to intermittent hypoxia and to test the hypotheses that short-term potentiation (STP) and ventilatory long term facilitation (vLTF) are state dependent; and that neurons with NMDA receptors in the dorso-ventral pontine respiratory group (dvPRG) modulate the development of STP and vLTF in rats. Low-doses of urethane anaesthesia (<1.3g/kg) that do not cause significant respiratory depression or reductions in sensitivity to hypoxia result in cycling between EEG states that superficially resemble wake and slow wave sleep in rats and are accompanied by changes in breathing pattern that closely resemble those seen when unanaesthetized rats cycle between wake and SWS. When changes between these states were accounted for, intermittent, poikilocapnic hypoxia did not produce a significant vLTF. However, there was a persistent STP of tidal volume and vLTF did develop after blockade of NMDAr in the region of the PBrKF complex by microinjection of MK-801. Blockade of NMDA-type glutamate receptor-mediated processes in the dorsal pons also caused animals to cycle into State III, but did not alter the response to either continuous or intermittent hypoxia indicating that the response to hypoxia was not state dependent. This shows that neurons in the region of the PRG inhibit STP and vLTF, but no longer do so if PRG NMDA receptor activation is blocked.

Glossopharyngeal long-term facilitation requires serotonin 5-HT2 and NMDA receptors in rats

Although the glossopharyngeal nerve (IX) is mainly a sensory nerve, it innervates stylopharyngeus and some other pharyngeal muscles, whose excitations would likely improve upper airway patency since electrical IX stimulation increases pharyngeal airway size. As acute intermittent hypoxia (AIH) induces hypoglossal and genioglossal long-term facilitation (LTF), we hypothesized that AIH induces glossopharyngeal LTF, which requires serotonin 5-HT(2) and NMDA receptors. Integrated IX activity was recorded in anesthetized, vagotomized, paralyzed and ventilated rats before, during and after 5 episodes of 3-min isocapnic 12% O(2) with 3-min intervals of 50% O(2). Either saline, ketanserin (5-HT(2) antagonist, 2mg/kg) or MK-801 (NMDA antagonist, 0.2mg/kg) was (i.v.) injected 30-60 min before AIH. Both phasic and tonic IX activities were persistently increased (both P<0.05) after AIH in vehicle, but not ketanserin or MK-801, rats. Hypoxic glossopharyngeal responses were minimally changed after either drug. These data suggest that AIH induces both phasic and tonic glossopharyngeal LTF, which requires activation of 5-HT(2) and NMDA receptors.

Propofol abolished the phrenic long-term facilitation in rats

The aim was to investigate the effect of propofol anesthesia on the phrenic long-term facilitation (pLTF) in rats. We hypothesized that pLTF would be abolished during propofol-compared with urethane anesthesia. Fourteen adult, male, anesthetized, vagotomized, paralyzed, and mechanically ventilated Sprague-Dawley rats (seven per group), were exposed to the acute intermittent hypoxia (AIH) protocol. Peak phrenic nerve activity (PNA), burst frequency (f), and breathing rhythm parameters (Ti, Te, Ttot) were analyzed during the first hypoxia (TH1), as well as at 15 (T15), 30 (T30), and 60min (T60) after the final hypoxic episode, and compared to the baseline values. In propofol-anesthetized rats no significant changes of PNA were recorded after the last hypoxic episode, i.e. no pLTF was induced. There was a significant increase of PNA (59.4+/-6.6%, P<0.001) in urethane-anesthetized group at T60. AIH did not elicit significant changes in f, Ti, Te, Ttot in either group at T15, T30, and T60. The pLTF, elicited by AIH, was induced in the urethane-anesthetized rats. On the contrary, pLTF was abolished in the propofol-anesthetized rats.

Progressive augmentation and ventilatory long-term facilitation are enhanced in sleep apnoea patients and are mitigated by antioxidant administration

Progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) of respiratory motor output are forms of respiratory plasticity that are initiated during exposure to intermittent hypoxia. The present study was designed to determine whether PA and vLTF are enhanced in obstructive sleep apnoea (OSA) participants compared to matched healthy controls. The study was also designed to determine whether administration of an antioxidant cocktail mitigates PA and vLTF. Thirteen participants with sleep apnoea and 13 controls completed two trials. During both trials participants were exposed to intermittent hypoxia which included twelve 4-min episodes of hypoxia (P(ETCO(2)), 50 mmHg; P(ETCO(2)), 4 mmHg above baseline) followed by 30 min of recovery. Prior to exposure to intermittent hypoxia, participants were administered, in a randomized fashion, either an antioxidant or a placebo cocktail. Baseline measures of minute ventilation during the placebo and antioxidant trials were not different between or within groups. During the placebo trial, PA was evident in both groups; however it was enhanced in the OSA group compared to control (last hypoxic episode 36.9 +/- 2.8 vs. 27.7 +/- 2.2 l min(-1); P Collapse

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MESH Headings

• Adult
• Antioxidants/administration & dosage
• Humans
• Hypoxia/blood
• Hypoxia/drug therapy
• Hypoxia/physiopathology
• Male
• Oxidative Stress/drug effects
• Oxidative Stress/physiology
• Polysomnography/methods
• Pulmonary Ventilation/drug effects
• Pulmonary Ventilation/physiology
• Sleep Apnea, Obstructive/blood
• Sleep Apnea, Obstructive/drug therapy
• Sleep Apnea, Obstructive/physiopathology
• Superoxide Dismutase/administration & dosage
• Time Factors
• Ubiquinone/administration & dosage
• Ubiquinone/analogs & derivatives
• Vitamin E/administration & dosage

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Grants

• R01 HL085537 NHLBI NIH HHS
• R01HL085537 NHLBI NIH HHS

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Affiliation(s)

Dorothy S Lee John D. Dingell Veterans Administration Medical Center, Wayne State University, Detroit, MI 48201, USA
M Safwan Badr
Jason H Mateika

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Shedding light on restoring respiratory function after spinal cord injury

Loss of respiratory function is one of the leading causes of death following spinal cord injury. Because of this, much work has been done in studying ways to restore respiratory function following spinal cord injury (SCI) – including pharmacological and regeneration strategies. With the emergence of new and powerful tools from molecular neuroscience, new therapeutically relevant alternatives to these approaches have become available, including expression of light sensitive proteins called channelrhodopsins. In this article we briefly review the history of various attempts to restore breathing after C2 hemisection, and focus on our recent work using the activation of light sensitive channels to restore respiratory function after experimental SCI. We also discuss how such light-induced activity can help shed light on the inner workings of the central nervous system respiratory circuitry that controls diaphragmatic function.

Vagal afferent innervation and remodeling in the aortic arch of young-adult fischer 344 rats following chronic intermittent hypoxia

Previously, we have shown that chronic intermittent hypoxia (CIH) impairs baroreflex control of heart rate and augments aortic baroreceptor afferent function. In the present study, we examined whether CIH induces structural changes of aortic afferent axons and terminals. Young-adult Fischer 344 (F344, 4 months old) rats were exposed to room air (RA) or CIH for 35-45 days. After 14-24 days of exposure, they received tracer DiI injection into the left nodose ganglion to anterogradely label vagal afferent nerves. After surgery, animals were returned to their cages to continue RA or CIH exposure. Twenty-one days after DiI injection, the animals were sacrificed and the aortic arch was examined using confocal microscopy. In both RA and CIH rats, we found that DiI-labeled vagal afferent axons entered the wall of the aortic arch, then fanned out and branched into large receptive fields with numerous terminals (flower-sprays, end-nets and free endings). Vagal afferent axons projected much more to the anterior wall than to the posterior wall. In general, the flower-sprays, end-nets and free endings were widely and similarly distributed in the aortic arch of both groups. However, several salient differences between RA and CIH rats were found. Compared to RA control, CIH rats appeared to have larger vagal afferent receptive fields. The CIH rats had many abnormal flower-sprays, end-nets, and free endings which were intermingled and diffused into "bush-like" structures. However, the total number of flower-sprays was comparable (P>0.05). Since there was a large variance of the size of flower-sprays, we only sampled the 10 largest flower-sprays from each animal. CIH substantially increased the size of large flower-sprays (P<0.01). Numerous free endings with enlarged varicosities were identified, resembling axonal sprouting structures. Taken together, our data indicate that CIH induces significant remodeling of afferent terminal structures in the aortic arch of F344 rats. We suggest that such an enlargement of vagal afferent terminals may contribute to altered aortic baroreceptor function following CIH.

Spinal NMDA receptor activation is necessary for de novo, but not the maintenance of, A2a receptor-mediated phrenic motor facilitation

Adenosine 2a (A2a) receptor agonists elicit persistent increases in phrenic nerve activity by transactivating the neurotrophin receptor, TrkB, near phrenic motoneurons. Our working model proposes that A2a receptor-mediated TrkB receptor activation strengthens glutamatergic synapses onto phrenic motoneurons. Activation of glutamate N-methyl d-aspartate (NMDA) receptors has been implicated in other models of phrenic motor plasticity. Thus we hypothesized that NMDA receptor activation also would contribute to A2a receptor-mediated phrenic motor facilitation. Adult male Sprague-Dawley rats were anesthetized with urethane, mechanically ventilated, neuromuscularly paralyzed, and bilaterally vagotomized. The A2a receptor agonist CGS-21680 and the NMDA receptor-channel blocker MK-801 were administered intrathecally over the C4 spinal segment. Phrenic nerve activity was recorded before, during, and after drug administration. MK-801 (concentration range 0.1, 1.0, 10.0, and 100 microM) was administered 30 min before CGS-21680 (50 microM). MK-801 dose-dependently blocked A2a receptor-mediated phrenic motor facilitation. When administered at 60 min post-CGS-21680, MK-801 prevented further increases in phrenic nerve activity compared with the CGS-21680 alone (CGS-21680 alone at 120 min: 114 +/- 19%; CGS-21680 and MK-801 at 60 min post-CGS-21680: 61 +/- 11%, above baseline, P < 0.05) but did not return phrenic motor output to baseline values. Our data suggest that NMDA receptor activation is necessary for de novo A2a receptor-mediated phrenic motor facilitation and that the maintenance of preexisting phrenic motor facilitation does not involve NMDA receptor-dependent mechanisms.

Intermittent hypoxia and respiratory plasticity in humans and other animals: does exposure to intermittent hypoxia promote or mitigate sleep apnoea?

This review focuses on two phenomena that are initiated during and after exposure to intermittent hypoxia. The two phenomena are referred to as long-term facilitation and progressive augmentation of respiratory motor output. Both phenomena are forms of respiratory plasticity. Long-term facilitation is characterized by a sustained elevation in respiratory activity after exposure to intermittent hypoxia. Progressive augmentation is characterized by a gradual increase in respiratory activity from the initial to the final hypoxic exposure. There is much speculation that long-term facilitation may have a significant role in individuals with sleep apnoea because this disorder is characterized by periods of upper airway collapse accompanied by intermittent hypoxia, one stimulus known to induce long-term facilitation. It has been suggested that activation of long-term facilitation may serve to mitigate apnoea by facilitating ventilation and, more importantly, upper airway muscle activity. We examine the less discussed but equally plausible situation that exposure to intermittent hypoxia might ultimately lead to the promotion of apnoea. There are at least two scenarios in which apnoea might be promoted following exposure to intermittent hypoxia. In both scenarios, long-term facilitation of upper airway muscle activity is initiated but ultimately rendered ineffective because of other physiological conditions. Thus, one of the primary goals of this review is to discuss, with support from basic and clinical studies, whether various forms of respiratory motor neuronal plasticity have a beneficial and/or a detrimental impact on breathing stability in individuals with sleep apnoea.

Overview: the neurochemistry of respiratory control

This special issue of Respiratory Physiology and Neurobiology surveys a broad range of topics focused on the neurochemical control of breathing. A variety of approaches have integrated the neurochemistry of breathing with the physiology of individual neurons, with the neuroanatomy of brainstem and forebrain respiratory circuits, and with the clinical pathology of respiratory disorders all of which has been fueled by the ongoing explosion of information in the molecular biology of the nervous system. Accordingly, substantial progress has identified neurotransmitters, neuromodulators, receptors, signaling cascades, trophic factors, hormones, and genes mediating normal and pathological breathing. Dynamic changes in the neurochemistry of breathing are addressed with respect to brainstem development, environmental challenges such as intermittent or chronic hypoxia, and as a function of the sleep-wake cycle. Respiratory disruption has also been identified in an increasing variety of genetic-based disorders and remarkable progress has been made in determining the affected genes and their mutations that negatively impact respiration.

Sleep fragmentation impairs ventilatory long-term facilitation via adenosine A1 receptors

Sleep fragmentation (SF), a primary feature of obstructive sleep apnoea (OSA), impairs hippocampal long-term potentiation and causes cognitive/attention deficits. However, its influence upon respiratory control has hardly been studied. This study examined the effect of SF on ventilatory long-term facilitation (LTF, a persistent augmentation of respiratory activity after episodic hypoxia) and the hypoxic ventilatory response (HVR), and investigated the role of adenosine A1 receptors in these SF effects in conscious adult male Sprague-Dawley rats. SF, confirmed by sleep architecture recordings, was achieved by periodic, forced locomotion in a rotating drum (30 s rotation/90 s stop for 24 h). LTF, elicited by five episodes of 5 min poikilocapnic hypoxia (10% O2) with 5 min intervals, was measured by plethysmography. Resting ventilation and metabolic rate were unchanged, HVR was reduced (150.6 +/- 3.5% versus 110.4 +/- 12.3%) and LTF was eliminated (22.6 +/- 0.5% versus -0.1 +/- 1.3%) shortly after 24 h SF. The SF-induced impairments were SF duration dependent, and completely reversible as HVR (< 24 h) and LTF (< 48 h) returned spontaneously to their pre-SF values. The SF-impaired HVR was improved (130.3 +/- 4.2%) and SF-eliminated LTF was restored (19.6 +/- 0.9%) by systemic injection of the adenosine A1 receptor antagonist 8-CPT (2.5 mg kg(-1)) approximately 30 min before LTF elicitation. Both HVR and LTF were also similarly impaired by 24 h total sleep deprivation or 24 h repeated cage tapping-induced SF, but not by a 24 h locomotion control protocol for SF. Collectively, these data suggest that: (1) 24 h SF impairs LTF and poikilocapnic HVR; (2) these impairments require A1 receptors; and (3) SF of OSA may exacerbate OSA via impaired ventilatory control mechanisms.