Episodic hypoxia induces long-term facilitation of neural drive to tongue protrudor and retractor muscles

Combination of acute intermittent hypoxia and intermittent transcutaneous electrical stimulation in obstructive sleep apnea: a randomized controlled crossover trial

Intermittent hypoxia (IH) and intermittent transcutaneous electrical stimulation (ITES) might benefit patients with obstructive sleep apnea (OSA). However, the therapeutic value of combined IH and ITES in OSA is unknown. In this prospective, randomized, controlled crossover study, normoxia (air exposure for 50 min before sleep and sham stimulation for 6 h during sleep), IH (5 repeats of 5 min 10-12 % O2 alternating with 5 min air for 50 min, and sham stimulation for 6 h), ITES (air exposure for 50 min and 6 repeats of 30 min transcutaneous electrical stimulation alternating with 30 min of sham stimulation for 6 h), and IH&ITES (10-12 % O2 alternating with air for 50 min and transcutaneous electrical stimulation alternating with sham stimulation for 6 h) were administered to patients with OSA over four single-night sessions. The primary endpoint was difference in OSA severity between the interventions according to apnea-hypopnea index (AHI) and oxygen desaturation index (ODI). The efficacy was response to IH, ITES, IH&ITES defined as a ≥50 % reduction in AHI compared with normoxia. Twenty participants (17 male, 3 female) completed the trial. The median (IQR) AHI decreased from 14.5 (10.8, 17.5) events/h with normoxia to 6.9 (3.9, 14.8) events/h with IH (p=0.020), 5.7 (3.4, 9.1) events/h with ITES (p=0.001), and 3.5 (1.8, 6.4) events/h with IH&ITES (p=0.001). AHI was significantly different between IH and IH&ITES (p=0.042) but not between ITES and IH&ITES (p=0.850). For mild-moderate OSA (n=17), IH, ITES, and IH&ITES had a significant effect on AHI (p=0.013, p=0.001, p=0.001, respectively) compared with normoxia, but there were no differences in post hoc pairwise comparisons between intervention groups. No serious adverse events were observed. In conclusion, IH, ITES, and IH&ITES significantly reduced OSA severity. IH&ITES showed better efficacy in mild-moderate OSA than IH and was comparable to ITES. Our data do not support recommending IH&ITES over ITES for OSA.

The hypoxic ventilatory response and hypoxic burden are predictors of the magnitude of ventilatory long-term facilitation in humans

Mild intermittent hypoxia initiates progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) in humans. The magnitude of these forms of plasticity might be influenced by anthropometric and physiological variables, as well as protocol elements. However, the impact of many of these variables on the magnitude of respiratory plasticity has not been established in humans. A meta-analysis was completed using anthropometric and physiological variables obtained from 124 participants that completed one of three intermittent hypoxia protocols. Simple correlations between the aggregate variables and the magnitude of PA and vLTF standardized to baseline was completed. Thereafter, the variables correlated to PA or vLTF were input into a multilinear regression equation. Baseline measures of the hypoxic ventilatory response was the sole predictor of PA (R = 0.370, P = 0.012). Similarly, this variable along with the hypoxic burden predicted the magnitude of vLTF (R = 0.546, P < 0.006 for both variables). In addition, the magnitude of PA was strongly correlated to vLTF (R = 0.617, P < 0.001). Anthropometric measures do not predict the magnitude of PA and vLTF in humans. Alternatively, the hypoxic ventilatory response was the sole predictor of PA, and in combination with the hypoxic burden, predicted the magnitude of vLTF. These influences should be considered in the design of mild intermittent hypoxia protocol studies in humans. Moreover, the strong correlation between PA and vLTF suggests that a common mechanistic pathway may have a role in the initiation of these forms of plasticity. KEY POINTS: Mild intermittent hypoxia initiates progressive augmentation (PA) and ventilatory long-term facilitation (vLTF) in humans. Many of the anthropometric and physiological variables that could impact the magnitude of these forms of plasticity are unknown. Anthropometric and physiological variables were measured from a total of 124 participants that completed one of three distinct intermittent hypoxia protocols. The variables correlated to PA or vLTF were input into a multilinear regression analysis. The hypoxic ventilatory response was the sole predictor of PA, while this variable in addition to the average hypoxic burden predicted the magnitude of vLTF. A strong correlation between PA and vLTF was also revealed. These influences should be considered in the design of mild intermittent hypoxia protocol studies in humans. Moreover, the strong correlation between PA and vLTF suggests that a common mechanistic pathway may have a role in the initiation of these forms of plasticity.

Randomised, controlled crossover trial of intermittent and continuous transcutaneous electrical stimulation of the genioglossus muscle for obstructive sleep apnoea

PURPOSE

Continuous transcutaneous electrical stimulation (CTES) of the genioglossus muscle may benefit patients with obstructive sleep apnoea (OSA). However, the therapeutic value of intermittent transcutaneous electrical stimulation (ITES) for OSA is unclear.

METHODS

This was a randomised, controlled, crossover study to compare the effects of ITES and CTES of the genioglossus muscle. Over three single-night sessions, participants were alternately subjected to three genioglossus stimulation modalities during sleep (sham, CTES and ITES). The apnoea-hypopnoea index (AHI) and oxygen desaturation index (ODI) were used for OSA diagnosis and to evaluate efficacy. A responder was defined as an individual with a ≥50% reduction in AHI together with <10 AHI events per hour and/or an ODI reduction of ≥25% between sham stimulation and electrical stimulation nights.

RESULTS

Fifteen men with OSA completed the study. Compared with sham, the median AHI with ITES decreased by 13.3 events/hour (95% CI 3.1 to 23.5, p=0.030) and by 7.3 events/hour (95% CI -3.9 to 18.5, p=0.825) with CTES. The median ODI was reduced by 9.25 events/hour (95% CI 0.5 to 18.0) with ITES and 3.3 events/hour (95% CI -5.6 to 12.2) with CTES; however, there was no significant difference between groups. Furthermore, ITES outperformed CTES with respect to longest apnoea duration (median (95% CI), 9.5 (0.0 to 19.0), p=0.011)) and the highest sleep efficiency (12.2 (2.7 to 21.7), p=0.009). Of the 15 participants, 8 responded to ITES and 3 responded to CTES (p=0.058), of whom all eight cases and two out of three cases had ODIs <5 events/hour, respectively. All participants tolerated ITES well.

CONCLUSIONS

ITES improved upper airway obstruction in patients with OSA, suggesting that further prospective validation of the intermittent approach is warranted.

TRIAL REGISTRATION NUMBER

ChiCTR2100050138.

Cardiorespiratory plasticity in humans following two patterns of acute intermittent hypoxia

NEW FINDINGS

What is the central question of this study? Do cardiorespiratory experience-dependent effects (EDEs) differ between two different stimulus durations of acute isocapnic intermittent hypoxia (IHx; 5-min vs. 90-s cycles between hypoxia and normoxia)? What is the main finding and its importance? There was long-term facilitation in ventilation and blood pressure in both IHx protocols, but there was no evidence of progressive augmentation or post-hypoxia frequency decline. Not all EDEs described in animal models translate to acute isocapnic IHx responses in humans, and cardiorespiratory responses to 5-min versus 90-s on/off IHx protocols are largely similar.

ABSTRACT

Peripheral respiratory chemoreceptors monitor breath-by-breath changes in arterial CO₂ and O₂ , and mediate ventilatory changes to maintain homeostasis. Intermittent hypoxia (IHx) elicits hypoxic ventilatory responses, with well-described experience-dependent effects (EDEs), derived mostly from animal work involving intermittent 5-min cycles of hypoxia and normoxia. These EDEs include post-hypoxia frequency decline (PHxFD), progressive augmentation (PA) and long-term facilitation (LTF). Comparisons of these EDEs between animal models and humans using similar IHx protocols are lacking. In addition, it is unknown whether shorter bouts of hypoxia, which may be more relevant to clinical conditions, elicit EDEs of similar magnitudes in humans. Respiratory (frequency, tidal volume and minute ventilation ( V ̇ I ) and cardiovascular (heart rate and mean arterial pressure (MAP)) variables were measured during and following two patterns of acute isocapnic IHx in 14 healthy human participants (four female): (1) 5 × 5 min and (2) 5 × 90 s on/off hypoxia. Participants' end-tidal P O 2 was clamped at 45 Torr during hypoxia and 100 Torr during normoxia. We found that (1) PHxFD and PA were not present in either IHx pattern (P > 0.14), (2) LTF was present in V ̇ I following both 5-min (P < 0.001) and 90-s isocapnic IHx trials (P < 0.001), and (3) LTF was present in MAP following 5-min isocapnic IHx (P < 0.001), and trended towards significance following 90-s IHx (P = 0.058). We demonstrate that acute isocapnic IHx alone may not elicit all of the EDEs that have been described in animal models. Additionally, ventilatory LTF occurred regardless of the length of hypoxia-normoxia cycles.

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.

Chronic intermittent hypoxia increases excitability and synaptic excitation of protrudor and retractor hypoglossal motoneurones

KEY POINTS

Dysfunctions in the hypoglossal control of tongue extrinsic muscles are implicated in obstructive sleep apnoea (OSA) syndrome. Chronic intermittent hypoxia (CIH), an important feature of OSA syndrome, produces deleterious effects on the motor control of oropharyngeal resistance, but whether the hypoglossal motoneurones innervating the tongue extrinsic muscles are affected by CIH is unknown. We show that CIH enhanced the respiratory-related activity of rat hypoglossal nerve innervating the protrudor and retractor tongue extrinsic muscles. Intracellular recordings revealed increases in respiratory-related firing frequency and synaptic excitation of inspiratory protrudor and retractor hypoglossal motoneurones after CIH. CIH also increased their intrinsic excitability, depolarised resting membrane potential and reduced K⁺ -dominated leak conductance. CIH affected the breathing-related synaptic control and intrinsic electrophysiological properties of protrudor and retractor hypoglossal motoneurones to optimise the neural control of oropharyngeal function.

ABSTRACT

Inspiratory-related tongue movements and oropharyngeal motor actions are controlled mainly by the protrudor and retractor extrinsic tongue muscles, which are innervated by the hypoglossal motoneurones. Chronic intermittent hypoxia (CIH), an important feature of obstructive sleep apnoea syndrome, produces detrimental effects on the contractile function of the tongue extrinsic muscles and the medullary inspiratory network of rodents. However, the impact of the CIH on the electrophysiological properties of protrudor and retractor hypoglossal motoneurones has not been described before. Using nerves and intracellular recordings in in situ preparation of rats (5 weeks old), we tested the hypothesis that CIH (FiO₂ of 0.06, SaO₂ 74%, during 30-40 s, every 9 min, 8 h/day for 10 days) increases the intrinsic excitability of protrudor and retractor motoneurones from the hypoglossal motor nucleus of rats. Recordings of hypoglossal nerve, before its bifurcation to innervate the tongue protrudor and retractor muscles, revealed that CIH enhances its pre-inspiratory, simultaneously with the presence of active expiration, and inspiratory activities. These changes were mediated by increases in the respiratory-related firing frequency and synaptic excitation of inspiratory protrudor and retractor hypoglossal motoneurones. Besides, CIH increases their intrinsic excitability and depolarises resting membrane potential by reducing a K⁺ -dominated leak conductance. In conclusion, CIH enhances the respiratory-related neural control of oropharyngeal function of rats by increasing the synaptic excitation, intrinsic excitability, and reducing leak conductance in both protrudor and retractor hypoglossal motoneurones. We propose that these network and cellular changes are important to optimise the oropharyngeal resistance in conditions related to intermittent hypoxia.

Development of ventilatory long-term facilitation is dependent on estrous cycle stage in adult female rats

Ventilatory long-term facilitation (vLTF) is a form of respiratory plasticity characterized by a progressive and sustained increase in minute ventilation over time following acute, intermittent hypoxia (AIH). Though vLTF has been repeatedly demonstrated in adult males (rats and humans), few studies have assessed vLTF in adult females and no studies have explored differential expression of vLTF across the normal female estrous cycle. We recently reported that AIH-induced plasticity of phrenic motor output (phrenic long-term facilitation, pLTF), a phenotypically similar form of respiratory plasticity presenting as a sustained increase in phrenic nerve amplitude, develops in adult female rats only during the proestrus stage of the estrous cycle, notable for high levels of serum estrogen. Here, we tested the hypothesis that AIH-induced vLTF would also be estrous-stage dependent; developing in female rats during proestrus, but not estrus. Barometric plethysmography in adult (4-5 months), normally cycling female rats revealed a progressive increase in minute ventilation for 60 min following AIH (5 × 5 min episodes; 10% O₂) during proestrus indicative of vLTF, while estrus rats showed no changes in minute ventilation over the same time period. The development of vLTF in proestrus rats was driven by changes in tidal volume production versus respiratory frequency consistent with prior studies. These data are the first to investigate differential vLTF expression across the estrous cycle in adult female rats and highlight the importance of female estrous cycle stage as a critical physiological variable to consider in studies of AIH-induced plasticity.

Intermittent but not sustained moderate hypoxia elicits long-term facilitation of hypoglossal motor output

Phrenic long-term facilitation (pLTF) is a form of serotonin-dependent respiratory motor plasticity induced by moderate acute intermittent hypoxia (AIH), but not by moderate acute sustained hypoxia (ASH) of similar cumulative duration. Thus, moderate AIH-induced pLTF is sensitive to the pattern of hypoxia. On the other hand, pLTF induced by severe AIH protocols is neither pattern sensitive nor serotonin dependent (it converts to an adenosine-dependent mechanism). Although moderate AIH also induces hypoglossal LTF (hLTF), no data are available concerning its sensitivity/insensitivity to the pattern of hypoxia. Since hLTF following moderate hypoxia is serotonin-dependent, we hypothesized that hLTF is pattern-sensitive, similar to serotonin-dependent pLTF. Integrated hypoglossal nerve activity was recorded in urethane-anesthetized, vagotomized, paralyzed, and ventilated rats exposed to isocapnic AIH (3, 5min episodes of 11% O₂) or ASH (a single 25min episode of 11% O₂). Similar to previous studies of pLTF, hypoglossal motor output was elevated for more than 1h following AIH (50±20%, p<0.01), but not ASH (-6±9%, p>0.05). Frequency LTF was not observed following either hypoxic exposure. Thus, in agreement with our hypothesis, hypoglossal LTF following moderate AIH is pattern-sensitive, similar to phrenic LTF.

Exposure to intermittent hypoxia and sustained hypercapnia reduces therapeutic CPAP in participants with obstructive sleep apnea

Our purpose was to determine whether exposure to mild intermittent hypoxia leads to a reduction in the therapeutic continuous positive airway pressure required to eliminate breathing events. Ten male participants were treated with twelve 2-min episodes of hypoxia ([Formula: see text] ≈50 mmHg) separated by 2-min intervals of normoxia in the presence of [Formula: see text] that was sustained 3 mmHg above baseline. During recovery from the last episode, the positive airway pressure was reduced in a stepwise fashion until flow limitation was evident. The participants also completed a sham protocol under normocapnic conditions, which mimicked the time frame of the intermittent hypoxia protocol. After exposure to intermittent hypoxia, the therapeutic pressure was significantly reduced (i.e., 5 cmH2O) without evidence of flow limitation (103.4 ± 6.3% of baseline, P = 0.5) or increases in upper airway resistance (95.6 ± 15.0% of baseline, P = 0.6). In contrast, a similar decrease in pressure was accompanied by flow limitation (77.0 ± 1.8% of baseline, P = 0.001) and an increase in upper airway resistance (167.2 ± 17.5% of baseline, P = 0.01) after the sham protocol. Consistent with the initiation of long-term facilitation of upper airway muscle activity, exposure to intermittent hypoxia reduced the therapeutic pressure required to eliminate apneic events that could improve treatment compliance. This possibility, coupled with the potentially beneficial effects of intermittent hypoxia on comorbidities linked to sleep apnea, suggests that mild intermittent hypoxia may have a multipronged therapeutic effect on sleep apnea.

NEW & NOTEWORTHY Our new finding is that exposure to mild intermittent hypoxia reduced the therapeutic pressure required to treat sleep apnea. These findings are consistent with previous results, which have shown that long-term facilitation of upper muscle activity can be initiated following exposure to intermittent hypoxia in humans.

Is normobaric hypoxia an effective treatment for sustaining previously acquired altitude acclimatization?

This study examined whether normobaric hypoxia (NH) treatment is more efficacious for sustaining high-altitude (HA) acclimatization-induced improvements in ventilatory and hematologic responses, acute mountain sickness (AMS), and cognitive function during reintroduction to altitude (RA) than no treatment at all. Seventeen sea-level (SL) residents (age = 23 ± 6 yr; means ± SE) completed in the following order: 1) 4 days of SL testing; 2) 12 days of HA acclimatization at 4,300 m; 3) 12 days at SL post-HA acclimatization (Post) where each received either NH (n = 9, [Formula: see text] = 0.122) or Sham (n = 8; [Formula: see text] = 0.207) treatment; and 4) 24-h reintroduction to 4,300-m altitude (RA) in a hypobaric chamber (460 Torr). End-tidal carbon dioxide pressure ([Formula: see text]), hematocrit (Hct), and AMS cerebral factor score were assessed at SL, on HA2 and HA11, and after 20 h of RA. Cognitive function was assessed using the SynWin multitask performance test at SL, on HA1 and HA11, and after 4 h of RA. There was no difference between NH and Sham treatment, so data were combined. [Formula: see text] (mmHg) decreased from SL (37.2 ± 0.5) to HA2 (32.2 ± 0.6), decreased further by HA11 (27.1 ± 0.4), and then increased from HA11 during RA (29.3 ± 0.6). Hct (%) increased from SL (42.3 ± 1.1) to HA2 (45.9 ± 1.0), increased again from HA2 to HA11 (48.5 ± 0.8), and then decreased from HA11 during RA (46.4 ± 1.2). AMS prevalence (%) increased from SL (0 ± 0) to HA2 (76 ± 11) and then decreased at HA11 (0 ± 0) and remained depressed during RA (17 ± 10). SynWin scores decreased from SL (1,615 ± 62) to HA1 (1,306 ± 94), improved from HA1 to HA11 (1,770 ± 82), and remained increased during RA (1,707 ± 75). These results demonstrate that HA acclimatization-induced improvements in ventilatory and hematologic responses, AMS, and cognitive function are partially retained during RA after 12 days at SL whether or not NH treatment is utilized.NEW & NOTEWORTHY This study demonstrates that normobaric hypoxia treatment over a 12-day period at sea level was not more effective for sustaining high-altitude (HA) acclimatization during reintroduction to HA than no treatment at all. The noteworthy aspect is that athletes, mountaineers, and military personnel do not have to go to extraordinary means to retain HA acclimatization to an easily accessible and relevant altitude if reexposure occurs within a 2-wk time period.

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.

Power spectral analysis of hypoglossal nerve activity during intermittent hypoxia-induced long-term facilitation in mice

Power spectral analyses of electrical signals from respiratory nerves reveal prominent oscillations above the primary rate of breathing. Acute exposure to intermittent hypoxia can induce a form of neuroplasticity known as long-term facilitation (LTF), in which inspiratory burst amplitude is persistently elevated. Most evidence indicates that the mechanisms of LTF are postsynaptic and also that high-frequency oscillations within the power spectrum show coherence across different respiratory nerves. Since the most logical interpretation of this coherence is that a shared presynaptic mechanism is responsible, we hypothesized that high-frequency spectral content would be unchanged during LTF. Recordings of inspiratory hypoglossal (XII) activity were made from anesthetized, vagotomized, and ventilated 129/SVE mice. When arterial O2 saturation (SaO2) was maintained >96%, the XII power spectrum and burst amplitude were unchanged for 90 min. Three, 1-min hypoxic episodes (SaO2 = 50 ± 10%), however, caused a persistent (>60 min) and robust (>400% baseline) increase in burst amplitude. Spectral analyses revealed a rightward shift of the signal content during LTF, with sustained increases in content above ∼125 Hz following intermittent hypoxia and reductions in power at lower frequencies. Changes in the spectral content during LTF were qualitatively similar to what occurred during the acute hypoxic response. We conclude that high-frequency content increases during XII LTF in this experimental preparation; this may indicate that intermittent hypoxia-induced plasticity in the premotor network contributes to expression of XII LTF.

Intermittent hypoxia: a low-risk research tool with therapeutic value in humans

Intermittent hypoxia has generally been perceived as a high-risk stimulus, particularly in the field of sleep medicine, because it is thought to initiate detrimental cardiovascular, respiratory, cognitive, and metabolic outcomes. In contrast, the link between intermittent hypoxia and beneficial outcomes has received less attention, perhaps because it is not universally understood that outcome measures following exposure to intermittent hypoxia may be linked to the administered dose. The present review is designed to emphasize the less recognized beneficial outcomes associated with intermittent hypoxia. The review will consider the role intermittent hypoxia has in cardiovascular and autonomic adaptations, respiratory motor plasticity, and cognitive function. Each section will highlight the literature that contributed to the belief that intermittent hypoxia leads primarily to detrimental outcomes. The second segment of each section will consider the possible risks associated with experimentally rather than naturally induced intermittent hypoxia. Finally, the body of literature indicating that intermittent hypoxia initiates primarily beneficial outcomes will be considered. The overarching theme of the review is that the use of intermittent hypoxia in research investigations, coupled with reasonable safeguards, should be encouraged because of the potential benefits linked to the administration of a variety of low-risk intermittent hypoxia protocols.

Effect of chronic intermittent hypoxia on the reflex recruitment of the genioglossus during airway obstruction in the anesthetized rat

We sought to test the hypothesis that chronic intermittent hypoxia (CIH)-a feature of sleep-disordered breathing in humans-impairs reflex recruitment of the genioglossus (GG, pharyngeal dilator) during obstructive airway events. Adult male Wistar rats were exposed to 20 cycles of normoxia and hypoxia (5% O2 at nadir) per hour, 8h a day for 7 days (CIH, N=7). The sham group (N=7) were exposed to normoxia in parallel. Following gas treatments, rats were anesthetized with an i.p. injection of urethane (1.5g/kg; 20%, w/v). Fine concentric needle electrodes were inserted into the GG and the costal diaphragm. Discriminated GG motor unit potentials and whole electromyograph (EMG), together with arterial blood pressure and arterial O2 saturation, were recorded during quiet basal breathing and during nasal airway occlusion. Airway occlusion significantly increased GG EMG activity in all animals; but there was no difference in the reflex response to airway occlusion between sham and CIH-treated animals (+105±22% vs. +105±17%, mean±SEM for area under the curve of integrated GG EMG, % increase from baseline, p=0.99). Occluded breaths were characterized by a significant increase in the firing frequency of phasically active units and the recruitment of large motor units that were quiescent under basal conditions. Though there are reports of impaired control of the upper airway following CIH in the rat, we conclude that reflexly evoked motor discharge to the GG is not affected by 7 days of CIH, a paradigm that we have shown increases apnea index in sleeping rats.

Pulmonary C-fiber activation attenuates respiratory-related tongue movements

The functional impact of pulmonary C-fiber activation on upper airway biomechanics has not been evaluated. Here, we tested the hypothesis that pulmonary C-fiber activation alters the respiratory-related control of tongue movements. The force produced by tongue movements was quantified in spontaneously breathing, anesthetized adult rats before and after stimulation of pulmonary C fibers via intrajugular delivery of capsaicin (0.625 and 1.25 μg/kg). Brief occlusion of the trachea was used to increase the respiratory drive to the tongue muscles, and hypoglossal (XII) nerve branches were selectively sectioned to denervate the protrusive and retrusive tongue musculature. Tracheal occlusion triggered inspiratory-related tongue retrusion in rats with XII nerves intact or following section of the medial XII nerve branch, which innervates the genioglossus muscle. Inspiratory-related tongue protrusion was only observed after section of the lateral XII branch, which innervates the primary tongue retrusor muscles. The tension produced by inspiratory-related tongue movement was significantly attenuated by capsaicin, but tongue movements remained retrusive, unless the medial XII branch was sectioned. Capsaicin also significantly delayed the onset of tongue movements such that tongue forces could not be detected until after onset of the inspiratory diaphragm activity. We conclude that altered neural drive to the tongue muscles following pulmonary C-fiber activation has a functionally significant effect on tongue movements. The diminished tongue force and delay in the onset of tongue movements following pulmonary C-fiber activation are potentially unfavorable for upper airway patency.

Recurrent laryngeal nerve activity exhibits a 5-HT-mediated long-term facilitation and enhanced response to hypoxia following acute intermittent hypoxia in rat

A progressive and sustained increase in inspiratory-related motor output (“long-term facilitation”) and an augmented ventilatory response to hypoxia occur following acute intermittent hypoxia (AIH). To date, acute plasticity in respiratory motor outputs active in the postinspiratory and expiratory phases has not been studied. The recurrent laryngeal nerve (RLN) innervates laryngeal abductor muscles that widen the glottic aperture during inspiration. Other efferent fibers in the RLN innervate adductor muscles that partially narrow the glottic aperture during postinspiration. The aim of this study was to investigate whether or not AIH elicits a serotonin-mediated long-term facilitation of laryngeal abductor muscles, and if recruitment of adductor muscle activity occurs following AIH. Urethane anesthetized, paralyzed, unilaterally vagotomized, and artificially ventilated adult male Sprague-Dawley rats were subjected to 10 exposures of hypoxia (10% O2 in N2, 45 s, separated by 5 min, n = 7). At 60 min post-AIH, phrenic nerve activity and inspiratory RLN activity were elevated (39 ± 11 and 23 ± 6% above baseline, respectively). These responses were abolished by pretreatment with the serotonin-receptor antagonist, methysergide ( n = 4). No increase occurred in time control animals ( n = 7). Animals that did not exhibit postinspiratory RLN activity at baseline did not show recruitment of this activity post-AIH ( n = 6). A repeat hypoxia 60 min after AIH produced a significantly greater peak response in both phrenic and RLN activity, accompanied by a prolonged recovery time that was also prevented by pretreatment with methysergide. We conclude that AIH induces neural plasticity in laryngeal motoneurons, via serotonin-mediated mechanisms similar to that observed in phrenic motoneurons: the so-called “Q-pathway”. We also provide evidence that the augmented responsiveness to repeat hypoxia following AIH also involves a serotonergic mechanism.

Effect of chronic intermittent hypoxia on noradrenergic activation of hypoglossal motoneurons

In obstructive sleep apnea patients, elevated activity of the lingual muscles during wakefulness protects the upper airway against occlusions. A possibly related form of respiratory neuroplasticity is present in rats exposed to acute and chronic intermittent hypoxia (CIH). Since rats exposed to CIH have increased density of noradrenergic terminals and increased α(1)-adrenoceptor immunoreactivity in the hypoglossal (XII) nucleus, we investigated whether these anatomic indexes of increased noradrenergic innervation translate to increased sensitivity of XII motoneurons to noradrenergic activation. Adult male Sprague-Dawley rats were subjected to CIH for 35 days, with O(2) level varying between 24% and 7% with 180-s period for 10 h/day. They were then anesthetized, vagotomized, paralyzed, and artificially ventilated. The dorsal medulla was exposed, and phenylephrine (2 mM, 10 nl) and then the α(1)-adrenoceptor antagonist prazosin (0.2 mM, 3 × 40 nl) were microinjected into the XII nucleus while XII nerve activity (XIIa) was recorded. The area under integrated XIIa was measured before and at different times after microinjections. The excitatory effect of phenylephrine on XII motoneurons was similar in sham- and CIH-treated rats. In contrast, spontaneous XIIa was more profoundly reduced following prazosin injections in CIH- than sham-treated rats [to 21 ± 7% (SE) vs. 40 ± 8% of baseline, P < 0.05] without significant changes in central respiratory rate, arterial blood pressure, or heart rate. Thus, consistent with increased neuroanatomic measures of noradrenergic innervation of XII motoneurons following exposure to CIH, prazosin injections revealed a stronger endogenous noradrenergic excitatory drive to XII motoneurons in CIH- than sham-treated anesthetized rats.

Impact of repeated daily exposure to intermittent hypoxia and mild sustained hypercapnia on apnea severity

We examined whether exposure to intermittent hypoxia (IH) during wakefulness impacted on the apnea/hypopnea index (AHI) during sleep in individuals with sleep apnea. Participants were exposed to twelve 4-min episodes of hypoxia in the presence of sustained mild hypercapnia each day for 10 days. A control group was exposed to sustained mild hypercapnia for a similar duration. The intermittent hypoxia protocol was completed in the evening on day 1 and 10 and was followed by a sleep study. During all sleep studies, the change in esophageal pressure (ΔPes) from the beginning to the end of an apnea and the tidal volume immediately following apneic events were used to measure respiratory drive. Following exposure to IH on day 1 and 10, the AHI increased above baseline measures (day 1: 1.95 ± 0.42 fraction of baseline, P ≤ 0.01, vs. day 10: 1.53 ± 0.24 fraction of baseline, P < 0.06). The indexes were correlated to the hypoxic ventilatory response (HVR) measured during the IH protocol but were not correlated to the magnitude of ventilatory long-term facilitation (vLTF). Likewise, ΔPes and tidal volume measures were greater on day 1 and 10 compared with baseline (ΔPes: -8.37 ± 0.84 vs. -5.90 ± 1.30 cmH(2)0, P ≤ 0.04; tidal volume: 1,193.36 ± 101.85 vs. 1,015.14 ± 119.83 ml, P ≤ 0.01). This was not the case in the control group. Interestingly, the AHI on day 10 (0.78 ± 0.13 fraction of baseline, P ≤ 0.01) was significantly less than measures obtained during baseline and day 1 in the mild hypercapnia control group. We conclude that enhancement of the HVR initiated by exposure to IH may lead to increases in the AHI during sleep and that initiation of vLTF did not appear to impact on breathing stability. Lastly, our results suggest that repeated daily exposure to mild sustained hypercapnia may lead to a decrease in breathing events.

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.

Identification of a novel form of noradrenergic-dependent respiratory motor plasticity triggered by vagal feedback

The respiratory control system is not just reflexive, it is smart, it learns, and, in fact, it has a memory. The respiratory system listens to and carefully remembers how previous stimuli affect breathing. Respiratory memory is laid down by adjusting synaptic strength between respiratory neurons. For example, repeated hypoxic bouts trigger a form of respiratory memory that functions to strengthen the ability of respiratory motoneurons to trigger contraction of breathing muscles. This type of respiratory plasticity is known as long-term facilitation (LTF). Although chemical feedback, such as hypoxia, initiates LTF, it is unknown whether natural modulation of mechanical feedback (from vagal inputs) also causes motor plasticity. Here, we used reverse microdialysis, electrophysiology, neuropharmacology, and histology to determine whether episodic modulation of vagally mediated mechanical feedback is able to induce respiratory LTF in anesthetized adult rats. We show that repeated obstructive apneas disrupt vagal feedback and trigger LTF of hypoglossal motoneuron activity and genioglossus muscle tone. This same stimulus does not cause LTF of diaphragm activity. Hypoxic episodes do not cause apnea-induced LTF; instead, LTF is triggered by modulation of vagal feedback. Unlike hypoxia-induced respiratory plasticity, vagus-induced LTF does not require 5-HT(2) receptors but instead relies on activation of α1-adrenergic receptors on hypoglossal motoneurons. In summary, we identify a novel form of hypoxia- and 5-HT-independent respiratory motor plasticity that is triggered by physiological modulation of vagal feedback and is mediated by α1-adrenergic receptor activation on (or near) hypoglossal motoneurons.

The hypoxic ventilatory response and ventilatory long-term facilitation are altered by time of day and repeated daily exposure to intermittent hypoxia

This study examined whether time of day and repeated exposure to intermittent hypoxia have an impact on the hypoxic ventilatory response (HVR) and ventilatory long-term facilitation (vLTF). Thirteen participants with sleep apnea were exposed to twelve 4-min episodes of isocapnic hypoxia followed by a 30-min recovery period each day for 10 days. On days 1 (initial day) and 10 (final day) participants completed the protocol in the evening (PM); on the remaining days the protocol was completed in the morning (AM). The HVR was increased in the morning compared with evening on the initial (AM 0.83 ± 0.08 vs. PM 0.64 ± 0.11 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.01) and final days (AM 1.0 ± 0.08 vs. PM 0.81 ± 0.09 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.01, where %SaO₂ refers to percent arterial oxygen saturation). Moreover, the magnitude of the HVR was enhanced following daily exposure to intermittent hypoxia in the morning (initial day 0.83 ± 0.08 vs. final day 1.0 ± 0.08 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.03) and evening (initial day 0.64 ± 0.11 vs. final day 0.81 ± 0.09 l·min⁻¹·%SaO₂⁻¹; P ≤ 0.03). vLTF was reduced in the morning compared with the evening on the initial (AM 19.03 ± 0.35 vs. PM 22.30 ± 0.49 l/min; P ≤ 0.001) and final (AM 20.54 ± 0.32 vs. PM 23.11 ± 0.54 l/min; P ≤ 0.01) days. Following daily exposure to intermittent hypoxia, vLTF was enhanced in the morning (initial day 19.03 ± 0.35 vs. final day 20.54 ± 0.32 l/min; P ≤ 0.01). We conclude that the HVR is increased while vLTF is decreased in the morning compared with the evening in individuals with sleep apnea and that the magnitudes of these phenomena are enhanced following daily exposure to intermittent hypoxia.

Acute intermittent hypoxia in rat in vivo elicits a robust increase in tonic sympathetic nerve activity that is independent of respiratory drive

Acute intermittent hypoxia (AIH) elicits long-term increases in respiratory and sympathetic outflow (long-term facilitation, LTF). It is still unclear whether sympathetic LTF is totally dependent on changes in respiration, even though respiratory drive modulates sympathetic nerve activity (SNA). In urethane-anaesthetized, vagotomized mechanically ventilated Sprague-Dawley rats, we investigated the effect of ten 45 s episodes of 10% O2-90% N(2) on splanchnic sympathetic nerve activity (sSNA) and phrenic nerve activity (PNA). We then tested whether or not hypoxic sympathetic chemoreceptor and baroreceptor reflexes were changed 60 min after AIH. We found that 17 animals manifested a sustained increase of sSNA (+51.2+/-4.7%) 60 min after AIH, but only 10 of these rats also expressed phrenic LTF compared with the time controls (rats not exposed to hypoxia, n=5). Inspiratory triggered averages of integrated sSNA showed respiratory modulation of SNA regardless of whether or not phrenic LTF had developed. The hypoxic chemoreceptor reflex was enhanced by 60 min after the development of AIH (peak change from 76.9+/-13.9 to 159.5+/-24.9%). Finally, sympathetic baroreceptor reflex sensitivity increased after sympathetic LTF was established (Gainmax from 1.79+/-0.18 to 2.60+/-0.28% mmHg1). Our findings indicate that respiratory-sympathetic coupling does contribute to sympathetic LTF, but that an additional tonic increase of sympathetic tone is also present that is independent of the level of PNA. Sympathetic LTF is not linked to the change in baroreflex function, since the baroreflex appears to be enhanced rather than impaired, but does play an important role in the enhancement of the hypoxic chemoreflex.

Phrenicotomy alters phrenic long-term facilitation following intermittent hypoxia in anesthetized rats

Intermittent hypoxia (IH) can induce a persistent increase in neural drive to the respiratory muscles known as long-term facilitation (LTF). LTF of phrenic inspiratory activity is often studied in anesthetized animals after phrenicotomy (PhrX), with subsequent recordings being made from the proximal stump of the phrenic nerve. However, severing afferent and efferent axons in the phrenic nerve has the potential to alter the excitability of phrenic motoneurons, which has been hypothesized to be an important determinant of phrenic LTF. Here we test the hypothesis that acute PhrX influences immediate and long-term phrenic motor responses to hypoxia. Phrenic neurograms were recorded in anesthetized, ventilated, and vagotomized adult male rats with intact phrenic nerves or bilateral PhrX. Data were obtained before (i.e., baseline), during, and after three 5-min bouts of isocapnic hypoxia. Inspiratory burst amplitude during hypoxia (%baseline) was greater in PhrX than in phrenic nerve-intact rats (P < 0.001). Similarly, burst amplitude 55 min after IH was greater in PhrX than in phrenic nerve-intact rats (175 + or - 9 vs. 126 + or - 8% baseline, P < 0.001). In separate experiments, phrenic bursting was recorded before and after PhrX in the same animal. Afferent bursting that was clearly observable in phase with lung deflation was immediately abolished by PhrX. The PhrX procedure also induced a form of facilitation as inspiratory burst amplitude was increased at 30 min post-PhrX (P = 0.01 vs. pre-PhrX). We conclude that, after PhrX, axotomy of phrenic motoneurons and, possibly, removal of phrenic afferents result in increased phrenic motoneuron excitability and enhanced LTF following IH.

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.

Preinspiratory and inspiratory hypoglossal motor output during hypoxia-induced plasticity in the rat

Respiratory-related discharge in the hypoglossal (XII) nerve is composed of preinspiratory (pre-I) and inspiratory (I) activity. Our first purpose was to test the hypothesis that hypoxia-induced plasticity in XII motor output is differentially expressed in pre-I vs. I XII bursting. Short-term potentiation (STP) of XII motor output was induced in urethane-anesthetized, vagotomized, and ventilated rats by exposure to isocapnic hypoxia (PaO2 of approximately 35 Torr). Both pre-I and I XII discharge abruptly increased at beginning of hypoxia (i.e., acute hypoxic response), and the relative increase in amplitude was much greater for pre-I (507+/-46% baseline) vs. I bursting (257+/-16% baseline; P<0.01). In addition, STP was expressed in I but not pre-I bursting following hypoxia. Specifically, I activity remained elevated following termination of hypoxia but pre-I bursting abruptly returned to prehypoxia levels. Our second purpose was to test the hypothesis that STP of I XII activity results from recruitment of inactive or "silent" XII motoneurons (MNs) vs. rate coding of active MNs. Single fiber recordings were used to classify XII MNs as I, expiratory-inspiratory, or silent based on baseline discharge patterns. STP of I XII activity following hypoxia was associated with increased discharge frequency in active I and silent MNs but not expiratory-inspiratory MNs. We conclude that the expression of respiratory plasticity is differentially regulated between pre-I and I XII activity. In addition, both recruitment of silent MNs and rate coding of active I MNs contribute to increases in XII motor output following hypoxia.

Effect of episodic hypoxia on the susceptibility to hypocapnic central apnea during NREM sleep

We hypothesized that episodic hypoxia (EH) leads to alterations in chemoreflex characteristics that might promote the development of central apnea in sleeping humans. We used nasal noninvasive positive pressure mechanical ventilation to induce hypocapnic central apnea in 11 healthy participants during stable nonrapid eye movement sleep before and after an exposure to EH, which consisted of fifteen 1-min episodes of isocapnic hypoxia (mean O(2) saturation/episode: 87.0 +/- 0.5%). The apneic threshold (AT) was defined as the absolute measured end-tidal PCO(2) (Pet(CO(2))) demarcating the central apnea. The difference between the AT and baseline Pet(CO(2)) measured immediately before the onset of mechanical ventilation was defined as the CO(2) reserve. The change in minute ventilation (V(I)) for a change in Pet(CO(2)) (DeltaV(I)/ DeltaPet(CO(2))) was defined as the hypocapnic ventilatory response. We studied the eupneic Pet(CO(2)), AT Pet(CO(2)), CO(2) reserve, and hypocapnic ventilatory response before and after the exposure to EH. We also measured the hypoxic ventilatory response, defined as the change in V(I) for a corresponding change in arterial O(2) saturation (DeltaV(I)/DeltaSa(O(2))) during the EH trials. V(I) increased from 6.2 +/- 0.4 l/min during the pre-EH control to 7.9 +/- 0.5 l/min during EH and remained elevated at 6.7 +/- 0.4 l/min the during post-EH recovery period (P < 0.05), indicative of long-term facilitation. The AT was unchanged after EH, but the CO(2) reserve declined significantly from -3.1 +/- 0.5 mmHg pre-EH to -2.3 +/- 0.4 mmHg post-EH (P < 0.001). In the post-EH recovery period, DeltaV(I)/DeltaPet(CO(2)) was higher compared with the baseline (3.3 +/- 0.6 vs. 1.8 +/- 0.3 l x min(-1) x mmHg(-1), P < 0.001), indicative of an increased hypocapnic ventilatory response. However, there was no significant change in the hypoxic ventilatory response (DeltaV(I)/DeltaSa(O(2))) during the EH period itself. In conclusion, despite the presence of ventilatory long-term facilitation, the increase in the hypocapnic ventilatory response after the exposure to EH induced a significant decrease in the CO(2) reserve. This form of respiratory plasticity may destabilize breathing and promote central apneas.

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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Phrenic motoneuron discharge patterns during hypoxia-induced short-term potentiation in rats

Hypoxia-induced short-term potentiation (STP) of respiratory motor output is manifested by a progressive increase in activity after the acute hypoxic response and a gradual decrease in activity on termination of hypoxia. We hypothesized that STP would be differentially expressed between physiologically defined phrenic motoneurons (PhrMNs). Phrenic nerve "single fiber" recordings were used to characterize PhrMN discharge in anesthetized, vagotomized and ventilated rats. PhrMNs were classified as early (Early-I) or late inspiratory (Late-I) according to burst onset relative to the contralateral phrenic neurogram during normocapnic baseline conditions. During hypoxia (F(I)O(2) = 0.12-0.14, 3 min), both Early-I and Late-I PhrMNs abruptly increased discharge frequency. Both cell types also showed a progressive increase in frequency over the remainder of hypoxia. However, Early-I PhrMNs showed reduced overall discharge duration and total spikes/breath during hypoxia, whereas Late-I PhrMNs maintained constant discharge duration and therefore increased the number of spikes/breath. A population of previously inactive (i.e., silent) PhrMNs was recruited 48 +/- 8 s after hypoxia onset. These PhrMNs had a Late-I onset, and the majority (8/9) ceased bursting promptly on termination of hypoxia. In contrast, both Early-I and Late-I PhrMNs showed post-hypoxia STP as reflected by greater discharge frequencies and spikes/breath during the post-hypoxic period (P < 0.01 vs. baseline). We conclude that the expression of phrenic STP during hypoxia reflects increased activity in previously active Early-I and Late-I PhrMNs and recruitment of silent PhrMNs. post-hypoxia STP primarily reflects persistent increases in the discharge of PhrMNs, which were active before hypoxia.

Long-term facilitation of upper airway muscle activity induced by episodic upper airway negative pressure and hypoxia in spontaneously breathing anaesthetized rats

Obstruction of the upper airway (UA) is associated with episodes of hypoxia and upper airway negative pressure (UANP). In the companion paper it is shown that episodic hypoxia elicits long-term facilitation (LTF) of tongue protrudor, retractor and respiratory pump muscle activity. However, whether repeated exposure to UANP also induces LTF is unknown. We hypothesized that repetitive exposure to UANP would induce LTF of UA and respiratory pump muscle activity and when coupled with hypoxia, as occurs when the UA obstructs, would lead to an even greater facilitation of muscle activity and the response to UANP. Experiments were performed in 24 anaesthetized, spontaneously breathing rats with intact vagi. To induce LTF, UANP stimuli (-10 cmH(2)O) of 5 s duration were delivered every 30 s for 3 min (+/- hypoxia). This was repeated eight times over 1 h, each 3 min episode separated by 5 min of normoxia. Genioglossus (GG), hyoglossus (HG) and diaphragm (Dia) muscle activity was recorded before, during and for 1 h following the last exposure to episodic UANP alone (n = 8), UANP and hypoxia together (n = 8) or normoxia alone (n = 8). During the final hour, single pulses of UANP were applied at 1 min and every 10 min thereafter to determine whether LTF of the response to UANP had been induced. Our results show that LTF of GG muscle activity and its response to UANP was induced following exposure to episodic UANP stimuli alone and UANP applied during hypoxia. However, there was no significant difference between these responses. Episodic UANP alone also induced LTF of HG muscle activity but this effect did not manifest until 40 min following the last episode of repeated UANP stimulation. In the presence of hypoxia, no LTF of HG muscle response to UANP was found. In conclusion, episodic UANP stimulation induces LTF of UA dilator and retractor tongue muscles, but no further facilitation occurs when coupled with hypoxia. This response may serve as an important protective mechanism of respiratory homeostasis during sleep, particularly in patients who suffer from obstructive sleep apnoea.

Neural deficits contribute to respiratory insufficiency in Pompe disease

Pompe disease is a severe form of muscular dystrophy due to glycogen accumulation in all tissues, especially striated muscle. Disease severity is directly related to the deficiency of acid alpha-glucosidase (GAA), which degrades glycogen in the lysosome. Respiratory dysfunction is a hallmark of the disease, muscle weakness has been viewed as the underlying cause, and the possibility of an associated neural contribution has not been evaluated previously. Therefore, we examined behavioral and neurophysiological aspects of breathing in 2 animal models of Pompe disease--the Gaa(-/-) mouse and a transgenic line (MTP) expressing GAA only in skeletal muscle, as well as a detailed analysis of the CNS in a Pompe disease patient. Glycogen content was elevated in the Gaa(-/-) mouse cervical spinal cord. Retrograde labeling of phrenic motoneurons showed significantly greater soma size in Gaa(-/-) mice vs. isogenic controls, and glycogen was observed in Gaa(-/-) phrenic motoneurons. Ventilation, assessed via plethysmography, was attenuated during quiet breathing and hypercapnic challenge in Gaa(-/-) mice (6 to >21 months of age) vs. controls. We confirmed that MTP mice had normal diaphragmatic contractile properties; however, MTP mice had ventilation similar to the Gaa(-/-) mice during quiet breathing. Neurophysiological recordings indicated that efferent phrenic nerve inspiratory burst amplitudes were substantially lower in Gaa(-/-) and MTP mice vs. controls. In human samples, we demonstrated similar pathology in the cervical spinal cord and greater accumulation of glycogen in spinal cord compared with brain. We conclude that neural output to the diaphragm is deficient in Gaa(-/-) mice, and therapies targeting muscle alone may be ineffective in Pompe disease.

Episodic hypoxia induces long-term facilitation of upper airway muscle activity in spontaneously breathing anaesthetized rats

We performed these experiments to determine if repeated exposure to episodic hypoxia induces long term facilitation (LTF) in anaesthetized spontaneously breathing rats. A previous study in spontaneously breathing rats was unable to demonstrate evidence of LTF with repeated hypoxia, but this may have been due to the low number of hypoxic episodes used. We hypothesized that with sufficient exposure, episodic hypoxia LTF of genioglossus (GG), hyoglossus (HG) and diaphragm (Dia) activities would be elicited. Experiments were performed in 24 anaesthetized spontaneously breathing rats with intact vagi. Peak and tonic GG, HG and Dia EMG activities were recorded before, during and for 1 h following exposure to eight (n = 8) or three (n = 8) episodes of isocapnic hypoxia ( = 0.1) each of 3 min duration. A third time control series was also performed with exposure to normoxia alone ( = 0.28, n = 8). Short-term potentiation of GG and HG muscle activity developed during the early period after repeated exposure to eight and three hypoxic episodes. LTF, however, occurred only after eight hypoxic episodes. This manifested as an increase in peak GG and Dia inspiratory muscle activity and tonic HG activity. LTF of respiratory breathing frequency was also induced, reflected by a reduction in inspiratory and expiratory time. These findings support our initial hypothesis that LTF in the anaesthetized, spontaneously breathing rat is dependent on the number of exposures to hypoxia and show that the responses to repetitive hypoxia are composed of both short and long-term facilitatory changes.

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.

Determinants of frequency long-term facilitation following acute intermittent hypoxia in vagotomized rats

Acute intermittent (AIH), but not acute sustained hypoxia (ASH) elicits a form of respiratory plasticity known as long-term facilitation (LTF). In anesthetized rats, LTF is expressed as increased respiratory-related nerve burst amplitude, with variable effects on burst frequency. We analyzed a large data set from multiple investigators using the same experimental protocol to determine factors influencing frequency LTF. Our meta-analysis revealed that AIH elicits both phrenic amplitude and frequency LTF in anesthetized and vagotomized rats, but frequency LTF is small in comparison with amplitude LTF (12% versus 60%, respectively). ASH elicits a small, but significant frequency and amplitude LTF (8% and 10%, respectively) that is not significantly different than controls. Similar to all published reports, analysis of this large data set confirms that phrenic amplitude LTF following AIH is significantly greater than ASH. Multiple regression analysis revealed a strong correlation between baseline burst frequency and frequency LTF. Variations in baseline burst frequency may contribute to variation in frequency LTF and may underlie the apparent effects of some drug treatments.

Impact of intermittent hypoxia on long-term facilitation of minute ventilation and heart rate variability in men and women: do sex differences exist?

Following exposure to intermittent hypoxia, respiratory motor activity and sympathetic nervous system activity may persist above baseline levels for over an hour. The present investigation was designed to determine whether sustained increases in minute ventilation and sympathovagal (S/V) balance, in addition to sustained depression of parasympathetic nervous system activity (PNSA), were greater in men compared with women following exposure to intermittent hypoxia. Fifteen healthy men and women matched for age, race, and body mass index were exposed to eight 4-min episodes of hypoxia during sustained hypercapnia followed by a 15-min end-recovery period. The magnitude of the increase in minute ventilation during the end-recovery period, compared with baseline, was similar in men and women (men, 1.52 +/- 0.03; women, 1.57 +/- 0.02 fraction of baseline; P < 0.0001). In contrast, depression of PNSA and increases in S/V balance were evident during the end-recovery period, compared with baseline, in men (PNSA, 0.66 +/- 0.06 fraction of baseline, P < 0.0001; S/V balance, 2.8 +/- 0.7 fraction of baseline, P < 0.03) but not in women (PNSA, 1.27 +/- 0.19 fraction of baseline, P = 0.3; S/V balance, 1.8 +/- 0.6 fraction of baseline, P = 0.2). We conclude that a sustained increase in minute ventilation, which is indicative of long-term facilitation, is evident in both men and women following exposure to intermittent hypoxia and that this response is independent of sex. In contrast, sustained alterations in autonomic nervous system activity were evident in men but not in women.

Long-term facilitation of genioglossus activity is present in normal humans during NREM sleep

Episodic hypoxia (EH) is followed by increased ventilatory motor output in the recovery period indicative of long-term facilitation (LTF). We hypothesized that episodic hypoxia evokes LTF of genioglossus (GG) muscle activity in humans during non-rapid eye movement sleep (NREM) sleep. We studied 12 normal non-flow limited humans during stable NREM sleep. We induced 10 brief (3 min) episodes of isocapnic hypoxia followed by 5 min of room air. Measurements were obtained during control, hypoxia, and at 5, 10, 20, 30 and 40 min of recovery, respectively, for minute ventilation (V(I)), supraglottic pressure (P(SG)), upper airway resistance (R(UA)) and phasic GG electromyogram (EMG(GG)). In addition, sham studies were conducted on room air. During hypoxia there was a significant increase in phasic EMG(GG) (202.7+/-24.1% of control, p<0.01) and in V (I) (123.0+/-3.3% of control, p<0.05); however, only phasic EMG(GG) demonstrated a significant persistent increase throughout the recovery. (198.9+/-30.9%, 203.6+/-29.9% and 205.4+/-26.4% of control, at 5, 10, and 20 min of recovery, respectively, p<0.01). In multivariate regression analysis, age and phasic EMG(GG) activity during hypoxia were significant predictors of EMG(GG) at recovery 20 min. No significant changes in any of the measured parameters were noted during sham studies.

CONCLUSION

(1) EH elicits LTF of GG in normal non-flow limited humans during NREM sleep, without concomitant ventilatory or mechanical LTF. (2) GG activity during the recovery period correlates with the magnitude of GG activation during hypoxia, and inversely with age.

Ventilatory long-term facilitation in mice can be observed during both sleep and wake periods and depends on orexin

Respiratory long-term facilitation (LTF) is a long-lasting (>1 h) augmentation of respiratory motor output that occurs even after cessation of hypoxic stimuli, is serotonin-dependent, and is thought to prevent sleep-disordered breathing such as sleep apnea. Raphe nuclei, which modulate several physiological functions through serotonin, receive dense projections from orexin-containing neurons in the hypothalamus. We examined possible contributions of orexin to ventilatory LTF by measuring respiration in freely moving prepro-orexin knockout mice (ORX-KO) and wild-type (WT) littermates before, during, and after exposure to intermittent hypoxia (IH; 5 x 5 min at 10% O2), sustained hypoxia (SH; 25 min at 10% O2), or sham stimulation. Respiratory data during quiet wakefulness (QW), slow wave sleep (SWS), and rapid-eye-movement sleep were separately calculated. Baseline ventilation before hypoxic stimulation and acute responses during stimulation did not differ between the ORX-KO and WT mice, although ventilation depended on vigilance state. Whereas the WT showed augmented minute ventilation (by 20.0 +/- 4.5% during QW and 26.5 +/- 5.3% during SWS; n = 8) for 2 h following IH, ORX-KO showed no significant increase (by -3.1 +/- 4.6% during QW and 0.3 +/- 5.2% during SWS; n = 8). Both genotypes showed no LTF after SH or sham stimulation. Sleep apnea indexes did not change following IH, even when LTF appeared in the WT mice. We conclude that LTF occurs during both sleep and wake periods, that orexin is necessary for eliciting LTF, and that LTF cannot prevent sleep apnea, at least in mice.

Inspiratory activation is not required for episodic hypoxia-induced respiratory long-term facilitation in postnatal rats

Episodic hypoxia causes repetitive inspiratory activation that induces a form of respiratory plasticity termed long-term facilitation (LTF). While LTF is a function of the hypoxic exposures and inspiratory activation, their relative importance in evoking LTF is unknown. The aims of this study were to: (1) dissociate the relative roles played by episodic hypoxia and respiratory activation in LTF; and (2) determine whether the magnitude of LTF varies as a function of hypoxic intensity. We did this by examining the effects of episodic hypoxia in postnatal rats (15-25 days old), which unlike adult rats exhibit a prominent hypoxia-induced respiratory depression. We quantified inspiratory phrenic nerve activity generated by the in situ working-heart brainstem before, during and for 60 min after episodic hypoxia. We demonstrate that episodic hypoxia evokes LTF despite the fact that it potently suppresses inspiratory activity during individual hypoxic exposures (P < 0.05). Specifically, we show that after episodic hypoxia (three 5 min periods of 10% O2) respiratory frequency increased to 40 +/- 3.3% above baseline values over the next 60 min (P < 0.001). Continuous hypoxia (15 min of 10% O2) had no lasting effects on respiratory frequency (P > 0.05). To determine if LTF magnitude was affected by hypoxic intensity, the episodic hypoxia protocol was repeated under three different O2 tensions. We demonstrate that the magnitude and time course of LTF depend on hypoxic severity, with more intense hypoxia inducing a more potent degree of LTF. We conclude that inspiratory activation is not required for LTF induction, and that hypoxia per se is the physiological stimulus for eliciting hypoxia-induced respiratory LTF.

Neural drive to tongue protrudor and retractor muscles following pulmonary C-fiber activation

Hypoglossal (XII) nerve recordings indicate that pulmonary C-fiber (PCF) receptor activation reduces inspiratory bursting and triggers tonic discharge. We tested three hypotheses related to this observation: 1) PCF receptor activation inhibits inspiratory activity in XII branches innervating both tongue protrudor muscles (medial branch; XIImed) and retractor muscles (lateral branch; XIIlat); 2) reduced XII neurogram amplitude reflects decreased XII motoneuron discharge rate; and 3) tonic XII activity reflects recruitment of previously silent motoneurons. Phrenic, XIImed, and XIIlat neurograms were recorded in anesthetized, paralyzed, and ventilated rats. Capsaicin delivered to the jugular vein reduced phrenic bursting at doses of 0.625 and 1.25 μg/kg but augmented bursting at 5 μg/kg. All doses reduced inspiratory amplitude in XIImed and XIIlat ( P < 0.05), and these effects were eliminated following bilateral vagotomy. Single-fiber recordings indicated that capsaicin causes individual XII motoneurons to either decrease discharge rate ( n = 101/153) or become silent ( n = 39/153). Capsaicin also altered temporal characteristics such that both XIImed and XIIlat inspiratory burst onset occurred after the phrenic burst ( P < 0.05). Increases in tonic discharge after capsaicin were greater in XIImed vs. XIIlat ( P < 0.05); single-fiber recordings indicated that tonic discharge reflected recruitment of previously silent motoneurons. We conclude that PCF receptor activation reduces inspiratory XII motoneuron discharge and transiently attenuates neural drive to both tongue protrudor and retractor muscles. However, tonic discharge appears to be selectively enhanced in tongue protrudor muscles. Accordingly, reductions in upper airway stiffness associated with reduced XII burst amplitude may be offset by enhanced tonic activity in tongue protrudor muscles.

Is there a link between intermittent hypoxia-induced respiratory plasticity and obstructive sleep apnoea?

Although neuroplasticity is an important property of the respiratory motor control system, its existence has been appreciated only in recent years and, as a result, its functional significance is not completely understood. The most frequently studied models of respiratory plasticity is respiratory long-term facilitation (LTF) following acute intermittent hypoxia and enhanced LTF following chronic intermittent hypoxia. Since intermittent hypoxia is a prominent feature of sleep-disordered breathing, LTF and/or enhanced LTF may compensate for factors that predispose to sleep-disordered breathing, particularly during obstructive sleep apnoea (OSA). Long-term facilitation has been studied most frequently in rats, and exhibits interesting properties consistent with a role in stabilizing breathing during sleep. Specifically, LTF: (1) is prominent in upper airway respiratory motor activity, suggesting that it stabilizes upper airways and maintains airway patency; (2) is most prominent during sleep in unanaesthetized rats; and (3) exhibits sexual dimorphism (greatest in young male and middle-aged female rats; smallest in middle-aged male and young female rats). Although these features are consistent with the hypothesis that upper airway LTF minimizes the prevalence of OSA in humans, there is little direct evidence for such an effect. Here we review advances in our understanding of LTF and its underlying mechanisms and present evidence concerning a potential role for LTF in maintaining upper airway patency, stabilizing breathing and preventing OSA in humans. Regardless of the relationship between LTF and OSA, a detailed understanding of cellular and synaptic mechanisms that underlie LTF may guide the development of new drugs to regulate upper airway tone, thereby offsetting the tendency for upper airway collapse characteristic of heavy snoring and OSA.

Long-term facilitation of ipsilateral but not contralateral phrenic output after cervical spinal cord hemisection

After chronic C2 spinal hemisection (C2HS), exposure to intermittent hypoxia (IH) evokes a persistent increase in phrenic output recorded ipsilateral to the injury (i.e., phrenic long-term facilitation, LTF; Golder and Mitchell, J. Neurosci. 25:2925-32, 2005). However, unilateral spinal cord injury induces compensatory increases in contralateral motoneuron activity that may reduce their capacity for further plasticity (i.e., a "ceiling effect"). We hypothesized that after chronic C2HS, LTF would be reduced in contralateral (vs. ipsilateral) phrenic output. Bilateral phrenic activity was recorded in three groups of anesthetized, paralyzed, vagotomized, and ventilated rats: uninjured, and 4 or 8 weeks following histologically verified C2HS. Baseline (BL) phrenic activity was established during normoxia and rats were then exposed to IH (5 x 3 min isocapnic hypoxia, 13-14% O2) followed by isocapnic normoxia; LTF was assessed 60-min post-IH. Uninjured animals showed an increase in inspiratory burst amplitude that was similar in the left (44 +/- 11%BL) and right phrenic nerves (39 +/- 13%BL). However, similar burst amplitude LTF did not occur in phrenic output recorded contralateral to C2HS at 4 (-10 +/- 7% BL) or 8 weeks post-C2HS (4 +/- 5% BL). In contrast, LTF of ipsilateral phrenic amplitude occurred at both 4 (44 +/- 11% BL) and 8 weeks post-C2HS (129 +/- 30% BL, P < 0.05). A persistent increase in phrenic burst frequency after IH (i.e., "frequency LTF") was observed in control (+9 +/- 3 burst/min, P < 0.05), but not C2HS rats. We conclude that compensatory responses to unilateral cervical spinal cord injury prevent the expression of LTF in contralateral phrenic motoneurons.

Long-term facilitation of ventilation and genioglossus muscle activity is evident in the presence of elevated levels of carbon dioxide in awake humans

We hypothesized that long-term facilitation (LTF) of minute ventilation and peak genioglossus muscle activity manifests itself in awake healthy humans when carbon dioxide is sustained at elevated levels. Eleven subjects completed two trials. During trial 1, baseline carbon dioxide levels were maintained during and after exposure to eight 4-min episodes of hypoxia. During trial 2, carbon dioxide was sustained 5 mmHg above baseline levels during exposure to episodic hypoxia. Seven subjects were exposed to sustained elevated levels of carbon dioxide in the absence of episodic hypoxia, which served as a control experiment. Minute ventilation was measured during trial 1, trial 2, and the control experiment. Peak genioglossus muscle activity was measured during trial 2. Minute ventilation during the recovery period of trial 1 was similar to baseline (9.3 +/- 0.5 vs. 9.2 +/- 0.7 l/min). Likewise, minute ventilation remained unchanged during the control experiment (beginning vs. end of control experiment, 14.4 +/- 1.7 vs. 14.7 +/- 1.4 l/min). In contrast, minute ventilation and peak genioglossus muscle activity during the recovery period of trial 2 was greater than baseline (minute ventilation: 28.4 +/- 1.7 vs. 19.6 +/- 1.0 l/min, P < 0.001; peak genioglossus activity: 1.6 +/- 0.3 vs. 1.0 fraction of baseline, P < 0.001). We conclude that exposure to episodic hypoxia is necessary to induce LTF of minute ventilation and peak genioglossus muscle activity and that LTF is only evident in awake humans in the presence of sustained elevated levels of carbon dioxide.

Impaired control of renal sympathetic nerve activity following neonatal intermittent hypoxia in rats

Apneas and recurring oxygen desaturations can occur in preterm infants and young children. To investigate long-term effects of neonatal intermittent hypoxia on baroreflex control of sympathetic nerve activity, we studied 5-7-month-old (adult) Sprague-Dawley rats exposed to chronic intermittent hypoxia (CIH, n=9; 8% O2 for 90 s alternating with 90 s 21% O2, 12h/day) for their first 30 postnatal days or controls exposed to normoxia (C, n=9). In adult CIH and C rats, baseline heart rate, mean arterial pressure, and plasma concentration of epinephrine and norepinephrine were similar. Baroreflex sensitivity was evaluated in anesthetized rats by changes in renal sympathetic nerve activity (RSNA) in response to i.v. infusions of phenylephrine (PE,1.5 microg/min/100g) and sodium nitroprusside (SNP, 1.5 microg/min/100g). Acute intermittent hypoxia (AIH, 18 min) induced elevations in RSNA by over 30% of baseline about three times more often in the CIH group than in the C group. After AIH, the gain of the baroreflex sympatho-excitatory response increased by approximately two times in C and did not change in CIH rats. The gain of sympatho-inhibitory responses to SNP at the maximum decrease in MAP was similar in the two groups in normoxia and was not affected by AIH. We conclude that postnatal intermittent hypoxia causes long-lasting impairment in chemoreceptor and baroreceptor control of renal nerve activity.

Influence of airway pressure on genioglossus activity during sleep in normal children

RATIONALE

Most children with obstructive sleep apnea are able to sustain stable breathing during portions of sleep, despite an anatomic predisposition toward airway collapse. This suggests that additional determinants of airway patency are active, such as neuromuscular compensation.

OBJECTIVES/METHODS

Using a custom intraoral surface electrode to record pharyngeal dilator muscle activity (the genioglossus [EMGgg]), we evaluated the muscle, ventilatory, and arousal responses to negative-pressure challenges during sleep in 19 healthy control children.

MEASUREMENTS AND MAIN RESULTS

In response to these challenges, we observed (1) marked variability in individual EMGgg responsiveness (peak EMGgg [mean+/-SD], 214+/-101% baseline), which was consistent within subjects; (2) a relationship between EMGgg activity and inspiratory flow and airway collapsibility; (3) reflex increases in flow (peak flow increase from challenge breaths 1-5 [mean+/-SD], 49+/-41% baseline) and respiratory rate often sufficient to sustain minute ventilation near baseline levels, without arousal; and (4) arousal threshold to be highest in stage 4, intermediate in stage 2, and lowest in REM sleep.

CONCLUSIONS

Healthy children have wide variation in upper airway neuromuscular compensatory responses and arousal thresholds that could represent intermediate phenotypes affecting the expression of sleep apnea. Children with robust upper airway neuromuscular responsiveness, or a very high arousal threshold, may be able to sustain minute ventilation when challenged with negative airway pressure.