Simulated apnoeas induce serotonin-dependent respiratory long-term facilitation in rats

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.

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.

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.

Acute morphine blocks spinal respiratory motor plasticity via long-latency mechanisms that require toll-like receptor 4 signalling

KEY POINTS

While respiratory complications following opioid use are mainly mediated via activation of mu opioid receptors, long-latency off-target signalling via innate immune toll-like receptor 4 (TLR4) may impair other essential elements of breathing control such as respiratory motor plasticity. In adult rats, pre-treatment with a single dose of morphine blocked long-term facilitation (LTF) of phrenic motor output via a long-latency TLR4-dependent mechanism. In the phrenic motor nucleus, morphine triggered TLR4-dependent activation of microglial p38 MAPK - a key enzyme that orchestrates inflammatory signalling and is known to undermine phrenic LTF. Morphine-induced LTF loss may destabilize breathing, potentially contributing to respiratory side effects. Therefore, we suggest minimizing TLR-4 signalling may improve breathing stability during opioid therapy.

ABSTRACT

Opioid-induced respiratory dysfunction is a significant public health burden. While respiratory effects are mediated via mu opioid receptors, long-latency off-target opioid signalling through innate immune toll-like receptor 4 (TLR4) may modulate essential elements of breathing control, particularly respiratory motor plasticity. Plasticity in respiratory motor circuits contributes to the preservation of breathing in the face of destabilizing influences. For example, respiratory long-term facilitation (LTF), a well-studied model of respiratory motor plasticity triggered by acute intermittent hypoxia, promotes breathing stability by increasing respiratory motor drive to breathing muscles. Some forms of respiratory LTF are exquisitely sensitive to inflammation and are abolished by even a mild inflammation triggered by TLR4 activation (e.g. via systemic lipopolysaccharides). Since opioids induce inflammation and TLR4 activation, we hypothesized that opioids would abolish LTF through a TLR4-dependent mechanism. In adult Sprague Dawley rats, pre-treatment with a single systemic injection of the prototypical opioid agonist morphine blocks LTF expression several hours later in the phrenic motor system - the motor pool driving diaphragm muscle contractions. Morphine blocked phrenic LTF via TLR4-dependent mechanisms because pre-treatment with (+)-naloxone - the opioid inactive stereoisomer and novel small molecule TLR4 inhibitor - prevented impairment of phrenic LTF in morphine-treated rats. Morphine triggered TLR4-dependent activation of microglial p38 MAPK within the phrenic motor system - a key enzyme that orchestrates inflammatory signalling and undermines phrenic LTF. Morphine-induced LTF loss may destabilize breathing, potentially contributing to respiratory side effects. We suggest minimizing TLR-4 signalling may improve breathing stability during opioid therapy by restoring endogenous mechanisms of plasticity within respiratory motor circuits.

Hypoxia-induced hypotension elicits adenosine-dependent phrenic long-term facilitation after carotid denervation

Moderate acute intermittent hypoxia (AIH) elicits a persistent, serotonin-dependent increase in phrenic amplitude, known as phrenic long-term facilitation (pLTF). Although pLTF was originally demonstrated by carotid sinus nerve stimulation, AIH still elicits residual pLTF in carotid denervated (CBX) rats via a distinct, but unknown mechanism. We hypothesized that exaggerated hypoxia-induced hypotension after carotid denervation leads to greater spinal tissue hypoxia and extracellular adenosine accumulation, thereby triggering adenosine 2A receptor (A_2A)-dependent pLTF. Phrenic activity, arterial pressure and spinal tissue oxygen pressure were measured in anesthetized CBX rats. Exaggerated hypoxia-induced hypotension after CBX was prevented via intravenous phenylephrine; without the hypotension, spinal tissue hypoxia during AIH was normalized, and residual pLTF was no longer observed. Spinal A_2A (MSX-3), but not serotonin 2 receptor (5-HT₂) inhibition (ketanserin), abolished residual pLTF in CBX rats. Thus, pLTF regulation may be altered in conditions impairing sympathetic activity and arterial pressure regulation, such as spinal cord injury.

Competing mechanisms of plasticity impair compensatory responses to repetitive apnoea

KEY POINTS

Intermittent reductions in respiratory neural activity, a characteristic of many ventilatory disorders, leads to inadequate ventilation and arterial hypoxia. Both intermittent reductions in respiratory neural activity and intermittent hypoxia trigger compensatory enhancements in inspiratory output when experienced separately, forms of plasticity called inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. Reductions in respiratory neural activity that lead to moderate, but not mild, arterial hypoxia occludes plasticity expression, indicating that concurrent induction of iMF and LTF impairs plasticity through cross-talk inhibition of their respective signalling pathways. Moderate hypoxia undermines iMF by enhancing NR2B-containing NMDA receptor signalling, which can be rescued by exogenous retinoic acid, a molecule necessary for iMF. These data suggest that in ventilatory disorders characterized by reduced inspiratory motor output, such as sleep apnoea, endogenous mechanisms of compensatory plasticity may be impaired, and that exogenously activating respiratory plasticity may be a novel strategy to improve breathing.

ABSTRACT

Many forms of sleep apnoea are characterized by recurrent reductions in respiratory neural activity, which leads to inadequate ventilation and arterial hypoxia. Both recurrent reductions in respiratory neural activity and hypoxia activate mechanisms of compensatory plasticity that augment inspiratory output and lower the threshold for apnoea, inactivity-induced inspiratory motor facilitation (iMF) and long-term facilitation (LTF), respectively. However, despite frequent concurrence of reduced respiratory neural activity and hypoxia, mechanisms that induce and regulate iMF and LTF have only been studied separately. Here, we demonstrate that recurrent reductions in respiratory neural activity ('neural apnoea') accompanied by cessations in ventilation that result in moderate (but not mild) hypoxaemia do not elicit increased inspiratory output, suggesting that concurrent induction of iMF and LTF occludes plasticity. A key role for NMDA receptor activation in impairing plasticity following concurrent neural apnoea and hypoxia is indicated since recurrent hypoxic neural apnoeas triggered increased phrenic inspiratory output in rats in which spinal NR2B-containing NMDA receptors were inhibited. Spinal application of retinoic acid, a key molecule necessary for iMF, bypasses NMDA receptor-mediated constraints, thereby rescuing plasticity following hypoxic neural apnoeas. These studies raise the intriguing possibility that endogenous mechanisms of compensatory plasticity may be impaired in some individuals with sleep apnoea, and that exogenously activating pathways giving rise to respiratory plasticity may be a novel pharmacological strategy to improve breathing.

The impact of intermittent or sustained carbon dioxide on intermittent hypoxia initiated respiratory plasticity. What is the effect of these combined stimuli on apnea severity?

The following review explores the effect that intermittent or sustained hypercapnia coupled to intermittent hypoxia has on respiratory plasticity. The review explores published work which suggests that intermittent hypercapnia leads to long-term depression of respiration when administered in isolation and prevents the initiation of long-term facilitation when administered in combination with intermittent hypoxia. The review also explores the impact that sustained hypercapnia alone and in combination with intermittent hypoxia has on the magnitude of long-term facilitation. After exploring the outcomes linked to intermittent hypoxia/hypercapnia and intermittent hypoxia/sustained hypercapnia the translational relevance of the outcomes as it relates to breathing stability during sleep is addressed. The likelihood that naturally induced cycles of intermittent hypoxia, coupled to oscillations in carbon dioxide that range between hypocapnia and hypercapnia, do not initiate long-term facilitation is addressed. Moreover, the conditions under which intermittent hypoxia/sustained hypercapnia could serve to improve breathing stability and mitigate co-morbidities associated with sleep apnea are considered.

Intermittent hypercapnic hypoxia during sleep does not induce ventilatory long-term facilitation in healthy males

Intermittent hypoxia-induced ventilatory neuroplasticity is likely important in obstructive sleep apnea pathophysiology. Although concomitant CO2levels and arousal state critically influence neuroplastic effects of intermittent hypoxia, no studies have investigated intermittent hypercapnic hypoxia effects during sleep in humans. Thus the purpose of this study was to investigate if intermittent hypercapnic hypoxia during sleep induces neuroplasticity (ventilatory long-term facilitation and increased chemoreflex responsiveness) in humans. Twelve healthy males were exposed to intermittent hypercapnic hypoxia (24 × 30 s episodes of 3% CO2and 3.0 ± 0.2% O2) and intermittent medical air during sleep after 2 wk washout period in a randomized crossover study design. Minute ventilation, end-tidal CO2, O2saturation, breath timing, upper airway resistance, and genioglossal and diaphragm electromyograms were examined during 10 min of stable stage 2 sleep preceding gas exposure, during gas and intervening room air periods, and throughout 1 h of room air recovery. There were no significant differences between conditions across time to indicate long-term facilitation of ventilation, genioglossal or diaphragm electromyogram activity, and no change in ventilatory response from the first to last gas exposure to suggest any change in chemoreflex responsiveness. These findings contrast with previous intermittent hypoxia studies without intermittent hypercapnia and suggest that the more relevant gas disturbance stimulus of concomitant intermittent hypercapnia frequently occurring in sleep apnea influences acute neuroplastic effects of intermittent hypoxia. These findings highlight the need for further studies of intermittent hypercapnic hypoxia during sleep to clarify the role of ventilatory neuroplasticity in the pathophysiology of sleep apnea.

NEW & NOTEWORTHY Both arousal state and concomitant CO2levels are known modulators of the effects of intermittent hypoxia on ventilatory neuroplasticity. This is the first study to investigate the effects of combined intermittent hypercapnic hypoxia during sleep in humans. The lack of neuroplastic effects suggests a need for further studies more closely replicating obstructive sleep apnea to determine the pathophysiological relevance of intermittent hypoxia-induced ventilatory neuroplasticity.

Reduced respiratory neural activity elicits a long-lasting decrease in the CO2 threshold for apnea in anesthetized rats

Two critical parameters that influence breathing stability are the levels of arterial pCO₂ at which breathing ceases and subsequently resumes - termed the apneic and recruitment thresholds (AT and RT, respectively). Reduced respiratory neural activity elicits a chemoreflex-independent, long-lasting increase in phrenic burst amplitude, a form of plasticity known as inactivity-induced phrenic motor facilitation (iPMF). The physiological significance of iPMF is unknown. To determine if iPMF and neural apnea have long-lasting physiological effects on breathing, we tested the hypothesis that patterns of neural apnea that induce iPMF also elicit changes in the AT and RT. Phrenic nerve activity and end-tidal CO₂ were recorded in urethane-anesthetized, ventilated rats to quantify phrenic nerve burst amplitude and the AT and RT before and after three patterns of neural apnea that differed in their duration and ability to elicit iPMF: brief intermittent neural apneas, a single brief "massed" neural apnea, or a prolonged neural apnea. Consistent with our hypothesis, we found that patterns of neural apnea that elicited iPMF also resulted in changes in the AT and RT. Specifically, intermittent neural apneas progressively decreased the AT with each subsequent neural apnea, which persisted for at least 60min. Similarly, a prolonged neural apnea elicited a long-lasting decrease in the AT. In both cases, the magnitude of the AT decrease was proportional to iPMF. In contrast, the RT was transiently decreased following prolonged neural apnea, and was not proportional to iPMF. No changes in the AT or RT were observed following a single brief neural apnea. Our results indicate that the AT and RT are differentially altered by neural apnea and suggest that specific patterns of neural apnea that elicit plasticity may stabilize breathing via a decrease in the AT.

Intermittent hypoxia initiated plasticity in humans: A multipronged therapeutic approach to treat sleep apnea and overlapping co-morbidities

Over the past three decades exposure to intermittent hypoxia (IH) has generally been considered a stimulus associated with a number of detrimental outcomes. However, there is sufficient evidence to link IH to many beneficial outcomes but they have largely been ignored, particularly in the field of sleep medicine in the United States. Recent reviews have postulated that this apparent contradiction is related to the severity and duration of exposure to IH; mild forms of IH initiate beneficial outcomes while severe forms of IH are coupled to detrimental consequences. In the present review we explore the role that IH has in initiating respiratory plasticity and the potential this form of plasticity has to mitigate obstructive sleep apnea (OSA) in humans. In taking this approach, we address the possibility that IH could serve as an adjunct therapy coupled with continuous positive airway pressure (CPAP) to treat OSA. Our working hypothesis is that exposure to mild IH leads to respiratory plasticity that manifests in increased stability of the upper airway, which could ultimately reduce the CPAP required to treat OSA. In turn, this reduction could increase CPAP compliance and extend the length of treatment each night, which might improve the magnitude of outcome measures. Improved treatment compliance coupled with the direct effect that IH has on numerous overlapping conditions (i.e. asthma, chronic obstructive pulmonary disease, spinal cord injury) may well lead to substantial improvements that exceed outcomes following treatment with CPAP alone. Overall, this review will consider evidence from the published literature which suggests that IH could serve as an effective multipronged therapeutic approach to treat sleep apnea and its overlapping co-morbidities.

Periodicity During Hypercapnic and Hypoxic Stimulus Is Crucial in Distinct Aspects of Phrenic Nerve Plasticity

This study was undertaken to determine pattern sensitivity of phrenic nerve plasticity in respect to different respiratory challenges. We compared long-term effects of intermittent and continuous hypercapnic and hypoxic stimuli, and combined intermittent hypercapnia and hypoxia on phrenic nerve plasticity. Adult, male, urethane-anesthetized, vagotomized, paralyzed, mechanically ventilated Sprague-Dawley rats were exposed to: acute intermittent hypercapnia (AIHc or AIHcO2), acute intermittent hypoxia (AIH), combined intermittent hypercapnia and hypoxia (AIHcH), continuous hypercapnia (CHc), or continuous hypoxia (CH). Peak phrenic nerve activity (pPNA) and burst frequency were analyzed during baseline (T0), hypercapnia or hypoxia exposures, at 15, 30, and 60 min (T60) after the end of the stimulus. Exposure to acute intermittent hypercapnia elicited decrease of phrenic nerve frequency from 44.25±4.06 at T0 to 35.29±5.21 at T60, (P=0.038, AIHc) and from 45.5±2.62 to 37.17±3.68 breaths/min (P=0.049, AIHcO2), i.e. frequency phrenic long term depression was induced. Exposure to AIH elicited increase of pPNA at T60 by 141.0±28.2 % compared to baseline (P=0.015), i.e. phrenic long-term facilitation was induced. Exposure to AIHcH, CHc, or CH protocols failed to induce long-term plasticity of the phrenic nerve. Thus, we conclude that intermittency of the hypercapnic or hypoxic stimuli is needed to evoke phrenic nerve plasticity.

Third International Congress on Epilepsy, Brain, and Mind: Part 2

Epilepsy is both a disease of the brain and the mind. Here, we present the second of two papers with extended summaries of selected presentations of the Third International Congress on Epilepsy, Brain and Mind (April 3-5, 2014; Brno, Czech Republic). Humanistic, biologic, and therapeutic aspects of epilepsy, particularly those related to the mind, were discussed. The extended summaries provide current overviews of epilepsy, cognitive impairment, and treatment, including brain functional connectivity and functional organization; juvenile myoclonic epilepsy; cognitive problems in newly diagnosed epilepsy; SUDEP including studies on prevention and involvement of the serotoninergic system; aggression and antiepileptic drugs; body, mind, and brain, including pain, orientation, the "self-location", Gourmand syndrome, and obesity; euphoria, obsessions, and compulsions; and circumstantiality and psychiatric comorbidities.

Electrophysiological properties of laryngeal motoneurones in rats submitted to chronic intermittent hypoxia

KEY POINTS

The respiratory control of the glottis by laryngeal motoneurones is characterized by inspiratory abduction and post-inspiratory adduction causing decreases and increases in upper airway resistance, respectively. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, exaggerated glottal abduction (before inspiration), associated with active expiration and decreased glottal adduction during post-inspiration. CIH increased the inspiratory and decreased the post-inspiratory laryngeal motoneurone activities, which is not associated to changes in their intrinsic electrophysiological properties. We conclude that the changes in the respiratory network after CIH seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia.

ABSTRACT

To keep an appropriate airflow to and from the lungs under physiological conditions a precise neural co-ordination of the upper airway resistance by laryngeal motoneurones in the nucleus ambiguus is essential. Chronic intermittent hypoxia (CIH), an important component of obstructive sleep apnoea, may alter these fine mechanisms. Here, using nerve and whole cell patch clamp recordings in in situ preparations of rats we investigated the effects of CIH on the respiratory control of the upper airway resistance, on the electrophysiological properties of laryngeal motoneurones in the nucleus ambiguus, and the role of carotid body (CB) afferents to the brainstem on the underlying mechanisms of these effects. CIH rats exhibited longer pre-inspiratory and lower post-inspiratory superior laryngeal nerve activities than control rats. These changes produced exaggerated glottal abduction (before inspiration) and decreased glottal adduction during post-inspiration, indicating a reduction of upper airway resistance during these respiratory phases after CIH. CB denervation abolished these changes produced by CIH. Regarding choline acetyltransferase positive-laryngeal motoneurones, CIH increased the firing frequency of inspiratory and decreased the firing frequency of post-inspiratory laryngeal motoneurones, without changes in their intrinsic electrophysiological properties. These data show that the effects of CIH on the upper airway resistance and laryngeal motoneurones activities are driven by the integrity of CB, which afferents induce changes in the central respiratory generators in the brainstem. These neural changes in the respiratory network seem to be an adaptive process required for an appropriated pulmonary ventilation and control of upper airway resistance under intermittent episodes of hypoxia.

Chronic Intermittent Hypoxia Blunts the Expression of Ventilatory Long Term Facilitation in Sleeping Rats

We have previously reported that chronic intermittent hypoxia (CIH), a central feature of human sleep-disordered breathing, causes respiratory instability in sleeping rats (Edge D, Bradford A, O'halloran KD. Adv Exp Med Biol 758:359-363, 2012). Long term facilitation (LTF) of respiratory motor outputs following exposure to episodic, but not sustained, hypoxia has been described. We hypothesized that CIH would enhance ventilatory LTF during sleep. We examined the effects of 3 and 7 days of CIH exposure on the expression of ventilatory LTF in sleeping rats. Adult male Wistar rats were exposed to 20 cycles of normoxia and hypoxia (5 % O(2) at nadir; SaO(2) ~ 80 %) per hour, 8 h per day for 3 or 7 consecutive days (CIH, N = 7 per group). Corresponding sham groups (N = 7 per group) were subjected to alternating cycles of air under identical experimental conditions in parallel. Following gas exposures, breathing during sleep was assessed in unrestrained, unanaesthetized animals using the technique of whole-body plethysmography. Rats were exposed to room air (baseline) and then to an acute IH (AIH) protocol consisting of alternating periods of normoxia (7 min) and hypoxia (FiO(2) 0.1, 5 min) for 10 cycles. Breathing was monitored during the AIH exposure and for 1 h in normoxia following AIH exposure. Baseline ventilation was elevated after 3 but not 7 days of CIH exposure. The hypoxic ventilatory response was equivalent in sham and CIH animals after 3 days but ventilatory responses to repeated hypoxic challenges were significantly blunted following 7 days of CIH. Minute ventilation was significantly elevated following AIH exposure compared to baseline in sham but not in CIH exposed animals. LTF, determined as the % increase in minute ventilation from baseline following AIH exposure, was significantly blunted in CIH exposed rats. In summary, CIH leads to impaired ventilatory responsiveness to AIH. Moreover, CIH blunts ventilatory LTF. The physiological significance of ventilatory LTF is context-dependent but it is reasonable to consider that it can potentially destabilize respiratory control, in view of the potential for LTF to give rise to hypocapnia. CIH-induced blunting of LTF may represent a compensatory mechanism subserving respiratory homeostasis. Our results suggest that CIH-induced increase in apnoea index (Edge D, Bradford A, O'halloran KD. Adv Exp Med Biol 758:359-363, 2012) is not related to enhanced ventilatory LTF. We conclude that the mature adult respiratory system exhibits plasticity and metaplasticity with potential consequences for the control of respiratory homeostasis. Our results may have implications for human sleep apnoea.

Raphe serotonergic neurons modulate genioglossus corticomotor activity in intermittent hypoxic rats

Background

Genioglossus activity is greater during wakefulness but decreases to a weaker state during sleep in obstructive sleep apnea syndrome (OSAS) patients, compared to healthy subjects. Previous studies suggested that the corticomotor control of the genioglossus was modified in OSAS patients. Intermittent hypoxia (IH), the typical pathophysiological change in OSAS, can induce genioglossus facilitation. The serotonergic neurons of the raphe dorsal (DRN) and magnus nuclei (RMg) are responsive to hypoxia and play important roles in the control of the genioglossus. However, it remains unknown whether DRN and RMg serotonergic neurons are responsible for the facilitated corticomotor activity of the genioglossus during IH. This study explored the influence of IH on the corticomotor activity of the genioglossus by transcranial magnetic stimulation (TMS), and the role of DRN and RMg serotonergic neurons in this effect.

Methods

Rats were exposed to IH and divided into two groups. In one group, anti-SERT-SAP was microinjected into the DRN and RMg respectively to kill serotonergic neurons. In the other group, artificial cerebrospinal fluid (ACSF) was injected. Comparisons were conducted between the two groups during four weeks of IH and four weeks after IH.

Results

Compared to the corresponding ACSF-injected group, the DRN lesion group and RMg lesion group showed longer TMS latencies and lower amplitudes during IH from the 1^st to the 28^th day. After 28 days of IH, longer latencies and lower amplitudes were seen only in the DRN lesion group.

Conclusion

These results indicate that DRN and RMg serotonergic neurons play different roles in the facilitation of genioglossus corticomotor activity induced by IH.

Prevention of sudden unexpected death in epilepsy: a realistic goal?

Sudden unexpected death in epilepsy (SUDEP) represents one of the most severe consequences of drug-resistant epilepsy, for which no evidence-based prevention is available. Development of effective prevention will depend on the following: (1) better understanding of the pathophysiology of SUDEP to define the most appropriate targets of intervention, and (2) identification of risk factors for SUDEP that would allow for the design of feasible clinical trials to test targeted interventions in high-risk populations. The most important known risk factor is the occurrence and frequency of generalized tonic-clonic seizure (GTCS), a seizure type that triggers the majority of witnessed SUDEP. Therefore, one likely way to prevent SUDEP is to minimize the risk of GTCS with optimal medical management and patient education. However, whether one might prevent SUDEP in patients with refractory epilepsy by using more frequent review of antiepileptic treatment and earlier referral for presurgical evaluation, remains to be seen. Another hypothetical strategy to prevent SUDEP is to reduce the risk of GTCS-induced postictal respiratory distress. This might be achieved by using lattice pillow, providing nocturnal supervision, reinforcing interictal serotoninergic tone, and lowering opiate- or adenosine-induced postictal brainstem depression. Promising interventions can be tested first on surrogate markers, such as postictal hypoxia in epilepsy monitoring units (EMUs), before SUDEP trials can be implemented. EMU safety should also be improved to avoid SUDEP occurrence in that setting. Finally, the development of ambulatory SUDEP prevention devices should be encouraged but raises a number of unsolved issues.

Inactivity-induced respiratory plasticity: protecting the drive to breathe in disorders that reduce respiratory neural activity

Multiple forms of plasticity are activated following reduced respiratory neural activity. For example, in ventilated rats, a central neural apnea elicits a rebound increase in phrenic and hypoglossal burst amplitude upon resumption of respiratory neural activity, forms of plasticity called inactivity-induced phrenic and hypoglossal motor facilitation (iPMF and iHMF), respectively. Here, we provide a conceptual framework for plasticity following reduced respiratory neural activity to guide future investigations. We review mechanisms giving rise to iPMF and iHMF, present new data suggesting that inactivity-induced plasticity is observed in inspiratory intercostals (iIMF) and point out gaps in our knowledge. We then survey conditions relevant to human health characterized by reduced respiratory neural activity and discuss evidence that inactivity-induced plasticity is elicited during these conditions. Understanding the physiological impact and circumstances in which inactivity-induced respiratory plasticity is elicited may yield novel insights into the treatment of disorders characterized by reductions in respiratory neural activity.

Hypoxia-induced phrenic long-term facilitation: emergent properties

As in other neural systems, plasticity is a hallmark of the neural system controlling breathing. One spinal mechanism of respiratory plasticity is phrenic long-term facilitation (pLTF) following acute intermittent hypoxia. Although cellular mechanisms giving rise to pLTF occur within the phrenic motor nucleus, different signaling cascades elicit pLTF under different conditions. These cascades, referred to as Q and S pathways to phrenic motor facilitation (pMF), interact via cross-talk inhibition. Whereas the Q pathway dominates pLTF after mild to moderate hypoxic episodes, the S pathway dominates after severe hypoxic episodes. The biological significance of multiple pathways to pMF is unknown. This review will discuss the possibility that interactions between pathways confer emergent properties to pLTF, including pattern sensitivity and metaplasticity. Understanding these mechanisms and their interactions may enable us to optimize intermittent hypoxia-induced plasticity as a treatment for patients that suffer from ventilatory impairment or other motor deficits.

Phrenic long-term facilitation after acute intermittent hypoxia requires spinal ERK activation but not TrkB synthesis

Acute intermittent hypoxia (AIH) elicits a form of spinal respiratory plasticity known as phrenic long-term facilitation (pLTF). pLTF requires spinal serotonin receptor-2 activation, the synthesis of new brain-derived neurotrophic factor (BDNF), and the activation of its high-affinity receptor tyrosine kinase, TrkB. Spinal adenosine 2A receptor activation elicits a distinct pathway to phrenic motor facilitation (pMF); this BDNF synthesis-independent pathway instead requires new synthesis of an immature TrkB isoform. Since hypoxia increases extracellular adenosine levels, we tested the hypothesis that new synthesis of TrkB and BDNF contribute to AIH-induced pLTF. Furthermore, given that signaling mechanisms "downstream" from TrkB are unknown in either mechanism, we tested the hypothesis that pLTF requires MEK/ERK and/or phosphatidylinositol 3-kinase (PI3K)/Akt activation. In anesthetized Sprague-Dawley rats, an intrathecal catheter at cervical level 4 was used to deliver drugs near the phrenic motor nucleus. Since pLTF was blocked by spinal injections of small interfering RNAs targeting BDNF mRNA but not TrkB mRNA, only new BDNF synthesis is required for AIH-induced pLTF. Pretreatment with a MEK inhibitor (U0126) blocked pLTF, whereas a PI3K inhibitor (PI-828) had no effect. Thus, AIH-induced pLTF requires MEK/ERK (not PI3K/AKT) signaling pathways. When U0126 was injected post-AIH, pLTF development was halted but not reversed, suggesting that ERK is critical for the development but not maintenance of pLTF. Thus, there are clear mechanistic distinctions between AIH-induced pLTF (i.e., BDNF synthesis and MEK/ERK dependent) versus adenosine 2A receptor-induced pMF (i.e., TrkB synthesis and PI3K/Akt dependent).

Severe acute intermittent hypoxia elicits phrenic long-term facilitation by a novel adenosine-dependent mechanism

Acute intermittent hypoxia [AIH; 3, 5-min episodes; 35-45 mmHg arterial PO(2) (Pa(O(2)))] elicits serotonin-dependent phrenic long-term facilitation (pLTF), a form of phrenic motor facilitation (pMF) initiated by G(q) protein-coupled metabotropic 5-HT(2) receptors. An alternate pathway to pMF is induced by G(s) protein-coupled metabotropic receptors, including adenosine A(2A) receptors. AIH-induced pLTF is dominated by the serotonin-dependent pathway and is actually restrained via inhibition from the adenosine-dependent pathway. Here, we hypothesized that severe AIH shifts pLTF from a serotonin-dependent to an adenosine-dependent form of pMF. pLTF induced by severe (25-30 mmHg Pa(O(2))) and moderate (45-55 mmHg Pa(O(2))) AIH were compared in anesthetized rats, with and without intrathecal (C4) spinal A(2A) (MSX-3, 130 ng/kg, 12 μl) or 5-HT receptor antagonist (methysergide, 300 μg/kg, 15 μl) injections. During severe, but not moderate AIH, progressive augmentation of the phrenic response during hypoxic episodes was observed. Severe AIH (78% ± 8% 90 min post-AIH, n = 6) elicited greater pLTF vs. moderate AIH (41% ± 12%, n = 8; P < 0.05). MSX-3 (28% ± 6%; n = 6; P < 0.05) attenuated pLTF following severe AIH, but enhanced pLTF following moderate AIH (86% ± 26%; n = 8; P < 0.05). Methysergide abolished pLTF after moderate AIH (12% ± 5%; n = 6; P = 0.035), but had no effect after severe AIH (66 ± 13%; n = 5; P > 0.05). Thus severe AIH shifts pLTF from a serotonin-dependent to an adenosine-dependent mechanism; the adenosinergic pathway inhibits the serotonergic pathway following moderate AIH. Here we demonstrate a novel adenosine-dependent pathway to pLTF following severe AIH. Shifts in the mechanisms of respiratory plasticity provide the ventilatory control system greater flexibility as challenges that differ in severity are confronted.

Expression and function of serotonin 2A and 2B receptors in the mammalian respiratory network

Neurons of the respiratory network in the lower brainstem express a variety of serotonin receptors (5-HTRs) that act primarily through adenylyl cyclase. However, there is one receptor family including 5-HT_2A, 5-HT_2B, and 5-HT_2C receptors that are directed towards protein kinase C (PKC). In contrast to 5-HT_2ARs, expression and function of 5-HT_2BRs within the respiratory network are still unclear. 5-HT_2BR utilizes a Gq-mediated signaling cascade involving calcium and leading to activation of phospholipase C and IP3/DAG pathways. Based on previous studies, this signal pathway appears to mediate excitatory actions on respiration. In the present study, we analyzed receptor expression in pontine and medullary regions of the respiratory network both at the transcriptional and translational level using quantitative RT-PCR and self-made as well as commercially available antibodies, respectively. In addition we measured effects of selective agonists and antagonists for 5-HT_2ARs and 5-HT_2BRs given intra-arterially on phrenic nerve discharges in juvenile rats using the perfused brainstem preparation. The drugs caused significant changes in discharge activity. Co-administration of both agonists revealed a dominance of the 5-HT_2BR. Given the nature of the signaling pathways, we investigated whether intracellular calcium may explain effects observed in the respiratory network. Taken together, the results of this study suggest a significant role of both receptors in respiratory network modulation.

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.

5-HT induces enhanced phrenic nerve activity via 5-HT(2A) receptor/PKC mechanism in anesthetized rats

Respiratory behavior expresses diverse forms of plasticity by altering breathing patterns. Failure of respiratory neuroplasticity often leads to malfunctions. Long-term facilitation (LTF), the most frequently studied model induced by episodic hypoxia to produce long-lasting enhancement of phrenic motor output, is thought to be serotonin 2A (5-HT(2A)) receptor-dependent. Previous studies have described 5-HT-induced prompt apnea in intact animals. However, the role of exogenous 5-HT in mediating respiratory neuroplasticity is less attended in vivo study. We hypothesized that an in vivo 5-HT challenge contributes to respiratory neuroplasticity. Here, we found that systemic bolus administration of 5-HT exerted an initial transient inhibition followed by marked facilitation, forming a biphasic pattern of phrenic nerve activity in artificially ventilated, midcervically vagotomized, and anesthetized adult rats. The facilitatory phase corresponded to the enhanced phrenic nerve activity that lasted for at least one hour after drug exposure, characterized as phrenic LTF (pLTF). The 5-HT-induced biphasic pattern and pLTF were 5-HT(2A) receptor-dependent and coupled to protein kinase C (PKC) activation. The initial inhibition of phrenic nerve activity was found to be nodose ganglion-associated, whereas the subsequent facilitation was carotid body-associated, establishing a peripheral inhibitory-facilitatory afferent balance. Immunoreactive expressions of 5-HT/5-HT(2A) receptors and phospho-PKC isoforms/PKC substrate provide morphological evidence of existence of a 5-HT/5-HT(2A) receptor/PKC mechanism in the nodose ganglion and the carotid body. We speculate that 5-HT challenge in vivo may contribute to respiratory neuroplasticity, to yield pLTF or augmented pLTF in animals with reduced or absent peripheral inhibitory inputs.

Experimental protocols and preparations to study respiratory long term facilitation

Respiratory long-term facilitation is a form of neuronal plasticity that is induced following exposure to intermittent hypoxia. Long-term facilitation is characterized by a progressive increase in respiratory motor output during normoxic periods that separate hypoxic episodes and by a sustained elevation in respiratory activity for up to 90min after exposure to intermittent hypoxia. This phenomenon is associated with increases in phrenic, hypoglossal or carotid sinus nerve inspiratory-modulated discharge. The examination of long-term facilitation has been steadily ongoing for approximately 3 decades. During this period of time a variety of animal models (e.g. cats, rats and humans), experimental preparations and intermittent hypoxia protocols have been used to study long-term facilitation. This review is designed to summarize the strengths and weaknesses of the models, preparations and protocols that have been used to study LTF over the past 30 years. The review is divided into two primary sections. Initially, the models and protocols used to study LTF in animals other than humans will be discussed, followed by a section specifically focused on human studies. Each section will begin with a discussion of various factors that must be considered when selecting an experimental preparation and intermittent hypoxia protocol to examine LTF. Model and protocol design recommendations will follow, with the goal of presenting a prevailing model and protocol that will ultimately ensure standardized comparisons across studies.

Protein kinase G-dependent mechanisms modulate hypoglossal motoneuronal excitability and long-term facilitation

Since protein kinase-dependent modulation of motoneuronal excitability contributes to adaptive changes in breathing, we hypothesized that cGMP-dependent pathways activating protein kinase G (PKG) modulate motoneuronal inspiratory drive currents and long-term plasticity. In a medullary slice preparation from neonatal rat (postnatal days 0-4) generating spontaneous respiratory-related rhythm, hypoglossal (XII) motoneuronal inspiratory drive currents and respiratory-related XII nerve activity were recorded. Focal application of a PKG activator, 8-bromoguanosine-3',5'-cyclomonophosphate (8-Br-cGMP), to voltage-clamped XII motoneurones decreased inspiratory drive currents. In the presence of tetrodotoxin (TTX), 8-Br-cGMP decreased the exogenous postsynaptic inward currents induced by focal application of AMPA. Intracellular dialysis of XII motoneurones with an inhibitory peptide to PKG (PKGI) increased endogenous inspiratory-drive currents and exogenous AMPA-induced currents. Application of 8-Br-cGMP with PKGI had no further effect on spontaneous or evoked currents, confirming that the observed effects were induced by PKG. However, PKG differentially increased longer-term plasticity. Three 3 min applications (separated by 5 min) of the α(1)-adrenergic agonist phenylephrine (PE) in combination with 8-Br-cGMP yielded greater in vitro long-term facilitation than PE alone. These data indicate the presence of a cGMP/PKG-dependent signalling pathway in XII motoneurones that modulates inspiratory drive currents and plasticity of XII motoneurones, possibly contributing to their adaptation during physiological challenges, such as sleep and exercise.

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.

Repeated obstructive apneas induce long-term facilitation of genioglossus muscle tone

The respiratory network is sensitive to both chemical and mechanosensory feedback and exhibits considerable plasticity when exposed to repeated respiratory perturbations. One example of respiratory plasticity is long-term facilitation (LTF) - a long-term enhancement in respiratory activity induced by intermittent hypoxia. Although intermittent hypoxia ranging from 5-min to 10-s has been shown to trigger respiratory LTF, it is unknown whether brief repeated airway obstruction as experience in obstructive sleep apnea could trigger LTF of respiratory motor outflow. Here we show that brief (15-s), repeated apneas trigger LTF of genioglossus muscle tone in rats. We also show that vagal feedback is important for the expression of apnea-induced LTF because it was not inducible when vagal feedback was impaired. Our results demonstrate a novel mechanism for triggering respiratory neuroplasticity and may have implications for maintenance of airway patency during sleep.

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.

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.

Serotonergic modulation of respiratory rhythmogenesis and central chemoreception

In addition to evidence supporting serotonergic modulation of respiratory rhythmogenesis, serotonergic mechanisms play a role in central respiratory chemoreception. We examined the role of serotonin 5HT1A receptors in respiratory rhythmicity and central respiratory chemosensitivity in in vitro brainstem preparations of the bullfrog tadpole, Rana catesbeiana. Spontaneous respiratory motor output was recorded from cranial nerve 7 at control bath pH (7.8) and hypercapnic bath pH (7.4) as bath concentrations of a 5HT1A receptor agonist were steadily increased from 0.5 to 25 microM. Activation of the 5HT1A receptor significantly altered the respiratory burst cycle. Significant increases in both gill and lung burst cycle were observed in response to bath application of 8-OH-DPAT; gill burst cycle in response to 8-OH-DPAT was influenced by bath pH, as gill burst cycle at bath pH 7.8 was not significantly increased at 0.5 or 5.0 microM 8-OH-DPAT. However, when the pH was reduced to 7.4 gill burst cycle was significantly increased at these same bath concentrations of 8-OH-DPAT. Gill burst amplitude was not altered in response to bath application of 8-OH-DPAT; however, lung burst amplitude was significantly decreased at 25.0 microM 8-OH-DPAT at bath pH 7.8. These data indicate that 5HT1A receptors are involved in neural respiratory rhythmogenic and chemoreceptive circuits in the bullfrog tadpole, and support the hypothesis that abnormalities in serotonergic systems may be an underlying component of Sudden Infant Death Syndrome.

Sleep apnea in young abstinent recreational MDMA ("ecstasy") consumers

BACKGROUND

Methylenedioxymethamphetamine (MDMA, "ecstasy") is a popular recreational drug of abuse and a selective brain serotonin neurotoxin. Functional consequences of MDMA neurotoxicity have defied ready characterization. Obstructive sleep apnea (OSA) is a common form of sleep-disordered breathing in which brain serotonin dysfunction may play a role. The present study sought to determine whether abstinent recreational MDMA users have an increased prevalence of OSA.

METHODS

We studied 71 medically healthy recreational MDMA users and 62 control subjects using all-night sleep polysomnography in a controlled inpatient research setting. Rates of apneas, hypopneas, and apnea hypopnea indices were compared in the 2 groups, controlling for body mass index, age, race, and gender.

RESULTS

Recreational MDMA users who had been drug free for at least 2 weeks had significantly increased rates of obstructive sleep apnea and hypopnea compared with controls. The odds ratio (95% confidence interval) for sleep apnea (mild, moderate, and severe combined) in MDMA users during non-REM sleep was 8.5 (2.4-30.4), which was greater than that associated with obesity [6.9 (1.7-28.2)]. Severity of OSA was significantly related to lifetime MDMA exposure.

CONCLUSIONS

These findings suggest that prior recreational methylenedioxymethamphetamine use increases the risk for obstructive sleep apnea and lend support to the notion that brain serotonin neuronal dysfunction plays a role in the pathophysiology of sleep apnea.

Medullary serotonin defects and respiratory dysfunction in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is defined as the sudden and unexpected death of an infant less than 12 months of age that occurs during sleep and remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. It is the leading cause of postneonatal mortality in the developed world. The cause of SIDS is unknown, but is postulated to involve impairment of brainstem-mediated homeostatic control. Extensive evidence from animal studies indicates that serotonin (5-HT) neurons in the medulla oblongata play a role in the regulation of multiple aspects of respiratory and autonomic function. A subset of SIDS infants have several abnormalities in medullary markers of 5-HT function and genetic polymorphisms impacting the 5-HT system, informing the hypothesis that SIDS results from a defect in 5-HT brainstem-mediated control of respiratory (and autonomic) regulation. Here we review the evidence from postmortem human studies and animal studies to support this hypothesis and discuss how the pathogenesis of SIDS is likely to originate in utero during fetal development.

Intermittent hypoxia induces functional recovery following cervical spinal injury

Respiratory-related complications are the leading cause of death in spinal cord injury (SCI) patients. Few effective SCI treatments are available after therapeutic interventions are performed in the period shortly after injury (e.g. spine stabilization and prevention of further spinal damage). In this review we explore the capacity to harness endogenous spinal plasticity induced by intermittent hypoxia to optimize function of surviving (spared) neural pathways associated with breathing. Two primary questions are addressed: (1) does intermittent hypoxia induce plasticity in spinal synaptic pathways to respiratory motor neurons following experimental SCI? and (2) can this plasticity improve respiratory function? In normal rats, intermittent hypoxia induces serotonin-dependent plasticity in spinal pathways to respiratory motor neurons. Early experiments suggest that intermittent hypoxia also enhances respiratory motor output in experimental models of cervical SCI (cervical hemisection) and that the capacity to induce functional recovery is greater with longer durations post-injury. Available evidence suggests that intermittent hypoxia-induced spinal plasticity has considerable therapeutic potential to treat respiratory insufficiency following chronic cervical spinal injury.

Reactive oxygen species and respiratory plasticity following intermittent hypoxia

The neural network controlling breathing exhibits plasticity in response to environmental or physiological challenges. For example, while hypoxia initiates rapid and robust increases in respiratory motor output to defend against hypoxemia, it also triggers persistent changes, or plasticity, in chemosensory neurons and integrative pathways that transmit brainstem respiratory activity to respiratory motor neurons. Frequently studied models of hypoxia-induced respiratory plasticity include: (1) carotid chemosensory plasticity and metaplasticity induced by chronic intermittent hypoxia (CIH), and (2) acute intermittent hypoxia (AIH) induced phrenic long-term facilitation (pLTF) in naïve and CIH preconditioned rats. These forms of plasticity share some mechanistic elements, although they differ in anatomical location and the requirement for CIH preconditioning. Both forms of plasticity require serotonin receptor activation and formation of reactive oxygen species (ROS). While the cellular sources and targets of ROS are not well known, recent evidence suggests that ROS modify the balance of protein phosphatase and kinase activities, shifting the balance towards net phosphorylation and favoring cellular reactions that induce and/or maintain plasticity. Here, we review possible sources of ROS, and the impact of ROS on phosphorylation events relevant to respiratory plasticity.

Serotonin and NMDA receptors in respiratory long-term facilitation

Some have postulated that long-term facilitation (LTF), a persistent augmentation of respiratory activity after episodic hypoxia, may play a beneficial role in helping stabilize upper airway patency in obstructive sleep apnea (OSA) patients. However, the neuronal and cellular mechanisms underlying this plasticity of respiratory motor behavior are still poorly understood. The main purpose of this review is to summarize recent findings about serotonin and NMDA receptors involved in both LTF and its enhancement after chronic intermittent hypoxia (CIH). The potential roles of these receptors in the initiation, formation and/or maintenance of LTF, as well as the CIH effect on LTF, will be discussed. As background, different paradigms for the stimulus protocol, different patterns of LTF expression and their mechanistic implications in LTF will also be discussed.

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.