Intermittent hypoxia induces functional recovery following cervical spinal injury

Prolonged intermittent hypoxia differentially regulates phrenic motor neuron serotonin receptor expression in rats following chronic cervical spinal cord injury

Low-dose (< 2 h/day), acute intermittent hypoxia (AIH) elicits multiple forms of serotonin-dependent phrenic motor plasticity and is emerging as a promising therapeutic strategy to restore respiratory and non-respiratory motor function after spinal cord injury (SCI). In contrast, high-dose (> 8 h/day), chronic intermittent hypoxia (CIH) undermines some forms of serotonin-dependent phrenic motor plasticity and elicits pathology. CIH is a hallmark of sleep disordered breathing, which is highly prevalent in individuals with cervical SCI. Interestingly, AIH and CIH preconditioning differentially impact phrenic motor plasticity. Although mechanisms of AIH-induced plasticity in the phrenic motor system are well-described in naïve rats, we know little concerning how these mechanisms are affected by chronic SCI or intermittent hypoxia preconditioning. Thus, in a rat model of chronic, incomplete cervical SCI (lateral spinal hemisection at C2 (C2Hx), we assessed serotonin type 2A, 2B and 7 receptor expression in and near phrenic motor neurons and compared: 1) intact vs. chronically injured rats; and 2) the impact of preconditioning with varied "doses" of intermittent hypoxia (IH). While there were no effects of chronic injury or intermittent hypoxia alone, CIH affected multiple receptors in rats with chronic C2Hx. Specifically, CIH preconditioning (8 h/day; 28 days) increased serotonin 2A and 7 receptor expression exclusively in rats with chronic C2Hx. Understanding the complex, context-specific interactions between chronic SCI and CIH and how this ultimately impacts phrenic motor plasticity is important as we leverage AIH-induced motor plasticity to restore breathing and other non-respiratory motor functions in people with chronic SCI.

Acute intermittent hypoxia increases maximal motor unit discharge rates in people with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for enhancing neuroplasticity and motor function in respiratory and limb musculature. Thus far, AIH-induced improvements in strength have been reported for upper and lower limb muscles after chronic incomplete cervical spinal cord injury (iSCI), but the underlying mechanisms have been elusive. We used high-density surface EMG (HDsEMG) to determine if motor unit discharge behaviour is altered after 15 × 60 s exposures to 9% inspired oxygen, interspersed with 21% inspired oxygen (AIH), compared to breathing only 21% air (SHAM). We recorded HDsEMG from the biceps and triceps brachii of seven individuals with iSCI during maximal elbow flexion and extension contractions, and motor unit spike trains were identified using convolutive blind source separation. After AIH, elbow flexion and extension torque increased by 54% and 59% from baseline (P = 0.003), respectively, whereas there was no change after SHAM. Across muscles, motor unit discharge rates increased by ∼4 pulses per second (P = 0.002) during maximal efforts, from before to after AIH. These results suggest that excitability and/or activation of spinal motoneurons is augmented after AIH, providing a mechanism to explain AIH-induced increases in voluntary strength. Pending validation, AIH may be helpful in conjunction with other therapies to enhance rehabilitation outcomes after incomplete spinal cord injury, due to these enhancements in motor unit function and strength. KEY POINTS: Acute intermittent hypoxia (AIH) causes increases in muscular strength and neuroplasticity in people living with chronic incomplete spinal cord injury (SCI), but how it affects motor unit discharge rates is unknown. Motor unit spike times were identified from high-density surface electromyograms during maximal voluntary contractions and tracked from before to after AIH. Motor unit discharge rates were increased following AIH. These findings suggest that AIH can facilitate motoneuron function in people with incomplete SCI.

A single sequence of intermittent hypoxia does not alter stretch reflex excitability in able-bodied individuals

Spasticity attributable to exaggerated stretch reflex pathways, particularly affecting the ankle plantar flexors, often impairs overground walking in persons with incomplete spinal cord injury. Compelling evidence from rodent models underscores how exposure to acute intermittent hypoxia (AIH) can provide a unique medium to induce spinal plasticity in key inhibitory pathways mediating stretch reflex excitability and potentially affect spasticity. In this study, we quantify the effects of a single exposure to AIH on the stretch reflex in able-bodied individuals. We hypothesized that a single sequence of AIH will increase the stretch reflex excitability of the soleus muscle during ramp-and-hold angular perturbations applied to the ankle joint while participants perform passive and volitionally matched contractions. Our results revealed that a single AIH exposure did not significantly change the stretch reflex excitability during both passive and active matching conditions. Furthermore, we found that able-bodied individuals increased their stretch reflex response from passive to active matching conditions after both sham and AIH exposures. Together, these findings suggest that a single AIH exposure might not engage inhibitory pathways sufficiently to alter stretch reflex responses in able-bodied persons. However, the generalizability of our present findings requires further examination during repetitive exposures to AIH along with potential reflex modulation during functional movements, such as overground walking.

Intermittent hypoxia training effectively protects against cognitive decline caused by acute hypoxia exposure

Intermittent hypoxia training (IHT) is a promising approach that has been used to induce acclimatization to hypoxia and subsequently lower the risk of developing acute mountain sickness (AMS). However, the effects of IHT on cognitive and cerebrovascular function after acute hypoxia exposure have not been characterized. In the present study, we first confirmed that the simplified IHT paradigm was effective at relieving AMS at 4300 m. Second, we found that IHT improved participants' cognitive and neural alterations when they were exposed to hypoxia. Specifically, impaired working memory performance, decreased conflict control function, impaired cognitive control, and aggravated mental fatigue induced by acute hypoxia exposure were significantly alleviated in the IHT group. Furthermore, a reversal of brain swelling induced by acute hypoxia exposure was visualized in the IHT group using magnetic resonance imaging. An increase in cerebral blood flow (CBF) was observed in multiple brain regions of the IHT group after hypoxia exposure as compared with the control group. Based on these findings, the simplified IHT paradigm might facilitate hypoxia acclimatization, alleviate AMS symptoms, and increase CBF in multiple brain regions, thus ameliorating brain swelling and cognitive dysfunction.

Pattern sensitivity of ampakine-hypoxia interactions for evoking phrenic motor facilitation in anesthetized rat

Repeated hypoxic episodes can produce a sustained (>60 min) increase in neural drive to the diaphragm. The requirement of repeated hypoxic episodes (vs. a single episode) to produce phrenic motor facilitation (pMF) can be removed by allosteric modulation of α-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptors using ampakines. We hypothesized that the ampakine-hypoxia interaction resulting in pMF requires that ampakine dosing precedes the onset of hypoxia. Phrenic nerve recordings were made from urethane-anesthetized, mechanically ventilated, and vagotomized adult male Sprague-Dawley rats during isocapnic conditions. Ampakine CX717 (15 mg/kg iv) was given immediately before (n = 8), during (n = 8), or immediately after (n = 8) a 5-min hypoxic episode (arterial oxygen partial pressure 40-45 mmHg). Ampakine before hypoxia (Aprior) resulted in a sustained increase in inspiratory phrenic burst amplitude (i.e., pMF) reaching +70 ± 21% above baseline (BL) after 60 min. This was considerably greater than corresponding values in the groups receiving ampakine during hypoxia (+28 ± 47% above BL, P = 0.005 vs. Aprior) or after hypoxia (+23 ± 40% above BL, P = 0.005 vs. Aprior). Phrenic inspiratory burst rate, heart rate, and systolic, diastolic, and mean arterial pressure (mmHg) were similar across the three treatment groups (all P > 0.3, treatment effect). We conclude that the presentation order of ampakine and hypoxia impacts the magnitude of pMF, with ampakine pretreatment evoking the strongest response. Ampakine pretreatment may have value in the context of hypoxia-based neurorehabilitation strategies.NEW & NOTEWORTHY Phrenic motor facilitation (pMF) is evoked after repeated episodes of brief hypoxia. pMF can also be induced when an allosteric modulator of AMPA receptors (ampakine) is intravenously delivered immediately before a single brief hypoxic episode. Here we show that ampakine delivery before hypoxia (vs. during or after hypoxia) evokes the largest pMF with minimal impact on arterial blood pressure and heart rate. Ampakine pretreatment may have value in the context of hypoxia-based neurorehabilitation strategies.

Acute intermittent hypoxia enhances strength, and modulates spatial distribution of muscle activation in persons with chronic incomplete spinal cord injury

Acute intermittent hypoxia (AIH) is an emerging technique for facilitating neural plasticity in individuals with chronic incomplete spinal cord injury (iSCI). A single sequence of AIH enhances hand grip strength and ankle plantarflexion torque, but underlying mechanisms are not yet clear. We sought to examine how AIH-induced changes in magnitude and spatial distribution of the electromyogram (EMG) of the biceps and triceps brachii contributes to improved strength. Seven individuals with iSCI visited the laboratory on two occasions, and received either AIH or Sham AIH intervention in a randomized order. AIH consisted of 15 brief (∼60s) periods of low oxygen (fraction of inspired O2 = 0.09) alternating with 60s of normoxia, whereas Sham AIH consisted of repeated exposures to normoxic air. High-density surface EMG of biceps and triceps brachii was recorded during maximal elbow flexion and extension. We then generated spatial maps which distinguished active muscle regions prior to and 60 min after AIH or Sham AIH. After an AIH sequence, elbow flexion and extension forces increased by 91.7 ± 88.4% and 51.7 ± 57.8% from baseline, respectively, whereas there was no difference after Sham AIH. Changes in strength were associated with an altered spatial distribution of EMG and increased root mean squared EMG amplitude in both biceps and triceps brachii muscles. These data suggest that altered motor unit activation profiles may underlie improved volitional strength after a single dose of AIH and warrant further investigation using single motor unit analysis techniques to further elucidate mechanisms of AIH-induced plasticity.

Novel role for non-invasive neuromodulation techniques in central respiratory dysfunction

Respiration is a crucial steady-state function of human life. Central nervous system injury can damage the central respiratory pattern generator (CRPG) or interrupt its outflow, leading to central respiratory paralysis and dysfunction, which can endanger the patient's life. At present, there is no effective means to reverse this process. Commonly used non-invasive neuromodulation techniques include repetitive transcranial magnetic stimulation (rTMS), transcranial direct current stimulation (tDCS) and so forth, which have been widely applied in nervous system diseases and their various secondary symptoms, but rarely in respiratory function. Clinical and animal studies have confirmed that TMS is also suitable for investigating the excitability and plasticity of ascending corticospinal respiratory pathways. In addition, although rTMS and tDCS differ in their respective mechanisms, both can regulate respiratory networks in healthy individuals and in diseased states. In this review, we provide an overview of the physiology of respiration, the use of TMS to assess the excitability of corticophrenic pathways in healthy individuals and in central respiratory disorders, followed by an overview of the animal and clinical studies of rTMS, tDCS and so forth in regulating respiratory circuits and the possible mechanisms behind them. It was found that the supplementary motor area (SMA) and the phrenic motor neuron (PMN) may be key regulatory areas. Finally, the challenges and future research directions of neuroregulation in respiratory function are proposed. Through understanding how neuromodulation affects the respiratory neural circuit non-invasively, we can further explore the therapeutic potential of this neuromodulation strategy, so as to promote the recovery of respiratory function after central nervous system diseases or injury.

Intermittent Hypoxia Differentially Regulates Adenosine Receptors in Phrenic Motor Neurons with Spinal Cord Injury

Cervical spinal cord injury (cSCI) impairs neural drive to the respiratory muscles, causing life- threatening complications such as respiratory insufficiency and diminished airway protection. Repetitive "low dose" acute intermittent hypoxia (AIH) is a promising strategy to restore motor function in people with chronic SCI. Conversely, "high dose" chronic intermittent hypoxia (CIH; ∼8 h/night), such as experienced during sleep apnea, causes pathology. Sleep apnea, spinal ischemia, hypoxia and neuroinflammation associated with cSCI increase extracellular adenosine concentrations and activate spinal adenosine receptors which in turn constrains the functional benefits of therapeutic AIH. Adenosine 1 and 2A receptors (A₁, A_2A) compete to determine net cAMP signaling and likely the tAIH efficacy with chronic cSCI. Since cSCI and intermittent hypoxia may regulate adenosine receptor expression in phrenic motor neurons, we tested the hypotheses that: 1) daily AIH (28 days) downregulates A_2A and upregulates A₁ receptor expression; 2) CIH (28 days) upregulates A_2A and downregulates A₁ receptor expression; and 3) cSCI alters the impact of CIH on adenosine receptor expression. Daily AIH had no effect on either adenosine receptor in intact or injured rats. However, CIH exerted complex effects depending on injury status. Whereas CIH increased A₁ receptor expression in intact (not injured) rats, it increased A_2A receptor expression in spinally injured (not intact) rats. The differential impact of CIH reinforces the concept that the injured spinal cord behaves in distinct ways from intact spinal cords, and that these differences should be considered in the design of experiments and/or new treatments for chronic cSCI.

Respiratory plasticity following spinal cord injury: perspectives from mouse to man

The study of respiratory plasticity in animal models spans decades. At the bench, researchers use an array of techniques aimed at harnessing the power of plasticity within the central nervous system to restore respiration following spinal cord injury. This field of research is highly clinically relevant. People living with cervical spinal cord injury at or above the level of the phrenic motoneuron pool at spinal levels C3-C5 typically have significant impairments in breathing which may require assisted ventilation. Those who are ventilator dependent are at an increased risk of ventilator-associated co-morbidities and have a drastically reduced life expectancy. Pre-clinical research examining respiratory plasticity in animal models has laid the groundwork for clinical trials. Despite how widely researched this injury is in animal models, relatively few treatments have broken through the preclinical barrier. The three goals of this present review are to define plasticity as it pertains to respiratory function post-spinal cord injury, discuss plasticity models of spinal cord injury used in research, and explore the shift from preclinical to clinical research. By investigating current targets of respiratory plasticity research, we hope to illuminate preclinical work that can influence future clinical investigations and the advancement of treatments for spinal cord injury.

Tetraplegia is associated with increased hypoxic ventilatory response during nonrapid eye movement sleep

People with cervical spinal cord injury (SCI) are likely to experience chronic intermittent hypoxia while sleeping. The physiological effects of intermittent hypoxia on the respiratory system during spontaneous sleep in individuals with chronic cervical SCI are unknown. We hypothesized that individuals with cervical SCI would demonstrate higher short- and long-term ventilatory responses to acute intermittent hypoxia (AIH) exposure than individuals with thoracic SCI during sleep. Twenty participants (10 with cervical SCI [9 male] and 10 with thoracic SCI [6 male]) underwent an AIH and sham protocol during sleep. During the AIH protocol, each participant experienced 15 episodes of isocapnic hypoxia using mixed gases of 100% nitrogen (N2 ) and 40% carbon dioxide (CO2 ) to achieve an oxygen saturation of less than 90%. This was followed by two breaths of 100% oxygen (O2 ). Measurements were collected before, during, and 40 min after the AIH protocol to obtain ventilatory data. During the sham protocol, participants breathed room air for the same amount of time that elapsed during the AIH protocol and at approximately the same time of night. Hypoxic ventilatory response (HVR) during the AIH protocol was significantly higher in participants with cervical SCI than those with thoracic SCI. There was no significant difference in minute ventilation (V.E. ), tidal volume (V.T. ), or respiratory frequency (f) during the recovery period after AIH in cervical SCI compared to thoracic SCI groups. Individuals with cervical SCI demonstrated a significant short-term increase in HVR compared to thoracic SCI. However, there was no evidence of ventilatory long-term facilitation following AIH in either group.

Diaphragmatic Activity and Respiratory Function Following C3 or C6 Unilateral Spinal Cord Contusion in Mice

The majority of spinal cord injuries (SCIs) are cervical (cSCI), leading to a marked reduction in respiratory capacity. We aimed to investigate the effect of hemicontusion models of cSCI on both diaphragm activity and respiratory function to serve as preclinical models of cervical SCI. Since phrenic motoneuron pools are located at the C3-C5 spinal level, we investigated two models of preclinical cSCI mimicking human forms of injury, namely, one above (C3 hemicontusion-C3HC) and one below phrenic motoneuron pools (C6HC) in wild-type swiss OF-1 mice, and we compared their effects on respiratory function using whole-body plethysmography and on diaphragm activity using electromyography (EMG). At 7 days post-surgery, both C3HC and C6HC damaged spinal cord integrity above the lesion level, suggesting that C6HC potentially alters C5 motoneurons. Although both models led to decreased diaphragmatic EMG activity in the injured hemidiaphragm compared to the intact one (-46% and -26% in C3HC and C6HC, respectively, both p = 0.02), only C3HC led to a significant reduction in tidal volume and minute ventilation compared to sham surgery (-25% and -20% vs. baseline). Moreover, changes in EMG amplitude between respiratory bursts were observed post-C3HC, reflecting a change in phrenic motoneuronal excitability. Hence, C3HC and C6HC models induced alteration in respiratory function proportionally to injury level, and the C3HC model is a more appropriate model for interventional studies aiming to restore respiratory function in cSCI.

Considerations about Hypoxic Changes in Neuraxis Tissue Injuries and Recovery

Hypoxia represents the temporary or longer-term decrease or deprivation of oxygen in organs, tissues, and cells after oxygen supply drops or its excessive consumption. Hypoxia can be (para)-physiological-adaptive-or pathological. Thereby, the mechanisms of hypoxia have many implications, such as in adaptive processes of normal cells, but to the survival of neoplastic ones, too. Ischemia differs from hypoxia as it means a transient or permanent interruption or reduction of the blood supply in a given region or tissue and consequently a poor provision with oxygen and energetic substratum-inflammation and oxidative stress damages generating factors. Considering the implications of hypoxia on nerve tissue cells that go through different ischemic processes, in this paper, we will detail the molecular mechanisms by which such structures feel and adapt to hypoxia. We will present the hypoxic mechanisms and changes in the CNS. Also, we aimed to evaluate acute, subacute, and chronic central nervous hypoxic-ischemic changes, hoping to understand better and systematize some neuro-muscular recovery methods necessary to regain individual independence. To establish the link between CNS hypoxia, ischemic-lesional mechanisms, and neuro-motor and related recovery, we performed a systematic literature review following the" Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA") filtering method by interrogating five international medical renown databases, using, contextually, specific keywords combinations/"syntaxes", with supplementation of the afferent documentation through an amount of freely discovered, also contributive, bibliographic resources. As a result, 45 papers were eligible according to the PRISMA-inspired selection approach, thus covering information on both: intimate/molecular path-physiological specific mechanisms and, respectively, consequent clinical conditions. Such a systematic process is meant to help us construct an article structure skeleton giving a primary objective input about the assembly of the literature background to be approached, summarised, and synthesized. The afferent contextual search (by keywords combination/syntaxes) we have fulfilled considerably reduced the number of obtained articles. We consider this systematic literature review is warranted as hypoxia's mechanisms have opened new perspectives for understanding ischemic changes in the CNS neuraxis tissue/cells, starting at the intracellular level and continuing with experimental research to recover the consequent clinical-functional deficits better.

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.

Phrenic motor neuron survival below cervical spinal cord hemisection

Cervical spinal cord injury (cSCI) severs bulbospinal projections to respiratory motor neurons, paralyzing respiratory muscles below the injury. C2 spinal hemisection (C2Hx) is a model of cSCI often used to study spontaneous and induced plasticity and breathing recovery post-injury. One key assumption is that C2Hx dennervates motor neurons below the injury, but does not affect their survival. However, a recent study reported substantial bilateral motor neuron death caudal to C2Hx. Since phrenic motor neuron (PMN) death following C2Hx would have profound implications for therapeutic strategies designed to target spared neural circuits, we tested the hypothesis that C2Hx minimally impacts PMN survival. Using improved retrograde tracing methods, we observed no loss of PMNs at 2- or 8-weeks post-C2Hx. We also observed no injury-related differences in ChAT or NeuN immunolabeling within labelled PMNs. Although we found no evidence of PMN loss following C2Hx, we cannot rule out neuronal loss in other motor pools. These findings address an essential prerequisite for studies that utilize C2Hx as a model to explore strategies for inducing plasticity and/or regeneration within the phrenic motor system, as they provide important insights into the viability of phrenic motor neurons as therapeutic targets after high cervical injury.

Respiratory Training and Plasticity After Cervical Spinal Cord Injury

While spinal cord injuries (SCIs) result in a vast array of functional deficits, many of which are life threatening, the majority of SCIs are anatomically incomplete. Spared neural pathways contribute to functional and anatomical neuroplasticity that can occur spontaneously, or can be harnessed using rehabilitative, electrophysiological, or pharmacological strategies. With a focus on respiratory networks that are affected by cervical level SCI, the present review summarizes how non-invasive respiratory treatments can be used to harness this neuroplastic potential and enhance long-term recovery. Specific attention is given to "respiratory training" strategies currently used clinically (e.g., strength training) and those being developed through pre-clinical and early clinical testing [e.g., intermittent chemical stimulation via altering inhaled oxygen (hypoxia) or carbon dioxide stimulation]. Consideration is also given to the effect of training on non-respiratory (e.g., locomotor) networks. This review highlights advances in this area of pre-clinical and translational research, with insight into future directions for enhancing plasticity and improving functional outcomes after SCI.

Intermittent Hypoxia Induces Greater Functional Breathing Motor Recovery as a Fixed Rather Than Varied Duration Treatment after Cervical Spinal Cord Injury in Rats

Intermittent hypoxia treatment (IH) has been shown to improve respiratory function in both pre-clinical animal models and human subjects following spinal cord injury (SCI), historically consisting of alternating and equal intervals of hypoxic and normoxic exposure. We describe such a procedure as fixed duration IH (FD-IH) and modulation of its severity, intermittency, and post-injury time-point of application differentially affects expression of breathing motor plasticity. As such, the established IH protocol exhibits similarity to instrumental conditioning and can be described as behavioral training through reinforcement. Findings from the field of operant conditioning, a form of more advanced learning, inspire the consideration that FD-IH protocols may be improved through exchanging fixed for varied durations of hypoxia between reinforcement. Thus, we hypothesized that varied duration intermittent hypoxia treatment (VD-IH) would induce greater breathing motor recovery ipsilateral to injury than FD-IH after cervical SCI in rats. To test this hypothesis, we treated animals with VD-IH or FD-IH for 5 days at 1 week and at 8 weeks following cervical SCI, then assessed breathing motor output by diaphragm electromyography (EMG) recording, and compared between groups. At 1 week post-injury, VD-IH-exposed animals trended slightly toward exhibiting greater levels of respiratory recovery in the hemidiaphragm ipsilateral to lesion than did FD-IH-treated animals, but at 8 weeks FD-IH produced significantly greater respiratory motor output than did VD-IH. Thus, these results identify a novel sensitivity of respiratory motor function to variations in the IH protocol that may lead to development of more effective treatments following SCI.

Intermittent hypoxia and respiratory recovery in pre-clinical rodent models of incomplete cervical spinal cord injury

Impaired respiratory function is a common and devastating consequence of cervical spinal cord injury. Accordingly, the development of safe and effective treatments to restore breathing function is critical. Acute intermittent hypoxia has emerged as a promising therapeutic strategy to treat respiratory insufficiency in individuals with spinal cord injury. Since the original report by Bach and Mitchell (1996) concerning long-term facilitation of phrenic motor output elicited by brief, episodic exposure to reduced oxygen, a series of studies in animal models have led to the realization that acute intermittent hypoxia may have tremendous potential for inducing neuroplasticity and functional recovery in the injured spinal cord. Advances in our understanding of the neurobiology of acute intermittent hypoxia have prompted us to begin to explore its effects in human clinical studies. Here, we review the basic neurobiology of the control of breathing and the pathophysiology and respiratory consequences of two common experimental models of incomplete cervical spinal cord injury (i.e., high cervical hemisection and mid-cervical contusion). We then discuss the impact of acute intermittent hypoxia on respiratory motor function in these models: work that has laid the foundation for translation of this promising therapeutic strategy to clinical populations. Lastly, we examine the limitations of these animal models and intermittent hypoxia and discuss how future work in animal models may further advance the translation and therapeutic efficacy of this treatment.

Efficacy and time course of acute intermittent hypoxia effects in the upper extremities of people with cervical spinal cord injury

Spinal cord injuries (SCI) disrupt neural pathways between the brain and spinal cord, causing impairment of motor function and loss of independent mobility. Spontaneous plasticity in spared neural pathways improves function but is often insufficient to restore normal function. One unique approach to augment plasticity in spinal synaptic pathways is acute intermittent hypoxia (AIH), meaning brief exposure to mild bouts of low oxygen, interspersed with normoxia. While the administration of AIH elicits rapid plasticity and enhances volitional somatic motor output in the lower-limbs of people with incomplete SCI, it is not known if AIH-induced neuroplasticity is equally prevalent in spinal motor pathways regulating upper-extremity motor-function. In addition, how long the motor effects are retained following AIH has not yet been established. The goal of this research was to investigate changes in hand strength and upper-limb function elicited by episodic hypoxia, and to establish how long these effects were sustained in persons with incomplete cervical SCI. We conducted a randomized, blinded, placebo-controlled and cross-over design study consisting of a single AIH or sham AIH session in 14 individuals with chronic, incomplete cervical SCI. In a subset of six participants, we also performed a second protocol to determine the cumulative effects of repetitive AIH (i.e., two consecutive days). In both protocols, hand dynamometry and clinical performance tests were performed pre- and post-exposure. We found that a single AIH session enhanced bilateral grip and pinch strength, and that this effect peaked ~3 h post-intervention. The strength change was substantially higher after AIH versus sham AIH. These findings demonstrate the potential of AIH to improve upper-extremity function in persons with chronic SCI, although follow-up studies are needed to investigate optimal dosage and duration of effect.

Novel approach to an early assessment of a patient's potential for neurological remission after acute spinal cord injury: Analysis of hemoglobin concentration dynamics

Context/objective: Examining hemoglobin (Hb) dynamics with regard to the potential of neurological remission in patients with traumatic spinal cord injury (TSCI).Design: Prospective Clinical Observational Study.Setting: BG Trauma Centre Ludwigshafen, Department of Paraplegiology, Rhineland-Palatinate, Germany.Methods: From 2011 to 2017 a total of 80 patients with acute spinal injury were enrolled and divided into three groups: initial neurological impairment either with (G1; n = 33) or without subsequent neurological remission (G0; n = 35) and vertebral fractures without initial neurological impairment as control group (C; n = 12). Blood samples were taken for 3 months at 11 time-points after injury. Analyses were performed using routine diagnostics.Outcome measures: Multiple logistic regression was used to determine the prognostic value of Hb regarding neurological remission respecting clinical covariates.Results: Data showed elevated mean Hb concentrations in G1 from the third day to 1 month compared to G0, Hb levels were significantly higher in G1 after 3 days (P = 0.03, G1 > G0). The final multiple logistic regression model based on this data predicting the presence of neurological remission resulted in an AUC (area under the curve) of 80.5% (CI: 67.8%-93.2%) in the ROC (receiver operating characteristic) analysis.Conclusion: Elevated Hb concentrations are associated with a higher likelihood of neurological remission. Elevated concentrations of Hb in G1 compared to G0 over time might be linked to both a better initial oxygen supply response and a decreased ECM (extracellular matrix) degradation highlighting the role of Hb as a valuable biomarker for neural regeneration after TSCI.

Acute intermittent hypoxia as a potential adjuvant to improve walking following spinal cord injury: evidence, challenges, and future directions

Purpose of Review

The reacquisition and preservation of walking ability are highly valued goals in spinal cord injury (SCI) rehabilitation. Recurrent episodes of breathing low oxygen (i.e., acute intermittent hypoxia, AIH) is a potential therapy to promote walking recovery after incomplete SCI via endogenous mechanisms of neuroplasticity. Here, we report on the progress of AIH, alone or paired with other treatments, on walking recovery in persons with incomplete SCI. We evaluate the evidence of AIH as a therapy ready for clinical and home use and the real and perceived challenges that may interfere with this possibility.

Recent Findings

Repetitive AIH is a safe and an efficacious treatment to enhance strength, walking speed and endurance, as well as, dynamic balance in persons with chronic, incomplete SCI.

Summary

The potential for AIH as a treatment for SCI remains high, but further research is necessary to understand treatment targets and effectiveness in a large cohort of persons with SCI.

Enhancing neural activity to drive respiratory plasticity following cervical spinal cord injury

Cervical spinal cord injury (SCI) results in permanent life-altering sensorimotor deficits, among which impaired breathing is one of the most devastating and life-threatening. While clinical and experimental research has revealed that some spontaneous respiratory improvement (functional plasticity) can occur post-SCI, the extent of the recovery is limited and significant deficits persist. Thus, increasing effort is being made to develop therapies that harness and enhance this neuroplastic potential to optimize long-term recovery of breathing in injured individuals. One strategy with demonstrated therapeutic potential is the use of treatments that increase neural and muscular activity (e.g. locomotor training, neural and muscular stimulation) and promote plasticity. With a focus on respiratory function post-SCI, this review will discuss advances in the use of neural interfacing strategies and activity-based treatments, and highlights some recent results from our own research.

Enhanced recovery of breathing capacity from combined adenosine 2A receptor inhibition and daily acute intermittent hypoxia after chronic cervical spinal injury

Daily acute intermittent hypoxia (dAIH) improves breathing capacity after C2 spinal hemisection (C2HS) in rats. Since C2HS disrupts spinal serotonergic innervation below the injury, adenosine-dependent mechanisms underlie dAIH-induced functional recovery 2weeks post-injury. We hypothesized that dAIH-induced functional recovery converts from an adenosine-dependent to a serotonin-dependent, adenosine-constrained mechanism with chronic injury. Eight weeks post-C2HS, rats began dAIH (10, 5-min episodes, 10.5% O2; 5-min intervals; 7days) followed by AIH 3× per week (3×wAIH) for 8 additional weeks with/without systemic A2A receptor inhibition (KW6002) on each AIH exposure day. Tidal volume (VT) and bilateral diaphragm (Dia) and T2 external intercostal motor activity were assessed in unanesthetized rats breathing air and during maximum chemoreflex stimulation (MCS: 7% CO2, 10.5% O2). Nine weeks post-C2HS, dAIH increased VT versus time controls (p<0.05), an effect enhanced by KW6002 (p<0.05). dAIH increased bilateral Dia activity (p<0.05), and KW6002 enhanced this effect in contralateral (p<0.05) and ipsilateral Dia activity (p<0.001), but not T2 inspiratory activity. Functional benefits of combined AIH plus systemic A2A receptor inhibition were maintained for 4weeks. Thus, in rats with chronic injuries: 1) dAIH improves VT and bilateral diaphragm activity; 2) VT recovery is enhanced by A2A receptor inhibition; and 3) functional recovery with A2A receptor inhibition and AIH "reminders" last 4weeks. Combined dAIH and A2A receptor inhibition may be a simple, safe, and effective strategy to accelerate/enhance functional recovery of breathing capacity in patients with respiratory impairment from chronic spinal injury.

Intermittent Hypoxia Does not Elicit Memory Impairment in Spinal Cord Injury Patients

There is a critical need for new therapeutic strategies to restore motor function in patients with spinal cord injuries (SCIs), without unwanted effects. Intermittent hypoxia (IH) induces plasticity in spared synaptic pathways to motor neurons below the level of injury, which can be harnessed to elicit motor recovery in incomplete SCI patients. However, there is conflicting evidence regarding the effects of IH on memory function. The aim of this study was to assess episodic verbal and visual memory function with the Complutense verbal learning test (TAVEC) and the Rey-Osterrieth Complex Figure Test (ROCF), respectively, before and after a 4-week protocol of repetitive IH combined with body weight-supported treadmill training (BWSTT) in incomplete ASIA C and D SCI subjects. Subjects received either IH (cycling 9%/21% FiO2 every 1.5 min, 15 cycles per day) or continued normoxia (Nx, 21% FiO2) combined with 45 min of BWSTT for 5 consecutive days, followed by 3 times per week IH and BWSTT for 3 additional weeks. ROCF Z scores between IH plus BWSTT and Nx plus BWSTT were not significantly different (p = .43). Compared with baseline, IH and BWSTT group showed a significantly greater (p < .05) verbal memory performance for immediate, short-term, and long-term recall; however, it was not different from Nx plus BWSTT group in all verbal memory components (p > .05). Our results suggest that a 4-week protocol of moderate IH does not elicit visual or verbal memory impairment. Thus, repetitive IH may be a safe therapeutic approach to incomplete spinal cord injury patients, without deleterious cognitive effects.

Reorganization of Respiratory Descending Pathways following Cervical Spinal Partial Section Investigated by Transcranial Magnetic Stimulation in the Rat

High cervical spinal cord injuries lead to permanent respiratory deficits. One preclinical model of respiratory insufficiency in adult rats is the C2 partial injury which causes unilateral diaphragm paralysis. This model allows the investigation of a particular population of respiratory bulbospinal axons which cross the midline at C3-C6 spinal segment, namely the crossed phrenic pathway. Transcranial magnetic stimulation (TMS) is a non-invasive technique that can be used to study supraspinal descending respiratory pathways in the rat. Interestingly, a lateral C2 injury does not affect the amplitude and latency of the largest motor-evoked potential recorded from the diaphragm (MEPdia) ipsilateral to the injury in response to a single TMS pulse, compared to a sham animal. Although the rhythmic respiratory activity on the contralateral diaphragm is preserved at 7 days post-injury, no diaphragm activity can be recorded on the injured side. However, a profound reorganization of the MEPdia evoked by TMS can be observed. The MEPdia is reduced on the non-injured rather than the injured side. This suggests an increase in ipsilateral phrenic motoneurons excitability. Moreover, correlations between MEPdia amplitude and spontaneous contralateral diaphragmatic activity were observed. The larger diaphragm activity correlated with a larger MEPdia on the injured side, and a smaller MEPdia on the non-injured side. This suggests, for the first time, the occurrence of a functional neuroplasticity process involving changes in motoneuron excitability balance between the injured and non-injured sides at a short post-lesional delay.

Effect of acute intermittent hypoxia treatment on ventilatory load compensation and magnitude estimation of inspiratory resistive loads in an individual with chronic incomplete cervical spinal cord injury

CONTEXT

Spinal cord injury (SCI) causes disruption of the efferent input to and afferent input from respiratory muscles, which impairs respiratory motor and sensory functions, respectively. This disturbs the injured individual's ability to respond to ventilatory loads and may alter the respiratory perceptual sensitivity of applied loads. Acute intermittent hypoxia with elevated CO(2) (AIH treatment) has been shown to induce ventilatory long-term facilitation in individuals with chronic SCI. This study evaluated the effect of ten days of AIH treatment on ventilatory load compensation and respiratory perceptual sensitivity to inspiratory resistive loads (IRL), in an individual with chronic, incomplete cervical SCI.

METHODS

Case report and literature review.

FINDINGS

We report a case of a 55-year-old female with a C4 chronic, incomplete SCI (American Spinal Injury Association Impairment Scale D). The subject underwent evaluation at four time-points: Baseline, Post Sham, AIH Day 1 and AIH Day 10. Significant improvements in airflow generated in response to applied IRL were found after AIH treatment compared to Baseline. There were no significant changes in the respiratory perceptual sensitivity to applied IRL after AIH treatment.

CLINICAL RELEVANCE

Rehabilitative interventions after SCI demand restoration of the respiratory motor function. However, they must also ensure that the respiratory perceptual sensitivity of the injured individual does not hinder their capability to compensate to ventilatory challenges.

Phrenic long-term facilitation requires PKCθ activity within phrenic motor neurons

Acute intermittent hypoxia (AIH) induces a form of spinal motor plasticity known as phrenic long-term facilitation (pLTF); pLTF is a prolonged increase in phrenic motor output after AIH has ended. In anesthetized rats, we demonstrate that pLTF requires activity of the novel PKC isoform, PKCθ, and that the relevant PKCθ is within phrenic motor neurons. Whereas spinal PKCθ inhibitors block pLTF, inhibitors targeting other PKC isoforms do not. PKCθ is highly expressed in phrenic motor neurons, and PKCθ knockdown with intrapleural siRNAs abolishes pLTF. Intrapleural siRNAs targeting PKCζ, an atypical PKC isoform expressed in phrenic motor neurons that underlies a distinct form of phrenic motor plasticity, does not affect pLTF. Thus, PKCθ plays a critical role in spinal AIH-induced respiratory motor plasticity, and the relevant PKCθ is localized within phrenic motor neurons. Intrapleural siRNA delivery has considerable potential as a therapeutic tool to selectively manipulate plasticity in vital respiratory motor neurons.

Repetitive acute intermittent hypoxia does not promote generalized inflammatory gene expression in the rat CNS

Modest protocols of repetitive acute intermittent hypoxia (rAIH) enhance motor function in patients with chronic incomplete spinal injury. Since chronic intermittent hypoxia (CIH) elicits neuroinflammation, there is potential for rAIH to have similar effects. Thus, we tested the hypothesis that rAIH has minimal impact on microglial inflammatory gene expression, but up-regulates key neurotrophic factor expression in a CNS region-specific manner. Using real time PCR, we evaluated mRNA levels of inflammatory and neurotrophic factors in immunomagnetically-isolated microglia from rat frontal cortex, brainstem and upper and lower cervical spinal cord following rAIH (ten, 5-min episodes, thrice weekly, 4 weeks). In agreement with our hypothesis, rAIH had no significant impact on microglial inflammatory gene expression in any region studied. On the other hand, neurotrophic factor expression was altered in a gene- and region-specific pattern. These results have important implications for the safety of rAIH as a potential therapy to enhance neuroplasticity and motor function in patients with spinal injury or other neurologic disorders.

Neuromechanical principles underlying movement modularity and their implications for rehabilitation

Neuromechanical principles define the properties and problems that shape neural solutions for movement. Although the theoretical and experimental evidence is debated, we present arguments for consistent structures in motor patterns, i.e., motor modules, that are neuromechanical solutions for movement particular to an individual and shaped by evolutionary, developmental, and learning processes. As a consequence, motor modules may be useful in assessing sensorimotor deficits specific to an individual and define targets for the rational development of novel rehabilitation therapies that enhance neural plasticity and sculpt motor recovery. We propose that motor module organization is disrupted and may be improved by therapy in spinal cord injury, stroke, and Parkinson's disease. Recent studies provide insights into the yet-unknown underlying neural mechanisms of motor modules, motor impairment, and motor learning and may lead to better understanding of the causal nature of modularity and its underlying neural substrates.

Intermittent hypoxia and neurorehabilitation

In recent years, it has become clear that brief, repeated presentations of hypoxia [i.e., acute intermittent hypoxia (AIH)] can boost the efficacy of more traditional therapeutic strategies in certain cases of neurologic dysfunction. This hypothesis derives from a series of studies in animal models and human subjects performed over the past 35 yr. In 1980, Millhorn et al. (Millhorn DE, Eldridge FL, Waldrop TG. Respir Physiol 41: 87-103, 1980) showed that electrical stimulation of carotid chemoafferent neurons produced a persistent, serotonin-dependent increase in phrenic motor output that outlasts the stimulus for more than 90 min (i.e., a "respiratory memory"). AIH elicits similar phrenic "long-term facilitation" (LTF) by a mechanism that requires cervical spinal serotonin receptor activation and de novo protein synthesis. From 2003 to present, a series of studies demonstrated that AIH can induce neuroplasticity in the injured spinal cord, causing functional recovery of breathing capacity after cervical spinal injury. Subsequently, it was demonstrated that repeated AIH (rAIH) can induce recovery of limb function, and the functional benefits of rAIH are greatest when paired with task-specific training. Since uncontrolled and/or prolonged intermittent hypoxia can elicit pathophysiology, a challenge of intermittent hypoxia research is to ensure that therapeutic protocols are well below the threshold for pathogenesis. This is possible since many low dose rAIH protocols have induced functional benefits without evidence of pathology. We propose that carefully controlled rAIH is a safe and noninvasive modality that can be paired with other neurorehabilitative strategies including traditional activity-based physical therapy or cell-based therapies such as intraspinal transplantation of neural progenitors.

Daily acute intermittent hypoxia elicits functional recovery of diaphragm and inspiratory intercostal muscle activity after acute cervical spinal injury

A major cause of mortality after spinal cord injury is respiratory failure. In normal rats, acute intermittent hypoxia (AIH) induces respiratory motor plasticity, expressed as diaphragm (Dia) and second external intercostal (T2 EIC) long-term facilitation (LTF). Dia (not T2 EIC) LTF is enhanced by systemic adenosine 2A (A2A) receptor inhibition in normal rats. We investigated the respective contributions of Dia and T2 EIC to daily AIH-induced functional recovery of breathing capacity with/without A2A receptor antagonist (KW6002, i.p.) following C2 hemisection (C2HS). Rats received daily AIH (dAIH: 10, 5-min episodes, 10.5% O2; 5-min normoxic intervals; 7 successive days beginning 7days post-C2HS) or daily normoxia (dNx) with/without KW6002, followed by weekly (reminder) presentations for 8weeks. Ventilation and EMGs from bilateral diaphragm and T2 EIC muscles were measured with room air breathing (21% O2) and maximum chemoreceptor stimulation (

MCS

7% CO2, 10.5% O2). dAIH increased tidal volume (VT) in C2HS rats breathing room air (dAIH+vehicle: 0.47±0.02, dNx+vehicle: 0.40±0.01ml/100g; p<0.05) and MCS (dAIH+vehicle: 0.83±0.01, dNx+vehicle: 0.73±0.01ml/100g; p<0.001); KW6002 had no significant effect. dAIH enhanced contralateral (uninjured) diaphragm EMG activity, an effect attenuated by KW6002, during room air breathing and MCS (p<0.05). Although dAIH enhanced contralateral T2 EIC EMG activity during room air breathing, KW6002 had no effect. dAIH had no statistically significant effects on diaphragm or T2 EIC EMG activity ipsilateral to injury. Thus, two weeks post-C2HS: 1) dAIH enhances breathing capacity by effects on contralateral diaphragm and T2 EIC activity; and 2) dAIH-induced recovery is A2A dependent in diaphragm, but not T2 EIC. Daily AIH may be a useful in promoting functional recovery of breathing capacity after cervical spinal injury, but A2A receptor antagonists (e.g. caffeine) may undermine its effectiveness shortly after injury.

Spinal metaplasticity in respiratory motor control

A hallmark feature of the neural system controlling breathing is its ability to exhibit plasticity. Less appreciated is the ability to exhibit metaplasticity, a change in the capacity to express plasticity (i.e., “plastic plasticity”). Recent advances in our understanding of cellular mechanisms giving rise to respiratory motor plasticity lay the groundwork for (ongoing) investigations of metaplasticity. This detailed understanding of respiratory metaplasticity will be essential as we harness metaplasticity to restore breathing capacity in clinical disorders that compromise breathing, such as cervical spinal injury, motor neuron disease and other neuromuscular diseases. In this brief review, we discuss key examples of metaplasticity in respiratory motor control, and our current understanding of mechanisms giving rise to spinal plasticity and metaplasticity in phrenic motor output; particularly after pre-conditioning with intermittent hypoxia. Progress in this area has led to the realization that similar mechanisms are operative in other spinal motor networks, including those governing limb movement. Further, these mechanisms can be harnessed to restore respiratory and non-respiratory motor function after spinal injury.

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.

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.

Restoring function after spinal cord injury: towards clinical translation of experimental strategies

Spinal cord injury is currently incurable and treatment is limited to minimising secondary complications and maximising residual function by rehabilitation. Improved understanding of the pathophysiology of spinal cord injury and the factors that prevent nerve and tissue repair has fuelled a move towards more ambitious experimental treatments aimed at promoting neuroprotection, axonal regeneration, and neuroplasticity. By necessity, these new options are more invasive. However, in view of recent advances in spinal cord injury research and demand from patients, clinicians, and the scientific community to push promising experimental treatments to the clinic, momentum and optimism exist for the translation of candidate experimental treatments to clinical spinal cord injury. The ability to rescue, reactivate, and rewire spinal systems to restore function after spinal cord injury might soon be within reach.

Adenosine 2A receptor inhibition enhances intermittent hypoxia-induced diaphragm but not intercostal long-term facilitation

Acute intermittent hypoxia (AIH) elicits diaphragm (Dia) and second external intercostal (T2 EIC) long-term facilitation (LTF) in normal unanesthetized rats. Although AIH-induced phrenic LTF is serotonin dependent, adenosine constrained in anesthetized rats, this has not been tested in unanesthetized animals. Cervical (C2) spinal hemisection (C2HS) abolishes phrenic LTF because of loss of serotonergic inputs 2 weeks post-injury, but LTF returns 8 weeks post-injury. We tested three hypotheses in unanesthetized rats: (1) systemic adenosine 2aA (A2A) receptor inhibition with intraperitoneal (IP) KW6002 enhances Dia and T2 EIC LTF in normal rats; (2) Dia and T2 EIC LTF are expressed after chronic (8 weeks), but not acute (1 week) C2HS; and (3) KW6002 enhances Dia and T2 EIC LTF after chronic (not acute) C2HS. Electromyography radiotelemetry was used to record Dia and T2 EIC activity during normoxia (21% O2), before and after AIH (10, 5-min 10.5% O2, 5-min intervals). In normal rats, KW6002 enhanced DiaLTF versus AIH alone (33.1±4.6% vs. 22.1±6.4% baseline, respectively; p<0.001), but had no effect on T2 EIC LTF (p>0.05). Although Dia and T2 EIC LTF were not observed 2 weeks post-C2HS, LTF was observed in contralateral (uninjured) Dia and T2 EIC 8 weeks post-C2HS (18.7±2.7% and 34.9±4.9% baseline, respectively; p<0.05), with variable ipsilateral expression. KW6002 had no significant effects on contralateral Dia (p=0.447) or T2 EIC LTF (p=0.796). We conclude that moderate AIH induces Dia and T2 EIC LTF after chronic, but not acute cervical spinal injuries. A single A2A receptor antagonist dose enhances AIH-induced Dia LTF in normal rats, but this effect is not significant in chronic (8 weeks) C2HS unanesthetized rats.

Unexpected benefits of intermittent hypoxia: enhanced respiratory and nonrespiratory motor function

Intermittent hypoxia (IH) is most often thought of for its role in morbidity associated with sleep-disordered breathing, including central nervous system pathology. However, recent evidence suggests that the nervous system fights back in an attempt to minimize pathology by increasing the expression of growth/trophic factors that confer neuroprotection and neuroplasticity. For example, even modest ("low dose") IH elicits respiratory motor plasticity, increasing the strength of respiratory contractions and breathing. These low IH doses upregulate hypoxia-sensitive growth/trophic factors within respiratory motoneurons but do not elicit detectable pathologies such as hippocampal cell death, neuroinflammation, or systemic hypertension. Recent advances have been made toward understanding cellular mechanisms giving rise to IH-induced respiratory plasticity, and attempts have been made to harness the benefits of low-dose IH to treat respiratory insufficiency after cervical spinal injury. Our recent realization that IH also upregulates growth/trophic factors in nonrespiratory motoneurons and improves limb (or leg) function after incomplete chronic spinal injuries suggests that IH-induced plasticity is a general feature of motor systems. Collectively, available evidence suggests that low-dose IH may represent a safe and effective treatment to restore lost motor function in diverse clinical disorders that impair motor function.

Therapeutic potential of intermittent hypoxia: a matter of dose

Intermittent hypoxia (IH) has been the subject of considerable research in recent years, and triggers a bewildering array of both detrimental and beneficial effects in multiple physiological systems. Here, we review the extensive literature concerning IH and its impact on the respiratory, cardiovascular, immune, metabolic, bone, and nervous systems. One major goal is to define relevant IH characteristics leading to safe, protective, and/or therapeutic effects vs. pathogenesis. To understand the impact of IH, it is essential to define critical characteristics of the IH protocol under investigation, including potentially the severity of hypoxia within episodes, the duration of hypoxic episodes, the number of hypoxic episodes per day, the pattern of presentation across time (e.g., within vs. consecutive vs. alternating days), and the cumulative time of exposure. Not surprisingly, severe/chronic IH protocols tend to be pathogenic, whereas any beneficial effects are more likely to arise from modest/acute IH exposures. Features of the IH protocol most highly associated with beneficial vs. pathogenic outcomes include the level of hypoxemia within episodes and the number of episodes per day. Modest hypoxia (9-16% inspired O2) and low cycle numbers (3-15 episodes per day) most often lead to beneficial effects without pathology, whereas severe hypoxia (2-8% inspired O2) and more episodes per day (48-2,400 episodes/day) elicit progressively greater pathology. Accumulating evidence suggests that "low dose" IH (modest hypoxia, few episodes) may be a simple, safe, and effective treatment with considerable therapeutic potential for multiple clinical disorders.

A murine model of cervical spinal cord injury to study post-lesional respiratory neuroplasticity

A cervical spinal cord injury induces permanent paralysis, and often leads to respiratory distress. To date, no efficient therapeutics have been developed to improve/ameliorate the respiratory failure following high cervical spinal cord injury (SCI). Here we propose a murine pre-clinical model of high SCI at the cervical 2 (C2) metameric level to study diverse post-lesional respiratory neuroplasticity. The technique consists of a surgical partial injury at the C2 level, which will induce a hemiparalysis of the diaphragm due to a deafferentation of the phrenic motoneurons from the respiratory centers located in the brainstem. The contralateral side of the injury remains intact and allows the animal recovery. Unlike other SCIs which affect the locomotor function (at the thoracic and lumbar level), the respiratory function does not require animal motivation and the quantification of the deficit/recovery can be easily performed (diaphragm and phrenic nerve recordings, whole body ventilation). This pre-clinical C2 SCI model is a powerful, useful, and reliable pre-clinical model to study various respiratory and non-respiratory neuroplasticity events at different levels (molecular to physiology) and to test diverse putative therapeutic strategies which might improve the respiration in SCI patients.

Mechanism of sympathetic activation and blood pressure elevation in humans and animals following acute intermittent hypoxia

Sleep apnea is associated with repeated episodes of hypoxemia, causing marked increase in sympathetic nerve activity and blood pressure. Considerable evidence suggests that intermittent hypoxia (IH) resulting from apnea is the primary stimulus for sympathetic overactivity in sleep apnea patients. Several IH protocols have been developed either in animals or in humans to investigate mechanisms underlying the altered autonomic regulation of the circulation. Most of these protocols involve several days (10-40 days) of IH exposure, that is, chronic intermittent hypoxia (CIH). Recent data suggest that a single session of IH exposure, that is, acute intermittent hypoxia (AIH), is already capable of increasing tonic sympathetic nerve output (sympathetic long-term facilitation, LTF) and altering chemo- and baroreflexes with or without elevation of blood pressure. This indicates that IH alters the autonomic neurocirculatory at a very early time point, although the mechanisms underlying this neuroplasticity have not been explored in detail. The purpose of this chapter is to briefly review the effects of AIH on sympathetic LTF and alteration of autonomic reflexes in comparison with the studies from CIH studies. We will also discuss the potential central and peripheral mechanism underlying sympathetic LTF.

Intermittent hypoxia-induced respiratory long-term facilitation is dominated by enhanced burst frequency, not amplitude, in spontaneously breathing urethane-anesthetized neonatal rats

Acute intermittent hypoxia (AIH) triggers a form of respiratory plasticity known as long-term facilitation (LTF), which is manifested as a progressive increase in respiratory motor activity that lasts for minutes to hours after the hypoxic stimulus is removed. Respiratory LTF has been reported in numerous animal models, but it appears to be influenced by a variety of factors (e.g., species, age, and gender). While most studies focusing on respiratory LTF have been conducted in adult (including young adult) rat preparations, little is known about the influence of postnatal maturation on AIH-induced respiratory LTF. To begin to address this issue, we examined diaphragm EMG activity in response to and at 5-min intervals for 60 min following three 5-min episodes of hypoxia (8% O2) in urethane-anesthetized spontaneously breathing P14-P15 neonatal rats (n=15). For these experiments, the hypoxic episodes were separated by hyperoxia (40% O2), and all rats were continuously supplied with ~4% CO2. During the AIH trials, burst frequency was increased by ~20-90% above baseline in each of the rats examined while changes in burst amplitude were highly variable. Following the AIH episodes, respiratory LTF was characterized by predominantly an increase in burst frequency (fLTF) ranging from ~10% to 55%, with most rats exhibiting a 20-40% increase. In seven rats, however, an increase in amplitude (ampLTF) (~10%, n=3; ~20%, n=3; ~30%, n=1) was also noted. These data suggest that in contrast to observations in anesthetized ventilated adult rats, in anesthetized spontaneously breathing P14-P15 neonatal rats, respiratory LTF is dominated by fLTF, not ampLTF.

Daily intermittent hypoxia enhances walking after chronic spinal cord injury: a randomized trial

Objectives:

To test the hypothesis that daily acute intermittent hypoxia (dAIH) and dAIH combined with overground walking improve walking speed and endurance in persons with chronic incomplete spinal cord injury (iSCI).

Methods:

Nineteen subjects completed the randomized, double-blind, placebo-controlled, crossover study. Participants received 15, 90-second hypoxic exposures (dAIH, fraction of inspired oxygen [Fio₂] = 0.09) or daily normoxia (dSHAM, Fio₂ = 0.21) at 60-second normoxic intervals on 5 consecutive days; dAIH was given alone or combined with 30 minutes of overground walking 1 hour later. Walking speed and endurance were quantified using 10-Meter and 6-Minute Walk Tests. The trial is registered at ClinicalTrials.gov (NCT01272349).

Results:

dAIH improved walking speed and endurance. Ten-Meter Walk time improved with dAIH vs dSHAM after 1 day (mean difference [MD] 3.8 seconds, 95% confidence interval [CI] 1.1–6.5 seconds, p = 0.006) and 2 weeks (MD 3.8 seconds, 95% CI 0.9–6.7 seconds, p = 0.010). Six-Minute Walk distance increased with combined dAIH + walking vs dSHAM + walking after 5 days (MD 94.4 m, 95% CI 17.5–171.3 m, p = 0.017) and 1-week follow-up (MD 97.0 m, 95% CI 20.1–173.9 m, p = 0.014). dAIH + walking increased walking distance more than dAIH after 1 day (MD 67.7 m, 95% CI 1.3–134.1 m, p = 0.046), 5 days (MD 107.0 m, 95% CI 40.6–173.4 m, p = 0.002), and 1-week follow-up (MD 136.0 m, 95% CI 65.3–206.6 m, p < 0.001).

Conclusions:

dAIH ± walking improved walking speed and distance in persons with chronic iSCI. The impact of dAIH is enhanced by combination with walking, demonstrating that combinatorial therapies may promote greater functional benefits in persons with iSCI.

Classification of evidence:

This study provides Class I evidence that transient hypoxia (through measured breathing treatments), along with overground walking training, improves walking speed and endurance after iSCI.

Intraspinal transplantation and modulation of donor neuron electrophysiological activity

Rat fetal spinal cord (FSC) tissue, naturally enriched with interneuronal progenitors, was introduced into high cervical, hemi-resection (Hx) lesions. Electrophysiological analyses were conducted to determine if such grafts exhibit physiologically-patterned neuronal activity and if stimuli which increase respiratory motor output also alter donor neuron bursting. Three months following transplantation, the bursting activity of FSC neurons and the contralateral phrenic nerve were recorded in anesthetized rats during a normoxic baseline period and brief respiratory challenges. Spontaneous neuronal activity was detected in 80% of the FSC transplants, and autocorrelation of action potential spikes revealed distinct correlogram peaks in 87% of neurons. At baseline, the average discharge frequency of graft neurons was 13.0 ± 1.7 Hz, and discharge frequency increased during a hypoxic respiratory challenge (p<0.001). Parallel studies in unanesthetized rats showed that FSC tissue recipients had larger inspiratory tidal volumes during brief hypoxic exposures (p<0.05 vs. C2Hx rats). Anatomical connectivity was explored in additional graft recipients by injecting a transsynaptic retrograde viral tracer (pseudorabies virus, PRV) directly into matured transplants. Neuronal labeling occurred throughout graft tissues and also in the host spinal cord and brainstem nuclei, including those associated with respiratory control. These results underscore the neuroplastic potential of host-graft interactions and training approaches to enhance functional integration within targeted spinal circuitry.

Common mechanisms of compensatory respiratory plasticity in spinal neurological disorders

In many neurological disorders that disrupt spinal function and compromise breathing (e.g. ALS, cervical spinal injury, MS), patients often maintain ventilatory capacity well after the onset of severe CNS pathology. In progressive neurodegenerative diseases, patients ultimately reach a point where compensation is no longer possible, leading to catastrophic ventilatory failure. In this brief review, we consider evidence that common mechanisms of compensatory respiratory plasticity preserve breathing capacity in diverse clinical disorders, despite the onset of severe pathology (e.g. respiratory motor neuron denervation and/or death). We propose that a suite of mechanisms, operating at distinct sites in the respiratory control system, underlies compensatory respiratory plasticity, including: (1) increased (descending) central respiratory drive, (2) motor neuron plasticity, (3) plasticity at the neuromuscular junction or spared respiratory motor neurons, and (4) shifts in the balance from more to less severely compromised respiratory muscles. To establish this framework, we contrast three rodent models of neural dysfunction, each posing unique problems for the generation of adequate inspiratory motor output: (1) respiratory motor neuron death, (2) de- or dysmyelination of cervical spinal pathways, and (3) cervical spinal cord injury, a neuropathology with components of demyelination and motor neuron death. Through this contrast, we hope to understand the multilayered strategies used to "fight" for adequate breathing in the face of mounting pathology.

Repeated intravenous doxapram induces phrenic motor facilitation

Doxapram is a respiratory stimulant used to treat hypoventilation. Here we investigated whether doxapram could also trigger respiratory neuroplasticity. Specifically, we hypothesized that intermittent delivery of doxapram at low doses would lead to long-lasting increases (i.e., facilitation) of phrenic motor output in anesthetized, vagotomized, and mechanically-ventilated rats. Doxapram was delivered intravenously in a single bolus (2 or 6mg/kg) or as a series of 3 injections (2mg/kg) at 5min intervals. Control groups received pH-matched saline injections (vehicle) or no treatment (anesthesia time control). Doxapram evoked an immediate increase in phrenic output in all groups, but a persistent increase in burst amplitude only occurred after repeated dosing with 2mg/kg. At 60min following the last injection, phrenic burst amplitude was 168±24% of baseline (%BL) in the group receiving 3 injections (P<0.05 vs. controls), but was 103±8%BL and 112±4%BL in the groups receiving a single dose of 2 or 6mg/kg, respectively. Following bilateral section of the carotid sinus nerves, the acute phrenic response to doxapram (2mg/kg) was reduced by 68% suggesting that at low doses the drug was acting primarily via the carotid chemoreceptors. We conclude that intermittent application of doxapram can trigger phrenic neuroplasticity, and this approach might be of use in the context of respiratory rehabilitation following neurologic injury.

Intermittent hypoxia training after C2 hemisection modifies the expression of PTEN and mTOR

In this study, we examined modulations in phosphatase and tensin homolog (PTEN) and mammalian target of rapamycin (mTOR) protein expression after a lateral C2 hemisection and subsequent intermittent hypoxia (IH) exposure and training, which initiates respiratory motor plasticity and recovery. PTEN and mTOR are significant molecules within a signaling pathway that directly influences dendritic sprouting, axonal plasticity, and regeneration. Expression levels of PTEN, mTOR and downstream effectors within this pathway were investigated, and it was found that following injury and IH exposure the expression of these molecules was significantly altered. This study directly demonstrates the implementation and feasibility of a non-invasive strategy to modulate the expression levels of intrinsic signaling molecules known to influence plasticity and regeneration in the CNS.

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.

Spinal vascular endothelial growth factor (VEGF) and erythropoietin (EPO) induced phrenic motor facilitation after repetitive acute intermittent hypoxia

Vascular endothelial growth factor (VEGF) and erythropoietin (EPO) exert neurotrophic and neuroprotective effects in the CNS. We recently demonstrated that VEGF, EPO and their receptors (VEGF-R2, EPO-R) are expressed in phrenic motor neurons, and that cervical spinal VEGF-R2 and EPO-R activation elicit long-lasting phrenic motor facilitation (pMF). Since VEGF, VEGF-R, EPO, and EPO-R are hypoxia-regulated genes, and repetitive exposure to acute intermittent hypoxia (rAIH) up-regulates these molecules in phrenic motor neurons, we tested the hypothesis that 4 weeks of rAIH (10 episodes per day, 3 days per week) enhances VEGF- or EPO-induced pMF. We confirm that cervical spinal VEGF and EPO injections elicit pMF. However, neither VEGF- nor EPO-induced pMF was affected by rAIH pre-conditioning (4 wks). Although our data confirm that spinal VEGF and EPO may play an important role in respiratory plasticity, we provide no evidence that rAIH amplifies their impact. Further experiments with more robust protocols are warranted.

Cervical spinal erythropoietin induces phrenic motor facilitation via extracellular signal-regulated protein kinase and Akt signaling

Erythropoietin (EPO) is typically known for its role in erythropoiesis but is also a potent neurotrophic/neuroprotective factor for spinal motor neurons. Another trophic factor regulated by hypoxia-inducible factor-1, vascular endothelial growth factor (VEGF), signals via ERK and Akt activation to elicit long-lasting phrenic motor facilitation (pMF). Because EPO also signals via ERK and Akt activation, we tested the hypothesis that EPO elicits similar pMF. Using retrograde labeling and immunohistochemical techniques, we demonstrate in adult, male, Sprague Dawley rats that EPO and its receptor, EPO-R, are expressed in identified phrenic motor neurons. Intrathecal EPO at C4 elicits long-lasting pMF; integrated phrenic nerve burst amplitude increased >90 min after injection (63 ± 12% baseline 90 min after injection; p < 0.001). EPO increased phosphorylation (and presumed activation) of ERK (1.6-fold vs controls; p < 0.05) in phrenic motor neurons; EPO also increased pAkt (1.6-fold vs controls; p < 0.05). EPO-induced pMF was abolished by the MEK/ERK inhibitor U0126 [1,4-diamino-2,3-dicyano-1,4-bis(o-aminophenylmercapto)butadiene] and the phosphatidylinositol 3-kinase/Akt inhibitor LY294002 [2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one], demonstrating that ERK MAP kinases and Akt are both required for EPO-induced pMF. Pretreatment with U0126 and LY294002 decreased both pERK and pAkt in phrenic motor neurons (p < 0.05), indicating a complex interaction between these kinases. We conclude that EPO elicits spinal plasticity in respiratory motor control. Because EPO expression is hypoxia sensitive, it may play a role in respiratory plasticity in conditions of prolonged or recurrent low oxygen.

Increased atypical PKC expression and activity in the phrenic motor nucleus following cervical spinal injury

Atypical protein kinase C (aPKC) isoforms are expressed in phrenic motor neurons, a group of motor neurons critical for breathing. Following C2 cervical hemisection (C2HS), spontaneous plasticity occurs in crossed-spinal synaptic pathways to phrenic motor neurons, at least partially restoring inspiratory phrenic activity below the injury. Since aPKCs are necessary for synaptic plasticity in other systems, we tested the hypothesis that C2HS increases aPKC expression and activity in spinal regions associated with the phrenic motor nucleus. C2 laminectomy (sham) or C2HS was performed on adult, male Lewis rats. Ventral spinal segments C3-5 were harvested 1, 3 or 28 days post-surgery, and prepared for aPKC enzyme activity assays and immunoblots. Ventral cervical aPKC activity was elevated 1 and 28, but not 3, days post-C2HS (1 day: 63% vs sham ipsilateral to injury; p<0.05; 28 day: 426% vs sham; p<0.05; no difference in ipsilateral vs contralateral response). Total PKCζ/ι protein expression was unchanged by C2HS, but total and phosphorylated PKMζ (constitutively active PKCζ isoform) increased ipsilateral to injury 28 days post-C2HS (p<0.05). Ipsilateral aPKC activity and expression were strongly correlated (r(2)=0.675, p<0.001). In a distinct group of rats, immunohistochemistry confirmed that aPKCs are expressed in neurons 28 days post-C2HS, including large, presumptive phrenic motor neurons; aPKCs were not detected in adjacent microglia (OX-42 positive cells) or astrocytes (GFAP positive cells). Changes in aPKC expression in the phrenic motor nucleus following C2HS suggests that aPKCs may contribute to functional recovery following cervical spinal injury.

Repetitive intermittent hypoxia induces respiratory and somatic motor recovery after chronic cervical spinal injury

Spinal injury disrupts connections between the brain and spinal cord, causing life-long paralysis. Most spinal injuries are incomplete, leaving spared neural pathways to motor neurons that initiate and coordinate movement. One therapeutic strategy to induce functional motor recovery is to harness plasticity in these spared neural pathways. Chronic intermittent hypoxia (CIH) (72 episodes per night, 7 nights) increases synaptic strength in crossed spinal synaptic pathways to phrenic motoneurons below a C2 spinal hemisection. However, CIH also causes morbidity (e.g., high blood pressure, hippocampal apoptosis), rendering it unsuitable as a therapeutic approach to chronic spinal injury. Less severe protocols of repetitive acute intermittent hypoxia may elicit plasticity without associated morbidity. Here we demonstrate that daily acute intermittent hypoxia (dAIH; 10 episodes per day, 7 d) induces motor plasticity in respiratory and nonrespiratory motor behaviors without evidence for associated morbidity. dAIH induces plasticity in spared, spinal pathways to respiratory and nonrespiratory motor neurons, improving respiratory and nonrespiratory (forelimb) motor function in rats with chronic cervical injuries. Functional improvements were persistent and were mirrored by neurochemical changes in proteins that contribute to respiratory motor plasticity after intermittent hypoxia (BDNF and TrkB) within both respiratory and nonrespiratory motor nuclei. Collectively, these studies demonstrate that repetitive acute intermittent hypoxia may be an effective and non-invasive means of improving function in multiple motor systems after chronic spinal injury.