Synergy Between Acute Intermittent Hypoxia and Task-Specific Training

Abstract

Acute intermittent hypoxia (AIH) and task-specific training (TST) synergistically improve motor function after spinal cord injury; however, mechanisms underlying this synergistic relationship are unknown. We propose a hypothetical working model of neural network and cellular elements to explain AIH-TST synergy. Our goal is to forecast experiments necessary to advance our understanding and optimize the neurotherapeutic potential of AIH-TST.

Summary for table of contents:

This article puts forward a hypothetical working model of synergy in motor plasticity induced by combined acute intermittent hypoxia and task-specific training.

INTRODUCTION

Plasticity is a fundamental property of neural systems, defined as a persistent change in neural system morphology and/or function based on prior experience. Experiences initiating neuroplasticity include: synaptic activity, exercise/physical activity, environmental challenges such altitude sojourn, and the onset of neurological disease or injury.

Spinal cord injury (SCI) impairs motor function, leaving many patients with chronic disability. Rehabilitation interventions seek to harness activity-dependent plasticity to partially restore function after neurological insult; unfortunately, functional gains are limited. Combinatorial therapies may enhance plasticity and functional improvements. One emerging method of augmenting the impact of task-specific exercise training (TST) is therapeutic (low-dose) acute intermittent hypoxia (AIH) (1). In this brief review, we consider knowns and unknowns regarding the disproportionate benefits of combined AIH-TST, and propose a working model to explain the synergy (to produce a combined effect greater than the sum of individual treatment effects) between these treatments.

Concepts driving our model are largely derived from decades of basic science research on the phrenic motor system and/or other neural systems such as the hippocampus. Whether the same mechanisms pertain to other motor systems or to humans is unknown, representing key areas for future investigation. In rodents, brief (≤ 5 minutes) episodic (3–15 exposures per day) periods of moderate hypoxia (inspired oxygen: 9–15%) increases phrenic nerve activity for more than 90 minutes post-AIH (2) – an effect known as phrenic long-term facilitation (LTF). Acute intermittent hypoxia should not be confused with the chronic intermittent hypoxia associated with sleep disordered breathing (15, 60 second hypoxic episodes, >5 episodes per hour; ~8 hours per day; many days to years).

Intermittent electrical stimulation of axons from chemoafferent neurons activates brainstem neural networks that quickly (and reversibly) increase breathing (i.e. chemoreflex), and triggers slower mechanisms of spinal synaptic plasticity. Specifically, serotonergic neurons in the medullary raphe nuclei are activated by carotid chemoafferent neurons, initiating and orchestrating phrenic motor plasticity (3). Raphe neurons project to spinal motor nuclei where they release serotonin on or near alpha-motor neurons. Serotonin type 2 receptors (5-HT₂) then activate intracellular signaling cascades that ultimately strengthen synapses between brainstem pre-motor and spinal alpha-motor neurons in an activity-independent manner (4). These neural network and intracellular mechanisms of motor plasticity underlie the therapeutic potential of AIH. Indeed, single or repeated AIH presentations improve breathing in rodent models of cervical SCI (5).

After decades of work investigating AIH-induced plasticity in the phrenic motor system, we came to realize that AIH effects are not unique to phrenic motor neurons. In fact, AIH influences diverse motor neuron pools, including hypoglossal respiratory (6), and locomotor limb motor neurons (1). For example, repetitive AIH improves both forelimb and respiratory function in rats with cervical SCI (7). Similar cellular mechanisms are thought to underlie plasticity in each of the relevant motor pools. However, forelimb improvements are minimal with AIH or TST presented alone. Significant functional benefits are observed only when AIH is delivered prior to TST on each training day (8). Since combined AIH plus TST produces an effect greater than the sum of individual treatments, there is synergy in their impact on motor plasticity.

Human AIH trials following SCI are underway. To date, nine studies have been published demonstrating efficacy in leg/ankle strength (9), hand function (10), locomotion (11), dynamic balance (12), and breathing (13). Moreover, combined AIH preceding overground walking practice (i.e. TST) improves walking function more than the sum of their individual effects when presented alone (11). Thus, the most effective rehabilitation strategy may incorporate AIH as a pre-conditioning stimulus (or ‘plasticity primer’) to amplify the benefits from TST (8, 11).

While functional improvements with combined AIH-TST are impressive (e.g. more than two-fold greater increase in walking endurance with daily AIH and walking practice versus daily AIH alone [37 vs. 17%] (11)), little evidence exists concerning mechanisms giving rise to their synergy. Understanding such mechanisms in multiple motor systems is of considerable importance since it may enable: 1) optimization of AIH-TST therapy; 2) development of new treatments for other neuromuscular disorders that compromise movement (e.g. amyotrophic lateral sclerosis, Pompe disease, multiple sclerosis); and 3) conceptual translation to other forms of neuromodulation and/or plasticity, such as closed-loop vagal nerve, epidural or transcutaneous spinal cord stimulation. In this review, we present a hypothetical model of motor plasticity with the aim of guiding new research, enabling a greater understanding of the synergistic relationship between AIH and TST. Before we present our model, we provide additional background on core concepts guiding model development.

ACUTE INTERMITTENT HYPOXIA-INDUCED MOTOR PLASTICITY

Over the past 30 years, considerable progress has been made towards a comprehensive understanding of cellular mechanisms giving rise to AIH-induced phrenic motor plasticity. This work inspired, and continues to guide translation of basic science discoveries into human clinical trials, including investigations of non-respiratory motor systems. Human studies demonstrating improved limb function following AIH (9) were directly influenced by earlier studies in rodent models that shaped our current knowledge (14).

Neural network mechanisms.

During hypoxia, peripheral chemoreceptors located at the carotid artery bifurcation activate chemoafferent neurons that synapse in the brainstem nucleus of the solitary tract (NTS). Second order NTS neurons project to contralateral respiratory neurons of the ventral respiratory column, including rhythm generating neurons of the pre-Bötzinger complex and pre-motor neurons of the ventral respiratory group. Projections from NTS (direct) or ventral respiratory group (indirect) neurons activate midline serotonergic neurons of the caudal raphe nuclei. Serotonergic neurons in the raphe nuclei project broadly, including the spinal cord where they release serotonin in the phrenic motor nucleus. Cervical spinal 5-HT₂ receptor activation is both necessary (15) and sufficient (16) for moderate AIH-induced phrenic LTF. Although serotonin type 7 (5-HT₇) receptor activation elicits similar phrenic motor facilitation (17), its cellular mechanism is completely distinct. Overall, moderate AIH-induced phrenic LTF requires caudal raphe neuron activity, serotonin release and 5-HT₂ receptor activation in the phrenic motor nucleus vs. tissue hypoxia per se.

In contrast to moderate AIH, the same AIH protocol consisting of severe hypoxic episodes (arterial oxygen pressure < 30 mmHg) elicits a phenotypically similar, but distinct form of phrenic LTF that requires spinal tissue hypoxia and adenosine accumulation (18), presumably from glial adenosine release. In this situation, adenosine 2A (A_2A) receptor activation on phrenic motor neurons initiates a distinct intracellular cascade leading to phrenic motor facilitation (18). Although the practical utility of severe AIH protocols in humans is limited due to safety concerns, it is important to recognize that subthreshold A_2A receptor activation competes with and undermines the serotonin-induced mechanism necessary for functional improvement (see below).

Intracellular mechanisms.

At least two opposing phrenic LTF mechanisms arise from AIH protocols that vary in severity and/or duration of hypoxic episodes. Both mechanisms operate within phrenic motor neurons, but differ in their reliance upon specific serotonin and adenosine receptors that trigger completely distinct intracellular signaling cascades. These signaling cascades are named for the G proteins canonically coupled to the initiating receptor, the Q (Gq) and S (Gs) pathways to phrenic motor facilitation, respectively (14).

With moderate AIH, the serotonin-dependent Q-pathway dominates phrenic LTF. Serotonin activates 5-HT₂ receptors, initiating a signaling cascade that includes extracellular related kinase/mitogen activated protein kinase (ERK/MAPK) activity (19), de novo brain-derived neurotrophic factor (BDNF) protein synthesis, activation the high-affinity BDNF receptor TrkB within phrenic motor neurons (20), and downstream signaling via the protein kinase C isoform, PKCθ. We postulate that PKCθ phosphorylates NMDA and/or AMPA (21) receptors, increasing glutamate receptor currents and/or trafficking/insertion, thereby strengthening bulbospinal synaptic inputs to phrenic motor neurons.

With severe AIH, A_2A receptors activate adenyl cyclase, increasing cyclic adenosine monophosphate (cAMP) formation. High cAMP levels activate a pathway including cAMP exchange protein (EPAC), PI3 kinase/Akt and mammalian target of rapamycin (mTOR). Subsequent synthesis of an immature TrkB isoform strengthens synaptic inputs to phrenic motor neurons. Activation of 5-HT₇ receptors initiates the same mechanism (22).

The Q- and S-pathways interact via powerful cross-talk inhibition (23). Thus, net plasticity reflects the inhibitory balance of opposing pathways. Where moderate AIH favors the Q-pathway (24), subthreshold S-pathway activation constrains its expression (23). On the other hand, severe AIH shifts the balance to S-pathway dominance (18, 22); Q-pathway activation constrains its expression (22). With intermediate AIH, the two pathways cancel one another, and phrenic LTF is not observed (22). Similarly, with sustained moderate hypoxia, time-dependent adenosine accumulation and S-pathway activation undermines the Q-pathway, canceling phrenic LTF (25) (please see Devinney, Fields (26) and Perim, Fields (22) for further information).

Because of these complex interactions, protocol details are a key factor determining phrenic LTF expression. Harnessing AIH as a therapeutic modality requires understanding of this balance, and how to minimize pathway competition. Translational efforts to date have focused on moderate AIH due to fears of tissue hypoxia; hence, successful treatment strategies utilizing AIH must optimize BDNF synthesis. Since TST and moderate AIH both increase spinal BDNF, AIH-induced BDNF synthesis may prime the nervous system, accentuating therapeutic benefits of subsequent TST – a concept elaborated below.

TASK-SPECIFIC TRAINING-INDUCED MOTOR PLASTICITY

Physical exercise promotes plasticity and improves neuromotor function with neurological injury and/or disease. One established mechanism of exercise-induced neuroplasticity is activity-dependent production of growth/trophic factors. However, exercise alone is not always sufficient to support complete functional recovery of specific tasks. The success of exercise-based neurorehabilitation depends on congruency between the trained motor function and the targeted motor networks (i.e. TST).

Task-specific training is repetitive functional practice of a motor task specific to the intended outcome. For example, when cats with SCI are trained to stand, standing ability improves but not stepping ability (27). This form of task-specific exercise can result in the acquisition of new motor skills, refinement of existing motor skills or partial restoration of lost motor skills. Further, TST may promote recovery by triggering neuroplasticity in specific neural circuits, including changes in pre- and/or post-synaptic function (28), neuromodulation via monoamines (e.g. serotonin, norepinephrine) (29), and/or new synapse formation (30).

Supraspinal plasticity.

Task-specific training-induced plasticity may occur at supraspinal sites. Indeed, sensory and motor cortices are reorganized by TST (31). Synaptic plasticity changes cortical maps (32) and may strengthen specific motor circuits practiced during TST. Other supraspinal structures, such as the cerebellum, may also exhibit TST-induced plasticity.

The neurotrophin, BDNF, is implicated in TST-induced cortical/cerebellar plasticity and motor learning. During skilled motor training, cortical neurons upregulate BDNF and undergo synaptic potentiation and/or synaptogenesis, thereby reinforcing practiced skills. Although exogenous BDNF applied to the motor cortex of rats with unilateral SCI fails to elicit functional recovery, exogenous BDNF coupled with TST improves task performance (33). Thus, in certain situations, combined BDNF and TST elicits recovery of motor function.

Spinal plasticity.

Task-specific training also strengthens spinal functions, including altered spinal reflex strength. For example, strength training augments Hoffmann (H) reflex amplitude during a maximum voluntary contraction (34). Increased spinal reflex strength after 4 weeks of training at least partially results from a net increase in excitatory synaptic inputs onto motor neurons (35). H-reflex amplitude also exhibits plasticity after operant conditioning (36). Down-conditioning affects motor neuron axon conduction velocity and firing threshold (37), the size and number of GABA terminals on motor neurons, and alters ventral spinal GABA interneurons (38). Mechanisms of H-reflex up-conditioning are less well understood, but may involve increased strength in glutamatergic terminals on motor neurons. Thus, acquiring or improving motor skill performance may necessitate refinement of a specific motor plan (supraspinal), increased spinal motor output, or both.

Brain-derived neurotrophic factor.

Treadmill and/or running wheel exercise increase BDNF in the hippocampus (39) and spinal cord (40–42). Increased BDNF synthesis may relate to increased neural network activity or medullary serotonergic neuron activation during exercise (43, 44). Lumbar BDNF mRNA is upregulated by reflex training in spinally transected rats (42). Therefore, TST may increase spinal BDNF expression in specific neurons via activity and/or serotonin-dependent mechanisms, potentially strengthening excitatory synapses onto motor neurons. Although both AIH and TST can increase spinal BDNF synthesis, their specific effects, in magnitude and/or distribution, are not known. In our model, we suggest that AIH has a more wide-spread impact on BDNF expression, priming the system for additional, localized BDNF synthesis in specific neural circuits activated by TST. Combined interventions may elevate BDNF within neurons/synapses above a functional “threshold” for plasticity, giving rise to unique therapeutic opportunities. Such task-specific, BDNF-dependent plasticity synergy may occur in cortical and/or spinal elements of the motor circuit. For the purposes of this review, we focus on spinal elements; however, since AIH increases BDNF in the motor cortex (45), we do not exclude the possibility of additional cortical mechanisms.

COMBINED ACUTE INTERMITTENT HYPOXIA AND TASK-SPECIFIC TRAINING

While there has been steady growth in our understanding of AIH-induced plasticity and its implications for neurorehabilitation, recent evidence suggests that therapeutic benefits are greatest with paired AIH-TST, at least with non-respiratory motor behaviors (8, 11, 46). Unfortunately, few studies have combined AIH and TST (Table), and even fewer directly demonstrate that combined training is more effective than either treatment alone (8, 11). More research in this area is warranted. Nevertheless, we base our hypothesis on these carefully designed experiments.

Table 1:

Previous Investigations of AIH-TST.

Author(s)	Year	Experimental Preparation	Intervention	Outcomes Measures	Results
Lovett-Barr et al (7)	2012	Rodent	• 7 days of AIH combined with ladder walking practice	• Spontaneous breathing (frequency, volume, minute ventilation) • Forelimb function during horizontal ladder walking • Expression of BDNF and TrkB in respiratory and non-respiratory motor nuclei	• Horizontal ladder walking performance: Daily AIH + ladder walking practice = ~30% ↓ foot slip errors compared to pre-treatment. Normoxia + ladder walking practice = ↔ (no change) • Growth factors: Daily AIH = ↑ BDNF and TrkB in respiratory (C4) and non-respiratory (C7) motor nuclei • Breathing: Daily AIH = ↑ minute ventilation (primarily via tidal volume) versus normoxia-treated rodents
Hayes et al (11)	2014	Human	• Block 1: 5 consecutive days of AIH or normoxia separated by 2 weeks • Block 2: 5 consecutive days of AIH + walking or normoxia + walking separated by 2 weeks	• Walking endurance: 6MWT • Walking speed: 10MWT	• 10MWT: Block 1: Significant ↑ after AIH compared to baseline and significantly ↑ than normoxia; Block 2: ↔ after AIH compared to baseline, but significant difference between change from baseline compared to normoxia • 6MWT: Block 1: Significant ↑ after AIH compared to baseline, but ↔ compared to normoxia; Block 2: Significant ↑ after AIH + walking compared to baseline and significantly ↑ than normoxia + walking
Prosser-Loose et al (8)	2015	Rodent	• 7 consecutive days of either: a) AIH or normoxia; b) AIH + TST (ladder walking practice) or normoxia + TST; c) AIH + non-TST (treadmill exercise) or normoxia + non-TST	• Number of foot-slip errors on horizontal ladder walking task	• AIH vs. normoxia: ↔ in number of errors post-treatment and ↔ between groups (~80% errors) • AIH + TST vs. normoxia + TST: Significantly ↓ errors post-treatment (~50% improvement) and significant difference between groups • AIH + non-TST vs. normoxia + non-TST: ↔ post-treatment and no difference between groups (~75% errors)
Navarette-Opazo et al (46)	2017	Human	• 4 weeks of AIH + BWSTT or normoxia + BWSTT	• Walking endurance: 6MWT • Walking speed: 10MWT	• 10MWT: Significant difference in absolute change from baseline between groups (AIH = −10 s, normoxia = −2 s) • 6MWT: Significant difference in absolute change from baseline between groups (AIH = +43 m, normoxia = +6 m)
Trumbower et al (9)	2017	Human	• 5 days of AIH + TST (hand opening practice) or normoxia + TST	• Box and block test • Jebsen-Taylor hand function test • Motion analysis (hand opening) • Surface EMG (hand muscles)	• Significantly ↑ box and block test scores after AIH + TST versus normoxia + TST • 4/6 participants ↑ Jebsen-Taylor hand function test after AIH + TST versus normoxia + TST • Significantly ↑ maximum hand opening after AIH + TST versus normoxia + TST (↑ only seen in 5/6 participants) • Significant ↑ in muscle co-activation in 5/6 participants after AIH + TST. ↔ after normoxia + TST

Abbreviations: 6MWT = six-minute walk test; 10MWT = 10-m-walk test; AIH = acute intermittent hypoxia; BDNF = brain derived neurotrophic factor; BWSTT = body weight supported treadmill training; TST = task-specific training.

In a rodent model of forelimb function following cervical SCI, AIH and TST (horizontal ladder walking) were ineffective when applied separately; functional benefits were observed only when applied together (8). In this same study, non-task-specific treadmill training failed to improve ladder walking performance, even when combined with AIH (8). Thus, AIH was effective when presented in combination with TST, but not non-specific exercise training. In humans with chronic, incomplete SCI, 5 consecutive days of AIH marginally improved walking speed, while daily walking practice had minimal impact (11). However, when AIH was presented 30 minutes prior to overground walking practice, functional benefits in walking endurance were greatly enhanced 3 days post-treatment (11). These two studies (one rodent, one human) were the inspiration for the working model and hypothesis presented here.

Neither rodent nor human experiments of combinatorial therapy have been thoroughly investigated. Many other motor functions remain to be explored, including the respiratory motor pools that formed much of our current understanding of AIH-induced motor plasticity. Since AIH exerts widespread effects on motor neuron BDNF expression (7, 45), our contention is that AIH acts as a global priming mechanism for motor plasticity, enhancing localized plasticity mechanisms triggered by TST. By priming the system, AIH augments subsequent activity-dependent plasticity, improving motor recovery in specific tasks in people with neurological injury/disease. We hypothesize that combined AIH-TST is more effective than the sum of benefits elicited by each treatment presented alone (i.e. they are synergistic) because: 1) AIH broadly elevates BDNF synthesis in diverse neural circuits, and 2) TST further elevates BDNF above a functional threshold for motor plasticity only in neural circuits activated during the specific motor task practiced. In Figure 1, a neural network model of AIH and TST activated pathways and their convergence onto spinal motor neurons is illustrated. In Figure 2, we illustrate a working, intracellular model of signaling cascades within task-targeted motor neurons. Functional outcomes from the neurochemical convergence of sequenced AIH-TST are illustrated in Figure 3.

Figure 1: Neural Network Model.

Postulated neural circuits activated by acute intermittent hypoxia (AIH) and task-specific training (TST). Hypoxia stimulates carotid body (CB) chemoreceptors and their chemoafferent neurons, activating second order medullary neurons in the nucleus tractus solitarii (NTS). These second order neurons project to the central pattern generator (CPG) of the ventral respiratory column (signified by an oscillator and integral sign). Either direct (NTS) or indirect projections from ventral respiratory group neurons activate serotonergic neurons of the caudal raphe nuclei. Descending projections to spinal motor nuclei and subsequent serotonin release initiate and orchestrate plasticity in phrenic (PMn) and limb (LMn) motor neurons. During TST, primary motor cortex (M1) neurons with direct projections to PMn, LMn and/or their pre-motor neurons are activated; although they could activate raphe and spinal CPG neurons, we do not postulate that those relay projections play a major role in TST-induced motor plasticity. Dashed lines indicate unknown pathways.

Figure 2: Cellular Model.

Postulated intracellular signaling cascades giving rise to AIH- and TST-induced motor plasticity. In A: AIH-activated cellular cascades (elevated BDNF) and increased motor neuron output (bottom trace). Serotonin release and receptor activation are the major drivers of the AIH-induced cellular cascade. In B: TST-activated cellular cascades (elevated BDNF) and increased motor neuron output (bottom trace). In this case, BDNF synthesis is not quite as high (signified as less bold) and motor output is not elevated as strongly versus AIH. We postulate that serotonin plays a less prominent role, and that the more relevant cascade is activity-dependent increases in intracellular calcium and CaMK activation – an alternate mechanism of increased BDNF synthesis; this activity driven effect would occur exclusively in neurons relevant to TST. In C: Combined AIH-TST converges both mechanisms, but only in neurons activated during TST. Thus, BDNF and motor activity in these task-relevant neurons are greater than that elicited by either treatment alone. Abbreviations: AIH = acute intermittent hypoxia; BDNF = brain-derived neurotrophic factor; CaMK = Ca²⁺-calmodulin-dependent protein kinase; TrkB = tyrosine kinase B; TST = task-specific training.

Figure 3: Working Hypothetical Model of Motor Plasticity.

With a postulated sigmoid BDNF dose-response curve, AIH and TST each elicit modest plasticity when delivered alone. However, combined AIH-TST elicits greater elevations in BDNF (but only within task-relevant neurons), increasing plasticity and improving motor function more than the combined value predicted from each alone. Abbreviations: AIH = acute intermittent hypoxia; BDNF = brain-derived neurotrophic factor; TrkB = tyrosine kinase B; TST = task-specific training.

Neural network model.

The network model consists of peripheral and central nervous system elements activated by AIH and TST, converging on targeted alpha-motor neurons (Figure 1). We focus on serotonergic and glutamatergic pathways.

During hypoxia, spinal serotonin release in motor nuclei is triggered by carotid chemoreceptor activation, and synaptic activation of second order neurons in the NTS, ventral respiratory group and caudal raphe nuclei. Serotonergic raphe neurons, which send descending projections to spinal gray matter, are activated by rhythmic motor behaviors such as walking, chewing and breathing (43). While serotonin could play a common role with both AIH and TST, accounting for their synergy, we do not favor this idea since treadmill training (i.e. non-task-specific exercise) increases spinal serotonin release, but does not enhance horizontal ladder walking (8). To explain available data, we propose that TST-induced neural activity (at glutamatergic synapses) is a necessary component of the AIH-TST synergy.

During TST, descending cortical projections activate alpha-motor neurons directly or indirectly via glutamatergic synaptic transmission. The resulting neuronal activity may up-regulate BDNF within alpha-motor neurons. This increase in BDNF would occur exclusively within neurons activated by the specific task. Combining AIH- and TST-induced BDNF effects within specific neuron populations may evoke selective, task-appropriate plasticity.

Cellular model.

The cellular model (Figure 2) describes potential mechanisms of AIH- and TST-convergence on targeted alpha-motor neurons from an intracellular signaling perspective. We posit that this convergence encapsulates the synergistic plasticity-inducing relationship of AIH and TST. Our model is not intended to portray comprehensive cellular pathways, but emphasizes principle aspects of our working hypothesis – that convergence onto common cellular elements in the same (task-specific) cells elevates BDNF levels above an effective threshold necessary for plasticity. We propose that these cascades are initiated via AIH-activated serotonergic, and TST-activated glutamatergic receptors. Despite initial activation of different neurochemical receptors (serotonin vs. glutamate), both AIH and TST ultimately converge on post-synaptic glutamate receptor density and/or current conductance for the maintenance of plasticity.

Knowledge concerning cellular mechanisms of AIH-induced motor plasticity is based on decades of studies on the phrenic motor system; relatively little is known concerning AIH effects on non-respiratory motor systems. In humans, AIH enhances motor evoked potentials in the first dorsal interosseous muscle at a site between the medulla and motor neuron synapse (47), consistent with our understanding of AIH-induced phrenic LTF. The effects of AIH appear to be synergistic with spike-timing dependent plasticity since their combined effects are greater than their individual effects in most subjects. In a rodent model of cervical SCI, seven days of AIH elevates BDNF and TrkB in both phrenic and non-respiratory motor nuclei (7), similar to intact rats. Despite parallels in neurochemical plasticity, direct comparisons of AIH-induced plasticity in respiratory vs. non-respiratory motor systems are not available.

We propose that AIH-induced serotonin-dependent BDNF synthesis is general to alpha-motor neurons, and that the independent effects of AIH on BDNF synthesis are sufficient to induce motor plasticity in some (e.g. phrenic, interosseous), but not all motor systems. We hypothesize that, in both situations, plasticity synergy and specificity can be achieved when TST triggers additional BDNF synthesis exclusively within the task-specific motor circuit.

Task-specific training elevates BDNF in the brain (48) and spinal cord (40). Increased BDNF/TrkB signaling triggers new synthesis of synapsin I, indicating possible synaptogenesis. Exercise also increases BDNF in skeletal muscles, including the neuromuscular junction (41), which can be retrogradely transported from muscle to the spinal cord through motor neuron axons, potentially supporting BDNF-induced plasticity.

TST activates post-synaptic glutamatergic NMDA and AMPA receptors. When glutamate receptors are phosphorylated, NMDA receptor currents and/or AMPA receptor trafficking to the post-synaptic membrane are augmented. Calcium influx and activation of Ca²⁺-calmodulin-dependent kinase (CaMK) can lead to glutamate receptor phosphorylation, thereby enhancing NMDA and AMPA receptor conductances (49). This mechanism contributes to a classic example activity-dependent synaptic plasticity, known as hippocampal long-term potentiation (LTP). Activity-dependent LTP is associated with spatial learning and memory (50). Brain-derived neurotrophic factor is required for at least some forms of hippocampal LTP (51). Thus, both activity-dependent hippocampal synaptic plasticity and AIH-induced phrenic motor plasticity require BDNF/TrkB signaling. Similar activity- and BDNF-dependent plasticity could occur in task-specific motor circuits because calcium is increased and CaMK activated exclusively in TST-relevant neuronal populations.

In our model, TST alone increases BDNF expression, but only to a limited extent (Figure 3). Activity-dependent increases in BDNF expression would be localized within the relevant (activated) motor circuit, and may or may not be sufficient to elicit some minor degree of motor plasticity (depending upon the shape/slope of the stimulus dose-response curve). Acute intermittent hypoxia pre-conditioning may establish a higher baseline level of spinal BDNF, eliciting greater, non-specific plasticity in multiple (i.e. non-task-specific) motor systems. On the other hand, a global increase in baseline BDNF would set the stage for enhanced TST effects, but only in neural circuits activated during the practiced task. In this way, plasticity would be unique to the specific task, accounting for synergy between AIH and TST. Because AIH elevates BDNF via translational regulation, it requires many minutes to hours in order to function as a plasticity priming stimulus. As a result, paired AIH-TST would ideally be separated by 30 to 60 minutes, giving adequate time for BDNF protein synthesis (8, 11).

Hypothesis testing.

Our model and hypothesis were developed to explain recent rodent and human experiments demonstrating AIH-TST synergy (8, 11). We propose that AIH and TST mechanisms converge at the level of alpha-motor neurons, increasing BDNF synthesis more than in other motor pools (i.e. those not activated by TST). There is currently no empirical evidence supporting the notion that TST elevates BDNF within specific, activated vs. non-activated motor neurons. This premise is the crux of our hypothesis is thus a prime target for future mechanistic experimentation.

From another perspective, respiratory infections and ventilatory insufficiency are leading causes of morbidity and mortality following cervical SCI. Improvements in respiratory function following daily AIH have been observed (13), yet the impact of combined treatments have not been explored. The use of respiratory strength training, voluntary (isocapnic) or CO₂-driven chemoreflex hyperpnea offer simple task-specific strategies to target neural pathways involved in breathing. Combined AIH pre-conditioning and respiratory TST may improve functional benefits.

Finally, although our discussion has focused spinal alpha-motor neuron plasticity, the possibility remains that synergy may result instead in the motor cortex, or other regions affected both by AIH and TST. These possibilities await further study and experimental verification.

SUMMARY

After spinal trauma, motor functions are impaired. Recovery depends on multiple factors, including the level and extent of injury. Since spontaneous compensatory mechanisms and conventional rehabilitation are insufficient to fully restore function, new strategies are needed.

Acute intermittent hypoxia is a novel intervention with promise to improve respiratory and non-respiratory motor function after SCI by promoting plasticity through targeted increases in BDNF/TrkB signaling. In non-respiratory motor systems, two studies (one rodent, one human) demonstrate that AIH effects are amplified considerably when combined with TST (8, 11). The AIH-TST synergy is specific to the engaged motor function, and does not appear to be generalizable to off-target motor functions. Although AIH-TST synergy has been demonstrated in limb functions (ladder walking in rats; over-ground walking in humans), we do not know if similar synergies occur in breathing function.

Since mechanisms of AIH-TST synergy are only poorly understood, we offer hypothetical working models of both neural network and cellular mechanisms. Our model is highly speculative, although grounded in neuroscience literature; our hope is that these models will provoke future experiments. In this review, we proposed that AIH primes the nervous system for plasticity by broadly elevating BDNF/TrkB signaling in diverse neuron pools whereas TST selectively activates neural circuits, further increasing BDNF only in task-relevant neuronal circuits via activity-dependent mechanisms. Due to the limited number of studies on this topic, we advise caution – the model requires extensive testing.

KEY POINTS.

• Following spinal cord injury, combined acute intermittent hypoxia (AIH) and task-specific training (TST) enhances motor function beyond that expected from either treatment alone.

• Mechanisms underlying this synergistic therapeutic strategy are unknown.

• We propose a hypothetical working model including both neural network and cellular elements to explain synergy between AIH and TST.

• Our model is intended to spur needed mechanistic studies and accelerate progress towards clinical application of AIH as a plasticity primer in neurorehabilitation.