Plasticity in respiratory motor control: intermittent hypoxia and hypercapnia activate opposing serotonergic and noradrenergic modulatory systems

Microglia regulate motor neuron plasticity via reciprocal fractalkine/adenosine signaling

Microglia are innate CNS immune cells that play key roles in supporting key CNS functions including brain plasticity. We now report a previously unknown role for microglia in regulating neuroplasticity within spinal phrenic motor neurons, the neurons driving diaphragm contractions and breathing. We demonstrate that microglia regulate phrenic long-term facilitation (pLTF), a form of respiratory memory lasting hours after repetitive exposures to brief periods of low oxygen (acute intermittent hypoxia; AIH) via neuronal/microglial fractalkine signaling. AIH-induced pLTF is regulated by the balance between competing intracellular signaling cascades initiated by serotonin vs adenosine, respectively. Although brainstem raphe neurons release the relevant serotonin, the cellular source of adenosine is unknown. We tested a model in which hypoxia initiates fractalkine signaling between phrenic motor neurons and nearby microglia that triggers extracellular adenosine accumulation. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF; in contrast, severe AIH drives pLTF by a unique, adenosine-dominant mechanism. Phrenic motor neuron fractalkine knockdown, cervical spinal fractalkine receptor inhibition on nearby microglia, and microglial depletion enhance serotonin-dominant pLTF with moderate AIH but suppress adenosine-dominant pLTF with severe AIH. Thus, microglia play novel functions in the healthy spinal cord, regulating hypoxia-induced neuroplasticity within the motor neurons responsible for breathing.

Ketamine plus Alcohol: What We Know and What We Can Expect about This

Drug abuse has become a public health concern. The misuse of ketamine, a psychedelic substance, has increased worldwide. In addition, the co-abuse with alcohol is frequently identified among misusers. Considering that ketamine and alcohol share several pharmacological targets, we hypothesize that the consumption of both psychoactive substances may synergically intensify the toxicological consequences, both under the effect of drugs available in body systems and during withdrawal. The aim of this review is to examine the toxicological mechanisms related to ketamine plus ethanol co-abuse, as well the consequences on cardiorespiratory, digestive, urinary, and central nervous systems. Furthermore, we provide a comprehensive discussion about the probable sites of shared molecular mechanisms that may elicit additional hazardous effects. Finally, we highlight the gaps of knowledge in this area, which deserves further research.

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

KEY POINTS

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

ABSTRACT

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

Blockade of alpha2-adrenergic receptors in the caudal raphe region enhances the renal sympathetic nerve activity response to acute intermittent hypercapnia in rats

The study investigated the role of alpha2-adrenergic receptors of the caudal raphe region in the sympathetic and cardiovascular responses to the acute intermittent hypercapnia (AIHc). Urethane-anesthetized, vagotomized, mechanically ventilated Sprague-Dawley rats (n=38) were exposed to the AIHc protocol (5×3 min, 15 % CO2+50 % O2) in hyperoxic background (50 % O2). alpha2-adrenergic receptor antagonist-yohimbine was applied intravenously (1 mg/kg, n=9) or microinjected into the caudal raphe region (2 mM, n=12) prior to exposure to AIHc. Control groups of animals received saline intravenously (n=7) or into the caudal raphe region (n=10) prior to exposure to AIHc. Renal sympathetic nerve activity (RSNA), mean arterial pressure (MAP) and heart rate (HR) were monitored before exposure to the AIHc protocol (T0), during five hypercapnic episodes (THc1-5) and at 15 min following the end of the last hypercapnic episode (T15). Following intravenous administration of yohimbine, RSNA was significantly greater during THc1-5 and at T15 than in the control group (P<0.05). When yohimbine was microinjected into the caudal raphe region, AIHc elicited greater increases in RSNA during THc1-5 when compared to the controls (THc1: 138.0+/-4.0 % vs. 123.7+/-4.8 %, P=0.032; THc2: 137.1+/-5.0 % vs. 124.1+/-4.5 %, P=0.071; THc3: 143.1+/-6.4 % vs. 122.0±4.8 %, P=0.020; THc4: 146.1+/-6.2 % vs. 120.7+/-5.7 %, P=0.007 and THc5: 143.2+/-7.7 % vs. 119.2+/-7.2 %, P=0.038). During THc1-5, significant decreases in HR from T0 were observed in all groups, while changes in MAP were observed in the group that received yohimbine intravenously. These findings suggest that blockade of the alpha2-adrenegic receptors in the caudal raphe region might have an important role in sympathetic responses to AIHc.

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

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

Respiratory neuroplasticity: Mechanisms and translational implications of phrenic motor plasticity

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

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.

Ampakine pretreatment enables a single hypoxic episode to produce phrenic motor facilitation with no added benefit of additional episodes

Repeated short episodes of hypoxia produce a sustained increase in phrenic nerve output lasting well beyond acute intermittent hypoxia (AIH) exposure (i.e., phrenic long-term facilitation; pLTF). Pretreatment with ampakines, drugs which allosterically modulate AMPA receptors, enables a single brief episode of hypoxia to produce pLTF, lasting up to 90 min after hypoxia. Here, we tested the hypothesis that ampakine pretreatment would enhance the magnitude of pLTF evoked by repeated bouts of hypoxia. Phrenic nerve output was recorded in urethane-anesthetized, mechanically ventilated, and vagotomized adult male Sprague-Dawley rats. Initial experiments demonstrated that ampakine CX717 (15 mg/kg iv) caused an acute increase in phrenic nerve inspiratory burst amplitude reaching 70 ± 48% baseline (BL) after 2 min (P = 0.01). This increased bursting was not sustained (2 ± 32% BL at 60 min, P = 0.9). When CX717 was delivered 2 min before a single episode of isocapnic hypoxia (5 min, [Formula: see text] = 44 ± 9 mmHg), facilitation of phrenic nerve burst amplitude occurred (96 ± 62% BL at 60 min, P < 0.001). However, when CX717 was given 2 min before three, 5-min hypoxic episodes ([Formula: see text] = 45 ± 6 mmHg) pLTF was attenuated and did not reach statistical significance (24 ± 29% BL, P = 0.08). In the absence of CX717 pretreatment, pLTF was observed after three (74 ± 33% BL at 60 min, P < 0.001) but not one episode of hypoxia (1 ± 8% BL at 60 min, P = 0.9). We conclude that pLTF is not enhanced when ampakine pretreatment is followed by repeated bouts of hypoxia. Rather, the combination of ampakine and a single hypoxic episode appears to be ideal for producing sustained increase in phrenic motor output.NEW & NOTEWORTHY Pretreatment with ampakine CX717 created conditions that enabled an acute bout of moderate hypoxia to evoke phrenic motor facilitation, but this response was not observed when ampakine pretreatment was followed by intermittent hypoxia. Thus, in anesthetized and spinal intact rats, the combination of ampakine and one bout of hypoxia appears ideal for triggering respiratory neuroplasticity.

Adenosine A2a receptors modulate TrkB receptor-dependent respiratory plasticity in neonatal rats

Neuroplasticity is a fundamental property of the respiratory control system, enabling critical adaptations in breathing to meet the challenges, but little is known whether neonates express neuroplasticity similar to adults. We tested the hypothesis that, similar to adults, tyrosine receptor kinase B (TrkB) or adenosine A2a receptor activation in neonates are independently sufficient to elicit respiratory motor facilitation, and that co-induction of TrkB and A2a receptor-dependent plasticity undermines respiratory motor facilitation. TrkB receptor activation with 7,8-dihydroxyflavone (DHF) in neonatal brainstem-spinal cord preparations induced a long-lasting increase in respiratory motor output in 55 % of preparations, whereas adenosine A2a receptor activation with CGS21680 only sporadically induced respiratory motor plasticity. CGS21680 and DHF co-application prevented DHF-dependent respiratory motor facilitation, whereas co-application of MSX-3 (adenosine A2a receptor antagonist) and DHF more rapidly induced respiratory motor plasticity. Collectively, these data suggest that mechanisms underlying respiratory neuroplasticity may be only partially operational in early neonatal life, and that adenosine A2a receptor activation undermines TrkB-induced respiratory plasticity.

Daily acute intermittent hypoxia enhances phrenic motor output and stimulus-evoked phrenic responses in rats

Plasticity is a hallmark of the respiratory neural control system. Phrenic long-term facilitation (pLTF) is one form of respiratory plasticity characterized by persistent increases in phrenic nerve activity following acute intermittent hypoxia (AIH). Although there is evidence that key steps in the cellular pathway giving rise to pLTF are localized within phrenic motor neurons (PMNs), the impact of AIH on the strength of breathing-related synaptic inputs to PMNs remains unclear. Further, the functional impact of AIH is enhanced by repeated/daily exposure to AIH (dAIH). Here, we explored the effects of AIH vs. 2 weeks of dAIH preconditioning on spontaneous and evoked responses recorded in anesthetized, paralyzed (with pancuronium bromide) and mechanically ventilated rats. Evoked phrenic potentials were elicited by respiratory cycle-triggered lateral funiculus stimulation at C2 delivered prior to- and 60 min post-AIH (or an equivalent time in controls). Charge-balanced biphasic pulses (100 µs/phase) of progressively increasing intensity (100 to 700 µA) were delivered during the inspiratory and expiratory phases of the respiratory cycle. Although robust pLTF (~60% from baseline) was observed after a single exposure to moderate AIH (3 x 5 min; 5 min intervals), there was no effect on evoked phrenic responses, contrary to our initial hypothesis. However, in rats preconditioned with dAIH, baseline phrenic nerve activity and evoked responses were increased, suggesting that repeated exposure to AIH enhances functional synaptic strength when assessed using this technique. The impact of daily AIH preconditioning on synaptic inputs to PMNs raises interesting questions that require further exploration.

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

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

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

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

Baseline Arterial CO2 Pressure Regulates Acute Intermittent Hypoxia-Induced Phrenic Long-Term Facilitation in Rats

Moderate acute intermittent hypoxia (mAIH) elicits a progressive increase in phrenic motor output lasting hours post-mAIH, a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). mAIH-induced pLTF is initiated by activation of spinally-projecting raphe serotonergic neurons during hypoxia and subsequent serotonin release near phrenic motor neurons. Since raphe serotonergic neurons are also sensitive to pH and CO₂, the prevailing arterial CO₂ pressure (PaCO₂) may modulate their activity (and serotonin release) during hypoxic episodes. Thus, we hypothesized that changes in background PaCO₂ directly influence the magnitude of mAIH-induced pLTF. mAIH-induced pLTF was evaluated in anesthetized, vagotomized, paralyzed and ventilated rats, with end-tidal CO₂ (i.e., a PaCO₂ surrogate) maintained at: (1) ≤39 mmHg (hypocapnia); (2) ∼41 mmHg (normocapnia); or (3) ≥48 mmHg (hypercapnia) throughout experimental protocols. Although baseline phrenic nerve activity tended to be lower in hypocapnia, short-term hypoxic phrenic response, i.e., burst amplitude (Δ = 5.1 ± 1.1 μV) and frequency responses (Δ = 21 ± 4 bpm), was greater than in normocapnic (Δ = 3.6 ± 0.6 μV and 8 ± 4, respectively) or hypercapnic rats (Δ = 2.0 ± 0.6 μV and −2 ± 2, respectively), followed by a progressive increase in phrenic burst amplitude (i.e., pLTF) for at least 60 min post mAIH. pLTF in the hypocapnic group (Δ = 4.9 ± 0.6 μV) was significantly greater than in normocapnic (Δ = 2.8 ± 0.7 μV) or hypercapnic rats (Δ = 1.7 ± 0.4 μV). In contrast, although hypercapnic rats also exhibited significant pLTF, it was attenuated versus hypocapnic rats. When pLTF was expressed as percent change from maximal chemoreflex stimulation, all pairwise comparisons were found to be statistically significant (p < 0.05). We conclude that elevated PaCO₂ undermines mAIH-induced pLTF in anesthetized rats. These findings contrast with well-documented effects of PaCO₂ on ventilatory LTF in awake humans.

Synergy between Acute Intermittent Hypoxia and Task-Specific Training

Acute intermittent hypoxia (AIH) and task-specific training (TST) synergistically improve motor function after spinal cord injury; however, mechanisms underlying this synergistic relation 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.

Effects of state anxiety on gait: a 7.5% carbon dioxide challenge study

We used the 7.5% carbon dioxide (CO₂) model of anxiety induction to investigate the effects of state anxiety on normal gait and gait when navigating an obstacle. Healthy volunteers (n = 22) completed a walking task during inhalations of 7.5% CO₂ and medical air (placebo) in a within-subjects design. The order of inhalation was counterbalanced across participants and the gas was administered double-blind. Over a series of trials, participants walked the length of the laboratory, with each trial requiring participants to navigate through an aperture (width adjusted to participant size), with gait parameters measured via a motion capture system. The main findings were that walking speed was slower, but the adjustment in body orientation was greater, during 7.5% CO₂ inhalation compared to air. These findings indicate changes in locomotor behaviour during heightened state anxiety that may reflect greater caution when moving in an agitated state. Advances in sensing technology offer the opportunity to monitor locomotor behaviour, and these findings suggest that in doing so, we may be able to infer emotional states from movement in naturalistic settings.

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

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

Ampakine pretreatment enables a single brief hypoxic episode to evoke phrenic motor facilitation

Phrenic long-term facilitation (LTF) is a sustained increase in phrenic motor output occurring after exposure to multiple (but not single) hypoxic episodes. Ampakines are a class of drugs that enhance AMPA receptor function. Ampakines can enhance expression of neuroplasticity, and the phrenic motor system is fundamentally dependent on excitatory glutamatergic currents. Accordingly, we tested the hypothesis that combining ampakine pretreatment with a single brief hypoxic exposure would result in phrenic motor facilitation lasting well beyond the period of hypoxia. Phrenic nerve output was recorded in urethane-anesthetized, ventilated, and vagotomized adult Sprague-Dawley rats. Ampakine CX717 (15 mg/kg iv; n = 8) produced a small increase in phrenic inspiratory burst amplitude and frequency, but values quickly returned to predrug baseline. When CX717 was followed 2 min later by a 5-min exposure to hypoxia (n = 8; PaO2 ~45 mmHg), a persistent increase in phrenic inspiratory burst amplitude (i.e., phrenic motor facilitation) was observed up to 60 min posthypoxia (103 ± 53% increase from baseline). In contrast, when hypoxia was preceded by vehicle injection (10% 2-hydroxypropyl-β-cyclodextrin; n = 8), inspiratory phrenic bursting was similar to baseline values at 60 min. Additional experiments with another ampakine (CX1739, 15 mg/kg) produced comparable results. We conclude that pairing low-dose ampakine treatment with a single brief hypoxic exposure can evoke sustained phrenic motor facilitation. This targeted approach for enhancing respiratory neuroplasticity may have value in the context of hypoxia-based neurorehabilitation strategies.NEW & NOTEWORTHY A single brief episode of hypoxia (e.g., 3-5 min) does not evoke long-lasting increases in respiratory motor output after the hypoxia is concluded. Ampakines are a class of drugs that enhance AMPA receptor function. We show that pairing low-dose ampakine treatment with a single brief hypoxic exposure can evoke sustained phrenic motor facilitation after the acute hypoxic episode.

Effect of chronic stress on cardiovascular and ventilatory responses activated by both chemoreflex and baroreflex in rats

Chronic stress results in physiological and somatic changes. It has been recognized as a risk factor for several types of cardiovascular dysfunction and changes in autonomic mechanisms, such as baroreflex and chemoreflex activity. However, the effects of different types of chronic stress on these mechanisms are still poorly understood. Therefore, in the present study, we investigated, in adult male rats, the effect of repeated restraint stress (RRS) or chronic variable stress (CVS) on baroreflex, chemoreflex and heart rate variability in a protocol of 14 days of stress sessions. Exposure to RRS and CVS indicated no changes in the basal level of either arterial pressure or heart rate. However, RRS and CVS were able to attenuate sympathovagal modulation and spontaneous baroreflex gain. Additionally, only RRS was able to increase the power of the low-frequency band of the systolic blood pressure spectrum, as well as the slope of linear regression of baroreflex bradycardic and tachycardic responses induced by vasoactive compounds. Additionally, our study is one of the first to show that exposure to RRS and CVS decreases the magnitude of the pressor response and potentiates respiratory responses to chemoreflex activation, which can trigger cardiovascular and respiratory pathologies. Furthermore, the basal respiratory parameters, such as minute ventilation and tidal volume, were significantly decreased by both protocols of chronic stress. However, only CVS increased the basal respiratory frequency. In this way, the findings of the present study demonstrate the impact of chronic stress in terms of not only depressive-like behavior but also alterations of the autonomic baroreflex responses and cardiocirculatory variability (systolic blood pressure and pulse interval).Our results provide evidence that chronic stress promotes autonomic dysregulation, and impairment of baroreflex, chemoreflex and heart rate variability.

Carotid Bodies and the Integrated Cardiorespiratory Response to Hypoxia

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

Circulatory control of phrenic motor plasticity

Acute intermittent hypoxia (AIH) elicits distinct mechanisms of phrenic motor plasticity initiated by brainstem neural network activation versus local (spinal) tissue hypoxia. With moderate AIH (mAIH), hypoxemia activates the carotid body chemoreceptors and (subsequently) brainstem neural networks associated with the peripheral chemoreflex, including medullary raphe serotonergic neurons. Serotonin release and receptor activation in the phrenic motor nucleus then elicits phrenic long-term facilitation (pLTF). This mechanism is independent of tissue hypoxia, since electrical carotid sinus nerve stimulation elicits similar serotonin-dependent pLTF. In striking contrast, severe AIH (sAIH) evokes a spinal adenosine-dependent, serotonin-independent mechanism of pLTF. Spinal tissue hypoxia per se is the likely cause of sAIH-induced pLTF, since local tissue hypoxia elicits extracellular adenosine accumulation. Thus, any physiological condition exacerbating spinal tissue hypoxia is expected to shift the balance towards adenosinergic pLTF. However, since these mechanisms compete for dominance due to mutual cross-talk inhibition, the transition from serotonin to adenosine dominant pLTF is rather abrupt. Any factor that compromises spinal cord circulation will limit oxygen availability in spinal cord tissue, favoring a shift in the balance towards adenosinergic mechanisms. Such shifts may arise experimentally from treatments such as carotid denervation, or spontaneous hypotension or anemia. Many neurological disorders, such as spinal cord injury or stroke compromise local circulatory control, potentially modulating tissue oxygen, adenosine levels and, thus, phrenic motor plasticity. In this brief review, we discuss the concept that local (spinal) circulatory control and/or oxygen delivery regulates the relative contributions of distinct pathways to phrenic motor plasticity.

Brainstem Nuclei Associated with Mediating Apnea-Induced Respiratory Motor Plasticity

The respiratory control system is plastic. It has a working memory and is capable of retaining how respiratory stimuli affect breathing by regulating synaptic strength between respiratory neurons. For example, repeated airway obstructions trigger a form of respiratory plasticity that strengthens inspiratory activity of hypoglossal (XII) motoneurons. This form of respiratory plasticity is known as long-term facilitation (LTF) and requires noradrenaline released onto XII motoneurons. However, the brainstem regions responsible for this form of LTF remain unidentified. Here, we used electrophysiology, neuropharmacology and immunohistochemistry in adult rats to identify the brainstem regions involved in mediating LTF. First, we show that repeated airway obstructions induce LTF of XII motoneuron activity and that inactivation of the noradrenergic system prevents LTF. Second, we show that noradrenergic cells in the locus coeruleus (LC), which project to XII motoneurons, are recruited during LTF induction. Third, we show that targeted inactivation of noradrenergic LC cells during LTF induction prevents LTF. And lastly, we show that the nucleus tractus solitarius (NTS), which has known projections to the LC, is critical for LTF because its inactivation prevents LTF. Our results suggest that both the LC and NTS are involved in mediating apnea-induced LTF, and we hypothesize that a NTS → LC → XII circuit mechanism mediates this form of respiratory motor plasticity.

Integration of Transplanted Neural Precursors with the Injured Cervical Spinal Cord

Cervical spinal cord injuries (SCI) result in devastating functional consequences, including respiratory dysfunction. This is largely attributed to the disruption of phrenic pathways, which control the diaphragm. Recent work has identified spinal interneurons as possible contributors to respiratory neuroplasticity. The present work investigated whether transplantation of developing spinal cord tissue, inherently rich in interneuronal progenitors, could provide a population of new neurons and growth-permissive substrate to facilitate plasticity and formation of novel relay circuits to restore input to the partially denervated phrenic motor circuit. One week after a lateralized, C3/4 contusion injury, adult Sprague-Dawley rats received allografts of dissociated, developing spinal cord tissue (from rats at gestational days 13-14). Neuroanatomical tracing and terminal electrophysiology was performed on the graft recipients 1 month later. Experiments using pseudorabies virus (a retrograde, transynaptic tracer) revealed connections from donor neurons onto host phrenic circuitry and from host, cervical interneurons onto donor neurons. Anatomical characterization of donor neurons revealed phenotypic heterogeneity, though donor-host connectivity appeared selective. Despite the consistent presence of cholinergic interneurons within donor tissue, transneuronal tracing revealed minimal connectivity with host phrenic circuitry. Phrenic nerve recordings revealed changes in burst amplitude after application of a glutamatergic, but not serotonergic antagonist to the transplant, suggesting a degree of functional connectivity between donor neurons and host phrenic circuitry that is regulated by glutamatergic input. Importantly, however, anatomical and functional results were variable across animals, and future studies will explore ways to refine donor cell populations and entrain consistent connectivity.

Phrenic long-term depression evoked by intermittent hypercapnia is modulated by serotonergic and adrenergic receptors in raphe nuclei

Intermittent hypercapnia evokes prolonged depression of phrenic nerve activity (phrenic long-term depression, pLTD). This study was undertaken to investigate the role of 5-HT and α2-adrenergic receptors in the initiation of pLTD. Adult male urethane-anesthetized, vagotomized, paralyzed, and mechanically ventilated Sprague-Dawley rats were exposed to a protocol of acute intermittent hypercapnia (AIHc; 5 episodes of 15% CO2in air, each episode lasting 3 min). The experimental group received microinjection of the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT), the broad-spectrum 5-HT antagonist methysergide, or the α2-adrenergic antagonist yohimbine, whereas the control group received microinjection of 0.9% saline into the caudal raphe region. Peak phrenic nerve activity (pPNA) and burst frequency ( f) were analyzed during baseline (T0), during 5 hypercapnic episodes (THc1–THc5), and at 15, 30, and 60 min after the end of the last hypercapnic episode. In the control group, pPNA decreased 60 min after the end of the last hypercapnic episode compared with baseline values, i.e., pLTD developed ( P = 0.023). In the 8-OH-DPAT group, pPNA significantly decreased at T15, T30, and T60 compared with baseline values, i.e., pLTD developed ( P = 0.01). In the methysergide and yohimbine groups, AIHc did not evoke significant changes of the pPNA at T15, T30, and T60 compared with baseline values. In conclusion, activation of 5-HT1Areceptors accentuated induction of pLTD, whereas blockade of α2-adrenergic receptors prevented development of pLTD following AIHc in anesthetized rats. These results suggest that chemical modulation of 5-HT and α2-adrenergic receptors in raphe nuclei affects hypercapnia-induced pLTD, offering important insights in understanding the mechanisms involved in development of respiratory plasticity.

NEW & NOTEWORTHY Hypercapnia is a concomitant feature of many breathing disorders, including obstructive sleep apnea. In this study, acute intermittent hypercapnia evoked development of phrenic long-term depression (pLTD) 60 min after the last hypercapnic episode that was preserved if the selective 5-HT1Areceptor agonist 8-hydroxy-2-(dipropylamino)tetralin hydrobromide was microinjected in the caudal raphe region before the hypercapnic stimulus. This study highlights that both 5-HT and adrenergic receptor activation is needed for induction of pLTD in urethane-anesthetized rats following intermittent hypercapnia exposure.

The effect of caffeine on the ventilatory response to hypercarbia in preterm infants

BackgroundWe tested the hypotheses that caffeine therapy would increase the ventilatory response to hypercarbia in infants above the effect of maturation and those with a weaker ventilatory response to hypercarbia would be more likely to subsequently develop apnea that required treatment.MethodsInfants born at less than 34 weeks of gestation underwent a steady-state hypercarbic challenge using 0, 2, and 4% carbon dioxide soon after birth that was repeated at weekly intervals. The results of the initial study were compared between infants who did or did not subsequently develop apnea requiring treatment with caffeine.ResultsTwenty-six infants born at a median gestation of 32 (range 31-33) weeks were assessed. Caffeine administration was associated with an increase in CO₂ sensitivity, and the mean increase was 15.3 (95% CI: 1-30) ml/kg/min/% CO₂. Fourteen infants subsequently developed apnea treated with caffeine. After controlling for gestational age and birth weight, they had significantly lower carbon dioxide sensitivity at their initial study compared with those who did not require treatment.ConclusionCaffeine administration was associated with an increase in the ventilatory response to hypercarbia. An initial weaker ventilatory response to hypercarbia was associated with the subsequent development of apnea requiring treatment with caffeine.

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

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

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

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

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

Intermittent Hypoxia Enhances Functional Connectivity of Midcervical Spinal Interneurons

Brief, intermittent oxygen reductions [acute intermittent hypoxia (AIH)] evokes spinal plasticity. Models of AIH-induced neuroplasticity have focused on motoneurons; however, most midcervical interneurons (C-INs) also respond to hypoxia. We hypothesized that AIH would alter the functional connectivity between C-INs and induce persistent changes in discharge. Bilateral phrenic nerve activity was recorded in anesthetized and ventilated adult male rats and a multielectrode array was used to record C4/5 spinal discharge before [baseline (BL)], during, and 15 min after three 5 min hypoxic episodes (11% O₂, H1-H3). Most C-INs (94%) responded to hypoxia by either increasing or decreasing firing rate. Functional connectivity was examined by cross-correlating C-IN discharge. Correlograms with a peak or trough were taken as evidence for excitatory or inhibitory connectivity between C-IN pairs. A subset of C-IN pairs had increased excitatory cross-correlations during hypoxic episodes (34%) compared with BL (19%; p < 0.0001). Another subset had a similar response following each episode (40%) compared with BL (19%; p < 0.0001). In the latter group, connectivity remained elevated 15 min post-AIH (30%; p = 0.0002). Inhibitory C-IN connectivity increased during H1-H3 (4.5%; p = 0.0160), but was reduced 15 min post-AIH (0.5%; p = 0.0439). Spike-triggered averaging indicated that a subset of C-INs is synaptically coupled to phrenic motoneurons and excitatory inputs to these "pre-phrenic" cells increased during AIH. We conclude that AIH alters connectivity of the midcervical spinal network. To our knowledge, this is the first demonstration that AIH induces plasticity within the propriospinal network.SIGNIFICANCE STATEMENT Acute intermittent hypoxia (AIH) can trigger spinal plasticity associated with sustained increases in respiratory, somatic, and/or autonomic motor output. The impact of AIH on cervical spinal interneuron (C-IN) discharge and connectivity is unknown. Our results demonstrate that AIH recruits excitatory C-INs into the spinal respiratory (phrenic) network. AIH also enhances excitatory and reduces inhibitory connections among the C-IN network. We conclude that C-INs are part of the respiratory, somatic, and/or autonomic response to AIH, and that propriospinal plasticity may contribute to sustained increases in motor output after AIH.

Impact of glutamatergic and serotonergic neurotransmission on diaphragm muscle activity after cervical spinal hemisection

Incomplete cervical spinal cord hemisection at C2 (SH) disrupts descending excitatory drive to phrenic motoneurons, paralyzing the ipsilateral diaphragm muscle. Spontaneous recovery over time is associated with increased phrenic motoneuron expression of glutamatergic N-methyl-d-aspartate (NMDA) and serotonergic 5-HT2A receptors. We hypothesized that NMDA and 5-HT2A receptor-mediated neurotransmission play a role in ipsilateral diaphragm muscle activity post-SH. Adult male Sprague-Dawley rats were implanted with bilateral diaphragm EMG electrodes for chronic EMG recordings up to 28 days post-SH (SH 28D). The extent of recovery was calculated by peak root-mean-square (RMS) EMG amplitude. In all animals, absence of ipsilateral activity was verified at 3 days post-SH. Diaphragm EMG activity was also recorded during exposure to hypoxia-hypercapnia (10% O₂-5% CO₂). In SH animals displaying recovery of ipsilateral diaphragm EMG activity at SH 28D, cervical spinal cord segments containing the phrenic motor nucleus (C3-C5) were surgically exposed and either the NMDA receptor antagonist d-2-amino-5-phosphonovalerate (d-AP5; 100 mM, 30 μl) or 5-HT2A receptor antagonist ketanserin (40 mM, 30 μl) was instilled intrathecally. Following d-AP5, diaphragm EMG amplitude was reduced ipsilaterally, during both eupnea (42% of pre-d-AP5 value; P = 0.007) and hypoxia-hypercapnia (31% of pre-d-AP5 value; P = 0.015), with no effect on contralateral EMG activity or in uninjured controls. Treatment with ketanserin did not change ipsilateral or contralateral RMS EMG amplitude in SH animals displaying recovery at SH 28D. Our results suggest that spinal glutamatergic NMDA receptor-mediated neurotransmission plays an important role in ipsilateral diaphragm muscle activity after cervical spinal cord injury.NEW & NOTEWORTHY Spontaneous recovery following C2 spinal hemisection (SH) is associated with increased phrenic motoneuron expression of glutamatergic and serotonergic receptors. In this study, we show that pharmacological inhibition of glutamatergic N-methyl-d-aspartate (NMDA) receptors blunts ipsilateral diaphragm activity post-SH. In contrast, pharmacological inhibition of serotonergic 5-HT2A receptors does not change diaphragm EMG activity post-SH. Our results suggest that NMDA receptor-mediated glutamatergic neurotransmission plays an important role in enhancing rhythmic respiratory-related diaphragm activity after spinal cord injury.

Prolonged Exposures to Intermittent Hypoxia Promote Visceral White Adipose Tissue Inflammation in a Murine Model of Severe Sleep Apnea: Effect of Normoxic Recovery

Study Objective

Increased visceral white adipose tissue (vWAT) mass results in infiltration of inflammatory macrophages that drive inflammation and insulin resistance. Patients with obstructive sleep apnea (OSA) suffer from increased prevalence of obesity, insulin resistance, and metabolic syndrome. Murine models of intermittent hypoxia (IH) mimicking moderate-severe OSA manifest insulin resistance following short-term IH. We examined in mice the effect of long-term IH on the inflammatory cellular changes within vWAT and the potential effect of normoxic recovery (IH-R).

Methods

Male C57BL/6J mice were subjected to IH for 20 weeks, and a subset was allowed to recover in room air (RA) for 6 or 12 weeks (IH-R). Stromal vascular fraction was isolated from epididymal vWAT and mesenteric vWAT depots, and single-cell suspensions were prepared for flow cytometry analyses, reactive oxygen species (ROS), and metabolic assays.

Results

IH reduced body weight and vWAT mass and IH-R resulted in catch-up weight and vWAT mass. IH-exposed vWAT exhibited increased macrophage counts (ATMs) that were only partially improved in IH-R. IH also caused a proinflammatory shift in ATMs (increased Ly6c(hi)(+) and CD36(+) ATMs). These changes were accompanied by increased vWAT insulin resistance with only partial improvements in IH-R. In addition, ATMs exhibited increased ROS production, altered metabolism, and changes in electron transport chain, which were only partially improved in IH-R.

Conclusion

Prolonged exposures to IH during the sleep period induce pronounced vWAT inflammation and insulin resistance despite concomitant vWAT mass reductions. These changes are only partially reversible after 3 months of normoxic recovery. Thus, long-lasting OSA may preclude complete reversibility of metabolic changes.

Effect of acute intermittent hypoxia on motor function in individuals with chronic spinal cord injury following ibuprofen pretreatment: A pilot study

INTRODUCTION

Acute intermittent hypoxia (AIH) enhances lower extremity motor function in humans with chronic incomplete spinal cord injury (SCI). AIH-induced spinal plasticity is inhibited by systemic inflammation in animal models. Since SCI is frequently associated with systemic inflammation in humans, we tested the hypothesis that pretreatment with the anti-inflammatory agent ibuprofen enhances the effects of AIH.

METHODS

A randomized, double-blinded, placebo-controlled crossover design was used. Nine adults (mean age 51.1 ± 13.1 years) with chronic motor-incomplete SCI (7.7 ± 6.3 years post-injury) received a single dose of ibuprofen (800 mg) or placebo, 90 minutes prior to AIH. For AIH, 9% O₂ for 90 seconds was interspersed with 21% O₂ for 60 seconds. Maximal voluntary ankle plantar flexion isometric torque was assessed prior to, and at 0, 30, and 60 minutes post-AIH. Surface electromyography (EMG) of plantar flexor muscles was also recorded.

RESULTS

Torque increased significantly after AIH at 30 (P = 0.007; by ∼20%) and 60 (P < 0.001; by ∼30%) minutes post-AIH versus baseline. Ibuprofen did not augment the effects of AIH. EMG activity did not increase significantly after AIH; however, there was a significant association between increases in torque and EMG in both gastrocnemius (R² = 0.17, P < 0.005) and soleus (R² = 0.17, P < 0.005) muscles.

CONCLUSIONS

AIH systematically increased lower extremity torque in individuals with chronic incomplete SCI, but there was no significant effect of ibuprofen pretreatment. Our study re-confirms the ability of AIH to enhance leg strength in persons with chronic incomplete SCI.

Time Domains of the Hypoxic Ventilatory Response and Their Molecular Basis

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

Raphe gene expression changes implicate immune-related functions in ventilatory plasticity following carotid body denervation in rats

The regulation of blood gases in mammals requires precise feedback mechanisms including chemoreceptor feedback from the carotid bodies. Carotid body denervation (CBD) leads to immediate hypoventilation (increased PaCO₂) in adult rats, but over a period of days and weeks ventilation normalizes due in part to central (brain) mechanisms. Here, we tested the hypothesis that functional ventilatory recovery following CBD correlated with significant shifts in medullary raphe gene expression of molecules/pathways associated with known or novel forms of neuroplasticity. Tissue punches were obtained from snap frozen brainstems collected from rats 1-2days or 14-15days post-sham or post-bilateral CBD surgery (verified by physiologic measurements), and subjected to mRNA sequencing to identify, quantify, and statistically compare gene expression level differences among these groups of rats. We found the greatest number of gene expression changes acutely after CBD (154 genes), with fewer changes in the weeks after CBD (69-80 genes) and the fewest changes in expression among the time control groups (39 genes). Little or no changes were observed for multiple genes associated with serotonin- or glutamate receptor-dependent forms of neuroplasticity. However, an unbiased assessment of gene expression changes using a bioinformatics pathway analysis highlighted multiple changes in gene expression in signaling pathways associated with immune function. These included several growth factors and cytokines associated with peripheral and innate immune systems. Thus, these medullary raphe gene expression data support a role for immune-related signaling pathways in the functional restoration of blood gas control after CBD, but little or no role for serotonin- or glutamate receptor-mediated plasticity.

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

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

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.

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.

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.

Function and modulation of premotor brainstem parasympathetic cardiac neurons that control heart rate by hypoxia-, sleep-, and sleep-related diseases including obstructive sleep apnea

Parasympathetic cardiac vagal neurons (CVNs) in the brainstem dominate the control of heart rate. Previous work has determined that these neurons are inherently silent, and their activity is largely determined by synaptic inputs to CVNs that include four major types of synapses that release glutamate, GABA, glycine, or serotonin. Whereas prior reviews have focused on glutamatergic, GABAergic and glycinergic pathways, and the receptors in CVNs activated by these neurotransmitters, this review focuses on the alterations in CVN activity with hypoxia-, sleep-, and sleep-related cardiovascular diseases including obstructive sleep apnea.

Effects of chronic sleep fragmentation on wake-active neurons and the hypercapnic arousal response

STUDY OBJECTIVES

Delayed hypercapnic arousals may occur in obstructive sleep apnea. The impaired arousal response is expected to promote more pronounced oxyhemoglobin desaturations. We hypothesized that long-term sleep fragmentation (SF) results in injury to or dysfunction of wake-active neurons that manifests, in part, as a delayed hypercapnic arousal response.

DESIGN

Adult male mice were implanted for behavioral state recordings and randomly assigned to 4 weeks of either orbital platform SF (SF4wk, 30 events/h) or control conditions (Ct4wk) prior to behavioral, histological, and locus coeruleus (LC) whole cell electrophysiological evaluations.

MEASUREMENTS AND RESULTS

SF was successfully achieved across the 4 week study, as evidenced by a persistently increased arousal index, P < 0.01 and shortened sleep bouts, P < 0.05, while total sleep/wake times and plasma corticosterone levels were unaffected. A multiple sleep latency test performed at the onset of the dark period showed a reduced latency to sleep in SF4wk mice (P < 0.05). The hypercapnic arousal latency was increased, Ct4wk 64 ± 5 sec vs. SF4wk 154 ± 6 sec, P < 0.001, and remained elevated after a 2 week recovery (101 ± 4 sec, P < 0.001). C-fos activation in noradrenergic, orexinergic, histaminergic, and cholinergic wake-active neurons was reduced in response to hypercapnia (P < 0.05-0.001). Catecholaminergic and orexinergic projections into the cingulate cortex were also reduced in SF4wk (P < 0.01). In addition, SF4wk resulted in impaired LC neuron excitability (P < 0.01).

CONCLUSIONS

Four weeks of sleep fragmentation (SF4wk) impairs arousal responses to hypercapnia, reduces wake neuron projections and locus coeruleus neuronal excitability, supporting the concepts that some effects of sleep fragmentation may contribute to impaired arousal responses in sleep apnea, which may not reverse immediately with therapy.

Neuronal control of breathing: sex and stress hormones

There is a growing public awareness that hormones can have a significant impact on most biological systems, including the control of breathing. This review will focus on the actions of two broad classes of hormones on the neuronal control of breathing: sex hormones and stress hormones. The majority of these hormones are steroids; a striking feature is that both groups are derived from cholesterol. Stress hormones also include many peptides which are produced primarily within the paraventricular nucleus of the hypothalamus (PVN) and secreted into the brain or into the circulatory system. In this article we will first review and discuss the role of sex hormones in respiratory control throughout life, emphasizing how natural fluctuations in hormones are reflected in ventilatory metrics and how disruption of their endogenous cycle can predispose to respiratory disease. These effects may be mediated directly by sex hormone receptors or indirectly by neurotransmitter systems. Next, we will discuss the origins of hypothalamic stress hormones and their relationship with the respiratory control system. This relationship is 2-fold: (i) via direct anatomical connections to brainstem respiratory control centers, and (ii) via steroid hormones released from the adrenal gland in response to signals from the pituitary gland. Finally, the impact of stress on the development of neural circuits involved in breathing is evaluated in animal models, and the consequences of early stress on respiratory health and disease is discussed.

Vulnerability of neonatal respiratory neural control to sustained hypoxia during a uniquely sensitive window of development

The first postnatal weeks represent a period of development in the rat during which the respiratory neural control system may be vulnerable to aberrant environmental stressors. In the present study, we investigated whether sustained hypoxia (SH; 11% O2) exposure starting at different postnatal ages differentially modifies the acute hypoxic (HVR) and hypercapnic ventilatory response (HCVR). Three different groups of rat pups were exposed to 5 days of SH, starting at either postnatal age 1 (SH1-5), 11 (SH11-15), or 21 (SH21-25) days. Whole body plethysmography was used to assess the HVR and HCVR the day after SH exposure ended. The primary results indicated that 1) the HVR and HCVR of SH11-15 rats were absent or attenuated (respectively) compared with age-matched rats raised in normoxia; 2) there was a profoundly high (∼84% of pups) incidence of unexplained mortality in the SH11-15 rats; and 3) these phenomena were unique to the SH11-15 group with no comparable effect of the SH exposure on the HVR, HCVR, or mortality in the younger (SH1-5) or older (SH21-25) rats. These results share several commonalities with the risk factors thought to underlie the etiology of sudden infant death syndrome, including 1) a vulnerable neonate; 2) a critical period of development; and 3) an environmental stressor.

Pontine mechanisms of respiratory control

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

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.

Spinal 5-HT7 receptors and protein kinase A constrain intermittent hypoxia-induced phrenic long-term facilitation

Phrenic long-term facilitation (pLTF) is a form of serotonin-dependent respiratory plasticity induced by acute intermittent hypoxia (AIH). pLTF requires spinal Gq protein-coupled serotonin-2 receptor (5-HT2) activation, new synthesis of brain-derived neurotrophic factor (BDNF) and activation of its high-affinity receptor, TrkB. Intrathecal injections of selective agonists for Gs protein-coupled receptors (adenosine 2A and serotonin-7; 5-HT7) also induce long-lasting phrenic motor facilitation via TrkB "trans-activation." Since serotonin released near phrenic motor neurons may activate multiple serotonin receptor subtypes, we tested the hypothesis that 5-HT7 receptor activation contributes to AIH-induced pLTF. A selective 5-HT7 receptor antagonist (SB-269970, 5mM, 12 μl) was administered intrathecally at C4 to anesthetized, vagotomized and ventilated rats prior to AIH (3, 5-min episodes, 11% O2). Contrary to predictions, pLTF was greater in SB-269970 treated versus control rats (80 ± 11% versus 45 ± 6% 60 min post-AIH; p<0.05). Hypoglossal LTF was unaffected by spinal 5-HT7 receptor inhibition, suggesting that drug effects were localized to the spinal cord. Since 5-HT7 receptors are coupled to protein kinase A (PKA), we tested the hypothesis that PKA inhibits AIH-induced pLTF. Similar to 5-HT7 receptor inhibition, spinal PKA inhibition (KT-5720, 100 μM, 15 μl) enhanced pLTF (99 ± 15% 60 min post-AIH; p<0.05). Conversely, PKA activation (8-br-cAMP, 100 μM, 15 μl) blunted pLTF versus control rats (16 ± 5% versus 45 ± 6% 60 min post-AIH; p<0.05). These findings suggest a novel mechanism whereby spinal Gs protein-coupled 5-HT7 receptors constrain AIH-induced pLTF via PKA activity.