Systemic inflammation inhibits serotonin receptor 2-induced phrenic motor facilitation upstream from BDNF/TrkB signaling

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

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

Disinhibition does not play a role in endomorphin-2-induced changes in inspiratory motoneuron output produced by in vitro neonatal rat preparations

Low level activation of mu-opioid receptors (MORs) in neonatal rat brainstem-spinal cord preparations increases inspiratory burst amplitude recorded on cervical spinal roots. We tested whether: (1) MOR activation with an endogenous ligand, such as endomorphin-2, increases inspiratory burst amplitude, (2) disinhibition of GABAergic or glycinergic inhibitory synaptic transmission is involved, and (3) inflammation alters endomorphin-2 effects. Using neonatal rat (P0-P3) brainstem-spinal cord preparations, bath-applied endomorphin-2 (10-200 nM) increased inspiratory burst amplitude and decreased burst frequency. Blockade of GABAA receptors (picrotoxin), glycine receptors (strychnine), or both (picrotoxin and strychnine) did not abolish endomorphin-2-induced effects. In preparations isolated from neonatal rats injected 3 h previously with lipopolysaccharide (LPS, 0.1 mg/kg), endomorphin-2 continued to decrease burst frequency but abolished the burst amplitude increase. Collectively, these data indicate that disinhibition of inhibitory synaptic transmission is unlikely to play a role in endomorphin-2-induced changes in inspiratory motor output, and that different mechanisms underlie the endomorphin-2-induced increases in inspiratory burst amplitude and decreases in burst frequency.

Targeting the Microbiome to Improve Gut Health and Breathing Function After Spinal Cord Injury

Spinal cord injury (SCI) is a devastating condition characterized by impaired motor and sensory function, as well as internal organ pathology and dysfunction. This internal organ dysfunction, particularly gastrointestinal (GI) complications, and neurogenic bowel, can reduce the quality of life of individuals with an SCI and potentially hinder their recovery. The gut microbiome impacts various central nervous system functions and has been linked to a number of health and disease states. An imbalance of the gut microbiome, i.e., gut dysbiosis, contributes to neurological disease and may influence recovery and repair processes after SCI. Here we examine the impact of high cervical SCI on the gut microbiome and find that transient gut dysbiosis with persistent gut pathology develops after SCI. Importantly, probiotic treatment improves gut health and respiratory motor function measured through whole-body plethysmography. Concurrent with these improvements was a systemic decrease in the cytokine tumor necrosis factor-alpha and an increase in neurite sprouting and regenerative potential of neurons. Collectively, these data reveal the gut microbiome as an important therapeutic target to improve visceral organ health and respiratory motor recovery after SCI.

Research Highlights

Cervical spinal cord injury (SCI) causes transient gut dysbiosis and persistent gastrointestinal (GI) pathology.Treatment with probiotics after SCI leads to a healthier GI tract and improved respiratory motor recovery.Probiotic treatment decreases systemic tumor necrosis factor-alpha and increases the potential for sprouting and regeneration of neurons after SCI.The gut microbiome is a valid target to improve motor function and secondary visceral health after SCI.

Mild inflammation impairs acute intermittent hypoxia-induced phrenic long-term facilitation by a spinal adenosine-dependent mechanism

Inflammation undermines neuroplasticity, including serotonin-dependent phrenic long-term facilitation (pLTF) following moderate acute intermittent hypoxia (mAIH: 3, 5-min episodes, arterial Po2: 40-50 mmHg; 5-min intervals). Mild inflammation elicited by a low dose of the TLR-4 receptor agonist, lipopolysaccharide (LPS; 100 µg/kg, ip), abolishes mAIH-induced pLTF by unknown mechanisms. In the central nervous system, neuroinflammation primes glia, triggering ATP release and extracellular adenosine accumulation. As spinal adenosine 2 A (A2A) receptor activation impairs mAIH-induced pLTF, we hypothesized that spinal adenosine accumulation and A2A receptor activation are necessary in the mechanism whereby LPS impairs pLTF. We report that 24 h after LPS injection in adult male Sprague Dawley rats: 1) adenosine levels increase in ventral spinal segments containing the phrenic motor nucleus (C3-C5; P = 0.010; n = 7/group) and 2) cervical spinal A2A receptor inhibition (MSX-3, 10 µM, 12 µL intrathecal) rescues mAIH-induced pLTF. In LPS vehicle-treated rats (saline, ip), MSX-3 enhanced pLTF versus controls (LPS: 110 ± 16% baseline; controls: 53 ± 6%; P = 0.002; n = 6/group). In LPS-treated rats, pLTF was abolished as expected (4 ± 6% baseline; n = 6), but intrathecal MSX-3 restored pLTF to levels equivalent to MSX-3-treated control rats (120 ± 14% baseline; P < 0.001; n = 6; vs. LPS controls with MSX-3: P = 0.539). Thus, inflammation abolishes mAIH-induced pLTF by a mechanism that requires increased spinal adenosine levels and A2A receptor activation. As repetitive mAIH is emerging as a treatment to improve breathing and nonrespiratory movements in people with spinal cord injury or ALS, A2A inhibition may offset undermining effects of neuroinflammation associated with these neuromuscular disorders.NEW & NOTEWORTHY Mild inflammation undermines motor plasticity elicited by mAIH. In a model of mAIH-induced respiratory motor plasticity (phrenic long-term facilitation; pLTF), we report that inflammation induced by low-dose lipopolysaccharide undermines mAIH-induced pLTF by a mechanism requiring increased cervical spinal adenosine and adenosine 2 A receptor activation. This finding advances the understanding of mechanisms impairing neuroplasticity, potentially undermining the ability to compensate for the onset of lung/neural injury or to harness mAIH as a therapeutic modality.

BDNF-induced phrenic motor facilitation shifts from PKCθ to ERK dependence with mild systemic inflammation

Moderate acute intermittent hypoxia (mAIH) elicits a form of phrenic motor plasticity known as phrenic long-term facilitation (pLTF), which requires spinal 5-HT2 receptor activation, ERK/MAP kinase signaling, and new brain-derived neurotrophic factor (BDNF) synthesis. New BDNF protein activates TrkB receptors that normally signal through PKCθ to elicit pLTF. Phrenic motor plasticity elicited by spinal drug administration (e.g., BDNF) is referred to by a more general term: phrenic motor facilitation (pMF). Although mild systemic inflammation elicited by a low lipopolysaccharide (LPS) dose (100 µg/kg; 24 h prior) undermines mAIH-induced pLTF upstream from BDNF protein synthesis, it augments pMF induced by spinal BDNF administration through unknown mechanisms. Here, we tested the hypothesis that mild inflammation shifts BDNF/TrkB signaling from PKCθ to alternative pathways that enhance pMF. We examined the role of three known signaling pathways associated with TrkB (MEK/ERK MAP kinase, PI3 kinase/Akt, and PKCθ) in BDNF-induced pMF in anesthetized, paralyzed, and ventilated Sprague Dawley rats 24 h post-LPS. Spinal PKCθ inhibitor (TIP) attenuated early BDNF-induced pMF (≤30 min), with minimal effect 60-90 min post-BDNF injection. In contrast, MEK inhibition (U0126) abolished BDNF-induced pMF at 60 and 90 min. PI3K/Akt inhibition (PI-828) had no effect on BDNF-induced pMF at any time. Thus, whereas BDNF-induced pMF is exclusively PKCθ-dependent in normal rats, MEK/ERK is recruited by neuroinflammation to sustain, and even augment downstream plasticity. Because AIH is being developed as a therapeutic modality to restore breathing in people living with multiple neurological disorders, it is important to understand how inflammation, a common comorbidity in many traumatic or degenerative central nervous system disorders, impacts phrenic motor plasticity.NEW & NOTEWORTHY We demonstrate that even mild systemic inflammation shifts signaling mechanisms giving rise to BDNF-induced phrenic motor plasticity. This finding has important experimental, biological, and translational implications, particularly since BDNF-dependent spinal plasticity is being translated to restore breathing and nonrespiratory movements in diverse clinical disorders, such as spinal cord injury (SCI) and amyotrophic lateral sclerosis (ALS).

Fractalkine/CX3CR1-Dependent Modulation of Synaptic and Network Plasticity in Health and Disease

CX3CR1 is a G protein-coupled receptor that is expressed exclusively by microglia within the brain parenchyma. The only known physiological CX3CR1 ligand is the chemokine fractalkine (FKN), which is constitutively expressed in neuronal cell membranes and tonically released by them. Through its key role in microglia-neuron communication, the FKN/CX3CR1 axis regulates microglial state, neuronal survival, synaptic plasticity, and a variety of synaptic functions, as well as neuronal excitability via cytokine release modulation, chemotaxis, and phagocytosis. Thus, the absence of CX3CR1 or any failure in the FKN/CX3CR1 axis has been linked to alterations in different brain functions, including changes in synaptic and network plasticity in structures such as the hippocampus, cortex, brainstem, and spinal cord. Since synaptic plasticity is a basic phenomenon in neural circuit integration and adjustment, here, we will review its modulation by the FKN/CX3CR1 axis in diverse brain circuits and its impact on brain function and adaptation in health and disease.

Gut microbiome and neurosurgery: Implications for treatment

Introduction

The aim of this review is to summarize the current understanding of the gut-brain axis (GBA), its impact on neurosurgery, and its implications for future treatment.

Background

An abundance of research has established the existence of a collection of pathways between the gut microbiome and the central nervous system (CNS), commonly known as the GBA. Complicating this relationship, the gut microbiome bacterial diversity appears to change with age, antibiotic exposure and a number of external and internal factors.

Methods

In this paper, we present the current understanding of the key protective and deleterious roles the gut microbiome plays in the pathogenesis of several common neurosurgical concerns.

Results

Specifically, we examine how spinal cord injury, traumatic brain injury and stroke may cause gut microbial dysbiosis. Furthermore, this link appears to be bidirectional as gut dysbiosis contributes to secondary CNS injury in each of these ailment settings. This toxic cycle may be broken, and the future secondary damage rescued by timely, therapeutic, gut microbiome modification. In addition, a robust gut microbiome appears to improve outcomes in brain tumour treatment. There are several primary routes by which microbiome dysbiosis may be ameliorated, including faecal microbiota transplant, oral probiotics, bacteriophages, genetic modification of gut microbiota and vagus nerve stimulation.

Conclusion

The GBA represents an important component of patient care in the field of neurosurgery. Future research may illuminate ideal methods of therapeutic microbiome modulation in distinct pathogenic settings.

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.

Nonsteroidal anti-inflammatory drug (ketoprofen) delivery differentially impacts phrenic long-term facilitation in rats with motor neuron death induced by intrapleural CTB-SAP injections

Intrapleural injections of cholera toxin B conjugated to saporin (CTB-SAP) selectively eliminates respiratory (e.g., phrenic) motor neurons, and mimics motor neuron death and respiratory deficits observed in rat models of neuromuscular diseases. Additionally, microglial density increases in the phrenic motor nucleus following CTB-SAP. This CTB-SAP rodent model allows us to study the impact of motor neuron death on the output of surviving phrenic motor neurons, and the underlying mechanisms that contribute to enhancing or constraining their output at 7 days (d) or 28d post-CTB-SAP injection. 7d CTB-SAP rats elicit enhanced phrenic long-term facilitation (pLTF) through the Gs-pathway (inflammation-resistant in naïve rats), while pLTF is elicited though the Gq-pathway (inflammation-sensitive in naïve rats) in control and 28d CTB-SAP rats. In 7d and 28d male CTB-SAP rats and controls, we evaluated the effect of cyclooxygenase-1/2 enzymes on pLTF by delivery of the nonsteroidal anti-inflammatory drug, ketoprofen (IP), and we hypothesized that pLTF would be unaffected by ketoprofen in 7d CTB-SAP rats, but pLTF would be enhanced in 28d CTB-SAP rats. In anesthetized, paralyzed and ventilated rats, pLTF was surprisingly attenuated in 7d CTB-SAP rats and enhanced in 28d CTB-SAP rats (both p < 0.05) following ketoprofen delivery. Additionally in CTB-SAP rats: 1) microglia were more amoeboid in the phrenic motor nucleus; and 2) cervical spinal inflammatory-associated factor expression (TNF-α, BDNF, and IL-10) was increased vs. controls in the absence of ketoprofen (p < 0.05). Following ketoprofen delivery, TNF-α and IL-10 expression was decreased back to control levels, while BDNF expression was differentially affected over the course of motor neuron death in CTB-SAP rats. This study furthers our understanding of factors (e.g., cyclooxygenase-1/2-induced inflammation) that contribute to enhancing or constraining pLTF and its implications for breathing following respiratory motor neuron death.

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

KEY POINTS

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

ABSTRACT

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

Protocol-Specific Effects of Intermittent Hypoxia Pre-Conditioning on Phrenic Motor Plasticity in Rats with Chronic Cervical Spinal Cord Injury

"Low-dose" acute intermittent hypoxia (AIH; 3-15 episodes/day) is emerging as a promising therapeutic strategy to improve motor function after incomplete cervical spinal cord injury (cSCI). Conversely, chronic "high-dose" intermittent hypoxia (CIH; > 80-100 episodes/day) elicits multi-system pathology and is a hallmark of sleep apnea, a condition highly prevalent in individuals with cSCI. Whereas daily AIH (dAIH) enhances phrenic motor plasticity in intact rats, it is abolished by CIH. However, there have been no direct comparisons of prolonged dAIH versus CIH on phrenic motor outcomes after chronic cSCI. Thus, phrenic nerve activity and AIH-induced phrenic long-term facilitation (pLTF) were assessed in anesthetized rats. Experimental groups included: 1) intact rats exposed to 28 days of normoxia (Nx28; 21% O2; 8 h/day), and three groups with chronic C2 hemisection (C2Hx) exposed to either: 2) Nx28; 3) dAIH (dAIH28; 10, 5-min episodes of 10.5% O2/day; 5-min intervals); or 4) CIH (IH28-2/2; 2-min episodes; 2-min intervals; 8 h/day). Baseline ipsilateral phrenic nerve activity was reduced in injured versus intact rats but unaffected by dAIH28 or IH28-2/2. There were no group differences in contralateral phrenic activity. pLTF was enhanced bilaterally by dAIH28 versus Nx28 but unaffected by IH28-2/2. Whereas dAIH28 enhanced pLTF after cSCI, it did not improve baseline phrenic output. In contrast, unlike shorter protocols in intact rats, CIH28-2/2 did not abolish pLTF in chronic C2Hx. Mechanisms of differential responses to dAIH versus CIH are not yet known, particularly in the context of cSCI. Further, it remains unclear whether enhanced phrenic motor plasticity can improve breathing after cSCI.

The homeoviscous adaptation to dietary lipids (HADL) model explains controversies over saturated fat, cholesterol, and cardiovascular disease risk

SFAs play the leading role in 1 of the greatest controversies in nutrition science. Relative to PUFAs, SFAs generally increase circulating concentrations of LDL cholesterol, a risk factor for atherosclerotic cardiovascular disease (ASCVD). However, the purpose of regulatory mechanisms that control the diet-induced lipoprotein cholesterol dynamics is rarely discussed in the context of human adaptive biology. We argue that better mechanistic explanations can help resolve lingering controversies, with the potential to redefine aspects of research, clinical practice, dietary advice, public health management, and food policy. In this paper we propose a novel model, the homeoviscous adaptation to dietary lipids (HADL) model, which explains changes in lipoprotein cholesterol as adaptive homeostatic adjustments that serve to maintain cell membrane fluidity and hence optimal cell function. Due to the highly variable intake of fatty acids in humans and other omnivore species, we propose that circulating lipoproteins serve as a buffer to enable the rapid redistribution of cholesterol molecules between specific cells and tissues that is necessary with changes in dietary fatty acid supply. Hence, circulating levels of LDL cholesterol may change for nonpathological reasons. Accordingly, an SFA-induced raise in LDL cholesterol in healthy individuals could represent a normal rather than a pathologic response. These regulatory mechanisms may become disrupted secondarily to pathogenic processes in association with insulin resistance and the presence of other ASCVD risk factors, as supported by evidence showing diverging lipoprotein responses in healthy individuals as opposed to those with metabolic disorders such as insulin resistance and obesity. Corresponding with the model, we suggest alternative contributing factors to the association between elevated LDL cholesterol concentrations and ASCVD, involving dietary factors beyond SFAs, such as an increased endotoxin load from diet-gut microbiome interactions and subsequent chronic low-grade inflammation that interferes with fine-tuned signaling pathways.

Systemic inflammation suppresses spinal respiratory motor plasticity via mechanisms that require serine/threonine protein phosphatase activity

Background

Inflammation undermines multiple forms of neuroplasticity. Although inflammation and its influence on plasticity in multiple neural systems has been extensively studied, its effects on plasticity of neural networks controlling vital life functions, such as breathing, are less understood. In this study, we investigated the signaling mechanisms whereby lipopolysaccharide (LPS)-induced systemic inflammation impairs plasticity within the phrenic motor system—a major spinal respiratory motor pool that drives contractions of the diaphragm muscle. Here, we tested the hypotheses that lipopolysaccharide-induced systemic inflammation (1) blocks phrenic motor plasticity by a mechanism that requires cervical spinal okadaic acid-sensitive serine/threonine protein phosphatase (PP) 1/2A activity and (2) prevents phosphorylation/activation of extracellular signal-regulated kinase 1/2 mitogen activated protein kinase (ERK1/2 MAPK)—a key enzyme necessary for the expression of phrenic motor plasticity.

Methods

To study phrenic motor plasticity, we utilized a well-characterized model for spinal respiratory plasticity called phrenic long-term facilitation (pLTF). pLTF is characterized by a long-lasting, progressive enhancement of inspiratory phrenic nerve motor drive following exposures to moderate acute intermittent hypoxia (mAIH). In anesthetized, vagotomized and mechanically ventilated adult Sprague Dawley rats, we examined the effect of inhibiting cervical spinal serine/threonine PP 1/2A activity on pLTF expression in sham-vehicle and LPS-treated rats. Using immunofluorescence optical density analysis, we compared mAIH-induced phosphorylation/activation of ERK 1/2 MAPK with and without LPS-induced inflammation in identified phrenic motor neurons.

Results

We confirmed that mAIH-induced pLTF is abolished 24 h following low-dose systemic LPS (100 μg/kg, i.p.). Cervical spinal delivery of the PP 1/2A inhibitor, okadaic acid, restored pLTF in LPS-treated rats. LPS also prevented mAIH-induced enhancement in phrenic motor neuron ERK1/2 MAPK phosphorylation. Thus, a likely target for the relevant okadaic acid-sensitive protein phosphatases is ERK1/2 MAPK or its upstream activators.

Conclusions

This study increases our understanding of fundamental mechanisms whereby inflammation disrupts neuroplasticity in a critical population of motor neurons necessary for breathing, and highlights key roles for serine/threonine protein phosphatases and ERK1/2 MAPK kinase in the plasticity of mammalian spinal respiratory motor circuits.

Differential mechanisms are required for phrenic long-term facilitation over the course of motor neuron loss following CTB-SAP intrapleural injections

Selective elimination of respiratory motor neurons using intrapleural injections of cholera toxin B fragment conjugated to saporin (CTB-SAP) mimics motor neuron death and respiratory deficits observed in rat models of neuromuscular diseases. This CTB-SAP model allows us to study the impact of motor neuron death on the output of surviving phrenic motor neurons. After 7(d) days of CTB-SAP, phrenic long-term facilitation (pLTF, a form of respiratory plasticity) is enhanced, but returns towards control levels at 28d. However, the mechanism responsible for this difference in magnitude of pLTF is unknown. In naïve rats, pLTF predominately requires 5-HT2 receptors, the new synthesis of BDNF, and MEK/ERK signaling; however, pLTF can alternatively be induced via A2A receptors, the new synthesis of TrkB, and PI3K/Akt signaling. Since A2A receptor-dependent pLTF is enhanced in naïve rats, we suggest that 7d CTB-SAP treated rats utilize the alternative mechanism for pLTF. Here, we tested the hypothesis that pLTF following CTB-SAP is: 1) TrkB and PI3K/Akt, not BDNF and MEK/ERK, dependent at 7d; and 2) BDNF and MEK/ERK, not TrkB and PI3K/Akt, dependent at 28d. Adult Sprague Dawley male rats were anesthetized, paralyzed, ventilated, and were exposed to acute intermittent hypoxia (AIH; 3, 5 min bouts of 10.5% O2) following bilateral, intrapleural injections at 7d and 28d of: 1) CTB-SAP (25 μg), or 2) un-conjugated CTB and SAP (control). Intrathecal C4 delivery included either: 1) small interfering RNA that targeted BDNF or TrkB mRNA; 2) UO126 (MEK/ERK inhibitor); or 3) PI828 (PI3K/Akt inhibitor). Our data suggest that pLTF in 7d CTB-SAP treated rats is elicited primarily through TrkB and PI3K/Akt-dependent mechanisms, whereas BDNF and MEK/ERK-dependent mechanisms induce pLTF in 28d CTB-SAP treated rats. This project increases our understanding of respiratory plasticity and its implications for breathing following motor neuron death.

Sex- and Region-Specific Differences in the Transcriptomes of Rat Microglia from the Brainstem and Cervical Spinal Cord

The neural control system underlying breathing is sexually dimorphic with males being more vulnerable to dysfunction. Microglia also display sex differences, and their role in the architecture of brainstem respiratory rhythm circuitry and modulation of cervical spinal cord respiratory plasticity is becoming better appreciated. To further understand the molecular underpinnings of these sex differences, we performed RNA sequencing of immunomagnetically isolated microglia from brainstem and cervical spinal cord of adult male and female rats. We used various bioinformatics tools (Gene Ontology, Kyoto Encyclopedia of Genes and Genomes, Reactome, STRING, MAGICTRICKS) to functionally categorize identified gene sets, as well as to pinpoint common transcriptional gene drivers that may be responsible for the observed transcriptomic differences. We found few sex differences in the microglial transcriptomes derived from the brainstem, but several hundred genes were differentially expressed by sex in cervical spinal microglia. Comparing brainstem and spinal microglia within and between sexes, we found that the major factor guiding transcriptomic differences was central nervous system (CNS) location rather than sex. We further identified key transcriptional drivers that may be responsible for the transcriptomic differences observed between sexes and CNS regions; enhancer of zeste homolog 2 emerged as the predominant driver of the differentially downregulated genes. We suggest that functional gene alterations identified in metabolism, transcription, and intercellular communication underlie critical microglial heterogeneity and sex differences in CNS regions that contribute to respiratory disorders categorized by dysfunction in neural control. These data will also serve as an important resource data base to advance our understanding of innate immune cell contributions to sex differences and the field of respiratory neural control. SIGNIFICANCE STATEMENT: The contributions of central nervous system (CNS) innate immune cells to sexually dimorphic differences in the neural circuitry controlling breathing are poorly understood. We identify key transcriptomic differences, and their transcriptional drivers, in microglia derived from the brainstem and the C3-C6 cervical spinal cord of healthy adult male and female rats. Gene alterations identified in metabolism, gene transcription, and intercellular communication likely underlie critical microglial heterogeneity and sex differences in these key CNS regions that contribute to the neural control of breathing.

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

Purpose of Review

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

Recent Findings

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

Summary

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

Use of viruses for interrogating viscera-specific projections in central nervous system

Each internal organ may perform many different functions under central regulation, yet how these processes are coordinated is poorly understood. The last three decades have witnessed a renaissance in tract tracing with genetically engineered strains of viruses that rapidly interrogate viscera-specific projections in the CNS. The application of novel methods to study cell type-specific projections through trans-synaptically transmitted virus 'label' highlights projections exclusively originating from neurons expressing a very specific molecular phenotype. This has opened the door to neuroanatomical studies interrogating organ-specific projections in the CNS at an unprecedented scale. In this contribution to the Special Issue we present an overview of the present state and of future opportunities in charting viscera-brain specific connectivity and in linking brain circuits to internal organ function.

Spinal AMP kinase activity differentially regulates phrenic motor plasticity

Acute intermittent hypoxia (AIH) elicits phrenic motor plasticity via multiple distinct cellular mechanisms. With moderate AIH, phrenic motor facilitation (pMF) requires G_q protein-coupled serotonin type 2 receptor activation, ERK MAP kinase activity, and new synthesis of brain-derived neurotrophic factor. In contrast, severe AIH elicits pMF by an adenosine-dependent mechanism that requires exchange protein activated by cAMP, Akt, and mammalian target of rapamycin (mTOR) activity, followed by new tyrosine receptor kinase B protein synthesis; this same pathway is also initiated by G_s protein-coupled serotonin 7 receptors (5-HT₇). Because the metabolic sensor AMP-activated protein kinase (AMPK) inhibits mTOR-dependent protein synthesis, and mTOR signaling is necessary for 5-HT₇ but not 5-HT₂ receptor-induced pMF, we hypothesized that spinal AMPK activity differentially regulates pMF elicited by these distinct receptor subtypes. Serotonin type 2A receptor [5-HT_2A; (±)-2,5-dimethoxy-4-iodoamphetamine hydrochloride] or 5-HT₇ (AS-19) receptor agonists were administered intrathecally at C₄ (3 injections, 5-min intervals) while recording integrated phrenic nerve activity in anesthetized, vagotomized, paralyzed, and ventilated rats. Consistent with our hypothesis, spinal AMPK activation with 2-deoxyglucose or metformin blocked 5-HT₇, but not 5-HT_2A receptor-induced pMF; in both cases, pMF inhibition was reversed by spinal administration of the AMPK inhibitor compound C. Thus, AMPK differentially regulates cellular mechanisms of serotonin-induced phrenic motor plasticity.NEW & NOTEWORTHY Spinal AMP-activated protein kinase (AMPK) overactivity, induced by local 2-deoxyglucose or metformin administration, constrains serotonin 7 (5-HT₇) receptor-induced (but not serotonin type 2A receptor-induced) respiratory motor facilitation, indicating that metabolic challenges might regulate specific forms of respiratory motor plasticity. Pharmacological blockade of spinal AMPK activity restores 5-HT₇ receptor-induced respiratory motor facilitation in the presence of either 2-deoxyglucose or metformin, showing that AMPK is an important regulator of 5-HT₇ receptor-induced respiratory motor plasticity.

Effects of inflammation on the developing respiratory system: Focus on hypoglossal (XII) neuron morphology, brainstem neurochemistry, and control of breathing

Breathing is fundamental to life and any adverse change in respiratory function can endanger the health of an organism or even be fatal. Perinatal inflammation is known to adversely affect breathing in preterm babies, but lung infection/inflammation impacts all stages of life from birth to death. Little is known about the role of inflammation in respiratory control, neuronal morphology, or neural function during development. Animal models of inflammation can provide understanding and insight into respiratory development and how inflammatory processes alter developmental phenotype in addition to providing insight into new treatment modalities. In this review, we focus on recent work concerning the development of neurons, models of perinatal inflammation with an emphasis on two common LPS-based models, inflammation and its impact on development, and current and potential treatments for inflammation within the respiratory control circuitry of the mammalian brainstem. We have also discussed models of inflammation in adults and have specifically focused on hypoglossal motoneurons (XII) and neurons of the nucleus tractus solitarii (nTS) as these nuclei have been studied more extensively than other brainstem nuclei participating in breathing and airway control. Understanding the impact of inflammation on the developmental aspects of respiratory control and breathing pattern is critical to addressing problems of cardiorespiratory dysregulation in disease and this overview points out many gaps in our current knowledge.

Prednisolone Pretreatment Enhances Intermittent Hypoxia-Induced Plasticity in Persons With Chronic Incomplete Spinal Cord Injury

Objective. To test the hypothesis that an anti-inflammatory corticosteroid drug enhances spinal motor plasticity induced by acute intermittent hypoxia (AIH) in persons with chronic incomplete spinal cord injury (iSCI). Methods. Fourteen subjects with incomplete spinal cord injury (ASIA level C or D; mean age = 46 years) participated in a randomized, double-blinded, crossover, and placebo-controlled study. Subjects received either 60 mg oral prednisolone or a matching placebo, 1 hour before administration of AIH (15, 60-second hypoxic exposures; fraction of inspired oxygen [F_iO₂] = 0.09). Changes in voluntary ankle strength, lower extremity electromyograms (EMG), and serum inflammatory biomarkers were quantified. Results. Maximal ankle plantarflexion torque was significantly higher following prednisolone + AIH versus placebo + AIH (mean difference [MD] 9, 11, and 7 newton meter [N∙m] at 30, 60, and 120 minutes post-AIH, respectively; all Ps <.02). Soleus surface EMG during maximal voluntary contraction was also significantly increased following prednisolone + AIH (MD 3.5, P = .02 vs placebo + AIH), while activity of other leg muscles remained unchanged. Individuals had significantly higher levels of the anti-inflammatory serum biomarker interleukin-10 after prednisolone versus placebo (P = .004 vs placebo + AIH). Conclusions. Pretreatment with prednisolone increased the capacity for AIH-induced functional motor plasticity, suggesting that suppression of inflammation enhances the efficacy of AIH administration in individuals with spinal cord injury. Clinical trial registration number: NCT03752749.

Viral Mimetic-Induced Inflammation Abolishes Q-Pathway, but Not S-Pathway, Respiratory Motor Plasticity in Adult Rats

Inflammation arises from diverse stimuli eliciting distinct inflammatory profiles, yet little is known about the effects of different inflammatory stimuli on respiratory motor plasticity. Respiratory motor plasticity is a key feature of the neural control of breathing and commonly studied in the form of phrenic long-term facilitation (pLTF). At least two distinct pathways can evoke pLTF with differential sensitivities to bacterial-induced inflammation. The Q-pathway is abolished by bacterial-induced inflammation, while the S-pathway is inflammation-resistant. Since viral-induced inflammation is common and elicits distinct temporal inflammatory gene profiles compared to bacterial inflammation, we tested the hypothesis that inflammation induced by a viral mimetic (polyinosinic:polycytidylic acid, polyIC) would abolish Q-pathway-evoked pLTF, but not S-pathway-evoked pLTF. Further, we hypothesized Q-pathway impairment would occur later relative to bacterial-induced inflammation. PolyIC (750 μg/kg, i.p.) transiently increased inflammatory genes in the cervical spinal cord (3 h), but did not alter medullary and splenic inflammatory gene expression, suggesting region specific inflammation after polyIC. Dose-response experiments revealed 750 μg/kg polyIC (i.p.) was sufficient to abolish Q-pathway-evoked pLTF at 24 h (17 ± 15% change from baseline, n = 5, p > 0.05). However, polyIC (750 μg/kg, i.p.) at 3 h was not sufficient to abolish Q-pathway-evoked pLTF (67 ± 21%, n = 5, p < 0.0001), suggesting a unique temporal impairment of pLTF after viral-mimetic-induced systemic inflammation. A non-steroidal anti-inflammatory (ketoprofen, 12.5 mg/kg, i.p., 3 h) restored Q-pathway-evoked pLTF (64 ± 24%, n = 5, p < 0.0001), confirming the role of inflammatory signaling in pLTF impairment. On the contrary, S-pathway-evoked pLTF was unaffected by polyIC-induced inflammation (750 μg/kg, i.p., 24 h; 72 ± 25%, n = 5, p < 0.0001) and was not different from saline controls (65 ± 32%, n = 4, p = 0.6291). Thus, the inflammatory-impairment of Q-pathway-evoked pLTF is generalizable between distinct inflammatory stimuli, but differs temporally. On the contrary, S-pathway-evoked pLTF is inflammation-resistant. Therefore, in situations where respiratory motor plasticity may be used as a tool to improve motor function, strategies targeting S-pathway-evoked plasticity may facilitate therapeutic outcomes.