Long-term Facilitation Research Articles

Ventilatory Long-Term Facilitation at Rest Increases the Feedforward Contribution to Subsequent Exercise Ventilatory Responses.

The respiratory control system exhibits neural plasticity, adjusting future ventilatory responses based on experience. We tested the hypothesis that ventilatory long-term facilitation induced by hypercapnic acute intermittent hypoxia (AIH) at rest enhances subsequent ventilatory responses to steady-state exercise. Fourteen healthy adults (age = 27 ± 5 years; 7 males) participated in the study. On day 1, pulmonary function testing was performed. On days 2 and 3, in a pseudo-randomized counterbalanced order, participants were exposed to AIH or Sham; AIH consisted of 15, 1-min hypoxic episodes with 1.5-min room air intervals. Mild hypercapnia (end-tidal PCO2 clamped ∼3 mmHg above baseline) was sustained throughout AIH and Sham and for 40 min after. Approximately 20-30 min later, participants performed continuous mild to moderate constant-load cycle exercise in room air at 30, 60 and 90 W for 5 min each. Minute ventilation (⩒I) increased by 3.6 ± 1.2 L·min-1 after AIH versus baseline and was significantly greater than Sham (P = 0.013), signifying the onset of ventilatory long-term facilitation. Although ⩒I during subsequent steady-state exercise was not significantly different between AIH and Sham (P = 0.511), the slope of the relationship between ⩒I and CO2 production rate (i.e., the system gain) and the calculated feedforward exercise gain were significantly increased (P = 0.021 and P < 0.001, respectively). Consequently, end-tidal PCO2 was regulated ∼1 mmHg lower across all exercise workloads after AIH vs. Sham (P = 0.006). Thus, ventilatory plasticity induced at rest alters future ventilatory responses to mild or moderate steady-state exercise.

Role of 5-hydroxytryptamine type 3 receptors in aerobic exercise-induced improvement of memory and hippocampal synaptic plasticity.

Aerobic exercise facilitates synaptic plasticity, thereby improving cognitive functions such as learning and memory. The 5-hydroxytryptamine system has been indicated in these processes. 5-Hydroxytryptamine type 3 receptors are necessary for exercise-induced hippocampal neurogenesis. Some antipsychotic drugs with 5-hydroxytryptamine type 3 receptor antagonistic properties may impede the amelioration of cognitive impairment and hippocampal plasticity induced by exercise. However, the mechanisms underlying the facilitation of synaptic plasticity by aerobic exercise have not yet been elucidated. In this study, we found that 5-hydroxytryptamine type 3 receptors played an important role in aerobic exercise-mediated improvement of hippocampal-dependent spatial and exploratory memory in mice. While 5-hydroxytryptamine type 3 receptors did not affect baseline neurogenesis in the hippocampal dentate gyrus, 5-hydroxytryptamine type 3 receptors were required for aerobic exercise-induced neurogenesis and astrocyte proliferation in this region. In addition, 5-hydroxytryptamine type 3 receptors were crucial for maintaining long-term potentiation in the CA1, dentate gyrus, and CA3 regions of the hippocampus. The long-term potentiation changes induced by aerobic exercise in sub-regions of the hippocampus were heterogeneous: 5-hydroxytryptamine type 3 receptors were essential for aerobic exercise to enhance long-term potentiation in the CA3, but not the CA1 or dentate gyrus, regions of the hippocampus. Furthermore, aerobic exercise up-regulated 5-hydroxytryptamine type 3 receptor expression and increased brain-derived neurotrophic factor release in the hippocampus in a 5-hydroxytryptamine type 3 receptor-dependent manner. These results suggest that aerobic exercise increases hippocampal dentate gyrus neurogenesis and astrocyte proliferation via the up-regulation of 5-hydroxytryptamine type 3 receptors, leading to more brain-derived neurotrophic factor production and release from these cells, which results in long-term potentiation facilitation in the hippocampal CA3 region and help improve memory. Our findings provide insight into the mechanisms by which physical activity enhances memory and may have implications for improving memory through modulating 5-hydroxytryptamine type 3 receptor.

Microglia regulate motor neuron plasticity via reciprocal fractalkine and adenosine signaling

We report an important role for microglia in regulating neuroplasticity within phrenic motor neurons. Brief episodes of low oxygen (acute intermittent hypoxia; AIH) elicit a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF) that is regulated by the balance of competing serotonin vs adenosine-initiated cellular mechanisms. Serotonin arises from brainstem raphe neurons, but the source of adenosine is unknown. We tested if hypoxic episodes initiate phrenic motor neuron to microglia fractalkine signaling that evokes extracellular adenosine formation using a well-defined neurophysiology preparation in male rats. With moderate AIH, phrenic motor neuron adenosine 2A receptor activation undermines serotonin-dominant pLTF whereas severe AIH induces pLTF by the adenosine-dependent mechanism. Consequently, phrenic motor neuron fractalkine knockdown, microglial fractalkine receptor inhibition, and microglial ablation enhance moderate AIH, but suppress severe AIH-induced pLTF. We conclude, microglia play important roles in healthy spinal cords, regulating plasticity in motor neurons responsible for breathing.

Ethanol abolishes ventilatory long-term facilitation and blunts the ventilatory response to hypoxia in female rats

Obstructive sleep apnea (OSA) is a breathing disorder in which airway obstruction during sleep leads to periodic bouts of inadequate (hypopneic) or absent (apneic) ventilation despite neurorespiratory effort. Repetitive apneic and hypopneic exposures can induce intermittent hypoxemia and lead to a host of maladaptive behavioral and physiological outcomes. Intermittent hypoxia treatment (IH), which consists of alternating exposure to hypoxic and normal air, can induce a long-lasting increase in breathing motor outputs called long term facilitation (LTF). IH models key aspects of the hypoxemia experienced during OSA and LTF might serve to prevent OSA or ameliorate its severity by stimulating ventilatory output during or after apnea/hypopnea. Ethanol consumption prior to sleep exacerbates existing OSA, but it is unknown how ethanol affects LTF expression. Thus, we hypothesized that ethanol treatment would attenuate LTF expression and the magnitude of the ventilatory response during acute hypoxic exposure. We administered either low-dose (0.8 g/kg) or high-dose (3 g/kg) ethanol or saline to adult female Sprague-Dawley rats through intraperitoneal injection and then measured subjects’ ventilatory output by whole-body plethysmography during baseline, a 5 by 3-minute moderate IH protocol (hypoxia: FiO2 = 0.11, Normoxia: room air), and for one hour following the end of IH. Results indicate that low-dose ethanol abolishes LTF of respiratory rate and minute ventilation and trends suggest that low-dose ethanol might attenuate respiratory rate and minute ventilation during acute hypoxic exposure. While high-dose ethanol significantly diminished subjects’ respiratory rate and minute ventilation during hypoxia, LTF expression was not significantly different between high-dose ethanol and saline-treated subjects. Overall, data indicate that ethanol exposure dramatically attenuates LTF expression following IH treatment and impairs ventilatory responses to hypoxia in a dose-dependent manner. Such findings inspire further consideration of ethanol’s negative effects upon endogenous compensatory mechanisms for repeated hypoxic exposure, both in the context of OSA and beyond.

Acute intermittent hypoxia elicits sympathetic neuroplasticity independent of peripheral chemoreflex activation and spinal cord tissue hypoxia in a rodent model of high-thoracic spinal cord injury

The loss of medullary control of spinal circuits controlling the heart and blood vessels is a unifying mechanism linking both hemodynamic instability and the risk for cardiovascular diseases (CVD) following spinal cord injury (SCI). As such, new avenues to regulate sympathetic activity are essential to mitigate CVD in this population. Acute intermittent hypoxia (AIH) induces a type of neuroplasticity known as long-term facilitation (LTF), a persistent increase in nerve activity post-AIH in spinal motor circuits. Whether LTF occurs within the sympathetic circuit following SCI is largely unknown. We aimed to test whether AIH elicits sympathetic LTF (i.e., sLTF) and attenuates hypoactivity in sub-lesional splanchnic sympathetic circuits in a male rat model of SCI. In 3 experimental series, we tested whether 1) high-thoracic contusion SCI induces hypoactivity in splanchnic sympathetic nerve activity, 2) AIH elicits sLTF following SCI, and 3) sLTF requires carotid chemoreflex activation or spinal cord tissue hypoxia. Our results indicate that a single-session of AIH therapy (10 × 1 min of FiO2 = 0.1, interspersed with 2 min of FiO2 = 1.0) delivered at 2 weeks following SCI attenuates SCI-induced sympathetic hypoactivity by eliciting sLTF 90 min post-treatment that is independent of peripheral chemoreflex activation and/or spinal cord hypoxia. These findings advance our mechanistic understanding of AIH in the field and yield new insights into factors underpinning AIH-induced sLTF following SCI in a rat model. Our findings also set the stage for the chronic application of AIH to alleviate secondary complications resulting from sympathetic hypoactivity following SCI.

Enhanced phrenic motor neuron BDNF expression elicited by daily acute intermittent hypoxia is undermined in rats with chronic cervical spinal cord injury

Acute intermittent hypoxia (AIH) elicits spinal neuroplasticity and is emerging as a potential therapeutic modality to improve respiratory and non-respiratory motor function in people with chronic incomplete spinal cord injury (SCI). Brain-derived neurotrophic factor (BDNF) is necessary and sufficient for moderate AIH-induced phrenic long-term facilitation, a well-studied form of respiratory motor plasticity. Repetitive daily AIH (dAIH) enhances BDNF expression within the phrenic motor neurons of normal rats, but its effects on BDNF after chronic cervical spinal cord injury (cSCI) are unknown. In contrast to AIH, chronic intermittent hypoxia (CIH), simulating that experienced during sleep apnea, elicits neuropathology and undermines plasticity. Here, we tested the hypothesis that daily AIH vs CIH differentially regulate phrenic motor neuron BDNF expression in spinally intact and injured rats. Rats with and without C2 hemisection (C2Hx; 8 weeks post-injury) were exposed to 28 days of: 1) sham normoxia (Nx, 21 % O2); 2) daily AIH (dAIH: 10, 5 min episodes of 10.5 % O2 per day; 5 min normoxic intervals); 3) mild CIH (CIH5/5: 5 min of 10.5 % O2, 5 min intervals, 8 hrs/day); or 4) moderate CIH (CIH2/2: 2 min of 10.5 % O2, 2 min intervals, 8 hrs/day). After 28 days of daily exposure (i.e., 12 weeks post-injury), BDNF immunoreactivity was assessed within phrenic motor neurons identified via retrograde cholera toxin B fragment labeling. In intact rats, daily AIH increased BDNF protein levels in phrenic motor neurons (∼31 %) but not in rats with C2Hx. CIH had no effects on phrenic motor neuron BDNF levels in intact rats, although there was a trend towards increased phrenic motor neuron BDNF after C2Hx, suggesting the need for further study. Since dAIH effects on phrenic motor neuron BDNF are not observed in rats with chronic cervical SCI, the potential of dAIH to enhance BDNF-dependent phrenic motor plasticity may be suppressed by conditions prevailing with chronic cSCI.

Patient-derived tau and amyloid-β facilitate long-term depression in vivo: role of tumour necrosis factor-α and the integrated stress response.

Alzheimer's disease is characterized by a progressive cognitive decline in older individuals accompanied by the deposition of two pathognomonic proteins amyloid-β and tau. It is well documented that synaptotoxic soluble amyloid-β aggregates facilitate synaptic long-term depression, a major form of synaptic weakening that correlates with cognitive status in Alzheimer's disease. Whether synaptotoxic tau, which is also associated strongly with progressive cognitive decline in patients with Alzheimer's disease and other tauopathies, also causes facilitation remains to be clarified. Young male adult and middle-aged rats were employed. Synaptotoxic tau and amyloid-β were obtained from different sources including (i) aqueous brain extracts from patients with Alzheimer's disease and Pick's disease tauopathy; (ii) the secretomes of induced pluripotent stem cell-derived neurons from individuals with trisomy of chromosome 21; and (iii) synthetic amyloid-β. In vivo electrophysiology was performed in urethane anaesthetized animals. Evoked field excitatory postsynaptic potentials were recorded from the stratum radiatum in the CA1 area of the hippocampus with electrical stimulation to the Schaffer collateral-commissural pathway. To study the enhancement of long-term depression, relatively weak low-frequency electrical stimulation was used to trigger peri-threshold long-term depression. Synaptotoxic forms of tau or amyloid-β were administered intracerebroventricularly. The ability of agents that inhibit the cytokine tumour necrosis factor-α or the integrated stress response to prevent the effects of amyloid-β or tau on long-term depression was assessed after local or systemic injection, respectively. We found that diffusible tau from Alzheimer's disease or Pick's disease patients' brain aqueous extracts or the secretomes of trisomy of chromosome 21 induced pluripotent stem cell-derived neurons, like Alzheimer's disease brain-derived amyloid-β and synthetic oligomeric amyloid-β, potently enhanced synaptic long-term depression in live rats. We further demonstrated that long-term depression facilitation by both tau and amyloid-β was age-dependent, being more potent in middle-aged compared with young animals. Finally, at the cellular level, we provide pharmacological evidence that tumour necrosis factor-α and the integrated stress response are downstream mediators of long-term depression facilitation by both synaptotoxic tau and amyloid-β. Overall, these findings reveal the promotion of an age-dependent synaptic weakening by both synaptotoxic tau and amyloid-β. Pharmacologically targeting shared mechanisms of tau and amyloid-β synaptotoxicity, such as tumour necrosis factor-α or the integrated stress response, provides an attractive strategy to treat early Alzheimer's disease.

Understanding health-related quality of life after hypospadias repair: A qualitative study with pre-adolescent males and parents

Hypospadias is a common disease that affects approximately 1 in every 200 live male births in the United States, and long-term studies of individuals who have undergone repair demonstrate complication rates of 15%-70%. The Hypospadias-Specific Health-related Quality of Life (HRQOL) Conceptual Framework for youth and adults suggests that additional morbidity may be incurred from poor psychological, social, and sexual health. The current study sought to clarify hypospadias-specific HRQOL and care priorities in a pre-pubertal population. This IRB-approved, semi-structured interview study used rigorous qualitative research methods. Eligible patients were English-speaking 8-12-year-old males with hypospadias and their parents. Families completed a demographic questionnaire and separate youth and parent 30-min telephone interviews. We used hybrid thematic analysis to develop an operational codebook, analyze participant responses, and generate conceptual themes. Mixed methods analysis was used to explore patterns of experiences across groups defined by socioeconomic level. We interviewed 10 parents and 8 children (Median age 9 years, Range 8-11). We generated three overarching themes: Penile Factors, Psychosocial Concerns, and Expectations of Surgery and the Healthcare Team. These highest-order themes were generated for youth, parent-proxy, and parent self-reported experiences, and there were different sub-themes for each participant type (Figure). Youth were focused on avoidance of disclosure and the psychological impact of self-comparisons and embarrassment, while the parental perspective centered on worries about future fertility, complications, psychological health, and normality. Some youth and parents from disadvantaged neighborhoods or those with public insurance indicated a need for more education on normal penile functions and provision of strategies for long-term self-monitoring and facilitation of long-term follow-up on mixed methods analysis. These findings add insight into the multifaceted experiences of pre-pubertal youth and families dealing with hypospadias, and underscore the consistent, wide-ranging interplay between medical, psychological, and social concerns. Patterns in themes across socioeconomic status and insurance coverage suggest that access to information and quality care may vary significantly and could contribute to health disparities. Urologists should employ an individualized approach to counseling and care delivery. Future studies will seek to characterize care priorities in pubertal and post-pubertal age groups to design developmentally adjusted support tools for youth and adults with hypospadias and their families.

Designing an implementation science clinical trial to integrate hypertension and cardiovascular diseases care into existing HIV services package in Botswana (InterCARE)

BackgroundDespite success in HIV treatment, diagnosis and management of hypertension (HTN) and cardiovascular disease (CVD) remains suboptimal among people living with HIV (PLWH) in Botswana, with an overall HTN control of only 19% compared to 98% HIV viral suppressed. These gaps persist despite CVD primary care national guidelines and availability of free healthcare including antihypertensive medications. Our study aims to develop and test strategies to close the HTN care gap in PLWH, through integration into HIV care, leveraging the successful national HIV care and treatment program and strategies.MethodsThe InterCARE trial is a cluster randomized controlled hybrid type 2 effectiveness-implementation trial at 14 sites designed to enroll 4652 adults living with HIV and HTN plus up to 2326 treatment partners. Primary outcomes included effectiveness (HTN control) and implementation outcomes using the Reach Effectiveness Adoption Implementation and Maintenance framework, with explanatory mixed methods used to understand variability in outcomes.InterCARE trial’s main strategies include healthcare worker HTN and CVD care training plus long-term practice facilitation, electronic health record (EHR) documentation of key indicators and use of reminders, and use of treatment partners to provide social support to people living with HIV and HTN. InterCARE started with formative research to identify contextual factors influencing care gaps using the Consolidated Framework for Implementation Research. Results were used to adapt initial and develop additional implementation strategies to address barriers and leverage facilitators. The package was pilot tested in two clinics, with findings used to further adapt or add strategies for the clinical trial.DiscussionIf successful, the InterCARE model can be scaled up to HIV clinics nationwide to improve diagnosis, management, and support in Botswana. The trial will provide insights for scale-up of HTN integration into HIV care in the region.Trial registrationClinicalTrials.gov reference NCT05414526. Registered 18 May 2022, https://clinicaltrials.gov/study/NCT05414526?term=NCT05414526.&rank=1.

BREATHE DMD: boosting respiratory efficacy after therapeutic hypoxic episodes in Duchenne muscular dystrophy.

Duchenne muscular dystrophy (DMD) is a fatal genetic neuromuscular disorder, characterised by progressive decline in skeletal muscle function due to the secondary consequences of dystrophin deficiency. Weakness extends to the respiratory musculature, and cardiorespiratory failure is the leading cause of death in men with DMD. Intermittent hypoxia has emerged as a potential therapy to counteract ventilatory insufficiency by eliciting long-term facilitation of breathing. Mechanisms of sensory and motor facilitation of breathing have been well delineated in animal models. Various paradigms of intermittent hypoxia have been designed and implemented in human trials culminating in clinical trials in people with spinal cord injury and amyotrophic lateral sclerosis. Application of therapeutic intermittent hypoxia to DMD is considered together with discussion of the potential barriers to progression owing to the complexity of this devastating disease. Notwithstanding the considerable challenges and potential pitfalls of intermittent hypoxia-based therapies for DMD, we suggest it is incumbent on the research community to explore the potential benefits in pre-clinical models. Intermittent hypoxia paradigms should be implemented to explore the proclivity to express respiratory plasticity with the longer-term aim of preserving and potentiating ventilation in pre-clinical models and people with DMD.

Sex hormone supplementation improves breathing and restores respiratory neuroplasticity following C2 hemisection in rats.

In addition to loss of sensory and motor function below the level of the lesion, traumatic spinal cord injury (SCI) may reduce circulating steroid hormones that are necessary for maintaining normal physiological function for extended time periods. For men, who comprise nearly 80% of new SCI cases each year, testosterone is the most abundant circulating sex steroid. SCI often results in significantly reduced testosterone production and may result in chronic low testosterone levels. Testosterone plays a role in respiratory function and the expression of respiratory neuroplasticity. When testosterone levels are low, young adult male rats are unable to express phrenic long-term facilitation (pLTF), an inducible form of respiratory neuroplasticity invoked by acute, intermittent hypoxia (AIH). However, testosterone replacement can restore this respiratory neuroplasticity. Complicating the interpretation of this finding is that testosterone may exert its influence in three possible ways: 1) directly through androgen receptor (AR) activation, 2) through conversion to dihydrotestosterone (DHT) by way of the enzyme 5α-reductase, or 3) through conversion to 17β-estradiol (E2) by way of the enzyme aromatase. DHT signals via AR activation similar to testosterone, but with higher affinity, while E2 activates local estrogen receptors. Evidence to date supports the idea that exogenous testosterone supplementation exerts its influence through estrogen receptor signaling under conditions of low circulating testosterone. Here we explored both recovery of breathing function (measured with whole body barometric plethysmography) and the expression of AIH-induced pLTF in male rats following C2-hemisection SCI. One week post injury, rats were supplemented with either E2 or DHT for 7days. We hypothesized that E2 would enhance ventilation and reveal pLTF following AIH in SCI rats. To our surprise, though E2 did beneficially impact overall breathing recovery following C2-hemisection, both E2 supplementation and DHT restored the expression of AIH-induced pLTF 2weeks post-SCI.

Ethanol pre-exposure attenuates the hypoxic ventilatory response and may attenuate expression of long term facilitation following intermittent hypoxia treatment in adult rats

Obstructive sleep apnea (OSA) is a highly prevalent breathing disorder in which airway collapse or obstruction during sleep leads to periodic bouts of inadequate (hypopneic) or absent (apneic) ventilation despite neurorespiratory effort. Repetitive apneic and hypopneic exposures during sleep can induce intermittent hypoxemia, sympathetic activation, and sleep fragmentation which can lead to a host of maladaptive behavioral and physiological outcomes. Intermittent hypoxia, which consists of alternating exposure to hypoxia and normal oxygen levels (normoxia), can induce a long-lasting increase in breathing motor output called long term facilitation (LTF) and is currently being investigated as a therapeutic treatment. Interestingly, experts in the field of LTF have noted how IH models some key aspects of OSA. It has been well established through clinical studies that ethanol consumption prior to sleep exacerbates existing OSA, increasing the apnea/hypopnea index and worsening arterial blood oxygen desaturations during the night. However, it is unknown whether ethanol affects expression of LTF following IH treatment. If LTF might serve as a means of increasing ventilatory output to compensate for periods of apnea and hypopnea during OSA, ethanol might not only worsen the severity of OSA, but impede individuals’ compensatory response. Thus, for this study we hypothesized that ethanol treatment would attenuate LTF expression and the magnitude of the hypoxic ventilatory response during IH treatment. Using adult female Sprague-Dawley rats, we administered either low-dose (0.8 g/kg) or high-dose (3 g/kg) ethanol through intraperitoneal injection to model a 2-drink nightcap or binge drinking-type ethanol exposure, respectively. Immediately afterwards, we measured subjects’ ventilatory output during baseline, then during a 5 by 3-minute moderate IH protocol, and for one hour following the end of IH using whole-body plethysmography. Finally, we took venous blood samples to assay serum ethanol concentration at the end of the approximately 2.5 hour plethysmography measurement. Results from the high-dose ethanol group indicate that ethanol pre-exposure might attenuate respiratory rate and minute volume LTF in comparison to saline-treated control. Furthermore, high-dose ethanol attenuated the hypoxic ventilatory response with respect to respiratory rate and minute volume. Low dosage of ethanol attenuated subjects’ HVR of respiratory rate and minute volume, but conflictingly trends towards increasing LTF of tidal volume while diminishing LTF of respiratory rate when compared to saline control. Overall, our findings indicate that high levels of ethanol exposure dramatically diminish the acute ventilatory response to hypoxic exposure and may impede long-term increases in ventilatory output following IH treatment. On the other hand, lower doses of ethanol have a subtler effect on the ventilatory response to hypoxia. Future directions will include analysis of additional parameters of ventilatory function to gain a more detailed understanding of how ethanol effects respiratory plasticity in the context of intermittent hypoxic exposure. NIH 5T32 AA027488 to ALSUniversity of Kentucky Endowment to WJA. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Mechanisms Underlying Long-Term Facilitation in the Carotid Body

Glutamate and γ-aminobutyric acid (GABA) are major modulators of excitatory and inhibitory transmission in the mammalian brain. Glutamate transmission is critical for neural plasticity, learning and memory, whereas GABA plays a fundamental role in regulating neuronal excitability and facilitating the generation of neural oscillations. Our data indicates that both glutamate and GABA are released in the carotid body (CB) to modulates its response to hypoxia. Objectives: We set out to investigate whether repeated stimulation with glutamate and GABA induces lasting effects on CB afferent discharge. Hypothesis: We hypothesised that glutamate/GABA mediates long-term facilitation (LTF) of CB afferent activity. Methods: We mined high-throughput RNA sequence data and used immunohistochemistry to map components underlying neuroplasticity in the CB in Wistar and Spontaneously-hypertensive rats (SHR). CSN discharge was measured as a functional readout of CB function from an ex vivo arterially perfused carotid artery-CB preparation. Data: We catalogue the expression of glutamate receptors (R) and transporters in the rat and mouse CB. We found N-methyl-D-aspartic acid receptor (NMDAR), α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPAR) and transporters to be localised to the CB chemosensory cells. Targeted administration of 15 mM glutamate activated the carotid sinus nerve ( p=0.003). Repetitive [5 times, 2-minute duration, 5-minute intervals] application of 15 mM glutamate evoked LTF of CB afferent discharge as demonstrated by both a 12-fold increase in basal firing frequency (0.78 ± 0.37 vs 1232 ± 596 spikes 1s−1; p = 0.004) over 60 minutes ( p = 0.004) and amplification of the evoked response to chemical hypoxia (CN−; 1.23 μmol bolus) by 2-fold ( p = 0.009). The stimulation paradigm was specific as neither application of glutamate, 5 times for 1-minute at 5-minute intervals, nor a single glutamate exposure evoked LTF. In contrast, repeated application of 15mM GABA [3 times for 1-minute at 5-minute intervals] evoked long-term depression (LTD) of CB afferent activity as seen by a reduced basal discharge from 10.5 to 3.5 spikes 1s−1 ( p = 0.0004) and a 2-fold suppression of the response to chemical hypoxia ( p = 3.71 x 10−5). Summary of the data: Repetitive exposure to glutamate using a specific protocol increased basal discharge of CB afferent output akin to LTF and caused sensitisation of evoked responses to CN−. Repeated exposures of GABA depressed the CB basal discharge akin to LTD. A statement of the conclusions: It is conceivable that processes such as LTP and LTD operate in the CB to modulate the set point of peripheral chemoreceptor sensitivity and its state-dependent tonicity; the role of LTP in peripheral chemoreflex sensitisation in disease remains to be determined. Research was supported by the Health Research Council of New Zealand and the Sidney Taylor Trust. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Ventilatory long-term facilitation following acute intermittent hypoxia in unanesthetized rats: influence of short hypoxic episodes and changes in metabolic rate

Plasticity is a hallmark feature of the neural system controlling breathing. Acute intermittent hypoxia (AIH) elicits plasticity in multiple respiratory motor pools ( e.g., phrenic, intercostal, XII, etc.) and has emerged as a promising therapeutic approach to restore lost respiratory (and non-respiratory) function in people with spinal cord injury and other clinical disorders that compromise movement. In anesthetized rats during the rest phase, AIH-induced phrenic long-term facilitation is enhanced when the AIH consists of short (1-min) versus longer (5-min) moderate hypoxic episodes (PaO2 ~40-50 mmHg), even with the same cumulative duration of hypoxia (15-min). However, since respiratory plasticity is influenced by a variety of behavioral and physiological factors such as sensory feedback, lung/chest wall mechanics and arousal state, the effectiveness of specific AIH protocols may differ when implemented in unanesthetized rats. Since it is unknown how hypoxic episode duration impacts ventilatory long-term facilitation (vLTF) in unanesthetized rats during the rest phase, we initially tested the hypothesis that short (1-min) hypoxic episodes (FIO2 = 0.09) elicit vLTF in young male Sprague-Dawley rats via whole-body plethysmography; metabolic CO2 production was also assessed. Following 15, 1-min hypoxic episodes, significant vLTF was observed, and this effect was more pronounced when ventilation was normalized to metabolic rate ( i.e. VE/VCO2; p &lt; 0.05). AIH-induced vLTF was accompanied by hypometabolism and a post-hypoxic temperature decrease (p &lt; 0.01), consistent with previous work. Additional studies will be performed to compare these results with “traditional” AIH protocols ( i.e., 5-min hypoxic episodes), and across the rats’ rest versus active phase. In perspective, these results set the stage for future investigations concerning the optimization of AIH protocols in unanesthetized rodent models, including rodent models of injury or disease. NIH HL148030 &amp; 147554 (Mitchell). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Acute Intermittent Hypoxia with Hypercapnia has Complex effects on Phrenic Long-Term Facilitation that Depend on Delivery in the Rest versus Active Phase in Rats

Exploration of mechanisms giving rise to respiratory motor plasticity, such as phrenic long-term facilitation (pLTF) following acute intermittent hypoxia (AIH), has inspired new therapeutic approaches to treat individuals affected by conditions such as spinal cord injury or other clinical disorders. One key step in the translation between rodent models and humans is the apparent need for hypercapnia (either baseline or within hypoxic episodes) during AIH. Whereas AIH consisting of 15, 1-minute hypoxic episodes (1-minute intervals) elicits robust pLTF in rats, similar protocols have minimal impact in humans—unless PaCO2 levels are increased. Although prior studies report that elevated baseline CO2 actually impairs AIH-induced pLTF in rats, the impact of hypercapnia within hypoxic episodes is not known. Thus, we compared AIH with and without concurrent hypercapnia (AIH versus AIHH) on pLTF in anesthetized rats. Since our laboratory recently reported profound time-of-day effects on pLTF magnitude and/or mechanisms, we also compared AIH versus AIHH-induced pLTF during the daily rest and active phase. We hypothesized that AIHH would enhance pLTF versus AIH in both phases, suggesting it may be a more effective intervention to elicit respiratory motor plasticity and respiratory rehabilitation. pLTF was assessed in anesthetized, paralyzed, vagotomized and ventilated male Sprague-Dawley rats ( n=6 per group) exposed to moderate AIH (arterial PO2 = 40-55mmHg) consisting of: 1) 15, 1-minute hypoxic episodes with concurrent hypercapnia (arterial PCO2 ~50mmHg) or 15, 1-minute episodes of isocapnic hypoxia (baseline arterial PCO2 ~2 mmHg above the recruitment threshold; ~40-42 mmHg). We also compared AIHH versus AIH during the active phase using prior published data (AIHH, n = 4; AIH, n = 7; data from Nair et al., Function, 2023 and Marciante et al., Function, 2023, respectively). Contrary to our hypothesis, pLTF was attenuated 90 minutes post-AIHH (37±8%) versus AIH (83±4%; p&lt;0.001) in the rest phase. In striking contrast, during the active phase, pLTF was greater with AIHH (60±11%) versus AIH (30±4%; P=0.012). These unexpected outcomes demonstrate that time of day (or the rest/active phase) must be considered before adding hypercapnia to an AIH protocol. Although the mechanisms of this difference remain unclear, our results make clear that there is still a great deal to learn about mechanisms of AIH-induced respiratory motor plasticity, including the complex interplay between hypoxia, hypercapnia and their impact on pLTF. Supported by: NIH R01 HL147554 &amp; R01 HL148030 (PI: G. Mitchell). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

APOE Genotype Differentially Regulates Respiratory Motor Plasticity in the Active vs Rest phase

Acute intermittent hypoxia (AIH) elicits respiratory motor plasticity and is emerging as a promising strategy to improve breathing and other (non-respiratory) movements in clinical disorders that end life due to respiratory insuffciency ( e.g., spinal cord injury &amp; ALS). AIH-induced phrenic long-term facilitation (pLTF) is the most commonly studied form of respiratory motor plasticity and presents as a persistent increase in phrenic nerve activity after AIH. Specific factors ( e.g. aging, inflammation, time of day) undermine AIH-induced pLTF. However, little information is available concerning the role of specific genes that regulate AIH-induced phrenic motor plasticity. Apolipoproteins (ApoE) are highly expressed in the central nervous system and function as lipid carriers. The APOE gene is tri-allelic ( E2, E3 and E4); ApoE2 and E3 proteins are associated with neuroprotective and neuro-neutral phenotypes, respectively. ApoE4 impairs synaptic plasticity and is implicated in neurodegeneration, particularly Alzheimer’s Disease. Indeed, we recently discovered that specific APOE alleles predict human expression of mAIH-induced plasticity in cortico-spinal pathways to the phrenic/diaphragm motor system. Initial experiments using human APOE4 knock-in rats demonstrate a causal effect in limiting phrenic motor plasticity (vs APOE3) during the active phase. However, the impact of APOE genotype on respiratory plasticity across the daily rest/active cycle is unknown. We tested the hypothesis that APOE4 undermines AIH-induced pLTF vsAPOE3 in a time-of-day-dependent manner. Anesthetized, vagotomized, paralyzed and ventilated adult (3-4 months) male Sprague-Dawley rats with knock-in humanized (h) APOE3 and h APOE4 were presented with AIH (15,1 min hypoxic episodes; arterial PO2 = 40-50mmHg). During the active phase, AIH-induced pLTF was significantly greater in h APOE3 (33±11%) vs h APOE4 rats (-26±7%; p&lt;0.001; both n=6). In contrast, rest phase AIH-induced pLTF was similar in both h APOE3 and h APOE4 rats (121±12% and 122±5%, respectively; both n=7). These experiments demonstrate a causal relationship whereby APOE genotype and time-of-day interact to undermine AIH-induced pLTF in a manner that is unique to the active phase in rats. Since AIH has emerged as a promising neurotherapeutic to preserve/restore breathing in clinical disorders such as spinal cord injury and neuromuscular disease, understanding how genetic factors and time-of-day of AIH delivery impact phrenic motor plasticity will help identify individuals most/least likely to respond to treatment and reveal new targets for precision interventions of AIH. NIH R01HL148030 (GSM), NIH R01HL149800 (GSM), NIH T32HL134621-5 (ABM). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Phrenic Motor Neuron Fractalkine Signaling to Nearby Microglia Regulates Phrenic Long-Term Facilitation

Although microglia are traditionally regarded as innate CNS immune cells, recent studies demonstrate that microglia also play key roles in normal CNS functions. For example, microglia engage in bidirectional communication with other cell types ( e.g., neurons) to regulate diverse processes including neuronal excitability, architecture and synaptic plasticity. Although neuron-microglia interactions regulate hippocampal synaptic plasticity, little is known concerning the role of microglia in regulating respiratory motor plasticity. One well-studied form of respiratory motor plasticity is phrenic long-term facilitation (pLTF), a prolonged increase in phrenic motor output following acute intermittent hypoxia (AIH). Whereas AIH consisting of moderate hypoxic episodes (mAIH) elicits pLTF by a serotonin-dominant, adenosine-constrained mechanism, severe AIH (sAIH) elicits pLTF by an adenosine-dominant, serotonin-constrained mechanism. Neurons interact with microglia via release of fractalkine (CX3CL1), activating responses from nearby microglia, the only CNS cell-type known to express fractalkine receptors (CX3CR1). Cervical spinal fractalkine protein injections elicit phrenic motor facilitation similar to pLTF and trigger glial ATP/adenosine release, consistent with the hypothesis that neuronal fractalkine release during hypoxia elicits extracellular adenosine accumulation, thereby modulating AIH-induced pLTF. Here, we tested the hypothesis that phrenic motor neuron fractalkine protein underlies the adenosine-constraint to mAIH-induced pLTF, and drives sAIH-induced pLTF. In adult male Sprague Dawley rats, siRNA knockdown of phrenic motor neuron fractalkine mRNA and protein was achieved via intrapleural injections of siRNAs targeting fractalkine mRNA for 3 successive days. Fractalkine knock-down: 1) enhanced serotonin-dependent mAIH-induced pLTF (100±7%; n=6; vs controls: 50±5%; n=7; p&lt;0.001), but 2) attenuated adenosine-dependent, sAIH-induced pLTF (23±12%; n=5; vs controls: 88±7%; n=7; p=0.001). These findings suggest that hypoxia triggers dose-dependent phrenic motor neuron fractalkine release, activating CX3CR1 on nearby microglia. Subsequent microglial ATP release and conversion to adenosine activates phrenic motor neuron adenosine 2A receptors which: 1) undermines mAIH-induced (serotonin-dominant) pLTF; or 2) reaches levels suffcient to drive sAIH-induced (adenosine-dominant) pLTF. These findings increase our understanding of how spinal intercellular interactions regulate AIH-induced phrenic motor plasticity. NIH R01HL148030 (GSM), NIH R01HL149800 (GSM), NIH T32HL134621-5 (ABM). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Spinal TNF-α receptor expression in a rodent model of respiratory motor neuron death

Intrapleural injection of cholera toxin B fragment conjugated to saporin (CTB-SAP) selectively kills respiratory motor neurons, and mimics aspects of neurodegenerative diseases including breathing deficits. In this model, phrenic long-term facilitation (pLTF) induced by acute intermittent hypoxia is enhanced following 7 days (d) of CTB-SAP vs. control rats, whereas pLTF in 28d CTB-SAP treated rats is comparable to pLTF in controls suggesting that the underlying mechanisms of plasticity at both time points differ. Inflammation is a hallmark of neurodegenerative diseases and is known to prevent pLTF. Furthermore, ketoprofen (NSAID) delivery attenuates pLTF in 7d CTB-SAP-treated rats, but enhances it in 28d CTB-SAP-treated rats; this suggests that inflammation contributes to pLTF in 7d CTB-SAP-treated rats, but undermines pLTF in 28d CTB-SAP-treated rats. In addition, cervical spinal inflammatory marker gene expression ( e.g., TNF-a) is increased in CTB-SAP rats, and pilot data suggest that soluble TNF receptor 1 (sTNFR1) inhibition differentially affects pLTF (Nichols and Smith , ibid). Thus, the goal of the current study is to investigate TNFR1 as a pharmacological target that can modulate inflammation to maximize pLTF and breathing in CTB-SAP rats. We hypothesized that C4 TNFR1 expression would be increased on phrenic motor neurons and glial cells of CTB-SAP rats vs. controls. To address this, we studied TNFR1 expression on phrenic motor neurons and glial cells using immunohistochemistry, confocal microscopy, and ImageJ analysis in male Sprague Dawley rats 7 and 28 days after intrapleural injection of: CTB-SAP or un-conjugated CTB and SAP (control); n=8/group. Our preliminary data (n=4/group) suggest that phrenic TNFR1 expression is increased in 7d CTB-SAP rats vs. controls (p&lt;0.05), but is unaffected in 28d CTB-SAP rats vs. controls (p&gt;0.05). Further data analysis is underway for phrenic as well as glial TNFR1 expression in CTB-SAP rats vs. controls. If C4 TNFR1 expression is increased, this would suggest that TNFR1 is a viable target for therapeutic intervention in CTB-SAP rats, and potentially in patients with breathing deficits due to respiratory motor neuron death. NSF Computational Neuroscience grant, MU CVM COR grant, and Spinal Cord Injury &amp; Disease Research Program grant. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

Phrenic Long-term Facilitation Following Severe Acute Intermittent Hypoxia Is Preserved in Geriatric Male Rats

Acute intermittent hypoxia (AIH) elicits a form of respiratory motor plasticity known as phrenic long-term facilitation (pLTF). Both moderate and severe AIH elicit phenotypically similar pLTF but do so via completely distinct cellular mechanisms. With moderate AIH (mAIH), pLTF arises from a serotonin-dominant, adenosine-constrained mechanism. In aged rats, mAIH-induced pLTF is impaired due to increased basal spinal adenosine levels, which constrain serotonin-dependent pLTF. With severe AIH (sAIH), pLTF arises from an adenosine-dominant, serotonin-constrained mechanism. Since spinal adenosine levels are elevated in aged male rats, we hypothesized that age would actually enhance sAIH-induced- pLTF. Young (~3.5 month) and aged (~20 month) male Sprague Dawley rats were urethane anesthetized, artificially ventilated, vagotomized, paralyzed, and exposed to sAIH (3, 5-minute episodes; arterial Po2 = 25-30 mmHg). Integrated phrenic nerve burst activity was measured before (baseline), during hypoxic episodes and 60 minutes after sAIH. Neither baseline phrenic burst amplitude, the short-term hypoxic phrenic response nor pLTF magnitude (assessed as % change in phrenic burst amplitude from baseline to 60 minutes post-sAIH) were different in aged versus young rats (120 ± 22% vs. 98 ± 17%, respectively). Thus, aged male rats preserve the capacity for adenosine driven phrenic motor plasticity. Although we cannot conclude that sAIH-induced pLTF is actually increased, age effects on adenosine versus serotonin-driven plasticity are clearly different. NIH HL148030. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

N-Methyl-D-aspartate receptor (NMDAR) activity in the nucleus tractus solitarii (nTS) is required for the full expression of phrenic long-term facilitation after acute intermittent hypoxia

Exposure to acute intermittent hypoxia (AIH) induces a persistent elevation of phrenic nerve activity during the normoxic period, or phrenic long-term facilitation (pLTF). We have previously shown that nTS activity is necessary for both the development and maintenance of pLTF. However, the underlying cellular mechanisms and pathways are not yet clear. Hypoxia induces the release of glutamate from chemoafferents in the nTS, where it binds to AMPA and NMDA receptors. Previous studies have shown that activation of NMDARs increases neuronal calcium influx and triggers long-term potentiation. The present study aimed to test the role of NMDARs in pLTF and, in particular, to determine whether activation of these receptors in the nTS is required for hypoxia-induced pLTF. Mean arterial pressure (MAP) and splanchnic sympathetic and phrenic nerve activity (sSNA and PhrNA) were recorded in anesthetized, vagotomized, neuromuscularly blocked, and artificially ventilated male Sprague-Dawley rats. Animals were exposed to 10 bouts of 10% O2 (45 sec, every 5 min; AIH). Time controls (TC) were exposed to a single hypoxia bout. Parameters were measured for an additional 1 hour after the end of AIH and compared to their initial baseline (Bsl) values or TC. The NMDAR antagonist MK-801 (3 mg kg (-1), i.v.) was injected systemically, or AP5 (10 mM, 60 nl) nanoinjected bilaterally into the nTS, after the development of AIH-induced pLTF. To test whether induction of AIH or blockade of NMDARs alters neuronal function of the nTS, nTS neuronal Ca2+ responses to AIH and MK-801 were measured by fiber photometry in 3 rats previously nanoinjected with a Ca2+ indicator GCaMP-expressing virus in the nTS. Minute PhrNA (MinPhrNA, frequency x amplitude) increased 1hr after AIH (% of Bsl, 438 ± 184%; mean ± SE; p &lt;0.01; t-test) and was significantly greater than TC (74 ± 15%Bsl p &lt;0.01; t-test), indicating pLTF. AIH also increased nTS neuronal Ca2+. Systemic blockade of NMDARs after the development of pLTF decreased MAP, sSNA, and nTS Ca2+ and eliminated pLTF, consistent with others. Bilateral nTS nanoinjection of AP5 reduced but did not abolish pLTF (before AP5: 449 ± 109% vs. after AP5: 371 ± 107%Bsl; p &lt;0.05; paired t-test), increased sSNA (before AP5: 192 ± 31% vs. after AP5: 219 ± 42%; p= 0.08; paired t-test) but did not alter MAP. nTS AP5 after TC had no effect on MAP, MinPhrNA (before AP5: 99 ± 46% vs. after AP5: 103 ± 43%; p = 0.44; t-test) or sSNA (before AP5: 244 ± 92% vs. after AP5: 245 ± 81%; =0.94; paired t-test). Altogether, these results show that systemic blockade of NMDARs eliminates pLTF, and our data suggest that nTS NMDARs are critical components for the maintenance of AIH-induced neuroplasticity. R01-HL-98602. This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.