Ventilatory and integrated physiological responses to chronic hypercapnia in goats

Confined spaces in space: Cerebral implications of chronic elevations of inspired carbon dioxide and implications for long-duration space travel

Cerebrovascular regulation is critically dependent upon the arterial partial pressure of carbon dioxide (P aC O 2 ${P_{{\mathrm{aC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ ), owing to its effect on cerebral blood flow, tissueP C O 2 ${P_{{\mathrm{C}}{{\mathrm{O}}_{\mathrm{2}}}}}$ , tissue proton concentration, cerebral metabolism and cognitive and neuronal function. In normal environments and in the absence of pathology, at least over acute time frames, hypercapnia is usually managed readily via the respiratory chemoreflex arcs and/or acid-base buffering capacity, such that there is minimal impact on cerebrovascular and neurological function. However, in non-normal environments, such as enclosed spaces, or with pathology, extended exposures to elevations inP aC O 2 ${P_{{\mathrm{aC}}{{\mathrm{O}}_{\mathrm{2}}}}}$ can be detrimental to cerebral health. Given the direct effect of protons on cellular function, even if pH is normalized, it is feasible that higher proton concentrations could still produce detrimental effects. Although it seems that humans can work safely in mildly hypercapnic environments for extended periods, chronic respiratory acidosis can cause bone demineralization, renal calcification, perinatal developmental abnormalities, systemic inflammation and impairments in cognitive function and visuomotor skills and can produce cerebral acidosis, potentially inducing sustained alterations in cerebral function. With the advancement of new initiatives in spaceflight, including proposed long-duration missions to Mars, the study of the effects of chronic inspired CO2 on human health is relevant. In this review, we draw on evidence from preclinical, physiological and clinical research in humans to summarize the cerebral ramifications of prolonged and chronic exposures to elevated partial pressures of inspired CO2 and respiratory acidosis.

Galaninergic and hypercapnia-activated neuronal projections to the ventral respiratory column

In mammals, the ventral respiratory column (VRC) plays a pivotal role in integrating neurochemically diverse inputs from brainstem and forebrain regions to generate respiratory motor patterns. VRC microinjection of the neuropeptide galanin has been reported to dampen carbon dioxide (CO2)-mediated chemoreflex responses. Additionally, we previously demonstrated that galaninergic neurons in the retrotrapezoid nucleus (RTN) are implicated in the adaptive response to hypercapnic stimuli, suggesting a link between RTN neuroplasticity and increased neuronal drive to the VRC. VRC neurons express galanin receptor 1, suggesting potential regulatory action by galanin, however, the precise galaninergic chemoreceptor-VRC circuitry remains to be determined. This study aimed to identify sources of galaninergic input to the VRC that contribute to central respiratory chemoreception. We employed a combination of retrograde neuronal tracing, in situ hybridisation and immunohistochemistry to investigate VRC-projecting neurons that synthesise galanin mRNA. In an additional series of experiments, we used acute hypercapnia exposure (10% CO2, 1 h) and c-Fos immunohistochemistry to ascertain which galaninergic nuclei projecting to the VRC are activated. Our findings reveal that a total of 30 brain nuclei and 51 subnuclei project to the VRC, with 12 of these containing galaninergic neurons, including the RTN. Among these galaninergic populations, only a subset of the RTN neurons (approximately 55%) exhibited activation in response to acute hypercapnia. Our findings highlight that the RTN is the likely source of galaninergic transmission to the VRC in response to hypercapnic stimuli.

Mild and moderate chronic hypercapnia elicit distinct transcriptomic responses of immune function in cardiorespiratory nuclei

Chronic hypercapnia (CH) is a hallmark of respiratory-related diseases, and the level of hypercapnia can acutely or progressively become more severe. Previously, we have shown time-dependent adaptations in steady-state physiology during mild (arterial Pco2 ∼55 mmHg) and moderate (∼60 mmHg) CH in adult goats, including transient (mild CH) or sustained (moderate CH) suppression of acute chemosensitivity suggesting limitations in adaptive respiratory control mechanisms as the level of CH increases. Changes in specific markers of glutamate receptor plasticity, interleukin-1ß, and serotonergic modulation within key nodes of cardiorespiratory control do not fully account for the physiological adaptations to CH. Here, we used an unbiased approach (bulk tissue RNA sequencing) to test the hypothesis that mild or moderate CH elicits distinct gene expression profiles in important brain stem regions of cardiorespiratory control, which may explain the contrasting responses to CH. Gene expression profiles from the brain regions validated the accuracy of tissue biopsy methodology. Differential gene expression analyses revealed greater effects of CH on brain stem sites compared with the medial prefrontal cortex. Mild CH elicited an upregulation of predominantly immune-related genes and predicted activation of immune-related pathways and functions. In contrast, moderate CH broadly led to downregulation of genes and predicted inactivation of cellular pathways related to the immune response and vascular function. These data suggest that mild CH leads to a steady-state activation of neuroinflammatory pathways within the brain stem, whereas moderate CH drives the opposite response. Transcriptional shifts in immune-related functions may underlie the cardiorespiratory network's capability to respond to acute, more severe hypercapnia when in a state of progressively increased CH.NEW & NOTEWORTHY Mild chronic hypercapnia (CH) broadly upregulated immune-related genes and a predicted activation of biological pathways related to immune cell activity and the overall immune response. In contrast, moderate CH primarily downregulated genes related to major histocompatibility complex signaling and vasculature function that led to a predicted inactivation of pathways involving the immune response and vascular endothelial function. The severity-dependent effect on immune responses suggests that neuroinflammation has an important role in CH and may be important in the maintenance of proper ventilatory responses to acute and chronic hypercapnia.

Effects of sub-lethal doses of fentanyl on vital physiologic functions and withdrawal-like behaviors in adult goats

Synthetic opioids like fentanyl have improved the standard of care for many patients in the clinical setting, but their abuse leads to tens of thousands of overdose deaths annually. The current opioid epidemic underscores a critical need for insights into the physiological effects of fentanyl on vital functions. High doses of opioids in small mammals cause opioid-induced respiratory depression (OIRD) leading to hypoventilation, hypoxemia, and hypercapnia. In addition, opioids can also increase the alveolar to arterial oxygen (A-a) gradient and airway dysfunction. However, little is known about the physiologic effects of sub-lethal doses of opioids in large mammals. Here we report the effects of a sub-lethal dose range of fentanyl (25-125 μg/kg; IV) on vital physiologic functions over 90 min (min) and withdrawal-like behaviors over the subsequent 4 h (h) in adult female goats (n = 13). Fentanyl induced decreases in breathing frequency in the first few min post-injection, but then led to a sustained increase in tidal volume, total ventilation, and blood pressure with a reduced heart rate for ≥90 min. These ventilatory changes resulted in time-dependent arterial hypocapnia and hypoxemia and an increased alveolar to arterial oxygen gradient ∼30 min post-injection indicative of impaired gas exchange in the lung. The predominant effects of fentanyl on breathing were stimulatory, underscored by an increased rate of rise of the diaphragm muscle activity and increased activation of upper airway, intercostal and abdominal muscles. Beginning 90 min post-injection we also quantified withdrawal-like behaviors over 4 h, demonstrating dose- and time-dependent increases in locomotor, biting, itching, and pawing behaviors. We conclude that fentanyl at sublethal doses induces multiple physiologic and behavior changes that emerge along different time courses suggesting multiple independent mechanisms underlying effects of opioids.

Association between pulmonary ventilatory function and mild cognitive impairment: A population-based study in rural China

Background

Mild cognitive impairment (MCI), a reversible intermediate state, plays an important role in the development and prevention of dementia. The relationship between pulmonary function and MCI risk has not yet been well-elucidated.

Methods

We included 2,947 rural Chinese residents aged ≥35 years who were free from a history of stroke, dementia, or other brain diseases and measured pulmonary ventilatory function using calibrated spirometry according to the recommended method. MCI was assessed with the Montreal Cognitive Assessment-Basic for Chinese scale. Logistic regression models and restricted cubic splines with covariate adjustment were performed to explore the association between pulmonary function and MCI risk.

Results

The prevalence of MCI increased with decreasing pulmonary function, from the lowest quartile to the highest quartile of pulmonary function: 63.9, 50.5, 43.8, and 43.6%, respectively. After adjustment for confounding factors, participants in the first quartile had a significantly increased risk of MCI (ORs, 1.691, 95% CI, 1.267-2.258), with the highest quartile as the reference. In the subgroup analysis, a significant association of pulmonary function and MCI was found in females and those with low physical activity. Meanwhile, we observed an L-shaped relationship between pulmonary function and MCI (P _non-linear = 0.032).

Conclusions

Poor pulmonary function was associated with an increased risk of MCI among rural Chinese adults, and presented a non-linear relationship. These findings remind us of the need for early cognitive assessment in local populations with lower pulmonary function.

Chronic moderate hypercapnia suppresses ventilatory responses to acute CO<sub>2</sub> challenges

Chronic hypercapnia (CH) is a hallmark of chronic lung disease, and CH increases the risk for acute-on-chronic exacerbations leading to greater hypoxemia/hypercapnia and poor health outcomes. However, the role of hypercapnia per se (duration and severity) in determining an individual's ability to tolerate further hypercapnic exacerbations is unknown. Our primary objective herein was to test the hypothesis that mild-to-moderate CH (arterial [Formula: see text] ∼50-70 mmHg) increases susceptibility to pathophysiological responses to severe acute CO₂ challenges. Three groups (GR) of adult female goats were studied during 14 days of exposure to room air (<i>GR 1</i>; control) or 6% inspired CO₂ (<i>GR 2</i>; mild CH), or 7 days of 6% inspired CO₂ followed by 7 days of 8% inspired CO₂ (<i>GR 3</i>; moderate CH). Consistent with previous reports, there were no changes in physiological parameters in <i>GR 1</i> (RA control), but mild CH (<i>GR 2</i>) increased steady-state ventilation and transiently suppressed CO₂/[H⁺] chemosensitivity. Further increasing InCO₂ from 6% to 8% (<i>GR 3</i>) transiently increased ventilation and arterial [H⁺]. Similar to mild CH, moderate CH increased ventilation to levels greater than predicted. However, in contrast to mild CH, acute ventilatory chemosensitivity was suppressed throughout the duration of moderate CH, and the arterial - mixed expired CO₂ gradient became negative. These data suggest that moderate CH limits physiological responses to acute severe exacerbations and provide evidence of recruitment of extrapulmonary systems (i.e., gastric CO₂ elimination) during times of moderate-severe hypercapnia.<b>NEW &amp; NOTEWORTHY</b> Moderate levels of chronic hypercapnia (CH; ∼70 mmHg) in healthy adult female goats elicited similar steady-state physiological adaptations compared with mild CH (∼55 mmHg). However, unlike mild CH, moderate CH chronically suppressed acute CO₂/[H⁺] chemosensitivity and reversed the arterial to mixed expired CO₂ gradient. These findings suggest that moderate CH suppresses vital mechanisms of ventilatory control and recruits additional physiological systems (i.e., gastric CO₂ release) to help buffer excess CO₂.

Ventilatory neural drive in chronically hypercapnic patients with COPD: effects of sleep and nocturnal noninvasive ventilation

Sleep brings major challenges for the control of ventilation in humans, particularly the regulation of arterial carbon dioxide pressure (P _aCO2 ). In patients with COPD, chronic hypercapnia is associated with increased mortality. Therefore, nocturnal high-level noninvasive positive-pressure ventilation (NIV) is recommended with the intention to reduce P _aCO2 down to normocapnia. However, the long-term physiological consequences of P _aCO2 "correction" on the mechanics of breathing, gas exchange efficiency and resulting symptoms (i.e. dyspnoea) remain poorly understood. Investigating the influence of sleep on the neural drive to breathe and its translation to the mechanical act of breathing is of foremost relevance to create a solid rationale for the use of nocturnal NIV. In this review, we critically discuss the mechanisms by which sleep influences ventilatory neural drive and mechanical consequences in healthy subjects and hypercapnic patients with advanced COPD. We then discuss the available literature on the effects of nocturnal NIV on ventilatory neural drive and respiratory mechanics, highlighting open avenues for further investigation.

Time course and magnitude of ventilatory and renal acid-base acclimatization following rapid ascent to and residence at 3,800 m over nine days

Rapid ascent to high altitude imposes an acute hypoxic and acid-base challenge, with ventilatory and renal acclimatization countering these perturbations. Specifically, ventilatory acclimatization improves oxygenation, but with concomitant hypocapnia and respiratory alkalosis. A compensatory, renally mediated relative metabolic acidosis follows via bicarbonate elimination, normalizing arterial pH(a). The time course and magnitude of these integrated acclimatization processes are highly variable between individuals. Using a previously developed metric of renal reactivity (RR), indexing the change in arterial bicarbonate concentration (Δ[HCO3-]a; renal response) over the change in arterial pressure of CO2 (Δ[Formula: see text]; renal stimulus), we aimed to characterize changes in RR magnitude following rapid ascent and residence at altitude. Resident lowlanders (n = 16) were tested at 1,045 m (day [D]0) prior to ascent, on D2 within 24 h of arrival, and D9 during residence at 3,800 m. Radial artery blood draws were obtained to measure acid-base variables: [Formula: see text], [HCO3-]a, and pHa. Compared with D0, [Formula: see text] and [HCO3-]a were lower on D2 (P < 0.01) and D9 (P < 0.01), whereas significant changes in pHa (P = 0.072) and RR (P = 0.056) were not detected. As pHa appeared fully compensated on D2 and RR did not increase significantly from D2 to D9, these data demonstrate renal acid-base compensation within 24 h at moderate steady-state altitude. Moreover, RR was strongly and inversely correlated with ΔpHa on D2 and D9 (r≤ -0.95; P < 0.0001), suggesting that a high-gain renal response better protects pHa. Our study highlights the differential time course, magnitude, and variability of integrated ventilatory and renal acid-base acclimatization following rapid ascent and residence at high altitude.NEW & NOTEWORTHY We assessed the time course, magnitude, and variability of integrated ventilatory and renal acid-base acclimatization with rapid ascent and residence at 3,800 m. Despite reductions in [Formula: see text] upon ascent, pHa was normalized within 24 h of arrival at 3,800 m through renal compensation (i.e., bicarbonate elimination). Renal reactivity (RR) was unchanged between days 2 and 9, suggesting a lack of plasticity at moderate steady-state altitude. RR was strongly correlated with ΔpHa, suggesting that a high-gain renal response better protects pHa.

Paraquat herbicide diminishes chemoreflex sensitivity, induces cardiac autonomic imbalance and impair cardiac function in rats

Paraquat (PQT) herbicide is widely used in agricultural practices despite being highly toxic to humans. It has been proposed that PQT exposure may promote cardiorespiratory impairment. However, the physiological mechanisms involved in cardiorespiratory dysfunction following PQT exposure are poorly known. We aimed to determine the effects of PQT on ventilatory chemoreflex control, cardiac autonomic control, and cardiac function in rats. Male Sprague-Dawley rats received two injections/week of PQT (5 mg·kg-1 ip) for 4 wk. Cardiac function was assessed through echocardiography and pressure-volume loops. Ventilatory function was evaluated using whole body plethysmography. Autonomic control was indirectly evaluated by heart rate variability (HRV). Cardiac electrophysiology (EKG) and exercise capacity were also measured. Four weeks of PQT administration markedly enlarged the heart as evidenced by increases in ventricular volumes and induced cardiac diastolic dysfunction. Indeed, end-diastolic pressure was significantly higher in PQT rats compared with control (2.42 ± 0.90 vs. 4.01 ± 0.92 mmHg, PQT vs. control, P < 0.05). In addition, PQT significantly reduced both the hypercapnic and hypoxic ventilatory chemoreflex response and induced irregular breathing. Also, PQT induced autonomic imbalance and reductions in the amplitude of EKG waves. Finally, PQT administration impaired exercise capacity in rats as evidenced by a ∼2-fold decrease in times-to-fatigue compared with control rats. Our results showed that 4 wk of PQT treatment induces cardiorespiratory dysfunction in rats and suggests that repetitive exposure to PQT may induce harmful mid/long-term cardiovascular, respiratory, and cardiac consequences.NEW & NOREWORTHY Paraquat herbicide is still employed in agricultural practices in several countries. Here, we showed for the first time that 1 mo paraquat administration results in cardiac adverse remodeling, blunts ventilatory chemoreflex drive, and promotes irregular breathing at rest in previously healthy rats. In addition, paraquat exposure induced cardiac autonomic imbalance and cardiac electrophysiology alterations. Lastly, cardiac diastolic dysfunction was overt in rats following 1 mo of paraquat treatment.

Physiological and neurochemical adaptations following abrupt termination of chronic hypercapnia in goats

Chronic hypercapnia (CH) is a hallmark of respiratory diseases such as chronic obstructive pulmonary disease. In such patients, mechanical ventilation is often used to restore normal blood-gas homeostasis. However, little is known regarding physiological changes and neuroplasticity within physiological control networks after termination of CH. Utilizing our goat model of increased inspired CO₂-induced CH, we determined whether termination of CH elicits time-dependent physiological and neurochemical changes within brain stem sites of physiological control. Thirty days of CH increased [Formula: see text] (+15 mmHg) and steady-state ventilation (SS V̇i; 283% of control). Within 24 h after terminating CH, SS V̇i, blood gases, arterial [H⁺], and most physiological measurements returned to control. However, the acute ventilatory chemoreflex (ΔV̇i/Δ[H⁺]) was greater than control, and measured SS V̇i exceeded ventilation predicted by arterial [H⁺] and ΔV̇i/Δ[H⁺]. Potentially contributing to these differences were increased excitatory neuromodulators serotonin and norepinephrine in the nucleus tractus solitarius, which contrasts with minimal changes observed at 24 h and 30 days of hypercapnia. Similarly, there were minimal changes found in markers of neuroinflammation and glutamate receptor-dependent neuroplasticity upon termination of CH, which were previously increased following 24 h of hypercapnia. Thus, following termination of CH: 1) ventilatory, renal, and other physiological functions rapidly return to control; 2) neuroplasticity within the ventilatory control network may contribute to the difference between measured vs. predicted ventilation and the elevation in the acute ventilatory [H⁺] chemoreflex; and 3) neuroplasticity is fundamentally distinct from acclimatization to CH.NEW & NOTEWORTHY In healthy adult goats, steady-state ventilation and most physiological measures return to control within 24 h after termination of chronic hypercapnia (CH). However, the acute [H⁺] chemoreflex is increased, and measured ventilation exceeds predicted ventilation. At 24 h of recovery, excitatory neuromodulators are above control, but other measured markers of neuroplasticity are unchanged from control. Our data suggest that CH elicits persistent physiological and neurochemical changes for up to 24 h after termination of CH.

Air Hunger: A Primal Sensation and a Primary Element of Dyspnea

The sensation that develops as a long breath hold continues is what this article is about. We term this sensation of an urge to breathe "air hunger." Air hunger, a primal sensation, alerts us to a failure to meet an urgent homeostatic need maintaining gas exchange. Anxiety, frustration, and fear evoked by air hunger motivate behavioral actions to address the failure. The unpleasantness and emotional consequences of air hunger make it the most debilitating component of clinical dyspnea, a symptom associated with respiratory, cardiovascular, and metabolic diseases. In most clinical populations studied, air hunger is the predominant form of dyspnea (colloquially, shortness of breath). Most experimental subjects can reliably quantify air hunger using rating scales, that is, there is a consistent relationship between stimulus and rating. Stimuli that increase air hunger include hypercapnia, hypoxia, exercise, and acidosis; tidal expansion of the lungs reduces air hunger. Thus, the defining experimental paradigm to evoke air hunger is to elevate the drive to breathe while mechanically restricting ventilation. Functional brain imaging studies have shown that air hunger activates the insular cortex (an integration center for perceptions related to homeostasis, including pain, food hunger, and thirst), as well as limbic structures involved with anxiety and fear. Although much has been learned about air hunger in the past few decades, much remains to be discovered, such as an accepted method to quantify air hunger in nonhuman animals, fundamental questions about neural mechanisms, and adequate and safe methods to mitigate air hunger in clinical situations. © 2021 American Physiological Society. Compr Physiol 11:1449-1483, 2021.

Unchanged cerebrovascular CO2 reactivity and hypercapnic ventilatory response during strict head-down tilt bed rest in a mild hypercapnic environment

KEY POINTS

Carbon dioxide levels are mildly elevated on the International Space Station and it is unknown whether this chronic exposure causes physiological changes to astronauts. We combined ∼4 mmHg ambient P C O 2 with the strict head-down tilt bed rest model of spaceflight and this led to the development of optic disc oedema in one-half of the subjects. We demonstrate no change in arterialized P C O 2 , cerebrovascular reactivity to CO₂ or the hypercapnic ventilatory response. Our data suggest that the mild hypercapnic environment does not contribute to the development of spaceflight associated neuro-ocular syndrome.

ABSTRACT

Chronically elevated carbon dioxide (CO₂ ) levels can occur in confined spaces such as the International Space Station. Using the spaceflight analogue 30 days of strict 6° head-down tilt bed rest (HDTBR) in a mild hypercapnic environment ( P C O 2 = ∼4 mmHg), we investigated arterialized P C O 2 , cerebrovascular reactivity and the hypercapnic ventilatory response in 11 healthy subjects (five females) before, on days 1, 9, 15 and 30 of bed rest (BR), and 6 and 13 days after HDTBR. During all HDTBR time points, arterialized P C O 2 was not significantly different from the pre-HDTBR measured in the 6° HDT posture, with a mean (95% confidence interval) increase of 1.2 mmHg (-0.2 to 2.5 mmHg, P = 0.122) on day 30 of HDTBR. Respiratory acidosis was never detected, although a mild metabolic alkalosis developed on day 30 of HDTBR by a mean (95% confidence interval) pH change of 0.032 (0.022-0.043; P < 0.001), which remained elevated by 0.021 (0.011-0.031; P < 0.001) 6 days after HDTBR. Arterialized pH returned to pre-HDTBR levels 13 days after BR with a change of -0.001 (-0.009 to 0.007; P = 0.991). Compared to pre-HDTBR, cerebrovascular reactivity during and after HDTBR did not change. Baseline ventilation, ventilatory recruitment threshold and the slope of the ventilatory response were similar between pre-HDTBR and all other time points. Taken together, these data suggest that the mildly increased ambient P C O 2 combined with 30 days of strict 6° HDTBR did not change arterialized P C O 2 levels. Therefore, the experimental conditions were not sufficient to elicit a detectable physiological response.

Adaptation of Respiratory-Related Brain Regions to Long-Term Hypercapnia: Focus on Neuropeptides in the RTN

Long-term hypercapnia is associated with respiratory conditions including obstructive sleep apnea, chronic obstructive pulmonary disease and obesity hypoventilation syndrome. Animal studies have demonstrated an initial (within hours) increase in ventilatory drive followed by a decrease in this response over the long-term (days–weeks) in response hypercapnia. Little is known about whether changes in the central respiratory chemoreflex are involved. Here we investigated whether central respiratory chemoreceptor neurons of the retrotrapezoid nucleus (RTN), which project to the respiratory pattern generator within the ventral respiratory column (VRC) have a role in the mechanism of neuroplasticity associated with long-term hypercapnia. Adult male C57BL/6 mice (n = 5/group) were used. Our aims were (1) to determine if galanin, neuromedin B and gastrin-releasing peptide gene expression is altered in the RTN after long-term hypercapnia. This was achieved using qPCR to measure mRNA expression changes of neuropeptides in the RTN after short-term hypercapnia (6 or 8 h, 5 or 8% CO₂) or long-term hypercapnia exposure (10 day, 5 or 8% CO₂), (2) in the mouse brainstem, to determine the distribution of preprogalanin in chemoreceptors, and the co-occurrence of the galanin receptor 1 (GalR1:Gi-coupled receptor) with inhibitory GlyT2 ventral respiratory column neurons using in situ hybridization (ISH) to better characterize galaninergic RTN-VRC circuitry, (3) to investigate whether long-term hypercapnia causes changes to recruitment (detected by cFos immunohistochemistry) of respiratory related neural populations including the RTN neurons and their galaninergic subset, in vivo. Collectively, we found that hypercapnia decreases neuropeptide expression in the RTN in the short-term and has the opposite effect over the long-term. Following long term hypercapnia, the number of RTN galanin neurons remains unchanged, and their responsiveness to acute chemoreflex is sustained; in contrast, we identified multiple respiratory related sites that exhibit blunted chemoreflex activation. GalR1 was distributed in 11% of preBötC and 30% of BötC glycinergic neurons. Our working hypothesis is that during long-term hypercapnia, galanin co-release from RTN neurons may counterbalance glutamatergic inputs to respiratory centers to downscale energetically wasteful hyperventilation, thereby having a role in neuroplasticity by contributing to a decrease in ventilation, through the inhibitory effects of galanin.

Midbrain and cerebral inflammatory and glutamatergic adaptations during chronic hypercapnia in goats

Cognitive impairment is associated with multiple human diseases that have in common chronic hypercapnia. However, the mechanisms leading to chronic hypercapnia-induced cognitive decline are not known. We have previously shown chronic hypercapnia through exposure to increased inspired CO₂ (6% InCO₂) in conscious goats caused an immediate (within hours) and sustained decline in cognitive performance during a shape discrimination test. Herein, within the same goats, we assessed markers of neuroinflammation and glutamate receptor expression/phosphorylation within CNS regions important for cognitive function following 24 hours (h) or 30 days (d) of chronic hypercapnia. Within 24 h, chronic hypercapnia increased expression of the inflammatory cytokine IL-1β in the orbitofrontal cortex and medial prefrontal cortex, but at 30d IL-1β levels were not different relative to time-matched goats exposed to room-air. Additionally, Iba1 expression (a marker of microglial activation) was unaltered by chronic hypercapnia in all regions tested. Finally, levels of the total and phosphorylated AMPA receptor subunit GluR₂ were reduced within the hippocampus at both 24 h and 30 d of hypercapnia, and reduced following 30 d within the anterior insular cortex. These data suggest that chronic hypercapnia leads to CNS site-dependent acute inflammatory responses and shifts in select glutamate receptor expression/phosphorylation in brain regions contributing to cognitive function. Such changes may be indicative of alterations in glutamatergic receptor-mediated signaling and neuronal dysfunction that contribute to declines in cognitive function associated with human diseases defined or marked by chronic CO₂ retention.

Brainstem serotonergic, catecholaminergic, and inflammatory adaptations during chronic hypercapnia in goats

Despite the prevalence of CO₂ retention in human disease, little is known about the adaptive neurobiological effects of chronic hypercapnia. We have recently shown 30-d exposure to increased inspired CO₂ (InCO₂) leads to a steady-state ventilation that exceeds the level predicted by the sustained acidosis and the acute CO₂/H⁺ chemoreflex, suggesting plasticity within respiratory control centers. Based on data showing brainstem changes in aminergic and inflammatory signaling during carotid body denervation-induced hypercapnia, we hypothesized chronic hypercapnia per se will lead to similar changes. We found that: 1) increased InCO₂ increased IL-1β in the medullary raphe (MR), ventral respiratory column, and cuneate nucleus after 24 h, but not after 30 d of hypercapnia; 2) the number of serotonergic and total neurons were reduced within the MR and ventrolateral medulla following 30 d of increased InCO₂; 3) markers of tryptophan metabolism were altered following 24 h, but not 30 d of InCO₂; and 4) there were few changes in brainstem amine levels following 24 h or 30 d of increased InCO₂. We conclude that these changes may contribute to initiating or maintaining respiratory neuroplasticity during chronic hypercapnia but alone do not account for ventilatory acclimatization to chronic increased InCO₂.-Burgraff, N. J., Neumueller, S. E., Buchholz, K. J., LeClaire, J., Hodges, M. R., Pan, L., Forster, H. V. Brainstem serotonergic, catecholaminergic, and inflammatory adaptations during chronic hypercapnia in goats.

Glutamate receptor plasticity in brainstem respiratory nuclei following chronic hypercapnia in goats

Patients that retain CO2 in respiratory diseases such as chronic obstructive pulmonary disease (COPD) have worse prognoses and higher mortality rates than those with equal impairment of lung function without hypercapnia. We recently characterized the time-dependent physiologic effects of chronic hypercapnia in goats, which suggested potential neuroplastic shifts in ventilatory control mechanisms. However, little is known about how chronic hypercapnia affects brainstem respiratory nuclei (BRN) that control multiple physiologic functions including breathing. Since many CNS neuroplastic mechanisms include changes in glutamate (AMPA (GluR) and NMDA (GluN)) receptor expression and/or phosphorylation state to modulate synaptic strength and network excitability, herein we tested the hypothesis that changes occur in glutamatergic signaling within BRN during chronically elevated inspired CO2 (InCO2 )-hypercapnia. Healthy goats were euthanized after either 24 h or 30 days of chronic exposure to 6% InCO2 or room air, and brainstems were rapidly extracted for western blot analyses to assess GluR and GluN receptor expression within BRN. Following 24-hr exposure to 6% InCO2 , GluR or GluN receptor expression were changed from control (P < 0.05) in the solitary complex (NTS & DMV),ventrolateral medulla (VLM), medullary raphe (MR), ventral respiratory column (VRC), hypoglossal motor nucleus (HMN), and retrotrapezoid nucleus (RTN). These neuroplastic changes were not found following 30 days of chronic hypercapnia. However, at 30 days of chronic hypercapnia, there was overall increased (P < 0.05) expression of glutamate receptors in the VRC and RTN. We conclude that time- and site-specific glutamate receptor neuroplasticity may contribute to the concomitant physiologic changes that occur during chronic hypercapnia.