Respiratory plasticity after perinatal hypercapnia in rats

Generation of a whole-brain hemodynamic response function and sex-specific differences in cerebral processing of mechano-sensation in mice detected by BOLD fMRI

BOLD fMRI has become a prevalent method to study cerebral sensory processing in rodent disease models, including pain and mechanical hypersensitivity. fMRI data analysis is frequently combined with a general-linear-model (GLM) -based analysis, which uses the convolution of a hemodynamic response function (HRF) with the stimulus paradigm. However, several studies indicated that the HRF differs across species, sexes, brain structures, and experimental factors, including stimulation modalities or anesthesia, and hence might strongly affect the outcome of BOLD analyzes. While considerable work has been done in humans and rats to understand the HRF, much less is known in mice. As a prerequisite to investigate mechano-sensory processing and BOLD fMRI data in male and female mice, we (1) designed a rotating stimulator that allows application of two different mechanical modalities, including innocuous von Frey and noxious pinprick stimuli and (2) determined and statistically compared HRFs across 30 brain structures and experimental conditions, including sex and, stimulus modalities. We found that mechanical stimulation lead to brain-wide BOLD signal changes thereby allowing extraction of HRFs from multiple brain structures. However, we did not find differences in HRFs across all brain structures and experimental conditions. Hence, we computed a whole-brain mouse HRF, which is based on 88 functional scans from 30 mice. A comparison of this mouse-specific HRF with our previously reported rat-derived HRF showed significantly slower kinetics in mice. Finally, we detected pronounced differences in cerebral BOLD activation between male and female mice with mechanical stimulation, thereby exposing divergent processing of noxious and innocuous stimuli in both sexes.

Development and Characterization of Inhaled Ethanol as a Novel Pharmacological Strategy Currently Evaluated in a Phase II Clinical Trial for Early-Stage SARS-CoV-2 Infection

Inhaled administration of ethanol in the early stages of COVID-19 would favor its location on the initial replication sites, being able to reduce the progression of the disease and improving its prognosis. Before evaluating the efficacy and safety of this novel therapeutic strategy in humans, its characterization is required. The developed 65° ethanol formulation is stable at room temperature and protected from light for 15 days, maintaining its physicochemical and microbiological properties. Two oxygen flows have been tested for its administration (2 and 3 L/min) using an automated headspace gas chromatographic analysis technique (HS-GC-MS), with that of 2 L/min being the most appropriate one, ensuring the inhalation of an ethanol daily dose of 33.6 ± 3.6 mg/min and achieving more stable concentrations during the entire treatment (45 min). Under these conditions of administration, the formulation has proven to be safe, based on histological studies of the respiratory tracts and lungs of rats. On the other hand, these results are accompanied by the first preclinical molecular imaging study with radiolabeled ethanol administered by this route. The current ethanol formulation has received approval from the Spanish Agency of Medicines and Medical Devices for a phase II clinical trial for early-stage COVID-19 patients, which is currently in the recruitment phase (ALCOVID-19; EudraCT number: 2020-001760-29).

Respiratory development in burrowing rodents: Effect of perinatal hypercapnia

Burrowing rodents have a blunted hypercapnic ventilatory response compared to non-burrowing rodents, but semi-fossorial ground squirrels and hamsters are not born with this blunted response when raised in room conditions. This study examined the hypercapnic ventilatory response of rats, hamsters, and ground squirrels raised in burrow-like hypercapnia (∼3 % CO₂) through development (embryonic day 16-18 to postnatal day 30) to determine if chronic hypercapnia exerts any effect on the developing and adult semi-fossorial response. Chronic hypercapnia attenuated the ventilatory response to 5 % CO₂ by 60 % (rats), 150 % (hamsters), and 70 % (squirrels) in newborns when compared to newborns raised in normal conditions. When raised in burrow conditions, squirrels and hamsters reached the blunted adult response ∼8-12 days sooner in development than their room air counterparts, while burrow-reared rats maintained a consistently blunted response until removal from chronic hypercapnia. Our study revealed no lasting effect of chronic hypercarbia on the ventilatory responses to CO₂ in burrowing rodents, but rather a change in the developmental profile such that the blunted adult response was reached earlier in development.

Cardiorespiratory and thermal responses to hypercapnia in chickens exposed to CO2 during embryonic development

The concentration of CO₂ in the environment surrounding the embryo impacts development and may also influence the cardiorespiratory responses after hatching. Therefore, we aimed to evaluate the cardiorespiratory and thermal responses to hypercapnia in chicks that were exposed to CO₂ during embryonic development, i.e., incubation. Embryos were incubated without and with a gradual increase in CO₂ concentration up to 1 % during the first ten days of incubation. Ten-day-old chicks (males and females) were again acutely exposed to hypercapnia (7 % CO₂), or to room air (normocapnia) and pulmonary ventilation, arterial pH and blood gases, arterial blood pressure and heart rate, body temperature (Tb) and oxygen consumption (V⋅O₂) were measured. Compared to control animals, male chicks incubated with 1 % CO₂ presented an attenuated ventilatory response to hypercapnia (P < 0.05), whereas no difference was found in the hypercapnic ventilatory response in both female chick groups (0 % vs 1 % CO₂ incubation). Hypercapnia induced bradycardia in all groups (P < 0.001). The CO₂ exposure during incubation did not alter the cardiovascular responses to hypercapnia in post-hatch animals. There were no significant effects of incubation treatment (0 % vs 1 % CO₂) or sex in the mean arterial pressure, Tb, and V⋅O₂ of animals in normocapnia and hypercapnia. As for the V⋅_E/V⋅O₂, hypercapnia caused an increase in both groups (P < 0.05), regardless of incubation treatment. In conclusion, among cardiorespiratory and metabolic variables, the ventilatory response to hypercapnia can be attenuated by pre-exposure to 1 % CO₂ during embryonic development, especially in male chicks up to 10 days.

Rodent models of respiratory control and respiratory system development-Clinical significance

The newborn infant's respiratory system must rapidly adapt to extra-uterine life. Neonatal rat and mouse models have been used to investigate early development of respiratory control and reactivity in both health and disease. This review highlights several rodent models of control of breathing and respiratory system development (including pulmonary function), discusses their translational strengths and limitations, and underscores the importance of creating clinically relevant models applicable to the human infant.

Combined effects of intermittent hyperoxia and intermittent hypercapnic hypoxia on respiratory control in neonatal rats

Chronic exposure to intermittent hyperoxia causes abnormal carotid body development and attenuates the hypoxic ventilatory response (HVR) in neonatal rats. We hypothesized that concurrent exposure to intermittent hypercapnic hypoxia would influence this plasticity. Newborn rats were exposed to alternating bouts of hypercapnic hypoxia (10% O₂/6% CO₂) and hyperoxia (30-40% O₂) (5 cycles h^-1, 24 h d^-1) through 13-14 days of age; the experiment was run twice, once in a background of 21% O₂ and once in a background of 30% O₂ (i.e., "relative hyperoxia"). Hyperoxia had only small effects on carotid body development when combined with intermittent hypercapnic hypoxia: the carotid chemoafferent response to hypoxia was reduced, but this did not affect the HVR. In contrast, sustained exposure to 30% O₂ reduced carotid chemoafferent activity and carotid body size which resulted in a blunted HVR. When given alone, chronic intermittent hypercapnic hypoxia increased carotid body size and reduced the hypercapnic ventilatory response but did not affect the HVR. Overall, it appears that intermittent hypercapnic hypoxia counteracted the effects of hyperoxia on the carotid body and prevented developmental plasticity of the HVR.

Intermittent hypercapnia enhances CO₂ responsiveness and overcomes serotonergic dysfunction

Serotonergic dysfunction compromises ventilatory chemosensitivity and may enhance vulnerability to pathologies such as the Sudden Infant Death Syndrome (SIDS). We have shown raphé contributions to central chemosensitivity involving serotonin (5-HT)-and γ-aminobutyric acid (GABA)-mediated mechanisms. We tested the hypothesis that mild intermittent hypercapnia (IHc) induces respiratory plasticity, due in part to strengthening of GABA mechanisms. Rat pups were IHc-pretreated (eight consecutive cycles; 5 min 5% CO2 - air, 10 min air) or constant normocapnia-pretreated as a control, each day for 5 consecutive days beginning at P12. We subsequently assessed CO2 responsiveness using the in situ perfused brainstem preparation. Hypercapnic responses were determined with and without pharmacological manipulation. Results show IHc-pretreatment induces plasticity sufficient for responsiveness despite removal of otherwise critical ketanserin-sensitive mechanisms. Responsiveness following IHc-pretreatment was absent if ketanserin was combined with GABAergic antagonism, indicating that plasticity depends on GABAergic mechanisms. We propose that IHc-induced plasticity could reduce the severity of reflex dysfunctions underlying pathologies such as SIDS.

Peripheral-central chemoreceptor interaction and the significance of a critical period in the development of respiratory control

Respiratory control entails coordinated activities of peripheral chemoreceptors (mainly the carotid bodies) and central chemosensors within the brain stem respiratory network. Candidates for central chemoreceptors include Phox2b-containing neurons of the retrotrapezoid nucleus, serotonergic neurons of the medullary raphé, and/or multiple sites within the brain stem. Extensive interconnections among respiratory-related nuclei enable central chemosensitive relay. Both peripheral and central respiratory centers are not mature at birth, but undergo considerable development during the first two postnatal weeks in rats. A critical period of respiratory development (∼P12-P13 in the rat) exists when abrupt neurochemical, metabolic, ventilatory, and electrophysiological changes occur. Environmental perturbations, including hypoxia, intermittent hypoxia, hypercapnia, and hyperoxia alter the development of the respiratory system. Carotid body denervation during the first two postnatal weeks in the rat profoundly affects the development and functions of central respiratory-related nuclei. Such denervation delays and prolongs the critical period, but does not eliminate it, suggesting that the critical period may be intrinsically and genetically determined.

Newborn ventilatory response to maternal chronic hypercapnia

This is a case of a neonate born with a respiratory acidosis with a compensatory metabolic alkalosis. This case demonstrates placental physiology of gas exchange as well as the blunted ventilatory response in the neonate from chronic hypercapnia.

Long-term effects of the perinatal environment on respiratory control

The respiratory control system exhibits considerable plasticity, similar to other regions of the nervous system. Plasticity is a persistent change in system behavior triggered by experiences such as changes in neural activity, hypoxia, and/or disease/injury. Although plasticity is observed in animals of all ages, some forms of plasticity appear to be unique to development (i.e., “developmental plasticity”). Developmental plasticity is an alteration in respiratory control induced by experiences during “critical” developmental periods; similar experiences outside the critical period will have little or no lasting effect. Thus complementary experiments on both mature and developing animals are generally needed to verify that the observed plasticity is unique to development. Frequently studied models of developmental plasticity in respiratory control include developmental manipulations of respiratory gas concentrations (O2and CO2). Environmental factors not specifically associated with breathing may also trigger developmental plasticity, however, including psychological stress or chemicals associated with maternal habits (e.g., nicotine, cocaine). Despite rapid advances in describing models of developmental plasticity in breathing, our understanding of fundamental mechanisms giving rise to such plasticity is poor; mechanistic studies of developmental plasticity are of considerable importance. Developmental plasticity may enable organisms to “fine tune” their phenotype to optimize the performance of this critical homeostatic regulatory system. On the other hand, developmental plasticity could also increase the risk of disease later in life. Future directions for studies concerning the mechanisms and functional implications of developmental plasticity in respiratory motor control are discussed.

Population genetic aspects and phenotypic plasticity of ventilatory responses in high altitude natives

Highland natives show unique breathing patterns and ventilatory responses at altitude, both at rest and during exercise. For many ventilatory traits, there is also significant variation between highland native groups, including indigenous populations in the Andes and Himalaya, and more recent altitude arrivals in places like Colorado. This review summarizes the literature in this area with some focus on partitioning putative population genetic differences from differences acquired through lifelong exposure to hypoxia. Current studies suggest that Tibetans have high resting ventilation (V (E)), and a high hypoxic ventilatory response (HVR), similar to altitude acclimatized lowlanders. Andeans, in contrast, show low resting V (E) and a low or "blunted" HVR, with little evidence that these traits are acquired via lifelong exposure. Resting V (E) of non-indigenous altitude natives is not well documented, but lifelong hypoxic exposure almost certainly blunts HVR in these groups through decreased chemosensitivity to hypoxia in a process known as hypoxic desensitization (HD). Together, these studies suggest that the time course of ventilatory response, and in particular the origin or absence of HD, depends on population genetic background i.e., the allele or haplotype frequencies that characterize a particular population. During exercise, altitude natives have lower V (E) compared to acclimatized lowland controls. Altitude natives also have smaller alveolar-arterial partial pressure differences P(AO2) - P(aO2) during exercise suggesting differences in gas exchange efficiency. Small P(AO2) - P(aO2) in highland natives of Colorado underscores the likely importance of developmental adaptation to hypoxia affecting structural/functional aspects of gas exchange with resultant changes in breathing pattern. However, in Andeans, at least, there is also evidence that low exercise V (E) is determined by genetic background affecting ventilatory control independent of gas exchange. Additional studies are needed to elucidate the effects of gene, environment, and gene-environment interaction on these traits, and these effects are likely to differ widely between altitude native populations.

Abnormal circulatory stress responses of preterm graduates

Preterm birth and chronic lung disease may increase the risk of hypertension and cardiovascular disease in infancy and adolescence. Here we looked for evidence of early circulatory dysfunction associated with these perinatal complications. We compared infants born at term (n = 12) with those born preterm with an uncomplicated neonatal course (n = 12) or diagnosed with bronchopulmonary dysplasia (BPD) (n = 10). We measured blood pressure (BP) (Finometer), and heart rate (HR) responses to 4 min of breathing 4% CO2 during quiet sleep. Hypercapnia accelerated HR and increased BP of term infants. Preterm infants either (i) had an exaggerated pressor but little or no HR response to CO2 (healthy or mild-moderate BPD) or (ii) had a diminished pressor response and accompanying decrease in HR (severe BPD). Short-term reflex cardiovascular control was consequently altered by premature birth, with potentially more serious aberrations associated with severe BPD. Most anomalies had not resolved by the time infants born preterm reached term age; some may be early signs of emerging long-term cardiovascular dysfunction.