Developmental changes in cardio-respiratory responses to hypoxia and hypercapnia in anesthetized low-birth-weight rats

Perinatal Hypoxemia and Oxygen Sensing

The development of the control of breathing begins in utero and continues postnatally. Fetal breathing movements are needed for establishing connectivity between the lungs and central mechanisms controlling breathing. Maturation of the control of breathing, including the increase of hypoxia chemosensitivity, continues postnatally. Insufficient oxygenation, or hypoxia, is a major stressor that can manifest for different reasons in the fetus and neonate. Though the fetus and neonate have different hypoxia sensing mechanisms and respond differently to acute hypoxia, both responses prevent deviations to respiratory and other developmental processes. Intermittent and chronic hypoxia pose much greater threats to the normal developmental respiratory processes. Gestational intermittent hypoxia, due to maternal sleep-disordered breathing and sleep apnea, increases eupneic breathing and decreases the hypoxic ventilatory response associated with impaired gasping and autoresuscitation postnatally. Chronic fetal hypoxia, due to biologic or environmental (i.e. high-altitude) factors, is implicated in fetal growth restriction and preterm birth causing a decrease in the postnatal hypoxic ventilatory responses with increases in irregular eupneic breathing. Mechanisms driving these changes include delayed chemoreceptor development, catecholaminergic activity, abnormal myelination, increased astrocyte proliferation in the dorsal respiratory group, among others. Long-term high-altitude residents demonstrate favorable adaptations to chronic hypoxia as do their offspring. Neonatal intermittent hypoxia is common among preterm infants due to immature respiratory systems and thus, display a reduced drive to breathe and apneas due to insufficient hypoxic sensitivity. However, ongoing intermittent hypoxia can enhance hypoxic sensitivity causing ventilatory overshoots followed by apnea; the number of apneas is positively correlated with degree of hypoxic sensitivity in preterm infants. Chronic neonatal hypoxia may arise from fetal complications like maternal smoking or from postnatal cardiovascular problems, causing blunting of the hypoxic ventilatory responses throughout at least adolescence due to attenuation of carotid body fibers responses to hypoxia with potential roles of brainstem serotonin, microglia, and inflammation, though these effects depend on the age in which chronic hypoxia initiates. Fetal and neonatal intermittent and chronic hypoxia are implicated in preterm birth and complicate the respiratory system through their direct effects on hypoxia sensing mechanisms and interruptions to the normal developmental processes. Thus, precise regulation of oxygen homeostasis is crucial for normal development of the respiratory control network. © 2021 American Physiological Society. Compr Physiol 11:1653-1677, 2021.

Impaired cardiorespiratory responses to hypercapnia in neonatal mice lacking PAC1 but not VPAC2 receptors

The evidence is mounting for a role for abnormal signaling of the stress peptide pituitary adenylate cyclase activating polypeptide (PACAP) and its canonical receptor PAC1 in the pathogenesis of sudden infant death syndrome. In this study, we investigated whether the PACAP receptors PAC1 or VPAC2 are involved in the neonatal cardiorespiratory response to hypercapnic stress. We used head-out plethysmography and surface ECG electrodes to assess cardiorespiratory responses to an 8% hypercapnic challenge in unanesthetized and spontaneously breathing 4-day-old PAC1 or VPAC2 knockout (KO) and wild-type mouse pups. We demonstrate that compared with WTs, breathing frequency (RR) and minute ventilation ([Formula: see text]) in PAC1 KO pups were significantly blunted in response to hypercapnia. Although heart rate was unaltered in PAC1 KO pups during hypercapnia, heart rate recovery posthypercapnia was impaired. In contrast, cardiorespiratory impairments in VPAC2 KO pups were limited to only an overall higher tidal volume (V_T), independent of treatment. These findings suggest that PACAP signaling through the PAC1 receptor plays a more important role than signaling through the VPAC2 receptor in neonatal respiratory responses to hypercapnia. Thus deficits in PACAP signaling primarily via PAC1 may contribute to the inability of infants to mount an appropriate protective response to homeostatic stressors in childhood disorders such as SIDS.

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Changes in MRI signal intensity during hypercapnic challenge under conscious and anesthetized conditions

Most functional magnetic resonance imaging (fMRI) studies in animals are conducted under anesthesia to minimize motion artifacts. However, methods and techniques have been developed recently for imaging fully conscious rats. Functional MRI studies on conscious animals report enhanced BOLD signal changes as compared to the anesthetized condition. In this study, rats were exposed to different concentrations of carbon dioxide (CO(2)) while conscious and anesthetized to test whether cerebrovascular reactivity may be contributing to these enhanced BOLD signal changes. Hypercapnia produced significantly greater increases in MRI signal intensity in fully conscious animals (6.7-13.3% changes) as when anesthetized with 1% isoflurane (3.2-4.9% changes). In addition, the response to hypercapnia was more immediate in the conscious condition (< 30s) with signal risetimes twice as fast as in the anesthetized state (60s). Both cortical and subcortical brain regions showed a robust, dose- dependent increase in MRI signal intensity with hypercapnic challenge while the animals were conscious but little or no change when anesthetized. Baseline variations in MRI signal were higher while animals were conscious but this was off set by greater signal intensity changes leading to a greater contrast-to-noise ratio, 13.1 in conscious animals, as compared to 8.0 in the anesthetized condition. In summary, cerebral vasculature appears to be more sensitive to hypercapnic challenge in the conscious condition resulting in enhanced T2* MRI signal intensity and the potential for better BOLD signal changes during functional imaging.

Development of CO(2)-response in the early newborn period in rat

We examined the respiratory response to moderate hypercapnia in rat pups during the first 10 days after birth and also studied immediate early gene expression to investigate whether areas described as chemosensitive in the adult rat are activated also in the early postnatal period. Breathing frequency increased in 1- and 3-day-old pups, but decreased in older animals in response to hypercapnia. Tidal volume and ventilation increased significantly in all age groups but relatively more in the 10-day-old pups as compared to younger animals. In situ hybridisation for c-fos mRNA revealed increased expression in several of the areas assigned as chemosensitive in the adult, including the caudal nucleus tractus solitarii and the ventral lateral medulla. In contrast, locus coeruleus and the majority of midline raphe neurons did not exhibit increased expression of c-fos mRNA. We conclude that the hypercapnic respiratory response tends to decrease during the first postnatal week, but thereafter increases on day 10 due to increased tidal volumes rather than changes in respiratory timing. We also speculate that differences in activation of chemosensitive brainstem neurons may be part of the maturation of the hypercapnic ventilatory response.

Developmental changes of (3)H-labelled mu-opioid receptors in brainstems of intra-uterine growth-restricted rats

The opioid mu-system is involved in brainstem-mediated respiratory control. Infants with intra-uterine growth restriction (IUGR) have more respiratory disorders in the early postnatal period. Using [(3)H]DAGO, a mu-selective ligand, and a computer-based image analysis of autoradiography, we compared the ontogeny and distribution of mu-opioid binding sites in the brainstem of IUGR and control rats in utero (E21), at birth (P0) and on postnatal days 1 (P1), P7, P10, P14 and P21. The ontogeny pattern was found to be similar in both groups. The density of the binding sites, which was low in E21, increased at P0, slightly declined at P1 and remained relatively constant thereafter. The distribution of DAGO-binding sites, also similar in both groups, was heterogeneous and was much denser in the dorsal areas of medulla and pons. In particular, binding sites were highly concentrated in nuclei involved in the cardio-respiratory function. However, DAGO-binding density was higher at all ages (except for P0 and P1) in IUGR than in control rats. Taken together, these results give at least a partial explanation for the effects of IUGR which lowers the Apgar score at birth and raises the incidence of respiratory disorders in infants.