Do genes on rat chromosomes 9, 13, 16, 18, and 20 contribute to regulation of breathing?

Systemic Oxygen Transport with Rest, Exercise, and Hypoxia: A Comparison of Humans, Rats, and Mice

The objective of this article is to compare and contrast the known characteristics of the systemic O₂ transport of humans, rats, and mice at rest and during exercise in normoxia and hypoxia. This analysis should help understand when rodent O₂ transport findings can-and cannot-be applied to human responses to similar conditions. The O₂ -transport system was analyzed as composed of four linked conductances: ventilation, alveolo-capillary diffusion, circulatory convection, and tissue capillary-cell diffusion. While the mechanisms of O₂ transport are similar in the three species, the quantitative differences are naturally large. There are abundant data on total O₂ consumption and on ventilatory and pulmonary diffusive conductances under resting conditions in the three species; however, there is much less available information on pulmonary gas exchange, circulatory O₂ convection, and tissue O₂ diffusion in mice. The scarcity of data largely derives from the difficulty of obtaining blood samples in these small animals and highlights the need for additional research in this area. In spite of the large quantitative differences in absolute and mass-specific O₂ flux, available evidence indicates that resting alveolar and arterial and venous blood PO₂ values under normoxia are similar in the three species. Additionally, at least in rats, alveolar and arterial blood PO₂ under hypoxia and exercise remain closer to the resting values than those observed in humans. This is achieved by a greater ventilatory response, coupled with a closer value of arterial to alveolar PO₂ , suggesting a greater efficacy of gas exchange in the rats. © 2018 American Physiological Society. Compr Physiol 8:1537-1573, 2018.

Acute and chronic effects of carotid body denervation on ventilation and chemoreflexes in three rat strains

Brown Norway (BN) rats have a relatively specific deficit in CO2 sensitivity. This deficit could be due to an abnormally weak carotid body contribution to CO2 sensitivity. Accordingly, we tested the hypothesis that CBD would have less of an effect on eupnoeic breathing and CO2 sensitivity in the BN rats compared to other rat strains.We measured ventilation and blood gases at rest (eupnoea) and during hypoxia (FIO2 =0.12) or hypercapnia (FICO2 =0.07) before and up to 23 days after bilateral or Sham CBD in BN, Sprague–Dawley (SD) and Dahl Salt-Sensitive (SS) rats. In all three rat strains, CBD elicited eupnoeic hypoventilation (PaCO2 +8.7–11.0 mmHg) 1–2 days post-CBD (P <0.05), and attenuated ventilatory responses to hypoxia (P <0.05) and venous sodium cyanide (NaCN; P<0.05), while sham CBD had no effect on resting breathing, blood gases or chemoreflexes (P >0.05). In contrast, CBD had no effect on CO2 sensitivity (˙VE/PaCO2) in all strains (P>0.05). Eupnoeic PaCO2 returned to pre-CBD values within 15–23 days post-CBD. Thus, the effects of CBD in rats (1) further support an important role for the carotid bodies in eupnoeic blood gas regulation, (2) suggest that the carotid bodies are not a major determinant of CO2 sensitivity in rats, and (3) may not support the concept of an interaction among the peripheral and central chemoreceptors in rats.

Vascular responses in aortic rings of a consomic rat panel derived from the Fawn Hooded Hypertensive strain

The present experiments, utilizing the high-throughput vascular protocol of PhysGen (Program for Genomic Applications) characterized the responses of aortic rings to vasoconstrictor (phenylephrine) and vasodilator (acetylcholine, sodium nitroprusside, and reduced tissue bath Po(2)) stimuli in consomic rat strains derived from a cross between the Fawn Hooded Hypertensive rat (FHH/EurMcwi) and the Brown Norway normotensive (BN/NHsdMcwi) rat. The effects of substituting individual BN chromosomes into the FHH genetic background were determined in animals that were maintained on a low-salt (0.4% NaCl) diet or switched to a high-salt (4% NaCl) diet for 3 wk. Sex-specific differences were evaluated in male and female consomic rats on similar dietary salt intake. Multiple chromosomes affected various vascular reactivity phenotypes in the FHH × BN consomic panel, and substantial salt-dependent changes in vascular reactivity and sex-specific differences in aortic reactivity were observed in individual consomic strains. However, compared with earlier studies of consomic rats derived from a cross between the BN rat and the Dahl salt-sensitive (SS) rat, only 3-7% of the vascular phenotypes were affected in a similar manner by substituting specific BN chromosomeschromosomes into the FHH genetic background versus the SS genetic background. The findings of the present study stress the potential value of consomic rat panels in gaining insight into genetic factors influencing vascular reactivity and suggest that the chromosomes that appear to be involved in the determination of aortic ring reactivity in different rodent models of hypertension are highly strain- and sex specific.

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.

Differences between three inbred rat strains in number of K+ channel-immunoreactive neurons in the medullary raphé nucleus

Ventilatory sensitivity to hypercapnia is greater in Dahl salt-sensitive (SS) rats than in Fawn Hooded hypertensive (FHH) and Brown Norway (BN) inbred rats. Since pH-sensitive potassium ion (K(+)) channels are postulated to contribute to the sensing and signaling of changes in CO(2)-H(+) in chemosensitive neurons, we tested the hypothesis that there are more pH-sensitive K(+) channel-immunoreactive (ir) neurons within the medullary raphé nuclei of the highly chemosensitive SS rats than in the other two strains. Medullary tissues from male and female BN, FHH, and SS rats were stained with cresyl violet or with antibodies targeting TASK-1, K(v)1.4, and Kir2.3 channels. K(+) channel-ir neurons were quantified and compared with the total neurons in the region. The total number of neurons in the medullary raphé 1) was greater in male FHH than the other male rats, 2) did not differ among the female rats, and 3) did not differ between sexes. The average number of K(+) channel-ir neurons per section was 30-60 neurons higher in the male SS than in the other rat strains. In contrast, for the females, the number of K(+) channel-ir neurons was greatest in the BN. We also found significant differences in the number of K(+) channel-ir neurons between sexes in SS (males > females) and BN (females > males) rats, but not the FHH strain. Our findings support the hypothesis for males but not for females, suggesting that both genetic background and sex are determinants of K(+) channel immunoreactivity of medullary raphé neurons, and that the expression of pH-sensitive K(+) channels in the medullary raphé does not correlate with the ventilatory sensitivity to hypercapnia.

Domperidone and ventilatory behavior: Sprague–Dawley versus Brown Norway rats

Domperidone, a dopamine D(2) receptor antagonist, is a tool for uncovering the tonic and dynamic effects of the peripheral dopaminergic system in unanesthestized animals. The hypothesis was that domperidone effects would vary between strains of the same species. Ventilatory behavior -- frequency and minute ventilation -- was measured by the plethysmographic method in unrestrained adult male Sprague-Dawley (SD: n=8) and Brown Norway (BN: n=8) rats before, during and after rapid transition to 100% O(2) after 5 min of 13% O(2)/3% CO(2). Tests were done 60 min after intraperitoneal injection of either vehicle (0.1% lactic acid in saline) or a dose of domperidone (0.1, 0.5, 1.0, or 5.0mg/kg) dissolved in vehicle, each on a separate day. Resting frequency and minute ventilation (mean+/-standard deviation) decreased after domperidone in the BN strain (e.g. 94.63/min+/-4.99 versus 87.37/min+/-9.59, p=0.42; 77.3 ml/min+/-9.25 versus 62.13 ml/min+/-11.5, p=0.019, respectively), but did not change in the SD. With increasing doses of domperidone the ventilatory response to hypoxia and reoxygenation became similar owing to a decrease in frequency and minute ventilation in the SD. At a dose altering SD hypoxic responses, the hypercapnic ventilatory response was not significantly affected. In conclusion, breathing frequency and minute ventilation over a challenge with hypoxia and reoxygenation differ with domperidone depending upon genetic background. We speculate that hypoxic ventilatory responses may be differently configured even among strains of the same species.

Postnatal developmental changes in CO2 sensitivity in rats

Ventilatory sensitivity to CO2 in awake adult Brown Norway (BN) rats is 50–75% lower than in adult Sprague-Dawley (SD) and salt-sensitive Dahl S (SS) rats. The purpose of the present study was to test the hypothesis that this difference would be apparent during the development of CO2 sensitivity. Four litters of each strain were divided into four groups such that rats were exposed to 7% inspired CO2 for 5 min in a plethysmograph every third day from postnatal day (P) 0 to P21 and again on P29 and P30. From P0 to P14, CO2 exposure increased pulmonary ventilation (V̇e) by 25–50% in the BN and SD strains and between 25 to over 200% in the SS strain. In all strains beginning around P15, the response to CO2 increased progressively reaching a peak at P19–21 when V̇e during hypercapnia was 175–225% above eucapnia. There were minimal changes in CO2 sensitivity between P21 and P30, and at both ages there were minimal between-strain differences. At P30, the response to CO2 in the SS and SD strains was near the adult response, but the response in the BN rats was 100% greater at P30 than in adults. We conclude that 1) CO2-sensing mechanisms, and/or mechanisms downstream from the chemoreceptors, change dramatically at the age in rats when other physiological systems are also maturing (∼P15), and 2) there is a high degree of age-dependent plasticity in CO2 sensitivity in rats, which differs between strains.

The use of mouse chromosome substitution strains to investigate the genetic regulation of pubertal timing

Identification of genes underlying complex traits such as pubertal timing, is vital to our understanding of fundamental human developmental processes. Animal models can provide an important adjunct to more traditional human investigations. Within this review, we discuss the use and advantages of chromosome substitution strains in the investigation of factors that regulate the timing of the onset of puberty.

Neuroanatomic deficits in congenital central hypoventilation syndrome

Congenital Central Hypoventilation Syndrome (CCHS) patients exhibit compromised autonomic regulation, reduced breathing drive during sleep, diminished ventilatory responses to chemoreceptor stimulation, and diminished air hunger perception. The syndrome provides an opportunity to partition neural processes regulating breathing and cardiovascular action. No obvious lesions appear with conventional magnetic resonance imaging; however, T2 relaxometry procedures can detect reduced cell or fiber density or diminished myelination not found with routine evaluation. High-resolution T1, proton density, and T2-weighted brain images were collected from 12 patients and 28 age- and gender-matched controls. Voxel-by-voxel T2 maps were generated from the proton density and T2-weighted images and evaluated by voxel-based-relaxometry procedures. Normalized and smoothed T2 maps were compared between groups using analysis of covariance at each voxel, with age and ventricle size included as covariates. Patients showed damaged or maldeveloped tissue, principally right-sided, including white matter from the level of the anterior cingulate cortex caudally to the level of the posterior cingulate and laterally to the posterior superior temporal cortex. Portions of the posterior, mid, and anterior cingulate, as well as the internal capsule, putamen, and globus pallidus and basal forebrain extending to the anterior and medial thalamus were affected. Deficits in the cingulum bundle and mid-hippocampus and ventral prefrontal cortex appeared, as well as the right cerebellar cortex and deep nuclei. Neuroanatomic deficiencies in limbic structures suggest a structural basis for reduced air hunger perception, thermoregulatory and autonomic deficiencies in the syndrome, while cerebellar deficits may also contribute to breathing and cardiovascular dysregulation.

Differences in time-dependent hypoxic phrenic responses among inbred rat strains

Hypoxic ventilatory responses differ between rodent strains, suggesting a genetic contribution to interindividual variability. However, hypoxic ventilatory responses consist of multiple time-dependent mechanisms that can be observed in different respiratory motor outputs. We hypothesized that strain differences would exist in discrete time-dependent mechanisms of the hypoxic response and, furthermore, that there may be differences between hypoglossal and phrenic nerve responses to hypoxia. Hypoglossal and phrenic nerve responses were assessed during and after a 5-min hypoxic episode in anesthetized, vagotomized, and ventilated rats from four inbred strains: Brown Norway (BN), Fischer 344 (FS), Lewis (LW), and Piebald-viral-Glaxo (PVG). During baseline, burst frequency was higher in PVG than LW rats ( P < 0.05), phrenic burst amplitude was higher in PVG vs. other strains ( P < 0.05), and hypoglossal burst amplitude was higher in PVG and BN vs. FS and LW ( P < 0.05). During hypoxia, burst frequency did not change in BN or LW rats, but it increased in PVG and FS rats. The phrenic amplitude response was smallest in PVG vs. other strains ( P < 0.05), and the hypoglossal response was similar among strains. Short-term potentiation posthypoxia was slowest in FS and fastest in LW rats ( P < 0.05). Posthypoxia frequency decline was absent in PVG, but it was observed in all other strains. Augmented breaths were observed during hypoxia in FS rats only. Thus genetic differences exist in the time domains of the hypoxic response, and these are differentially expressed in hypoglossal and phrenic nerves. Furthermore, genetic diversity observed in hypoxic ventilatory responses in unanesthetized rats may arise from multiple neural mechanisms.

Genetic determinants on rat chromosome 6 modulate variation in the hypercapnic ventilatory response using consomic strains

To understand the genetic basis of pathways involved in the control of breathing, a large scale, high-throughput study using chromosomal substitution strains of rats is underway. Eight new consomic rat stains (SS-2(BN), SS-4(BN), SS-6(BN), SS-7(BN), SS-8(BN), SS-11(BN), SS-12(BN), SS-14(BN), SS-Y(BN)), containing one homozygous BN/NHsdMcwi (BN) chromosome on a background of SS/JrHsdMcwi (SS), were created by PhysGen (http://pga.mcw.edu) Program for Genomic Applications. Male and female rats were studied using standard plethysmography under control conditions and during acute hypoxia (inspired oxygen fraction = 0.12) and hypercapnia (inspired CO(2) fraction = 0.07). The rats were also studied during treadmill exercise. Both male and female BN rats had a significantly lower ventilatory response during 7% CO(2) compared with SS rats of the same gender. SS-6(BN) female rats had a significantly reduced ventilatory response, similar to BN rats due primarily to a reduced tidal volume. Male SS-6(BN) rats had a significantly reduced tidal volume response to hypercapnia but a slightly increased frequency response during hypercapnia. Gene(s) on the Y chromosome may play a role in this increased frequency response in the male rats because the SS-Y(BN) hypercapnic ventilatory response involves a significantly increased frequency response. Several chromosomal substitutions slightly altered the ventilatory responses to hypoxia and exercise. However, genes on chromosomes 6 and Y of those studied are of primary importance in aspects of ventilatory control currently studied.

Hypoxia reveals posterior thalamic, cerebellar, midbrain, and limbic deficits in congenital central hypoventilation syndrome

Congenital central hypoventilation syndrome (CCHS) patients show deficient respiratory and cardiac responses to hypoxia and hypercapnia, despite apparently intact arousal responses to hypercapnia and adequate respiratory motor mechanisms, thus providing a model to evaluate functioning of particular brain mechanisms underlying breathing. We used functional magnetic resonance imaging to assess blood oxygen level-dependent signals, corrected for global signal changes, and evaluated them with cluster and volume-of-interest procedures, during a baseline and 2-min hypoxic (15% O(2), 85% N(2)) challenge in 14 CCHS and 14 age- and gender-matched control subjects. Hypoxia elicited significant (P < 0.05) differences in magnitude and timing of responses between groups in cerebellar cortex and deep nuclei, posterior thalamic structures, limbic areas (including the insula, amygdala, ventral anterior thalamus, and right hippocampus), dorsal and ventral midbrain, caudate, claustrum, and putamen. Deficient responses to hypoxia included no, or late, changes in CCHS patients with declining signals in control subjects, a falling signal in CCHS patients with no change in controls, or absent early transient responses in CCHS. Hypoxia resulted in signal declines but no group differences in hypothalamic and dorsal medullary areas, the latter being a target for PHOX2B, mutations of which occur in the syndrome. The findings extend previously identified posterior thalamic, midbrain, and cerebellar roles for normal mediation of hypoxia found in animal fetal and adult preparations and suggest significant participation of limbic structures in responding to hypoxic challenges, which likely include cardiovascular and air-hunger components. Failing structures in CCHS include areas additional to those associated with PHOX2B expression and chemoreceptor sites.

The fa leptin receptor mutation and the heritability of respiratory frequency in a Brown Norway and Zucker intercross

We sought to determine whether the fa (leptin receptor) mutation was a major determinant of the putative obesity effects on respiratory frequency in an intercross between the Brown Norway (low breathing frequency, nonobese strain) and the Zucker (moderately high breathing frequency, with the fa mutation) strains. The hypothesis was that rats bearing one (heterozygote) or two (homozygote) alleles of the Glu296Pro point mutation (fa) would have a uniformly high respiratory frequency in the second filial (F2) generation, compared with wild-type animals. In addition to breathing frequency, tidal volume and minute ventilation were assessed during baseline, acute hypoxic (10% O2-0% CO2-balance nitrogen), hypercapnic (93% O2-7% CO2), hyperoxic (100% O2-0% CO2), and combined (10% O2-3% CO2-balance nitrogen) challenges in fa homozygote (fa/fa; n = 24), fa heterozygote (fa/wt; n = 33), and wild-type (wt/wt; n = 19) animals. Phenotypes were adjusted with stepwise regression analyses for the effects of age, sex, length, and litter size. Broad-sense heritability was estimated by examining the variance of the traits in first filial and F2 generations. ANOVAs were used to determine the mode of inheritance of the fa allele in the F2 generation. As anticipated, weight demonstrated the greatest overall broad-sense heritability (77%) and was the result of the recessive mutation. Breathing parameters during the hypoxic, hypercapnic, and combined challenges demonstrated a wide range of heritability from 5 to 96%, with a very nonuniform proportion of heritability explained by the leptin receptor. At best, for frequency 4.5 min into the hypercapnic hypoxic challenge, approximately 20% of the total heritability (approximately 67%) could be attributed to an effect of the leptin receptor mutation. We conclude that, unlike its major effect on weight, the effect of the fa allele is not a major gene involved in the regulation of breathing frequency.