The area postrema of newborn swine is activated by hypercapnia: relevance to sudden infant death syndrome?

Genetic diseases: congenital central hypoventilation, Rett, and Prader-Willi syndromes

The present review summarizes current knowledge on three rare genetic disorders of respiratory control, congenital central hypoventilation syndrome (CCHS), Rett syndrome (RTT), and Prader-Willi syndrome (PWS). CCHS is characterized by lack of ventilatory chemosensitivity caused by PHOX2B gene abnormalities consisting mainly of alanine expansions. RTT is associated with episodes of tachypneic and irregular breathing intermixed with breathholds and apneas and is caused by mutations in the X-linked MECP2 gene encoding methyl-CpG-binding protein. PWS manifests as sleep-disordered breathing with apneas and episodes of hypoventilation and is caused by the loss of a group of paternally inherited genes on chromosome 15. CCHS is the most specific disorder of respiratory control, whereas the breathing disorders in RTT and PWS are components of a more general developmental disorder. The main clinical features of these three disorders are reviewed with special emphasis on the associated brain abnormalities. In all three syndromes, disease-causing genetic defects have been identified, allowing the development of genetically engineered mouse models. New directions for future therapies based on these models or, in some cases, on clinical experience are delineated. Studies of CCHS, RTT, and PWS extend our knowledge of the molecular and cellular aspects of respiratory rhythm generation and suggest possible pharmacological approaches to respiratory control disorders. This knowledge is relevant for the clinical management of many respiratory disorders that are far more prevalent than the rare diseases discussed here.

c-Fos immunoreactivity in the pig brain following deoxynivalenol intoxication: focus on NUCB2/nesfatin-1 expressing neurons

Deoxynivalenol (DON), produced by the cereal-contaminating Fusarium fungi, is a major trichothecene responsible for mycotoxicoses in farm animals, including swine. The main effect of DON-intoxication is food intake reduction and the consequent body weight loss. The present study aimed to identify brain structures activated during DON intoxication in pigs. To this goal, we used c-Fos staining which constitutes a useful approach to identify activated neurons. We showed that per os administration of Fusarium graminearum extracts (containing the equivalent of 1mg DON per kg of body weight) induced an increase in c-Fos immunoreactivity in several central structures, including the ventrolateral medulla (VLM), dorsal vagal complex (DVC), paraventricular nucleus of the hypothalamus (PVN), arcuate nucleus (Arc), supraoptic nucleus (SON) and amygdala (CeA). Moreover, we coupled c-Fos staining with phenotypic markers detection in order to specify the neuronal populations activated during DON intoxication. This phenotypic characterization revealed the activation of catecholaminergic but not of serotoninergic neurons in response to the toxin. In this context, we also paid a particular attention to NUCB2/nesfatin-1 positive cells, since nesfatin-1 is known to exert a satiety effect. We report here, for the first time in the pig brain, the presence of NUCB2/nesfatin-1 neurons in the VLM, DVC, PVN, Arc and SON, and their activation during DON intoxication. Taken together, these data show that DON stimulates the main structures involved in food intake in pigs and suggest that catecholaminergic and NUCB2/nesfatin-1 neurons could contribute in the anorexigenic effects of the mycotoxin.

Chemosensory responses to CO2 in multiple brain stem nuclei determined using a voltage-sensitive dye in brain slices from rats

We used epifluorescence microscopy and a voltage-sensitive dye, di-8-ANEPPS, to study changes in membrane potential during hypercapnia with or without synaptic blockade in chemosensory brain stem nuclei: the locus coeruleus (LC), the nucleus of the solitary tract, lateral paragigantocellularis nucleus, raphé pallidus, and raphé obscurus and, in putative nonchemosensitive nuclei, the gigantocellularis reticular nucleus and the spinotrigeminal nucleus. We studied the response to hypercapnia in LC cells to evaluate the performance characteristics of the voltage-sensitive dye. Hypercapnia depolarized many LC cells and the voltage responses to hypercapnia were diminished, but not eradicated, by synaptic blockade (there were intrinsically CO2-sensitive cells in the LC). The voltage response to hypercapnia was substantially diminished after inhibiting fast Na+ channels with tetrodotoxin. Thus action potential-related activity was responsible for most of the optical signal that we detected. We systematically examined CO2 sensitivity among cells in brain stem nuclei to test the hypothesis that CO2 sensitivity is a ubiquitous phenomenon, not restricted to nominally CO2 chemosensory nuclei. We found intrinsically CO2 sensitive neurons in all the nuclei that we examined; even the nonchemosensory nuclei had small numbers of intrinsically CO2 sensitive neurons. However, synaptic blockade significantly altered the distribution of CO2-sensitive cells in all of the nuclei so that the cellular response to CO2 in more intact preparations may be difficult to predict based on studies of intrinsic neuronal activity. Thus CO2-sensitive neurons are widely distributed in chemosensory and nonchemosensory nuclei and CO2 sensitivity is dependent on inhibitory and excitatory synaptic activity even within brain slices. Neuronal CO2 sensitivity important for the behavioral response to CO2 in intact animals will thus be determined as much by synaptic mechanisms and patterns of connectivity throughout the brain as by intrinsic CO2 sensitivity.

PHOX2B mutations and ventilatory control

The transcription factor PHOX2B is essential for the development of the autonomic nervous system. In humans, polyalanine expansion mutations in PHOX2B cause Congenital Central Hypoventilation Syndrome (CCHS), a rare life-threatening disorder characterized by hypoventilation during sleep and impaired chemosensitivity. CCHS is combined with comparatively less severe impairments of autonomic functions including thermoregulation, cardiac rhythm, and digestive motility. Respiratory phenotype analyses of mice carrying an invalidated Phox2b allele (Phox2b+/- mutant mice) or the Phox2b mutation (+7 alanine expansion) found in patients with CCHS (Phox2b(27Ala/+) mice) have shed light on the role for PHOX2B in breathing control and on the pathophysiological mechanisms underlying CCHS. Newborn mice that lacked one Phox2b allele (Phox2b+/-) had sleep apneas and depressed sensitivity to hypercapnia. However, these impairments resolved rapidly, whereas the CCHS phenotype is irreversible. Heterozygous Phox2b(27Ala/+) pups exhibited a lack of responsiveness to hypercapnia and unstable breathing; they died within the first few postnatal hours. The generation of mouse models of CCHS provides tools for evaluating treatments aimed at alleviating both the respiratory symptoms and all other autonomic symptoms of CCHS.

Neural control of breathing: insights from genetic mouse models

Recent studies described the in vivo ventilatory phenotype of mutant newborn mice with targeted deletions of genes involved in the organization and development of the respiratory-neuron network. Whole body flow barometric plethysmography is the noninvasive method of choice for studying unrestrained newborn mice. Breathing-pattern abnormalities with apneas occur in mutant newborn mice that lack genes involved in the development and modulation of rhythmogenesis. Studies of deficits in ventilatory responses to hypercapnia and/or hypoxia helped to identify genes involved in chemosensitivity to oxygen and carbon dioxide. Combined studies in mutant newborn mice and in humans have shed light on the pathogenesis of genetically determined respiratory-control abnormalities such as congenital central hypoventilation syndrome, Rett syndrome, and Prader-Willi syndrome. The development of mouse models has opened up the field of research into new treatments for respiratory-control disorders in humans.

Effects of smoking and fetal hypokinesia in early pregnancy

BACKGROUND

The aim of this study was to perform qualitative and quantitative ultrasound (US) monitoring of fetal movement in early pregnancy (gestational weeks 10-20) as a component of fetal behavior in women smokers.

METHODS

The study included three groups of 20 pregnant women each: non-smokers (group 1), smokers of an average of 10 cigarettes daily (group 2), and smokers of an average of >20 cigarettes daily (chronic smokers; group 3). Two-dimensional US study was performed once during gestational weeks 10-20 by the then standard method of fetal movement monitoring: 1) quantitative measurement of global fetal movements qualitatively verified as brisk or sluggish; 2) quantitative measurement of isolated spontaneous head movements; 3) quantitative measurement of isolated spontaneous arm movements; 4) quantitative measurement of isolated spontaneous leg movements; and 5) M-mode measurement of fetal heart rate.

RESULTS

The ratio of brisk to sluggish fetal movements was 82.8% to 17.2%, 79.01% to 20.98%, and 44.25% to 55.75% in groups 1, 2, and 3, respectively (p <0.001). The rate of isolated spontaneous head and arm movements and of the upper cerebral pattern (head and arm movements) was statistically significantly lower in group 3 as compared with groups 1 and 2 (p <0.001), whereas no statistical significance was recorded in isolated spontaneous leg movements (p >0.01). The rate of fetal tachycardia was also significant in group 3, whereas tachyarrhythmia was recorded in seven children born to group 3 mothers (p <0.001).

CONCLUSIONS

The present study of the effect of chronic tobacco hypoxia on the components of fetal behavior revealed a positive correlation between global and isolated fetal hypokinesia of the upper cerebral pattern (p <0.001), fetal tachycardia, and tachyarrhythmia in the group of mothers who were chronic smokers (p <0.001).

Enhanced c-Fos expression in the rostral ventral respiratory complex and rostral parapyramidal region by inhibition of the Na+/H+ exchanger type 3

Previous studies have shown that selective inhibition of Na+/H+ exchanger type 3 (NHE3) induces intracellular acidification and activates CO2/H+-sensitive medullary neurons, mimicking the responses evoked by hypercapnic stimuli. In addition, NHE3 blockers administration decreases the duration of apnoea induced by laryngeal stimulation, presumably by means of central chemoreceptor activation. To test the hypothesis that the central chemoreceptor network may be affected by NHE3 inhibition, brainstem c-Fos immunoreactive cell counting was performed after systemic administration of the NHE3 blocker AVE1599 (Aventis Pharma Deutschland GmbH) (2 mg/kg). The rostro-caudal quantitative c-Fos analysis showed a significant increase in the number of c-Fos positive cells in the rostral part of the ventral respiratory complex (VRC) as well as in the rostral part of the parapyramidal (Ppy) region. The VRC activated region (-4.2 to -3.2mm interaural) included the pre-Bötzinger complex, the rostral ventral respiratory group and the rostral ventrolateral medulla, all of them involved in cardiorespiratory control. The activated Ppy region corresponded with the rostral chemosensitive area, which elicits the strongest ventilatory response upon ventral medullary surface stimulation with H+/CO2. Most cells activated in Ppy after NHE3 inhibition were serotonergic. Hence, systemic application of NHE3 blockers may induce central chemoreceptors activation and an increase in the respiratory network activity in a similar way to known physiological stimuli such as hypercapnia. On the other hand, selective NHE3 blockers could be excellent tools for treatment of pathological states where central chemoreceptor function is diminished or absent, such as central hypoventilation syndrome or sudden infant death syndrome.

Comparative anatomical assessment of the piglet as a model for the developing human medullary serotonergic system

Because the piglet is frequently used as a model for developmental disorders of the medullary serotonergic (5-HT) system in the human infant, this review compares the topography and developmental profile of selected 5-HT markers between humans in the first year of life and piglets in the first 60 days of life. The distribution of tryptophan hydroxylase-immunoreactive 5-HT neurons in the human infant medulla is very similar, but not identical, to that in the piglet. One notable difference is the presence of compact clusters of 5-HT neurons at the ventral surface of the piglet medulla. While it lacks these distinctive clusters, the human infant medulla contains potentially homologous 5-HT neurons scattered along the ventral surface embedded in the arcuate nucleus. Each species shows evidence of age-related changes in the 5-HT system, but the changes are different in nature; in the human infant, statistically significant age-related changes are observed in the proportional distribution of medullary 5-HT cells, while in the piglet, statistically significant age-related changes are observed in the levels of 5-HT receptor binding in certain medullary nuclei. Analyses of 5-HT receptor binding profiles in selected nuclei in the two species suggest that the equivalent postnatal ages for 5-HT development in piglets and human infants are, respectively, 4 days and 1 month, 12 days and 4 months, 30 days and 6 months, and 60 days and 12 months. Collectively, when certain species differences are considered, these data support the use of the piglet as a model for the human infant medullary 5-HT system.

Sleep-disordered Breathing in Newborn Mice Heterozygous for the Transcription Factor Phox2b

RATIONALE

Central congenital hypoventilation syndrome (CCHS) is a rare autosomal dominant syndrome present from birth, and characterized by depressed ventilation during sleep. Heterozygous mutations of the homeobox gene Phox2b were recently found in a very high proportion of patients.

OBJECTIVES

To determine whether newborn mice with heterozygous targeted deletion of the transcription factor Phox2b would display sleep-disordered breathing.

METHODS

We measured breathing pattern using whole-body plethysmography in wild-type and mutant 5-day-old mice, and we classified sleep-wake states using nuchal EMG and behavioral scores.

RESULTS

We found that sleep apnea total time was approximately six times longer (8.9 +/- 12 vs. 1.5 +/- 2.2 seconds, p < 0.0015), and ventilation during active sleep was 21% lower (18.4 +/- 5.1 vs. 23.3 +/- 5.5 ml/g/second, p < 0.006) in mutant than in wild-type pups. During wakefulness, apnea time and ventilation were not significantly different between mutant and wild-type pups. Mutant and wild-type pups showed highly similar sleep-wake states.

CONCLUSION

Although their respiratory phenotype was much less severe than CCHS, the Phox2b(+/-) mutant mice showed sleep-disordered breathing, which partially modeled the key feature of CCHS.

Organization of the newborn piglets vagal motor complex: insights into integrated autonomic control mechanisms

Pediatric disorders frequently exhibit dysregulation of sympatho-vagal activity, and impaired control of cardiovascular vagal networks. Factors influencing the maturation of vagal networks are of special interest because they normally protect the heart and circulation, facilitate digestion, and preserve visceral metabolism. At present, scant literature exists regarding the development of vagal innervation of the heart. This study in neonatal swine, Sus scrofa, mapped the normal anatomy of vagal motor cell groups, with special focus on the origins of cardiomotor neurons. Right cardiac nerve branches, or the right thoracic vagal trunk were resected, inserted into capillary glass vials filled with 2% FluoroGold (FG) tracer solutions, and sealed to prevent leakage (false positives). Dorsal and ventral vagal complexes were identified on cross-sectioned tissues incubated in a well-characterized specific FG antibody. Thoracic and abdominal vagal motoneurons were cytologically heterogeneous, and predominantly medium-sized, polygonal cell bodies. Discrete longitudinal cell columns were observed, as well as organized arrays of elongate spindle-shaped cells in formation. Long axes and dendrites appeared to orient toward incoming peripheral sensory and central afferents, and were juxtaposed to cerebral microvasculature. The piglets' dorsal vagal complex is: (i) thick and long, comparable to ruminants, in contrast to much shorter lengths in non-ruminants, and (ii) the chief source of vagal motoneurons, forming discrete, topographically organized parasympathetic cell groups with distinct dendritic arbors. The cardiac motor subnucleus is localized to a highly restricted areal subunit of nucleus ambiguus' external formation in the vicinity of the obex. On the other hand, dorsal cardiac vagal motoneurons were few in number and diffusely distributed. Dorsal vagal motoneurons of neonatal swine likely projected primarily to the enteric nervous system, exerting excitatory influence over gastrointestinal activity.

Hypercapnia induces long-term changes in postganglionic renal nerve activity in the piglet

In developing swine, time and frequency domain analyses were used to compare changes in discharge features of efferent phrenic and postganglionic renal nerve activities evoked by prolonged (1 h) exposure to severe hypercapnia (10% CO2, balance O2), before and after combined carotid sinus and aortic depressor nerve (CSN-AOD) sectioning. With intact CSN-AOD innervation, respiration-related activity in renal nerve discharge was rare (3 of 11 animals) during baseline periods with intact innervation, but was observed in most cases (10 of 11 animals) during baseline following denervation. Renal nerve respiration-related activity was recruited by hypercapnic stimulation in animals with intact CSN-AOD innervation, and was augmented in denervated animals with ongoing respiratory activity. Phrenic nerve discharge was markedly augmented during hypercapnia, whether CSN-AOD innervation was intact or not, and it did not exhibit a post-hypercapnic depression. Autopower spectra of renal nerve activity revealed the presence of two coexisting rhythms, 2-6 and 7-13 Hz, which were present whether CSN-AOD innervation was intact or not. The hypercapnic-induced increases of activity in the 2-6 and 7-13 Hz bands were not comparable, with the latter region exhibiting a much more robust response to hypercapnia, especially following CSN-AOD denervation. Thus, prolonged exposure to hypercapnia evoked changes in renal nerve discharge that involved increased coupling to neuronal ensembles shaping central inspiratory activity and those generating central sympathetic outflows, especially to networks generating 7-13 Hz rhythm. Such changes may permit more efficient modulation of innervated structures during exposure to stressors.

Developmental changes in heart rate variability during exposure to prolonged hypercapnia in piglets

The hypothesis that hypercapnia-induced differences in heart rate variability (HRV) would emerge during early maturation was tested using a developing porcine model. Piglets were randomly assigned to either exposed (10% CO2 for 1 h) or control (100% O2) conditions, and then to one of three study groups: (a) 5-8 days old, (b) 13-15 days old, (c) 26-34 days old. Experiments were performed on pairs of age-and litter-matched animals that were anesthetized, paralyzed, and artificially ventilated. HRV was evaluated using power spectral analysis, SD of differences between successive RR intervals, and cardiac interval analysis. Statistical comparisons of simultaneously studied animals were made at baseline, 15 and 55 min after onset of hypercapnia, and 2 h after offset of hypercapnia. Our analyses revealed that only HRV of 26-34-day-old animals differed significantly from values of control animals. Cardiac intervals of those animals were distributed in such a manner that hypercapnia likely elicited coactivation of sympathetic and parasympathetic systems. Comparison of the distribution of cardiac intervals for other animals showed that 5-8-day-old animals had high frequency of balanced intervals at baseline that remained so during hypercapnia. Given that such coactivation may be neuroprotective, the paucity of balanced intervals in 13-15-day-old animals could mean that the end of the second postnatal week is associated with increased vulnerability.

Differential CO(2)-induced c-fos gene expression in the nucleus tractus solitarii of inbred mouse strains

Genetic determinants confer variation between inbred mouse strains with respect to the magnitude and pattern of ventilation during hypercapnic challenge. Specifically, inheritance patterns derived from low-responsive C3H/HeJ (C3) and high-responsive C57BL/6J (B6) mouse strains suggest that differential hypercapnic ventilatory sensitivity (HCVS) is controlled by two independent genes. The present study also tests whether differential neuronal activity in respiratory control regions of the brain is positively associated with strain variation in HCVS. With the use of whole body plethysmography, ventilation was assessed in C3 and B6 strains at baseline and during 30 min of hypercapnia (inspired CO(2) fraction = 0.15, inspired O(2) fraction = 0.21 in N(2)). Subsequently, in situ hybridization histochemistry was performed to determine changes in c-fos gene expression in the commissural subnucleus of the nucleus tractus solitarius (NTS). During hypercapnia, breathing frequency and tidal volume were significantly (P < 0.01) different between strains: C3 mice showed a slow, deep-breathing pattern relative to a rapid, shallow phenotype of B6 mice. CO(2)-induced increase in c-fos gene expression was significantly (P < 0.01) greater in NTS regions of B6 compared with C3 mice. In this genetic model of differential HCVS, the results suggest that a genomic basis for varied hypercapnic chemoreception or transduction confers greater afferent neuronal activity in the caudal NTS for high-responsive B6 mice compared with low-responsive C3 mice.

The human nucleus of the solitary tract: visceral pathways revealed with an "in vitro" postmortem tracing method

Visceral relay neurons in the nucleus of the solitary tract (NTS) regulate behavior and autonomic reflex functions. NTS projections have been extensively characterized in animal studies but not in humans. For the first time, NTS fiber trajectories in the human medulla oblongata were revealed with an "in vitro" postmortem tracing method. Local intramedullary pathways were labeled by direct pressure injections of free horseradish peroxidase centered on the medial subnucleus at a level adjacent to true obex. Labeled elements were resolved by peroxidase histochemistry as a dark brown intracellular reaction product. A prominent transtegmental system of axons emerged from the NTS injection sites and entered the intermediate reticular zone, a region corresponding to an autonomic reflex center in other mammals. A medial system of axons arched across the dorsomedial reticular formation toward the dorsal medullary raphe and projected ventrally toward the nucleus gigantocellularis. A small lateral fiber trajectory coursed towards the dorsomedial zone of spinal trigeminal nucleus caudalis. Presumptive terminals appeared as dustings of fine punctate processes within the NTS, dorsomotor nucleus and reticular formation. NTS projections in humans resemble those identified in other mammals including primates. Axonal tracing studies predict that visceral impulses in humans may transmit over evolutionarily conserved pathways involved in autonomic feedback control and stress adaptation.