Alteration of serotonergic receptors in the brain stems of human patients with respiratory disorders

The vicious spiral in Sudden Infant Death Syndrome

Sudden Infant Death Syndrome (SIDS) is the sudden and unexpected death of an otherwise healthy infant less than 1 year of age where the cause of death remains unexplained after a thorough post-mortem investigation and evaluation of the circumstances. Epidemiological, clinical, biochemical, immunological and pathological evidence indicates that three factors must coincide for SIDS to occur: a vulnerable developmental stage of the immune system and central nervous system in the infant, predisposing factors, and external trigger events. This model is referred to as the fatal triangle or triple risk hypothesis. The concept of a vicious spiral in SIDS, starting with the fatal triangle and ending in death, is proposed as a model to understand the death mechanism. The vicious spiral is initiated by a mucosal infection and immune activation in the upper respiratory and digestive tracts, increased production of cytokines, and an overstimulation of the immature and rapidly developing immune system. A second trigger is the prone sleeping position, which may lead to rebreathing and hypercapnia, in addition to intensify the immune stimulation. In susceptible infants, this induces an aberrant cytokine production that affects sleep regulation, induces hyperthermia, and disrupts arousal mechanisms. In turn, this initiates downregulation of respiration and hypoxemia, which is worsened by nicotine. Inefficient autoresuscitation results in severe hypoxia and accumulation of hypoxic markers which, if not prevented by a normally functioning serotonergic network, contribute to a self-amplifying vicious spiral that eventually leads to coma and death. The purpose of this review is to summarize the research that underpins the concept of the vicious spiral.

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part II. Age-associated alterations in serotonin receptor binding profiles within medullary nuclei supporting cardiorespiratory homeostasis

The failure of chemoreflexes, arousal, and/or autoresuscitation to asphyxia may underlie some sudden infant death syndrome (SIDS) cases. In Part I, we showed that some SIDS infants had altered 5-hydroxytryptamine (5-HT)2A/C receptor binding in medullary nuclei supporting chemoreflexes, arousal, and autoresuscitation. Here, using the same dataset, we tested the hypotheses that the prevalence of low 5-HT1A and/or 5-HT2A/C receptor binding (defined as levels below the 95% confidence interval of controls-a new approach), and the percentages of nuclei affected are greater in SIDS versus controls, and that the distribution of low binding varied with age of death. The prevalence and percentage of nuclei with low 5-HT1A and 5-HT2A/C binding in SIDS were twice that of controls. The percentage of nuclei with low 5-HT2A/C binding was greater in older SIDS infants. In >80% of older SIDS infants, low 5-HT2A/C binding characterized the hypoglossal nucleus, vagal dorsal nucleus, nucleus of solitary tract, and nuclei of the olivocerebellar subnetwork (important for blood pressure regulation). Together, our findings from SIDS infants and from animal models of serotonergic dysfunction suggest that some SIDS cases represent a serotonopathy. We present new hypotheses, yet to be tested, about how defects within serotonergic subnetworks may lead to SIDS.

Sudden Infant Death Syndrome: Risk Factors and Newer Risk Reduction Strategies

Sudden infant death syndrome (SIDS) continues to be one of the top causes of infant death in the U.S. Despite significant public health initiatives focused on high-risk populations to enhance sleep environments and techniques. The SIDS rate has remained stable in recent years. Risk factors and newer risk reduction strategies for SIDS are the focus of this review article. We conducted a comprehensive literature search on Medline, Cochrane, Embase, and Google Scholar until July 2022. The following search strings and Medical Subject Heading (MeSH) terms were used: "SIDS," "Sudden Infant Death" and "SUID". We explored the literature on SIDS for its epidemiology, pathophysiology, the role of various etiologies and their influence, associated complications leading to SIDS, and preventive and treatment modalities. Despite a more than 50% drop-in rates since the start of the "Back to Sleep" campaign in 1994, sudden infant death syndrome (SIDS) continues to be the top cause of post-neonatal mortality in the United States, despite continued educational initiatives that support safe sleep and other risk reduction strategies. The new American Academy of Pediatrics guidelines for lowering the risk of SIDS include a lot of emphasis on sleeping habits, bedding, and environment but also include elements that are frequently ignored (i.e., prenatal care, smoking, alcohol and drug use, and childhood vaccinations). This study highlights these less-frequently discussed aspects and identifies treatments that have produced beneficial behavioral shifts that benefit newborns as well as their mothers' health and wellbeing.

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part I. Tissue-based evidence for serotonin receptor signaling abnormalities in cardiorespiratory- and arousal-related circuits

The sudden infant death syndrome (SIDS), the leading cause of postneonatal infant mortality in the United States, is typically associated with a sleep period. Previously, we showed evidence of serotonergic abnormalities in the medulla (e.g. altered serotonin (5-HT)1A receptor binding), in SIDS cases. In rodents, 5-HT2A/C receptor signaling contributes to arousal and autoresuscitation, protecting brain oxygen status during sleep. Nonetheless, the role of 5-HT2A/C receptors in the pathophysiology of SIDS is unclear. We hypothesize that in SIDS, 5-HT2A/C receptor binding is altered in medullary nuclei that are key for arousal and autoresuscitation. Here, we report altered 5-HT2A/C binding in several key medullary nuclei in SIDS cases (n = 58) compared to controls (n = 12). In some nuclei the reduced 5-HT2A/C and 5-HT1A binding overlapped, suggesting abnormal 5-HT receptor interactions. The data presented here (Part 1) suggest that a subset of SIDS is due in part to abnormal 5-HT2A/C and 5-HT1A signaling across multiple medullary nuclei vital for arousal and autoresuscitation. In Part II to follow, we highlight 8 medullary subnetworks with altered 5-HT receptor binding in SIDS. We propose the existence of an integrative brainstem network that fails to facilitate arousal and/or autoresuscitation in SIDS cases.

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Influence of developmental nicotine exposure on serotonergic control of breathing-related motor output

Serotonin plays an important role in the development of brainstem circuits that control breathing. Here, we test the hypothesis that developmental nicotine exposure (DNE) alters the breathing-related motor response to serotonin (5HT). Pregnant rats were exposed to nicotine or saline, and brainstem-spinal cord preparations from 1- to 5-day-old pups were studied in a split-bath configuration, allowing drugs to be applied selectively to the medulla or spinal cord. The activity of the fourth cervical ventral nerve roots (C4VR), which contain axons of phrenic motoneurons, was recorded. We applied 5HT alone or together with antagonists of 5HT1A, 5HT2A, or 5HT7 receptor subtypes. In control preparations, 5HT applied to the medulla consistently reduced C4VR frequency and this reduction could not be blocked by any of the three antagonists. In DNE preparations, medullary 5HT caused a large and sustained frequency increase (10 min), followed by a sustained decrease. Notably, the transient increase in frequency could be blocked by the independent addition of any of the antagonists. Experiments with subtype-specific agonists suggest that the 5HT7 subtype may contribute to the increased frequency response in the DNE preparations. Changes in C4VR burst amplitude in response to brainstem 5HT were uninfluenced by DNE. Addition of 5HT to the caudal chamber modestly increased phasic and greatly increased tonic C4VR activity, but there were no effects of DNE. The data show that DNE alters serotonergic signaling within brainstem circuits that control respiratory frequency but does not functionally alter serotonin signaling in the phrenic motoneuron pool.

Eupnea and gasping in vivo are facilitated by the activation of 5-HT2A receptors

Eupnea and gasping in infancy depend on central nervous system (CNS) serotonin (5-hydroxytryptamine; 5-HT). Although previous in vitro preparations have provided some evidence that 5-HT acts through type 2 A receptors (5-HT_2A) to facilitate eupnea and gasping, here the hypothesis addressed is that 5-HT_2A receptor activation is necessary for eupnea and the proper generation of gasping in vivo. To test this, we administered 2,5-dimethoxy-4-iodoamphetamine (DOI; 0.25 mg/kg i.p.), a 5-HT_2A agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.25 mg/kg i.p.), a 5-HT_1A agonist, or vehicle (saline) to 7-9-day-old tryptophan hydroxylase 2 knockout (TPH2^-/-) mice. A second experiment assessed the effect of MDL-11,939 (MDL; 10 mg/kg i.p.), the specific 5-HT_2A antagonist, or vehicle (DMSO) on the gasping of wild-type (TPH2^+/+) animals. Drugs were given 15 min prior to five episodes of severe hypoxia that elicited gasping. TPH2^-/- breathed more slowly but had the same V̇e and V̇e/V̇o₂ compared with TPH2^+/+. As previously reported, the gasping of TPH2^-/- was significantly delayed (P < 0.001) and occurred at a significantly lower frequency compared with TPH2^+/+ (P = 0.04). For both genotypes, DOI hastened eupneic frequency but had no effect on V̇e or V̇e/V̇o₂. The gasping of TPH2^-/-, although unaffected by 8-OH-DPAT, was indistinguishable from the gasping of TPH2^+/+ following DOI. In TPH2^+/+, application of MDL led to hypoventilation (P = 0.01), a delay in the appearance of gasping (P = 0.005), and reduced gasp frequency (P = 0.05). These data show that, in vivo, 5-HT_2A receptors facilitate both eupnea and gasping. As has been shown in vitro, 5-HT_2A probably promotes gasping by exciting hypoxia-resistant pacemaker neurons.NEW & NOTEWORTHY Previous in vitro studies suggest that 5-HT_2A receptors contribute to eupnea and are necessary for fictive gasping. The current study shows that the impaired gasping displayed by neonatal TPH2^-/- mice, deficient in CNS serotonin, is restored by 5-HT_2A receptor activation. Following 5-HT_2A blockade, wild-type mice hypoventilated and their gasping resembled that of TPH2^-/- mice. This study shows that both eupnea and gasping in vivo rely on the activation of 5-HT_2A receptors.

Revisiting the Neuropathology of Sudden Infant Death Syndrome (SIDS)

Background: Sudden infant death syndrome (SIDS) is one of the leading causes of infant mortality in the United States (US). The extent to which SIDS manifests with an underlying neuropathological mechanism is highly controversial. SIDS correlates with markers of poor prenatal and postnatal care, generally rooted in the lack of access and quality of healthcare endemic to select racial and ethnic groups, and thus can be viewed in the context of health disparities. However, some evidence suggests that at least a subset of SIDS cases may result from a neuropathological mechanism. To explain these issues, a triple-risk hypothesis has been proposed, whereby an underlying biological abnormality in an infant facing an extrinsic risk during a critical developmental period SIDS is hypothesized to occur. Each SIDS decedent is thus thought to have a unique combination of these risk factors leading to their death. This article reviews the neuropathological literature of SIDS and uses machine learning tools to identify distinct subtypes of SIDS decedents based on epidemiological data.

Methods: We analyzed US Period Linked Birth/Infant Mortality Files from 1990 to 2017 (excluding 1992–1994). Using t-SNE, an unsupervised machine learning dimensionality reduction algorithm, we identified clusters of SIDS decedents. Following identification of these groups, we identified changes in the rates of SIDS at the state level and across three countries.

Results: Through t-SNE and distance based statistical analysis, we identified three groups of SIDS decedents, each with a unique peak age of death. Within the US, SIDS is geographically heterogeneous. Following this, we found low birth weight and normal birth weight SIDS rates have not been equally impacted by implementation of clinical guidelines. We show that across countries with different levels of cultural heterogeneity, reduction in SIDS rates has also been distinct between decedents with low vs. normal birth weight.

Conclusions: Different epidemiological and extrinsic risk factors exist based on the three unique SIDS groups we identified with t-SNE and distance based statistical measurements. Clinical guidelines have not equally impacted the groups, and normal birth weight infants comprise more of the cases of SIDS even though low birth weight infants have a higher SIDS rate.

The Serotonin Brainstem Hypothesis for the Sudden Infant Death Syndrome

The sudden infant death syndrome (SIDS) is the leading cause of postneonatal infant mortality in the United States today, with an overall rate of 0.39/1000 live births. It is defined as the sudden and unexpected death of an infant <12 months of age that remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. The serotonin brainstem hypothesis has been a leading hypothesis for SIDS over the last 2 decades. Our laboratory has studied this hypothesis over time with a variety of tissue techniques, including tissue receptor autoradiography, high performance liquid chromatography, Western blot analysis, immunocytochemistry, and proteomics. The purpose of this article is to review the progress in our laboratory toward supporting this hypothesis. We conclude that an important subset of SIDS infants has serotonergic abnormalities resulting from a "core lesion" in the medullary reticular formation comprised of nuclei that contain serotonin neurons. This lesion could lead to a failure of protective brainstem responses to homeostatic challenges during sleep in a critical developmental period which cause sleep-related sudden death.

Serotonin and substance P: Synergy or competition in the control of breathing

Numerous neurotransmitters identified in the central nervous system play role in ventilatory control. This mini-review focuses on the respiratory effects of two neurotransmitters: serotonin (5-HT) and substance P (SP). We discuss their co-localization in medullary raphe nuclei, expression of proper receptors within the specific regions of respiratory related structures and contribution to respiratory rhythmogenesis.

Prenatal intermittent hypoxia sensitizes the laryngeal chemoreflex, blocks serotoninergic shortening of the reflex, and reduces 5-HT3 receptor binding in the NTS in anesthetized rat pups

We tested the hypothesis that exposure to intermittent hypoxia (IH) during pregnancy would prolong the laryngeal chemoreflex (LCR) and diminish the capacity of serotonin (5-hydroxytryptamine; 5-HT) to terminate the LCR. Prenatal exposure to IH was associated with significant prolongation of the LCR in younger, anesthetized, postnatal day (P) rat pups age P8 to P16 compared to control, room air (RA)-exposed rat pups of the same age. Serotonin microinjected into the NTS shortened the LCR in rat pups exposed to RA during gestation, but 5-HT failed to shorten the LCR in rat pups exposed to prenatal IH. Given these observations, we tested the hypothesis that prenatal hypoxia would decrease binding to 5-HT3 receptors in the nucleus of the solitary tract (NTS) where 5-HT acts to shorten the LCR. Serotonin 3 receptor binding was reduced in younger rat pups exposed to IH compared to control, RA-exposed rat pups in the age range P8 to P12. Serotonin 3 receptor binding was similar in older animals (P18-P24) regardless of gas exposure during gestation. The failure of the 5-HT injected into the NTS to shorten the LCR was correlated with a developmental decrease in 5-HT3 receptor binding in the NTS associated with exposure to prenatal IH. In summary, prenatal IH sensitized reflex apnea and blunted processes that terminate reflex apneas in neonatal rat pups, processes that are essential to prevent death following apneas such as those seen in babies who died of SIDS.

Impact of inflammation on developing respiratory control networks: rhythm generation, chemoreception and plasticity

The respiratory control network in the central nervous system undergoes critical developmental events early in life to ensure adequate breathing at birth. There are at least three "critical windows" in development of respiratory control networks: 1) in utero, 2) newborn (postnatal day 0-4 in rodents), and 3) neonatal (P10-13 in rodents, 2-4 months in humans). During these critical windows, developmental processes required for normal maturation of the respiratory control network occur, thereby increasing vulnerability of the network to insults, such as inflammation. Early life inflammation (induced by LPS, chronic intermittent hypoxia, sustained hypoxia, or neonatal maternal separation) acutely impairs respiratory rhythm generation, chemoreception and increases neonatal risk of mortality. These early life impairments are also greater in young males, suggesting sex-specific impairments in respiratory control. Further, neonatal inflammation has a lasting impact on respiratory control by impairing adult respiratory plasticity. This review focuses on how inflammation alters respiratory rhythm generation, chemoreception and plasticity during each of the three critical windows. We also highlight the need for additional mechanistic studies and increased investigation into how glia (such as microglia and astrocytes) play a role in impaired respiratory control after inflammation. Understanding how inflammation during critical windows of development disrupt respiratory control networks is essential for developing better treatments for vulnerable neonates and preventing adult ventilatory control disorders.

Variability of the medullary arcuate nucleus in humans

INTRODUCTION

The arcuate nucleus is a component of the ventral medullary surface involved in chemoreception and breathing control. The hypoplasia of this nucleus is a very frequent finding in victims of sudden unexplained fetal and infant death (from the last weeks of pregnancy to the first year of life). On the contrary, this developmental alteration is rarely present in age-matched controls who died of defined causes. These observations lead to hypothesize that a well-developed and functional arcuate nucleus is generally required to sustain life. The aim of this study was to investigate whether the arcuate nucleus maintains the same supposed function throughout life.

METHODS

We carried out neuropathological examinations of brainstems obtained from 25 adult subjects, 18 males and 7 females, aged between 34 and 89 years, who died from various causes.

RESULTS

For almost half of the cases (44%) microscopic examinations of serial histological sections of medulla oblongata showed a normal cytoarchitecture of the arcuate nucleus, extending along the pyramids. For the remaining 56% of cases, various degrees of hypodevelopment of this nucleus were observed, validated through the application of quantitative morphometric investigations, from decreased area, neuron number and volume, to full aplasia.

CONCLUSIONS

These unexpected findings indicate that the involvement of the arcuate nucleus in chemoreception in adulthood is questionable, given the possibility of living until late age without this nucleus. This opens new perspectives for researchers on the role and function of the arcuate nucleus in humans from birth to old age.

Neuropathological Developments in Sudden Infant Death Syndrome

A wide variety of neuropathological abnormalities have been investigated in infants who have died of sudden infant death syndrome (SIDS). Issues which detracted from early studies included failure to use uniform definitions of SIDS and lack of appropriately matched control populations. Development of the triple risk model focused attention on the concept of an inherent susceptibility to unexpected death in certain infants, with research demonstrating a role for the neurotransmitter serotonin within the brainstem. However, it now appears that neuropathological abnormalities in SIDS infants are more complex than a simple serotonergic deficiency in certain medullary nuclei but instead could involve failure of an integrated network of neurochemical transmitters in a variety of subcortical locations. The following overview examines recent research developments looking particularly at the potential role of the peptide neurotransmitter substance P and its neurokinin-1 receptor in multiple nuclei within the brainstem, asymmetry and microdysgenesis of the hippocampus, and decreased orexin levels within dorsomedial, perifornical, and lateral levels in the hypothalamus. Whether such research will lead to identifiable biomarker for infants at risk of SIDS is yet to be established. Use of standardized and consistent methods of classifying and categorizing infant deaths will be pivotal in generating reproducible research results.

Medullary Serotonin Neuron Abnormalities in an Australian Cohort of Sudden Infant Death Syndrome

Serotonin (5-hydroxytryptamine [5-HT]) neurons in the medulla oblongata project extensively to key autonomic and respiratory nuclei in the brainstem and spinal cord regulating critical homeostatic functions. Multiple abnormalities in markers of 5-HT function in the medulla in sudden infant death syndrome (SIDS) have been reported, informing the hypothesis that at least a subset of SIDS cases is caused by deficits in 5-HT function resulting in impaired homeostatic responses to potentially life-threatening events during sleep. To investigate medullary 5-HT defects in SIDS further, we undertook qualitative analysis immunohistochemical assessment of 5-HT neuron expression within the medulla of SIDS infants (n41) and nonSIDS controls (n = 28) in an independent cohort from Forensic Science South Australia. Compared with controls SIDS cases had significantly higher 5-HT neuron numbers and density in addition to significantly altered 5-HT neuron morphology. Thus, for the first time, we replicated and corroborated previous observations of a significant abnormality in medullary 5-HT neuron expression in SIDS in a separate independent SIDS cohort. This study further supports the hypothesis that medullary 5-HT defects contribute to the pathogenesis of a subset of SIDS victims and provides additional evidence of a more complex abnormality in 5-HT neuron dysfunction specifically within the different caudal and rostral medullary 5-HT domains.

Congenital central hypoventilation syndrome: a bedside-to-bench success story for advancing early diagnosis and treatment and improved survival and quality of life

The "bedside-to-bench" Congenital Central Hypoventilation Syndrome (CCHS) research journey has led to increased phenotypic-genotypic knowledge regarding autonomic nervous system (ANS) regulation, and improved clinical outcomes. CCHS is a neurocristopathy characterized by hypoventilation and ANS dysregulation. Initially described in 1970, timely diagnosis and treatment remained problematic until the first large cohort report (1992), delineating clinical presentation and treatment options. A central role of ANS dysregulation (2001) emerged, precipitating evaluation of genes critical to ANS development, and subsequent 2003 identification of Paired-Like Homeobox 2B (PHOX2B) as the disease-defining gene for CCHS. This breakthrough engendered clinical genetic testing, making diagnosis exact and early tracheostomy/artificial ventilation feasible. PHOX2B genotype-CCHS phenotype relationships were elucidated, informing early recognition and timely treatment for phenotypic manifestations including Hirschsprung disease, prolonged sinus pauses, and neural crest tumors. Simultaneously, cellular models of CCHS-causing PHOX2B mutations were developed to delineate molecular mechanisms. In addition to new insights regarding genetics and neurobiology of autonomic control overall, new knowledge gained has enabled physicians to anticipate and delineate the full clinical CCHS phenotype and initiate timely effective management. In summary, from an initial guarantee of early mortality or severe neurologic morbidity in survivors, CCHS children can now be diagnosed early and managed effectively, achieving dramatically improved quality of life as adults.

Medullary 5-HT neurons: Switch from tonic respiratory drive to chemoreception during postnatal development

Serotonin (5-HT) neurons contribute to respiratory chemoreception in adult mice, but it is unclear whether they play a similar role in neonatal mice. We studied breathing during development in Lmx1b^f/f/p mice, which lack 5-HT neurons. From postnatal days 1-7 (P1-P7), ventilation of Lmx1b^f/f/p mice breathing room air was 50% of WT mice (p<0.001). By P12, baseline ventilation increased to a level equal to WT mice. In contrast, the hypercapnic ventilatory response (HCVR) of neonatal Lmx1b^f/f/p and WT mice was equal to each other, but were both much less than adult WT mice. By P21 the HCVR of WT mice increased to near adult levels, but the HCVR of Lmx1b^f/f/p mice had not changed, and was 42% less than WT mice. Primary cell cultures were prepared from the ventromedial medulla of neonatal mice, and patch-clamp recordings were made from neurons identified as serotonergic by expression of a reporter gene. In parallel with developmental changes of the HCVR in vivo, 5-HT neurons had little chemosensitivity to acidosis until 12days in vitro (DIV), after which their response increased to reach a plateau around 25 DIV. Neonatal Lmx1b^f/f/p mice displayed high mortality and decreased growth rate, and this worsened in hypoxia. Mortality was decreased in hyperoxia. These results indicate that maturation of 5-HT neurons contributes to development of respiratory CO₂/pH chemoreception during the first few weeks of life in mice in vivo. A defect in the 5-HT system in early postnatal life decreases survival due in part to hypoxia.

Serotonin in the solitary tract nucleus shortens the laryngeal chemoreflex in anaesthetized neonatal rats

What is the central question of this study? Failure to terminate apnoea and arouse is likely to contribute to sudden infant death syndrome (SIDS). Serotonin is deficient in the brainstems of babies who died of SIDS. Therefore, we tested the hypothesis that serotonin in the nucleus of the solitary tract (NTS) would shorten reflex apnoea. What is the main finding and its importance? Serotonin microinjected into the NTS shortened the apnoea and respiratory inhibition associated with the laryngeal chemoreflex. Moreover, this effect was achieved through a 5-HT3 receptor. This is a new insight that is likely to be relevant to the pathogenesis of SIDS. The laryngeal chemoreflex (LCR), an airway-protective reflex that causes apnoea and bradycardia, has long been suspected as an initiating event in the sudden infant death syndrome. Serotonin (5-HT) and 5-HT receptors may be deficient in the brainstems of babies who die of sudden infant death syndrome, and 5-HT seems to be important in terminating apnoeas directly or in causing arousals or as part of the process of autoresuscitation. We hypothesized that 5-HT in the brainstem would limit the duration of the LCR. We studied anaesthetized rat pups between 7 and 21 days of age and made microinjections into the cisterna magna or into the nucleus of the solitary tract (NTS). Focal, bilateral microinjections of 5-HT into the caudal NTS significantly shortened the LCR. The 5-HT1a receptor antagonist, WAY 100635, did not affect the LCR consistently, nor did a 5-HT2 receptor antagonist, ketanserin, alter the duration of the LCR. The 5-HT3 specific agonist, 1-(3-chlorophenyl)-biguanide, microinjected bilaterally into the caudal NTS significantly shortened the LCR. Thus, endogenous 5-HT released within the NTS may curtail the respiratory depression that is part of the LCR, and serotonergic shortening of the LCR may be attributed to activation of 5-HT3 receptors within the NTS. 5-HT3 receptors are expressed presynaptically on C fibre afferents of the superior laryngeal nerve, and serotonergic shortening of the LCR may be mediated presynaptically by enhanced activation of inhibitory interneurons within the NTS.

Sudden Unexpected Death in Fetal Life Through Early Childhood

In March 2015, the Eunice Kennedy Shriver National Institute of Child Health and Human Development held a workshop entitled "Sudden Unexpected Death in Fetal Life Through Early Childhood: New Opportunities." Its objective was to advance efforts to understand and ultimately prevent sudden deaths in early life, by considering their pathogenesis as a potential continuum with some commonalities in biological origins or pathways. A second objective of this meeting was to highlight current issues surrounding the classification of sudden infant death syndrome (SIDS), and the implications of variations in the use of the term "SIDS" in forensic practice, and pediatric care and research. The proceedings reflected the most current knowledge and understanding of the origins and biology of vulnerability to sudden unexpected death, and its environmental triggers. Participants were encouraged to consider the application of new technologies and "omics" approaches to accelerate research. The major advances in delineating the intrinsic vulnerabilities to sudden death in early life have come from epidemiologic, neural, cardiac, metabolic, genetic, and physiologic research, with some commonalities among cases of unexplained stillbirth, SIDS, and sudden unexplained death in childhood observed. It was emphasized that investigations of sudden unexpected death are inconsistent, varying by jurisdiction, as are the education, certification practices, and experience of death certifiers. In addition, there is no practical consensus on the use of "SIDS" as a determination in cause of death. Major clinical, forensic, and scientific areas are identified for future research.

Synaptogenesis and Myelination in the Nucleus/Tractus Solitarius: Potential Role in Apnea of Prematurity, Congenital Central Hypoventilation, and Sudden Infant Death Syndrome

Fetuses as early as 15 weeks' gestation exhibit rhythmical respiratory movements shown by real-time ultrasonography. The nucleus/tractus solitarius is the principal brainstem respiratory center; other medullary nuclei also participate. The purpose was to determine temporal maturation of synaptogenesis. Delayed synaptic maturation may explain neurogenic apnea or hypoventilation of prematurity and some cases of sudden infant death syndrome. Sections of medulla oblongata were studied from 30 human fetal and neonatal brains 9 to 41 weeks' gestation. Synaptophysin demonstrated the immunocytochemical sequence of synaptogenesis. Other neuronal markers and myelin stain also were applied. The nucleus/tractus solitarius was similarly studied in fetuses with chromosomopathies, metabolic encephalopathies, and brain malformations. Synapse formation in the nucleus solitarius begins at about 12 weeks' gestation and matures by 15 weeks; myelination initiated at 33 weeks. Synaptogenesis was delayed in 3 fetuses with different conditions, but was not specific for only nucleus solitarius. Delayed synaptogenesis or myelination in the nucleus solitarius may play a role in neonatal hypoventilation, especially in preterm infants and in some sudden infant death syndrome cases.

Neurotoxicity of prenatal alcohol exposure on medullary pre-Bötzinger complex neurons in neonatal rats

Prenatal alcohol exposure disrupts the development of normal fetal respiratory function, but whether it perturbs respiratory rhythmical discharge activity is unclear. Furthermore, it is unknown whether the 5-hydroxytryptamine 2A receptor (5-HT_2AR) is involved in the effects of prenatal alcohol exposure. In the present study, pregnant female rats received drinking water containing alcohol at concentrations of 0%, 1%, 2%, 4%, 8% or 10% (v/v) throughout the gestation period. Slices of the medulla from 2-day-old neonatal rats were obtained to record respiratory rhythmical discharge activity. 5-HT_2AR protein and mRNA levels in the pre-Bötzinger complex of the respiratory center were measured by western blot analysis and quantitative RT-PCR, respectively. Compared with the 0% alcohol group, respiratory rhythmical discharge activity in medullary slices in the 4%, 8% and 10% alcohol groups was decreased, and the reduction was greatest in the 8% alcohol group. Respiratory rhythmical discharge activity in the 10% alcohol group was irregular. Thus, 8% was the most effective alcohol concentration at attenuating respiratory rhythmical discharge activity. These findings suggest that prenatal alcohol exposure attenuates respiratory rhythmical discharge activity in neonatal rats by downregulating 5-HT_2AR protein and mRNA levels.

Cytoarchitectural and functional abnormalities of the inferior colliculus in sudden unexplained perinatal death

The inferior colliculus is a mesencephalic structure endowed with serotonergic fibers that plays an important role in the processing of acoustic information. The implication of the neuromodulator serotonin also in the aetiology of sudden unexplained fetal and infant death syndromes and the demonstration in these pathologies of developmental alterations of the superior olivary complex (SOC), a group of pontine nuclei likewise involved in hearing, prompted us to investigate whether the inferior colliculus may somehow contribute to the pathogenetic mechanism of unexplained perinatal death. Therefore, we performed in a wide set of fetuses and infants, aged from 33 gestational weeks to 7 postnatal months and died of both known and unknown cause, an in-depth anatomopathological analysis of the brainstem, particularly of the midbrain. Peculiar neuroanatomical and functional abnormalities of the inferior colliculus, such as hypoplasia/structural disarrangement and immunonegativity or poor positivity of serotonin, were exclusively found in sudden death victims, and not in controls. In addition, these alterations were frequently related to dysgenesis of connected structures, precisely the raphé nuclei and the superior olivary complex, and to nicotine absorption in pregnancy. We propose, on the basis of these results, the involvement of the inferior colliculus in more important functions than those related to hearing, as breathing and, more extensively, all the vital activities, and then in pathological conditions underlying a sudden death in vulnerable periods of the autonomic nervous system development, particularly associated to harmful risk factors as cigarette smoking.

Assigning cause for sudden unexpected infant death

We have reached a conundrum in assigning cause of death for sudden unexpected infant deaths. We summarize the discordant perspectives and approaches and how they have occurred, and recommend a pathway toward improved consistency. This lack of consistency affects pediatricians and other health care professionals, scientific investigators, medical examiners and coroners, law enforcement agencies, families, and support or advocacy groups. We recommend that an interdisciplinary international committee be organized to review current approaches for assigning cause of death, and to identify a consensus strategy for improving consistency. This effort will need to encompass intrinsic risk factors or infant vulnerability in addition to known environmental risk factors including unsafe sleep settings, and must be sufficiently flexible to accommodate a progressively expanding knowledge base.

Dentate gyrus abnormalities in sudden unexplained death in infants: morphological marker of underlying brain vulnerability

Sudden unexplained death in infants, including the sudden infant death syndrome, is likely due to heterogeneous causes that involve different intrinsic vulnerabilities and/or environmental factors. Neuropathologic research focuses upon the role of brain regions, particularly the brainstem, that regulate or modulate autonomic and respiratory control during sleep or transitions to waking. The hippocampus is a key component of the forebrain-limbic network that modulates autonomic/respiratory control via brainstem connections, but its role in sudden infant death has received little attention. We tested the hypothesis that a well-established marker of hippocampal pathology in temporal lobe epilepsy-focal granule cell bilamination in the dentate, a variant of granule cell dispersion-is associated with sudden unexplained death in infants. In a blinded study of hippocampal morphology in 153 infants with sudden and unexpected death autopsied in the San Diego County medical examiner's office, deaths were classified as unexplained or explained based upon autopsy and scene investigation. Focal granule cell bilamination was present in 41.2% (47/114) of the unexplained group compared to 7.7% (3/39) of the explained (control) group (p < 0.001). It was associated with a cluster of other dentate developmental abnormalities that reflect defective neuronal proliferation, migration, and/or survival. Dentate lesions in a large subset of infants with sudden unexplained death may represent a developmental vulnerability that leads to autonomic/respiratory instability or autonomic seizures, and sleep-related death when the infants are challenged with homeostatic stressors. Importantly, these lesions can be recognized in microscopic sections prepared in current forensic practice. Future research is needed to determine the relationship between hippocampal and previously reported brainstem pathology in sudden infant death.

Neurochemical abnormalities in the brainstem of the Sudden Infant Death Syndrome (SIDS)

The brainstem has been a focus in Sudden Infant Death Syndrome (SIDS) research for 30 years. Physiological and animal model data show that cardiorespiratory, sleep, and arousal mechanisms are abnormal after exposure to SIDS risk factors or in infants who subsequently die from SIDS. As the brainstem houses the regulatory centres for these functions, it is the most likely site to find abnormalities. True to this hypothesis, data derived over the last 30 years shows that the brainstem of infants who died from SIDS exhibits abnormalities in a number of major neurotransmitter and receptor systems including: catecholamines, neuropeptides, acetylcholinergic, indole amines (predominantly serotonin and its receptors), amino acids (predominantly glutamate), brain derived neurotrophic growth factor (BDNF), and some cytokines. A pattern is emerging of particular brainstem nuclei being consistently affected including the dorsal motor nucleus of the vagus (DMNV), nucleus of the solitary tract (NTS), arcuate nucleus (AN) and raphe. We discuss the implications of these findings and directions that this may lead in future research.

Serotonin metabolites in the cerebrospinal fluid in sudden infant death syndrome

Forensic biomarkers are needed in sudden infant death syndrome (SIDS) to help identify this group among other sudden unexpected deaths in infancy. Previously, we reported multiple serotonergic (5-HT) abnormalities in nuclei of the medulla oblongata that help mediate protective responses to homeostatic stressors. As a first step toward their assessment as forensic biomarkers of medullary pathology, here we test the hypothesis that 5-HT-related measures are abnormal in the cerebrospinal fluid (CSF) of SIDS infants compared with those of autopsy controls. Levels of CSF 5-hydroxyindoleacetic acid (5-HIAA) and homovanillic acid (HVA), the degradative products of 5-HT and dopamine, respectively, were measured by high-performance liquid chromatography in 52 SIDS and 29 non-SIDS autopsy cases. Tryptophan (Trp) and tyrosine (Tyr), the substrates of 5-HT and dopamine, respectively, were also measured. There were no significant differences in 5-HIAA, Trp, HVA, or Tyr levels between the SIDS and non-SIDS groups. These data preclude the use of 5-HIAA, HVA, Trp, or Tyr measurements as CSF autopsy biomarkers of 5-HT medullary pathology in infants who have died suddenly and unexpectedly. They do, however, provide important information about monoaminergic measurements in human CSF at autopsy and their developmental profile in infancy that is applicable to multiple pediatric disorders beyond SIDS.

Serotonin gene variants are unlikely to play a significant role in the pathogenesis of the sudden infant death syndrome

Sudden infant death syndrome (SIDS) is defined as the sudden and unexpected death of an infant less than 12 months of age that is related to a sleep period and remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. The cause of SIDS is unknown, but a major subset of SIDS is proposed to result from abnormalities in serotonin (5-HT) and related neurotransmitters in regions of the lower brainstem that result in failure of protective homeostatic responses to life-threatening challenges during sleep. Multiple studies have implicated gene variants that affect different elements of 5-HT neurotransmission in the pathogenesis of these abnormalities in SIDS. In this review I discuss the data from these studies together with some new data correlating genotype with brainstem 5-HT neurochemistry in the same SIDS cases and conclude that these gene variants are unlikely to play a major role in the pathogenesis of the medullary 5-HT abnormalities observed in SIDS.

Time- and age-dependent effects of serotonin on gasping and autoresuscitation in neonatal mice

The role of brain stem serotonin (5-hydroxytryptamine, 5-HT) in autoresuscitation in neonatal life is unclear. We hypothesized that a specific loss of 5-HT would compromise gasping and autoresuscitation mainly in the second postnatal week and that acute restoration of 5-HT would reverse the defects. We exposed postnatal day (P)4-5, P8-9, and P11-12 tryptophan-hydroxylase-2 knockout (TPH2(-/-)) and wild-type littermates (WT) to 10 episodes of anoxia (97% N2, 3% CO2), measuring survival, gasp latency, gasp frequency (fB), and the time required to restore eupnea and heart rate. We also tested P8-9 TPH2(-/-) mice after restoring 5-HT with a single injection of 5-hydroxytryptophan (5-HTP) 1-2 h before testing or with multiple injections beginning 24 h before testing. At P4-5 and P8-9, but not at P11-12, gasp latency and the recovery of eupnea were delayed ~2- to 3-fold in TPH2(-/-) pups compared with WT (P < 0.001). At all ages, TPH2(-/-) pups displayed reduced gasp fB (~20-30%; P < 0.001) and delayed heart rate recovery (~60%; P = 0.002) compared with WT littermates. TPH2(-/-) survival was reduced compared with WT (P < 0.001), especially at P8-9 and P11-12 (P = 0.004). Whereas 1-2 h of 5-HTP treatment improved the gasp latency and fB of P8-9 TPH2(-/-) pups, improved cardiorespiratory recovery and survival required 24 h of treatment. Our data suggest that 5-HT operates over a long time span (24 h) to improve survival during episodic severe hypoxia. Early in development (P4-9), 5-HT is critical for both respiratory and cardiovascular components of autoresuscitation; later (P11-12), it is critical mainly for cardiovascular components. Nevertheless, the effect of 5-HT deficiency on survival is most striking from P8 to P12.

Gestational stress promotes pathological apneas and sex-specific disruption of respiratory control development in newborn rat

Recurrent apneas are important causes of hospitalization and morbidity in newborns. Gestational stress (GS) compromises fetal brain development. Maternal stress and anxiety during gestation are linked to respiratory disorders in newborns; however, the mechanisms remain unknown. Here, we tested the hypothesis that repeated activation of the neuroendocrine response to stress during gestation is sufficient to disrupt the development of respiratory control and augment the occurrence of apneas in newborn rats. Pregnant dams were displaced and exposed to predator odor from days 9 to 19 of gestation. Control dams were undisturbed. Experiments were performed on male and female rats aged between 0 and 4 d old. Apnea frequency decreased with age but was consistently higher in stressed pups than controls. At day 4, GS augmented the proportion of apneas with O(2) desaturations by 12%. During acute hypoxia (12% O(2)), the reflexive increase in breathing augmented with age; however, this response was lower in stressed pups. Instability of respiratory rhythm recorded from medullary preparations decreased with age but was higher in stressed pups than controls. GS reduced medullary serotonin (5-HT) levels in newborn pups by 32%. Bath application of 5-HT and injection of 8-OH-DPAT [(±)-8-hydroxy-2-di-(n-propylamino) tetralin hydrobromide; 5-HT(1A) agonist; in vivo] reduced respiratory instability and apneas; these effects were greater in stressed pups than controls. Sex-specific effects were observed. We conclude that activation of the stress response during gestation is sufficient to disrupt respiratory control development and promote pathological apneas in newborn rats. A deficit in medullary 5-HT contributes to these effects.

Congenital central hypoventilation syndrome and sudden infant death syndrome: disorders of autonomic regulation

Long considered a rare and unique disorder of respiratory control, congenital central hypoventilation syndrome has recently been further distinguished as a disorder of autonomic regulation. Similarly, more recent evidence suggests that sudden infant death syndrome is also a disorder of autonomic regulation. Congenital central hypoventilation syndrome typically presents in the newborn period with alveolar hypoventilation, symptoms of autonomic dysregulation and, in a subset of cases, Hirschsprung disease or tumors of neural crest origin or both. Genetic investigation identified PHOX2B, a crucial gene during early autonomic development, as disease defining for congenital central hypoventilation syndrome. Although sudden infant death syndrome is most likely defined by complex multifactorial genetic and environmental interactions, it is also thought to result from central deficits in the control of breathing and autonomic regulation. The purpose of this article is to review the current understanding of these autonomic disorders and discuss the influence of this information on clinical practice and future research directions.

Sudden infant death syndrome: an update and new perspectives of etiology

Sudden infant death syndrome (SIDS) is a condition in which an infant, usually in the early postnatal period and nearly always before 6 months of age, dies during sleep for unexplained reasons and the standard autopsy fails to disclose an etiology. Various physiological explanations of risk factors include the prone sleeping position, overheating by excessive bundling, viral upper respiratory tract infections, parental smoking at home, and birthing injury resulting in an insult to the inner ear and central chemoreceptor zone, an immaturity that involves CO2 chemoreceptors that regulate respiratory control. Neuropathological studies and theories implicate: (1) hypoplasia or defective transmitter function in the medullary arcuate nucleus, a derivative of the rhombencephalic lip of His; (2) synaptic or receptor immaturity of the nucleus of the fasciculus solitarius, the "pneumotaxic center"; and (3) functional impairment of the serotonergic raphé nuclei of the pontine and medullary ventral median septum and other serotonergic neurons of the brainstem. Additional neurological risk factors for SIDS include perinatal neuromuscular diseases, infantile epilepsies or status epilepticus, and genetic metabolic encephalopathies.

Caffeine improves the ability of serotonin-deficient (Pet-1-/-) mice to survive episodic asphyxia

BACKGROUND

In neonatal rodents, serotonin (5-HT) neurons are critical for successful autoresuscitation. We hypothesized that caffeine, a respiratory stimulant, would hasten the onset of gasping and improve autoresuscitation in 5-HT-deficient, Pet-1(-/-) mice.

METHODS

Using a head-out system and electrocardiogram, we measured respiratory and heart rate (HR) responses of Pet-1(-/-) rodents and their littermates during episodic asphyxia at postnatal days 8-9 (P8-9). After a baseline recording, we injected either vehicle or caffeine (i.p.) at doses of 1, 5, or 10 mg/kg. We then induced 10 brief (~30 s) episodes of asphyxia, each interspersed with 5 min of room air to allow autoresuscitation. In addition to measuring survival, we measured the duration of hypoxic apnea (time to initiate gasping) and time to recover eupnea and HR.

RESULTS

Caffeine had a dose-dependent effect of hastening the onset of gasping, recovery of breathing, and restoration of HR in Pet-1(-/-) (but not in wild-type) rodents, thereby improving survival across asphyxic episodes. Increased survival was strongly correlated with hastened onset of gasping.

CONCLUSION

Our data suggest that caffeine reduces mortality relating to asphyxia and 5-HT deficiency. These findings may be relevant for efforts to reduce the incidence of sudden infant death syndrome (SIDS), given that SIDS is associated with failed autoresuscitation and reduced brainstem 5-HT.

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.

The dorsal motor nucleus of the vagus (DMNV) in sudden infant death syndrome (SIDS): pathways leading to apoptosis

Sudden infant death syndrome (SIDS) remains the commonest cause of death in the post-neonatal period in the developed world. A leading hypothesis is that an abnormality in the brainstem of infants who succumb to SIDS, either causes or predisposes to failure to respond appropriately to an exogenous stressor. Neuronal apoptosis can lead to loss of cardiorespiratory reflexes, compromise of the infant's ability to respond to stressors such as hypoxia, and ultimately a sleep-related death. The dorsal motor nucleus of the vagus (DMNV) is a medullary autonomic nucleus where abnormalities have regularly been identified in SIDS research. This review collates neurochemical findings documented over the last 30 years, including data from our laboratory focusing on neuronal apoptosis and the DMNV, and provides potential therapeutic interventions targeting neurotransmitters, growth factors and/or genes.

Subtle alterations in breathing and heart rate control in the 5-HT1A receptor knockout mouse in early postnatal development

We hypothesized that absence of the 5-HT(1A) receptor would negatively affect the development of cardiorespiratory control. In conscious wild type (WT) and 5-HT(1A) receptor knockout (KO) mice, we measured resting ventilation (Ve), oxygen consumption (Vo(2)), heart rate (HR), breathing and HR variability, and the hypercapnic ventilatory response (HCVR) at postnatal day 5 (P5), day 15 (P15), and day 25 (P25). In KO mice compared with WT, we found a 17% decrease in body weight at only P5 (P < 0.01) and no effect on Vo(2). Ve was significantly (P < 0.001) lower at P5 and P25, but there was no effect on the HCVR. Breathing variability (interbreath interval), measured by standard deviation, the root mean square of the standard deviation (RMSSD), and the product of the major (L) and minor axes (T) of the Poincaré first return plot, was 57% to 187% higher only at P5 (P < 0.001). HR was 6-10% slower at P5 (P < 0.001) but 7-9% faster at P25 (P < 0.001). This correlated with changes in the spectral analysis of HR variability; the low frequency to high frequency ratio was 47% lower at P5 but 68% greater at P25. The RMSSD and (L × T) of HR variability were ~2-fold greater at P5 only (P < 0.001; P < 0.05). We conclude that 5-HT(1A) KO mice have a critical period of potential vulnerability at P5 when pups hypoventilate and have a slower respiratory frequency and HR with enhanced variability of both, suggesting abnormal maturation of cardiorespiratory control.

Neuroimaging of the periaqueductal gray: state of the field

This review and meta-analysis aims at summarizing and integrating the human neuroimaging studies that report periaqueductal gray (PAG) involvement; 250 original manuscripts on human neuroimaging of the PAG were identified. A narrative review and meta-analysis using activation likelihood estimates is included. Behaviors covered include pain and pain modulation, anxiety, bladder and bowel function and autonomic regulation. Methods include structural and functional magnetic resonance imaging, functional connectivity measures, diffusion weighted imaging and positron emission tomography. Human neuroimaging studies in healthy and clinical populations largely confirm the animal literature indicating that the PAG is involved in homeostatic regulation of salient functions such as pain, anxiety and autonomic function. Methodological concerns in the current literature, including resolution constraints, imaging artifacts and imprecise neuroanatomical labeling are discussed, and future directions are proposed. A general conclusion is that PAG neuroimaging is a field with enormous potential to translate animal data onto human behaviors, but with some growing pains that can and need to be addressed in order to add to our understanding of the neurobiology of this key region.

Expression of brain-derived neurotrophic factor and TrkB receptor in the sudden infant death syndrome brainstem

This study compared the expression of BDNF (proBDNF and rhBDNF forms) and its receptor TrkB, in the medulla of sudden infant death syndrome (SIDS) infants and infants who died from known causes (non-SIDS). This study also evaluated these markers in association with SIDS clinical risk factors including, sleep position, cigarette smoke exposure and gender. Brainstem tissue was immunohistochemically stained and quantitative analyses were made for eight nuclei of the caudal and rostral medulla. Compared to non-SIDS, SIDS infants had lower rhBDNF in the caudal nucleus of the solitary tract and higher TrkB in the caudal dorsal motor nucleus of the vagus. Within the SIDS cohort, prone sleep position was associated with lower rhBDNF in the caudal arcuate nucleus, and cigarette smoke exposure was associated with lower rhBDNF and TrkB in the inferior olivary nucleus. Abnormal expression of BDNF and TrkB suggests that neuroprotective functions of the BDNF/TrkB system may be reduced in respiratory-related nuclei of SIDS infants.

The serotonergic anatomy of the developing human medulla oblongata: implications for pediatric disorders of homeostasis

The caudal serotonergic (5-HT) system is a critical component of a medullary "homeostatic network" that regulates protective responses to metabolic stressors such as hypoxia, hypercapnia, and hyperthermia. We define anatomically the caudal 5-HT system in the human medulla as 5-HT neuronal cell bodies located in the raphé (raphé obscurus, raphé magnus, and raphé pallidus), extra-raphé (gigantocellularis, paragigantocellularis lateralis, intermediate reticular zone, lateral reticular nucleus, and nucleus subtrigeminalis), and ventral surface (arcuate nucleus). These 5-HT neurons are adjacent to all of the respiratory- and autonomic-related nuclei in the medulla where they are positioned to modulate directly the responses of these effector nuclei. In the following review, we highlight the topography and development of the caudal 5-HT system in the human fetus and infant, and its inter-relationships with nicotinic, GABAergic, and cytokine receptors. We also summarize pediatric disorders in early life which we term "developmental serotonopathies" of the caudal (as well as rostral) 5-HT domain and which are associated with homeostatic imbalances. The delineation of the development and organization of the human caudal 5-HT system provides the critical foundation for the neuropathologic elucidation of its disorders directly in the human brain.

Localization of 5-HT1A and 5-HT2A positive cells in the brainstems of control age-matched and Alzheimer individuals

Serotonin receptor 1A and 2A positive cells in postmortem brainstems were demonstrated via immunohistochemistry in eight control age-matched elderly individuals and eight Alzheimer patients. The 5-HT1A positive cells were found in substantia nigra, pontile nucleus, and vagal as well as dorsal raphe nucleus, while 5-HT2A receptor positive cells were found in motor, sensory and spinal trigeminal nuclei, pontile nucleus, substantia nigra, and nucleus solitarius. A comparison in density of positive cells per unit area was made between control age-matched and Alzheimer individuals. Statistically significant differences (p ≤ 0.01) in density were observed in 5-HT1A cells in pontile, dorsal raphe, and vagal nuclei between control age-matched and Alzheimer, and in 5-HT2A positive cells in the sensory trigeminal nucleus, between control and Alzheimer. This de novo study indicated the presence of 5-HT1A and 5-HT2A receptor positive cells in the above nuclei of human brainstem and revealed differences in density between control age-matched and Alzheimer, indicating possible functional derangements in Alzheimer patients in these areas. In addition, colocalization studies indicated that 5-HT1A receptors were in cholinergic cells and gamma-aminobutyric acid positive fibers were linked to 5-HT2A receptor positive cells. It is hoped that understanding these two important 5-HT receptors and their localization might lead to advances in future therapeutic development.

Genomic risk factors in sudden infant death syndrome

Sudden infant death syndrome (SIDS) is a major contributor to postneonatal infant death, and is the third leading cause of infant mortality in the USA. While public health efforts have reduced these deaths in recent years, the pathogenesis of SIDS remains unclear. Epidemiological data on SIDS-related deaths have suggested genetic factors, and many studies have attempted to identify SIDS-associated genes. This has resulted in a large body of literature implicating various genes and their encoded proteins and signaling pathways in numerous cohorts of various sizes and ethnicities. This review has undertaken a systematic evaluation of these studies, identifying the pathways that have been implicated in these studies, including central nervous system pathways, cardiac channelopathies, immune dysfunction, metabolism/energy pathways, and nicotine response. This review also explores how new genomic techniques will aid in advancing our knowledge of the genomic risk factors associated with SIDS, including SNPs and copy number variation. Last, this review explores how the current information can be applied to aid in our assessment of the at risk infant population.

Lack of association of the serotonin transporter polymorphism with the sudden infant death syndrome in the San Diego Dataset

Dysfunction of medullary serotonin (5-HT)-mediated respiratory and autonomic function is postulated to underlie the pathogenesis of the majority of sudden infant death syndrome (SIDS) cases. Several studies have reported an increased frequency of the LL genotype and L allele of the 5-HT transporter (5-HTT) gene promoter polymorphism (5-HTTLPR), which is associated with increased transcriptional activity and 5-HT transport in vitro, in SIDS cases compared with controls. These findings raise the possibility that this polymorphism contributes to or exacerbates existing medullary 5-HT dysfunction in SIDS. In this study, we tested the hypothesis that the frequency of LL genotype and L allele are higher in 179 SIDS cases compared with 139 controls of multiple ethnicities in the San Diego SIDS Dataset. We observed no significant association of genotype or allele with SIDS cases either in the total cohort or on stratification for ethnicity. These observations do not support previous findings that the L allele and/or LL genotype of the 5-HTTLPR are associated with SIDS.

Genetics of the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is currently defined as "the sudden unexpected death of an infant less than 1 year of age with onset of the fatal episode apparently occurring during sleep, that remains unexplained after a thorough investigation". SIDS, whose etiology remains rather vague, is still the major cause of death among infants between 1 month and 1 year of age in industrialized countries with varying incidences in different populations. Herein, after touching on definitory approaches and several current hypotheses concerning SIDS etiology, we focus on the triple risk model of SIDS and discuss two large classes of genetic factors potentially contributing to or predisposing for the generation of a vulnerable infant that, when encountering an environmental trigger, may succumb to SIDS. We conclude by acknowledging that for the integration of the vast and complex genetic evidence concerning SIDS, a lot more research will be required and we briefly discuss the potential use of recently presented animal models for functional studies of SIDS pathology.

Neuroanatomic relationships between the GABAergic and serotonergic systems in the developing human medulla

gamma-Amino butyric (GABA) critically influences serotonergic (5-HT) neurons in the raphé and extra-raphé of the medulla oblongata. In this study we hypothesize that there are marked changes in the developmental profile of markers of the human medullary GABAergic system relative to the 5-HT system in early life. We used single- and double-label immunocytochemistry and tissue receptor autoradiography in 15 human medullae from fetal and infant cases ranging from 15 gestational weeks to 10 postnatal months, and compared our findings with an extensive 5-HT-related database in our laboratory. In the raphé obscurus, we identified two subsets of GABAergic neurons using glutamic acid decarboxylase (GAD65/67) immunostaining: one comprised of small, round neurons; the other, medium, spindle-shaped neurons. In three term medullae cases, positive immunofluorescent neurons for both tryptophan hydroxylase and GAD65/67 were counted within the raphé obscurus. This revealed that approximately 6% of the total neurons counted in this nucleus expressed both GAD65/67 and TPOH suggesting co-production of GABA by a subset of 5-HT neurons. The distribution of GABA(A) binding was ubiquitous across medullary nuclei, with highest binding in the raphé obscurus. GABA(A) receptor subtypes alpha1 and alpha3 were expressed by 5-HT neurons, indicating the site of interaction of GABA with 5-HT neurons. These receptor subtypes and KCC2, a major chloride transporter, were differentially expressed across early development, from midgestation (20 weeks) and thereafter. The developmental profile of GABAergic markers changed dramatically relative to the 5-HT markers. These data provide baseline information for medullary studies of human pediatric disorders, such as sudden infant death syndrome.

Brainstem serotonergic deficiency in sudden infant death syndrome

CONTEXT

Sudden infant death syndrome (SIDS) is postulated to result from abnormalities in brainstem control of autonomic function and breathing during a critical developmental period. Abnormalities of serotonin (5-hydroxytryptamine [5-HT]) receptor binding in regions of the medulla oblongata involved in this control have been reported in infants dying from SIDS.

OBJECTIVE

To test the hypothesis that 5-HT receptor abnormalities in infants dying from SIDS are associated with decreased tissue levels of 5-HT, its key biosynthetic enzyme (tryptophan hydroxylase [TPH2]), or both.

DESIGN, SETTING, AND PARTICIPANTS

Autopsy study conducted to analyze levels of 5-HT and its metabolite, 5-hydroxyindoleacetic acid (5-HIAA); levels of TPH2; and 5-HT(1A) receptor binding. The data set was accrued between 2004 and 2008 and consisted of 41 infants dying from SIDS (cases), 7 infants with acute death from known causes (controls), and 5 hospitalized infants with chronic hypoxia-ischemia.

MAIN OUTCOME MEASURES

Serotonin and metabolite tissue levels in the raphé obscurus and paragigantocellularis lateralis (PGCL); TPH2 levels in the raphé obscurus; and 5-HT(1A) binding density in 5 medullary nuclei that contain 5-HT neurons and 5 medullary nuclei that receive 5-HT projections.

RESULTS

Serotonin levels were 26% lower in SIDS cases (n = 35) compared with age-adjusted controls (n = 5) in the raphé obscurus (55.4 [95% confidence interval {CI}, 47.2-63.6] vs 75.5 [95% CI, 54.2-96.8] pmol/mg protein, P = .05) and the PGCL (31.4 [95% CI, 23.7-39.0] vs 40.0 [95% CI, 20.1-60.0] pmol/mg protein, P = .04). There was no evidence of excessive 5-HT degradation assessed by 5-HIAA levels, 5-HIAA:5-HT ratio, or both. In the raphé obscurus, TPH2 levels were 22% lower in the SIDS cases (n = 34) compared with controls (n = 5) (151.2% of standard [95% CI, 137.5%-165.0%] vs 193.9% [95% CI, 158.6%-229.2%], P = .03). 5-HT(1A) receptor binding was 29% to 55% lower in 3 medullary nuclei that receive 5-HT projections. In 4 nuclei, 3 of which contain 5-HT neurons, there was a decrease with age in 5-HT(1A) receptor binding in the SIDS cases but no change in the controls (age x diagnosis interaction). The profile of 5-HT and TPH2 abnormalities differed significantly between the SIDS and hospitalized groups (5-HT in the raphé obscurus: 55.4 [95% CI, 47.2-63.6] vs 85.6 [95% CI, 61.8-109.4] pmol/mg protein, P = .02; 5-HT in the PGCL: 31.4 [95% CI, 23.7-39.0] vs 71.1 [95% CI, 49.0-93.2] pmol/mg protein, P = .002; TPH2 in the raphé obscurus: 151.2% [95% CI, 137.5%-165.0%] vs 102.6% [95% CI, 58.7%-146.4%], P = .04).

CONCLUSION

Compared with controls, SIDS was associated with lower 5-HT and TPH2 levels, consistent with a disorder of medullary 5-HT deficiency.

Potential Mechanisms of Failure in the Sudden Infant Death Syndrome

Current evidence suggests that multiple neural mechanisms contribute to the fatal lethal event in SIDS. The processes may develop from a range of otherwise seemingly-innocuous circumstances, such as unintended external airway obstruction or accidental extreme flexion of the head of an already-compromised structure of the infant upper airway. The fatal event may occur in a sleep state which can suppress muscle tone essential to restore airway patency or exert muscle action to overcome a profound loss of blood pressure. Neural processes that could overcome those transient events with reflexive compensation appear to be impaired in SIDS infants. The evidence ranges from subtle physiological signs that appear very early in life, to autopsy findings of altered neurotransmitter, including serotonergic, systems that have extensive roles in breathing, cardiovascular regulation, and thermal control. Determination of the fundamental basis of SIDS is critical to provide biologic plausibility to SIDS risk reduction messages and to develop specific prevention strategies.

Bioaminergic neuromodulation of respiratory rhythm in vitro

Bioamines, such as norepinephrine and serotonin are key neurotransmitters implicated in multiple physiological and pathological brain mechanisms. Evolutionarily, the bioaminergic neuromodulatory system is widely distributed throughout the brain and is among the earliest neurotransmitters to arise within the hindbrain. In both vertebrates and invertebrates, monoamines play a critical role in the control of respiration. In mammals, both norepinephrine and serotonin are involved in the maturation of the respiratory network, as well as in the neuromodulation of intrinsic and synaptic properties, that not only differentially alters the activity of individual respiratory neurons but also the activity of the network during normoxic and hypoxic conditions. Here, we review the basic noradrenergic and serotonergic pathways and their impact on the activity of the pre-Bötzinger Complex inspiratory neurons and network activity.

Serotonin-related FEV gene variant in the sudden infant death syndrome is a common polymorphism in the African-American population

An important subset of the sudden infant death syndrome (SIDS) is associated with multiple serotonergic (5-HT) abnormalities in regions of the medulla oblongata. The mouse ortholog of the fifth Ewing variant gene (FEV) is critical for 5-HT neuronal development. A putatively rare intronic variant [IVS2-191_190insA, here referred to as c.128-(191_192)dupA] has been reported as a SIDS-associated mutation in an African-American population. We tested this association in an independent dataset: 137 autopsied cases (78 SIDS, 59 controls) and an additional 296 control DNA samples from Coriell Cell Repositories. In addition to the c.128-(191_192)dupA variant, we observed an associated single-base deletion [c.128-(301-306)delG] in a subset of the samples. Neither of the two FEV variants showed significant association with SIDS in either the African-American subgroup or the overall cohort. Although we found a significant association of c.128-(191_192)dupA with SIDS when San Diego Hispanic SIDS cases were compared with San Diego Hispanic controls plus Mexican controls (p = 0.04), this became nonsignificant after multiple testing correction. Among Coriell controls, 33 of 99 (33%) African-American and 0 of 197 (0%) of the remaining controls carry the polymorphism (c.128-(191_192)dupA). The polymorphism seems to be a common, likely nonpathogenic, variant in the African-American population.

Optimisation of postmortem tissue preservation and alternative protocol for serotonin transporter gene polymorphisms amplification in SIDS and SIUD cases

The major obstacle to genetic research in SIUD (sudden intrauterine unexplained death) and SIDS (sudden infant death syndrome) cases is the complex characteristics of the human anatomic samples available. In fact, in Italy autopsies are performed at least 24 h post-mortem and tissues can be left in formalin for long fixation times (>4/5 days), thus compromising nucleic acids integrity. In this study we compared the quality of DNA and RNA extracted from tissues differently preserved. As expected, the DNA and RNA from formalin-fixed and paraffin-embedded tissues, formalin-acetic acid-alcohol tissues and ethanol tissues were of poor quality and not adequate for subsequent molecular analysis. The best results were obtained with RNAlater preserved tissues: this buffer was equivalent, if not superior, to freezing method for preservation of postmortem DNA and RNA. In addition, we introduce a new protocol for the amplification of the serotonin transporter gene promoter region (5-HTT) ideal to obtain the increase of specific product, avoiding artifacts formation.