A serotonin malfunction hypothesis by finding clear mutual relationships between several risk factors and symptoms associated with sudden infant death syndrome

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.

Impaired chemosensitivity of mouse dorsal raphe serotonergic neurons overexpressing serotonin 1A (Htr1a) receptors

Background

Serotonergic system participates in a wide range of physiological processes and behaviors, but its role is generally considered as modulatory and noncrucial, especially concerning life-sustaining functions. We recently created a transgenic mouse line in which a functional deficit in serotonin homeostasis due to excessive serotonin autoinhibition was produced by inducing serotonin 1A receptor (Htr1a) overexpression selectively in serotonergic neurons (Htr1a raphe-overexpressing or Htr1a^RO mice). Htr1a^RO mice exhibit episodes of autonomic dysregulation, cardiovascular crises and death, resembling those of sudden infant death syndrome (SIDS) and revealing a life-supporting role of serotonergic system in autonomic control. Since midbrain serotonergic neurons are chemosensitive and are implicated in arousal we hypothesized that their chemosensitivity might be impaired in Htr1a^RO mice.

Principal findings

Loose-seal cell-attached recordings in brainstem slices revealed that serotonergic neurons in dorsal raphe nucleus of Htr1a^RO mice have dramatically reduced responses to hypercapnic challenge as compared with control littermates. In control mice, application of 9% CO₂ produced an increase in firing rate of serotonergic neurons (0.260±0.041 Hz, n = 20, p = 0.0001) and application of 3% CO₂ decreased their firing rate (−0.142±0.025 Hz, n = 17, p = 0.0008). In contrast, in Htr1a^RO mice, firing rate of serotonergic neurons was not significantly changed by 9% CO₂ (0.021±0.034 Hz, n = 16, p = 0.49) and by 3% CO₂ (0.012±0.046 Hz, n = 12, p = 0.97).

Conclusions

Our findings support the hypothesis that chemosensitivity of midbrain serotonergic neurons provides a physiological mechanism for arousal responses to life-threatening episodes of hypercapnia and that functional impairment, such as excessive autoinhibition, of midbrain serotonergic neuron responses to hypercapnia may contribute to sudden death.

5HT1A receptors inhibit glutamate inputs to cardiac vagal neurons post-hypoxia/hypercapnia

Synaptic inputs to cardiac vagal neurons (CVNs) regulate parasympathetic activity to the heart. Previous work has shown insults such as hypoxia and hypercapnia (H/H) alter CVN activity by activating post-synaptic serotonergic, purinergic, and glutamatergic receptors in CVNs. This study examines the role of serotonergic 5HT1A receptors in modulating these excitatory neurotransmissions to CVNs during control conditions, H/H and recovery from H/H. Excitatory post-synaptic currents (EPSCs) were recorded from identified CVNs in vitro before, during and post H/H. The 5HT1A receptor antagonist WAY 100635 had no effect on EPSCs in CVNs before, and during H/H. However, during recovery from H/H inspiratory-related excitatory serotonergic and purinergic pathways were recruited to excite CVNs. However, when these serotonergic and purinergic pathways are blocked, the 5HT1A receptor antagonist WAY 100635 restores an excitatory glutamatergic neurotransmission to CVNs. This study indicates endogenous activation of serotonergic 5HT1A receptors diminishes glutamatergic neurotransmission to CVNs following H/H, likely via a presynaptic site of action.

Association of dopamine transporter and monoamine oxidase molecular polymorphisms with sudden infant death syndrome and stillbirth: new insights into the serotonin hypothesis

Recent findings demonstrated the role of neurotransmitters in the aetiopathogenesis of sudden unexpected deaths in infancy. Although genes involved in serotonin metabolism have been proposed as risk factors for sudden infant death syndrome (SIDS), the contribution of additional neurotransmitters and genes different from the serotonin transporter (SLC6A4, 5-HTT) has not been investigated. Considering the common metabolic pathway and synergism between dopamine and serotonin, the role of dopamine transporter (SLC6A3, DAT) and monoamine oxidase A (MAOA) genes in SIDS and stillbirth (sudden intrauterine unexplained death, SIUD) was investigated. Genotypes and allelic frequencies of DAT and MAOA were determined in 20 SIDS and five stillbirth cases and compared with 150 controls. No association was found between DAT polymorphisms and SIDS either at genotype (P = 0.64) or allelic (P = 0.86) level; however, a highly significant association was found between MAOA genotypes (P = 0.047) and alleles (P = 0.002) regulating different expression patterns (3R/3R vs 3.5R/3.5R + 4R/4R) in SIDS + SIUD and controls. Analysis of combined 5-HTTLPR (serotonin transporter linked polymorphic region)/MAOA genotypes revealed that frequency of L/L-4R/4R genotype combination was eightfold higher in SIDS + SIUD than in controls (P < 0.001). Findings are discussed considering the metabolic association among DAT, 5-HTT and MAOA with special emphasis on the linked action of 5-HTT/MAOA in regulating serotonin metabolism of SIDS and SIUD infants.

Genes regulating the serotonin metabolic pathway in the brain stem and their role in the etiopathogenesis of the sudden infant death syndrome

Genotypes and allelic frequencies of TPH2, 5-HTTLPR, the 5-HTT (SLC6A4) intron 2 variable-number tandem repeat (VNTR) region, and the MAOA VNTR region were determined in brain-stem samples of 20 "genuine" SIDS cases and compared with results obtained from 150 healthy controls. The SNP G1463A responsible for 80% functionality loss of TPH2 (tryptophan hydroxylase 2) was not detected, neither in SIDS infants nor in the controls. In contrast, a strict relation was found between the 5-HTTLPR genotype and its allelic frequencies with SIDS cases. The L/L genotype and the long allele (L) of the promoter region of the serotonin transporter were significantly associated (likelihood ratio (LR) test, p<0.001) with the syndrome (L/L, 60% SIDS vs 14% controls; L, 80% SIDS vs 42.6% controls). Polymorphisms of the intron 2 VNTR of the same gene showed a trend for significant differences between genotypes 10/10 and 12/12 (LR test, p=0.068), with the L-12 haplotype being almost twofold in SIDS (44.5%) with respect to controls (23.4%). Differences were even higher considering the genotype combination L/L-12/12 (20% SIDS vs 2.6%), and variations among categories were statistically highly significant (p<0.001). Although additional differences were observed in the frequency of the MAOA (monoamine oxidase A) VNTR genotype 3R/3R between SIDS and controls (respectively 15% vs 26%), the results were not supported by statistical significance. Molecular polymorphisms are discussed considering their functional role in regulating serotonin synthesis (TPH2), neuronal reuptake (5-HTTLPR and 5-HTT intron 2), and catabolism (MAOA) in the nervous system of Italian SIDS infants. Comparisons are made with previous data obtained in different ethnic groups.

Mechanisms of pathogenesis in the Sudden Infant Death Syndrome

The likely processes of the Sudden Infant Death Syndrome (SIDS) were identified many years ago (apnea, failed arousal, failed autoresuscitation, etc.). The neurophysiological basis of these processes and the neurophysiological reasons some infants die of SIDS and others do not are, however, only emerging now. We reviewed recent studies that have shed light on the way in which epidemiological risk factors, genetics, neurotransmitter receptor defects and neonatal cardiorespiratory reflex responses interact to lead to sudden death during sleep in a small number of normal appearing infants. As a result of this review and analysis, we hypothesize that the neurophysiological basis of SIDS resides in a persistence of fetal reflex responses into the neonatal period, amplification of inhibitory cardiorespiratory reflex responses and reduced excitatory cardiorespiratory reflex responses. The hypothesis we developed explores the ways in which multiple subtle abnormalities interact to lead to sudden death and emphasizes the difficulty of ante-mortem identification of infants at risk for SIDS, although identification of infants at risk remains an essential goal of SIDS research.

Baroreceptor-mediated inhibition of respiration after peripheral and central administration of a 5-HT1Areceptor agonist in neonatal piglets

Inhibition of neurones in the ventral medulla accentuates the respiratory inhibition associated with acute blood pressure elevation in piglets. Activation of presynaptic 5-HT(1A) receptors inhibits serotonergic neurones in the ventral medulla and caudal raphé, and we tested the hypothesis that administration of 8-hydroxydipropylaminotetralin (8-OH-DPAT), a 5-HT(1A) agonist, within the rostroventral medulla and caudal raphé would enhance baroreceptor-mediated inhibition of respiratory activity in decerebrate, neonatal piglets. Baroreceptor stimulation was achieved by inflating a balloon in the distal aorta to elevate carotid blood pressure. After two to four control trials of baroreceptor stimulation, each piglet was given either a single intravenous (i.v.) dose of 10 microg kg(-1) 8-OH-DPAT or treated by adding 10 or 30 mm 8-OH-DPAT to the dialysate for approximately 10 min to inhibit serotonergic neurones, after which the baroreceptor stimulation trials were repeated. Baroreceptor stimulation reduced respiratory activity, particularly the respiratory frequency, which diminished from 35.7 +/- 3.3 to 33.8 +/- 3.1 breaths min(-1) (P < 0.02) and, following i.v. 8-OH-DPAT, baroreceptor-mediated inhibition of respiratory output was significantly accentuated (P < 0.05); the respiratory frequency declined from 34.5 +/- 3.6 to 26.5 +/- 2.9 breaths min(-1). Increasing aortic blood pressure reduced the respiratory frequency (P < 0.01), but focal dialysis of 10 or 30 mm 8-OH-DPAT had, on average, no effect on the ventilatory inhibition associated with an acute elevation of blood pressure. We conclude that activation of 5-HT(1A) receptors after systemic administration of 8-OH-DPAT enhanced baroreflex-mediated inhibition of ventilation, but this effect cannot be attributed to 5-HT(1A) receptor activation within the rostroventral medulla and caudal raphé.

Alterations of serotonin synaptic proteins in brain regions of neonatal Rhesus monkeys exposed to perinatal environmental tobacco smoke

Serotonin (5HT) systems play important roles in brain development, and early perturbations of 5HT receptor expression produce permanent changes in 5HT synaptic function and associated behaviors. We exposed pregnant Rhesus monkeys to environmental tobacco smoke (ETS) during gestation and for up to 3 months postnatally and examined the expression of 5HT(1A) and 5HT(2) receptors, and of the presynaptic 5HT transporter in brain regions containing 5HT projections (frontal, temporal and occipital cortex) and cell bodies (midbrain). Perinatal ETS exposure elicited upregulation of 5HT(1A) receptor expression without parallel changes in the other two proteins, a pattern consistent with specific 5HT receptor dysregulation, rather than universal disruption of 5HT synaptic development. The effects seen here for ETS in a primate model are virtually identical in direction, magnitude and regional selectivity to those obtained previously for prenatal nicotine administration in rats. Specifically, early 5HT(1A) overexpression alters the program for future synaptic and behavioral 5HT responses, thus providing a mechanistic link for the shared effects of ETS and nicotine on a specific pathway responsible for behavioral anomalies associated with perinatal tobacco exposure. These results reinforce the need to reduce ETS exposure of pregnant women and young children.

Endogenous 5-HT(1/2) systems and the newborn rat respiratory control. A comparative in vivo and in vitro study

Consequences of 5-HT(1/2) systems blockade by methysergide on newborn rats respiratory drive were evaluated in vivo with unrestrained animals and in vitro using brainstem-spinal cord preparations. A decrease in respiratory frequency until a plateau level was observed under both in vivo (82.8 +/- 0.6% of control values) and in vitro (76.8 +/- 0.8% of control values) conditions whereas an increase in inspiratory amplitude (135.1 +/- 2.1% of control values) was only retrieved in vivo. By the use of the c-fos expression analysis, we correlated these effects with neuronal activity changes, particularly, in vivo in two key structures between the respiratory ponto-medullary network and the peripheral or suprapontine afferences, namely the commissural subnucleus of the nucleus of the solitary tract and the lateral parabrachial nucleus. Thus, peripheral and suprapontine inputs seem to be of a primeval importance in the respiratory influence of endogenous 5-HT. Besides, as 5-HT is involved in the respiratory perturbations that occur in sudden infant death syndrome (SIDS), our results suggest a participation of peripheral and suprapontine inputs in these disorders.

Serotonergic receptors in the midbrain correlated with physiological data on sleep apnea in SIDS victims

BACKGROUND

Recently it has been reported that serotonin and related matters are associated with the sudden infant death syndrome (SIDS), which is still the main cause of postneonatal infant death. To further explore this claim, the correlation between serotonin receptors in the brainstem and sleep apnea in SIDS victims was investigated.

MATERIALS AND METHODS

Among 27,000 infants studied prospectively to characterize their sleep-wake behavior, 38 infants died under 6 months of age including 26 cases of SIDS. All the infants had been recorded during one night in a pediatric sleep laboratory some 3-12 weeks before death. The frequency and duration of sleep apnea were analyzed. Brainstem material was collected and immunohistochemistry on 5-hydroxy tryptamine 1A (5HT1A) receptor was carried out. The density of 5HT1A receptor-positive neurons was measured quantitatively. Nonparametric analysis of the density of 5HT1A receptor-positive neurons was carried out between SIDS and non-SIDS cases. Correlation analyses were performed between the density of 5HT1A receptor-positive neurons and the data on sleep apnea.

RESULTS

There was no correlation between the pathological data on 5HT1A receptors and the physiological data on sleep apnea in SIDS victims.

CONCLUSIONS

No correlation between pathological findings of serotonin and physiological findings of sleep apnea were not in agreement with the association of sleep apnea in pathophysiology of SIDS.

Arsenic methylation by micro-organisms isolated from sheepskin bedding materials

Sudden infant death syndrome (SIDS) has been associated with the volatilization of arsenic, antimony or phosphorus compounds from infants' bedding material by micro-organisms, the so-called 'toxic gas hypothesis'. The volatilization of arsenic by aerobic micro-organisms isolated from new sheepskin bedding material, as well as on material used by a healthy infant and by an infant who perished of SIDS, was examined. Three fungi were isolated from a piece of sheepskin bedding material on which an infant perished of SIDS, which methylated arsenic to form trimethylarsenic(V) species, precursors to volatile trimethylarsine. These three fungi were identified as Scopulariopsis koningii, Fomitopsis pinicola and Penicillium gladioli by their 26S-ribosomal RNA polymerase chain reaction products. These fungi were not previously known to methylate arsenic. The volatilization of arsenic by these three fungi was then examined. Only P. gladioli volatilized arsenic and only under conditions such that the production of sufficient trimethylarsine to be acutely toxic to an infant is unlikely. S. brevicaulis grew on the sheepskin bedding material and evolved a trace amount of trimethylarsine. Known human pathogens such as Mycobacterium neoaurum and Acinetobacter junii were isolated from used bedding.