Changes in cerebrospinal fluid monoamine metabolites, tryptophan, and gamma-aminobutyric acid during the 1st year of life in normal infants. comparison with victims of sudden infant death syndrome

An age- and sex-dependent role of catecholaminergic neurons in the control of breathing and hypoxic chemoreflex during postnatal development

The respiratory system undergoes significant development during the postnatal phase. Maturation of brainstem catecholaminergic (CA) neurons is important for the control and modulation of respiratory rhythmogenesis, as well as for chemoreception in early life. We demonstrated an inhibitory role for CA neurons in CO₂ chemosensitivity in neonatal and juvenile male and female rats, but information regarding their role in the hypoxic ventilatory response (HVR) is lacking. We evaluated the contribution of brainstem CA neurons in the HVR during postnatal (P) development (P7-8, P14-15 and P20-21) in male and female rats through chemical injury with conjugated saporin anti-dopamine beta-hydroxylase (DβH-SAP, 420 ng·μL^-1) injected in the fourth ventricle. Ventilation (V̇_E) and oxygen consumption were recorded one week after the lesion in unanesthetized rats during exposure to normoxia and hypoxia. Hypoxia reduced breathing variability in P7-8 control rats of both sexes. At P7-8, the HVR for lesioned males and females increased 27% and 24%, respectively. Additionally, the lesion reduced the normoxic breathing variability in both sexes at P7-8, but hypoxia partially reverted this effect. For P14-15, the increase in V̇_E during hypoxia was 30% higher for male and 24% higher for female lesioned animals. A sex-specific difference was detected at P20-21, as lesioned males exhibited a 24% decrease in the HVR, while lesioned females experienced a 22% increase. Furthermore, the hypoxia-induced body temperature reduction was attenuated in P20-21 lesioned females. We conclude that brainstem CA neurons modulate the HRV during the postnatal phase, and possibly thermoregulation during hypoxia.

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

KEY POINTS

The brainstem catecholaminergic (CA) modulation on ventilation changes with development. We determined the role of the brainstem CA system in ventilatory control under normocapnic and hypercapnic conditions during different phases of development [postnatal day (P)7-8, P14-15 and P20-21] in male and female Wistar rats. Brainstem CA neurones produce a tonic inhibitory drive that affects breathing frequency in P7-8 rats and provide an inhibitory drive during hypercapnic conditions in both males and females at P7-8 and P14-15. In pre-pubertal rats, brainstem CA neurones become excitatory for the CO₂ ventilatory response in males but remain inhibitory in females. Diseases such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome have been associated with abnormalities in the functioning of CA neurones; therefore, the results of the present study contribute to a better understanding of this system.

ABSTRACT

The respiratory network undergoes significant development during the postnatal phase, including the maturation of the catecholaminergic (CA) system. However, postnatal development of this network and its effect on the control of pulmonary ventilation ( V̇E ) is not fully understood. We investigated the involvement of brainstem CA neurones in respiratory control during postnatal development [postnatal day (P)7-8, P14-15 and P20-21], in male and female rats, through chemical injury with conjugated saporin anti-dopamine β-hydroxylase (DβH-SAP). Thus, DβH-SAP (420 ng μL^-1 ), saporin (SAP) or phosphate buffered solution (PBS) was injected into the fourth ventricle of neonatal Wistar rats of both sexes. V̇E and oxygen consumption were recorded 1 week after the injections in unanaesthetized neonatal and juvenile rats during room air and hypercapnia. The resting ventilation was higher in both male and female P7-8 lesioned rats by 33%, with a decrease in respiratory variability being observed in males. The hypercapnic ventilatory response (HCVR) was altered in male and female lesioned rats at all postnatal ages. At P7-8, the HCVR for males and females was increased by 37% and 30%, respectively. For both sexes at P14-15 rats, the increase in V̇E during hypercapnia was 37% higher for lesioned rats. A sex-specific difference in HCRV was observed at P20-21, with lesioned males showing a 33% decrease, and lesioned females showing an increase of 33%. We conclude that brainstem CA neurones exert a tonic inhibitory effect on V̇E in the early postnatal days of the life of a rat, increase variability in P7-8 males and modulate HCRV during the postnatal phase.

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.

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.

5-HT2 receptors modulate excitatory neurotransmission to cardiac vagal neurons within the nucleus ambiguus evoked during and after hypoxia

To examine the role of 5-HT2 receptors in the central cardiorespiratory network, and in particular the respiratory modulation of parasympathetic activity to the heart, we used an in vitro medullary slice that allowed simultaneous examination of rhythmic inspiratory-related activity recorded from hypoglossal rootlet and excitatory inspiratory-related neurotransmission to cardioinhibitory vagal neurons (CVNs) within the nucleus ambiguus (NA). Focal application of ketanserin, a 5-HT2 receptor antagonist, did not significantly alter the frequency of spontaneous excitatory postsynaptic excitatory currents (EPSCs) in CVNs in control conditions. However, ketanserin diminished spontaneous excitatory neurotransmission to CVNs during hypoxia. The inhibitory action of ketanserin was on 5-HT3 mediated EPSCs during hypoxia since these responses were blocked by the 5-HT3 receptor antagonist ondansetron. In addition, a robust inspiratory-related excitatory neurotransmission was recruited during recovery from hypoxia. Focal application of ketanserin during this posthypoxia period evoked a significant augmentation of the frequency of inspiratory-related, but not spontaneous EPSCs in CVNs. This excitatory effect of ketanserin was prevented by application of the purinergic receptor blocker pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS). These results demonstrate 5-HT2 receptors differentially modulate excitatory neurotransmission to CVNs during and after hypoxia. Activation of 5-HT2 receptors acts to maintain excitatory neurotransmission to CVNs during hypoxia, likely via presynaptic facilitation of 5-HT3 receptor-mediated neurotransmission to CVNs. However, activation of 5HT2 receptors diminishes the subsequent inspiratory-related excitatory neurotransmission to CVNs that is recruited during the recovery from hypoxia likely exerting an inhibitory action on inspiratory-related purinergic signaling.

Medullary serotonin defects and respiratory dysfunction in 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 occurs during sleep and remains unexplained after a complete autopsy, death scene investigation, and review of the clinical history. It is the leading cause of postneonatal mortality in the developed world. The cause of SIDS is unknown, but is postulated to involve impairment of brainstem-mediated homeostatic control. Extensive evidence from animal studies indicates that serotonin (5-HT) neurons in the medulla oblongata play a role in the regulation of multiple aspects of respiratory and autonomic function. A subset of SIDS infants have several abnormalities in medullary markers of 5-HT function and genetic polymorphisms impacting the 5-HT system, informing the hypothesis that SIDS results from a defect in 5-HT brainstem-mediated control of respiratory (and autonomic) regulation. Here we review the evidence from postmortem human studies and animal studies to support this hypothesis and discuss how the pathogenesis of SIDS is likely to originate in utero during fetal development.

The role of 5-HT3 and other excitatory receptors in central cardiorespiratory responses to hypoxia: implications for sudden infant death syndrome

Although brainstem serotonergic (5-HT) systems are involved in the protective responses to hypoxia, abnormalities of 5-HT function are strongly implicated in SIDS, and the neurochemical mechanisms by which 5-HT receptors influence brainstem cardiorespiratory responses to hypoxia remains unclear. This study focuses on the role of excitatory neurotransmission, including 5-HT3 signaling, to cardiac vagal neurons (CVNs) that dominate the control of heart rate. Excitatory synaptic inputs to CVNs, located in the nucleus ambiguus (NA), were recorded simultaneously with respiratory activity in in vitro brainstem slices. During control conditions excitatory inputs to CVNs were blocked by application of NMDA and AMPA/kainate glutamatergic receptor antagonists, whereas the 5-HT3 and purinergic receptor antagonists ondansetron and pyridoxal-phosphate-6-azophenyl-2',4'-disulfonic acid (PPADS), respectively, had no effect. However, during hypoxia ondansetron inhibited excitatory neurotransmission to CVNs. In recovery from hypoxia, spontaneous and respiratory-related excitatory events were blocked by glutamatergic and purinergic receptor blockers, respectively, whereas ondancetron had no effect. These results demonstrate that hypoxia recruits a 5-HT pathway to CVNs that activates 5-HT3 receptors on CVNs to maintain parasympathetic cardiac activity during hypoxia. Exaggeration of this 5-HT neurotransmission could increase the incidence of bradycardia and risk of sudden infant death during hypoxia.

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.

Endogenous noradrenaline affects the maturation and function of the respiratory network: Possible implication for SIDS

Breathing is a vital, rhythmic motor act that is required for blood oxygenation and oxygen delivery to the whole body. Therefore, the brainstem network responsible for the elaboration of the respiratory rhythm must function from the very first moments of extrauterine life. In this review, it is shown that the brainstem noradrenergic system plays a pivotal role in both the modulation and the maturation of the respiratory rhythm generator. Compelling evidence are reported demonstrating that genetically induced alterations of the noradrenergic system in mice affect the prenatal maturation and the perinatal function of the respiratory rhythm generator and have drastic consequences on postnatal survival. Sudden Infant Death Syndrome (SIDS), the leader cause of infant death in industrialised countries, may result from cardiorespiratory disorders during sleep. As several cases of SIDS have been observed in infants having noradrenergic deficits, a possible link between prenatal alteration of the noradrenergic system, altered maturation and function of the respiratory network and SIDS is suggested.

Modulation of the respiratory rhythm generator by the pontine noradrenergic A5 and A6 groups in rodents

The aim of the present review is to summarise available studies dealing with the respiratory control exerted by pontine noradrenergic neurones in neonatal and adult mammals. During the perinatal period, in vitro studies on neonatal rodents have shown that A5 and A6 neurones exert opposite modulations onto the respiratory rhythm generator, inhibitory and facilitatory respectively, that the anatomical support for these modulations already exists at birth, and that genetically induced alterations in the formation of A5 and A6 neurones affect the maturation of the respiratory rhythm generator, leading to lethal respiratory deficits at birth. The A5-A6 modulation of the respiratory rhythm generator is not transient, occurring solely during the perinatal period but it persists throughout life: A5 and A6 neurones display a respiratory-related activity, receive inputs from and send information to the medullary respiratory centres and contribute to the adaptation of adult breathing to physiological needs.

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.

Phox2a gene, A6 neurons, and noradrenaline are essential for development of normal respiratory rhythm in mice

Although respiration is vital to the survival of all mammals from the moment of birth, little is known about the genetic factors controlling the prenatal maturation of this physiological process. Here we investigated the role of the Phox2a gene that encodes for a homeodomain protein involved in the generation of noradrenergic A6 neurons in the maturation of the respiratory network. First, comparisons of the respiratory activity of fetuses delivered surgically from heterozygous Phox2a pregnant mice on gestational day 18 showed that the mutants had impaired in vivo ventilation, in vitro respiratory-like activity, and in vitro respiratory responses to central hypoxia and noradrenaline. Second, pharmacological studies on wild-type neonates showed that endogenous noradrenaline released from pontine A6 neurons potentiates rhythmic respiratory activity via alpha1 medullary adrenoceptors. Third, transynaptic tracing experiments in which rabies virus was injected into the diaphragm confirmed that A6 neurons were connected to the neonatal respiratory network. Fourth, blocking the alpha1 adrenoceptors in wild-type dams during late gestation with daily injections of the alpha1 adrenoceptor antagonist prazosin induced in vivo and in vitro neonatal respiratory deficits similar to those observed in Phox2a mutants. These results suggest that noradrenaline, A6 neurons, and the Phox2a gene, which is crucial for the generation of A6 neurons, are essential for development of normal respiratory rhythm in neonatal mice. Metabolic noradrenaline disorders occurring during gestation therefore may induce neonatal respiratory deficits, in agreement with the catecholamine anomalies reported in victims of sudden infant death syndrome.

The human raphe nuclei and the serotonergic system

The raphe nuclei are distributed near the midline of the brainstem along its entire rostro-caudal extension. The serotonergic neurons are their main neuronal components, although a proportion of them lie in subdivisions of the lateral reticular formation. They develop from mesopontine and medullary primordia, and the resulting grouping into rostral and caudal clusters is maintained into adulthood, and is reflected in the connectivity. Thus, the mesencephalon and rostral pons, neurons within the rostral raphe complex (caudal linear, dorsal raphe, and median raphe nuclei) project primarily to the forebrain. By contrast, in the caudal pons and medulla oblongata, neurons within the caudal raphe complex (raphe magnus, raphe obscurus, raphe pallidus nuclei and parts of the adjacent lateral reticular formation) project to the brainstem nuclei and to the spinal cord. The median raphe and dorsal raphe nuclei provide parallel and overlapping projections to many forebrain structures with axon fibers exhibiting distinct structural and functional characteristics. The caudal group of the serotonergic system projects to the brainstem, and, by three parallel projections, to the dorsal, intermediate and ventral columns in the spinal cord. The serotonergic axons arborize over large areas comprising functionally diverse targets. Some projections form classical chemical synapses while many do not, thus contributing to the so-called paracrine or volume transmission. The serotonergic projections participate in the regulation of different functional (motor, somatosensory, limbic) systems; and have been associated with a wide range of neuropsychiatric and neurological disorders. Finally, recent experimental data support the role of serotonin in modulating brain development, such that a dysfunction in serotonergic transmission during early life could lead to long lasting structural and functional alterations.

The triple risk hypotheses in sudden infant death syndrome

Sudden infant death syndrome (SIDS) victims were regarded as normal as a matter of definition (Beckwith 1970) until 1952 when Kinney and colleagues argued for elimination of the clause, "unexpected by history." They argued that "not all SIDS victims were normal," and referred to their hypothesis that SIDS results from brain abnormalities, which they postulated "to originate in utero and lead to sudden death during a vulnerable postnatal period." Bergman (1970) argued that SIDS did not depend on any "single characteristic that ordains a infant for death," but on an interaction of risk factors with variable probabilities. Wedgwood (1972) agreed and grouped risk factors into the first "triple risk hypothesis" consisting of general vulnerability, age-specific risks, and precipitating factors. Raring (1975), based on a bell-shaped curve of age of death (log-transformed), concluded that SIDS was a random process with multifactorial causation. Rognum and Saugstad (1993) developed a "fatal triangle" in 1993, with groupings similar to those of Wedgwood, but included mucosal immunity under a vulnerable developmental stage of the infant. Filiano and Kinney (1994) presented the best known triple risk hypothesis and emphasized prenatal injury of the brainstem. They added a qualifier, "in at least a subset of SIDS," but, the National Institute of Child Health and Development SIDS Strategic Plan 2000, quoting Kinney's work, states unequivocally that "SIDS is a developmental disorder. Its origins are during fetal development." Except for the emphasis on prenatal origin, all 3 triple risk hypotheses are similar. Interest in the brainstem of SIDS victims began with Naeye's 1976 report of astrogliosis in 50% of all victims. He concluded that these changes were caused by hypoxia and were not the cause of SIDS. He noted an absence of astrogliosis in some older SIDS victims, compatible with a single, terminal episode of hypoxia without previous hypoxic episodes, prenatal or postnatal. Kinney and colleagues (1983) reported gliosis in 22% of their SIDS victims. Subsequently, they instituted studies of neurotransmitter systems in the brainstem, particularly the muscarinic (1995) and serotenergic systems (2001). The major issue is when did the brainstem abnormalities, astrogliosis, or neurotransmitter changes occur and whether either is specific to SIDS. There is no published method known to us of determining the time of origin of these markers except that the injury causing astrogliosis must have occurred at least 4 days before death (Del Bigio and Becker, 1994). Because the changes in neurotransmitter systems found in the arcuate nucleus in SIDS victims were also found in the chronic controls with known hypoxia, specificity of these markers for SIDS has not been established. It seems likely that the "acute control" group of Kinney et al (1995) died too quickly to develop gliosis or severe depletion of the neurotransmitter systems. We can conclude that the acute controls had no previous episodes of severe hypoxia, unlike SIDS or their "chronic controls." Although the average muscarinic cholinergic receptor level in the SIDS victim was significantly less than in the acute controls, the difference was only 27%, and only 21 of 41 SIDS victims had values below the mean of the acute controls. The study of the medullary serotonergic network by Kinney et al (2001) revealed greater reductions in the SIDS victims than in acute controls, but the questions of cause versus effect of the abnormalities, and whether they occurred prenatally or postnatally, remain unanswered. Hypoplasia of the arcuate nucleus was stated to occur in 5% of their SIDS cases by Kinney et al (2001), but this is a "primary developmental defect" according to Matturri et al (2002) with a larger series, many of whom were stillbirths. These cases should not be included under the rubric of SIDS, by definition. There are difficulties with Filiano and Kinney's (1994) explanation of the age at death distribution of SIDS. They postulate that the period between 1 and 6 months represents an unstable time for virtually all physiologic systems. However, this period demonstrates much less instability than does the neonatal period, when most deaths from congenital defects and severe maternal anemia occur. We present data for infants born to mothers who were likely to have suffered severe anemia as a consequence of placenta previa, abruptio placentae, and excessive bleeding during pregnancy; these infants presumably are at increased risk of hypoxia and brainstem injury. The total neonatal mortality rate in these 3 groups of infants is 4 times greater than the respective postneonatal mortality, and in the postneonatal period the non-SIDS mortality rate is between 14 and 22 times greater than the postneonatal SIDS rate in these 3 groups. A preponderance of deaths in the neonatal period is also found for congenital anomalies, a category that logically should include infants who experienced prenatal hypoxia or ischemia; this distribution of age of death is very different from that for SIDS, which mostly spares the first month and peaks between 2 and 3 months of age. Finally, evidence inconsistent with prenatal injury as a frequent cause of SIDS comes from prospective studies of ventilatory control in neonates who subsequently died of SIDS; no significant respiratory abnormalities in these infants have been found (Waggener et al 1990; Schectman et al 1991). We conclude that none of the triple risk hypotheses presented so far have significantly improved our understanding of the cause of SIDS. Bergman's and Raring's concepts of multifactorial causation with interaction of risk factors with variable probabilities is less restrictive and more in keeping with the large number of demonstrated risk factors and their varying prevalence. If prenatal hypoxic damage of the brainstem occurred, it seems likely that the infant so afflicted would be at risk for SIDS, but it is even more likely that their death would occur in the neonatal period, as we have demonstrated in infants who have known maternal risk factors that involve severe anemia. This is in contrast to the delay until the postneonatal period of most SIDS deaths. A categorical statement that the origin of SIDS is prenatal is unwarranted by the evidence. Brainstem abnormalities have not been shown to cause SIDS, but are more likely a nonspecific effect of hypoxia.

Abnormal phrenic motoneuron activity and morphology in neonatal monoamine oxidase A-deficient transgenic mice: possible role of a serotonin excess

In rodent neonates, the neurotransmitter serotonin (5-HT) modulates the activity of both the medullary respiratory rhythm generator and the cervical phrenic motoneurons. To determine whether 5-HT also contributes to the maturation of the respiratory network, experiments were conducted in vitro on the brainstem-spinal cord preparation of neonatal mice originating from the control strain (C3H) and the monoamine oxidase A-deficient strain, which has a brain perinatal 5-HT excess (Tg8). At birth, the Tg8 respiratory network is unable to generate a respiratory pattern as stable as that produced by the C3H network, and the modulation by 5-HT of the network activity present in C3H neonates is lacking in Tg8 neonates. In addition, the morphology of the phrenic motoneurons is altered in Tg8 neonates; the motoneuron dendritic tree loses the C3H bipolar aspect but exhibits an increased number of spines and varicosities. These abnormalities were prevented in Tg8 neonates by treating pregnant Tg8 dams with the 5-HT synthesis inhibitor p-chlorophenylalanine or a 5-HT(2A) receptor antagonist but were induced in wild-type neonates by treating C3H dams with a 5-HT(2A) receptor agonist. We conclude that 5-HT contributes, probably via 5-HT(2A) receptors, to the normal maturation of the respiratory network but alters it when present in excess. Disorders affecting 5-HT metabolism during gestation may therefore have deleterious effects on newborns.

Central effects of 5-HT on respiratory rhythm in newborn rats in vivo

The role of 5-HT in inducing apnoeas (a major element in sudden infant death syndrome) is controversial because while 5-HT is a respiratory depressant in vivo, it evokes respiratory analeptic effects when applied to the isolated brainstem of the newborn rat. In decerebrate newborn rats, the electrical activity of the diaphragm and that of a hypoglossally innervated tongue muscle, as well as the cardiac frequency (Fc), were recorded before and after the application of 5-HT and related agents to the floor of the IVth ventricle. To assess the spontaneous variability in inspiratory frequency (Fi) and Fc, a sham group was studied. A decrease in Fi was observed in response to 5-HT. This respiratory depressant effect was associated with an activation of the tongue muscle, but there was no change in Fc. Application of agonists elicited a small increase in Fi linked to activation of 5-HT1A receptors, and decreases in both Fi and the activity of the tongue muscle resulting predominantly from activation of 5-HT2 receptors. The decrease in Fi was much smaller in newborn rats than that reported in newborn kittens. Indeed, in newborn rats, we did not observe long-lasting apnoeas. Our results differ from those obtained from the newborn rat in vitro, inasmuch as in vivo 5-HT essentially depressed the respiratory rhythm generator. The role of the afferent system appears to be crucial in modulating the action of 5-HT.