Involvement of the rostral ventro-lateral medulla in respiratory rhythm genesis during the peri-natal period: an in vitro study in newborn and fetal rats

Respiratory rhythm generation, hypoxia, and oxidative stress-Implications for development

Encountered in a number of clinical conditions, repeated hypoxia/reoxygenation during the neonatal period can pose both a threat to immediate survival as well as a diminished quality of living later in life. This review focuses on our current understanding of central respiratory rhythm generation and the role that hypoxia and reoxygenation play in influencing rhythmogenesis. Here, we examine the stereotypical response of the inspiratory rhythm from the preBötzinger complex (preBötC), basic neuronal mechanisms that support rhythm generation during the peri-hypoxic interval, and the physiological consequences of inspiratory network responsivity to hypoxia and reoxygenation, acute and chronic intermittent hypoxia, and oxidative stress. These topics are examined in the context of Sudden Infant Death Syndrome, apneas of prematurity, and neonatal abstinence syndrome.

[Electrophysiological, molecular and genetic identifications of the pre-Bötzinger complex]

From birth onwards, rhythmic breathing is required for blood oxygenation and survival in mammals. During their lifespan, human or mouse or elephant will spontaneously produce several hundreds of millions of respiratory movements. The central nervous command responsible for these spontaneous rhythmic movements is elaborated by a complex neural network extending within the brainstem. In the medulla, a special part of this network contains respiratory pacemaker neurons that play a crucial role in respiratory rhythmogenesis: the pre-Bötzinger complex. This review summarizes and discusses the main electrophysiological, molecular and genetic mechanisms contributing to the function and the perinatal maturation of the pre-Bötzinger complex.

Monoamine oxidases in development

Monoamine oxidases (MAOs) are flavoproteins of the outer mitochondrial membrane that catalyze the oxidative deamination of biogenic and xenobiotic amines. In mammals there are two isoforms (MAO-A and MAO-B) that can be distinguished on the basis of their substrate specificity and their sensitivity towards specific inhibitors. Both isoforms are expressed in most tissues, but their expression in the central nervous system and their ability to metabolize monoaminergic neurotransmitters have focused MAO research on the functionality of the mature brain. MAO activities have been related to neurodegenerative diseases as well as to neurological and psychiatric disorders. More recently evidence has been accumulating indicating that MAO isoforms are expressed not only in adult mammals, but also before birth, and that defective MAO expression induces developmental abnormalities in particular of the brain. This review is aimed at summarizing and critically evaluating the new findings on the developmental functions of MAO isoforms during embryogenesis.

Intercostal and abdominal respiratory motoneurons in the neonatal rat spinal cord: spatiotemporal organization and responses to limb afferent stimulation

Respiration requires the coordinated rhythmic contractions of diverse muscles to produce ventilatory movements adapted to organismal requirements. During fast locomotion, locomotory and respiratory movements are coordinated to reduce mechanical conflict between these functions. Using semi-isolated and isolated in vitro brain stem-spinal cord preparations from neonatal rats, we have characterized for the first time the respiratory patterns of all spinal intercostal and abdominal motoneurons and explored their functional relationship with limb sensory inputs. Neuroanatomical and electrophysiological procedures were initially used to locate intercostal and abdominal motoneurons in the cord. Intercostal motoneuron somata are distributed rostrocaudally from C(7)-T(13) segments. Abdominal motoneuron somata lie between T(8) and L(2). In accordance with their soma distributions, inspiratory intercostal motoneurons are recruited in a rostrocaudal sequence during each respiratory cycle. Abdominal motoneurons express expiratory-related discharge that alternates with inspiration. Lesioning experiments confirmed the pontine origin of this expiratory activity, which was abolished by a brain stem transection at the rostral boundary of the VII nucleus, a critical area for respiratory rhythmogenesis. Entrainment of fictive respiratory rhythmicity in intercostal and abdominal motoneurons was elicited by periodic low-threshold dorsal root stimulation at lumbar (L(2)) or cervical (C(7)) levels. These effects are mediated by direct ascending fibers to the respiratory centers and a combination of long-projection and polysynaptic descending pathways. Therefore the isolated brain stem-spinal cord in vitro generates a complex pattern of respiratory activity in which alternating inspiratory and expiratory discharge occurs in functionally identified spinal motoneuron pools that are in turn targeted by both forelimb and hindlimb somatic afferents to promote locomotor-respiratory coupling.

Anatomical and functional development of the pre-Bötzinger complex in prenatal rodents

Developmental anomalies of central respiratory neural control contribute to newborn mortality and morbidity. Elucidation of the cellular, molecular, trophic, and genetic mechanisms involved in the formation and function of respiratory nuclei during prenatal development will provide a foundation for understanding pathologies. The pre-Bötzinger Complex (pre-BötC) is a specific group of neurons located in the ventrolateral medulla that is critical for respiratory rhythmogenesis. Thus it has become a major focus of research. Here, we provide an overview of current knowledge regarding the anatomical and functional emergence of the rodent pre-BötC during the prenatal period.

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.

Perinatal maturation of the respiratory rhythm generator in mammals: from experimental results to computational simulation

The survival of neonatal mammals requires a correct function of the respiratory rhythm generator (RRG), and therefore, the processes that control its prenatal maturation are of vital importance. In humans, lambs and rodents, foetal breathing movements (FBMs) occur early during gestation, are episodic, sensitive to bioamines, central hypoxia and inputs from CNS upper structures, and evolve with developmental age. In vitro, the foetal rodent RRG studied in preparations where the upper CNS structures are lacking continuously produces a rhythmic command, which is sensitive to hypoxia and bioaminergic inputs. The rhythm is slow with variable periods 4 days before birth. It becomes faster 2 days before birth, similar to the postnatal rhythm. Compelling evidence suggests that a region of the RRG called the preBötzinger complex (PBC) contains respiratory pacemaker neurones which play a primary role in perinatal rhythmogenesis. Although the RRG functions during early gestation, no pacemakers are found in the putative PBC area and its electrical stimulation and lesion do not affect the early foetal rhythm. To know whether the early foetal and perinatal rhythms originate from either pacemaker neurones or network connection properties, and to know which maturational processes might explain the appearance of PBC pacemakers and the rhythm increase during perinatal development, we computationally modelled maturing RRG. Our model shows that both network noise and persistent sodium conductance are crucial for rhythmogenesis and that a slight increase in the persistent sodium conductance can solve the pacemaker versus network dilemma in a noisy network.

Developmental changes in the spatio-temporal pattern of respiratory neuron activity in the medulla of late fetal rat

We investigated how the spatio-temporal pattern of respiratory neuron network activity in the ventral medulla changes during the late fetal period of rat. Brainstem-spinal cord preparations isolated from rat fetuses on embryonic days 17-21 (E17-E21) were stained with a voltage-sensitive dye for optical image analysis of neuronal activity of the ventral medulla. The spatio-temporal pattern of respiratory neuron activity in the preparation from E20 to E21 was basically identical to that of neonatal rat; pre-inspiratory activity in a limited region of the rostral ventrolateral medulla, the para-facial region, preceded by several hundred milliseconds the onset of inspiratory activity in the more caudal ventrolateral medulla, the pre-Bötzinger complex level. In contrast, in E17-E18 specimens, pre-inspiratory activity could not be detected in the rostral medulla at the level of the facial nucleus. Neuronal activity appeared to begin at the pre-Bötzinger complex level shortly before onset of the inspiratory burst. Strong activity then developed in the facial nucleus and peaked in the post-inspiratory phase. The transition of these patterns of respiratory activity occurred at E19. We conclude that the changes in the spatio-temporal pattern of neuronal activity reflect developmental changes in the cellular elements underlying rhythm generation in the fetal respiratory neuron network. We suggest that the pre-inspiratory neuron network of the para-facial region in the rostral ventrolateral medulla functions as the rhythm generator after E19/20.

Un modèle amphibian pour l’étude du développement du contrôle de la respiration

Recent medical advances have made it possible for babies to survive premature birth at increasingly earlier developmental stages. This population requires costly and sophisticated medical care to address the problems associated with immaturity of the respiratory system. In addition to pulmonary complications, respiratory instability and apnea reflecting immaturity of the respiratory control system are major causes of hospitalization and morbidity in this highly vulnerable population. These medical concerns, combined with the curiosity of physiologists, have contributed to the expansion of research in respiratory neurobiology. While most researchers working in this field commonly use rodents as an animal model, recent research using in vitro brainstem preparation from bullfrogs (Rana catesbeiana) have revealed the technical advantages of this animal model, and shown that the basic principles underlying respiratory control and its ontogeny are very similar between these two groups of vertebrates. The present review highlights the recent advances in the area of research with a focus on intermittent (episodic) breathing and the role of serotonergic and GABAergic modulation of respiratory activity during development.

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.

Perinatal maturation of the mouse respiratory rhythm-generator: in vivo and in vitro studies

In vivo (plethysmography) and in vitro (en bloc preparations) experiments were performed from embryonic day 16 (E16) to postnatal day 9 (P9) in order to analyse the perinatal maturation of the respiratory rhythm-generator in mice. At E16, delivered foetuses did not ventilate and survive but at E18 they breathed at about 110 cycles/min with respiratory cycles of variable individual duration. From E18 to P0-P2, the respiratory cycles stabilised without changes in the breathing parameters. However, these increased several-fold during the next days. Hypoxia increased breathing frequency from E18-P5 and only significantly affected ventilation from P3 onwards. At E16, in vitro medullary preparations (pons resection) produced rhythmic phrenic bursts at a low frequency (about 5 cycles/min) with variable cycle duration. At E18, their frequency doubled but cycle duration remained variable. After birth, the frequency did not change although cycle duration stabilised. At E18 and P0-P2, the in vitro frequency decreased by around 50% under hypoxia, increased by 40-50% under noradrenaline or substance P and was permanently depressed by the pontine A5 areas. At E16 however, hypoxia had no effects, both noradrenaline and substance P drastically increased the frequency and area A5 inhibition was not expressed at this time. At E18 and P0-P2, electrical stimulation and electrolytic lesion of the rostral ventrolateral medulla affected the in vitro rhythm but failed to induce convincing effects at E16. Thus, a major maturational step in respiratory rhythmogenesis occurs between E16-E18, in agreement with the concept of multiple rhythmogenic mechanisms.

The murine neurokinin NK1 receptor gene contributes to the adult hypoxic facilitation of ventilation

Substance P and neurokinin-1 receptors (NK1) modulate the respiratory activity and are expressed early during development. We tested the hypothesis that NK1 receptors are involved in prenatal development of the respiratory network by comparing the resting respiratory activity and the respiratory response to hypoxia of control mice and mutant mice lacking the NK1 receptor (NK1-/-). In vitro and in vivo experiments were conducted on neonatal, young and adult mice from wild-type and NK1-/- strains. In the wild strain, immunohistological, pharmacological and electrophysiological studies showed that NK1 receptors were expressed within medullary respiratory areas prior to birth and that their activation at birth modulated central respiratory activity and the membrane properties of phrenic motoneurons. Both the membrane properties of phrenic motoneurons and the respiratory activity generated in vitro by brainstem-spinal cord preparation from NK1-/- neonate mice were similar to that from the wild strain. In addition, in vivo ventilation recordings by plethysmography did not reveal interstrain differences in resting breathing parameters. The facilitation of ventilation by short-lasting hypoxia was similar in wild and NK1-/- neonates but was significantly weaker in adult NK1-/- mice. Results demonstrate that NK1 receptors do appear to be necessary for a normal respiratory response to short-lasting hypoxia in the adult. However, NK1 receptors are not obligatory for the prenatal development of the respiratory network, for the production of the rhythm, or for the regulation of breathing by short-lasting hypoxia in neonates.

Development of the rat respiratory neuron network during the late fetal period

We studied developmental changes in respiratory-like C4 activity and respiratory-related neurons in the ventrolateral medulla (VLM) of brainstem-spinal cord preparations from rat fetuses after embryonic day 16 (E16). In addition to respiratory nerve activity, non-respiratory activity was recorded from the C4 ventral root of preparations before E19. The burst duration of respiratory nerve discharge increased markedly at E19/20. Subtypes of neurons similar to newborn respiratory neurons were found in preparations with prolonged burst duration (more than 400 ms) after E20. These subtypes were not evident in preparations with short burst duration (less than 300 ms) before E19. About 60% of the inspiratory neurons in E17-19 preparations produced voltage-dependent burst activity, which was preserved in low Ca(2+)/high Mg(2+) synaptic blockade solution. In about 11% of the inspiratory neurons of E18-19 preparations, activation of one neuron induced activation of the inspiratory neuron network and generation of a full C4 inspiratory burst. The present findings suggest that respiratory neuron networks mature functionally to the level of the neonatal respiratory neuron networks during gestation period E19/20. Potentiation of synaptic interaction between respiratory neurons, causing developmental changes in the burst pattern, might be involved in the maturation process during late fetal stages.

Perinatal changes of I(h) in phrenic motoneurons

The hyperpolarization-activated cationic current (I(h)) was characterized and its maturation studied on phrenic motoneurons (PMNs), from reduced preparations of foetal (E18 and E21) and newborn (P0-P3) rats, using the whole-cell patch-clamp technique. In voltage-clamp mode, 2-s hyperpolarizing steps (5-mV, -50 to -110 mV) elicited a noninactivating inward current, blocked by external application of Cs+ or ZD 7288. At -110 mV, Ih current density averaged 0.67 +/- 0.41 pA/pF at E18, reached a transient peak at E21 (1.38 +/- 0.11 pA/pF) and decreased at P0-P3 (0.77 +/- 0.22 pA/pF). V1/2 was similar at E18 and E21 (-79 mV) but was significantly hyperpolarized at P0-P3 (-90 mV). The time constant of activation was voltage-dependent, and significantly faster at E21. Reversal potential was similar at all ages when estimated by extrapolation or tail current procedures. It was positively shifted by 25 +/- 6 mV when external potassium was raised from 3 to 10 m M, suggesting a similar sensitivity to K+ from E18 to P0-3. Cs(+) or ZD 7288 applications on PMNs at rest in current-clamp mode, in a partitioned chamber, induced a 10 +/- 2 mV hyperpolarization at E18 and E21, and an 8 +/- 2 mV hyperpolarization at P0-3. The area of the central respiratory drive potential or current was increased by 33 and 31%, respectively, at E21, but was not significantly modified at E18 and P0-3. Our data suggest a critical period during the perinatal maturation of Ih during which it is transiently upregulated and attenuates the influence of the central respiratory drive on PMNs just prior to birth.

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.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

Nucleus ambiguus and bulbospinal ventral respiratory group neurons in the neonatal rat

The in vitro brainstem-spinal cord preparation of the neonatal rat is an important model system for studies of the respiratory control system, yet there have not been studies to anatomically characterize respiratory neuron populations in the neonate. Fluorescent retrograde tracers were used to identify bulbospinal neurons of the ventral respiratory group and motoneurons of nucleus ambiguus in neonatal rats. Fluoro-Gold injections into the C4 ventral horn labeled bulbospinal neurons within a densely packed column within the ventrolateral intermediate reticular nucleus from the level of the pyramidal decussation to the facial nucleus. This cell column corresponded closely to the location of the ventral respiratory group of the adult rat. In particular, neurons were labeled in regions corresponding to the rostral ventral respiratory group and the Bötzinger complex. Unlike adult rats, the preBötzinger complex also contained many bulbospinal neurons. Fluoro-Gold-labeled neurons were also located in the medial reticular nuclei, raphe pallidus, and obscurus and spinal vestibular nucleus. As in adult rats, bulbospinal ventral respiratory group neurons overlapped with cervical vagal motoneurons in the external formation, and partially with those in the loose formation, but not with those in the semicompact or compact formation of nucleus ambiguus. These results indicate that the distribution of bulbospinal ventral respiratory group neurons corresponds with that observed in physiological studies of neonatal rats.

Substance P and central respiratory activity: a comparative in vitro study on foetal and newborn rat

Experiments were performed in vitro on foetal (embryonic days 18 to 21, E18-21) and newborn rat (postnatal days 0 to 3, P0-3) brainstem spinal cord preparations to analyse the perinatal developmental changes in the effects induced by substance P. Superfusion of the preparations with SP-containing artificial cerebrospinal fluid (aCSF) induced significant increase in the respiratory frequency of newborn rats (10-9 M), whereas concentration up to 10-7 M induced no change in foetal preparations. A whole cell patch clamp approach was used to record intracellularly from phrenic motoneurones. In newborn or E20-21 foetal rats SP-containing aCSF depolarised the phrenic motoneurones, increased their input resistance, reduced the rheobase current and shifted the frequency-intensity curves upward. In E18 foetal rats, no change was evoked by SP. A peptidase inhibitor mixture was used to block the enzymatic degradation of endogenous SP. This mixture was ineffective in changing the respiratory frequency in newborn and foetal preparations. In newborn rat phrenic motoneurones, the peptidase inhibitor mixture induced changes similar to those caused by SP but no change was induced in foetal rats. These results indicate that SP may modulate (i) the activity of the respiratory rhythm generator in newborn but not in foetal rats, and (ii) the activity of phrenic motoneurones at E20, E21 and in newborn rats but not at E18. Results obtained using the peptidase inhibitor mixture suggest that endogenous SP is probably not involved in the control of the respiratory rhythm in the prenatal period, but may influence the activity of the phrenic motoneurones after birth.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Serotonin levels are abnormally elevated in the fetus of the monoamine oxidase-A-deficient transgenic mouse

Developmental changes in levels of serotonin, L-tryptophan and 5-hydroxyindol acetic acid (5-HIAA) were measured by high pressure liquid chromatography (HPLC) in the forebrain, brainstem and cervical cord of fetal, neonatal and adult mice from the wild strain C3H and the transgenic strain Tg8, created from the C3H line by the disruption of the gene encoding monoamine oxidase A. The results indicated that the absence of monoamine oxidase A activity in Tg8 mice results in abnormally high 5-hydroxytryptamine (5-HT) levels in all the central nervous structures and at all the studied developmental ages. Since serotonin levels were 4-5 times larger in Tg8 than in C3H mice at gestational day 20, comparing the central network function at birth of C3H and Tg8 neonates should shed some light on the role of serotonin in prenatal network maturation.

The pre-Bötzinger complex and phase-spanning neurons in the adult rat

To characterise respiratory neurons in the pre-Bötzinger complex of adult rats, extracellular recordings were made from 302 respiratory neurons in the ventral respiratory group of sodium pentobarbitone anaesthetised adult rats. Neurons were located 0 to 1.6 mm caudal to the facial nucleus, and ventral to the nucleus ambiguus. The pre-Bötzinger complex comprised expiratory neurons (22%, 22/100), inspiratory neurons (37%, 37/100) and phase-spanning neurons (41%, 41/100). In contrast, 80% (125/157) of Bötzinger neurons were expiratory, and 80% (36/45) of rostral ventral respiratory group neurons were inspiratory. Rostrocaudally, the pre-Bötzinger complex extended about 400 microns, starting at the caudal pole of the nucleus ambiguus compact formation. The pre-Bötzinger complex was also characterised by a predominance of propriobulbar neurons (81%, 13/16). Furthermore, 68% (33/48) of expiratory-inspiratory neurons found were located within the pre-Bötzinger complex. The variety of neuronal subtypes in the pre-Bötzinger complex, including many firing during the expiratory-inspiratory transition is consistent with the hypothesis that this nucleus plays a key role in respiratory rhythm generation in the adult rat.

PreBötzinger complex and pacemaker neurons: hypothesized site and kernel for respiratory rhythm generation

Identification of the sites and mechanisms underlying the generation of respiratory rhythm is of longstanding interest to physiologists and neurobiologists. Recently, with the development of novel experimental preparations, especially in vitro en bloc and slice preparations of rodent brainstem, progress has been made In particular, a site in the ventrolateral medulla, the preBötzinger Complex, is hypothesized to contain neuronal circuits generating respiratory rhythm. Lesions or disruption of synaptic transmission within the preBötzinger Complex, either in vivo or in vitro, can abolish respiratory activity. Furthermore, the persistence of respiratory rhythm following interference with postsynaptic inhibition and the subsequent discovery of neurons with endogenous bursting properties within the preBötzinger Complex have led to the hypothesis that rhythmogenesis results from synchronized activity of pacemaker or group-pacemaker neurons.

Rostral ventrolateral medulla and respiratory rhythmogenesis in mice

To compare the mechanisms governing perinatal respiratory rhythmogenesis in mice and rats, we adapted to the neonatal mouse the in vitro brainstem-spinal cord preparation of the neonatal rat. In mouse preparations retaining the pons, phrenic root did not show any rhythmic activity. Elimination of the pons induced phrenic rhythmic bursts which (1) induced respiratory chest movements (rib cage kept attached to the spinal cord), (2) were abolished by spinal cord transection, (3) could be prematurely induced by rostral ventro-lateral medulla (RVLM) stimulation, (4) occurred in phase with the bursting firing of RVLM neurons, and (5) were abolished by RVLM lesion. Then, the RVLM appears crucial for respiratory rhythmogenesis in both species; some results suggest however that vagal and pontine respiratory controls might not be identical in mice and rats.

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

Gasping is a critical mechanism for survival in that it serves as a mechanism for autoresuscitation when eupnea fails. Eupnea and gasping are separable patterns of automatic ventilatory activity in all mammalian species from the day of birth. The neurogenesis of the gasp is dependent on the discharge of neurons in the rostroventral medulla. This gasping center overlaps a region termed "the pre-Bötzinger complex." Neuronal activities of this complex, characterized in an in vitro brain stem spinal cord preparation of the neonatal rat, have been hypothesized to underlie respiratory rhythm generation. Yet, the rhythmic activity of this in vitro preparation is markedly different from eupnea but identical with gasping in vivo. In eupnea, medullary neuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of the pontomedullary circuit defining eupneic ventilatory activity. However, these medullary neuronal activities do not appear critical for the neurogenesis of eupnea, per se.

Perinatal developmental changes in respiratory activity of medullary and spinal neurons: an in vitro study on fetal and newborn rats

Experiments were performed in vitro on fetal and newborn rat brainstem-spinal cord preparations to analyse the perinatal developmental changes in inspiratory motor output. The amplitude of the inspiratory bursts of the whole C4 ventral root (global extracellular recording), the firing patterns of 80 medullary inspiratory neurons (unitary extracellular recording) and the firing and membrane properties of 71 respiratory neurons in the C4 ventral horn (whole-cell recording) were analysed at embryonic day 18 (E18), 21 (E21) and post natal days 0 to 3 (P0-3). At E18, the amplitude of the C4 bursts was weak and variable from one respiratory cycle to the next, as well as the discharge pattern of most of the medullary inspiratory neurons. C4 motoneurons were immature, very excitable and displaying variable inspiratory discharges, but already able to deliver sustained bursts of potentials when depolarised. At E21 and P0-3, the amplitude of the C4 bursts was increased and stable, most of the medullary inspiratory neurons already were able to generate a stable firing pattern and C4 motoneurons showed maturational changes in terms of the resting potential, spike amplitude and input membrane resistance. This work suggests that the short period extending from E18 to E21 is a critical maturational period for the medullary respiratory network which becomes able to elaborate a stable respiratory motor output.

Characterizations and comparisons of eupnoea and gasping in neonatal rats

1. Our purpose was to characterize the ventilatory patterns of eupnoea and gasping in the neonatal rat. This study was precipitated by reports, using in vitro brainstem spinal cord preparations, that only a single pattern is present in neonatal rats. 2. In anaesthetized or decerebrate rat pups aged less than 13 days, eupnoea was characterized by a sudden onset of inspiratory activity and then a more gradual rise to peak levels. Following vagotomy, frequency fell and peak phrenic activity and tidal volume increased. The rate of rise of inspiratory activity also rose, but peak levels were still achieved during the latter half of inspiration. Vagal efferent activity exhibited bursts during both inspiration and the early expiration. This basic eupnoeic rhythm was not altered after sectioning of the carotid sinus nerves. 3. Upon exposure to hypoxia or anoxia, phrenic activity, tidal volume and frequency initially increased and then declined. In many animals, ventilatory activity then ceased, but later returned with a gasping pattern. 4. Gasping was characterized by a sudden onset of phrenic activity, which reached a peak intensity during the early portion of inspiration. The expiratory burst of vagal activity was eliminated. 5. Reductions of body temperature from 37 to 27 degrees C resulted in prolongations of inspiration and expiration and decreases of phrenic amplitude; phasic phrenic activity completely disappeared in some animals. Upon exposure to anoxia, gasping was observed, even in animals in which phrenic activity had disappeared in hyperoxia. 6. We conclude that, from the day of birth, rats can exhibit eupnoea and gasping patterns which are very similar to those of adult animals. 7. The rhythmic neural activities of the in vitro brainstem-spinal cord preparation, reported by others, differ markedly from eupnoea but are identical with gasping. We therefore conclude that this preparation is not suitable for investigation of the mechanisms that generate eupnoeic breathing.

Generation of respiratory rhythm and pattern in mammals: insights from developmental studies

Our understanding of the cellular, synaptic and network mechanisms underlying respiratory rhythm generation in mammals is progressing rapidly as researchers focus on a site hypothesized as the source of rhythm generation, the preBötzinger complex, in the rostral ventrolateral medulla. Furthermore, ontogenetic and modulatory factors affecting respiratory neuronal circuits are receiving considerable attention, as postnatal development of motor systems becomes increasingly apparent.

Studies of the respiratory center using isolated brainstem-spinal cord preparations

It has been ten years since a brainstem-spinal cord preparation isolated from a newborn rat was introduced for study of the mammalian respiratory center. Here, I briefly summarize first, these studies, which include the tissue condition of in vitro preparations, respiratory reflexes, pharmacology, rhythm generation, respiratory chemoreception, phrenic motoneurons, regulation from pons, and development of a respiratory center. In the latter half of this paper, I focus on the neural mechanisms of respiratory rhythm generation. A current hypothesis for the central pattern generator of respiration proposed by the author's group is that the respiratory rhythm generator, composed of pre-inspiratory neurons in the rostral ventrolateral medulla, produces the primary rhythm of respiration and triggers an inspiratory pattern generator composed of inspiratory neurons in the rostral and the caudal ventrolateral medulla.