5-Hydroxytryptamine modulates central respiratory activity in the newborn rat: an in vitro study

The serotonergic system and the control of breathing during development

Serotonin (5-hydroxytryptamine 5-HT) was first discovered in the late 1940's as an endogenous bioactive amine capable of inducing vasoconstriction, and in the mid-1950's was found in the brain. It was in these early years that some of the first demonstrations were made regarding a role for brain 5-HT in neurological function and behavior, including data implicating reduced brain levels of 5-HT in clinical depression. Since that time, advances in molecular biology and physiological approaches in basic science research have intensely focused on 5-HT in the brain, and the many facets of its role during embryonic development, post-natal maturation, and neural function in adulthood continues to be established. This review focuses on what is known about the developmental roles for the 5-HT system, which we define as the neurons producing 5-HT along with pre-and post-synaptic receptors, in a vital homeostatic motor behavior - the control of breathing. We will cover what is known about the embryonic origins and fate specification of 5-HT neurons, and how the 5-HT system influences pre- and post-natal maturation of the ventilatory control system. In addition, we will focus on the role of the 5-HT system in specific respiratory behaviors during fetal, neonatal and postnatal development, and the relevance of dysfunction in this system in respiratory-related human pathologies including Sudden Infant Death Syndrome (SIDS).

Current Perspectives for the use of Gonane Progesteronergic Drugs in the Treatment of Central Hypoventilation Syndromes

BACKGROUND

Central alveolar hypoventilation syndromes (CHS) encompass neurorespiratory diseases resulting from congenital or acquired neurological disorders. Hypercapnia, acidosis, and hypoxemia resulting from CHS negatively affect physiological functions and can be lifethreatening. To date, the absence of pharmacological treatment implies that the patients must receive assisted ventilation throughout their lives.

OBJECTIVE

To highlight the relevance of determining conditions in which using gonane synthetic progestins could be of potential clinical interest for the treatment of CHS.

METHODS

The mechanisms by which gonanes modulate the respiratory drive were put into the context of those established for natural progesterone and other synthetic progestins.

RESULTS

The clinical benefits of synthetic progestins to treat respiratory diseases are mixed with either positive outcomes or no improvement. A benefit for CHS patients has only recently been proposed. We incidentally observed restoration of CO2 chemosensitivity, the functional deficit of this disease, in two adult CHS women by desogestrel, a gonane progestin, used for contraception. This effect was not observed by another group, studying a single patient. These contradictory findings are probably due to the complex nature of the action of desogestrel on breathing and led us to carry out mechanistic studies in rodents. Our results show that desogestrel influences the respiratory command by modulating the GABAA and NMDA signaling in the respiratory network, medullary serotoninergic systems, and supramedullary areas.

CONCLUSION

Gonanes show promise for improving ventilation of CHS patients, although the conditions of their use need to be better understood.

The role of serotonin in respiratory function and dysfunction

Serotonin (5-HT) is a neuromodulator-transmitter influencing global brain function. Past and present findings illustrate a prominent role for 5-HT in the modulation of ponto-medullary autonomic circuits. 5-HT is also involved in the control of neurotrophic processes during pre- and postnatal development of neural circuits. The functional implications of 5-HT are particularly illustrated in the alterations to the serotonergic system, as seen in a wide range of neurological disorders. This article reviews the role of 5-HT in the development and control of respiratory networks in the ponto-medullary brainstem. The review further examines the role of 5-HT in breathing disorders occurring at different stages of life, in particular, the neonatal neurodevelopmental diseases such as Rett, sudden infant death and Prader-Willi syndromes, adult diseases such as sleep apnoea and mental illness linked to neurodegeneration.

Medullary serotonin neurons and central CO2 chemoreception

Serotonergic (5-HT) neurons are putative central respiratory chemoreceptors, aiding in the brain's ability to detect arterial changes in PCO2 and implement appropriate ventilatory responses to maintain blood homeostasis. These neurons are in close proximity to large medullary arteries and are intrinsically chemosensitive in vitro, characteristics expected for chemoreceptors. 5-HT neurons of the medullary raphé are stimulated by hypercapnia in vivo, and their disruption results in a blunted hypercapnic ventilatory response. More recently, data collected from transgenic and knockout mice have provided further insight into the role of 5-HT in chemosensitivity. This review summarizes current evidence in support of the hypothesis that 5-HT neurons are central chemoreceptors, and addresses arguments made against this role. We also briefly explore the relationship between the medullary raphé and another chemoreceptive site, the retrotrapezoid nucleus, and discuss how they may interact during hypercapnia to produce a robust ventilatory response.

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.

SEROTONERGIC MECHANISMS IN AMYOTROPHIC LATERAL SCLEROSIS

Serotonin (5-HT) has been intimately linked with global regulation of motor behavior, local control of motoneuron excitability, functional recovery of spinal motoneurons as well as neuronal maturation and aging. Selective degeneration of motoneurons is the pathological hallmark of amyotrophic lateral sclerosis (ALS). Motoneurons that are preferentially affected in ALS are also densely innervated by 5-HT neurons (e.g., trigeminal, facial, ambiguus, and hypoglossal brainstem nuclei as well as ventral horn and motor cortex). Conversely, motoneuron groups that appear more resistant to the process of neurodegeneration in ALS (e.g., oculomotor, trochlear, and abducens nuclei) as well as the cerebellum receive only sparse 5-HT input. The glutamate excitotoxicity theory maintains that in ALS degeneration of motoneurons is caused by excessive glutamate neurotransmission, which is neurotoxic. Because of its facilitatory effects on glutaminergic motoneuron excitation, 5-HT may be pivotal to the pathogenesis and therapy of ALS. 5-HT levels as well as the concentrations 5-hydroxyindole acetic acid (5-HIAA), the major metabolite of 5-HT, are reduced in postmortem spinal cord tissue of ALS patients indicating decreased 5-HT release. Furthermore, cerebrospinal fluid levels of tryptophan, a precursor of 5-HT, are decreased in patients with ALS and plasma concentrations of tryptophan are also decreased with the lowest levels found in the most severely affected patients. In ALS progressive degeneration of 5-HT neurons would result in a compensatory increase in glutamate excitation of motoneurons. Additionally, because 5-HT, acting through presynaptic 5-HT1B receptors, inhibits glutamatergic synaptic transmission, lowered 5-HT activity would lead to increased synaptic glutamate release. Furthermore, 5-HT is a precursor of melatonin, which inhibits glutamate release and glutamate-induced neurotoxicity. Thus, progressive degeneration of 5-HT neurons affecting motoneuron activity constitutes the prime mover of the disease and its progression and treatment of ALS needs to be focused primarily on boosting 5-HT functions (e.g., pharmacologically via its precursors, reuptake inhibitors, selective 5-HT1A receptor agonists/5-HT2 receptor antagonists, and electrically through transcranial administration of AC pulsed picotesla electromagnetic fields) to prevent excessive glutamate activity in the motoneurons. In fact, 5HT1A and 5HT2 receptor agonists have been shown to prevent glutamate-induced neurotoxicity in primary cortical cell cultures and the 5-HT precursor 5-hydroxytryptophan (5-HTP) improved locomotor function and survival of transgenic SOD1 G93A mice, an animal model of ALS.

Mecp2 deficiency disrupts norepinephrine and respiratory systems in mice

Rett syndrome is a severe X-linked neurological disorder in which most patients have mutations in the methyl-CpG binding protein 2 (MECP2) gene and suffer from bioaminergic deficiencies and life-threatening breathing disturbances. We used in vivo plethysmography, in vitro electrophysiology, neuropharmacology, immunohistochemistry, and biochemistry to characterize the consequences of the MECP2 mutation on breathing in wild-type (wt) and Mecp2-deficient (Mecp2-/y) mice. At birth, Mecp2-/y mice showed normal breathing and a normal number of medullary neurons that express tyrosine hydroxylase (TH neurons). At approximately 1 month of age, most Mecp2-/y mice showed respiratory cycles of variable duration; meanwhile, their medulla contained a significantly reduced number of TH neurons and norepinephrine (NE) content, even in Mecp2-/y mice that showed a normal breathing pattern. Between 1 and 2 months of age, all unanesthetized Mecp2-/y mice showed breathing disturbances that worsened until fatal respiratory arrest at approximately 2 months of age. During their last week of life, Mecp2-/y mice had a slow and erratic breathing pattern with a highly variable cycle period and frequent apneas. In addition, their medulla had a drastically reduced number of TH neurons, NE content, and serotonin (5-HT) content. In vitro experiments using transverse brainstem slices of mice between 2 and 3 weeks of age revealed that the rhythm produced by the isolated respiratory network was irregular in Mecp2-/y mice but could be stabilized with exogenous NE. We hypothesize that breathing disturbances in Mecp2-/y mice, and probably Rett patients, originate in part from a deficiency in noradrenergic and serotonergic modulation of the medullary respiratory network.

Vagal pulmonary afferents and central respiratory effects of 5-HT in newborn rats

In decerebrate newborn rats, serotonin (5-HT) is a respiratory depressant via activation of 5-HT2 receptors, whereas it evokes respiratory stimulant effects when applied to the isolated brainstem obtained from the newborn rat. This discrepancy could be due to deafferentation in the in vitro preparation. The aim of our study was to analyse the role of vagal afferents in the modulation of central respiratory effects of 5-HT. In decerebrate cervically or abdominally bivagotomized newborn rats aged between 0 and 3 days, we recorded electrical activity from the diaphragm and from a hypoglossally innervated tongue muscle, as well as cardiac frequency (Fc), before and after application of 5-HT to the floor of the IVth ventricle. The effects of related agents (a 5-HT1A agonist, 8-OH DPAT, and a 5-HT2 agonist, DOI) were studied in cervically bivagotomized animals. For comparison, and to assess the spontaneous variability in inspiratory frequency (Fi) and Fc, sham groups were studied. Each group comprised ten newborn rats. In cervically bivagotomized newborn rats, 5-HT induces a significant increase in Fi, which is the opposite to that observed in decerebrate newborn rats with intact vagi. This respiratory effect is mediated in particular, via activation of 5-HT1A. By contrast, in abdominally bivagotomized newborn rats, a decrease in Fi was observed in response to 5-HT (as previously described in decerebrate animals with intact vagi). We conclude that pulmonary vagal afferents modulate the central respiratory action of 5-HT in decerebrate newborn rats, explaining the conflicting results between in vivo and in vitro experiments.

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.

Endogenous activation of serotonin-2A receptors is required for respiratory rhythm generation in vitro

Endogenous amines and peptides continuously modulate the activity of neuronal networks and are required even for their normal operation. The respiratory rhythm generator, localized in the pre-Bötzinger complex, is not an exception. This network is modulated by various neurotransmitters, including serotonin (5-HT). In this study, we isolated the respiratory network in brainstem slices and demonstrate that the endogenous activation of 5-HT(2A) is required for the generation of the respiratory rhythm in vitro. At the network level, activation of 5-HT(2A) receptors with 4-iodo-2,5-dimethoxyamphetamine or the 5-HT uptake blocker alaproclate increased the frequency of respiratory activity. Blockade of endogenously activated 5-HT(2A) receptors with three different antagonists decreased the frequency, amplitude, and regularity of respiratory population activity, an effect that was blocked by protein kinase C (PKC) activators. At the cellular level, blockade of 5-HT(2A) receptors reduced the action potential discharge in all examined respiratory neurons, which was associated with a reduction in the fast and the persistent sodium current. Continuous application of 5-HT(2A)-receptor antagonists differentially affected pacemaker neurons. Pacemaker activity was eliminated in cadmium-insensitive pacemaker neurons. In cadmium-sensitive pacemaker neurons, the frequency of pacemaker activity was unaffected and the amplitude of pacemaker bursts was enhanced. It is assumed that cadmium-insensitive pacemakers rely on the persistent sodium current, whereas cadmium-sensitive pacemakers depend on the activation of calcium currents. We conclude that endogenously activated 5-HT(2A) receptors are required for maintaining fictive respiratory activity in the brainstem slice by modulating sodium conductances via a PKC pathway.

Theophylline-induced respiratory recovery following cervical spinal cord hemisection is augmented by serotonin 2 receptor stimulation

Cervical spinal cord hemisection leads to a disruption of bulbospinal innervation of phrenic motoneurons resulting in paralysis of the ipsilateral hemidiaphragm. We have previously demonstrated separate therapeutic roles for theophylline, and more recently serotonin (5-HT) as modulators to phrenic nerve motor recovery; mechanisms that likely occur via adenosine A1 and 5-HT2 receptors, respectively. The present study was designed to specifically determine if concurrent stimulation of 5-HT2 receptors may enhance motor recovery induced by theophylline alone. Adult female rats (250-350 g; n=7 per group) received a left cervical (C2) hemisection that resulted in paralysis of the ipsilateral hemidiaphragm. Twenty-four hours later rats were given systemic theophylline (15 mg/kg, i.v.), resulting in burst recovery in the ipsilateral phrenic nerve. Theophylline-induced recovery was enhanced with the 5-HT2A/2C receptor agonist, (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI; 1.0 mg/kg). DOI-evoked augmentation of theophylline-induced recovery was attenuated following subsequent injection of the 5-HT2 receptor antagonist, ketanserin (2.0 mg/kg). In a separate group, rats were pretreated with ketanserin, which did not prevent subsequent theophylline-induced respiratory recovery. However, pretreatment with ketanserin did prevent DOI-induced augmentation of the theophylline-evoked phrenic nerve burst recovery. Lastly, using immunocytochemistry and in situ hybridization, we showed for the first time a positive co-localization of adenosine A1 receptor mRNA and immunoreactivity with phrenic motoneurons of the cervical ventral horns. Taken together, the results of the present study suggest that theophylline may induce motor recovery likely at adenosine A1 receptors located at the level of the spinal cord, and the concurrent stimulation of converging 5-HT2 receptors may augment the response.

Long-term deprivation of substance P in PPT-A mutant mice alters the anoxic response of the isolated respiratory network

The aim of this study was to elucidate the role of the neuromodulator substance P and its related tachykinin neurokinin A (NKA) in the homeostasis of respiratory activity. Respiratory activities, in form of fictive eupneic and sigh activities, were recorded extracellularly from the preBötzinger complex (PBC) in normoxic and anoxic conditions using medullary slice preparations. The effect of a blockade of endogenous substance P was assessed by an acute pharmacological blockade of the receptors with spantide in wild-type animals and by the use of preprotachykinin-A (PPT-A) mutants. These mutants lack from birth the PPT-A gene, which codes for the precursor of substance P and NKA. Spantide treatment reduced frequency (-37%, n = 9) and regularity (twofold) of eupneic-like respiratory activity under normoxic conditions, whereas in PPT-A mutants, eupneic-like activity was under normoxic conditions not significantly different from the wild-type mice (WT). The response to short anoxic episodes (5 min) was characterized in the WT by an increase in respiratory frequencies at the onset of anoxia (ratio anoxic/control frequency = 1.9 +/- 0.2, n = 18). This anoxic ratio was unaltered in the presence of spantide (ratio = 2.3 +/- 0.4, n = 8) but increased in the mutant (ratio = 4.1, n = 15). We conclude that endogenously released substance P is important for the maintenance of regular respiratory activity. Short-term blockade of substance P receptors decreases the frequency and regularity of rhythmic activity. Long-term deficiency in substance P leads to compensatory mechanisms that result in an apparently normal respiratory activity under normoxic conditions but a significantly altered response of the respiratory network during anoxia.

Noradrenergic receptors and in vitro respiratory rhythm: possible interspecies differences between mouse and rat neonates

Similar in vitro experiments were performed on brainstem-spinal cord preparations from mouse and rat neonates to compare the noradrenergic regulations of the respiratory network. In preparations retaining the pons, rhythmic phrenic bursts occurred in rats but not in mice. Transection of the pons, electrolytic lesions and noradrenaline applications showed that the pontine noradrenergic A5 group inhibited the respiratory rhythm generator in both species but the inhibition was especially potent in mice. After pons elimination, noradrenaline applications to the medulla decreased the respiratory frequency in rats but increased it in mice. Noradrenergic agent applications revealed that the frequency changes implicated medullary alpha 1 and alpha 2 noradrenergic receptors in mice and rats, respectively. Thus, interspecies differences seem to exist in the noradrenergic regulations of the rat and mouse medullary respiratory networks.

Serotonin elicits long-lasting enhancement of rhythmic respiratory activity in turtle brain stems in vitro

Brain stem preparations from adult turtles were used to determine how bath-applied serotonin (5-HT) alters respiration-related hypoglossal activity in a mature vertebrate. 5-HT (5-20 microM) reversibly decreased integrated burst amplitude by approximately 45% (P < 0.05); burst frequency decreased in a dose-dependent manner with 20 microM abolishing bursts in 9 of 13 preparations (P < 0.05). These 5-HT-dependent effects were mimicked by application of a 5-HT(1A) agonist, but not a 5-HT(1B) agonist, and were abolished by the broad-spectrum 5-HT antagonist, methiothepin. During 5-HT (20 microM) washout, frequency rebounded to levels above the original baseline for 40 min (P < 0.05) and remained above baseline for 2 h. A 5-HT(3) antagonist (tropesitron) blocked the post-5-HT rebound and persistent frequency increase. A 5-HT(3) agonist (phenylbiguanide) increased frequency during and after bath application (P < 0.05). When phenylbiguanide was applied to the brain stem of brain stem/spinal cord preparations, there was a persistent frequency increase (P < 0.05), but neither spinal-expiratory nor -inspiratory burst amplitude were altered. The 5-HT(3) receptor-dependent persistent frequency increase represents a unique model of plasticity in vertebrate rhythm generation.

Serotonin(2) receptors mediate respiratory recovery after cervical spinal cord hemisection in adult rats

The aim of the present study was to specifically investigate the involvement of serotonin [5-hydroxytryptamine (5-HT(2))] receptors in 5-HT-mediated respiratory recovery after cervical hemisection. Experiments were conducted on C(2) spinal cord-hemisected, anesthetized (chloral hydrate, 400 mg/kg ip), vagotomized, pancuronium- paralyzed, and artificially ventilated female Sprague-Dawley rats in which CO(2) levels were monitored and maintained. Twenty-four hours after spinal hemisection, the ipsilateral phrenic nerve displayed no respiratory-related activity indicative of a functionally complete hemisection. Intravenous administration of the 5-HT(2A/2C)-receptor agonist (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI) induced respiratory-related activity in the phrenic nerve ipsilateral to hemisection under conditions in which CO(2) was maintained at constant levels and augmented the activity induced under conditions of hypercapnia. The effects of DOI were found to be dose dependent, and the recovery of activity could be maintained for up to 2 h after a single injection. DOI-induced recovery was attenuated by the 5-HT(2)-receptor antagonist ketanserin but not with the 5-HT(2C)-receptor antagonist RS-102221, suggesting that 5-HT(2A) and not necessarily 5-HT(2C) receptors may be involved in the induction of respiratory recovery after cervical spinal cord injury.

5-HT acting on 5-HT(1/2) receptors does not participate in the in vitro hypoxic respiratory depression

The involvement of serotoninergic mechanisms in the central respiratory depression produced by hypoxia was studied in the newborn rat brainstem-spinal cord preparation. The respiratory frequency measured by the C4 ventral root activity was recorded. 5-HT (30 microM) superfusion elicited a rapid increase in respiratory frequency, prevented by a treatment with methysergide (a 5-HT(1/2) receptor antagonist) (40 microM). To investigate the possible participation of 5-HT in hypoxic respiratory depression, this concentration of methysergide was added to the bathing medium during hypoxia. Methysergide did not modify the decrease in respiratory frequency produced by hypoxia. In order to ensure that other 5-HT subtype receptors were not involved in hypoxic respiratory depression, 5-HT was added to the bath during hypoxic-methysergide tests; no effect on respiratory frequency was observed. These results suggest that in the newborn rat brainstem-spinal cord preparation, serotoninergic mechanisms are not involved in the elaboration of the in vitro respiratory response to hypoxia.

Distribution of Serotonin 2A and 2C Receptor mRNA Expression in the Cervical Ventral Horn and Phrenic Motoneurons Following Spinal Cord Hemisection

Cervical spinal cord injury leads to a disruption of bulbospinal innervation from medullary respiratory centers to phrenic motoneurons. Animal models utilizing cervical hemisection result in inhibition of ipsilateral phrenic nerve activity, leading to paralysis of the hemidiaphragm. We have previously demonstrated a role for serotonin (5-HT) as one potential modulator of respiratory recovery following cervical hemisection, a mechanism that likely occurs via 5-HT2A and/or 5-HT2C receptors. The present study was designed to specifically examine if 5-HT2A and/or 5-HT2C receptors are colocalized with phrenic motoneurons in both intact and spinal-hemisected rats. Adult female rats (250-350 g; n = 6 per group) received a left cervical (C2) hemisection and were injected with the fluorescent retrograde neuronal tracer Fluorogold into the left hemidiaphragm. Twenty-four hours later, animals were killed and spinal cords processed for in situ hybridization and immunohistochemistry. Using (35)S-labeled cRNA probes, cervical spinal cords were probed for 5-HT2A and 5-HT2C receptor mRNA expression and double-labeled using an antibody to Fluorogold to detect phrenic motoneurons. Expression of both 5-HT2A and 5-HT2C receptor mRNA was detected in motoneurons of the cervical ventral horn. Despite positive expression of both 5-HT2A and 5-HT2C receptor mRNA-hybridization signal over phrenic motoneurons, only 5-HT2A silver grains achieved a signal-to-noise ratio representative of colocalization. 5-HT2A mRNA levels in identified phrenic motoneurons were not significantly altered following cervical hemisection compared to sham-operated controls. Selective colocalization of 5-HT2A receptor mRNA with phrenic motoneurons may have implications for recently observed 5-HT2A receptor-mediated regulation of respiratory activity and/or recovery in both intact and injury-compromised states.

5-Hydroxytryptamine(2A) and 5-hydroxytryptamine(1B) receptors are differently affected by the monoamine oxidase A-deficiency in the Tg8 transgenic mouse

In brainstem-spinal cord preparations of neonatal control C3H and transgenic Tg8 mice where deletion of the gene encoding monoamine oxidase-A results in serotonin (5-hydroxytryptamine (HT)) excess, whole cell recordings of identified phrenic motoneurons (Phr Mns) were performed to study the modulation of their activity by 5-HT. In C3H mice, a dual effect was observed: (i) a facilitation via 5-HT(2A) receptors and (ii) a decrease of the transmission of the central inspiratory drive via 5-HT(1B) receptors. In Tg8 mice, the 5-HT(2A)-mediated facilitation was present but the 5-HT(1B)-mediated decrease was lacking. Therefore, the conservation of the 5-HT(2A) response vs. the loss of the 5-HT(1B) one suggest that the two types of receptors respond differently to 5-HT level changes.

Control of respiration in the isolated central nervous system of the neonatal opossum, Monodelphis domestica

Respiration represents an unusual motor activity with respect to its development. As newly born mammals enter the world, their limb movements are not coordinated; time and experience are required for effective performance to be achieved. Yet the rhythm of respiration is of necessity functionally perfected and unfailing at birth. Inspiratory and expiratory motor neurons are already able to fire at appropriate rates, under the command of rhythmically active neurons in the medulla. In this review, we discuss refinements of control present in the newborn opossum, particularly with respect to mechanisms that allow adaptation of respiration to changes in the level of activity or in the outside environment. Our own studies have been aimed at analyzing respiration at the earliest stages, and at establishing the way in which important variables influence inspiration and expiration. To this end, we have used the central nervous system (CNS) of a neonatal opossum, isolated in its entirety and maintained in culture. Although the opossum is unable to walk and highly immature at birth, its respiration is regular and unfailing. The isolated CNS survives, undergoes development, and maintains its neural activity and fine structure in vitro. Moreover, fictive respiration persists for over a day or longer at rates similar to those of the intact pup. The effects of altered pH, of increased temperature, and of drugs known to alter respiratory rhythm in intact animals can be measured directly, by electrical recordings made from medullary neurons or ventral roots. As in a slice, fluids of different composition can be applied focally, through micropipettes to the surface of the ventral medulla, or diffusely to the brainstem, With highly localized application of procaine hydrochloride (2%) to selected areas of the ventral medulla, the respiratory rhythm is reduced or abolished. As in adult mammals, both the rate and the amplitude of respiration simultaneously increase in response to lowered pH (6.5-.7.1) or to topical application of 1.0 microM carbachol. Conversely, as expected, the rate and amplitude decrease in response to increased pH (pH 7.5-7.7), or 100 microM scopolamine. Two characteristic features of the control of respiration in the neonatal opossum are evident from such tests. First, changes in rate are achieved by changes in the duration of the expiratory phase of respiration. This result suggests that the timing of the respiratory cycle in the neonatal opossum is controlled by an expiratory instead of an inspiratory "off-switch". Second, the rate and the amplitude of the respiratory excursions can be controlled independently, depending on the stimulus. For example, an increase in temperature increases the rate of fictive respiration without changing its amplitude, whereas noradrenaline decreases the rate while increasing the amplitude. Thus, changes of timing and amplitude need not go hand in hand. The opossum CNS offers a favorable preparation for the analysis of neural mechanisms that generate and modulate a motor rhythm, as the animal develops from embryonic to adult stages.

alpha1-adrenergic receptor-induced slow rhythmicity in nonrespiratory cervical motoneurons of neonatal rat spinal cord

Previous studies have reported that the alpha1-adrenergic system can activate spinal rhythm generators belonging to the central respiratory network. In order to analyse alpha1-adrenergic effects on both cranial and spinal motoneuronal activity, phenylephrine (1-800 microM) was applied to in vitro preparations of neonatal rat brainstem-spinal cord. High concentration of phenylephrine superfusion exerted multiple effects on spinal cervical outputs (C2-C6), consisting of a lengthening of respiratory period and an increase in inspiratory burst duration. Furthermore, in 55% of cases a slow motor rhythm recorded from the same spinal outputs was superimposed on the inspiratory activity. However, this phenylephrine-induced slow motor rhythm generated at the spinal level was observed neither in inspiratory cranial nerves (glossopharyngeal, vagal and hypoglossal outputs) nor in phrenic nerves. Whole-cell patch-clamp recordings were carried out on cervical motoneurons (C4-C5), to determine first which motoneurons were involved in this slow rhythm, and secondly the cellular events underlying direct phenylephrine effects on motoneurons. In all types of motoneurons (inspiratory and nonrespiratory) phenylephrine induced a prolonged depolarization with an increase in neuronal excitability. However, only nonrespiratory motoneurons showed additional rhythmic membrane depolarizations (with spiking) occurring in phase with the slow motor rhythm recorded from the ventral root. Furthermore the tonic depolarization produced in all motoneurons results from an inward current [which persists in the presence of tetrodotoxin (TTX)] associated with a decrease in neuron input conductance, with a reversal potential varying as a Nernstian function of extracellular K+ concentration. Our results indicate that the alpha1-adrenoceptor activation: (i) affects both the central respiratory command (i.e. respiratory period and inspiratory burst duration) and spinal inspiratory outputs; (ii) induces slow spinal motor rhythmicity, which is unlikely to be related to the respiratory system; and (iii), increases motoneuronal excitability, probably through a decrease in postsynaptic leak K+ conductance.

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.

Effects of the serotonin synthesis inhibitor p-CPA on the expression of the crossed phrenic phenomenon 4 h following C2 spinal cord hemisection

The present study assesses the effects of para-chlorophenylalanine (p-CPA), a serotonin-depleting drug, on the recovery of respiratory-related activity in the phrenic nerve induced by asphyxia 4 h following ipsilateral C2 hemisection in young adult rats. HPLC analysis was used to quantify levels of serotonin (5-HT), dopamine (DA), norepinephrine, and the 5-HT metabolite, 5-hydroxyindoleacetic acid, in the C4 segment of the spinal cord, all of which were significantly lower in p-CPA-treated hemisected rats compared to hemisected controls receiving saline. Hemisection alone was found to significantly increase 5-HT levels and significantly decrease DA levels compared to normal controls. Eight of eight saline-injected rats expressed recovery of respiratory-related activity in the ipsilateral phrenic nerve during asphyxia 4 h following hemisection, while only 4/8 rats in the p-CPA-treated group expressed recovery in the ipsilateral nerve. Quantification of integrated phrenic nerve wave-forms indicated that the mean amplitude of respiratory-related activity in the ipsilateral phrenic nerve was significantly lower in p-CPA-treated rats than in saline controls. In addition, saline controls demonstrated significant increases in mean respiratory frequency and mean amplitude of contralateral phrenic nerve activity during asphyxia, compared to normocapnia. However, p-CPA-treated rats did not express significant differences in either mean respiratory frequency or mean amplitude of integrated respiratory wave-forms during asphyxia, compared to normocapnia. The results suggest that p-CPA treatment attenuates the recovery of respiratory-related activity in the phrenic nerve 4 h following ipsilateral C2 hemisection and attenuates asphyxia-induced increases in respiratory frequency and respiratory burst amplitude recorded from the contralateral phrenic nerve.

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.

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.

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.

Effects of CO2 and pH on the spinal respiratory rhythm generator in vitro

In vitro brainstem spinal cord preparations isolated from newborn rats were used to separately test the effects of modifications of FCO2 and pH of artificial cerebrospinal fluid on the frequency and amplitude of spinal respiratory activity recorded from C2-C8 ventral roots. Different substances such as L-glutamic acid (3 x 10[-3] M), N-methyl-D-aspartic acid (5 x 5 x 10[-6] M), amphetamine (6 mg/100 ml), 5-hydroxytryptophane (10[-3] M), or modified K+ (10[-3] M) were tested for their capacity to elicit stable changes in spinal respiratory activity over a long time period (more than 30 min) and with high frequency of occurrence, i.e., in at least 50% of the cases. None of the above drugs were found to be suitable for the investigation of the chemosensitivity of the spinal respiratory generator (sRG) because they were only able to maintain spinal respiratory activity for around 15 min. Given these data, the previously used procedure of activation through initial deep diethyl ether anaesthesia of newborn rats was employed [3] to test the chemosensitivity of the sRG because this treatment resulted in the maintenance of spinal respiratory activity with a regular pattern for 30 min, even if it occurred in only 25% of the preparations. After an increase in FCO2 from 5 to 7% (at constant pH 7.4), a significant (p < 0.05) enhancement of the mean frequency was observed on spinal respiratory bursting in both brainstem spinal cord and isolated spinal cord preparations. The changes in burst amplitude, however, were quite variable from one experiment to the other. At constant FCO2 (5%), a decrease in pH from 7.4 to 7.2 enhanced spinal respiratory frequency on brainstem spinal cord or isolated spinal cord preparations, while an increase in pH from 7.4 to 7.6 decreased it. Under these pH conditions, we did not observe any reproducible variations in spinal burst amplitude. From these results, we conclude that this spinal generator is chemosensitive to both CO2 and [H+], suggesting that it belongs to the respiratory system. Our data provide evidence for the existence of spinal CO2 and/or H+ chemoreceptors.

Serotonin and motor activity

The activity of brain serotonergic neurons in both the pontine-mesencephalic and medullary groups is positively correlated with the level of behavioral arousal and/or the behavioral state. This, in turn, appears to be related to the level of tonic motor activity, especially as manifested in antigravity muscles and other muscle groups associated with gross motor activity. In addition, a subset of serotonergic neurons displays a further increase in activity in association with repetitive, central pattern generator mediated responses. Accumulating evidence indicates that this relation to motor activity is related both to the co-activation of the sympathetic nervous system and to the modulation of afferent inputs.

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.

Tachykinins and central respiratory activity: an in vitro study on the newborn rat

The newborn rat brainstem-spinal cord preparation was used to study the effects of tachykinins on the activity of the respiratory rhythm generator in vitro and to characterize the receptors involve. Substance P and tachykinin NK1 and NK3 receptor agonists induced a concentration-dependent increase in the respiratory frequency (10(-9)-10(-7) M), whereas the respiratory frequency was only slightly affected by the tachykinin NK2 receptor agonist. Pre-treatments with tachykinin NK1 receptor antagonists (SR140333, (S)1-¿2-[3-(3.4-dichlorophenyl) -1-(3-isopropoxyphenylacetyl)piperidin-n-3-yl]ethyl¿-4-ph eny l-1-azoniabicyclo [2,2,2]octane chloride; GR82334, pGlu-Ala- Asp-Pro-Asn-Lys-Phe-Tyr-(S-S)Pro-Leu(spiro-gamma-lactam)-Trp-NH2) reduced the substance P-induced increases in the respiratory frequency but the tachykinin NK2 receptor antagonist (SR48968, ((S)-N-methyl-N-[4-4-acetylamino-4-phenylpiperidine)-2-(3,4-dichlorop hen yl) butyl]benzamide); MEN 10376, Asp-Tyr-D-Trp-Val-D-Trp-Lys-NH2) had no effect; the increase in the respiratory frequency induced by the tachykinin NK3 receptor agonist was not affected by a pre-treatment with tachykinin NK1 and NK2 receptor antagonists. These result indicate that tachykinin NK1 and NK3 receptors may be involved in the control of the respiratory frequency.

Central effects of 5-HT on activity of respiratory and hypoglossally innervated muscles in newborn kittens

1. In decerebrate kittens (n = 29), electrical activity was studied in the 3rd intercartilaginous (inspiratory), the 9th internal intercostal (expiratory) and the hypoglossally innervated muscles (geniohyoid m. and sternohyoid m.) evoked by the application of 5-HT (n = 16) or related agents (5-HT1A agonist, 8-OH-DPAT (n = 6) and 5-HT2 agonist, DOI floor of the IVth ventricle. 2. The application of a control solution (n = 2) produced no significant changes either in minute inspiratory frequency (Fi) or in the electrical activity of the muscles studied. Except for these controls, only one trial with one dose of one drug was performed in a given kitten. 3. A dose-related decrease in Fi was observed in response to 5-HT. Low doses (50-500 nmol, n1 = 8) induced a long-lasting bradypnoea; high doses (5000-10,000 nmol, n2 = 8) induced prolonged periods of apnoea. 4. The apnoeas observed in tracheotomized (n = 3) or non-tracheotomized (n2 = 8) kittens were mainly of central origin and linked to the lengthening of expiratory time. The expiratory muscle activation came on with the reinforcement of the activity of hypoglossally innervated muscles. 5. Application of agonists showed that both the 5-HT-dependent modulation of Fi and the effects of 5-HT on the activity of the muscles studied resulted predominantly from activation of 5-HT2 receptors.

Central effects of 5-HT on respiratory and hypoglossal activities in the adult cat

The activities of the diaphragmatic, internal intercostal and hypoglossal-innervated muscles were studied in adult decerebrate cats in response to 5-HT and related agents (8-OH-DPAT and DOI). The drugs were placed on the floor of the IVth ventricle. The mean respiratory frequency (Fi) increased (124-193% of the control value) within 3 min of the 5-HT application, and decreased thereafter (30-90%). The mean Ti and Te changed similarly, but opposite to Fi. With some delay, the hypoglossal-innervated muscles were tonically activated or exhibited increased activities. Methysergide pretreatment completely blocked the effect of 5-HT on all the respiratory parameters and the hypoglossal-innervated muscles activities. The responses to 8-OH-DPAT and DOI indicate that 5-HT modulates the respiratory frequency via activation of both 5-HT1A and 5-HT2 receptors. Nevertheless, the effect of 5-HT on both the expiratory and hypoglossal-innervated muscles seems to depend on 5-HT2 receptors activation only.

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.

Endogenous serotonin modulates the fetal respiratory rhythm: an in vitro study in the rat

The aim of the present work was to know whether the excitatory modulation of the central respiratory rhythm generator by serotonin (5-HT) previously found to occur in the newborn rat, is already functional during the fetal life. Experiments were performed at embryonic day 18 (D18) and 20-21 (D20-21; full-term day 21) on the fetal rat brainstem-spinal cord preparation in which the ability to generate central respiratory activity in vitro persists. Replacing the normal medium which bathed the preparation by a medium containing 5-HT increased the respiratory frequency (RF) within 2-3 min in a dose-dependent manner in both D18 and D20-21 fetuses but the effect was particularly drastic at D18. Applying a medium containing the 5-HT antagonist, methysergide, to block the effect of endogenous 5-HT, if any, reduced the RF within 2-3 min and the reduction was especially drastic at D18 where respiratory arrests occurred for several minutes in most of the experiments. Applying a medium containing either the 5-HT reuptake inhibitor fluoxetine to potentiate the effect of endogenous 5-HT or the 5-HT precursor, L-tryptophan, to activate 5-HT biosynthesis mechanisms, increased the RF. To define the type of 5-HT receptors involved in the modulation of the RF, experiments were conducted with specific 5-HT agonists and antagonists. Both 5-HT1 (8-OH-DPAT, buspirone) and 5-HT2 agonists (DOI, alpha-methyl-5-HT) increased the RF but only the 5-HT1A agonist 8-OH-DPAT was efficient at submicromolar concentrations. Applying the 5-HT1A antagonist NAN-190 alone decreased the RF and even elicited respiratory arrests while the 5-HT2 antagonist ketanserin was inefficient. NAN-190 pre-treatment blocked the increase in the RF due to 8-OH-DPAT and 5-HT. Taken as a whole these results clearly indicate that endogenous 5-HT exerts an excitatory modulation on the respiratory rhythm generator via activation of medullary 5-HT1A receptors well before birth, as soon as D18 where the modulation is particularly potent.

Further evidence that various 5-HT receptor subtypes modulate central respiratory activity: in vitro studies with SR 46349B

The ability of SR 46349B [trans,4-([3Z)3-(2-dimethylaminoethyl)oxyimino- 3(2-flurophenyl)propen-1-yl]phenol hemifumarate], a 5-HT2 receptor antagonist, to block the changes in respiratory activity induced by serotonin was analysed by using brain stem-spinal cord preparations from newborn rats. The increases in respiratory frequency elicited by serotonin (and 5-HT1A receptor agonist) were not suppressed by SR 46349B. The tonic discharge of cervical motoneurons and the depression of inspiratory hypoglossal activity elicited by serotonin (and 5-HT2 receptor agonist) were reduced in a dose-dependent manner by SR 46349B. These results confirm that activation of 5-HT1A and 5-HT2 receptors influences respiratory frequency and motoneuronal output, respectively.

Long-term facilitation of inspiratory intercostal nerve activity following carotid sinus nerve stimulation in cats

1. Repeated carotid sinus nerve (CSN) stimulation evokes a serotonin-dependent long-term facilitation (LTF) of phrenic nerve activity in cats. To determine whether CSN stimulation-evoked LTF is a general property of spinal inspiratory motoneurones, phrenic and inspiratory internal intercostal (IIC) nerve activities were recorded in nine cats (eight anaesthetized; one decerebrate), which were vagotomized, paralysed, thoracotomized and ventilated with O2; airway CO2 was controlled by means of of a servo-respirator. Baseline conditions were established by setting the arterial CO2 pressure (Pa,CO2) at approximately 2 mmHg above the threshold for IIC activity. One CSN was stimulated (3 times threshold, 25 Hz, 0.5 ms duration) with five (2 min) trains, each separated by 5 min. 2. The peak integrated phrenic activity was elevated by 33% whereas IIC activity was elevated by 226% above baseline, 90 min post-stimulation (P < 0.05). The results were similar when expressed as a percentage of the maximal neural activities (elicited by combined hypercapnia and CSN stimulation), although differences between the nerves were less pronounced. The burst frequency was not change following stimulation. 3. In five additional cats that were pretreated with the serotonin receptor antagonist, methysergide maleate (0.5-1 mg kg-1, I.V.), the CO2 thresholds of the phrenic (12 mmHg) and IIC nerves (22 mmHg) were increased (P < 0.05), and LTF could not be elicited in either neurogram. 4. Successive CSN stimulation episodes evoked a previously undescribed phenomenon. Although the peak integrated phrenic activity was unchanged (90-95% of maximal), IIC activity increased progressively during successive stimulus episodes (66-90% of maximal; P < 0.05). However, after methysergide treatment, the initial stimulus-evoked phrenic response decreased to 58% of maximal and both neurograms exhibited progressive augmentation of the stimulus-evoked response. As stimulus-evoked augmentation does not require serotonin, it is independent of LTF. 5. We conclude that CSN stimulation-evoked LTF of IIC activity exceeds that of phrenic activity. Since LTF requires the neuromodulator serotonin and is expressed predominantly by changes in burst pattern formation versus rhythm generation, serotonin may exert a greater influence on IIC relative to phrenic respiratory motor output. A unique mechanism is described whereby successive CSN stimulus episodes cause progressively increasing responses in both neurograms.

Electrical activation and inhibition of respiration in vitro

The present studies employed the neonatal rat rib-attached brain stem-spinal cord preparation to examine the effects of electrical stimulation of the medulla and the pons on respiratory-like activity. The investigation focused on determining whether electrical stimulation of the medulla can be used to modulate respiratory-like activity, whether electrical stimulation of the pons can inhibit respiratory-like activity, and how the preparation responds when both the medulla and the pons are stimulated simultaneously. The results suggest that: (1) stimulation of the ventromedial part of the medulla entrains the onset and the frequency of respiratory-like electromyographic (EMG) bursting most effectively and drives the respiratory rate as high as 0.4 Hz; (2) both ventromedial and ventrolateral pontine stimulation inhibited respiratory-like EMG bursting, but only ventrolateral pontine stimulation was followed by post-stimulation inhibition; (3) when the medulla and the pons were stimulated simultaneously, pontine stimulation-induced inhibition outweighed medullary stimulation-induced activation and resulted in a temporary cessation of respiratory-like EMG bursting.

Characteristics of slow bursting activities recorded in cervical ventral roots in the in vitro brainstem-spinal cord preparation of the neonatal rat

The aim of the present work was to disclose, through pharmacological activation of an isolated central nervous system maintained in vitro, spinal locomotor and respiratory-like activities inferred from an in vivo rabbit preparation. In a brainstem-spinal cord preparation in neonatal rats (0-3 days old), medullary respiratory activity occurred spontaneously in the cervical ventral roots. During 5-hydroxytryptophan (5-HTP) superfusion (0.2 mM), a slower rhythm with longer burst duration developed in the same ventral roots, with the pre-existing long-lasting slow bursting (LLSB) activity. At the same time, locomotor bursts were recorded from lumbar ventral roots. The LLSB activity was mainly recorded in cervical ventral roots, but they could also be encountered at the lumbar level, where they were eliminated after thoracic transection. The LLSB activity and the locomotor bursting were maintained after a C1 or C2 spinal transection, whereas medullary activity disappeared. Bilateral recording of the three types of rhythmic activity demonstrated that the LLSB activity and the medullary respiratory bursting typically displayed a synchronous bilateral coupling, whereas at caudal levels an alternate bilateral pattern was the rule for locomotor activity. Lactic acid could reinduce LLSB activity if introduced after it had just disappeared during the washout phase following 5-HTP superfusion. These results strongly suggest that the LLSB activity that originates from cervical generators belongs to the respiratory system, and not to locomotor activity. Finally, similar results in an in vivo rabbit preparation have been obtained through pharmacological activation. This preparation appears to be a suitable model for the analysis of this cervical burst generator and for the study of interactions among the different pattern generators.

Serotonergic modulation of the respiratory rhythm generator at birth: an in vitro study in the rat

In order to investigate the mechanisms through which serotonin (5-HT) modulates the activity of the respiratory rhythm generator, respiratory activity was recorded from cervical ventral roots of the superfused isolated brainstem-spinal cord preparation of the newborn rat. Replacing the normal bathing medium by a medium containing 5-HT (30 microM) increased the respiratory frequency by 70% of the control value. Intact pontomedullary structures are necessary for this effect to take place, however, since the 5-HT-induced increases in respiratory frequency were no longer observed after elimination (section and electrolytic lesion) of the caudal ventro-lateral pons containing the A5 areas. Local applications of 5-HT (dual bath, microdialysis and microinjection experiments) revealed, however, that 5-HT acts at the medullary level and that its effects are not due to a diffuse action on all the neurons of the medullary respiratory centers but to a specific action focusing on structures located in the rostral ventro-lateral medulla.

In vitro study of central respiratory-like activity of the fetal rat

A fetal rat brain stem-spinal cord in vitro preparation (15-20 days) which retains for several hours respiratory-like discharges on cervical and cranial ventral roots has been developed for analysing fetal central respiratory activity. Two different patterns of easily distinguishable rhythmic activity were recorded. The first, of spinal origin, appeared every 2-10 min as long bursts of potentials (3-30 s) on cervical, but not hypoglossal, roots. The second pattern corresponded to brief bursts (1 s) of potentials occurring on both cervical and hypoglossal roots at a frequency ranging from 3-4 cycles min-1. The second type of activity was likely to be respiratory since it originated from the medulla, and behaved similarly to the respiratory activity recorded in vitro from newborn rats. The fetal respiratory-like activity was never observed at day 15, appeared at day 16 in 30% of the preparations with fluctuating frequency and amplitude bursts, and stabilised at day 20 where it was usually present and resembled newborn rat respiratory activity: its frequency was stable but was reduced by withdrawal of CO2 and pH stimuli and modulated by a pontine noradrenergic influence. This fetal preparation offers many advantages for studying the ontogeny of the central respiratory activity because of the background knowledge available on the adult and newborn rat respiratory centers and the possibility of performing electrophysiological, morphological and pharmacological fetal studies directly at the central level without any feedback from the periphery.

Compared effects of serotonin on cervical and hypoglossal inspiratory activities: an in vitro study in the newborn rat

1. Experiments were performed on the brain stem-spinal cord preparation of newborn rats, in which the phrenic and hypoglossal nerves continue to show rhythmic respiratory activity in vitro, in order to compare the effects of serotonin (5-HT) on both activities and to analyse the mechanisms responsible for the depression by 5-HT of the hypoglossal activity. 2. Under control conditions, simultaneous recordings of the inspiratory discharges of hypoglossal and cervical roots showed that the two bursts did not start simultaneously and had different patterns (time-to-peak and peak values); this suggests that both pools of motoneurons did not share the same central drive(s). 3. Adding 5-HT and related agents to the bathing medium delayed and depressed the hypoglossal inspiratory discharge via activation of 5-HT2 receptors since these effects were elicited by 5-HT2 agonists (alpha-methyl-5-HT and 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane-HCl (DOI)) but not by 5-HT1 agonists (RU 24969 and (+/-)-8-hydroxy-2-(di-N-propylamino)tetralin hydrobromide (8-OH-DPAT)). The 5-HT depression of the hypoglossal discharge was prevented by applying a pretreatment with a specific 5-HT2 antagonist (ketanserin). Parallel to the hypoglossal discharge decrease, 5-HT elicited a permanent cervical root discharge along with a persistent inspiratory bursting. Adding the 5-HT precursor L-tryptophan to the bathing medium depressed the hypoglossal (XII) discharge without affecting the cervical one. 4. Local application of 5-HT within the hypoglossal motor nucleus decreased the hypoglossal output, revealing that the 5-HT depression of the hypoglossal discharge was at least partly mediated by the 5-HT effects at the level of the motoneurons. Local application of 5-HT within the cervical motor nucleus elicited a permanent firing in the cervical root with a persistent inspiratory bursting. 5. Intracellular analysis confirmed the existence of differences in central respiratory drive between cervical and hypoglossal motoneurons under control conditions, as well as differences in response to 5-HT. All the hypoglossal motoneurons became silent under 5-HT bathing, and showed no change in the input membrane resistance, a moderate depolarization, and a delayed central respiratory drive with a decreased amplitude. The cervical motoneurons became more active during inspiration, despite a decrease in the amplitude of the central respiratory drive, which was compensated for by a large depolarization and an increased input membrane resistance. Some cervical motoneurons even fired at a low rate during expiration.(ABSTRACT TRUNCATED AT 400 WORDS)