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

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

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

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

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

Exposure to toxics during pregnancy and childhood and asthma in children: A pilot study

Environmental factors, pesticides, alcohol and smoking are linked to asthma in children. The association of toxic substances exposure with asthma has not been evaluated. Our objective is to assess such associations among children aged less than 16 years old. This is a cross-sectional study, conducted between January and May 2015, using a sample of Lebanese students from private schools in Beirut and Mount Lebanon. Out of 700 distributed questionnaires, 527 (75.2%) were returned to us. Verbal informed consent was also obtained from all parents prior to participating in the study. A significant association was found between waterpipe smoking and diagnosed asthma (p = 0.003; OR_a = 13.25; 95% CI 2.472–71.026). Alcohol during pregnancy, waterpipe smoking during pregnancy and parents respiratory problems significantly increased the risk of respiratory problems by approximately 5 times, 6 times and 2 times respectively (p = 0.016; OR_a = 4.889; 95% CI 1.339–17.844, p = 0.021; OR_a = 6.083; 95% CI 1.314–28.172, p = 0.004; OR_a = 1.748; 95% CI 1.197–2.554 respectively). Waterpipe smoking, alcohol during pregnancy, recurrent otitis and humidity at home seem to be significantly correlated with asthma in children. Spreading awareness by health care professionals is needed to permit a reduction of the prevalence of these allergic diseases, especially asthma, in children.

The Onset of the Fetal Respiratory Rhythm: An Emergent Property Triggered by Chemosensory Drive?

The mechanisms responsible for the onset of respiratory activity during fetal life are unknown. The onset of respiratory rhythm may be a consequence of the genetic program of each of the constituents of the respiratory network, so they start to interact and generate respiratory cycles when reaching a certain degree of maturation. Alternatively, generation of cycles might require the contribution of recently formed sensory inputs that will trigger oscillatory activity in the nascent respiratory neural network. If this hypothesis is true, then sensory input to the respiratory generator must be already formed and become functional before the onset of fetal respiration. In this review, we evaluate the timing of the onset of the respiratory rhythm in comparison to the appearance of receptors, neurotransmitter machinery, and afferent projections provided by two central chemoreceptive nuclei, the raphe and locus coeruleus nuclei.

Antidepressant use during pregnancy and asthma in the offspring

BACKGROUND AND OBJECTIVES

It has been suggested that maternal depression during pregnancy is associated with asthma in the offspring, but the role of medical treatment of depression is not known. Our goal was to examine whether prenatal antidepressant use increases the risk of asthma in the offspring.

METHODS

A cohort study was performed among all live singletons born in Denmark between 1996 and 2007. Mothers who had a diagnosis of depressive disorder and/or who used antidepressants 1 year before or during the index pregnancy were identified. Using a Cox proportional hazards regression model, we estimated the hazard ratio (HR) for asthma in the offspring after antidepressant use during pregnancy.

RESULTS

Of the 733,685 children identified, 84,683 had a diagnosis of asthma. A total of 21,371 children were exposed to prenatal maternal depression (ie, a diagnosis of depressive disorder or use of antidepressants 1 year before or during pregnancy). Prenatal maternal depression was associated with childhood asthma (HR: 1.25 [95% confidence interval (CI): 1.20-1.30]). Overall, 8895 children were exposed to antidepressants in utero. Compared with children born to mothers with prenatal depression and no antidepressant use during pregnancy, the HR for asthma after any antidepressant use during pregnancy was 1.00 (95% CI: 0.93-1.08). HRs after use of selective serotonin reuptake inhibitors only, newer antidepressants only, and older antidepressants only were 0.95 (95% CI: 0.88-1.03), 1.11 (95% CI: 0.89-1.39), and 1.26 (95% CI: 1.02-1.55), respectively.

CONCLUSIONS

Antidepressant use during pregnancy generally did not increase the risk of asthma. Only use of older antidepressants was associated with an increased risk of asthma.

Peripheral-central chemoreceptor interaction and the significance of a critical period in the development of respiratory control

Respiratory control entails coordinated activities of peripheral chemoreceptors (mainly the carotid bodies) and central chemosensors within the brain stem respiratory network. Candidates for central chemoreceptors include Phox2b-containing neurons of the retrotrapezoid nucleus, serotonergic neurons of the medullary raphé, and/or multiple sites within the brain stem. Extensive interconnections among respiratory-related nuclei enable central chemosensitive relay. Both peripheral and central respiratory centers are not mature at birth, but undergo considerable development during the first two postnatal weeks in rats. A critical period of respiratory development (∼P12-P13 in the rat) exists when abrupt neurochemical, metabolic, ventilatory, and electrophysiological changes occur. Environmental perturbations, including hypoxia, intermittent hypoxia, hypercapnia, and hyperoxia alter the development of the respiratory system. Carotid body denervation during the first two postnatal weeks in the rat profoundly affects the development and functions of central respiratory-related nuclei. Such denervation delays and prolongs the critical period, but does not eliminate it, suggesting that the critical period may be intrinsically and genetically determined.

The vesicular glutamate transporter VGLUT3 contributes to protection against neonatal hypoxic stress

Neonates respond to hypoxia initially by increasing ventilation, and then by markedly decreasing both ventilation (hypoxic ventilatory decline) and oxygen consumption (hypoxic hypometabolism). This latter process, which vanishes with age, reflects a tight coupling between ventilatory and thermogenic responses to hypoxia. The neurological substrate of hypoxic hypometabolism is unclear, but it is known to be centrally mediated, with a strong involvement of the 5-hydroxytryptamine (5-HT, serotonin) system. To clarify this issue, we investigated the possible role of VGLUT3, the third subtype of vesicular glutamate transporter. VGLUT3 contributes to glutamate signalling by 5-HT neurons, facilitates 5-HT transmission and is expressed in strategic regions for respiratory and thermogenic control. We therefore assumed that VGLUT3 might significantly contribute to the response to hypoxia. To test this possibility, we analysed this response in newborn mice lacking VGLUT3 using anatomical, biochemical, electrophysiological and integrative physiology approaches. We found that the lack of VGLUT3 did not affect the histological organization of brainstem respiratory networks or respiratory activity under basal conditions. However, it impaired respiratory responses to 5-HT and anoxia, showing a marked alteration of central respiratory control. These impairments were associated with altered 5-HT turnover at the brainstem level. Furthermore, under cold conditions, the lack of VGLUT3 disrupted the metabolic rate, body temperature, baseline breathing and the ventilatory response to hypoxia. We conclude that VGLUT3 expression is dispensable under basal conditions but is required for optimal response to hypoxic stress in neonates.

Chronic serotonin-norepinephrine reuptake transporter inhibition modifies basal respiratory output in adult mouse in vitro and in vivo

Respiratory disturbances are a common feature of panic disorder and present as breathing irregularity, hyperventilation, and increased sensitivity to carbon dioxide. Common therapeutic interventions, such as tricyclic (TCA) and selective serotonin reuptake inhibitor (SSRI) antidepressants, have been shown to ameliorate not only the psychological components of panic disorder but also the respiratory disturbances. These drugs are also prescribed for generalized anxiety and depressive disorders, neither of which are characterized by respiratory disturbances, and previous studies have demonstrated that TCAs and SSRIs exert effects on basal respiratory activity in animal models without panic disorder symptoms. Whether serotonin-norepinephrine reuptake inhibitors (SNRIs) have similar effects on respiratory activity remains to be determined. Therefore, the current study was designed to investigate the effects of chronic administration of the SNRI antidepressant venlafaxine (VHCL) on basal respiratory output. For these experiments, we recorded phrenic nerve discharge in an in vitro arterially-perfused adult mouse preparation and diaphragm electromyogram (EMG) activity in an in vivo urethane-anesthetized adult mouse preparation. We found that following 28-d VHCL administration, basal respiratory burst frequency was markedly reduced due to an increase in expiratory duration (T(E)), and the inspiratory duty cycle (T(I)/T(tot)) was significantly shortened. In addition, post-inspiratory and spurious expiratory discharges were seen in vitro. Based on our observations, we suggest that drugs capable of simultaneously blocking both 5-HT and NE reuptake transporters have the potential to influence the respiratory control network in patients using SNRI therapy.

Decreased GABAB receptor function in the cerebellum and brain stem of hypoxic neonatal rats: role of glucose, oxygen and epinephrine resuscitation

Background-

Hypoxia during the first week of life can induce neuronal death in vulnerable brain regions usually associated with an impairment of cognitive function that can be detected later in life. The neurobiological changes mediated through neurotransmitters and other signaling molecules associated with neonatal hypoxia are an important aspect in establishing a proper neonatal care.

Methods-

The present study evaluated total GABA, GABA_B receptor alterations, gene expression changes in GABA_B receptor and glutamate decarboxylase in the cerebellum and brain stem of hypoxic neonatal rats and the resuscitation groups with glucose, oxygen and epinephrine. Radiolabelled GABA and baclofen were used for receptor studies of GABA and GABA_B receptors respectively and Real Time PCR analysis using specific probes for GABA_B receptor and GAD mRNA was done for gene expression studies.

Results-

The adaptive response of the body to hypoxic stress resulted in a reduction in total GABA and GABA_B receptors along with decreased GABA_B receptor and GAD gene expression in the cerebellum and brain stem. Hypoxic rats supplemented with glucose alone and with oxygen showed a reversal of the receptor alterations and changes in GAD. Resuscitation with oxygen alone and epinephrine was less effective in reversing the receptor alterations.

Conclusions-

Being a source of immediate energy, glucose can reduce the ATP-depletion-induced changes in GABA and oxygenation, which helps in encountering hypoxia. The present study suggests that reduction in the GABA_B receptors functional regulation during hypoxia plays an important role in central nervous system damage. Resuscitation with glucose alone and glucose and oxygen to hypoxic neonatal rats helps in protecting the brain from severe hypoxic damage.

Chapter 3--networks within networks: the neuronal control of breathing

Breathing emerges through complex network interactions involving neurons distributed throughout the nervous system. The respiratory rhythm generating network is composed of micro networks functioning within larger networks to generate distinct rhythms and patterns that characterize breathing. The pre-Bötzinger complex, a rhythm generating network located within the ventrolateral medulla assumes a core function without which respiratory rhythm generation and breathing cease altogether. It contains subnetworks with distinct synaptic and intrinsic membrane properties that give rise to different types of respiratory rhythmic activities including eupneic, sigh, and gasping activities. While critical aspects of these rhythmic activities are preserved when isolated in in vitro preparations, the pre-Bötzinger complex functions in the behaving animal as part of a larger network that receives important inputs from areas such as the pons and parafacial nucleus. The respiratory network is also an integrator of modulatory and sensory inputs that imbue the network with the important ability to adapt to changes in the behavioral, metabolic, and developmental conditions of the organism. This review summarizes our current understanding of these interactions and relates the emerging concepts to insights gained in other rhythm generating networks.

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.

Fluoxetine treatment abolishes the in vitro respiratory response to acidosis in neonatal mice

Background

To secure pH homeostasis, the central respiratory network must permanently adapt its rhythmic motor drive to environment and behaviour. In neonates, it is commonly admitted that the retrotrapezoid/parafacial respiratory group of neurons of the ventral medulla plays the primary role in the respiratory response to acidosis, although the serotonergic system may also contribute to this response.

Methodology/Principal Findings

Using en bloc medullary preparations from neonatal mice, we have shown for the first time that the respiratory response to acidosis is abolished after pre-treatment with the serotonin-transporter blocker fluoxetine (25–50 µM, 20 min), a commonly used antidepressant. Using mRNA in situ hybridization and immunohistology, we have also shown the expression of the serotonin transporter mRNA and serotonin-containing neurons in the vicinity of the RTN/pFRG of neonatal mice.

Conclusions

These results reveal that the serotonergic system plays a pivotal role in pH homeostasis. Although obtained in vitro in neonatal mice, they suggest that drugs targeting the serotonergic system should be used with caution in infants, pregnant women and breastfeeding mothers.

Alterations in cortical GABAB receptors in neonatal rats exposed to hypoxic stress: role of glucose, oxygen, and epinephrine resuscitation

Hypoxia in neonates can cause permanent brain damage by gene and receptor level alterations mediated through changes in neurotransmitters. The present study evaluated GABA(B) receptor alterations, gene expression changes in glutamate decarboxylase and hypoxia-inducible factor 1A in the cerebral cortex of hypoxic neonatal rats and the resuscitation groups with glucose, oxygen, and epinephrine. Under hypoxic stress, a significant decrease in total GABA and GABA(B) receptors, GABA(B) and GAD gene expression was observed in the cerebral cortex, which accounts for the respiratory inhibition. Hypoxia-inducible factor 1A was upregulated under hypoxia to maintain body homeostasis. Hypoxic rats supplemented with glucose alone and with oxygen showed a reversal of the receptor alterations and changes in GAD and HIF-1A to near control. Being a source of immediate energy, glucose can reduce the ATP-depletion-induced changes in GABA and oxygenation, which helps in encountering hypoxia. Resuscitation with oxygen alone and epinephrine was less effective in reversing the receptor alterations. Thus, our study suggests that reduction in the GABA(B) receptors functional regulation during hypoxia plays an important role in cortical damage. Resuscitation with glucose alone and glucose and oxygen to hypoxic neonatal rats helps in protecting the brain from severe hypoxic damage.

5-HT3 receptor-dependent modulation of respiratory burst frequency, regularity, and episodicity in isolated adult turtle brainstems

To determine the role of central serotonin 5-HT(3) receptors in respiratory motor control, respiratory motor bursts were recorded from hypoglossal (XII) nerve rootlets on isolated adult turtle brainstems during bath-application of 5-HT(3) receptor agonists and antagonists. mCPBG and PBG (5-HT(3) receptor agonists) acutely increased XII burst frequency and regularity, and decreased bursts/episode. Tropisetron and MDL72222 (5-HT(3) antagonists) increased bursts/episode, suggesting endogenous 5-HT(3) receptor activation modulates burst timing in vitro. Tropisetron blocked all mCPBG effects, and the PBG-induced reduction in bursts/episode. Tropisetron application following mCPBG application did not reverse the long-lasting (2h) mCPBG-induced decrease in bursts/episode. We conclude that endogenous 5-HT(3) receptor activation regulates respiratory frequency, regularity, and episodicity in turtles and may induce a form of respiratory plasticity with the long-lasting changes in respiratory regularity.

Bioaminergic neuromodulation of respiratory rhythm in vitro

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

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.

Neurochemical and physiological correlates of a critical period of respiratory development in the rat

Despite its vital importance to life, respiration is not mature at birth in mammals, but rather, it undergoes a great deal of growth, refinement, and adjustments postnatally. Many adjustments do not follow smooth paths, but assume abrupt changes during certain postnatal periods that may render the animal less capable of responding to respiratory stressors. The present review focuses on neurochemical and physiological correlates of a critical period of respiratory development in the rat. In addition to an imbalanced expression of reduced excitatory and enhanced inhibitory neurotransmitters, a switch in the expressions of gamma-aminobutyric acid (GABA)A receptor subunits from alpha3 to alpha1 occurs around postnatal day (P)12 in the pre-Bötzinger nucleus and the ventrolateral subnucleus of the solitary tract nucleus. Possible subunit switches in a number of other neurotransmitter receptors are discussed. These neurochemical changes are paralleled by ventilatory adjustments at the end of the second postnatal week. At P13 and under normoxia, respiratory frequency reaches its peak before assuming a gradual fall, and both tidal volume and minute ventilation exhibit a significant rise prior to a plateau or a gradual decline until P21. The response to acute hypoxia is markedly reduced between P12 and P16, being lowest at P13. Thus, the end of the second postnatal week can be considered as a critical period of respiratory development, during which multiple neurochemical and physiological adjustments and switches are orchestrated at the same time, rendering the system extremely dynamic but, at the same time, vulnerable to externally imposed perturbations and insults. The critical period embodies a time of multi-system, multifaceted growth and adjustments. It is a plastic, transitional period that is also a part of the normal development of the respiratory system.

Developmental plasticity of the carotid chemoafferent pathway in rats that are hypoxic during the prenatal period

The chemoreflex pathway undergoes postnatal maturation, and the perinatal environment plays a critical role in shaping respiratory control system. We investigated the role of prenatal hypoxia on the maturation of the chemoreflex neural circuits regulating ventilation in rat. Effects of hypoxia (10% O2) from the 5th to the 20th day of gestation were studied on male offspring at birth and on postnatal days 3, 7, 21 and 68. Maturation of the respiratory control system was assessed by in vivo tyrosine hydroxylase (TH) activity measurement in peripheral chemoreceptors (carotid bodies, petrosal ganglia), and in brainstem catecholaminergic cell groups (A2C2c and A1C1 areas in the medulla, A5 and A6 areas in the pons). Resting ventilation and ventilatory response to hypoxia were evaluated as functional sequelae. In peripheral structures, prenatal hypoxia reduced TH activity within the first postnatal week and enhanced it later. In contrast, in central areas, prenatal hypoxia upregulated TH activity within the first postnatal week and downregulated it later. The in vivo TH activity impairment is therefore tissue specific, with an opposite effect on the peripheral and central neural circuits. A shift of the effect of prenatal hypoxia occurred between 1 and 3 weeks, indicating a postnatal temporal effect of prenatal hypoxia. An important period in the development of the chemoafferent pathway occurred between the first and the third postnatal week. Functionally, prenatal hypoxia impaired resting ventilation and ventilatory response to hypoxia. The alterations of the catecholaminergic components of the chemoafferent pathway resulting from prenatal hypoxia might contribute to impair postnatal respiratory behaviour.

Arrest of 5HT neuron differentiation delays respiratory maturation and impairs neonatal homeostatic responses to environmental challenges

Serotonin (5HT) is a powerful modulator of respiratory circuitry in vitro but its role in the development of breathing behavior in vivo is poorly understood. Here we show, using 5HT neuron-deficient Pet-1 (Pet-1(-/-)) neonates, that serotonergic function is required for the normal timing of postnatal respiratory maturation. Plethysmographic recordings reveal that Pet-1(-/-) mice are born with a depressed breathing frequency and a higher incidence of spontaneous and prolonged respiratory pauses relative to wild type littermates. The wild type breathing pattern stabilizes by postnatal day 4.5, while breathing remains depressed, highly irregular and interrupted more frequently by respiratory pauses in Pet-1(-/-) mice. Analysis of in vitro hypoglossal nerve discharge indicates that instabilities in the central respiratory rhythm generator contribute to the abnormal Pet-1(-/-) breathing behavior. In addition, the breathing pattern in Pet-1(-/-) neonates is susceptible to environmental conditions, and can be further destabilized by brief exposure to hypoxia. By postnatal day 9.5, however, breathing frequency in Pet-1(-/-) animals is only slightly depressed compared to wild type, and prolonged respiratory pauses are rare, indicating that the abnormalities seen earlier in the Pet-1(-/-) mice are transient. Our findings provide unexpected insight into the development of breathing behavior by demonstrating that defects in 5HT neuron development can extend and exacerbate the period of breathing instability that occurs immediately after birth during which respiratory homeostasis is vulnerable to environmental challenges.

Postnatal changes in ventilation during normoxia and acute hypoxia in the rat: implication for a sensitive period

Previously, we found heightened expression of inhibitory neurochemicals and depressed expression of excitatory neurochemicals with a sudden drop in metabolic activity around postnatal day (P) 12 in rat brainstem respiratory nuclei, suggesting that this period is a critical window during which respiratory control or regulation may be distinctly different. To test this hypothesis, the hypoxic ventilatory responses (HVR) to 10% oxygen were tested in rats every day from P0 to P21. Our data indicate that (1) during normoxia (N), breathing frequency (f) increased with age, peaking at P13, followed by a gradual decline, whereas both tidal volume (V(T)) and minute ventilation (.V(E) ) significantly increased in the second postnatal week, followed by a progressive increase in V(T) and a relative plateau in .V(E); (2) during 5 min of hypoxia (H), .V(E) exhibited a biphasic response from P3 onward. Significantly, the ratio of .V(E)(H) to .V(E)(N) was generally > 1 during development, except for P13-16, when it was < 1 after the first 1-2 min, with the lowest value at P13; (3) the H : N ratio for f, V(T) and .V(E) during the first 30 s and the last minute of hypoxia all showed a distinct dip at P13, after which the V(T) and .V(E) values rose again, while the f values declined through P21; and (4) the H : N ratios for f, V(T) and .V(E) averaged over 5 min of hypoxia all exhibited a sudden fall at P13. The f ratio remained low thereafter, while those for V(T) and .V(E) increased again with age until P21. Thus, hypoxic ventilatory response is influenced by both f and V(T) before P13, but predominantly by V(T) after P13. The striking changes in normoxic ventilation as well as HVR at or around P13, together with our previous neurochemical and metabolic data, strongly suggests that the end of the second postnatal week is a critical period of development for brainstem respiratory nuclei in the rat.

Maturational changes in neuromodulation of central pathways underlying hypoxic ventilatory response

The neuromodulator systems mediating the central component of the hypoxic ventilatory response (HVR) during development are complex and diverse. The early component of the HVR is mediated through N-methyl-D-aspartate (NMDA) glutamate receptors in the caudal brainstem. The intracellular downstream signal transductions of the NMDA receptors involve protein kinase C (PKC), neuronal nitric oxide synthase (nNOS) and tyrosine kinase (TK). Activation of NMDA receptors will also lead to activation of the early gene transcription factors including AP-1 (c-fos, c-jun) and NF-kappaB which may play a role in modulation of the subsequent response to hypoxia. NMDA receptors in the caudal brainstem play a critical role in the development of the HVR and increasing dependency on NMDA receptors emerges over time. Similarly, hypoxia-induced PKC, NOS and c-Fos activation in the caudal brainstem is relatively weak in the immature animals, but this activation increases with age and the strength of the response appears to increase concomitantly with the appearance of NMDA expression. Several neurotransmitters including adenosine, gamma-aminobutyric acid (GABA), serotonin and opioids are involved in the late component of the HVR. In addition, the late phase of the HVR is mediated in part through platelet-derived growth factor (PDGF)-beta receptors. PDGF-beta receptor activation is an important contributor of the hypoxic ventilatory depression at all postnatal ages, but its role is more critical in the developing animals. Maturation of these neuromodulators, especially the NMDA and PDGF-beta receptors-mediated pathways, occurs primarily during the early postnatal period. Perturbation of these developmental processes may result in short-term or sustained alterations to the HVR and may also affect neuronal survival during hypoxia.

Paroxetine influences respiration in rats: implications for the treatment of panic disorder

Since hyperventilation and shortness of breath are characteristic features of panic attacks, and since the attacks can be elicited by CO(2) inhalation, an involvement of central or peripheral chemoreceptors in the pathophysiology of panic disorder has been suggested. Prompted by clinical reports suggesting that the susceptibility to spontaneous as well as CO(2)-induced anxiety and hyperventilation is attenuated by serotonin reuptake inhibitors (SRIs), we undertook the present study in order to explore the possible effect of an SRI, paroxetine, on baseline respiration and CO(2)-induced hyperventilation in freely moving Wistar rats. A significant increase in baseline respiratory rate was seen both after 5 and 15 weeks of treatment with paroxetine. CO(2) exposure induced a dose-dependent increase in respiratory rate, but not tidal volume, in both paroxetine-treated rats and controls; this response was reduced after 15 weeks of paroxetine treatment, but not after 5 weeks of treatment. We suggest that an influence on the regulation of respiration may be of importance for the anti-panic effect of SRIs.

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.

Serotonergic modulation of respiratory motor output during tadpole development

To test the hypothesis that serotonin (5-hydroxytryptamine; 5-HT)-receptor activation elicits age-dependent changes in respiratory motor output, we compared the effects of 5-HT bath application (5-HT concentration = 0.5-25 microM) onto in vitro brain stem preparations from pre- and postmetamorphic bullfrog tadpoles. Recording of motor output related to gill and lung ventilation showed that 5-HT elicits a dose-dependent depression of gill burst frequency in both groups. In contrast, the lung burst frequency response was stage dependent; an increase in lung burst frequency at low 5-HT concentration (< or =0.5 microM) was observed only in the postmetamorphic group. Higher 5-HT concentrations decreased lung burst frequency in all preparations. Gill burst frequency attenuation is mediated (at least in part) by 5-HT(1A)-receptor activation in an age-dependent fashion. We conclude that serotonergic modulation of respiratory motor output 1) changes during tadpole development and 2) is distinct for gill and lung ventilation.

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.

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.

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.

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.

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.

Prolongation in expiration evoked from ventrolateral pons of adult rats

Activation of neurons in the ventrolateral (vl) pons was hypothesized to alter the breathing pattern because previous studies demonstrated apneusis after inhibiting neuronal activity with bilateral muscimol (10 mM) microinjections into the vl pons (17). The excitatory amino acid L-glutamate (10 mM) was microinjected (10-100 nl) into the vl pons in anesthetized, vagotomized, paralyzed, and ventilated adult rats (n = 8). In four of these animals, the target site was approached from the ventral surface of the pons to avoid penetrating the dorsolateral (dl) pons. The expiratory phase was prolonged transiently and concurrently with the microinjection. The location of the injection sites included the A5 area, was independent of the approach, and was distinct from the dl pons. These results complement our previous data and indicate that neurons located in the vl pons influence respiration specifically by prolonging expiration when activated and by delaying the inspiratory-to-expiratory phase transition when inhibited.

Serotonergic and noradrenergic effects on respiratory neural discharge in the medullary slice preparation of neonatal rats

Rhythmically active medullary slice preparations isolated from neonatal rats (postnatal days 0-3, P0-P3) were used to study the modulation of respiraory rhythmogenesis and hypoglossal (XII) nerve discharge by serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline (NA). 5-HT, NA and their respective receptor agonists and antagonists were applied either to the bathing medium or focally via pressure injection into regions encompassing the pre-Botzinger complex or XII motoneurons. The effects of endogenously released 5-HT were also studied by chemical stimulation of neurons within the raphe obscurus. The frequency of respiratory burst discharge was increased when 5-HT was applied: (1) to the bathing medium (37+/-16%; 30 "mu"M; P < 0.05); (2) via pressure injection into the region of the pre-Botzinger complex (22 +/- 14%; < 25 pmol; P < 0. 05); or (3) endogenously released in response to activation of neurons within the raphe obscurus via pressure injection of (R,S)- "alpha"-amino-3-hydroxy-5-methylisoxazole-4-propionic acid hydrobromide (AMPA, 34 +/- 15%; P < 0.05) or 5-HT (33 +/- 5%; P < 0. 05). All of these effects were antagonized by bath application of methysergide (30-40 "mu"M). NA caused a reduction of respiratory burst frequency when applied to the bathing medium (40 +/- 15%; 100 "mu"M; P < 0.05) or when pressure injected into the region of the pre-Botzinger complex (22 +/- 11%; < 25 pmol; P < 0.05). These effects were blocked by the bath application of the "alpha"2-receptor antagonist idazoxan (2 "mu"M). 5-HT and NA both caused an augmentation of tonic discharge of XII nerves when applied either to the bathing medium or via pressure injection into the XII motoneuron pool. The 5-HT-induced XII nerve tonic discharge was mimicked by the 5-HT2 receptor agonist R(-)2-(2, 5-dimethoxy-4-iodophenyl) (DOI.HCl, 5 "mu"M) and blocked by the 5-HT2 receptor antagonist ketanserine tartrate (30-40 "mu"M). The NA-induced XII nerve tonic discharge was mimicked by the "alpha"1-receptor agonist phenylephrine HCl (500 nM) and blocked by the "alpha"1-receptor antagonist prozasin HCl (1 "mu"M).

Localization of chemosensitive structures in the isolated brainstem of adult guinea-pig

1. Central respiratory chemosensitivity has been intensively examined but some questions remain unsolved; namely, what is the nature of the stimulus (fixed acid and/or CO2) and where is the site of brainstem chemosensitivity (near the ventral medullary surface or structures deeper within the brainstem)? To examine these questions, we used the in vitro isolated brainstem of adult guinea-pig perfused independently through the basilar artery and the bath. 2. Respiratory motor output was recorded with a suction electrode from cranial hypoglossal (XII) roots. Changes in pH and CO2 in the Krebs perfusate were made by changing either the bicarbonate concentration or the PCO2 saturating the Krebs solution. 3. Changes in basilar artery perfusate consisting of (i) an acidifying increase in PCO2 (hypercapnic acidic Krebs solution), (ii) an increase in PCO2 with no change in pH (hypercapnic Krebs solution), or (iii) a decrease in pH with no change in PCO2 (acidic Krebs solution) evoked increases in respiratory frequency and a concomitant decrease in inspiratory burst amplitude. 4. Bath superfusion with hypercapnic acidic Krebs solution increased the inspiratory burst amplitude with no effect on respiratory burst frequency. 5. Bath superfusion with hypercapnic non-acidic Krebs solution increased the inspiratory burst amplitude and decreased the respiratory frequency, while normocapnic acidic Krebs solution increased the respiratory frequency with no change in burst amplitude. 6. These results show that respiratory responses to changes in CO2 and pH depend upon the sites of action. While a CO2 increase or a pH decrease affected the respiratory frequency in the deep brainstem structures (perfused through the basilar artery), CO2 respiratory chemosensitivity at the ventral surface could be differentiated from the hydrogen ion chemosensitivity. This suggests that different mechanisms mediated respiratory responses when deep versus superficial brainstem structures were stimulated.

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.

Serotonin 1A-receptor activation suppresses respiratory apneusis in the cat

Malfunction of inhibitory synaptic processes in the brainstem result in abnormal prolonged inspiration (apneusis). Since we previously found that the serotonin (5-hydroxytryptamine; 5-HT) 5-HT1A receptor agonist 8-hydroxy-dipropylaminotetralin (8-OH-DPAT) shortens inspiratory discharges, we tested its ability to suppress apneusis. We recorded phrenic nerve activity and the membrane potential of medullary expiratory (E-2) and postinspiratory (PI) neurons in 14 anaesthetized, paralyzed, artificially ventilated cats. Systemic hypoxia or i.v. injection of pentobarbital sodium or the N-methyl-D-aspartate (NMDA) receptor blocker ketamine induced apneustic phrenic nerve discharges, delayed depolarization to threshold of E-2 neurons and prolonged hyperpolarization in PI neurons. 8-OH-DPAT (10-40 micrograms/kg i.v.) produced partial to complete restoration of normal phrenic nerve discharges and membrane potential.

A 'pneumotaxic centre' in rats

Electrical and chemical lesions in the ventrolateral pons produced apneustic breathing in anesthetized, vagotomized, paralyzed, ventilated adult rats (n = 13). Apneustic breathing did not develop if the vagi remained intact and was reversed partially with vagal (proximal end) stimulation. Physiologically, these data are similar to those obtained following dorsolateral pontine lesion in rat and other mammalian species and support the hypothesis that pontine neurons influence breathing similarly across mammalian species.

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

The involvement of the rostral ventro-lateral medulla (RVLM) in respiratory rhythm genesis was analysed on brain stem-spinal cord preparations from newborn and fetal rats in which the ability to generate central respiratory activity in vitro persists. The respiratory rhythm (around 5 per min) was stable in preparations from newborn and pre-term fetal (D20-21) rats but very variable in young fetuses (D18). In newborn and D20-21 fetal rats, RVLM electrical stimulation delivered during mid-expiration initiated premature inspiration while RVLM electrolytic lesions suppressed the respiratory rhythm. In D18 fetuses, RVLM stimulation had no effect and strong stimulations evoked only diffuse activation. Extracellular recordings of the activity of 423 RVLM neurons showed that this area contained numerous inspiratory neurons in all the age groups studied. Low Ca(2+)-high Mg2+ medium bathing (assumed to block synaptic transmission) abolished the inspiratory bursts in the cervical roots and most of the 99 RVLM inspiratory neurons investigated. In newborn and D20-21 rats, however, 7 of the 68 RVLM inspiratory neurons tested behaved like respiratory pacemakers, since they continued to fire with a bursting pattern, while in the D18 preparations, none of 31 did so. These experiments confirm that the RVLM is a crucial site in respiratory rhythm genesis in newborn rats, and suggest that this may also be the case in D20-21 fetuses, but probably not in D18 fetuses.

Pattern generation

Significant advances have been made in understanding the cellular mechanisms for pattern generation in both invertebrate and vertebrate preparations. In a number of preparations, slow neuromodulators have been shown not only to modify network function, but to be intimately involved in development and/or normal function of the neural network and its associated behavior. The mechanisms underlying coordination between multiple pattern-generating networks, including switching of neurons from one network to another, are now being studied. Several new quantitative models of network function have been developed, and modeling is now an important component of research in this field.