Altered respiratory activity and respiratory regulations in adult monoamine oxidase A-deficient mice

Prenatal fluoxetine has long lasting, differential effects on respiratory control in male and female rats

Serotonin (5-HT) is an important modulator of brain networks that control breathing. The selective serotonin reuptake inhibitor fluoxetine (FLX) is the first-line antidepressant drug prescribed during pregnancy. We investigated the effects of prenatal FLX on baseline breathing, ventilatory and metabolic responses to hypercapnia and hypoxia as well as number of brainstem 5-HT and tyrosine hydroxylase (TH) neurons of rats during postnatal development (P0-82). Prenatal FLX exposure of males showed a lower baseline that appeared in juveniles and remained in adulthood, with no sleep-wake state dependency. Prenatal FLX exposure of females did not affect baseline breathing. Juvenile male FLX rats showed increased CO₂ and hypoxic ventilatory responses, normalizing by adulthood. Alterations in juvenile-FLX treated males were associated with greater number of 5-HT neurons in the ROB and RMAG. Adult FLX-exposed males showed greater number of 5-HT neurons in the RPA and TH neurons in the A5, while reduced number of TH neurons in A7. Prenatal FLX exposure of female rats was associated with greater hyperventilation induced by hypercapnia at P0-2 and juveniles whereas P12-14 and adult FLX (NREM sleep) rats showed an attenuation of the hypercapnic hyperventilation.FLX-exposed females had fewer 5-HT neurons in the RPA and reduced TH A6 density at P0-2; and greater number of TH neurons in the A7 at P12-14. These data indicate that prenatal FLX exposure affects the number of neurons of some monoaminergic regions in the brain and results in long lasting, sex specific changes in baseline breathing pattern and ventilatory responses to respiratory challenges.

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).

Perinatal Fluoxetine Exposure Impairs the CO2 Chemoreflex. Implications for Sudden Infant Death Syndrome

High serotonin levels during pregnancy affect central nervous system development. Whether a commonly used antidepressant such as fluoxetine (a selective serotonin reuptake inhibitor) taken during pregnancy may adversely affect respiratory control in offspring has not been determined. The objective was to determine the effect of prenatal-perinatal fluoxetine exposure on the respiratory neural network in offspring, particularly on central chemoreception. Osmotic minipumps implanted into CF-1 mice on Days 5-7 of pregnancy delivered 7 milligrams per kilogram per day of fluoxetine, achieving plasma levels within the range found in patients. Ventilation was assessed in offspring at postnatal Days 0-40 using head-out body plethysmography. Neuronal activation was evaluated in the raphe nuclei and in the nucleus tractus solitarius by c-Fos immunohistochemistry during normoxic eucapnia and hypercapnia (10% CO2). Respiratory responses to acidosis were evaluated in brainstem slices. Prenatal-perinatal fluoxetine did not affect litter size, birth weight, or the postnatal growth curve. Ventilation under eucapnic normoxic conditions was similar to that of control offspring. Fluoxetine exposure reduced ventilatory responses to hypercapnia at P8-P40 (P < 0.001) but not at P0-P5. At P8, it reduced hypercapnia-induced neuronal activation in raphe nuclei (P < 0.05) and nucleus tractus solitarius (P < 0.01) and the acidosis-induced increase in the respiratory frequency in brainstem slices (P < 0.05). Fluoxetine applied acutely on control slices did not modify their respiratory response to acidosis. We concluded that prenatal-perinatal fluoxetine treatment impairs central respiratory chemoreception during postnatal life. These results are relevant in understanding the pathogenesis of respiratory failures, such as sudden infant death syndrome, associated with brainstem serotonin abnormalities and the failure of respiratory chemoreflexes.

Role of glutamate and serotonin on the hypoxic ventilatory response in high-altitude-adapted plateau Pika

The highland "plateau Pika" is considered to be adapted to chronic hypoxia. We hypothesized that glutamate N-methyl-D-aspartate (NMDA) and non-NMDA receptors, nitric oxide (NO) synthase, and serotonin are involved in hypoxic ventilatory response (HVR) in Pikas. We tested the effects of NMDA (memantine) and non-NMDA receptors (DNQX) antagonists, NO synthase inhibitor (L-NAME), and selective serotonin reuptake inhibitors (fluoxetine) on ventilation and HVR in Pikas. Ventilatory parameters were measured before and after drug (or vehicle) injections in conscious Pikas at their natural living altitude (PIO2 86 mmHg) and after a hypoxic challenge (PIO2 57 mmHg, 3 min) to assess the influence of peripheral chemoreceptor on HVR. Minute ventilation (VI) and tidal volume (Vt) increased during hypoxic challenge after vehicle injection, whereas the Ti/Ttot ratio remained unchanged. The increase in VI and Vt observed with vehicle at PIO2-57, when compared with PIO2-86, was inhibited after memantine and fluoxetine injection, whereas the DNQX injection increased HVR. At PIO2-57, L-NAME induced an increase in the Ti/Ttot ratio when compared with vehicle. Therefore, the glutamate through NMDA-R/AMPA receptor bindings and serotonin pathway are implicated at the peripheral chemoreceptor level in HVR in Pikas. However, NO influences the ventilatory pattern of Pikas at their habitual living altitude.

Ventilatory long-term facilitation is evident after initial and repeated exposure to intermittent hypoxia in mice genetically depleted of brain serotonin

Our study was designed to determine if central nervous system (CNS) serotonin is required for the induction of ventilatory long-term facilitation (LTF) in intact, spontaneously breathing mice. Nineteen tryptophan hydroxylase 2-deficient (Tph2(-/-)) mice, devoid of serotonin in the CNS, and their wild-type counterparts (Tph2(+/+)) were exposed to intermittent hypoxia each day for 10 consecutive days. The ventilatory response to intermittent hypoxia was greater in the Tph2(+/+) compared with the Tph2(-/-) mice (1.10 ± 0.10 vs. 0.77 ± 0.01 ml min(-1)·percent(-1) oxygen; P ≤ 0.04). Ventilatory LTF, caused by increases in breathing frequency, was evident in Tph2(+/+) and Tph2(-/-) mice following exposure to intermittent hypoxia each day; however, the magnitude of the response was greater in the Tph2(+/+) compared with the Tph2(-/-) mice (1.11 ± 0.02 vs. 1.05 ± 0.01 normalized to baseline on each day; P ≤ 0.01). The magnitude of ventilatory LTF increased significantly from the initial to the finals days of the protocol in the Tph2(-/-) (1.06 ± 0.02 vs. 1.11 ± 0.03 normalized to baseline on the initial days; P ≤ 0.004) but not in the Tph2(+/+) mice. This enhanced response was mediated by increases in tidal volume. Body temperature and metabolic rate did not account for differences in the magnitude of ventilatory LTF observed between groups after acute and repeated daily exposure to intermittent hypoxia. We conclude that ventilatory LTF, after acute exposure to intermittent hypoxia, is mediated by increases in breathing frequency and occurs in the absence of serotonin, although the magnitude of the response is diminished. This weakened response is enhanced following repeated daily exposure to intermittent hypoxia, via increases in tidal volume, to a similar magnitude evident in Tph2(+/+) mice. Thus the magnitude of ventilatory LTF following repeated daily exposure to intermittent hypoxia is not dependent on the presence of CNS serotonin.

Raphé tauopathy alters serotonin metabolism and breathing activity in terminal Tau.P301L mice: possible implications for tauopathies and Alzheimer's disease

Tauopathies, including Alzheimer's disease are the most frequent neurodegenerative disorders in elderly people. Patients develop cognitive and behaviour defects induced by the tauopathy in the forebrain, but most also display early brainstem tauopathy, with oro-pharyngeal and serotoninergic (5-HT) defects. We studied these aspects in Tau.P301L mice, that express human mutant tau protein and develop tauopathy first in hindbrain, with cognitive, motor and upper airway defects from 7 to 8 months onwards, until premature death before age 12 months. Using plethysmography, immunohistochemistry and biochemistry, we examined the respiratory and 5-HT systems of aging Tau.P301L and control mice. At 8 months, Tau.P301L mice developed upper airway dysfunction but retained normal respiratory rhythm and normal respiratory regulations. In the following weeks, Tau.P301L mice entered terminal stages with reduced body weight, progressive limb clasping and lethargy. Compared to age 8 months, terminal Tau.P301L mice showed aggravated upper airway dysfunction, abnormal respiratory rhythm and abnormal respiratory regulations. In addition, they showed severe tauopathy in Kolliker-Fuse, raphé obscurus and raphé magnus nuclei but not in medullary respiratory-related areas. Although the raphé tauopathy concerned mainly non-5-HT neurons, the 5-HT metabolism of terminal Tau.P301L mice was altered. We propose that the progressive raphé tauopathy affects the 5-HT metabolism, which affects the 5-HT modulation of the respiratory network and therefore the breathing pattern. Then, 5-HT deficits contribute to the moribund phenotype of Tau.P301L mice, and possibly in patients suffering from tauopathies, including Alzheimer's disease.

Monoamine oxidase A regulates neural differentiation of murine embryonic stem cells

Monoamine oxidase (MAO) A is the major metabolizing enzyme of serotonin (5-hydroxytryptamine, 5-HT) which regulates early brain development. In this study, wild-type (WT) and MAO A(neo) embryonic stem (ES) cell lines were established from the inner cell mass of murine blastocysts and their characteristics during ES and differentiating stages were studied. Our results show that the differentiation to neural cells in MAO A(neo) ES cells was reduced compared to WT, suggesting MAO A played a regulatory role in stem cells neural differentiation.

Monoamine oxidase a expression is vital for embryonic brain development by modulating developmental apoptosis

Monoamine oxidases (MAO-A, MAO-B) metabolize biogenic amines and have been implicated in neuronal apoptosis. Although apoptosis is an important process in embryo development, the role of MAO isoenzymes has not been investigated in detail. We found that expression of MAO-A and MAO-B can be detected early on during embryo development. Expression levels remained constant until around midgestation but then dropped to almost undetectable levels toward birth. Similar expression kinetics were observed in the brain. Isoform-specific expression silencing of MAO-A mediated by siRNA during in vitro embryogenesis induced developmental defects, as indicated by a reduction of the crown rump length and impaired cerebral development. These alterations were paralleled by elevated serotonin levels. Similar abnormalities were observed when embryos were cultured in the presence of the MAO-A inhibitor clorgyline or when the transcriptional inhibitor of MAO-A expression R1 was overexpressed. In contrast, no such alterations were detected when expression of MAO-B was knocked down. To explore the underlying mechanisms for the developmental abnormalities in MAO-A knockdown embryos, we quantified the degree of developmental apoptosis in the developing brain. MAO-A knockdown reduced the number of apoptotic cells in the neuroepithelium, which coincided with impaired activation of caspases 3 and 9. Moreover, we observed reduced cyclin D1 levels as an indicator of impaired cell proliferation in MAO-A knockdown embryos. This data highlights MAO-A as a vital regulator of embryonic brain development.

Differences in serotoninergic metabolism possibly contribute to differences in breathing phenotype of FVB/N and C57BL/6J mice

Mouse readiness for gene manipulation allowed the production of mutants with breathing defects reminiscent of breathing syndromes. As C57BL/6J and FVB/N inbred strains were often used as background strains for producing mutants, we compared their breathing pattern from birth onwards. At birth, in vivo and in vitro approaches revealed robust respiratory rhythm in FVB/N, but not C57BL/6J, neonates. With aging, rhythm robustness difference persisted, and interstrain differences in tidal volume, minute ventilation, breathing regulations, and blood-gas parameters were observed. As serotonin affected maturation and function of the medullary respiratory network, we examined the serotoninergic metabolism in the medulla of C57BL/6J and FVB/N neonates and aged mice. Interstrain differences in serotoninergic metabolism were observed at both ages. We conclude that differences in serotoninergic metabolism possibly contribute to differences in breathing phenotype of FVB/N and C57BL/6J mice.

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.

Do monoamine-synthesizing cells constitute a complex network of oxygen sensors?

Oxygen represents an essential molecule for organisms. Because of this, sophisticated systems of sensors have evolved to monitor oxygenation of tissues. We propose that monoamine-synthesizing cells represent an important part of this system. It is well known that the carotid body, which contains chromaffin cells, serves as a chemical sensor of blood oxygenation. Similarly, the activity of adrenal medullary chromaffin cells is increased during hypoxia. Moreover, neurons located in the central nervous system containing catecholamines, serotonin, and histamine are also sensitive to hypoxia. On the basis of this common sensitivity of monoamine-synthesizing cells to changes in oxygenation we propose the hypothesis that these cells constitute a widely distributed network of sensors that monitor oxygen levels. The role of monoamine-synthesizing cells in monitoring tissue oxygen supply during both physiological and pathological conditions is also discussed.

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.

Ondansetron and fluoxetine reduce sleep apnea in mice lacking monoamine oxidase A

Prospective clinical trials addressing the role of serotonin (5-HT) in sleep apnea have indicated that the 5-HT uptake inhibitor fluoxetine is beneficial to some patients with obstructive apnea, whereas the 5-HT(3) receptor antagonist ondansetron seems of little value despite its efficacy in rat and dog models of sleep apnea (central and obstructive). Here, we examined the effect of these drugs in transgenic mice lacking monoamine oxidase A (Tg8), which exhibit approximately 3-fold higher rates of central sleep apnea than their wild-type counterparts (C3H), linked to their enhanced 5-HT levels. Acute ondansetron (2 mg kg(-1), intraperitoneal), acute fluoxetine (16 mg kg(-1)) and 13-day chronic fluoxetine (1 or 16 mg kg(-1)) decreased by approximately 80% the total (spontaneous and post-sigh) apnea index in Tg8 mice during non-rapid eye movement sleep, with no statistically significant effect on apnea in C3H mice. Our study shows that both drugs reduce the frequency of apneic episodes attributable to increased monoamine levels in this model of MAOA deficiency, and suggests that both may be effective in some patients with central sleep apneas.

Breathing disorders in Rett syndrome: progressive neurochemical dysfunction in the respiratory network after birth

Disorders of respiratory control are a prominent feature of Rett syndrome (RTT), a severely debilitating condition caused by mutations in the gene encoding methyl-CpG-binding protein 2 (MECP2). RTT patients present with a complex respiratory phenotype that can include periods of hyperventilation, apnea, breath holds terminated by Valsalva maneuvers, forced and deep breathing and apneustic breathing, as well as abnormalities of heart rate control and cardiorespiratory integration. Recent studies of mouse models of RTT have begun to shed light on neurologic deficits that likely contribute to respiratory dysfunction including, in particular, defects in neurochemical signaling resulting from abnormal patterns of neurotransmitter and neuromodulator expression. The authors hypothesize that breathing dysregulation in RTT results from disturbances in mechanisms that modulate the respiratory rhythm, acting either alone or in combination with more subtle disturbances in rhythm and pattern generation. This article reviews the evidence underlying this hypothesis as well as recent efforts to translate our emerging understanding of neurochemical defects in mouse models of RTT into preclinical trials of potential treatments for respiratory dysfunction in this disease.

Contributions of 5-HT neurons to respiratory control: neuromodulatory and trophic effects

Serotonin (5-hydroxytryptamine; 5-HT) is a neurotransmitter produced by a small number of neurons in the midbrain, pons and medulla. These neurons project widely throughout the neuraxis, where they release 5-HT and co-localized neuropeptides such as substance P (SP) and thyrotropin-releasing hormone (TRH). Each of these chemicals produce effects largely through G protein-coupled receptors, second messenger systems and subsequent neuromodulatory effects on target neurons. Emerging evidence suggests that 5-HT has additional modes of action during development and in adult mammals, including trophic effects (neurogenesis, cell differentiation, proliferation, migration and maturation) and influences on synaptic plasticity. Here, we discuss some of the neuromodulatory and trophic roles of 5-HT in general and in the context of respiratory control, as well as the regulation of release of modulatory neurotransmitters from 5-HT neurons. Future directions of study are also discussed.

Prenatal activation of 5-HT2A receptor induces expression of 5-HT1B receptor in phrenic motoneurons and alters the organization of their premotor network in newborn mice

In newborn mice of the control [C3H/HeJ (C3H)] and monoamine oxidase A-deficient (Tg8) strains, in which levels of endogenous serotonin (5-HT) were drastically increased, we investigated how 5-HT system dysregulation affected the maturation of phrenic motoneurons (PhMns), which innervate the diaphragm. First, using immunocytochemistry and confocal microscopy, we observed a 5-HT(2A) receptor (5-HT(2A)-R) expression in PhMns of both C3H and Tg8 neonates at the somatic and dendritic levels, whereas 5-HT(1B) receptor (5-HT(1B)-R) expression was observed only in Tg8 PhMns at the somatic level. We investigated the interactions between 5-HT(2A)-R and 5-HT(1B)-R during maturation by treating pregnant C3H mice with a 5-HT(2A)-R agonist (2,5-dimethoxy-4-iodoamphetamine hydrochloride). This pharmacological overactivation of 5-HT(2A)-R induced a somatic expression of 5-HT(1B)-R in PhMns of their progeny. Conversely, treatment of pregnant Tg8 mice with a 5-HT(2A)-R antagonist (ketanserin) decreased the 5-HT(1B)-R density in PhMns of their progeny. Second, using retrograde transneuronal tracing with rabies virus injected into the diaphragm of Tg8 and C3H neonates, we studied the organization of the premotor network driving PhMns. The interneuronal network monosynaptically connected to PhMns was much more extensive in Tg8 than in C3H neonates. However, treatment of pregnant C3H mice with 2,5-dimethoxy-4-iodoamphetamine hydrochloride switched the premotoneuronal network of their progeny from a C3H- to a Tg8-like pattern. These results show that a prenatal 5-HT excess affects, via the overactivation of 5-HT(2A)-R, the expression of 5-HT(1B)-R in PhMns and the organization of their premotor network.

Calcium-activated potassium currents differentially modulate respiratory rhythm generation

The pre-Bötzinger complex (PBC) generates eupnea and sighs in normoxia and gasping during hypoxia through particular mixtures of intrinsic and synaptic properties. Among intrinsic properties, little is known about the role of Ca(2+)-activated potassium channels in respiratory rhythms generation. To examine this role, we tested the effects of openers and blockers of the large-conductance (BK) and small-conductance (SK) Ca(2+)-activated potassium channels on the respiratory rhythms recorded both in vitro and in vivo, as well as on the discharge pattern of respiratory neurons in the PBC. Activation of SK channels with 1-ethyl-2-benzimidazolinone (1-EBIO) abolished sigh-like activity and inhibited eupneic-like activity, whereas blockade of SK channels with apamine (APA) increased frequency in both rhythms. In hypoxia, APA did not affect the transition to gasping-like activity. At the cellular level, activation of SK channels abolished pacemaker activity and decreased non-pacemaker neurons discharge; opposite effects were observed with SK blockade. In contrast to SK channel modulation, either activation or blockade of BK channels with NS 1619 or iberiotoxin and paxilline, respectively, produced mild effects on eupneic-like and sigh-like bursts during normoxia in vitro. However, BK blockers prevented the changes associated with the transition to gasping-like activity in vitro and perturbed gasping generation and autoresuscitation in vivo. At the cellular level BK channel modulation did not affect respiratory neurons discharge. We conclude that K(Ca) participate in rhythm generation in a state-dependent manner; SK channels are preferentially involved in rhythm generation in normoxia whereas BK channels participate in the transition to gasping generation in hypoxia.

Necdin gene, respiratory disturbances and Prader-Willi syndrome

Prader-Willi Syndrome (PWS) is a complex neurogenetic disease with various symptoms, including breathing deficits and possible alteration of serotonin (5HT) metabolism. As PWS results from the absence of paternal expression of several imprinted genes among which NECDIN (Ndn), we examined whether Ndn deficiency in mice induced breathing and 5HT deficits. In vivo, Ndn-deficient mice (Ndn-/-) had irregular breathing, severe apneas and blunted respiratory response to hypoxia. In vitro, medullary preparations from Ndn-/- neonates produced a respiratory-like rhythm that was highly irregular, frequently interrupted and abnormally regulated by central hypoxia. In wild type (wt) and Ndn-/- neonates, immunohistofluorescence and biochemistry revealed that medullary 5HT neurons expressed Ndn in wt and that the medulla contained abnormally high levels of 5HT in Ndn-/-. Thus, our preliminary results fully confirm a primary role of Ndn in PWS, revealing that Ndn-deficiency in mice induces respiratory and 5HT alterations reminiscent of PWS.

Oral treatment with desipramine improves breathing and life span in Rett syndrome mouse model

Rett syndrome is a neurodevelopmental disease due to Mecp2 gene mutations that is associated to complex neurological symptoms, with bioaminergic deficits and life-threatening apneas related to sudden and unexpected death. In male mice, Mecp2-deficiency similarly induces medullary bioaminergic deficits, severe apneas and short life span. Here, we show that long-term oral treatment of Mecp2-deficient male mice with desipramine, an old drug of clinical use known to block norepinephrine uptake and to strengthen its synaptic effects, significantly alleviates their breathing symptoms and prolongs their life span. Although these mouse results identify desipramine as the first oral pharmacological treatment potentially able to alleviate breathing symptoms of Rett syndrome, we recommend further studies of desipramine effects in Mecp2-deficient mice before attempting any clinical trials in Rett patients.

Movement disorders and neurochemical changes in zebrafish larvae after bath exposure to fluoxetine (PROZAC)

This study examines the effects of the selective serotonin reuptake inhibitor (SSRI), fluoxetine (PROZAC), on the ontogeny of spontaneous swimming activity (SSA) in developing zebrafish. The development of zebrafish motor behavior consists of four sequential locomotor patterns that develop over 1-5 days post fertilization (dpf), with the final pattern, SSA, established at 4-5 dpf. In stage specific experiments, larvae were exposed to 4.6 microM fluoxetine for 24 h periods beginning at 24 h post fertilization (hpf) and extending through 5 dpf. From 1-3 dpf, there was no effect on SSA or earlier stages of motor development, i.e., spontaneous coiling, evoked coiling and burst swimming. Fluoxetine exposure at 3 dpf for 24 h resulted in a transient decrease in SSA through 7 dpf with a complete recovery by 8 dpf. Larvae exposed to 4.6 microM fluoxetine for 24 h on 4 or 5 dpf showed a significant decrease in SSA by day 6 with no recovery through 14 dpf. Although SSA was significantly affected 24 h after fluoxetine exposure, there was little or no effect on pectoral fin movement. These results demonstrate both a stage specific and a long term effect of 4.6 microM fluoxetine exposure in 4 and 5 dpf larvae. Reverse transcriptase polymerase chain reaction (RT-PCR) was performed to determine the relative levels of a serotonin transporter protein (SERT) transcript and the serotonin 1A (5-HT(1A)) receptor transcript in developing embryos/larvae over 1-6 dpf. Both transcripts were present at 24 hpf with the relative concentration of SERT transcript showing no change over the developmental time range. The relative concentration of the 5-HT(1A) receptor transcript, however, showed a two-tiered pattern of concentration. RT-PCR was also used to detect potential changes in the SERT and 5-HT(1A) receptor transcripts in 6 dpf larvae after a 24 h exposure to 4.6 microM fluoxetine on 5 dpf. Three separate regions of the CNS were individually analyzed, two defined brain regions and spinal cord. The two brain regions showed no effect on transcript levels subsequent to fluoxetine exposure, however, the spinal cord showed a significant decrease in both transcripts. These results suggest a correlation between decreased concentration of SERT and 5-HT(1A) receptor transcripts in spinal cord and decreased SSA subsequent to fluoxetine exposure.

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.

Acute role of the brain-derived neurotrophic factor (BDNF) on the respiratory neural network activity in mice in vitro

In humans, several pathologies are associated with disturbances of the respiratory control, some of them including alteration in the brain-derived neurotrophic factor (BDNF) signalling pathway. BDNF has long been known as a neurotrophic factor involved in survival, differentiation and maintenance of neuronal populations in the peripheral and central nervous system. More recently BDNF has also been discovered to be a potent neuromodulator with acute effects on neuronal excitability and synaptic plasticity. Animals deleted for the gene encoding BDNF exhibit respiratory alteration suggesting an important but yet undefined role of the neurotrophin in respiratory rhythmogenesis either by a trophic and/or an acute action. The possibility that BDNF might exert an acute regulatory role on the rhythmic activity of the respiratory generator of the pre-Bötzinger complex has been recently examined in newborn mice in vitro. Results obtained, reviewed in the present paper, will help getting insights in respiratory rhythm regulatory mechanisms that involve BDNF signalling.

The development of the medullary serotonergic system in early human life

The serotonergic (5-HT) neurons of the medulla oblongata are postulated to comprise a system that modulates homeostatic function in response to metabolic imbalances in the internal milieu in a state-dependent manner. In this study, we define the baseline development of the topography of the human medullary 5-HT system in 30 cases ranging from the embryonic period through infancy. We used immunocytochemical techniques with the PH8 antibody which recognizes the key 5-HT synthetic enzyme, tryptophan hydroxylase, and computer-based methods of cell quantitation. In the infant medulla, 5-HT neurons were distributed in raphé, extra-raphé, and ventral positions that place these neurons adjacent to, or intermingled with, the neurons in the lower cranial nerve nuclei and reticular formation that directly mediate respiration, upper airway reflexes, and autonomic function. Along the ventral and ventrolateral surface, 5-HT neurons formed two lateral and one midline "columns" in the rostrocaudal axis that are homologous in position to chemosensitive 5-HT neurons in rats, and that correspond in part to the classic respiratory chemosensitive fields. Serotonergic neurons comprised a subpopulation of the arcuate nucleus along the ventral surface; their short processes directly abutted the surface, suggesting a role for them in monitoring carbon dioxide levels in the cerebrospinal fluid. The medullary 5-HT system began to form in the embryo, with the raphé primordia appearing as early as 7 weeks (the earliest time-point available). By 10-12 weeks, the lateral tegmental 5-HT neurons clustered into the early primordia of extra-raphé subnuclei. By 20 weeks, the "adult-like" topography of the medullary 5-HT system was in place, with subtle (quantitative) changes occurring thereafter. Thus, protracted changes occur from the prenatal period through infancy. These data provide a foundation for 5-HT neuronal analysis in pediatric brainstem disorders, as proposed in the sudden infant death syndrome.

Sex-specific densities of estrogen receptors alpha and beta in the subnuclei of the nucleus tractus solitarius, hypoglossal nucleus and dorsal vagal motor nucleus weanling rats

In rats ventilatory responses to N-methyl-d-aspartate (NMDA) receptor modulation are sexually dimorphic and may be altered by manipulating brain levels of estrogen receptors. Here we used image analysis and immunohistochemistry in weanling male and female rats to quantitate areas and densities of ER alpha and ER beta-positive neurons within medullary regions associated with cardiopulmonary regulation including the hypoglossal nucleus, subnuclei of the nucleus of the solitary tract (NTS), and the dorsal motor nucleus of the vagus. Weanling rats were selected because ventilation, metabolic rate, and body and brain weights are comparable at this age and there are no large fluctuations in plasma hormone levels. Females, relative to males, had smaller areas in the A2 region and parts of the NTS. Counts and densities for ER alpha were greater in females than males in almost all regions studied. In contrast sex differences in ER beta were found in fewer nuclei, but in those higher counts and densities were noted in females. In general, ER beta-positive neurons in the brainstem regions examined were less prevalent than ER alpha neurons. Thus, in weanling rats sex affected ER alpha and ER beta neuronal densities in brainstem regions associated with cardiopulmonary regulation that may be responsible for sex differences in control of breathing.

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

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

Genetic alterations of the murine serotonergic gene pathway: the neurodevelopmental basis of anxiety

The relative contribution of genetic and environmental factors in the configuration of behavioral differences is among the most prolonged and contentious controversies in intellectual history. Although current views emphasize the joint influence of genes and environmental sources during early brain development, the physiological complexities of multiple gene-gene and gene-environment interactions in the developmental neurobiology of fear and anxiety remain elusive. Variation in genes coding for proteins that control serotonin (5-hydroxytryptamine, 5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction as well as cellular pathways have been implicated in the genetics of anxiety and related disorders. This review selects anxiety and avoidance as paradigmatic traits and behaviors, and it focuses on mouse models that have been modified by deletion of genes coding for key players of serotonergic neurotransmission. In particular, pertinent approaches regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, and monoamine oxidase A and other genes related to 5-HT signaling will be discussed and major findings highlighted.

Anxiety-related traits in mice with modified genes of the serotonergic pathway

The neurobiology of anxiety is complex, reflecting the cumulative physiological effects of multiple genes. These genes are interactive with each other and with the environment in which they are expressed. Variation in genes coding for proteins that control serotonin (5-HT) system development and plasticity, establish 5-HT neuron identity, and modulate 5-HT receptor-mediated signal transduction and cellular pathways have been implicated in the genetics of anxiety and related disorders. Here, we selected anxiety and avoidance as paradigmatic traits and behavior and cover both traditional studies with inbred murine strains and selected lines which have been modified by gene knockout technologies. The design of a mouse model partially or completely lacking a gene of interest during all stages of development (constitutive knockout) or in a spatio-temporal context (conditional knockout) is among the prime strategies directed at elucidating the role of genetic factors in fear and anxiety. In many cases, knockout mice have been able to confirm what has already been anticipated based on pharmacological studies. In other instances, knockout studies have changed views of the relevance of 5-HT homeostasis in brain development and plasticity as well as processes underlying emotional behavior. In this review, we discuss the pertinent literature regarding phenotypic changes in mice bearing inactivation mutations of 5-HT receptors, 5-HT transporter, monoamine oxidase A and other components of the serotonergic pathway. Finally, we attempt to identify future directions of genetic manipulation in animal models to advance our understanding of brain dysregulation characteristic of anxiety disorders.

Nasal trigeminal inputs release the A5 inhibition received by the respiratory rhythm generator of the mouse neonate

Experiments were performed on neonatal mice to analyze why, in vitro, the respiratory rhythm generator (RRG) was silent and how it could be activated. We demonstrated that in vitro the RRG in intact brain stems is silenced by a powerful inhibition arising from the pontine A5 neurons through medullary alpha(2) adrenoceptors and that in vivo nasal trigeminal inputs facilitate the RRG as nasal continuous positive airway pressure increases the breathing frequency, whereas nasal occlusion and nasal afferent anesthesia depress it. Because nasal trigeminal afferents project to the A5 nuclei, we applied single trains of negative electric shocks to the trigeminal nerve in inactive ponto-medullary preparations. They induced rhythmic phrenic bursts during the stimulation and for 2-3 min afterward, whereas repetitive trains produced on-going rhythmic activity up to the end of the experiments. Electrolytic lesion or pharmacological inactivation of the ipsilateral A5 neurons altered both the phrenic burst frequency and occurrence after the stimulation. Extracellular unitary recordings and trans-neuronal tracing experiments with the rabies virus show that the medullary lateral reticular area contains respiratory-modulated neurons, not necessary for respiratory rhythmogenesis, but that may provide an excitatory pathway from the trigeminal inputs to the RRG as their electrolytic lesion suppresses any phrenic activity induced by the trigeminal nerve stimulation. The results lead to the hypothesis that the trigeminal afferents in the mouse neonate involve at least two pathways to activate the RRG, one that may act through the medullary lateral reticular area and one that releases the A5 inhibition received by the RRG.

Functional genomics and the comparative physiology of hypoxia

Comparative physiology has proven a powerful approach to our understanding of how animals function under hypoxic conditions and to identifying potential adaptations to environmental oxygen levels. This review considers the potential for using a similar comparative approach with functional genomics to understand the genetic basis of such physiological processes and evolutionary adaptations. Comparative functional genomics is currently limited by genome data, which are available for only a few model organisms. However, comparative studies between model organisms of the same species having slightly different genomes (e.g., in-bred strains of laboratory rodents, transgenic mice, and consomic rats) demonstrate the types of results, as well as the analytical challenges, that are possible if comparative functional genomics is applied to more species. Results from wild and domestic animal studies suggest new models to investigate physiological and evolutionary responses to oxygen levels with functional genomics.

Tissue PO2 and the effects of hypoxia on the generation of locomotor-like activity in the in vitro spinal cord of the neonatal mouse

The neonatal mouse en bloc spinal cord-brainstem preparation used in combination with advances in mouse genomics provides a novel strategy for studying the spinal control of locomotion. How well the mouse en bloc preparation is oxygenated however, is unknown. This is an important consideration given that (a) other superfused mammalian en bloc preparations have anoxic cores and (b) hypoxia can have profound effects on neuronal activity. Here we measure the level of tissue oxygenation in the mouse preparation and determine how neuronal activity within the spinal cord is influenced by poor superfusion and/or low oxygen. To measure tissue oxygenation, oxygen depth profiles were obtained (P0-1 and P2-3; Swiss Webster mice). At P0-1, spinal cords were oxygenated throughout under resting conditions. When fictive locomotor activity was evoked (5-HT 10 microM, dopamine 50 microM, NMA 5 microM), there was a substantial reduction in tissue PO(2) starting within 5 min of drug application. Following washout, the PO(2) slowly returned to control levels over a period of 30 min. The experiments described above were repeated using P2-3 preparations. In this older age group, the spinal cord preparations had a hypoxic/anoxic core that was exacerbated during metabolically demanding tasks such as drug-evoked rhythmic activity. To examine how an anoxic core affects neuronal activity within the spinal cord we either altered the flow-rate or manipulated superfusate PO(2). When the flow-rate was reduced a transient disruption in the rhythmicity of drug-induced locomotion occurred during the first 15 min (P0-1 preparations). However, the motor output adapted and stabilized. During prolonged superfusion with hypoxic artificial cerebrospinal fluid on the other hand, both the motor bursts in spinal nerves and the activity of most neurons near the center of the tissue were abolished.Overall, this study suggests that while oxygenation of P0-P1 preparations is adequate for studies of locomotor function, oxygenation of older preparations is more problematic. Our data also show that neonatal spinal neurons require oxygen to maintain activity; and the spinal locomotor rhythm generator continues to function providing the peripheral tissue of the cord is oxygenated. Together, these results are consistent with the results of a previous study which suggest that the locomotor pattern generator is located close to the surface of the spinal cord.

Monoamine oxidase A-deficiency and noradrenergic respiratory regulations in neonatal mice

In vitro experiments were performed on brainstem-spinal cord preparations from mouse neonates to compare the noradrenergic regulations of the respiratory network in the control C3H/HeJ strain and the transgenic Tg8 strain which has been created from the C3H/HeJ strain by deletion of the gene encoding monoamine oxidase A (MAOA), the main enzyme for serotonin degradation. In both control and MAOA-deficient strains, we show: (i). that the pontine A5 area exerts a potent inhibitory modulation on the respiratory rhythm generator; (ii). that noradrenaline application induces a tonic phrenic activity; and (iii). that noradrenaline increases the respiratory rhythm. The latter effect is however delayed and weak in the Tg8 strain. Therefore, MAOA-deficiency has only slightly altered the noradrenergic regulations of the respiratory network.

Monoaminergic control of vasopressin and VIP expression in the mouse suprachiasmatic nucleus

We studied the effects of serotonin and noradrenaline on the expression of arginine-vasopressin (AVP) and vasoactive intestinal peptide (VIP) in the suprachiasmatic nucleus (SCN). We used transgenic Tg8 mice knockout for the MAO-A (monoamine oxidase A) gene, which are characterized by increased amounts of serotonin and noradrenaline in brain compared to wild-type mice (C3H). The MAO-A deficiency caused an increase in AVP and VIP expression (determined by immunohistochemistry, enzyme immunoassay, and in situ hybridization) compared to C3H mice. The number of peptidergic neurons was also increased. Inhibiting serotonin or noradrenaline synthesis in Tg8 mice by the administration of parachlorophenylalanine or alpha-methylparatyrosine, respectively, the amounts of AVP, VIP and their mRNAs were decreased, but not the number of peptidergic neurons. This study indicates that serotonin and noradrenaline stimulate AVP and VIP expression, and could participate in the differentiation of the neurochemical phenotype in the mouse SCN.

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.

Breathing: rhythmicity, plasticity, chemosensitivity

Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.

Neuroplasticity in respiratory motor control

Although recent evidence demonstrates considerable neuroplasticity in the respiratory control system, a comprehensive conceptual framework is lacking. Our goals in this review are to define plasticity (and related neural properties) as it pertains to respiratory control and to discuss potential sites, mechanisms, and known categories of respiratory plasticity. Respiratory plasticity is defined as a persistent change in the neural control system based on prior experience. Plasticity may involve structural and/or functional alterations (most commonly both) and can arise from multiple cellular/synaptic mechanisms at different sites in the respiratory control system. Respiratory neuroplasticity is critically dependent on the establishment of necessary preconditions, the stimulus paradigm, the balance between opposing modulatory systems, age, gender, and genetics. Respiratory plasticity can be induced by hypoxia, hypercapnia, exercise, injury, stress, and pharmacological interventions or conditioning and occurs during development as well as in adults. Developmental plasticity is induced by experiences (e.g., altered respiratory gases) during sensitive developmental periods, thereby altering mature respiratory control. The same experience later in life has little or no effect. In adults, neuromodulation plays a prominent role in several forms of respiratory plasticity. For example, serotonergic modulation is thought to initiate and/or maintain respiratory plasticity following intermittent hypoxia, repeated hypercapnic exercise, spinal sensory denervation, spinal cord injury, and at least some conditioned reflexes. Considerable work is necessary before we fully appreciate the biological significance of respiratory plasticity, its underlying cellular/molecular and network mechanisms, and the potential to harness respiratory plasticity as a therapeutic tool.

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

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

Muscimol inhibition of medullary raphé neurons decreases the CO2 response and alters sleep in newborn piglets

Medullary raphé neurons are chemosensitive in vitro (Wang et al., J. Physiol. Lond. 511 (1998)), are involved in the ventilatory response to CO(2) in vivo (Dreshaj et al., Respir. Physiol. 111 (1998); Nattie and Li, J. Appl. Physiol. 90 (2001)), and are abnormal in many Sudden Infant Death Syndrome (SIDS) victims (Panigrahy et al., J. Neuropathol. Exp. Neurol. 59 (2000)). In this study we determine whether the ventilatory response to CO(2) is altered when medullary raphé neuronal function is focally and reversibly inhibited in chronically instrumented newborn piglets. Ventilation was measured by whole body plethysmography in room air and in 5% CO(2) before and during microdialysis of muscimol, a gamma-amino butyric acid (GABA(A)) receptor agonist, into the medullary raphé. Muscimol (10 mM in the dialysate), had no effect on eupneic ventilation, but reduced significantly the CO(2) response by 17% during wakefulness. Sleep cycling was also disrupted, as characterized by a significant increase in the percentage of time spent awake and a significant decrease in the percentage of time spent in NREM sleep. Disturbances of medullary raphé function can alter central chemoreception and normal sleep architecture, which may contribute to the pathogenesis of SIDS.

Postural modifications and neuronal excitability changes induced by a short-term serotonin depletion during neonatal development in the rat

Serotonin (5-HT) plays an important role both in the development and in the recovery of locomotion after spinalization in vertebrates. We investigated the contribution of the serotonergic system to the maturation of the lumbar motoneurons and networks in the neonatal rat. A 5-HT synthesis inhibitor, p-chlorophenylalanine (PCPA), was administered daily from the first postnatal day (P0) onward. This protocol depleted serotonin in the spinal cord within 3-4 d, as demonstrated by immunohistochemistry. PCPA-treated rats exhibited postural changes characterized by lesser flexion at the knee and ankle levels and lesser extension of the hip. Posture was asymmetric, suggesting possible deficits in the interlimb coordination. Intracellular recordings were made at P3-5 from motoneurons innervating different hindlimb muscles, using the in vitro brainstem-spinal cord-nerve-attached preparation. In PCPA-treated rats, the conduction velocity of motoneurons was increased, and their excitability was decreased (because of higher rehobase and input conductance) compared with sham animals. In accordance with postural observations, changes were more pronounced in hip extensor/knee flexor than in ankle extensor motoneurons. The maturation of repetitive firing properties was stopped by PCPA treatment, although PCPA, applied in vitro, had no effect on membrane properties. The spontaneous endogenously generated activity, which is a characteristic of immature networks, was increased in PCPA-treated rats, suggesting that developing lumbar networks are sensitive to 5-HT levels. Serotonin may play a critical role during development in regulating the balance between the excitability of motoneurons and that of interneurons. Interneuronal excitability is crucial for the activity-dependent development of spinal cord networks.

Heart rate dynamics in monoamine oxidase-A- and -B-deficient mice

Heart rate (HR) dynamics were investigated in mice deficient in monoamine oxidase A and B, whose phenotype includes elevated tissue levels of norepinephrine, serotonin, dopamine, and phenylethylamine. In their home cages, spectral analysis of R-R intervals revealed more pronounced fluctuations at all frequencies in the mutants compared with wild-type controls, with a particular enhancement at 1-4 Hz. No significant genotypic differences in HR variability (HRV) or entropies calculated from Poincaré plots of the R-R intervals were noted. During exposure to the stress of a novel environment, HR increased and HRV decreased in both genotypes. However, mutants, unlike controls, demonstrated a rapid return to baseline HR during the 10-min exposure. Such modulation may result from an enhanced vagal tone, as suggested by the observation that mutants responded to cholinergic blockade with a decrease in HRV and a prolonged tachycardia greater than controls. Monoamine oxidase-deficient mice may represent a useful experimental model for studying compensatory mechanisms responsible for changes in HR dynamics in chronic states of high sympathetic tone.

Central pattern generators and the control of rhythmic movements

Central pattern generators are neuronal circuits that when activated can produce rhythmic motor patterns such as walking, breathing, flying, and swimming in the absence of sensory or descending inputs that carry specific timing information. General principles of the organization of these circuits and their control by higher brain centers have come from the study of smaller circuits found in invertebrates. Recent work on vertebrates highlights the importance of neuro-modulatory control pathways in enabling spinal cord and brain stem circuits to generate meaningful motor patterns. Because rhythmic motor patterns are easily quantified and studied, central pattern generators will provide important testing grounds for understanding the effects of numerous genetic mutations on behavior. Moreover, further understanding of the modulation of spinal cord circuitry used in rhythmic behaviors should facilitate the development of new treatments to enhance recovery after spinal cord damage.