Inhibitions mediated by glycine and GABAA receptors shape the discharge pattern of bulbar respiratory neurons

GABAergic Inhibition of Presynaptic Ca2+ Transients in Respiratory PreBötzinger Neurons in Organotypic Slice Cultures

GABAergic somatodendritic inhibition in the preBötzinger complex (preBötC), a medullary site for the generation of inspiratory rhythm, is involved in respiratory rhythmogenesis and patterning. Nevertheless, whether GABA acts distally on presynaptic terminals, evoking presynaptic inhibition is unknown. Here, we begin to address this problem by measuring presynaptic Ca²⁺ transients in preBötC neurons, under rhythmic and non-rhythmic conditions, with two variants of genetically encoded Ca²⁺ indicators (GECIs). Organotypic slice cultures from newborn mice, containing the preBötC, were drop-transduced with jGCaMP7s, or injected with jGCaMP7f-labeling commissural preBötC neurons. Then, Ca²⁺ imaging combined with whole-cell patch-clamp or field stimulation was obtained from inspiratory preBötC neurons. We found that rhythmically active neurons expressed synchronized Ca²⁺ transients in soma, proximal and distal dendritic regions, and punctate synapse-like structures. Expansion microscopy revealed morphologic characteristics of bona fide synaptic boutons of the en passant and terminal type. Under non-rhythmic conditions, we found that bath application of the GABA_A receptor agonist muscimol, and local microiontophoresis of GABA, reduced action potential (AP)-evoked and field stimulus-evoked Ca²⁺ transients in presynaptic terminals in inspiratory neurons and commissural neurons projecting to the contralateral preBötC. In addition, under rhythmic conditions, network rhythmic activity was suppressed by muscimol, while the GABA_A receptor antagonist bicuculline completely re-activated spontaneous activity. These observations demonstrate that the preBötC includes neurons that show GABAergic inhibition of presynaptic Ca²⁺ transients, and presynaptic inhibition may play a role in the network activity that underlies breathing.

Photocontrol of Endogenous Glycine Receptors In Vivo

Glycine receptors (GlyRs) are indispensable for maintaining excitatory/inhibitory balance in neuronal circuits that control reflexes and rhythmic motor behaviors. Here we have developed Glyght, a GlyR ligand controlled with light. It is selective over other Cys-loop receptors, is active in vivo, and displays an allosteric mechanism of action. The photomanipulation of glycinergic neurotransmission opens new avenues to understanding inhibitory circuits in intact animals and to developing drug-based phototherapies.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

Developmentally regulated KCC2 phosphorylation is essential for dynamic GABA-mediated inhibition and survival

Despite its importance for γ-aminobutyric acid (GABA) inhibition and involvement in neurodevelopmental disease, the regulatory mechanisms of the K+/Cl- cotransporter KCC2 (encoded by SLC12A5) during maturation of the central nervous system (CNS) are not entirely understood. Here, we applied quantitative phosphoproteomics to systematically map sites of KCC2 phosphorylation during CNS development in the mouse. KCC2 phosphorylation at Thr906 and Thr1007, which inhibits KCC2 activity, underwent dephosphorylation in parallel with the GABA excitatory-inhibitory sequence in vivo. Knockin mice expressing the homozygous phosphomimetic KCC2 mutations T906E/T1007E (Kcc2E/E ), which prevented the normal developmentally regulated dephosphorylation of these sites, exhibited early postnatal death from respiratory arrest and a marked absence of cervical spinal neuron respiratory discharges. Kcc2E/E mice also displayed disrupted lumbar spinal neuron locomotor rhythmogenesis and touch-evoked status epilepticus associated with markedly impaired KCC2-dependent Cl- extrusion. These data identify a previously unknown phosphorylation-dependent KCC2 regulatory mechanism during CNS development that is essential for dynamic GABA-mediated inhibition and survival.

Role of fast inhibitory synaptic transmission in neonatal respiratory rhythmogenesis and pattern formation

Several studies have investigated the general role of chloride-based neurotransmission (GABA_A and glycinergic signaling) in respiratory rhythmogenesis and pattern formation. In several brain regions, developmental alterations in these signaling pathways have been shown to be mediated by changes in cation-chloride cotransporter (CC) expression. For instance, CC expression changes during the course of neonatal development in medullary respiratory nuclei and other brain/spinal cord regions in a manner which decreases the cellular import, and increases the export, of chloride ions, shifting reversal potentials for chloride to progressively more negative values with maturation. In slice preparations of the same, this is related to an excitatory-to-inhibitory shift of GABA_A- and glycinergic signaling. In medullary slices, GABA_A-/glycinergic signaling in the early neonatal period is excitatory, becoming inhibitory over time. Additionally, blockade of the Na⁺/K⁺/2Cl^- cotransporter, which imports these ions via secondary active transport, converts excitatory response to inhibitory ones. These effects have not yet been demonstrated at the individual respiratory-related neuron level to occur in intact (in vivo or in situ) animal preparations, which in contrast to slices, possess normal network connectivity and natural sources of tonic drive. Developmental changes in respiratory rhythm generating and pattern forming pontomedullary respiratory circuitry may contribute to critical periods, during which there exist increased risk for perinatal respiratory disturbances of central, obstructive, or hypoxia/hypercapnia-induced origin, including the sudden infant death syndrome. Thus, better characterizing the neurochemical maturation of the central respiratory network will enhance our understanding of these conditions, which will facilitate development of targeted therapies for respiratory disturbances in neonates and infants.

Probing the function of glycinergic neurons in the mouse respiratory network using optogenetics

Glycine is a primary inhibitory transmitter in the ventral medullary respiratory network, but the functional role of glycinergic neurons for breathing remains a matter of debate. We applied optogenetics to selectively modulate glycinergic neuron activity within regions of the rostral ventral respiratory column (VRC). Responses of the phrenic nerve activity to the light-driven stimulation were studied in the working heart-brainstem preparation from adult glycine transporter 2 Cre mice (GlyT2-Cre), which received a unilateral injection of a Cre-dependent AAV virus into Bötzinger and preBötzinger Complex. Sustained light stimulation from the ventral medullary surface resulted in a substantial depression of the phrenic nerve (PN) frequency, which in most cases was compensated by an increase in PN amplitude. Periodic, burst stimulation with variable intervals could alter and reset respiratory rhythm. We conclude that unilateral activation of the rostral VRC glycinergic neurons can significantly affect respiratory pattern by lengthening the expiratory interval and modulating phase transition.

Phrenic motoneurons: output elements of a highly organized intraspinal network

pontomedullary respiratory network generates the respiratory pattern and relays it to bulbar and spinal respiratory motor outputs. The phrenic motor system controlling diaphragm contraction receives and processes descending commands to produce orderly, synchronous, and cycle-to-cycle-reproducible spatiotemporal firing. Multiple investigators have studied phrenic motoneurons (PhMNs) in an attempt to shed light on local mechanisms underlying phrenic pattern formation. I and colleagues (Marchenko V, Ghali MG, Rogers RF. Am J Physiol Regul Integr Comp Physiol 308: R916–R926, 2015.) recorded PhMNs in unanesthetized, decerebrate rats and related their activity to simultaneous phrenic nerve (PhN) activity by creating a time-frequency representation of PhMN-PhN power and coherence. On the basis of their temporal firing patterns and relationship to PhN activity, we categorized PhMNs into three classes, each of which emerges as a result of intrinsic biophysical and network properties and organizes the orderly contraction of diaphragm motor fibers. For example, early inspiratory diaphragmatic activation by the early coherent burst generated by high-frequency PhMNs may be necessary to prime it to overcome its initial inertia. We have also demonstrated the existence of a prominent role for local intraspinal inhibitory mechanisms in shaping phrenic pattern formation. The objective of this review is to relate and synthesize recent findings with those of previous studies with the aim of demonstrating that the phrenic nucleus is a region of active local processing, rather than a passive relay of descending inputs.

Oscillation patterns are enhanced and firing threshold is lowered in medullary respiratory neuron discharges by threshold doses of a μ-opioid receptor agonist

μ-Opioid receptors are distributed widely in the brain stem respiratory network, and opioids with selectivity for μ-type receptors slow in vivo respiratory rhythm in lowest effective doses. Several studies have reported μ-opioid receptor effects on the three-phase rhythm of respiratory neurons, but there are until now no reports of opioid effects on oscillatory activity within respiratory discharges. In this study, effects of the μ-opioid receptor agonist fentanyl on spike train discharge properties of several different types of rhythm-modulating medullary respiratory neuron discharges were analyzed. Doses of fentanyl that were just sufficient for prolongation of discharges and slowing of the three-phase respiratory rhythm also produced pronounced enhancement of spike train properties. Oscillation and burst patterns detected by autocorrelation measurements were greatly enhanced, and interspike intervals were prolonged. Spike train properties under control conditions and after fentanyl were uniform within each experiment, but varied considerably between experiments, which might be related to variability in acid-base balance in the brain stem extracellular fluid. Discharge threshold was shifted to more negative levels of membrane potential. The effects on threshold are postulated to result from opioid-mediated disinhibition and postsynaptic enhancement of N-methyl-d- aspartate receptor current. Lowering of firing threshold, enhancement of spike train oscillations and bursts and prolongation of discharges by lowest effective doses of fentanyl could represent compensatory adjustments in the brain stem respiratory network to override opioid blunting of CO₂/pH chemosensitivity.

Defects of the Glycinergic Synapse in Zebrafish

Glycine mediates fast inhibitory synaptic transmission. Physiological importance of the glycinergic synapse is well established in the brainstem and the spinal cord. In humans, the loss of glycinergic function in the spinal cord and brainstem leads to hyperekplexia, which is characterized by an excess startle reflex to sudden acoustic or tactile stimulation. In addition, glycinergic synapses in this region are also involved in the regulation of respiration and locomotion, and in the nociceptive processing. The importance of the glycinergic synapse is conserved across vertebrate species. A teleost fish, the zebrafish, offers several advantages as a vertebrate model for research of glycinergic synapse. Mutagenesis screens in zebrafish have isolated two motor defective mutants that have pathogenic mutations in glycinergic synaptic transmission: bandoneon (beo) and shocked (sho). Beo mutants have a loss-of-function mutation of glycine receptor (GlyR) β-subunit b, alternatively, sho mutant is a glycinergic transporter 1 (GlyT1) defective mutant. These mutants are useful animal models for understanding of glycinergic synaptic transmission and for identification of novel therapeutic agents for human diseases arising from defect in glycinergic transmission, such as hyperekplexia or glycine encephalopathy. Recent advances in techniques for genome editing and for imaging and manipulating of a molecule or a physiological process make zebrafish more attractive model. In this review, we describe the glycinergic defective zebrafish mutants and the technical advances in both forward and reverse genetic approaches as well as in vivo visualization and manipulation approaches for the study of the glycinergic synapse in zebrafish.

Respiratory Rhythm Generation: Plasticity of a Neuronal Network

The exchange of gases between the external environment and the organism is controlled by a neural network of medullary neurons that produces rhythmic activity that ultimately leads to periodic contractions of thoracic, abdominal, and diaphragm muscles. This occurs in three neural phases: inspiration, postinspiration, and expiration. The present article deals with the mechanisms underlying respiratory rhythm generation and the processes of dynamic adjustment of respiratory activity by neuromodulation as it occurs during normoxia and hypoxia. The respiratory rhythm originates from the “pre-Bötzinger complex,” which is a morphologically defined region within the lower brainstem. There is a primary oscillating network consisting of reciprocally connected early-inspiratory and postinspiratory neurons, whereas various other subgroups of respiratory neurons shape the activity pattern. Rhythm generation and pattern formation result from neuronal interactions within the network, that is, from cooperative adjustments of intrinsic membrane properties and synaptic processes in the respiratory neurons. There is evidence that in neonatal mammals, as well as under certain pathological situations in adult mammals, the respiratory rhythm derives from early-inspiratory burster neurons that drive inspiratory output neurons. The respiratory network is influenced by a variety of neuromodulators. Stimulation of appropriate receptors mostly activates signal pathways that converge on cAMP-dependent protein kinase and protein kinase C. Both pathways exert modulatory effects on voltage- and ligand-controlled ion channels. Many neuromodulators are continuously released within the respiratory region or accumulated under pathological conditions such as hypoxia. The functional significance of such ongoing neuromodulation is seen in variations of network excitability. In this review, the authors concentrate on the modulators serotonin, adenosine, and opioids.

Perturbations of Respiratory Rhythm and Pattern by Disrupting Synaptic Inhibition within Pre-Bötzinger and Bötzinger Complexes

The pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes are the brainstem compartments containing interneurons considered to be critically involved in generating respiratory rhythm and motor pattern in mammals.

The pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes are the brainstem compartments containing interneurons considered to be critically involved in generating respiratory rhythm and motor pattern in mammals. Current models postulate that both generation of the rhythm and coordination of the inspiratory-expiratory pattern involve inhibitory synaptic interactions within and between these regions. Both regions contain glycinergic and GABAergic neurons, and rhythmically active neurons in these regions receive appropriately coordinated phasic inhibition necessary for generation of the normal three-phase respiratory pattern. However, recent experiments attempting to disrupt glycinergic and GABAergic postsynaptic inhibition in the pre-BötC and BötC in adult rats in vivo have questioned the critical role of synaptic inhibition in these regions, as well as the importance of the BötC, which contradicts previous physiological and pharmacological studies. To further evaluate the roles of synaptic inhibition and the BötC, we bilaterally microinjected the GABA_A receptor antagonist gabazine and glycinergic receptor antagonist strychnine into the pre-BötC or BötC in anesthetized adult rats in vivo and in perfused in situ brainstem–spinal cord preparations from juvenile rats. Muscimol was microinjected to suppress neuronal activity in the pre-BötC or BötC. In both preparations, disrupting inhibition within pre-BötC or BötC caused major site-specific perturbations of the rhythm and disrupted the three-phase motor pattern, in some experiments terminating rhythmic motor output. Suppressing BötC activity also potently disturbed the rhythm and motor pattern. We conclude that inhibitory circuit interactions within and between the pre-BötC and BötC critically regulate rhythmogenesis and are required for normal respiratory motor pattern generation.

Deficiency of GABAergic synaptic inhibition in the Kölliker-Fuse area underlies respiratory dysrhythmia in a mouse model of Rett syndrome

KEY POINTS

Life threatening breathing irregularity and central apnoeas are highly prevalent in children suffering from Rett syndrome. Abnormalities in inhibitory synaptic transmission have been associated with the physiopathology of this syndrome, and may underlie the respiratory disorder. In a mouse model of Rett syndrome, GABAergic terminal projections are markedly reduced in the Kölliker-Fuse nucleus (KF) in the dorsolateral pons, an important centre for control of respiratory rhythm regularity. Administration of a drug that augments endogenous GABA localized to this region of the pons reduced the incidence of apnoea and the respiratory irregularity of Rett female mice. Conversely, the respiratory disorder was recapitulated by blocking GABAergic transmission in the KF area of healthy rats. This study helps us understand the mechanism for generation of respiratory abnormality in Rett syndrome, pinpoints a brain site responsible and provides a clear anatomical target for the development of a translatable drug treatment. Central apnoeas and respiratory irregularity are a common feature in Rett syndrome (RTT), a neurodevelopmental disorder most often caused by mutations in the methyl-CpG-binding protein 2 gene (MECP2). We used a MECP2 deficient mouse model of RTT as a strategy to obtain insights into the neurobiology of the disease and into mechanisms essential for respiratory rhythmicity during normal breathing. Previously, we showed that, systemic administration of a GABA reuptake blocker in MECP2 deficient mice markedly reduced the occurrence of central apnoeas. Further, we found that, during central apnoeas, post-inspiratory drive (adductor motor) to the upper airways was enhanced in amplitude and duration in Mecp2 heterozygous female mice. Since the pontine Kölliker-Fuse area (KF) drives post-inspiration, suppresses inspiration, and can reset the respiratory oscillator phase, we hypothesized that synaptic inhibition in this area is essential for respiratory rhythm regularity. In this study, we found that: (i) Mecp2 heterozygous mice showed deficiency of GABA perisomatic bouton-like puncta and processes in the KF nucleus; (ii) blockade of GABA reuptake in the KF of RTT mice reduced breathing irregularity; (iii) conversely, blockade of GABAA receptors in the KF of healthy rats mimicked the RTT respiratory phenotype of recurrent central apnoeas and prolonged post-inspiratory activity. Our results show that reductions in synaptic inhibition within the KF induce rhythm irregularity whereas boosting GABA transmission reduces respiratory arrhythmia in a murine model of RTT. Our data suggest that manipulation of synaptic inhibition in KF may be a clinically important strategy for alleviating the life threatening respiratory disorders in RTT.

Defining inhibitory neurone function in respiratory circuits: opportunities with optogenetics?

Pharmacological and mathematical modelling studies support the view that synaptic inhibition in mammalian brainstem respiratory circuits is essential for generating normal and stable breathing movements. GABAergic and glycinergic neurones are known components of these circuits but their precise functional roles have not been established, especially within key microcircuits of the respiratory pre-Bötzinger (pre-BötC) and Bötzinger (BötC) complexes involved in phasic control of respiratory pump and airway muscles. Here, we review briefly current concepts of relevant complexities of inhibitory synapses and the importance of synaptic inhibition in the operation of these microcircuits. We highlight results and limitations of classical pharmacological studies that have suggested critical functions of synaptic inhibition. We then explore the potential opportunities for optogenetic strategies that represent a promising new approach for interrogating function of inhibitory circuits, including a hypothetical wish list for optogenetic approaches to allow expedient application of this technology. We conclude that recent technical advances in optogenetics should provide a means to understand the role of functionally select and regionally confined subsets of inhibitory neurones in key respiratory circuits such as those in the pre-BötC and BötC.

The role of spinal GABAergic circuits in the control of phrenic nerve motor output

While supraspinal mechanisms underlying respiratory pattern formation are well characterized, the contribution of spinal circuitry to the same remains poorly understood. In this study, we tested the hypothesis that intraspinal GABAergic circuits are involved in shaping phrenic motor output. To this end, we performed bilateral phrenic nerve recordings in anesthetized adult rats and observed neurogram changes in response to knocking down expression of both isoforms (65 and 67 kDa) of glutamate decarboxylase (GAD65/67) using microinjections of anti-GAD65/67 short-interference RNA (siRNA) in the phrenic nucleus. The number of GAD65/67-positive cells was drastically reduced on the side of siRNA microinjections, especially in the lateral aspects of Rexed's laminae VII and IX in the ventral horn of cervical segment C4, but not contralateral to microinjections. We hypothesize that intraspinal GABAergic control of phrenic output is primarily phasic, but also plays an important role in tonic regulation of phrenic discharge. Also, we identified respiration-modulated GABAergic interneurons (both inspiratory and expiratory) located slightly dorsal to the phrenic nucleus. Our data provide the first direct evidence for the existence of intraspinal GABAergic circuits contributing to the formation of phrenic output. The physiological role of local intraspinal inhibition, independent of descending direct bulbospinal control, is discussed.

KCC2-mediated regulation of respiration-related rhythmic activity during postnatal development in mouse medulla oblongata

GABA acts as inhibitory neurotransmitter in the adult central nervous system but as excitatory neurotransmitter during early postnatal development. This shift in GABA's action from excitation to inhibition is caused by a decrease in intracellular chloride concentration ([Cl(-)]i), which in turn is caused by changes in the relative expression levels of the K(+)-Cl(-) co-transporter (KCC2) and the Na(+), K(+)-2Cl(-) co-transporter (NKCC1) proteins. Previous studies have used slices containing the medullary pre-Bötzinger complex (pre-BötC) to record respiration-related rhythmic activity (RRA) from the hypoglossal nucleus (12 N). The role of GABAergic transmission in the regulation of medullary RRA neonatally, however, is yet to be determined. Here, we examined how GABA and chloride co-transporters contribute to RRA during development in the 12 N where inspiratory neurons reside. We recorded extracellular RRA in medullary slices obtained from postnatal day (P) 0-7 mice. RRA was induced by soaking slices in artificial cerebrospinal fluid (aCSF) containing 8mM-K(+). Application of GABA significantly increased the frequency of RRA after P3, whereas application of a KCC2 blocker (R (+)-[(2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-indenyl-5-yl)oxy]acetic acid (DIOA)) significantly decreased the frequency of RRA after P1. In addition, dense KCC2 immunolabeling was seen in the superior longitudinalis (SL) of the 12 N, which is responsible for retraction of the tongue, from P0 and P7. These results indicate that GABA administration can increase RRA frequency during the first week following birth. This in turn suggests that decreasing [Cl(-)]i levels caused by increasing KCC2 levels in the 12 N could play important roles in regulating the frequency of RRA during development.

Optogenetic perturbation of preBötzinger complex inhibitory neurons modulates respiratory pattern

Inhibitory neurons make up a substantial fraction of the neurons in the preBötzinger complex (preBötC), a site that is critical for mammalian eupneic breathing. We investigated the role of glycinergic preBötC neurons in respiratory rhythmogenesis in mice using optogenetically targeted excitation and inhibition. Channelrhodopsin-2 (ChR2) or Archaerhodopsin (Arch) were expressed in glycinergic preBötC neurons of glycine transporter 2 (Glyt2, also known as Slc6a5)-Cre mice. In ChR2-transfected mice, brief inspiratory-phase bilateral photostimulation targeting the preBötC prematurely terminated inspiration, whereas expiratory-phase photostimulation delayed the onset of the next inspiration. Prolonged photostimulation produced apneas lasting as long as the light pulse. Inspiratory-phase photoinhibition in Arch-transfected mice during inspiration increased tidal volume without altering inspiratory duration, whereas expiratory-phase photoinhibition shortened the latency until the next inspiration. During persistent apneas, prolonged photoinhibition restored rhythmic breathing. We conclude that glycinergic preBötC neurons modulate inspiratory pattern and are important for reflex apneas, but that the rhythm can persist after substantial dampening of their activity.

Respiratory rhythm generation in vivo

The cellular and circuit mechanisms generating the rhythm of breathing in mammals have been under intense investigation for decades. Here, we try to integrate the key discoveries into an updated description of the basic neural processes generating respiratory rhythm under in vivo conditions.

Effects of glycinergic inhibition failure on respiratory rhythm and pattern generation

Inhibitory interactions between neurons of the respiratory network are involved in rhythm generation and pattern formation. Using a computational model of brainstem respiratory networks, we investigated the possible effects of suppressing glycinergic inhibition on the activity of different respiratory neuron types. Our study revealed that progressive suppression of glycinergic inhibition affected all neurons of the network and disturbed neural circuits involved in termination of inspiration. Causal was a dysfunction of postinspiratory inhibition targeting inspiratory neurons, which often led to irregular preterm reactivation of these neurons, producing double or multiple short-duration inspiratory bursts. An increasing blockade of glycinergic inhibition led to apneustic inspiratory activity. Similar disturbances of glycinergic inhibition also occur during hypoxia. A clear difference in prolonged hypoxia, however, is that the rhythm terminates in expiratory apnea. The critical function of glycinergic inhibition for normal respiratory rhythm generation and the consequences of its reduction, including in pathological conditions, are discussed.

The cellular building blocks of breathing

Respiratory brainstem neurons fulfill critical roles in controlling breathing: they generate the activity patterns for breathing and contribute to various sensory responses including changes in O2 and CO2. These complex sensorimotor tasks depend on the dynamic interplay between numerous cellular building blocks that consist of voltage-, calcium-, and ATP-dependent ionic conductances, various ionotropic and metabotropic synaptic mechanisms, as well as neuromodulators acting on G-protein coupled receptors and second messenger systems. As described in this review, the sensorimotor responses of the respiratory network emerge through the state-dependent integration of all these building blocks. There is no known respiratory function that involves only a small number of intrinsic, synaptic, or modulatory properties. Because of the complex integration of numerous intrinsic, synaptic, and modulatory mechanisms, the respiratory network is capable of continuously adapting to changes in the external and internal environment, which makes breathing one of the most integrated behaviors. Not surprisingly, inspiration is critical not only in the control of ventilation, but also in the context of "inspiring behaviors" such as arousal of the mind and even creativity. Far-reaching implications apply also to the underlying network mechanisms, as lessons learned from the respiratory network apply to network functions in general.

Role of inhibition in respiratory pattern generation

Postsynaptic inhibition is a key element of neural circuits underlying behavior, with 20-50% of all mammalian (nongranule) neurons considered inhibitory. For rhythmic movements in mammals, e.g., walking, swimming, suckling, chewing, and breathing, inhibition is often hypothesized to play an essential rhythmogenic role. Here we study the role of fast synaptic inhibitory neurotransmission in the generation of breathing pattern by blocking GABA(A) and glycine receptors in the preBötzinger complex (preBötC), a site essential for generation of normal breathing pattern, and in the neighboring Bötzinger complex (BötC). The breathing rhythm continued following this blockade, but the lung inflation-induced Breuer-Hering inspiratory inhibitory reflex was suppressed. The antagonists were efficacious, as this blockade abolished the profound effects of the exogenously applied GABA(A) receptor agonist muscimol or glycine, either of which under control conditions stopped breathing in vagus-intact or vagotomized, anesthetized, spontaneously breathing adult rats. In vagotomized rats, GABA(A)ergic and glycinergic antagonists had little, if any, effect on rhythm. The effect in vagus-intact rats was to slow the rhythm to a pace equivalent to that seen after suppression of the aforementioned Breuer-Hering inflation reflex. We conclude that postsynaptic inhibition within the preBötC and BötC is not essential for generation of normal respiratory rhythm in intact mammals. We suggest the primary role of inhibition is in shaping the pattern of respiratory motor output, assuring its stability, and in mediating reflex or volitional apnea, but not in the generation of rhythm per se.

Computational modelling of 5-HT receptor-mediated reorganization of the brainstem respiratory network

Brainstem respiratory neurons express the glycine α(3) receptor (Glyα(3) R), which is a target of modulation by several serotonin (5-HT) receptor agonists. Application of the 5-HT(1A) receptor (5-HT(1A) R) agonist 8-OH-DPAT was shown (i) to depress cellular cAMP, leading to dephosphorylation of Glyα(3) R and augmentation of postsynaptic inhibition of neurons expressing Glyα(3) R (Manzke et al., 2010) and (ii) to hyperpolarize respiratory neurons through 5-HT-activated potassium channels. These processes counteract opioid-induced depression and restore breathing from apnoeas often accompanying pharmacotherapy of pain. The effect is postulated to rely on the enhanced Glyα(3) R-mediated inhibition of inhibitory neurons causing disinhibition of their target neurons. To evaluate this proposal and investigate the neural mechanisms involved, an established computational model of the brainstem respiratory network (Smith et al., 2007), was extended by (i) incorporating distinct subpopulations of inhibitory neurons (glycinergic and GABAergic) and their synaptic interconnections within the Bötzinger and pre-Bötzinger complexes and (ii) assigning the 5-HT(1A) R-Glyα(3) R complex to some of these inhibitory neuron types in the network. The modified model was used to simulate the effects of 8-OH-DPAT on the respiratory pattern and was able to realistically reproduce a number of experimentally observed responses, including the shift in the onset of post-inspiratory activity to inspiration and conversion of the eupnoeic three-phase rhythmic pattern into a two-phase pattern lacking the post-inspiratory phase. The model shows how 5-HT(1A) R activation can produce a disinhibition of inspiratory neurons, leading to the recovery of respiratory rhythm from opioid-induced apnoeas.

Multiple phases of excitation and inhibition in central respiratory drive potentials of thoracic motoneurones in the rat

Intracellular recordings were made from motoneurones with axons in the intercostal nerves of T9 or T10 in adult rats, with neuromuscular blockade and artificial ventilation, under hypercapnia and under either anaesthesia or decerebration. In nearly all motoneurones, central respiratory drive potentials (CRDPs) were seen, which included an excitatory wave in inspiration, in expiration, or in both of these. This was the case both for motoneurones with axons in the internal intercostal nerve (n = 81) and for those with axons in the external intercostal nerve (n = 5). In the decerebrates, motoneurones with purely inspiratory CRDPs were rare (1/44), but those excited in both phases (showing biphasic CRDPs) were common (22/44). For about one-third of biphasic CRDPs (11/30), the inspiratory depolarization was seen to reverse to a hyperpolarization when the motoneurone was depolarized, which was interpreted as indicating concurrent inhibition and excitation during this phase. A few motoneurones were seen where depolarization revealed signs of inhibition in both phases. The results confirm the novel observations of biphasic excitation in individual intercostal nerve branches, EMG sites and motor units reported in a companion paper. They also provide new insights into the functional roles of inhibition in motoneurones physiologically activated in natural rhythmic behaviours.

Mixed GABA-glycine synapses delineate a specific topography in the nucleus tractus solitarii of adult rat

Using combined morphological and electrophysiological approaches, we have determined the composition of inhibitory synapses of the nucleus tractus solitarii (NTS), a brainstem structure that is a gateway for many visceral sensory afferent fibres. Immunohistochemical experiments demonstrate that, in adult rat, GABA axon terminals are present throughout the NTS while mixed GABA-glycine axon terminals are strictly located to the lateral part of the NTS within subnuclei surrounding the tractus solitarius. Purely glycine axon terminals are rare in the lateral part of the NTS and hardly detected in its medial part. Electrophysiological experiments confirm the predominance of GABA inhibition throughout the NTS and demonstrate the existence of a dual inhibition involving the co-release of GABA and glycine restricted to the lateral part of NTS. Since GABA(A) and glycine receptors are co-expressed postsynaptically in virtually all the inhibitory axon terminals throughout the NTS, it suggests that the inhibition phenotype relies on the characteristics of the axon terminals. Our results also demonstrate that glycine is mostly associated with GABA within axon terminals and raise the possibility of a dynamic regulation of GABA/glycine release at the presynaptic level. Our data provide new information for understanding the mechanisms involved in the processing of visceral information by the central nervous system in adult animals.

The potency of different serotonergic agonists in counteracting opioid evoked cardiorespiratory disturbances

Serotonin receptor (5-HTR) agonists that target 5-HT(4(a))R and 5-HT(1A)R can reverse mu-opioid receptor (mu-OR)-evoked respiratory depression. Here, we have tested whether such rescuing by serotonin agonists also applies to the cardiovascular system. In working heart-brainstem preparations in situ, we have recorded phrenic nerve activity, thoracic sympathetic chain activity (SCA), vascular resistance and heart rate (HR) and in conscious rats, diaphragmatic electromyogram, arterial blood pressure (BP) and HR via radio-telemetry. In addition, the distribution of 5-HT(4(a))R and 5-HT(1A)R in ponto-medullary cardiorespiratory networks was identified using histochemistry. Systemic administration of the mu-OR agonist fentanyl in situ decreased HR, vascular resistance, SCA and phrenic nerve activity. Subsequent application of the 5-HT(1A)R agonist 8-OH-DPAT further enhanced bradycardia, but partially compensated the decrease in vascular resistance, sympathetic activity and restored breathing. By contrast, the 5-HT(4(a))R agonist RS67333 further decreased vascular resistance, HR and sympathetic activity, but partially rescued breathing. In conscious rats, administration of remifentanyl caused severe respiratory depression, a decrease in mean BP accompanied by pronounced bradyarrhythmia. 8-OH-DPAT restored breathing and prevented the bradyarrhythmia; however, BP and HR remained below baseline. In contrast, RS67333 further suppressed cardiovascular functions in vivo and only partially recovered breathing in some cases. The better recovery of mu-OR cardiorespiratory disturbance by 5-HT(1A)R than 5-HT(4(a))R is supported by the finding that 5-HT(1A)R was more densely expressed in key brainstem nuclei for cardiorespiratory control compared with 5-HT(4(a))R. We conclude that during treatment of severe pain, 5-HT(1A)R agonists may provide a useful tool to counteract opioid-mediated cardiorespiratory disturbances.

Serotonin targets inhibitory synapses to induce modulation of network functions

The cellular effects of serotonin (5-HT), a neuromodulator with widespread influences in the central nervous system, have been investigated. Despite detailed knowledge about the molecular biology of cellular signalling, it is not possible to anticipate the responses of neuronal networks to a global action of 5-HT. Heterogeneous expression of various subtypes of serotonin receptors (5-HTR) in a variety of neurons differently equipped with cell-specific transmitter receptors and ion channel assemblies can provoke diverse cellular reactions resulting in various forms of network adjustment and, hence, motor behaviour. Using the respiratory network as a model for reciprocal synaptic inhibition, we demonstrate that 5-HT(1A)R modulation primarily affects inhibition through glycinergic synapses. Potentiation of glycinergic inhibition of both excitatory and inhibitory neurons induces a functional reorganization of the network leading to a characteristic change of motor output. The changes in network operation are robust and help to overcome opiate-induced respiratory depression. Hence, 5-HT(1A)R activation stabilizes the rhythmicity of breathing during opiate medication of pain.

D1/D2-dopamine receptor agonist dihydrexidine stimulates inspiratory motor output and depresses medullary expiratory neurons

It is now accepted that dopamine plays an important neuromodulatory role in the central nervous control of respiration. D1, D2, and D4 subtypes of the receptor seem to be important players, but the assignment of various respiratory tasks to specific subtypes of the dopamine receptor is a work in progress. In the present investigation, dihydrexidine (DHD), a full dopamine receptor agonist with affinity for both D1- and D2-subtypes of receptor, was tested for its effects on inspiratory neurons and motor output and on membrane potential properties of medullary bulbospinal expiratory augmenting expiratory neurons in the pentobarbital anesthetized adult cat. The effects of DHD were compared with those of the highly selective D1-dopamine receptor (D1R) agonists SKF-38393 and 6-chloro-APB. DHD increased the intensity and duration of inspiratory motor output. Phrenic nerve discharge intensity was increased and prolonged, contributing to elevated inspiratory effort and duration when spontaneous breathing was monitored with tracheal pressure measurements. Intracellular recording from rostral medullary inspiratory neurons revealed that DHD, like SKF-38393, increases and prolongs inspiratory phase membrane depolarization, resulting in a longer and more intense discharge of action potentials. Remarkably, DHD had opposite effects on Aug-E neurons. Membrane potential was hyperpolarized, and action potential discharges were suppressed or abolished. In association with reduction of discharge intensity, action potential half width was reduced and after-hyperpolarization increased. The stimulatory action of DHD on inspiratory motor output is attributed to D1R effects, while the depression of Aug-E neurons seems to be linked to D2R actions on the postsynaptic membrane.

Effect of inspired oxygen on periodic breathing in methy-CpG-binding protein 2 (Mecp2) deficient mice

Rett syndrome (RTT) is a neurodevelopmental disorder caused by mutations in the X-linked gene methyl-CpG-binding protein 2 (Mecp2) that encodes a DNA binding protein involved in gene silencing. Periodic breathing (Cheyne-Stokes respiration) is commonly seen in RTT. Freely moving mice were studied with continuous recording of pleural pressure by telemetry. Episodes of periodic breathing in heterozygous Mecp2 deficient (Mecp2(+/-)) female mice (9.4 +/- 2.2 h(-1)) exceeded those in wild-type (Mecp2(+/+)) animals (2.5 +/- 0.4 h(-1)) (P = 0.010). Exposing Mecp2(+/-) animals to 40% oxygen increased the amount of periodic breathing from 118 +/- 25 s/30 min in air to 242 +/- 57 s/30 min (P = 0.001), and 12% oxygen tended to decrease it (67 +/- 29 s/30 min, P = 0.14). Relative hyperoxia and hypoxia did not affect the incidence of periodic breathing in Mecp2(+/+) animals. The ventilation/apnea ratio (V/A) was less at all levels of oxygen in heterozygous Mecp2(+/-) females compare with wild type (P = 0.003 to P < 0.001), indicating that their loop gain is larger. V/A in Mecp2(+/-) fell from 2.42 +/- 0.18 in normoxia to 1.82 +/- 0.17 in hyperoxia (P = 0.05) indicating an increase in loop gain with increased oxygen. Hyperoxia did not affect V/A in Mecp2(+/+) mice (3.73 +/- 0.28 vs. 3.5 +/- 0.28). These results show that periodic breathing in this mouse model of RTT is not dependent on enhanced peripheral chemoreceptor oxygen sensitivity. Rather, the breathing instability is of central origin.

Respiratory activity in brainstem of fetal mice lacking glutamate decarboxylase 65/67 and vesicular GABA transporter

The respiratory neural network in the mammalian medulla oblongata shows rhythmic activity before birth. GABA and glycine are considered to be involved in control of respiratory rhythm. Recently we have demonstrated respiratory failure in glutamic acid decarboxylase (GAD) 67-deficient mice [Tsunekawa N, Arata A, Obata K (2005) Development of spontaneous mouth/tongue movement and related neural activity, and their repression in mouse fetus lacking glutamate decarboxylase 67. Eur J Neurosci 21:173-178]. To further evaluate the involvement of GABA and glycine in fetal respiratory function, we studied neural activities in brainstem-spinal cord blocks prepared from GAD65-/-:67-/- and vesicular GABA transporter (VGAT)-/-mice on embryonic day 14 (E14)-E15 and E18. In these knockout mice, the synthesis of GABA and the vesicular release of GABA and glycine are completely absent, respectively. Spontaneous respiratory discharges were observed in the ventral roots at the cervical cord (C) 4 level from wild-type mice but not from the knockout mice on E18. Administration of substance P induced C4 discharges in GAD65-/-:67-/- preparations but not in VGAT-/- preparations. C4 discharges were observed in the knockout mice on E14-E15, although the frequency was lower than that in the wild-type. Neuronal activities in the respiratory network of the E18 brainstem were recorded using a "blind" patch-clamp technique. Expiratory and inspiratory neurons with their characteristic firing patterns were observed in the wild-type fetuses. Strychnine reversed inspiratory-phase hyperpolarization to large depolarization in expiratory neurons. On the other hand, neurons in the same area of the knockout mice fired spontaneously without any rhythm. Substance P induced hyperpolarizing potentials in medullary neurons of GAD65-/-:67-/- mice. Further administration of strychnine induced large depolarizing potentials. Rhythmic activities were not observed in VGAT-/- mice even in the presence of substance P and strychnine. These results indicate that the lack of GABA and glycine impairs the function of the respiratory network in mouse fetuses and the impairment progresses with fetal age.

Cannabinoid suppressed bicuculline-induced convulsion without respiratory depression in the brainstem-spinal cord preparation from newborn rats

Previous studies have suggested that cannabinoid compounds are anticonvulsants and that these compounds depress respiratory activity. However, the anticonvulsant potential of cannabinoids and their depressive effect on respiration have not been evaluated simultaneously. In the present study, we used a brainstem-spinal cord preparation model to investigate changes in inspiratory activity and the anticonvulsant effects of a cannabinoid receptor agonist, WIN 55, 212-2, in bicuculline-induced convulsion. Application of 10 microM WIN 55, 212-2 caused no change in inspiratory activity (6.9+/- 0.89 bursts/min vs. 8.0+/- 1.3 bursts/min, not significant) and decreased bicuculline-induced seizure-like nerve activity (number of seizure-like activities in 10 min, 11+/- 7.4 bursts vs. 1.5+/- 1.6 bursts, P< 0.01; average duration of seizure-like activity, 8.9+/- 4.0 sec vs. 4.7+/- 2.1 sec, P> 0.01). Our results suggest that administration of an appropriate dose of cannabinoid receptor agonist WIN 55,212-2 has an anticonvulsant effect but does not cause respiratory depression.

A combined blockade of glycine and calcium-dependent potassium channels abolishes the respiratory rhythm

In order to test whether glycinergic inhibition is essential for the in vivo respiratory rhythm, we analysed the discharge properties of neurones in the medullary respiratory network after blockade of glycine receptors in the in situ perfused brainstem preparation of mature wild type and oscillator mice with a deficient glycine receptor. In wild type mice, selective blockade of glycine receptors with low concentrations of strychnine (0.03-0.3 microM) provoked considerable changes in neuronal discharge characteristics: The cycle phase relationship of inspiratory, post-inspiratory and expiratory specific patterns of membrane potential changes was altered profoundly. Inspiratory, post-inspiratory and expiratory neurones developed a propensity for fast voltage oscillations that were accompanied by multiple burst discharges. These burst discharges were followed by "after-burst" hyperpolarisations that were capable of triggering secondary burst discharges. Blockade of glycine receptors and the "big" Ca2+-dependent K+-conductance by charybdotoxin (3.3 nM) resulted in loss of the respiratory rhythm, whilst only tonic discharge activity remained. In contrast, rhythmic activity was only weakened, but preserved after the "small" Ca2+-dependent activated K+ conductance was blocked with apamin (8 nM). Also low concentrations of pentobarbital sodium (6 mg/kg) abolished rhythmic respiratory activity after blockade of glycine receptors in the wild type mice and in glycine receptor deficient oscillator mice. The data imply that failure of glycine receptors provokes enhanced bursting behaviour of respiratory neurones, whilst the additional blockade of BKCa channels by charybdotoxin or with pentobarbital abolishes the respiratory rhythm.

Disturbance of neural respiratory control in neonatal mice lacking GABA synthesizing enzyme 67-kDa isoform of glutamic acid decarboxylase

To examine the role of GABA in the respiratory rhythm and pattern generation in neonatal mice, we analyzed the function of the respiratory control system of 67-kDa isoform of glutamic acid decarboxylase (GAD67)-deficient neonatal mice. In these mutant (GAD67-/-) mice, GABA levels in the brainstem were reduced to about 30% of those in wild-type (GAD67+/+) mice. In in vivo preparations, ventilatory parameters were analyzed by whole body plethysmography and electromyography of intercostal muscles. GAD67-/- mice exhibited abnormal respiratory patterns, i.e. irregular respiratory rhythm, and periodic gasp-like respiration followed by shallow breathing with short inspiratory duration and apnea. In in vitro GAD67-/- brainstem-spinal cord preparations, inspiratory C4 burst duration was shorter than that in GAD67+/+ preparations. Whole cell recordings revealed that activities of inspiratory neurons in the ventral medulla of GAD67-/- mice were characterized by a short depolarization period and a paucity of firing during the inspiratory phase. Superfusion of the in vitro GAD67-/- preparation with 10 microM GABA prolonged C4 burst duration and partly restored a normal pattern of inspiration, although the restoration was limited. These results indicate that reduced GABA levels during the perinatal period induce malfunction in the respiratory control system. We suggest that GABAergic transmission is not essential for basic respiratory rhythm generation but plays an important role in the maintenance of regular respiratory rhythm and normal inspiratory pattern in neonatal mice.

Glycine is used as a transmitter by decrementing expiratory neurons of the ventrolateral medulla in the rat

The medullary respiratory network involves various types of respiratory neurons. The present study focused on possible inhibitory neurons called decrementing expiratory (E-DEC) neurons and aimed to determine whether their transmitter is glycine or GABA. In Nembutal-anesthetized, neuromuscularly blocked, and artificially ventilated rats we labeled E-DEC neurons with Neurobiotin and processed the tissues for detection of mRNA encoding either glycine transporter 2 (GLYT2) as a marker for glycinergic neurons or glutamic acid decarboxylase isoform 67 (GAD67) as a marker for GABAergic neurons, using in situ hybridization. Of 38 E-DEC neurons that were labeled, cranial motoneurons (n = 14), which were labeled as control, were negative for either GLYT2 mRNA (n = 10) or GAD67 mRNA (n = 4). The other E-DEC neurons (n = 24) were non-motoneurons. Sixteen of them were examined for GLYT2 mRNA, and the majority (11 of 16) was GLYT2 mRNA-positive. The remaining E-DEC neurons (n = 8) were examined for GAD67 mRNA, and all of them were GAD67 mRNA-negative. The GLYT2 mRNA-positive E-DEC neurons were located in the ventrolateral medulla spanning the Bötzinger complex (BOT), the rostral ventral respiratory group (VRG), and the caudal VRG. We conclude that not only E-DEC neurons of the BOT but also many E-DEC neurons of the VRG are inhibitory and use glycine as a transmitter. Although the present negative data cannot rule out completely the release of GABA or co-release of glycine and GABA from E-DEC neurons, several lines of evidence suggest that the glycinergic process is primarily responsible for the phasic inhibition of the respiratory network during the expiratory phase.

Hypercapnia-induced activation of brainstem GABAergic neurons during early development

During early development, GABAergic mechanisms contribute to the regulation of respiratory timing in response to CO2. In 5-7 day old piglets, a double labeling technique was used to determine whether GABA-containing neurons are activated by normoxic hypercapnia (10% CO2, 21% O2, and 69% N2). The c-Fos gene encoded protein (c-Fos) was employed to localize CO2 activated cells within the piglet medulla oblongata. Parvalbumin was used as a marker for GABAergic neurons. In animals breathing room air, only scant c-Fos-like immunoreactive neurons were observed. A marked increase in c-Fos positive cells was induced after a 60 min exposure to hypercapnia. Colocalization studies revealed that hypercapnia significantly increased c-Fos expression in GABA-containing neurons in the medulla oblongata, especially in the ventral aspect of the medulla, within the Bötzinger region, the gigantocellular reticular nucleus, and the caudal raphe nuclei. Only a few double-labeled cells were observed within the nucleus tractus solitarius. Therefore, brainstem GABAergic neurons are part of the neural networks that respond to CO2 and may contribute to respiratory frequency responses to hypercapnia during early development.

Effects of GABA(A) and glycine receptor agonists on the medullary inspiratory neuronal activity during spontaneous augmented breaths in anesthetized rats

To clarify whether GABAergic or glycinergic transmission alters the activity of inspiratory neurons during spontaneous augmented breaths, we recorded the single unit activity from inspiratory neurons in the dorsal and ventral respiratory groups in the medulla of pentobarbital anesthetized rats and applied GABA(A) and glycine receptor agonists by iontophoresis using multibarrel microelectrodes. The spontaneous augmented breath was divided into two different phases; the first phase (phase I) resembled a normal inspiration but the second phase (phase II) indicated a marked increase in diaphragm electromyogram activity. During application of either muscimol or glycine, the discharge of inspiratory neurons during the phase I of spontaneous augmented breaths was suppressed, but the augmenting discharge of the phase II did not change significantly in any cell type of the neurons (I-augmenting, I-decrementing and I-other). These results suggested that the excitatory inputs to inspiratory neurons during the phase II of augmented breaths may not be significantly influenced by the activation of either GABA(A) receptors or glycine receptors.

Microinjections of glycine into the pre-Bötzinger complex inhibit phrenic nerve activity in the rat

Microinjections of L-glutamate were used to identify the pre-Bötzinger complex in urethane-anesthetized, immobilized, bilaterally vagotomized, artificially ventilated, adult male Wistar rats. Unilateral microinjections (20-30 nl) of L-glutamate into the pre-Bötzinger complex on either side elicited a bilateral continuous phrenic nerve discharge superimposed on which was an increase in burst-frequency. Neurokinin-1 receptor immunoreactivity in the semi-compact region of the nucleus ambiguus and the area immediately ventral to it indicated that the site of microinjections was in the general region of pre-Bötzinger complex. Unilateral microinjections of glycine into the pre-Bötzinger complex caused an inhibition of phrenic nerve activity bilaterally in a concentration-dependent manner. At lower concentrations (1 and 3 mM) phrenic nerve burst-frequency as well as burst-amplitude were decreased. At higher concentrations (6 mM), complete bilateral cessation of phrenic nerve activity was observed. The effects of glycine were prevented by a prior microinjection of strychnine (0.5 mM) into the pre-Bötzinger complex. The specificity of strychnine as an antagonist for glycine receptors was established by its lack effect on GABA(A) receptors; muscimol was used as a GABA(A) receptor agonist. Unilateral microinjections of muscimol (0.01 and 0.1 mM) into previously identified pre-Bötzinger complex also caused a bilateral decrease in phrenic nerve burst-frequency and burst-amplitude. At higher concentrations (0.3 and 1 mM) muscimol microinjections into the pre-Bötzinger elicited a complete bilateral cessation of phrenic nerve activity. The effects of muscimol were not altered by prior microinjections of strychnine (0.5 mM) at the same site. These results demonstrate pharmacologically the presence of glycine receptors in the pre-Bötzinger complex. The role of these receptors in the regulation of respiration remains to be elucidated.

Neurochemical perspectives on the control of breathing during sleep

A specific depression of minute ventilation occurs during sleep in normal subjects. This sleep-related ventilatory depression is partially related to mechanical events and upper airway atonia but some data also indicate that it is likely to be centrally mediated. This paper reviews the anatomical and neurochemical connections between sleep/wake- and respiratory-related areas in an attempt to identify the potential implication of sleep-related neurochemicals (serotonin, catecholamines, GABA, acetylcholine) in the sleep-related hypoventilation. The review of available data suggests that the sleep-related ventilatory depression depends upon the enhanced GABAergic activity together with a loss of suprapontine influence depending on the cessation of activity of the reticular formation. During REM sleep, an additional inhibitory activity emerges from the pontine cholinergic neurons, which contributes to the breathing irregularities and the associated depression of minute ventilation and ventilatory response to chemical stimuli. This model may contribute to a better understanding of the neurochemical environment of respiratory neurons during sleep, which remains a question of importance regarding the numerous pathological states that are linked to specific perturbations of breathing control during sleep.

Effects of functional knock-out of alpha 1 glycine-receptors on breathing movements in oscillator mice

The effects of a deficiency of glycinergic inhibition deriving from mutations of the glycine-receptor gene Glra1 on the breathing pattern of oscillator mice were studied. We compared the development of breathing frequency, tidal volume and minute ventilation from control mice (wild type- and heterozygous oscillator mice) with those of homozygous oscillator mice during early postnatal periods from p9 until p21. The changes of ventilation were correlated with body-weight and changes in blood-pH. During the second to third weeks of postnatal development, breathing frequency increased from 310 to 445.4 mm-1 in control mice. Oscillator mice reached a maximal value of 313.3 min-1 at p18 followed by a fast decrease to 233.0 min-1. This decrease is caused by a prolongation of expiratory duration. Tidal volume showed a steady increase from 6.6 to 15.1 microliters in control animals. In comparison, oscillator mice showed significant lower values after p14. After p15, minute ventilation of oscillator mice declined as compared with control animals leading to respiratory acidosis at p20.

Postnatal expression of neurotransmitters, receptors, and cytochrome oxidase in the rat pre-Bötzinger complex

The pre-Bötzinger complex (PBC) is postulated as the center of respiratory rhythmogenesis. Previously, we found a reduction or plateau of cytochrome oxidase (CO) activity in the PBC and other respiratory nuclei at postnatal days 3-4, despite a general increase of CO with age, suggesting a period of synaptic readjustment. The present study examined the expression of CO and a number of neurochemicals in the PBC at closer time intervals. At postnatal days 3-4 and, more prominently, at postnatal day 12, expression of CO, glutamate, and N-methyl-D-aspartate receptor subunit 1 was reduced, whereas expression of GABA, GABA(B) receptor, glycine receptor, and alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid receptor subunit 2 was increased. These findings are consistent with our hypothesis that decreased CO activity is associated with an increase in inhibitory drive (mediated by GABA and glycine, their receptors, and possibly blockage of Ca(2+) entry by glutamate receptor subunit 2) and a decrease in excitatory drive (mediated by glutamate and its receptors). Our findings point to two critical periods during postnatal development of the rat when their respiratory system may be more vulnerable to respiratory insults.

Physiological properties of late inspiratory neurons and their possible involvement in inspiratory off-switching in cats

To assess the functional significance of late inspiratory (late-I) neurons in inspiratory off-switching (IOS), membrane potential and discharge properties were examined in vagotomized, decerebrate cats. During spontaneous IOS, late-I neurons displayed large membrane depolarization and associated discharge of action potentials that started in late inspiration, peaked at the end of inspiration, and ended during postinspiration. Depolarization was decreased by iontophoresis of dizocilpine and eliminated by tetrodotoxin. Stimulation of the vagus nerve or the nucleus parabrachialis medialis (NPBM) also evoked depolarization of late-I neurons and IOS. Waves of spontaneous chloride-dependent inhibitory postsynaptic potentials (IPSPs) preceded membrane depolarization during early inspiration and followed during postinspiration and stage 2 expiration of the respiratory cycle. Iontophoresed bicuculline depressed the IPSPs. Intravenous dizocilpine caused a greatly prolonged inspiratory discharge of the phrenic nerve (apneusis) and suppressed late-inspiratory depolarization as well as early-inspiratory IPSPs, resulting in a small constant depolarization throughout the apneusis. NPBM or vagal stimulation after dizocilpine produced small, stimulus-locked excitatory postsynaptic potentials (EPSPs) in late-I neurons. Neurobiotin-labeled late-I neurons revealed immunoreactivity for glutamic acid decarboxylase as well as N-methyl-D-aspartate (NMDA) receptors. These results suggest that late-I neurons are GABAergic inhibitory neurons, while the effects of bicuculline and dizocilpine indicate that they receive periodic waves of GABAergic IPSPs and glutamatergic EPSPs. The data lead to the conclusion that late-I neurons play an important inhibitory role in IOS. NMDA receptors are assumed to augment and/or synchronize late-inspiratory depolarization and discharge of late-I neurons, leading to GABA release and consequently off-switching of bulbar inspiratory neurons and phrenic motoneurons.

The respiratory rhythm in mutant oscillator mice

Since glycinergic inhibition is important for respiratory rhythm generation in mature mammals, we tested the hypothesis that the loss of glycine receptors during postnatal development (P17-P23) of homozygous mutant oscillator mice (spd(ot)/spd(ot)) may result in serious impairment of respiratory rhythm. We measured breathing in a plethysmographic recording chamber on conscious oscillator mice and used an in situ perfused brainstem preparation to record phrenic nerve activity, as well as membrane properties of respiratory neurones. The deletion of glycinergic inhibition did not result in failure of respiratory rhythm: homozygous mutant oscillator mice continue to generate a disturbed respiratory rhythm until death. Postsynaptic activity and membrane potential trajectories of respiratory neurones revealed a persistence of GABAergic inhibition and changes in respiratory rhythm and pattern generation.

Chemosensitivity of medullary inspiratory neurones: a role for GABA(A) receptors?

This study tested the hypothesis that during hypercapnia partial removal of a tonic GABA-mediated inhibition contributes to the increase in activity of the ventrolateral medulla (VLM) inspiratory neurones. Extracellular recordings were taken from 22 inspiratory neurones in the VLM of rats anaesthetised with pentobarbitone and artificially ventilated. It was found that during hypercapnia, changes in the discharge pattern (i.e. an increase in the discharge frequency during the neurone's normally active phase) and firing frequency of the VLM inspiratory neurones were similar to those evoked by GABA(A) receptor antagonist bicuculline methiodide (BMI, 10 mM, 20 nA), applied ionophoretically in conditions of normocapnia. During hypercapnia BMI (20 nA) failed to evoke a further increase in firing of these neurones. This suggests that CO2-evoked activation of VLM inspiratory neurones may involve a withdrawal in part of a tonic GABA(A) receptor-mediated inhibition. This disinhibition may play a role in the hypercapnia-induced increase in ventilatory activity.

Studying rhythmogenesis of breathing: comparison of in vivo and in vitro models

In all mammalian species, breathing is controlled by a neuronal network within the lower brainstem. A component known as the ventral respiratory group produces rhythmic activity, which is transmitted to spinal motoneurons to produce a periodic contraction of respiratory muscles. A dispute about the mechanisms of 'normal' respiratory rhythm generation arose from the differences between experimental preparations that have been used to dissect the process. It is, therefore, essential to compare the various experimental approaches and to discuss the differences between experimental data. We conclude that the various preparations all have great value, but that they define different operational conditions of the network, including maturation of neurons and synaptic processes. We have taken note of these in formulating a 'maturational network-burster model' for rhythm generation that includes most features of the existing models of respiratory rhythm generation.

Involvement of cholinergic mechanisms in the central control of respiration in the cane toad, Bufo marinus

Chemical substrates, central sites and central mechanisms underlying the regulation of breathing in lower vertebrates have not been well characterized. The present study was undertaken to determine the effect of pH changes and cholinergic agents on the central control of respiration in the cane toad, Bufo marinus. Adult toads were anesthetized, catheterized and unidirectionally ventilated before exposing the brainstem. An airtight buccal cannula was also inserted through the tympanum to record buccal pressure. The animal was decerebrated, anesthetic removed and the responses to pH changes of solutions bathing the ventral surface of the medulla (VSM) were tested by superfusing the VSM with mock cerebrospinal fluid (mCSF) of pH 7.8-normal, 7.2-acidic and 8.4-basic. The acidic solution increased respiratory activity, the basic solution decreased activity and the normal solution had no effect. In addition, cholinergeric agents (acetylcholine-ACh, physostigmine-Phy, nicotine-Nic, and atropine-Atr) dissolved in mCSF were applied bilaterally onto the VSM using filter paper pledgets. ACh, Phy and Nic increased episodic breathing frequency by 14.3+/-9.7, 9.4+/-5.4 and 29.1+/-11.8 %, respectively, whereas, Atr caused a decrease (-26.6+/-16.6%). These agents had no effect on blood pressure. It is therefore, concluded that the VSM is pH sensitive and a cholinergic mechanism is involved in the central modulation of respiration in Bufo.

Immunoreactivity for glutamic acid decarboxylase and N-methyl-D-aspartate receptors of intracellularly labeled respiratory neurons in the cat

In adult cats, immunofluorescence images of glutamic acid decarboxylase (GAD) and N-methyl-D-aspartate (NMDA) receptors were achieved in the ventral respiratory group (VRG) neurons, which had been individually identified by in vivo intracellular recording and labeling with neurobiotin. Among augmenting inspiratory (aug-I), postinspiratory (post-I), and augmenting expiratory (aug-E) neurons labeled, GAD-immunoreactivity was demonstrated only in those neurons that were not antidromically activated (NAA) by stimulation of the vagus nerve and the C2-C3 spinal cord. Substantial immunoreactivity for NMDA receptors was presented in virtually all types of neurons, but lesser reactivity in aug-E bulbospinal neurons. These results suggest that the aug-I, post-I, and aug-E types of NAA neurons are gamma-aminobutyric acid (GABA)ergic and that NMDA receptors distribute in lesser degree in aug-E bulbospinal neurons than in other types of VRG neurons.

Relative magnitude of tonic and phasic synaptic excitation of medullary inspiratory neurons in dogs

The relative contribution of phasic and tonic excitatory synaptic drives to the augmenting discharge patterns of inspiratory (I) neurons within the ventral respiratory group (VRG) was studied in anesthetized, ventilated, paralyzed, and vagotomized dogs. Multibarrel micropipettes were used to record simultaneously single-unit neuronal activity and pressure microejected antagonists of GABAergic, glycinergic, N-methyl-D-aspartate (NMDA) and non-NMDA glutamatergic, and cholinergic receptors. The discharge patterns were quantified via cycle-trigger histograms. The findings suggest that two-thirds of the excitatory drive to caudal VRG I neurons is tonic and mediated by NMDA receptors and the other third is ramp-like phasic and mediated by non-NMDA receptors. Cholinergic receptors do not appear to be involved. The silent expiratory phase is produced by phasic inhibition of the tonic activity, and approximately 80% of this inhibition is mediated by gamma-aminobutyric acid receptors (GABA(A)) and approximately 20% by glycine receptors. Phasic I inhibition by the I decrementing neurons does not appear to contribute to the predominantly step-ramp patterns of these I neurons. However, this decrementing inhibition may be very prominent in controlling the rate of augmentation in late-onset I neurons and those with ramp patterns lacking the step component.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.

Reconfiguration of the neural network controlling multiple breathing patterns: eupnea, sighs and gasps [see comment]

Are different forms of breathing derived from one or multiple neural networks? We demonstrate that brainstem slices containing the pre-Bötzinger complex generated two rhythms when normally oxygenated, with striking similarities to eupneic ('normal') respiration and sighs. Sighs were triggered by eupneic bursts under control conditions, but not in the presence of strychnine (1 microM). Although all neurons received synaptic inputs during both activities, the calcium channel blocker cadmium (4 microM) selectively abolished sighs. In anoxia, sighs ceased, and eupneic activity was reconfigured into gasping, which like eupnea was insensitive to 4 microM cadmium. This reconfiguration was accompanied by suppression of synaptic inhibition. We conclude that a single medullary network underlies multiple breathing patterns.