Distribution and colocalization of neurotransmitters and receptors in the pre-Bötzinger complex of rats

Human IPSC-Derived PreBötC-Like Neurons and Development of an Opiate Overdose and Recovery Model

Opioid overdose is the leading cause of drug overdose lethality, posing an urgent need for investigation. The key brain region for inspiratory rhythm regulation and opioid-induced respiratory depression (OIRD) is the preBötzinger Complex (preBötC) and current knowledge has mainly been obtained from animal systems. This study aims to establish a protocol to generate human preBötC neurons from induced pluripotent cells (iPSCs) and develop an opioid overdose and recovery model utilizing these iPSC-preBötC neurons. A de novo protocol to differentiate preBötC-like neurons from human iPSCs is established. These neurons express essential preBötC markers analyzed by immunocytochemistry and demonstrate expected electrophysiological responses to preBötC modulators analyzed by patch clamp electrophysiology. The correlation of the specific biomarkers and function analysis strongly suggests a preBötC-like phenotype. Moreover, the dose-dependent inhibition of these neurons' activity is demonstrated for four different opioids with identified IC50's comparable to the literature. Inhibition is rescued by naloxone in a concentration-dependent manner. This iPSC-preBötC mimic is crucial for investigating OIRD and combating the overdose crisis and a first step for the integration of a functional overdose model into microphysiological systems.

Alterations in synapses and mitochondria induced by acute or chronic intermittent hypoxia in the pre-Bötzinger complex of rats: an ultrastructural triple-labeling study with immunocytochemistry and histochemistry

Introduction

The pre-Bötzinger complex (pre-BötC), a kernel of inspiratory rhythmogenesis, is a heterogeneous network with excitatory glutamatergic and inhibitory GABAergic and glycinergic neurons. Inspiratory rhythm generation relies on synchronous activation of glutamatergic neuron, whilst inhibitory neurons play a critical role in shaping the breathing pattern, endowing the rhythm with flexibility in adapting to environmental, metabolic, and behavioral needs. Here we report ultrastructural alterations in excitatory, asymmetric synapses (AS) and inhibitory, symmetric synapses (SS), especially perforated synapses with discontinuous postsynaptic densities (PSDs) in the pre-BötC in rats exposed to daily acute intermittent hypoxia (dAIH) or chronic (C) IH.

Methods

We utilized for the first time a combination of somatostatin (SST) and neurokinin 1 receptor (NK1R) double immunocytochemistry with cytochrome oxidase histochemistry, to reveal synaptic characteristics and mitochondrial dynamic in the pre-BötC.

Results

We found perforated synapses with synaptic vesicles accumulated in distinct pools in apposition to each discrete PSD segments. dAIH induced significant increases in the PSD size of macular AS, and the proportion of perforated synapses. AS were predominant in the dAIH group, whereas SS were in a high proportion in the CIH group. dAIH significantly increased SST and NK1R expressions, whereas CIH led to a decrease. Desmosome-like contacts (DLC) were characterized for the first time in the pre-BötC. They were distributed alongside of synapses, especially SS. Mitochondria appeared in more proximity to DLC than synapses, suggestive of a higher energy demand of the DLC. Findings of single spines with dual AS and SS innervation provide morphological evidence of excitation-inhibition interplay within a single spine in the pre-BötC. In particular, we characterized spine-shaft microdomains of concentrated synapses coupled with mitochondrial positioning that could serve as a structural basis for synchrony of spine-shaft communication. Mitochondria were found within spines and ultrastructural features of mitochondrial fusion and fission were depicted for the first time in the pre-BötC.

Conclusion

We provide ultrastructural evidence of excitation-inhibition synapses in shafts and spines, and DLC in association with synapses that coincide with mitochondrial dynamic in their contribution to respiratory plasticity in the pre-BötC.

Phase- and state-dependent modulation of breathing pattern by preBötzinger complex somatostatin expressing neurons

As neuronal subtypes are increasingly categorized, delineating their functional role is paramount. The preBötzinger complex (preBötC) subpopulation expressing the neuropeptide somatostatin (SST) is classified as mostly excitatory, inspiratory-modulated and not rhythmogenic. We further characterized their phenotypic identity: 87% were glutamatergic and the balance were glycinergic and/or GABAergic. We then used optogenetics to investigate their modulatory role in both anaesthetized and freely moving mice. In anaesthetized mice, short photostimulation (100 ms) of preBötC SST⁺ neurons modulated breathing-related variables in a combinatory phase- and state-dependent manner; changes in inspiratory duration, inspiratory peak amplitude (Amp), and phase were different at higher (≥2.5 Hz) vs. lower (<2.5 Hz) breathing frequency (f). Moreover, we observed a biphasic effect of photostimulation during expiration that is probabilistic, that is photostimulation given at the same phase in consecutive cycles can evoke opposite responses (lengthening vs. shortening of the phase). These unexpected probabilistic state- and phase-dependent responses to photostimulation exposed properties of the preBötC that were not predicted and cannot be readily accounted for in current models of preBötC pattern generation. In freely moving mice, prolonged photostimulation decreased f in normoxia, hypoxia or hypercapnia, and increased Amp and produced a phase advance, which was similar to the results in anaesthetized mice when f ≥ 2.5 Hz. We conclude that preBötC SST⁺ neurons are a key mediator of the extraordinary and essential lability of breathing pattern. KEY POINTS: PreBötzinger complex (preBötC) SST⁺ neurons, which modulate respiratory pattern but are not rhythmogenic, were transfected with channelrhodopsin to investigate phase- and state-dependent modulation of breathing pattern in anaesthetized and freely behaving mice in normoxia, hypoxia and hypercapnia. In anaesthetized mice, photostimulation during inspiration increased inspiratory duration and amplitude regardless of baseline f, yet the effects were more robust at higher f. In anaesthetized mice with low f (<2.5 Hz), photostimulation during expiration evoked either phase advance or phase delay, whereas in anaesthetized mice with high f (≥2.5 Hz) and in freely behaving mice in normoxia, hypoxia or hypercapnia, photostimulation always evoked phase advance. Phase- and state-dependency is a function of overall breathing network excitability. The f-dependent probabilistic modulation of breathing pattern by preBötC SST⁺ neurons was unexpected, requiring reconsideration of current models of preBötC function, which neither predict nor can readily account for such responses.

CX3CR1 mutation alters synaptic and astrocytic protein expression, topographic gradients, and response latencies in the auditory brainstem

The precise and specialized circuitry in the auditory brainstem develops through adaptations of cellular and molecular signaling. We previously showed that elimination of microglia during development impairs synaptic pruning that leads to maturation of the calyx of Held, a large encapsulating synapse that terminates on neurons of the medial nucleus of the trapezoid body (MNTB). Microglia depletion also led to a decrease in glial fibrillary acidic protein (GFAP), a marker for mature astrocytes. Here, we investigated the role of signaling through the fractalkine receptor (CX3CR1), which is expressed by microglia and mediates communication with neurons. CX3CR1^-/- and wild-type mice were studied before and after hearing onset and at 9 weeks of age. Levels of GFAP were significantly increased in the MNTB in mutants at 9 weeks. Pruning was unaffected at the calyx of Held, but we found an increase in expression of glycinergic synaptic marker in mutant mice at P14, suggesting an effect on maturation of inhibitory inputs. We observed disrupted tonotopic gradients of neuron and calyx size in MNTB in mutant mice. Auditory brainstem recording (ABR) revealed that CX3CR1^-/- mice had normal thresholds and amplitudes but decreased latencies and interpeak latencies, particularly for the highest frequencies. These results demonstrate that disruption of fractalkine signaling has a significant effect on auditory brainstem development. Our findings highlight the importance of neuron-microglia-astrocyte communication in pruning of inhibitory synapses and establishment of tonotopic gradients early in postnatal development.

Breathing matters

Breathing is a well-described, vital and surprisingly complex behaviour, with behavioural and physiological outputs that are easy to directly measure. Key neural elements for generating breathing pattern are distinct, compact and form a network amenable to detailed interrogation, promising the imminent discovery of molecular, cellular, synaptic and network mechanisms that give rise to the behaviour. Coupled oscillatory microcircuits make up the rhythmic core of the breathing network. Primary among these is the preBötzinger Complex (preBötC), which is composed of excitatory rhythmogenic interneurons and excitatory and inhibitory pattern-forming interneurons that together produce the essential periodic drive for inspiration. The preBötC coordinates all phases of the breathing cycle, coordinates breathing with orofacial behaviours and strongly influences, and is influenced by, emotion and cognition. Here, we review progress towards cracking the inner workings of this vital core.

Pre-Bötzinger complex: Generation and modulation of respiratory rhythm

INTRODUCTION

In mammals, the preBötzinger complex (preBötC) is a bilateral and symmetrical neural network located in the brainstem which is essential for the generation and modulation of respiratory rhythm. There are few human studies about the preBötC and, its relationship with neurological diseases has not been described. However, the importance of the preBötC in neural control of breathing and its potential participation in neurological diseases in humans, has been suggested based on pharmacological manipulation and lesion of the preBötC in animal models, both in vivo and in vitro.

METHOD

In this review, we describe the effects of some drugs on the inspiratory activity in vitro in a transverse slice that contains the preBötC, as well as some in vivo experiments. Drugs were classified according to their effects on the main neurotransmitter systems and their importance as stimulators or inhibitors of preBötC activity and therefore for the generation of the respiratory rhythm.

CONCLUSION

Clinical neurologists will find this information relevant to understanding how the central nervous system generates the respiratory rhythm and may also relate this information to the findings made in daily practice.

Blockade of neurokinin-1 receptors in the ventral respiratory column does not affect breathing but alters neurochemical release

Substance P (SP) and its receptor, neurokinin-1 (NK1R), have been shown to be excitatory modulators of respiratory frequency and to stabilize breathing regularity. Studies in anesthetized mice suggest that tonic activation of NK1Rs is particularly important when other excitatory inputs to the pre-Bötzinger complex in the ventral respiratory column (VRC) are attenuated. Consistent with these findings, muscarinic receptor blockade in the VRC of intact goats elicits an increase in breathing frequency associated with increases in SP and serotonin concentrations, suggesting an involvement of these substances in neuromodulator compensation. To gain insight on the contribution to breathing of endogenous SP and NK1R activation, and how NK1R modulates the release of other neurochemicals, we individually dialyzed antagonists to NK1R (133, 267, 500 μM Spantide; 3 mM RP67580) throughout the VRC of awake and sleeping goats. We found that NK1R blockade with either Spantide at any dose or RP67580 had no effect on breathing or regularity. Both antagonists significantly (P < 0.001) increased SP, while RP67580 also increased serotonin and glycine and decreased thyrotropin-releasing hormone concentrations in the dialysate. Taken together, these data support the concept of neuromodulator interdependence, and we believe that the loss of excitatory input from NK1Rs was locally compensated by changes in other neurochemicals.

Glycinergic inhibition to the medial nucleus of the trapezoid body shows prominent facilitation and can sustain high levels of ongoing activity

Neurons in the medial nucleus of the trapezoid body (MNTB) are well known for their prominent excitatory inputs, mediated by the calyx of Held. Less attention has been paid to the prominent inhibitory inputs that MNTB neurons also receive. Because of their auditory nature, both excitatory and inhibitory synapses are highly active in vivo. These high levels of activity are known to reduce excitatory synaptic currents considerably, such that in vivo synaptic currents produced by the calyx are smaller than typically measured in standard brain slice experiments. The goal of this study was to investigate the properties of the inhibitory inputs in the Mongolian gerbil (Meriones unguiculatus) under activity levels that correspond to those in the intact brain to facilitate a direct comparison between the two inputs. Our results suggest that inhibitory inputs to MNTB are largely mediated by a fast and phasic glycinergic component, and to a lesser degree by a GABAergic component. The glycinergic component can sustain prolonged high levels of activity. Even when challenged with stimulus patterns consisting of thousands of stimuli over tens of minutes, glycinergic inputs to MNTB maintain large conductances and fast decays and even facilitate substantially when the stimulation frequency is increased. The inhibition is mediated by a relatively small number of independent input fibers. The data presented here suggest that inhibitory inputs to MNTB sustain high levels of activity and need to be considered for a full understanding of mechanisms underlying processing of auditory information in MNTB.

Morphological characterization of respiratory neurons in the pre-Bötzinger complex

Although the pre-Bötzinger complex (preBötC) was defined as the inspiratory rhythm generator long ago, the functional-anatomical characterization of its neuronal components is still being achieved. Recent advances have identified the expression of molecular markers in the preBötC neurons that, however, are not exclusive to specific respiratory neuron subtypes and have not always been related to specific cell morphologies. Here, we evaluated the morphology and the axonal projections of electrophysiologically defined respiratory neurons in the preBötC using whole-cell recordings and intracellular biocytin labeling. We found that respiratory pacemaker neurons are larger than expiratory neurons and that inspiratory neurons are smaller than pacemaker and expiratory neurons. Other morphological features such as somata shapes or dendritic branching patterns were not found to be significantly different among the preBötC neurons sampled. We also found that both pacemaker and inspiratory nonpacemaker neurons, but not expiratory neurons, show extensive axonal projections to the contralateral preBötC and show signs of electrical coupling. Overall, our data suggest that there are morphological differences between subtypes of preBötC respiratory neurons. It will be important to take such differences in consideration since morphological differences would influence synaptic responses and action potential propagation.

Apnea of prematurity

Apnea of prematurity is a significant problem due to immaturity of the central neural control circuitry responsible for integrating afferent input and central rhythm. In this review, we provide an overview of the pathogenesis of apnea of prematurity--including our current understanding of the role that afferent input to the brain stem plays in synergy with the central pattern generation circuitry in the emergence of apnea of prematurity. We then discuss the interplay of apnea, bradycardia, desaturation, as well as the genesis of central, mixed, and obstructive apnea. Finally, we provide a summary of the physiological basis for current therapeutic approaches to treating apnea of prematurity, and conclude with an overview of proposed long-term consequences of the resultant intermittent hypoxic episodes.

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.

Airway inflammation and central respiratory control: results from in vivo and in vitro neonatal rat

In infants, respiratory infection elicits tachypnea. To begin to evaluate the role of brainstem cytokine expression in modulation of breathing pattern changes, we compared the pattern generated after endotracheal instillation of lipopolysaccharide (LPS) in in vivo rat pups to local pro-inflammatory cytokine injection in the nucleus tractus solitarius (nTS) in an in vitro en bloc brainstem spinal cord preparation. We hypothesized that both challenges would elicit similar changes in patterning of respiration. In anesthetized, spontaneously breathing rat pups, lipopolysaccharide (LPS) or saline was instilled in the airway of urethane-anesthetized rats (postnatal days 10-11). We recorded diaphragm EMG over the subsequent 2h and saw a 20-30% decrease in interburst interval (Te) at 20-80min post-injection in LPS-instilled animals with no significant change in Ti. In contrast, IL-1β injections into the nTS of en bloc in vitro brainstem-spinal cord preparations from 0 to 5 day-old pups maintained Ti and caused an increase in Te as early as 20min later, decreasing frequency for 80-120min after injection. Our results suggest that the neonatal respiratory response to the cytokine IL-1β mediated inflammatory response depends on the site of the inflammatory stimulus and that the direct effect of IL-1β in the nTS is to slow rather than increase rate.

Nitric oxide synthase expression in the medullary respiratory related nuclei and its involvement in CO-mediated central respiratory effects in neonatal rats

The present study was conducted in order to observe the potential participation of the nitric oxide synthase-NO pathway in CO-mediated regulation of respiration of neonatal rats. An immunofluorescent histochemical technique was used to examine the existence of the neuronal nitric oxide synthase, a key enzyme of synthesizing NO, in medullary respiratory nuclei. The rhythmic respiratory-like discharges of hypoglossal rootlets of medullary slices were recorded to test the role of the nitric oxide synthase in CO-mediated respiratory effects. We observed neuronal nitric oxide synthase expressed in the medullary respiratory nuclei in conjunction with CO lengthened expiratory duration, decreased respiratory frequency, and increased inspiratory amplitude. These CO-mediated respiratory effects could be partially eliminated by prior treatment of the slices with Nω-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase. The results suggest that nitric oxide synthase-NO pathway might be involved in the CO-mediated central regulation of respiration at the level of medulla oblongata in neonatal rats.

Effect of chronic pain on morphine-induced respiratory depression in mice

Respiratory depression is the most well-known and dangerous side-effect of opioid analgesics. Clinical investigations have revealed that this opioid-induced respiratory depression is less severe in patients with chronic pain, but the mechanisms that underlie this phenomenon are unknown. Therefore, the present study was designed to examine the influence of chronic pain on morphine-induced respiratory depression. Respiration was detected by double-chamber, flow-through whole-body plethysmography. Respiratory frequency was dose-dependently and significantly decreased after morphine administration. This effect peaked at 30 min after administration and lasted 3 h. In contrast, tidal volume was increased. Minute volume was significantly decreased by morphine at a higher dose, but not a lower dose. In nerve-ligated mice, a morphine-induced decrease in respiratory frequency was observed, whereas the increase of tidal volume was more prominent. A decrease in minute volume was not observed in nerve-ligated mice. This attenuation of the morphine-induced decrease in minute volume in nerve-ligated mice was reversed by treatment with the serotonin (5-HT)4a receptor antagonist GR125487. Moreover, treatment with the 5-HT4 receptor agonist mosapride antagonized the morphine-induced decrease in minute volume, due to the enhancement of tidal volume. Finally, the expression of 5-HT4a receptor in the brainstem was enhanced in nerve-ligated mice compared to that in sham-operated mice. These results suggest that the decrease in morphine-induced respiratory depression under chronic pain is mediated by the enhancement of 5-HT4a receptor systems in the brainstem.

Hypercapnia causes cellular oxidation and nitrosation in addition to acidosis: implications for CO2 chemoreceptor function and dysfunction

Cellular mechanisms of CO2 chemoreception are discussed and debated in terms of the stimuli produced during hypercapnic acidosis and their molecular targets: protons generated by the hydration of CO2 and dissociation of carbonic acid, which target membrane-bound proteins and lipids in brain stem neurons. The CO2 hydration reaction, however, is not the only reaction that CO2 undergoes that generates molecules capable of modifying proteins and lipids. Molecular CO2 also reacts with peroxynitrite (ONOO-), a reactive nitrogen species (RNS), which is produced from nitric oxide (*NO) and superoxide (*O2-). The CO2/ONOO- reaction, in turn, produces additional nitrosative and oxidative reactive intermediates. Furthermore, protons facilitate additional redox reactions that generate other reactive oxygen species (ROS). ROS/RNS generated by these redox reactions may act as additional stimuli of CO2 chemoreceptors since neurons in chemosensitive areas produce both *NO and *O2- and, therefore, ONOO-. Perturbing *NO, *O2-, and ONOO- activities in chemosensitive areas modulates cardiorespiration. Moreover, neurons in at least one chemosensitive area, the solitary complex, are stimulated by cellular oxidation. Together, these data raise the following two questions: 1) do pH and ROS/RNS work in tandem to stimulate CO2 chemoreceptors during hypercapnic acidosis; and 2) does nitrosative stress and oxidative stress contribute to CO2 chemoreceptor dysfunction? To begin considering these two issues and their implications for central chemoreception, this minireview has the following three goals: 1) summarize the nitrosative and oxidative reactions that occur during hypercapnic acidosis and isocapnic acidosis; 2) review the evidence that redox signaling occurs in chemosensitive areas; and 3) review the evidence that neurons in the solitary complex are stimulated by cellular oxidation.

Respiratory responses induced by blockades of GABA and glycine receptors within the Bötzinger complex and the pre-Bötzinger complex of the rabbit

The respiratory role of GABA(A), GABA(B) and glycine receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (preBötC) was investigated in alpha-chloralose-urethane anesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30-50 nl) of GABA and glycine receptor agonists and antagonists. GABA(A) receptor blockade by bicuculline (5mM) or gabazine (2mM) within the BötC induced strong depression of respiratory activity up to apnea. The latter was reversed by hypercapnia. Glycine receptor blockade by strychnine (5mM) within the BötC decreased the frequency and amplitude of phrenic bursts. Bicuculline microinjections into the preBötC caused decreases in respiratory frequency and the appearance of two alternating different levels of peak phrenic activity. Strychnine microinjections into the preBötC increased respiratory frequency and decreased peak phrenic amplitude. GABA(A), but not glycine receptor antagonism within the preBötC restored respiratory rhythmicity during apnea due to bicuculline or gabazine applied to the BötC. GABA(B) receptor blockade by CGP-35348 (50mM) within the BötC and the preBötC did not affect baseline respiratory activity, though microinjections of the GABA(B) receptor agonist baclofen (1mM) into the same regions altered respiratory activity. The results show that only GABA(A) and glycine receptors within the BötC and the preBötC mediate a potent control on both the intensity and frequency of inspiratory activity during eupneic breathing. This study is the first to provide evidence that these inhibitory receptors have a respiratory function within the BötC.

Kv3.3 immunoreactivity in the vestibular nuclear complex of the rat with focus on the medial vestibular nucleus: targeting of Kv3.3 neurones by terminals positive for vesicular glutamate transporter 1

Kv3 voltage-gated K(+) channels are important in shaping neuronal excitability and are abundant in the CNS, with each Kv3 gene exhibiting a unique expression pattern. Mice lacking the gene encoding for the Kv3.3 subunit exhibit motor deficits. Furthermore, mutations in this gene have been linked to the human disease spinocerebellar ataxia 13, associated with cerebellar and extra-cerebellar symptoms such as imbalance and nystagmus. Kv subunit localisation is important in defining their functional roles and thus, we investigated the distribution of Kv3.3-immunoreactivity in the vestibular nuclear complex of rats with particular focus on the medial vestibular nucleus (MVN). Kv3.3-immunoreactivity was widespread in the vestibular nuclei and was detected in somata, dendrites and synaptic terminals. Kv3.3-immunoreactivity was observed in distinct neuronal populations and dual labelling with the neuronal marker NeuN revealed 28.5+/-1.9% of NeuN labelled MVN neurones were Kv3.3-positive. Kv3.3-immunoreactivity co-localised presynaptically with the synaptic vesicle marker SV2, parvalbumin, the vesicular glutamate transporter VGluT2 and the glycine transporter GlyT2. VGluT1 terminals were scarce within the MVN (2.5+/-1.1 per 50 microm(2)) and co-localisation was not observed. However, 85.4+/-9.4% of VGluT1 terminals targeted and enclosed Kv3.3-immunoreactive somata. Presynaptic Kv3.3 co-localisation with the GABAergic marker GAD67 was also not observed. Cytoplasmic GlyT2 labelling was observed in a subset of Kv3.3-positive neurones. Electron microscopy confirmed a pre- and post-synaptic distribution of the Kv3.3 protein. This study provides evidence supporting a role for Kv3.3 subunits in vestibular processing by regulating neuronal excitability pre- and post-synaptically.

Identification of neurotransmitters and co-localization of transmitters in brainstem respiratory neurons

Identifying the major ionotropic neurotransmitter in a respiratory neuron is of critical importance in determining how the neuron fits into the respiratory system, whether in producing or modifying respiratory drive and rhythm. There are now several groups of respiratory neurons whose major neurotransmitters have been identified and in some of these cases, more than one transmitter has been identified in particular neurons. This review will describe the physiologically identified neurons in major respiratory areas that have been phenotyped for major ionotropic transmitters as well as those where more than one transmitter has been identified. Although the purpose of the additional transmitter has not been elucidated for any of the respiratory neurons, some examples from other systems will be discussed.

Glycine receptors in the striatum, globus pallidus, and substantia nigra of the human brain: An immunohistochemical study

Glycine receptors (GlyRs) are heteropentameric chloride ion channels that facilitate fast-response, inhibitory neurotransmission in the mammalian spinal cord and brain. GlyRs have four functional subunits, alpha1-3 and beta, which likely exist in heteromeric alphabeta combinations. Mutations in GlyR alpha1 and beta subunits are well known for their involvement in hyperekplexia, a paroxysmal motor disorder. In this study we present the first detailed immunohistochemical investigation at the regional, cellular, and subcellular levels of GlyRs in the human basal ganglia. The results show that GlyRs are present at the regional level in low concentrations in the striatum and globus pallidus and are present in the highest concentrations in the substantia nigra. At the cellular level, GlyRs are present only in discrete populations of neurons immunoreactive for choline acetyltransferase (ChAT), parvalbumin, and calretinin in the human striatum, on a subpopulation of parvalbumin- and calretinin-positive neurons in the globus pallidus, and in the substantia nigra GlyRs are present on approximately three-fourths of all pars compacta and one-third of all pars reticulata neurons. They also form a distinct band of immunoreactive neurons in the intermedullary layers of the globus pallidus. At the subcellular level in the substantia nigra pars reticulata (SNr), GlyRs show a localized distribution on the soma and dendrites that partially complements but does not overlap with the distribution of gamma-aminobutyric acid (GABA)A receptors. Our results demonstrate the precise cellular and subcellular localization of GlyRs in the human basal ganglia and suggest that glycinergic receptors may play an important complementary role to other inhibitory receptors in modulating cholinergic, dopaminergic, and GABAergic neuronal pathways in the basal ganglia.

Glycine transporter 1 expression in the ventral respiratory group is restricted to protoplasmic astrocytes

The lack of the glycine transporter 1 (GlyT1) leads to early postnatal death due to failure of respiratory network activity. Here we demonstrate a segregated expression of GlyT1 on different astroglial cell populations of the ventral respiratory group. In TgN(hGFAP-EGFP) mice a combined immunohistochemical and electrophysiological approach was used to define the cellular expression of GlyT1 in the respiratory network. EGFP-labeled cells with outwardly rectifying current-voltage relationship did not express glycine transporter 1, while GlyT1 was abundantly expressed in mature protoplasmic astrocytes, which are electrophysiologically characterized by a large potassium conductance, a more negative membrane potential and the expression of glutamate transporters. Taken together, the vital capacity for the clearance of extracellular glycine is restricted to a subpopulation of astroglial cells.

Immunohistochemical localization of GAD67-expressing neurons and processes in the rat brainstem: subregional distribution in the nucleus tractus solitarius

The role of gamma-aminobutyric acid (GABA) in homeostatic control in the brainstem, in particular, in the nucleus tractus solitarius (NTS), is well established. However, to date, there is no detailed description of the distribution of GABAergic neurons within the NTS. The goal of the current study was to reexamine the efficacy of immunohistochemical localization of glutamic acid decarboxylase (GAD) protein, specifically the 67-kDa isoform (GAD67), as a marker for GABAergic neurons in the medulla and to provide a detailed map of GAD67-immunoreactive (-ir) cells within rat NTS by using a recently developed mouse monoclonal antibody. We describe a distribution of GAD67-ir cells in the medulla similar to that reported previously from in situ hybridization study. GAD67-ir cells were localized in regions known to contain high GABA content, including the ventrolateral medulla, raphe nuclei, and area postrema, but were absent from all motor nuclei, although dense terminal labeling was discerned in these regions. In the NTS, GAD67-ir was localized in all subregions. Semiquantitative analysis of the GAD67-ir distribution in the NTS revealed greater numbers of GAD67-ir cells medial to the solitary tract. Finally, dense GAD67 terminal labeling was found in the medial, central, intermediate, commissural, and subpostremal subregions, whereas sparse labeling was observed in the ventral subregion. Our findings support the use of immunohistochemistry for GAD67 as a marker for the localization of GABAergic cells and terminal processes in the rat brainstem. Furthermore, the reported heterogeneous distribution of GAD67-ir in the NTS suggests differential inhibitory modulation of sensory processing.

Hypotension and short-term anaesthesia induce ERK1/2 phosphorylation in autonomic nuclei of the brainstem

The aims of this study were: first, to investigate the effects of anaesthesia on phosphorylated extracellular signal-regulated kinase (p-ERK)1/2-immunoreactivity (-ir) in the brainstem; second, to choose the best anaesthetic for p-ERK1/2 studies; and third, to determine the effect of short-term hypotension on p-ERK1/2-ir in the brainstem. Rats were anaesthetized with halothane, sodium pentobarbital or 100% CO2 narcosis, or were cervically dislocated and within 5 min perfused and the brains processed immunohistochemically for pERK1/2-ir. p-ERK1/2-ir was primarily observed in regions associated with cardiovascular and/or respiratory control. Several regions consistently showed dense p-ERK1/2 labelling, including a restricted region of the ventrolateral medulla (VLM). In contrast, other regions showed differential labelling depending on the mode of death. Cervical dislocation showed the least VLM labelling, limited to a discrete area approximately 0.6-1.4 mm caudal to the facial nucleus. Anaesthetics induced labelling throughout the VLM, with halothane inducing the most. Many p-ERK1/2-ir VLM neurons were catecholaminergic following halothane or sodium pentobarbital anaesthesia, but no double labelling was seen following cervical dislocation. Of the anaesthetics, sodium pentobarbital induced the least labelling and was used subsequently. Intravenous hydralazine was used to induce a 20-min period of hypotension, whereas arterial pressure did not change in vehicle-treated animals. Hydralazine evoked more pERK-ir neurons in specific regions, including the VLM, nucleus tractus solitarius (NTS), parabrachial nuclei, Kolliker-Fuse nucleus and locus coeruleus. Approximately twice as many p-ERK1/2-positive neurons were seen in the intermediate NTS and rostral VLM following hydralazine compared with the vehicle. In conclusion, p-ERK1/2-ir identifies neurons in central autonomic regions, and their number and distribution are markedly affected by anaesthetics, and are increased in some regions by short-term hypotension.

A group of neurokinin-1 receptor-immunoreactive neurons expressing phospho-extracellular signal-regulated protein kinases in the pre-Bötzinger complex of rats

Extracellular-signal-regulated kinases (ERKs) were described originally as critical regulators of cell growth and differentiation. ERKs have received increasing attention as important regulators of neuronal function, especially in synaptic plasticity and memory. We found a group of phospho-ERK1/2 (pERK1/2)-immunoreactive (ir) neurons in the ventrolateral medulla oblongata of adult rats. In the pre-Botzinger complex (pre-BotC) region, identified by neurokinin-1 receptor (NK1R) immunoreactivity, the distribution of pERK1/2-ir neurons seemed to overlap with that of NK1R-ir neurons. A detailed study combining immunofluorescence and immunogold-silver staining was designed to determine whether ERK-ir neurons were NK1R-ir and related to respiratory function in the pre-BotC. The pERK1/2 immunoreactivity was present mainly in somas and processes of small- to medium-sized neurons. Of pERK1/2-ir neurons in the pre-BotC, 19.7% were NK1R-ir. Some of the small, fusiform, NK1R-ir neurons, the putative rhythmogenic neurons, were also pERK1/2-ir. Under the electron microscope, immunogold particles indicative of pERK1/2 immunoreactivity were found in somas and dendrites in pre-BotC neurons. They were associated mainly with rough endoplasmic reticula and ribosomes. Immunogold particles could also be found in nuclei, but were absent from nucleoli. The existence of pERK1/2 in a subtype of NK1R-ir pre-BotC neurons implies that the pERK cascade may play a role in normal respiratory activity in adult rats. The pERK cascade may also participate in sympathoexcitatory activity, given that 42.2% of pERK1/2-ir neurons expressed tyrosine hydroxylase, a marker of C1 adrenergic neurons in the ventrolateral medulla oblongata.

Hypercapnic and hypoxic responses require intact neural transmission from the pre-Bötzinger complex

The central respiratory network that includes the pre-Bötzinger complex (pre-BötC), a region believed to contain rhythmogenic neurons, is capable of responding to fluctuations in CO2 and pH. However, the role of inputs from this site in mediating ventilatory responses to hypercapnia and/or hypoxia in nonsedated animals is not well established. Therefore, in the present study we tested the hypothesis that altered transmission from the pre-BötC to its target sites would decrease chemosensory responsiveness to acute hypercapnia and modulate the ventilatory response to hypoxia. Colchicine was used to block axonal transport. At 48 h after bilateral microinjections of colchicine into the pre-BötC (100 microg/uL, 100 nL/site), but not saline, the baseline frequency of breathing decreased; however, rhythmicity was not altered. In addition, there was a significant fall in the ventilatory response to hypercapnia (5 and 12% CO2) and hypoxia (8% O2). These findings indicate that, inputs from pre-BötC neurons are of critical importance in providing the normal ventilatory response to both hypercapnia and hypoxia.

Respiratory responses evoked by blockades of ionotropic glutamate receptors within the Bötzinger complex and the pre-Bötzinger complex of the rabbit

The respiratory role of excitatory amino acid (EAA) receptors within the Bötzinger complex (BötC) and the pre-Bötzinger complex (pre-BötC) was investigated in alpha-chloralose-urethane anaesthetized, vagotomized, paralysed and artificially ventilated rabbits by using bilateral microinjections (30-50 nL) of EAA receptor antagonists. Blockade of both N-methyl-D-aspartic acid (NMDA) and non-NMDA receptors by 50 mM kynurenic acid (KYN) within the BötC induced a pattern of breathing characterized by low-amplitude, high-frequency irregular oscillations superimposed on tonic phrenic activity and successively the disappearance of respiratory rhythmicity in the presence of intense tonic inspiratory discharges (tonic apnea). KYN microinjections into the pre-BötC caused similar respiratory responses that, however, never led to tonic apnea. Blockade of NMDA receptors by D(-)-2-amino-5-phosphonopentanoic acid (D-AP5; 1, 10 and 20 mM) within the BötC induced increases in respiratory frequency and decreases in peak phrenic amplitude; the highest concentrations caused tonic apnea insensitive to chemical stimuli. Blockade of non-NMDA receptors by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 1, 10 and 20 mM) within the BötC produced only less pronounced increases in respiratory frequency. Responses to D-AP5 in the pre-BötC were similar, although less pronounced than those elicited in the BötC and never characterized by tonic apnea. In the same region, CNQX provoked increases in respiratory frequency similar to those elicited in the BötC, associated with slight reductions in peak phrenic activity. The results show that EAA receptors within the investigated medullary subregions mediate a potent control on both the intensity and frequency of inspiratory activity, with a major role played by NMDA receptors.

Substance P and enkephalinergic synapses onto neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats

Our previous studies have demonstrated that neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons in the pre-Bötzinger Complex (pre-BötC), the hypothesized kernel of respiratory rhythmogenesis, receive both glutamatergic excitatory and GABAergic or glycinergic inhibitory inputs. Neuromodulators, such as substance P (SP) and opioids, play important roles in normal respiratory activity and respiratory disorders. The identification of the relationship between neurotransmitters and NK1R-ir neurons at the cellular level is essential for understanding the synaptic interaction within the pre-BötC network. Using immunofluorescence and immunogold-silver staining, we wished to exploit SP and enkephalin (ENK) immunoreactivity and their relationships with glutamate, GABA, glycine, or NK1R in the pre-BötC in adult Sprague-Dawley rats. The pre-BötC contained a substantial amount of SP-ir and ENK-ir boutons. They were largely colocalized with glutamate and much less so with GABA. Glycine immunoreactivity was rarely found in either SP-ir or ENK-ir boutons. A number of SP-ir boutons were ENK-ir as well. Synapses were commonly found between SP-ir or ENK-ir terminals and NK1R-ir neurons in the pre-BötC. Most of them were asymmetric. Symmetric synapses made up 10% of all synapses examined between SP-ir boutons and NK1R-ir neurons, and 19% of ENK/NK1R synapses. Colocalization of SP and/or ENK with glutamate in boutons in the pre-BötC implies the combined synaptic release of excitatory amino acid and neuropeptides, which may exert combined post-synaptic effects onto NK1R-ir neurons and contribute to respiratory activity.

Ontogeny of the pre-Bötzinger complex in perinatal rats

The aim of this study was to provide a systematic examination of the ontogenesis of the mammalian respiratory rhythm generating center, the pre-Bötzinger complex (pre-BötC). A combination of immunohistochemical markers and electrophysiological recordings was used to determine the time of inception of the pre-BötC and the developmental changes during the perinatal period in rats spanning from embryonic day 15 (E15) to postnatal day 7. The first clear indication of neurons immunopositive for neurokinin-1 receptors (NK1Rs) and somatostatin expression, two proposed markers for pre-BötC neurons, was at approximately E17. Birth dating of neurons in the ventrolateral medulla using 5-bromo-2'-deoxyuridine demonstrated that NK1R-positive neurons populating the area of the pre-BötC during late E16-E18 are born at E12.5-E13.5, approximately 2 d later than adjacent NK1R-positive neurons in the ventrolateral medulla. Extracellular recordings of neuronal populations within the pre-BötC of perinatal medullary slice preparations demonstrated that the onset of rhythmical respiratory discharge commences at approximately E17. Application of substance P, a ligand for NK1R receptors, to the media bathing E17 medullary slice and brainstem-spinal cord preparations resulted in a marked increase in respiratory frequency. These data provide insights into the ontogeny of the pre-BötC, giving fundamental information on the genesis, settlement, and inception of rhythmic activity within the group of neurons proposed to be responsible for the respiratory rhythm generation. Furthermore, this provides the foundation for further analyses of cell lineage, the transcriptional control of respiratory neuronal development, and electrophysiological and pharmacological properties of the pre-BötC during the prenatal period.

Tachykinins in the control of breathing by hypoxia: pre- and post-genomic era

This article highlights major findings from physiological and pharmacological studies conducted in the pre- and post-genomic era examining the roles of substance P (SP) and other tachykinins in the response of the carotid body to hypoxia, in the ventilatory response to hypoxia and in respiratory rhythm generation. In the post-genomic period, the hypoxic ventilatory responses of mice carrying targeted deletion of genes that affect synthesis or degradation or receptor interaction of SP have been examined by us and also by other investigators. A brief summary of the findings from these investigations will also be presented. The combined observations from the pre- and post-genomic era strongly support the involvement of SP and also other tachykinins in the control of respiration during hypoxia.

Two approaches to double post-embedding immunogold labeling of freeze-substituted tissue embedded in low temperature Lowicryl HM20 resin

Double labeling is used for localizing two antigens simultaneously in the same tissue. We have used two approaches to post-embedding immunogold labeling to investigate whether nerve terminals in the guinea-pig anteroventral cochlear nucleus (AVCN) that contain gamma-aminobutyric acid (GABA) or glycine are capable of retrieving the other amino acid as part of an investigation of colocalization of these putative neurotransmitters. For this, vibroslices of perfusion-fixed brain stem were freeze-substituted and embedded in the low temperature resin, Lowicryl HM20. Simultaneous labeling of ultrathin sections was then performed with a mixture of a rabbit primary antibody to GABA and a guinea-pig primary antibody to the glycine transporter, GLYT2, followed by labeling with a mixture of secondary antibodies (goat anti-rabbit IgG-30 nm gold, goat anti-guinea pig IgG-15 nm gold). This approach indicated that GLYT2 occurs in the plasma membrane of some terminals that contain GABA. The other approach involved sequential labeling of ultrathin sections with a rabbit primary antibody to the GABA transporter, GAT1, followed by an anti-rabbit secondary antibody conjugated to 15-nm gold particles. Sections were then treated with paraformaldehyde vapor to denature any free anti-IgG binding sites on the first antibody, and labeled with a primary antibody to glycine also raised in rabbit followed by an anti-rabbit secondary antibody conjugated to 30-nm gold particles. This approach indicated that GAT1 occurs in the plasma membrane of some terminals that contain glycine. Thus, these techniques can be used to localize heat-labile multiple antigens in the same tissue.

Defining ventral medullary respiratory compartments with a glutamate receptor agonist in the rat

The regional organization of the ventral respiratory group (VRG) was examined with respect to generation of respiratory rhythm (breathing frequency) versus control of the respiratory motor pattern on individual nerves. In urethane-anaesthetized, neuromuscularly blocked and vagotomized Sprague-Dawley rats, arterial blood pressure (ABP) and respiratory motor outputs (phrenic, pharyngeal branch of the vagus, or superior laryngeal nerves) were recorded. The VRG organization was mapped systematically using injections of the excitatory amino acid DL-homocysteic acid (DLH; 5-20 mM, 2-6 nl) from single- or double-barrel pipettes at 100-200 microm intervals between the facial nucleus and the calamus scriptorius. Recording of respiratory neurons through the injection pipette ensured that the pipette was located within the VRG. At the end of each experiment, the injection pipette was used to make an electrical lesion, thereby marking the electrode position for subsequent histological reconstruction of injection sites. Four rostrocaudal regions were identified: (1) a rostral bradypnoea area, at the level of the Bötzinger complex, in which respiratory rhythm slowed and ABP increased, (2) a tachypnoea/dysrhythmia area, at the level of the preBötzinger complex, in which breathing rate either increased or became irregular, with little or no change in ABP, (3) a caudal bradypnoea area at the level of the anterior part of the rostral VRG in which ABP decreased and (4) a caudal 'no effect' region in the posterior part of the rostral VRG. The peak amplitude of phrenic nerve activity decreased with injections into all three rostral regions. Changes in respiratory rhythm were associated with opposite changes in inspiratory (TI) and expiratory (TE) durations after injections into either the Bötzinger complex or anterior rostral VRG, while both TI and TE decreased after injections into the preBötzinger complex. Effects on selected cranial nerves were similar to those on the phrenic nerve except that tonic activity was elicited on the superior larygneal nerve ipsilateral to injections in the Bötzinger complex and on the pharyngeal branch of the vagus ipsilateral to injections in the preBötzinger complex. These data reinforce the subdivision of the VRG into functionally distinct compartments and suggest that a further subdivision of the rostral VRG is warranted. They also suggest that region-specific influences, especially on the pattern of cranial motor discharge, can be used to assist the identification of recording sites within the VRG. However, the postulated clear functional separation of rhythm- versus pattern-generating regions was not supported.

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.

Relationship between two types of vesicular glutamate transporters and neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats: light and electron microscopic studies

Our previous study demonstrated GABAergic and glycinergic synapses onto neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons in the pre-Bötzinger complex (pre-BötC), the hypothesized kernel of normal respiratory rhythmogenesis. In the present study, we aimed to identify glutamatergic synapses onto NK1R-ir pre-BötC neurons, as excitatory synaptic transmission is a prerequisite to normal respiratory rhythmogenesis. Two types of vesicular glutamate transporters (VGLUT), VGLUT1 and VGLUT2, have been recently implicated in glutamate-mediated transmission. The present study used immunofluorescence and immunogold-silver staining to determine the relationship between the transporters and NK1R-ir neurons in the pre-BötC of adult rats. Under the confocal laser-scanning microscope, VGLUT2-ir boutons were found to be widely distributed in the pre-BötC, some of which were in close apposition to NK1R-ir somas and dendrites. VGLUT1-ir boutons were relatively rare and only a few were found to be in close apposition to NK1R-ir somas and dendrites. Electron microscopic observation revealed that approximately 41% of VGLUT2-ir terminals were in close apposition to, or made asymmetric synapses with NK1R-ir somas and dendrites in the pre-BötC. On the other hand, 50.5% of NK1R-ir dendrites were closely apposed to, or synapsed with VGLUT2-ir terminals. Occasionally, VGLUT1-ir terminals were found in close apposition to NK1R-ir somas or dendrites, but we were unable to identify synapses between them. The present findings provide the morphological basis for excitatory synaptic inputs onto NK1R-ir neurons in the pre-BötC. VGLUT2 may be involved in a dominant excitatory synaptic pathway for normal respiratory rhythmogenesis.

GABAergic and glycinergic synapses onto neurokinin-1 receptor-immunoreactive neurons in the pre-Bötzinger complex of rats: light and electron microscopic studies

The pre-Bötzinger complex (preBötC) in the ventrolateral medulla is thought to be the kernel for respiratory rhythm generation. Neurons in the preBötC contain intense neurokinin-1 receptor (NK1R) immunoreactivity. Some of these neurons in the adult preBötC are presumed to be the pre-inspiratory interneurons that are essential for generating respiratory rhythm in the neonate. Chloride-mediated synaptic inhibition is critical for rhythmogenesis in the adult. The present study used immunofluorescence histochemistry and immunogold-silver staining to determine the inhibitory synaptic relationship between glutamic acid decarboxylase (GAD)- or glycine transporter 2 (GlyT2)-immunoreactive (ir) boutons and NK1R-ir neurons in the preBötC of adult rats. Under the confocal microscope, we found that GAD- and GlyT2-ir boutons were in close apposition to NK1R-ir somas and dendrites in the preBötC. Under the electron microscope, GAD- and GlyT2-ir terminals were in close apposition to NK1R-ir somas and dendrites. Symmetric synapses were identified between GAD- or GlyT2-ir terminals and NK1R-ir neurons. A total of 51.6% GAD-ir and 38.2% GlyT2-ir terminals were found to contact or make synapses with NK1R-ir profiles, respectively. GAD- and GlyT2-ir terminals synapsed not only upon NK1R-ir neurons but also upon NK1R immuno-negative neurons. NK1R-ir neurons received both symmetric (presumed inhibitory) and asymmetric (presumed excitatory) synapses. Thus, the present findings provide the morphological basis for inhibitory inputs to NK1R-ir neurons in the preBötC, consistent with the suggestion that chloride-mediated synaptic inhibition may contribute importantly to rhythm generation by controlling the membrane potential trajectory and resetting rhythmic bursting of the kernel neurons in the adult.

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.

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

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

Respiratory rhythm generation: converging concepts from in vitro and in vivo approaches?

The timing and activation pattern of breathing movements are determined by the respiratory network. This network is amenable to a variety of in vivo and in vitro approaches, which offers a unique opportunity to investigate multiple organizational levels. It is only recently, however, that concepts obtained under in vivo and in vitro conditions are being integrated into a coherent model of breathing behavior. For example, the pre-Bötzinger complex as an essential site for rhythm generation was first identified in vitro, but has since been verified in vivo. Conversely, timing signals provided by other central and peripheral neuronal areas have so far been investigated in vivo, but it is now possible to address these issues with more complex in vitro preparations. Several key issues remain unresolved. For example, to what extent is the respiratory pattern controlled independently of the underlying rhythm? Answers to this and other questions require a dissection of mechanisms that is only possible through a complementary combination of experimental approaches.

Depressor and tachypneic responses to chemical stimulation of the ventral respiratory group are reduced by ablation of neurokinin-1 receptor-expressing neurons

Our goal was to investigate whether the neurokinin-1 receptor (NK1R)-expressing cells of the rostral ventrolateral medulla (RVLM) regulate respiration and arterial pressure (AP). We examined the consequences of their ablation on the cardiorespiratory responses [phrenic nerve discharge (PND) and AP] caused by injecting dl-homocysteic acid (DLH) into the ventral respiratory group (VRG). In intact rats, DLH produced tachypnea only when injected into the pre-Bötzinger complex (pre-BötC). Injections into pre-BötC and rostral VRG (rVRG) caused hypotension, whereas injections into the Bötzinger region elevated AP. Selective unilateral ablation of RVLM NK1R-immunoreactive cells (97% loss within the pre-BötC and rVRG without loss of catecholaminergic neurons) was done by injecting saporin (SAP) conjugated with a selective NK1R agonist [Sar9, Met(O2)11]-substance P (SSP). Free SAP produced no lesion. Resting AP was normal in SAP- and SSP-SAP-treated rats, but the PND rate was slightly elevated in SSP-SAP-treated rats. The response of SAP-treated rats to DLH injection into VRG was normal and identical on each side, but tachypnea could not be elicited in the pre-BötC of SSP-SAP-treated rats on the toxin-injected side, and DLH caused a long-lasting apnea on the untreated side. The hypotension produced by DLH injection into pre-BötC and rVRG of SSP-SAP-treated rats was reduced on the lesioned side only. In conclusion, NK1R-expressing cells of the rostral ventrolateral medulla control both respiratory rhythm and blood pressure. However, there is no evidence yet that these two functions are regulated by the same NK1R-expressing neurons.

Neurokinin-1 receptor-expressing cells of the ventral respiratory group are functionally heterogeneous and predominantly glutamatergic

According to a recent theory (Gray et al., 1999) the neurokinin-1 receptor (NK1R)-immunoreactive (ir) neurons of the ventral respiratory group (VRG) are confined to the pre-Bötzinger complex (pre-BötC) and might be glutamatergic interneurons that drive respiratory rhythmogenesis. In this study we tested whether the NK1R-ir neurons of the VRG are glutamatergic. We also examined whether different groups of NK1R-ir neurons coexist in the VRG on the basis of whether these cells contain preproenkephalin (PPE) mRNA or project to the spinal cord. NK1R immunoreactivity was found in two populations of VRG neurons that are both predominantly glutamatergic because most of them contained vesicular glutamate transporter 2 mRNA (77 +/- 9%; n = 3). A group of small fusiform neurons (somatic cross section: 91 +/- 3.6 microm2) that has neither PPE mRNA nor spinal projections is primarily restricted to the pre-BötC. These cells may be the interneurons the destruction of which produces massive disruptions of the respiratory rhythm (Gray et al., 2001). The rest of the NK1R-ir neurons of the VRG are multipolar, are larger (somatic cross section: 252 +/- 15 microm2), and express high levels of PPE mRNA. Some of these cells located in the rostral half of the rostral VRG project to the spinal cord (C4 or T3). Using electrophysiological methods, we showed that these bulbospinal NK1R-ir neurons are slowly discharging inspiratory-augmenting neurons, suggesting that they may control phrenic or intercostal motor neurons. In summary, NK1R-expressing cells of the VRG are a heterogeneous group of predominantly glutamatergic neurons that include subpopulations of respiratory premotor neurons.

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.