Structural-functional properties of identified excitatory and inhibitory interneurons within pre-Botzinger complex respiratory microcircuits

The respiratory control mechanisms in the brainstem and spinal cord: integrative views of the neuroanatomy and neurophysiology

Respiratory activities are produced by medullary respiratory rhythm generators and are modulated from various sites in the lower brainstem, and which are then output as motor activities through premotor efferent networks in the brainstem and spinal cord. Over the past few decades, new knowledge has been accumulated on the anatomical and physiological mechanisms underlying the generation and regulation of respiratory rhythm. In this review, we focus on the recent findings and attempt to elucidate the anatomical and functional mechanisms underlying respiratory control in the lower brainstem and spinal cord.

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.

Voltage-Dependent Rhythmogenic Property of Respiratory Pre-Bötzinger Complex Glutamatergic, Dbx1-Derived, and Somatostatin-Expressing Neuron Populations Revealed by Graded Optogenetic Inhibition

The rhythm of breathing in mammals, originating within the brainstem pre-Bötzinger complex (pre-BötC), is presumed to be generated by glutamatergic neurons, but this has not been directly demonstrated. Additionally, developmental expression of the transcription factor Dbx1 or expression of the neuropeptide somatostatin (Sst), has been proposed as a marker for the rhythmogenic pre-BötC glutamatergic neurons, but it is unknown whether these other two phenotypically defined neuronal populations are functionally equivalent to glutamatergic neurons with regard to rhythm generation. To address these problems, we comparatively investigated, by optogenetic approaches, the roles of pre-BötC glutamatergic, Dbx1-derived, and Sst-expressing neurons in respiratory rhythm generation in neonatal transgenic mouse medullary slices in vitro and also more intact adult perfused brainstem-spinal cord preparations in situ. We established three different triple-transgenic mouse lines with Cre-driven Archaerhodopsin-3 (Arch) expression selectively in glutamatergic, Dbx1-derived, or Sst-expressing neurons for targeted photoinhibition. In each line, we identified subpopulations of rhythmically active, Arch-expressing pre-BötC inspiratory neurons by whole-cell recordings in medullary slice preparations in vitro, and established that Arch-mediated hyperpolarization of these inspiratory neurons was laser power dependent with equal efficacy. By site- and population-specific graded photoinhibition, we then demonstrated that inspiratory frequency was reduced by each population with the same neuronal voltage-dependent frequency control mechanism in each state of the respiratory network examined. We infer that enough of the rhythmogenic pre-BötC glutamatergic neurons also have the Dbx1 and Sst expression phenotypes, and thus all three phenotypes share the same voltage-dependent frequency control property.

Circuits in the Ventral Medulla That Phase-Lock Motoneurons for Coordinated Sniffing and Whisking

The exploratory behavior of rodents is characterized by stereotypical movements of the vibrissae, nose, and head, which are phase locked with rapid respiration, that is, sniffing. Here we review the brainstem circuitry that coordinates these actions and propose that respiration may act as a master clock for binding orofacial inputs across different sensory modalities.

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.

Inhibition, Not Excitation, Drives Rhythmic Whisking

Sniffing and whisking typify the exploratory behavior of rodents. These actions involve separate oscillators in the medulla, located respectively in the pre-Bötzinger complex (preBötC) and the vibrissa-related region of the intermediate reticular formation (vIRt). We examine how these oscillators synergize to control sniffing and whisking. We find that the vIRt contains glycinergic/GABAergic cells that rhythmically inhibit vibrissa facial motoneurons. As a basis for the entrainment of whisking by breathing, but not vice versa, we provide evidence for unidirectional connections from the preBötC to the vIRt. The preBötC further contributes to the control of the mystacial pad. Lastly, we show that bilateral synchrony of whisking relies on the respiratory rhythm, consistent with commissural connections between preBötC cells. These data yield a putative circuit in which the preBötC acts as a master clock for the synchronization of vibrissa, pad, and snout movements, as well as for the bilateral synchronization of whisking.

Somatostatin 2a receptors are not expressed on functionally identified respiratory neurons in the ventral respiratory column of the rat

Microinjection of somatostatin (SST) causes site-specific effects on respiratory phase transition, frequency, and amplitude when microinjected into the ventrolateral medulla (VLM) of the anesthetized rat, suggesting selective expression of SST receptors on different functional classes of respiratory neurons. Of the six subtypes of SST receptor, somatostatin 2a (sst2a ) is the most prevalent in the VLM, and other investigators have suggested that glutamatergic neurons in the preBötzinger Complex (preBötC) that coexpress neurokinin-1 receptor (NK1R), SST, and sst2a are critical for the generation of respiratory rhythm. However, quantitative data describing the distribution of sst2a in respiratory compartments other than preBötC, or on functionally identified respiratory neurons, is absent. Here we examine the medullary expression of sst2a with particular reference to glycinergic/expiratory neurons in the Bötzinger Complex (BötC) and NK1R-immunoreactive/inspiratory neurons in the preBötC. We found robust sst2a expression at all rostrocaudal levels of the VLM, including a large proportion of catecholaminergic neurons, but no colocalization of sst2a and glycine transporter 2 mRNA in the BötC. In the preBötC 54% of sst2a -immunoreactive neurons were also positive for NK1R. sst2a was not observed in any of 52 dye-labeled respiratory interneurons, including seven BötC expiratory-decrementing and 11 preBötC preinspiratory neurons. We conclude that sst2a is not expressed on BötC respiratory neurons and that phasic respiratory activity is a poor predictor of sst2a expression in the preBötC. Therefore, sst2a is unlikely to underlie responses to BötC SST injection, and is sparse or absent on respiratory neurons identified by classical functional criteria. J. Comp. Neurol. 524:1384-1398, 2016. © 2015 Wiley Periodicals, Inc.

Facing the challenge of mammalian neural microcircuits: taking a few breaths may help

Breathing in mammals is a seemingly straightforward behaviour controlled by the brain. A brainstem nucleus called the preBötzinger Complex sits at the core of the neural circuit generating respiratory rhythm. Despite the discovery of this microcircuit almost 25 years ago, the mechanisms controlling breathing remain elusive. Given the apparent simplicity and well-defined nature of regulatory breathing behaviour, the identification of much of the circuitry, and the ability to study breathing in vitro as well as in vivo, many neuroscientists and physiologists are surprised that respiratory rhythm generation is still not well understood. Our view is that conventional rhythmogenic mechanisms involving pacemakers, inhibition or bursting are problematic and that simplifying assumptions commonly made for many vertebrate neural circuits ignore consequential detail. We propose that novel emergent mechanisms govern the generation of respiratory rhythm. That a mammalian function as basic as rhythm generation arises from complex and dynamic molecular, synaptic and neuronal interactions within a diverse neural microcircuit highlights the challenges in understanding neural control of mammalian behaviours, many (considerably) more elaborate than breathing. We suggest that the neural circuit controlling breathing is inimitably tractable and may inspire general strategies for elucidating other neural microcircuits.

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.

Optogenetic excitation of preBötzinger complex neurons potently drives inspiratory activity in vivo

KEY POINTS

This study investigates the effects on ventilation of an excitatory stimulus delivered in a spatially and temporally precise manner to the inspiratory oscillator, the preBötzinger complex (preBötC). We used an adeno-associated virus expressing channelrhodopsin driven by the synapsin promoter to target the region of the preBötC. Unilateral optogenetic stimulation of preBötC increased respiratory rate, minute ventilation and increased inspiratory modulated genioglossus muscle activity. Unilateral optogenetic stimulation of preBötC consistently entrained respiratory rate up to 180 breaths min(-1) both in presence of ongoing respiratory activity and in absence of inspiratory activity. Unilateral optogenetic stimulation of preBötC induced a strong phase-independent Type 0 respiratory reset, with a short delay in the response of 100 ms. We identified a refractory period of ∼200 ms where unilateral preBötC optogenetic stimulation is not able to initiate the next respiratory event.

ABSTRACT

Understanding the sites and mechanisms underlying respiratory rhythmogenesis is of fundamental interest in the field of respiratory neurophysiology. Previous studies demonstrated the necessary and sufficient role of preBötzinger complex (preBötC) in generating inspiratory rhythms in vitro and in vivo. However, the influence of timed activation of the preBötC network in vivo is as yet unknown given the experimental approaches previously used. By unilaterally infecting preBötC neurons using an adeno-associated virus expressing channelrhodopsin we photo-activated the network in order to assess how excitation delivered in a spatially and temporally precise manner to the inspiratory oscillator influences ongoing breathing rhythms and related muscular activity in urethane-anaesthetized rats. We hypothesized that if an excitatory drive is necessary for rhythmogenesis and burst initiation, photo-activation of preBötC not only will increase respiratory rate, but also entrain it over a wide range of frequencies with fast onset, and have little effect on ongoing respiratory rhythm if a stimulus is delivered during inspiration. Stimulation of preBötC neurons consistently increased respiratory rate and entrained respiration up to fourfold baseline conditions. Furthermore, brief pulses of photostimulation delivered at random phases between inspiratory events robustly and consistently induced phase-independent (Type 0) respiratory reset and recruited inspiratory muscle activity at very short delays (∼100 ms). A 200 ms refractory period following inspiration was also identified. These data provide strong evidence for a fine control of inspiratory activity in the preBötC and provide further evidence that the preBötC network constitutes the fundamental oscillator of inspiratory rhythms.

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.

Midline section of the medulla abolishes inspiratory activity and desynchronizes pre-inspiratory neuron rhythm on both sides of the medulla in newborn rats

Each half of the medulla contains respiratory neurons that constitute two generators that control respiratory rhythm. One generator consists of the inspiratory neurons in the pre-Bötzinger complex (preBötC); the other, the pre-inspiratory (Pre-I) neurons in the parafacial respiratory group (pFRG), rostral to the preBötC. We investigated the contribution of the commissural fibers, connecting the respiratory rhythm generators located on the opposite side of the medulla to the generation of respiratory activity in brain stem-spinal cord preparation from 0- to 1-day-old rats. Pre-I neuron activity and the facial nerve and/or first lumbar (L1) root activity were recorded as indicators of the pFRG-driven rhythm. Fourth cervical ventral root (C4) root and/or hypoglossal (XII) nerve activity were recorded as indicators of preBötC-driven inspiratory activity. We found that a midline section that interrupted crossed fibers rostral to the obex irreversibly eliminated C4 and XII root activity, whereas the Pre-I neurons, facial nerve, and L1 roots remained rhythmically active. The facial and contralateral L1 nerve activities were synchronous, whereas right and left facial (and right and left L1) nerves lost synchrony. Optical recordings demonstrated that pFRG-driven burst activity was preserved after a midline section, whereas the preBötC neurons were no longer rhythmic. We conclude that in newborn rats, crossed excitatory interactions (via commissural fibers) are necessary for the generation of inspiratory bursts but not for the generation of rhythmic Pre-I neuron activity.

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.

Influence of developmental nicotine exposure on spike-timing precision and reliability in hypoglossal motoneurons

Smoothly graded muscle contractions depend in part on the precision and reliability of motoneuron action potential generation. Whether or not a motoneuron generates spikes precisely and reliably depends on both its intrinsic membrane properties and the nature of the synaptic input that it receives. Factors that perturb neuronal intrinsic properties and/or synaptic drive may compromise the temporal precision and the reliability of action potential generation. We have previously shown that developmental nicotine exposure (DNE) alters intrinsic properties and synaptic transmission in hypoglossal motoneurons (XIIMNs). Here we show that the effects of DNE also include alterations in spike-timing precision and reliability, and spike-frequency adaptation, in response to sinusoidal current injection. Current-clamp experiments in brainstem slices from neonatal rats show that DNE lowers the threshold for spike generation but increases the variability of spike-timing mechanisms. DNE is also associated with an increase in spike-frequency adaptation and reductions in both peak and steady-state firing rate in response to brief, square wave current injections. Taken together, our data indicate that DNE causes significant alterations in the input-output efficiency of XIIMNs. These alterations may play a role in the increased frequency of obstructive apneas and altered suckling strength and coordination observed in nicotine-exposed neonatal humans.

Statistical analysis and data mining of digital reconstructions of dendritic morphologies

Neuronal morphology is diverse among animal species, developmental stages, brain regions, and cell types. The geometry of individual neurons also varies substantially even within the same cell class. Moreover, specific histological, imaging, and reconstruction methodologies can differentially affect morphometric measures. The quantitative characterization of neuronal arbors is necessary for in-depth understanding of the structure-function relationship in nervous systems. The large collection of community-contributed digitally reconstructed neurons available at NeuroMorpho.Org constitutes a “big data” research opportunity for neuroscience discovery beyond the approaches typically pursued in single laboratories. To illustrate these potential and related challenges, we present a database-wide statistical analysis of dendritic arbors enabling the quantification of major morphological similarities and differences across broadly adopted metadata categories. Furthermore, we adopt a complementary unsupervised approach based on clustering and dimensionality reduction to identify the main morphological parameters leading to the most statistically informative structural classification. We find that specific combinations of measures related to branching density, overall size, tortuosity, bifurcation angles, arbor flatness, and topological asymmetry can capture anatomically and functionally relevant features of dendritic trees. The reported results only represent a small fraction of the relationships available for data exploration and hypothesis testing enabled by sharing of digital morphological reconstructions.

Substitution of extracellular Ca2+ by Sr2+ prolongs inspiratory burst in pre-Bötzinger complex inspiratory neurons

The pre-Bötzinger complex (preBötC) underlies inspiratory rhythm generation. As a result of network interactions, preBötC neurons burst synchronously to produce rhythmic premotor inspiratory activity. Each inspiratory burst consists of action potentials (APs) on top of a 10- to 20-mV synchronous depolarization lasting 0.3-0.8 s known as inspiratory drive potential. The mechanisms underlying the initiation and termination of the inspiratory burst are unclear, and the role of Ca(2+) is a matter of intense debate. To investigate the role of extracellular Ca(2+) in inspiratory burst initiation and termination, we substituted extracellular Ca(2+) with Sr(2+). We found for the first time an ionic manipulation that significantly interferes with burst termination. In a rhythmically active slice, we current-clamped preBötC neurons (Vm ≅ -60 mV) while recording integrated hypoglossal nerve (∫XIIn) activity as motor output. Substitution of extracellular Ca(2+) with either 1.5 or 2.5 mM Sr(2+) significantly prolonged the duration of inspiratory bursts from 653.4 ± 30.7 ms in control conditions to 981.6 ± 78.5 ms in 1.5 mM Sr(2+) and 2,048.2 ± 448.5 ms in 2.5 mM Sr(2+), with a concomitant increase in decay time and area. Substitution of extracellular Ca(2+) by Sr(2+) is a well-established method to desynchronize neurotransmitter release. Our findings suggest that the increase in inspiratory burst duration is determined by a presynaptic mechanism involving desynchronization of glutamate release within the network.

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.

Identification of the pre-Bötzinger complex inspiratory center in calibrated "sandwich" slices from newborn mice with fluorescent Dbx1 interneurons

Inspiratory active pre‐Bötzinger complex (preBötC) networks produce the neural rhythm that initiates and controls breathing movements. We previously identified the preBötC in the newborn rat brainstem and established anatomically defined transverse slices in which the preBötC remains active when exposed at one surface. This follow‐up study uses a neonatal mouse model in which the preBötC as well as a genetically defined class of respiratory interneurons can be identified and selectively targeted for physiological recordings. The population of glutamatergic interneurons whose precursors express the transcription factor Dbx1 putatively comprises the core respiratory rhythmogenic circuit. Here, we used intersectional mouse genetics to identify the brainstem distribution of Dbx1‐derived neurons in the context of observable respiratory marker structures. This reference brainstem atlas enabled online histology for generating calibrated sandwich slices to identify the preBötC location, which was heretofore unspecified for perinatal mice. Sensitivity to opioids ensured that slice rhythms originated from preBötC neurons and not parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN) cells because opioids depress preBötC, but not pFRG/RTN rhythms. We found that the preBötC is centered ~0.4 mm caudal to the facial motor nucleus in this Cre/lox reporter mouse during postnatal days 0–4. Our findings provide the essential basis for future optically guided electrophysiological and fluorescence imaging‐based studies, as well as the application of other Cre‐dependent tools to record or manipulate respiratory rhythmogenic neurons. These resources will ultimately help elucidate the mechanisms that promote respiratory‐related oscillations of preBötC Dbx1‐derived neurons and thus breathing.

Breathing movements emanate from Dbx1‐derived interneurons of the brainstem pre‐Bötzinger complex (preBötC). We generated a histology atlas of the medulla in newborn Dbx1 Cre/lox reporter mice and performed physiological tests to pinpoint the preBötC location and map the Dbx1 neuron distribution, which will facilitate neurobiological studies of respiratory rhythm generation.

When norepinephrine becomes a driver of breathing irregularities: how intermittent hypoxia fundamentally alters the modulatory response of the respiratory network

Neuronal networks are endogenously modulated by aminergic and peptidergic substances. These modulatory processes are critical for maintaining normal activity and adapting networks to changes in metabolic, behavioral, and environmental conditions. However, disturbances in neuromodulation have also been associated with pathologies. Using whole animals (in vivo) and functional brainstem slices (in vitro) from mice, we demonstrate that exposure to acute intermittent hypoxia (AIH) leads to fundamental changes in the neuromodulatory response of the respiratory network located within the preBötzinger complex (preBötC), an area critical for breathing. Norepinephrine, which normally regularizes respiratory activity, renders respiratory activity irregular after AIH. Respiratory irregularities are caused both in vitro and in vivo by AIH, which increases synaptic inhibition within the preBötC when norepinephrine is endogenously or exogenously increased. These irregularities are prevented by blocking synaptic inhibition before AIH. However, regular breathing cannot be reestablished if synaptic inhibition is blocked after AIH. We conclude that subtle changes in synaptic transmission can have dramatic consequences at the network level as endogenously released neuromodulators that are normally adaptive become the drivers of irregularity. Moreover, irregularities in the preBötC result in irregularities in the motor output in vivo and in incomplete transmission of inspiratory activity to the hypoglossus motor nucleus. Our finding has basic science implications for understanding network functions in general, and it may be clinically relevant for understanding pathological disturbances associated with hypoxic episodes such as those associated with myocardial infarcts, obstructive sleep apneas, apneas of prematurity, Rett syndrome, and sudden infant death syndrome.

The pronociceptive dorsal reticular nucleus contains mostly tonic neurons and shows a high prevalence of spontaneous activity in block preparation

Despite the importance and significant clinical impact of understanding information processing in the nociceptive system, the functional properties of neurons in many parts of this system are still unknown. In this work we performed whole cell patch-clamp recording in rat brain stem blocks to characterize the electrophysiological properties of neurons in the dorsal reticular nucleus (DRt), a region known to be involved in pronociceptive modulation. We also compared properties of DRt neurons with those in the adjacent parvicellular reticular nucleus and in neighboring regions outside the reticular formation. We found that neurons in the DRt and parvicellular reticular nucleus had similar electrophysiological properties and exhibited mostly toniclike firing patterns, whereas neurons outside the reticular formation showed a larger diversity of firing patterns. Interestingly, more than one-half of the neurons also showed spontaneous activity. While the general view of the reticular formation, being a loosely associated mesh of groups of neurons with diverse function, and earlier reports suggests more electrophysiological heterogeneity, we showed that this is indeed not the case. Our results indicate that functional difference of neurons in the reticular formation may mostly be determined by their connectivity profiles and not by their intrinsic electrophysiological properties. The dominance of tonic neurons in the DRt supports previous conclusions that these neurons encode stimulus intensity through their firing frequency, while the high prevalence of spontaneous activity most likely shapes nociceptive modulation by this brain stem region.

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.

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.

Activation of upper airway muscles during breathing and swallowing

The upper airway is a complex muscular tube that is used by the respiratory and digestive systems. The upper airway is invested with several small and anatomically peculiar muscles. The muscle fiber orientations and their nervous innervation are both extremely complex, and how the activity of the muscles is initiated and adjusted during complex behaviors is poorly understood. The bulk of the evidence suggests that the entire assembly of tongue and laryngeal muscles operate together but differently during breathing and swallowing, like a ballet rather than a solo performance. Here we review the functional anatomy of the tongue and laryngeal muscles, and their neural innervation. We also consider how muscular activity is altered as respiratory drive changes, and briefly address upper airway muscle control during swallowing.