Respiratory neuronal assemblies

Microcircuit Synchronization and Heavy-Tailed Synaptic Weight Distribution Augment preBötzinger Complex Bursting Dynamics

The preBötzinger Complex (preBötC) encodes inspiratory time as rhythmic bursts of activity underlying each breath. Spike synchronization throughout a sparsely connected preBötC microcircuit initiates bursts that ultimately drive the inspiratory motor patterns. Using minimal microcircuit models to explore burst initiation dynamics, we examined the variability in probability and latency to burst following exogenous stimulation of a small subset of neurons, mimicking experiments. Among various physiologically plausible graphs of 1000 excitatory neurons constructed using experimentally determined synaptic and connectivity parameters, directed Erdős-Rényi graphs with a broad (lognormal) distribution of synaptic weights best captured the experimentally observed dynamics. preBötC synchronization leading to bursts was regulated by the efferent connectivity of spiking neurons that are optimally tuned to amplify modest preinspiratory activity through input convergence. Using graph-theoretic and machine learning-based analyses, we found that input convergence of efferent connectivity at the next-nearest neighbor order was a strong predictor of incipient synchronization. Our analyses revealed a crucial role of synaptic heterogeneity in imparting exceptionally robust yet flexible preBötC attractor dynamics. Given the pervasiveness of lognormally distributed synaptic strengths throughout the nervous system, we postulate that these mechanisms represent a ubiquitous template for temporal processing and decision-making computational motifs.SIGNIFICANCE STATEMENT Mammalian breathing is robust, virtually continuous throughout life, yet is inherently labile: to adapt to rapid metabolic shifts (e.g., fleeing a predator or chasing prey); for airway reflexes; and to enable nonventilatory behaviors (e.g., vocalization, breathholding, laughing). Canonical theoretical frameworks-based on pacemakers and intrinsic bursting-cannot account for the observed robustness and flexibility of the preBötzinger Complex rhythm. Experiments reveal that network synchronization is the key to initiate inspiratory bursts in each breathing cycle. We investigated preBötC synchronization dynamics using network models constructed with experimentally determined neuronal and synaptic parameters. We discovered that a fat-tailed (non-Gaussian) synaptic weight distribution-a manifestation of synaptic heterogeneity-augments neuronal synchronization and attractor dynamics in this vital rhythmogenic network, contributing to its extraordinary reliability and responsiveness.

Modeling and simulation of vagal afferent input of the cough reflex

We employed computational modeling to investigate previously conducted experiments of the effect of vagal afferent modulation on the cough reflex in an anesthetized cat animal model. Specifically, we simulated unilateral cooling of the vagus nerve and analyzed characteristics of coughs produced by a computational model of brainstem cough/respiratory neuronal network. Unilateral vagal cooling was simulated by a reduction of cough afferent input (corresponding to unilateral vagal cooling) to the cough network. All these attempts resulted in only mild decreases in investigated cough characteristics such as cough number, amplitudes of inspiratory and expiratory cough efforts in comparison with experimental data. Multifactorial alterations of model characteristics during cough simulations were required to approximate cough motor patterns that were observed during unilateral vagal cooling in vivo. The results support the plausibility of a more complex NTS processing system for cough afferent information than has been proposed.

Neuronal mechanisms underlying opioid-induced respiratory depression: our current understanding

Opioid-induced respiratory depression (OIRD) represents the primary cause of death associated with therapeutic and recreational opioid use. Within the United States, the rate of death from opioid abuse since the early 1990s has grown disproportionally, prompting the classification as a nationwide "epidemic." Since this time, we have begun to unravel many fundamental cellular and systems-level mechanisms associated with opioid-related death. However, factors such as individual vulnerability, neuromodulatory compensation, and redundancy of opioid effects across central and peripheral nervous systems have created a barrier to a concise, integrative view of OIRD. Within this review, we bring together multiple perspectives in the field of OIRD to create an overarching viewpoint of what we know, and where we view this essential topic of research going forward into the future.

Volumetric mapping of the functional neuroanatomy of the respiratory network in the perfused brainstem preparation of rats

KEY POINTS

The functional neuroanatomy of the mammalian respiratory network is far from being understood since experimental tools that measure neural activity across this brainstem-wide circuit are lacking. Here, we use silicon multi-electrode arrays to record respiratory local field potentials (rLFPs) from 196-364 electrode sites within 8-10 mm³ of brainstem tissue in single arterially perfused brainstem preparations with respect to the ongoing respiratory motor pattern of inspiration (I), post-inspiration (PI) and late-expiration (E2). rLFPs peaked specifically at the three respiratory phase transitions, E2-I, I-PI and PI-E2. We show, for the first time, that only the I-PI transition engages a brainstem-wide network, and that rLFPs during the PI-E2 transition identify a hitherto unknown role for the dorsal respiratory group. Volumetric mapping of pontomedullary rLFPs in single preparations could become a reliable tool for assessing the functional neuroanatomy of the respiratory network in health and disease.

ABSTRACT

While it is widely accepted that inspiratory rhythm generation depends on the pre-Bötzinger complex, the functional neuroanatomy of the neural circuits that generate expiration is debated. We hypothesized that the compartmental organization of the brainstem respiratory network is sufficient to generate macroscopic local field potentials (LFPs), and if so, respiratory (r) LFPs could be used to map the functional neuroanatomy of the respiratory network. We developed an approach using silicon multi-electrode arrays to record spontaneous LFPs from hundreds of electrode sites in a volume of brainstem tissue while monitoring the respiratory motor pattern on phrenic and vagal nerves in the perfused brainstem preparation. Our results revealed the expression of rLFPs across the pontomedullary brainstem. rLFPs occurred specifically at the three transitions between respiratory phases: (1) from late expiration (E2) to inspiration (I), (2) from I to post-inspiration (PI), and (3) from PI to E2. Thus, respiratory network activity was maximal at respiratory phase transitions. Spatially, the E2-I, and PI-E2 transitions were anatomically localized to the ventral and dorsal respiratory groups, respectively. In contrast, our data show, for the first time, that the generation of controlled expiration during the post-inspiratory phase engages a distributed neuronal population within ventral, dorsal and pontine network compartments. A group-wise independent component analysis demonstrated that all preparations exhibited rLFPs with a similar temporal structure and thus share a similar functional neuroanatomy. Thus, volumetric mapping of rLFPs could allow for the physiological assessment of global respiratory network organization in health and disease.

Modulation of Cough Reflex by Gaba-Ergic Inhibition in Medullary Raphé of the Cat

We studied the effects of GABA receptor agonists microinjections in medullary raphé on the mechanically induced tracheobronchial cough response in anesthetized, unparalyzed, spontaneously breathing cats. The results suggest that GABA-ergic inhibition significantly contributes to the regulation of cough reflex by action of both GABA(A) and GABA(B) receptors. The data are consistent with inhomogeneous occurrence of GABA-ergic neurons in medullary raphé and their different involvement in the cough reflex control. Cells within rostral nucleus raphéobscurus with dominant role of GABA(A) receptors and neurons of rostral nucleus raphépallidus and caudal nucleus raphémagnus with dominant role of GABA(B) receptors participate in regulation of cough expiratory efforts. These cough control elements are distinct from cough gating mechanism. GABA-ergic inhibition in the raphé caudal to obex had insignificant effect on cough. Contradictory findings for GABA, muscimol and baclofen administration in medullary raphé suggest involvement of coordinated activity of GABA on multiple receptors affecting raphé neurons and/or the local neuronal circuits in the raphé modulating cough motor drive.

The Pathophysiology of Respiratory Failure: Control of Breathing, Respiratory Load, and Muscle Capacity

The purpose of this review is to provide an overview on how interactions between control of breathing, respiratory load, and muscle function may lead to respiratory failure. The mechanisms involved vary according to the underlying pathology, but respiratory failure is most often the result of an imbalance between the muscular pump and the mechanical load placed upon it. Changes in respiratory drive and response to CO2 seem to be important contributors to the pathophysiology of respiratory failure. Inspiratory muscle dysfunction is also frequent but is not a mandatory prerequisite to respiratory failure since increased load may also be sufficient to precipitate it. It is crucial to recognize these interactions to be able to timeously establish patients on mechanical ventilation and adapt the ventilator settings to their respiratory system physiology.

Neural Network Reconfigurations: Changes of the Respiratory Network by Hypoxia as an Example

Neural networks, including the respiratory network, can undergo a reconfiguration process by just changing the number, the connectivity or the activity of their elements. Those elements can be either brain regions or neurons, which constitute the building blocks of macrocircuits and microcircuits, respectively. The reconfiguration processes can also involve changes in the number of connections and/or the strength between the elements of the network. These changes allow neural networks to acquire different topologies to perform a variety of functions or change their responses as a consequence of physiological or pathological conditions. Thus, neural networks are not hardwired entities, but they constitute flexible circuits that can be constantly reconfigured in response to a variety of stimuli. Here, we are going to review several examples of these processes with special emphasis on the reconfiguration of the respiratory rhythm generator in response to different patterns of hypoxia, which can lead to changes in respiratory patterns or lasting changes in frequency and/or amplitude.

Respiratory neuron characterization reveals intrinsic bursting properties in isolated adult turtle brainstems (Trachemys scripta)

It is not known whether respiratory neurons with intrinsic bursting properties exist within ectothermic vertebrate respiratory control systems. Thus, isolated adult turtle brainstems spontaneously producing respiratory motor output were used to identify and classify respiratory neurons based on their firing pattern relative to hypoglossal (XII) nerve activity. Most respiratory neurons (183/212) had peak activity during the expiratory phase, while inspiratory, post-inspiratory, and novel pre-expiratory neurons were less common. During synaptic blockade conditions, ∼10% of respiratory neurons fired bursts of action potentials, with post-inspiratory cells (6/9) having the highest percentage of intrinsic burst properties. Most intrinsically bursting respiratory neurons were clustered at the level of the vagus (X) nerve root. Synaptic inhibition blockade caused seizure-like activity throughout the turtle brainstem, which shows that the turtle respiratory control system is not transformed into a network driven by intrinsically bursting respiratory neurons. We hypothesize that intrinsically bursting respiratory neurons are evolutionarily conserved and represent a potential rhythmogenic mechanism contributing to respiration in adult turtles.

Change in network connectivity during fictive-gasping generation in hypoxia: prevention by a metabolic intermediate

The neuronal circuit in charge of generating the respiratory rhythms, localized in the pre-Bötzinger complex (preBötC), is configured to produce fictive-eupnea during normoxia and reconfigures to produce fictive-gasping during hypoxic conditions in vitro. The mechanisms involved in such reconfiguration have been extensively investigated by cell-focused studies, but the actual changes at the network level remain elusive. Since a failure to generate gasping has been linked to Sudden Infant Death Syndrome (SIDS), the study of gasping generation and pharmacological approaches to promote it may have clinical relevance. Here, we study the changes in network dynamics and circuit reconfiguration that occur during the transition to fictive-gasping generation in the brainstem slice preparation by recording the preBötC with multi-electrode arrays and assessing correlated firing among respiratory neurons or clusters of respiratory neurons (multiunits). We studied whether the respiratory network reconfiguration in hypoxia involves changes in either the number of active respiratory elements, the number of functional connections among elements, or the strength of these connections. Moreover, we tested the influence of isocitrate, a Krebs cycle intermediate that has recently been shown to promote breathing, on the configuration of the preBötC circuit during normoxia and on its reconfiguration during hypoxia. We found that, in contrast to previous suggestions based on cell-focused studies, the number and the overall activity of respiratory neurons change only slightly during hypoxia. However, hypoxia induces a reduction in the strength of functional connectivity within the circuit without reducing the number of connections. Isocitrate prevented this reduction during hypoxia while increasing the strength of network connectivity. In conclusion, we provide an overview of the configuration of the respiratory network under control conditions and how it is reconfigured during fictive-gasping. Additionally, our data support the use of isocitrate to favor respiratory rhythm generation under normoxia and to prevent some of the changes in the respiratory network under hypoxic conditions.

Millisecond-timescale local network coding in the rat primary somatosensory cortex

Correlation among neocortical neurons is thought to play an indispensable role in mediating sensory processing of external stimuli. The role of temporal precision in this correlation has been hypothesized to enhance information flow along sensory pathways. Its role in mediating the integration of information at the output of these pathways, however, remains poorly understood. Here, we examined spike timing correlation between simultaneously recorded layer V neurons within and across columns of the primary somatosensory cortex of anesthetized rats during unilateral whisker stimulation. We used bayesian statistics and information theory to quantify the causal influence between the recorded cells with millisecond precision. For each stimulated whisker, we inferred stable, whisker-specific, dynamic bayesian networks over many repeated trials, with network similarity of 83.3±6% within whisker, compared to only 50.3±18% across whiskers. These networks further provided information about whisker identity that was approximately 6 times higher than what was provided by the latency to first spike and 13 times higher than what was provided by the spike count of individual neurons examined separately. Furthermore, prediction of individual neurons' precise firing conditioned on knowledge of putative pre-synaptic cell firing was 3 times higher than predictions conditioned on stimulus onset alone. Taken together, these results suggest the presence of a temporally precise network coding mechanism that integrates information across neighboring columns within layer V about vibrissa position and whisking kinetics to mediate whisker movement by motor areas innervated by layer V.

Optical analysis of circuitry for respiratory rhythm in isolated brainstem of foetal mice

Respiratory rhythms arise from neurons situated in the ventral medulla. We are investigating their spatial and functional relationships optically by measuring changes in intracellular calcium using the fluorescent, calcium-sensitive dye Oregon Green 488 BAPTA-1 AM while simultaneously recording the regular firing of motoneurons in the phrenic nerve in isolated brainstem/spinal cord preparations of E17 to E19 mice. Responses of identified cells are associated breath by breath with inspiratory and expiratory phases of respiration and depend on CO(2) and pH levels. Optical methods including two-photon microscopy are being developed together with computational analyses. Analysis of the spatial pattern of neuronal activity associated with respiratory rhythm, including cross-correlation analysis, reveals a network distributed in the ventral medulla with intermingling of neurons that are active during separate phases of the rhythm. Our experiments, aimed at testing whether initiation of the respiratory rhythm depends on pacemaker neurons, on networks or a combination of both, suggest an important role for networks.

Maximum decoding abilities of temporal patterns and synchronized firings: application to auditory neurons responding to click trains and amplitude modulated white noise

Simultaneous recordings of an increasing number of neurons have recently become available, but few methods have been proposed to handle this activity. Here, we extract and investigate all the possible temporal neural activity patterns based on synchronized firings of neurons recorded on multiple electrodes, or based on bursts of single-electrode activity in cat primary auditory cortex. We apply this to responses to periodic click trains or sinusoïdal amplitude modulated noise by obtaining for each pattern its temporal modulation transfer function. An algorithm that maximizes the mutual information between all patterns and stimuli subsequently leads to the identification of patterns that optimally decode modulation frequency (MF). We show that stimulus information contained in multi-electrode synchronized firing is not redundant with single-electrode firings and leads to improved efficiency of MF decoding. We also show that the combined use of firing rate and temporal codes leads to a better discrimination of the MF.

Central mechanisms I: plasticity of central pathways

Cough is the most common symptom for which individuals seek medical attention and spend health-care dollars. Despite the burden induced by cough, the current treatments for cough are only partially effective. Delineating the sites and mechanisms in the cough central network for changes in the cough reflex could lead to new therapeutic strategies and drug target sites for more effective treatments. The first synaptic target in the CNS for the cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS); these neurons reorganize the primary sensory information into a coherent output. The NTS neurons have been shown to undergo neuroplasticity under a variety of conditions, such as respiratory disorders, stress, and exposures to environmental pollutants. The NTS contains a rich innervation of substance P immunoreactive nerve terminals, suggesting that substance P might be important in altered cough reflex response. This chapter summarizes our current findings on the role of substance P in enhanced cough reflex as well as the potential NTS targets for the action of substance P.

Reconfiguration of the pontomedullary respiratory network: a computational modeling study with coordinated in vivo experiments

A large body of data suggests that the pontine respiratory group (PRG) is involved in respiratory phase-switching and the reconfiguration of the brain stem respiratory network. However, connectivity between the PRG and ventral respiratory column (VRC) in computational models has been largely ad hoc. We developed a network model with PRG-VRC connectivity inferred from coordinated in vivo experiments. Neurons were modeled in the "integrate-and-fire" style; some neurons had pacemaker properties derived from the model of Breen et al. We recapitulated earlier modeling results, including reproduction of activity profiles of different respiratory neurons and motor outputs, and their changes under different conditions (vagotomy, pontine lesions, etc.). The model also reproduced characteristic changes in neuronal and motor patterns observed in vivo during fictive cough and during hypoxia in non-rapid eye movement sleep. Our simulations suggested possible mechanisms for respiratory pattern reorganization during these behaviors. The model predicted that network- and pacemaker-generated rhythms could be co-expressed during the transition from gasping to eupnea, producing a combined "burst-ramp" pattern of phrenic discharges. To test this prediction, phrenic activity and multiple single neuron spike trains were monitored in vagotomized, decerebrate, immobilized, thoracotomized, and artificially ventilated cats during hypoxia and recovery. In most experiments, phrenic discharge patterns during recovery from hypoxia were similar to those predicted by the model. We conclude that under certain conditions, e.g., during recovery from severe brain hypoxia, components of a distributed network activity present during eupnea can be co-expressed with gasp patterns generated by a distinct, functionally "simplified" mechanism.

Functional connectivity in the pontomedullary respiratory network

Current models propose that a neuronal network in the ventrolateral medulla generates the basic respiratory rhythm and that this ventrolateral respiratory column (VRC) is profoundly influenced by the neurons of the pontine respiratory group (PRG). However, functional connectivity among PRG and VRC neurons is poorly understood. This study addressed four model-based hypotheses: 1) the respiratory modulation of PRG neuron populations reflects paucisynaptic actions of multiple VRC populations; 2) functional connections among PRG neurons shape and coordinate their respiratory-modulated activities; 3) the PRG acts on multiple VRC populations, contributing to phase-switching; and 4) neurons with no respiratory modulation located in close proximity to the VRC and PRG have widely distributed actions on respiratory-modulated cells. Two arrays of microelectrodes with individual depth adjustment were used to record sets of spike trains from a total of 145 PRG and 282 VRC neurons in 10 decerebrate, vagotomized, neuromuscularly blocked, ventilated cats. Data were evaluated for respiratory modulation with respect to efferent phrenic motoneuron activity and short-timescale correlations indicative of paucisynaptic functional connectivity using cross-correlation analysis and the "gravity" method. Correlogram features were found for 109 (3%) of the 3,218 pairs composed of a PRG and a VRC neuron, 126 (12%) of the 1,043 PRG-PRG pairs, and 319 (7%) of the 4,340 VRC-VRC neuron pairs evaluated. Correlation linkage maps generated for the data support our four motivating hypotheses and suggest network mechanisms for proposed modulatory functions of the PRG.

Phylogeny of vertebrate respiratory rhythm generators: the Oscillator Homology Hypothesis

A revolution is underway in our understanding of respiratory rhythm generation in mammals. Until recently, a major focus of research within the field has centered around the question of locating and elucidating the mechanism of rhythmogenesis of a single respiratory neuronal oscillator which is reiterated bilaterally within the brainstem. Now it appears that each hemisection may contain at least two oscillators that interact to generate the respiratory rhythm in mammals. Comparative studies have hinted at the existence of multiple respiratory oscillators in non-mammalian vertebrates for some time, raising the possibility of homologous oscillators. Here, we consider available tools to identify neuronal oscillators and critically review the evidence for the importance and existence of multiple respiratory oscillators in vertebrates. First focusing on a comparison between frogs and mammals, we then evaluate the hypothesis that ventilatory oscillators in extant tetrapods evolved from ancestral oscillators present in fish (the Oscillator Homology Hypothesis). While supporting data are incomplete, the Oscillator Homology Hypothesis will likely serve as a useful framework to motivate further studies of respiratory rhythm generation in lower vertebrates.

Neurogenesis of cough, other airway defensive behaviors and breathing: A holarchical system?

Cough and breathing are generated by a common muscular system. However, these two behaviors differ significantly in their mechanical features and regulation. The current conceptualization of the neurogenic mechanism for these behaviors holds that the multifunctional respiratory pattern generator undergoes reconfiguration to produce cough. Our previous results indicate the presence of a functional cough gate mechanism that controls the excitability of this airway defensive behavior, but is not involved in the regulation of breathing. We propose that the neurogenesis of cough, breathing, and other nonbreathing behaviors is controlled by a larger network, of which the respiratory pattern generator is part. This network we term a holarchical system. This system is governed by functional control elements known as holons, which confer unique regulatory features to each behavior. The cough gate is an example of such a holon. Neurons that participate in a cough holon may include behavior selective elements. That is, neurons that are either specifically recruited during cough and/or tonically-active neurons with little or no modulation during breathing but with significant alterations in discharge during coughing. We also propose that the holarchical system is responsible for the orderly expression of different airway defensive behaviors such that each motor task is executed in a temporally and mechanically discrete manner. We further propose that a holon controlling one airway defensive behavior can regulate the excitability of, and cooperate with, holons unique to other behaviors. As such, co-expression of multiple rhythmic behaviors such as cough and swallow can occur without compromising airway defense.

Central pathways of pulmonary and lower airway vagal afferents

Lung sensory receptors with afferent fibers coursing in the vagus nerves are broadly divided into three groups: slowly (SAR) and rapidly (RAR) adapting stretch receptors and bronchopulmonary C fibers. Central terminations of each group are found in largely nonoverlapping regions of the caudal half of the nucleus of the solitary tract (NTS). Second order neurons in the pathways from these receptors innervate neurons located in respiratory-related regions of the medulla, pons, and spinal cord. The relative ease of selective activation of SARs, and to a lesser extent RARs, has allowed for more complete physiological and morphological characterization of the second and higher order neurons in these pathways than for C fibers. A subset of NTS neurons receiving afferent input from SARs (termed pump or P-cells) mediates the Breuer-Hering reflex and inhibits neurons receiving afferent input from RARs. P-cells and second order neurons in the RAR pathway also provide inputs to regions of the ventrolateral medulla involved in control of respiratory motor pattern, i.e., regions containing a predominance of bulbospinal premotor neurons, as well as regions containing respiratory rhythm-generating neurons. Axon collaterals from both P-cells and RAR interneurons, and likely from NTS interneurons in the C-fiber pathway, project to the parabrachial pontine region where they may contribute to plasticity in respiratory control and integration of respiratory control with other systems, including those that provide for voluntary control of breathing, sleep-wake behavior, and emotions.

An identified central pattern-generating neuron co-ordinates sensory-motor components of respiratory behavior in Lymnaea

Defining the attributes of individual central pattern-generating (CPG) neurons underlying various rhythmic behaviors are fundamental to our understanding of how the brain controls motor programs, such as respiration and locomotion. To this end, we have explored a simple invertebrate preparation in which the neuronal basis of respiratory rhythmogenesis can be investigated from the whole animal to a single cell level. An identified dopaminergic neuron, termed right pedal dorsal 1 (RPeD1), is a component of the CPG network which controls hypoxia-driven, aerial respiration in the fresh water snail Lymnaea stagnalis. Using intact, semi-intact and isolated brain preparations, we have discovered that in addition to its role as a respiratory CPG neuron, RPeD1 co-ordinates sensory-motor input from the pneumostome (the respiratory orifice) at the water/air interface to initiate respiratory rhythm generation. An additional, novel role of RPeD1 was also found. Specifically, direct intracellular stimulation of RPeD1 induced pneumostome openings, in the absence of motor neuronal activity. To determine further the role of RPeD1 in the respiratory behavior of intact animals, either its axon was severed or the soma selectively killed. Many components of the respiratory behavior in the intact animals were found to be perturbed following RPeD1 axotomy or 'somatomy' (soma removed). Taken together, the data presented provide a direct demonstration that RPeD1 is a multifunctional CPG neuron, which also serves many additional roles in the control of breathing behavior, ranging from co-ordination of mechanosensory input to the motor control of the respiratory orifice.

Plasticity of brainstem mechanisms of cough

The cough reflex is a brainstem reflex, consisting of specific sensory afferent nerves which trigger the reflex, by transmitting the sensory input over vagal or laryngeal nerves to a brainstem circuitry which processes and ultimately transforms the sensory input into a complex motor output to generate cough. The first synaptic target for the primary cough-related sensory input is the second-order neurons in the nucleus tractus solitarius (NTS). This position in the reflex pathway and intricate local circuits within the nucleus make it a strategic site where the sensory information can be modified. Plasticity at this synapse will change the nature of the output--exaggerating it, suppressing it or transforming it into some other complex pattern. This review integrates evidence implicating the NTS in exaggerated cough with proof of the concept that NTS neurons undergo plasticity to contribute to an exaggeration of cough.

Two enhancements of the gravity algorithm for multiple spike train analysis

The gravity method for neuronal assembly analysis represents each neuron as a particle in N-space with a time varying charge that is a filtered version of the corresponding spike train, with appropriate rules for forces between and movements of the charged particles. Resulting trajectories reflect neuronal timing relationships. The usual short time constants in the filter restrict aggregation to highly synchronized neurons and reduce the sensitivity for delayed correlations; long time constants in the filter reduce selectivity. Here we describe an enhancement that modifies rules for assigning charge increment times to allow mixtures of short and long lag correlations. Charge increments for each pair are offset from the actual spike times by time lags defined by features in corresponding cross-correlograms; no such charge offsets are invoked if the correlogram is flat. Tuning increases charge products and aggregation of long lag correlated pairs. A second enhancement uses a new three-dimensional display of particle pair trajectories to parse the type of neuronal relationship. For each pair, we record and display the inter-particle distance and the distance each particle moves from its original location in the N-space. The resulting trajectories cluster according to the type of interaction between the represented neurons. Results from simulated networks and in vivo multi-site recordings show that these modifications detect assembly properties not identified by the standard methods.

Plasticity of central mechanisms for cough

Cough is associated with plasticity of putative cough afferent fibres, but whether plasticity in the brainstem network contributes is less well understood. A key site in the CNS network is the nucleus tractus solitarius (NTS), the first synaptic contact of the primary afferent fibres. We sought to develop a conscious guinea pig model to detect enhanced cough, to focus on the NTS as a potential site for plasticity, and to test a role for substance P in the NTS since the neuropeptide has been implicated in plasticity of the vagal afferent fibres. Guinea pigs were exposed to second-hand tobacco smoke (SHS) or filtered air (FA) from 1-6 weeks of age. At 5 weeks, cannulae were implanted in the NTS. At 6 weeks, either vehicle or a neurokinin 1 (NK-1) receptor antagonist was injected into the NTS of the conscious guinea pigs who were then exposed to citric acid aerosol. SHS exposure significantly enhanced citric acid-induced cough (56%, P<0.05), an effect attenuated by NTS NK-1 receptor blockade (P<0.05). The findings suggest that one possible mechanism for plasticity in cough is related to substance P effects in the NTS. Future studies will be required to investigate the possible mechanisms underlying the role of substance P as well as other mechanisms in generating SHS-induced cough.

Production of reflex cough by brainstem respiratory networks

Delineation of neural mechanisms involved in reflex cough is essential for understanding its many physiological and clinical complexities, and the development of more desirable antitussive agents. Brainstem networks that generate and modulate the breathing pattern are also involved in producing the motor patterns during reflex cough. Neurones of the ventrolateral medulla respiratory pattern generator mutually interact with neural networks in the pons, medulla and cerebellum to form a larger dynamic network. This paper discusses evidence from our laboratory and others supporting the involvement of the nucleus tractus solitarii, midline raphe nuclei and lateral tegmental field in the medulla, and the pontine respiratory group and cerebellum in the production of reflex cough. Gaps in our knowledge are identified to stimulate further research on this complicated issue.

Afferent modulation of neonatal rat respiratory rhythm in vitro: cellular and synaptic mechanisms

In mammals, expiration is lengthened by mid-expiratory lung inflation (Breuer-Hering Expiratory reflex; BHE). The central pathway mediating the BHE is paucisynaptic, converging on neurones in the rostral ventrolateral medulla. An in vitro neonatal rat brainstem-lung preparation in which mid-expiratory inflation lengthens expiration was used to study afferent modulation of respiratory neurone activity. Recordings were made from respiratory neurones in or near the pre-Bötzinger Complex (preBötC). Respiratory neurone membrane properties and BHE-induced changes in activity were characterized. Our findings suggest the following mechanisms for the BHE: (i) lung afferent signals strongly excite biphasic neurones that convey these signals to respiratory neurones in ventrolateral medulla; (ii) expiratory lengthening is mediated by inhibition of rhythmogenic and (pre)motoneuronal networks; and (iii) pre-inspiratory (Pre-I) neurones, some of which project to abdominal expiratory motoneurones, are excited during the BHE. These findings are qualitatively similar to studies of the BHE in vivo. Where there are differences, they can largely be accounted for by developmental changes and experimental conditions.

Emergent properties of CNS neuronal networks as targets for pharmacology: application to anticonvulsant drug action

CNS drugs may act by modifying the emergent properties of complex CNS neuronal networks. Emergent properties are network characteristics that are not predictably based on properties of individual member neurons. Neuronal membership within networks is controlled by several mechanisms, including burst firing, gap junctions, endogenous and exogenous neuroactive substances, extracellular ions, temperature, interneuron activity, astrocytic integration and external stimuli. The effects of many CNS drugs in vivo may critically involve actions on specific brain loci, but this selectivity may be absent when the same neurons are isolated from the network in vitro where emergent properties are lost. Audiogenic seizures (AGS) qualify as an emergent CNS property, since in AGS the acoustic stimulus evokes a non-linear output (motor convulsion), but the identical stimulus evokes minimal behavioral changes normally. The hierarchical neuronal network, subserving AGS in rodents is initiated in inferior colliculus (IC) and progresses to deep layers of superior colliculus (DLSC), pontine reticular formation (PRF) and periaqueductal gray (PAG) in genetic and ethanol withdrawal-induced AGS. In blocking AGS, certain anticonvulsants reduce IC neuronal firing, while other agents act primarily on neurons in other AGS network sites. However, the NMDA receptor channel blocker, MK-801, does not depress neuronal firing in any network site despite potently blocking AGS. Recent findings indicate that MK-801 actually enhances firing in substantia nigra reticulata (SNR) neurons in vivo but not in vitro. Thus, the MK-801-induced firing increases in SNR neurons observed in vivo may involve an indirect effect via disinhibition, involving an action on the emergent properties of this seizure network.

Postnatal changes in cytochrome oxidase expressions in brain stem nuclei of rats: implications for sensitive periods

Previously, we reported that cytochrome oxidase (CO) activity in the rat pre-Bötzinger complex (PBC) exhibited a plateau on postnatal days (P) 3-4 and a prominent decrease on P12 (Liu and Wong-Riley, J Appl Physiol 92: 923-934, 2002). These changes were correlated with a concomitant reduction in the expression of glutamate and N-methyl-d-aspartate receptor subunit 1 and an increase in GABA, GABAB, glycine receptor, and glutamate receptor 2. To determine whether changes were limited to the PBC, the present study aimed at examining the expression of CO in a number of brain stem nuclei, with or without known respiratory functions from P0 to P21 in rats: the ventrolateral subnucleus of the solitary tract nucleus, nucleus ambiguus, hypoglossal nucleus, nucleus raphe obscurus, dorsal motor nucleus of the vagus nerve, medial accessory olivary nucleus, spinal nucleus of the trigeminal nerve, and medial vestibular nucleus (MVe). Results indicated that, in all of the brain stem nuclei examined, CO activity exhibited a general increase with age from P0 to P21, with MVe having the slowest rise. Notably, in all of the nuclei examined except for MVe, there was a plateau or decrease at P3-P4 and a prominent rise-fall-rise pattern at P11-P13, similar to that observed in the PBC. In addition, there was a fall-rise-fall pattern at P15-P17 in these nuclei, instead of a plateau pattern in the PBC. Our data suggest that the two postnatal periods with reduced CO activity, P3-P4 and especially P12, may represent common sensitive periods for most of the brain stem nuclei with known or suspected respiratory control functions.

Commentary on the definition of eupnea and gasping

Despite clear qualitative differences, it has proven difficult to identify criteria that reliably differentiate eupnea and gasping--particularly when multiple species or experimental preparations are considered. From a motor control perspective, this is unsurprising. Three organizational rules are common to nearly all rhythmic activities: (1) the basic rhythm is produced by a small network of cells, (2) the activity of this network in isolation often differs dramatically from the behavior of the whole animal, and (3) the rhythmogenic networks responsible for related behaviors are not fixed and independent but dynamically modifiable and overlapping. In this context, it becomes clear that the definition of a particular pattern and the investigation of the mechanisms underlying its production are inseparable. Rather than attempting to rigidly apply criteria developed using any one experimental preparation, the classification of respiratory patterns must evolve alongside our understanding of how each pattern is produced-a process that is only aided by investigations using a variety of experimental preparations.

High frequency oscillations in respiratory networks: functionally significant or phenomenological?

Inspiratory activities, whether recorded from medullary neurons, motoneurons or motor nerves, feature prominent oscillations in high (50-120 Hz) and medium (15-50 Hz) frequency ranges. These oscillations have been extensively characterized and are considered signatures of respiratory network activity. Their functional significance, however, if any, remains unknown. Here we review the literature describing the nature and origin of these oscillations as well as their modulation during development and by mechanoreceptive and chemoreceptive feedback, respiratory- and non-respiratory-related behaviors, temperature and anesthesia. We then consider the potential significance of these oscillations for respiratory network function by drawing on analyses of distributed motor and sensory networks of the cortex where current interest in oscillatory activity, and the synchronization of neural discharge that can result, is based on the increased efficacy with which synchronous inputs influence neuronal output, and the role that synchronous activity may play in information coding. We speculate that synchronized oscillations at the network level help coordinate activity in distributed rhythm and pattern generating systems and at the muscle level enhance force development. Data most strongly support that oscillatory synaptic inputs play an important role in controlling timing and pattern of action potential output.

Learning in respiratory control

In this article, it is argued that learning participates to fulfill the metabolic requirements by adapting respiratory control to changing internal and external states. Recent classical-conditioning experiments in newborn mice or adult rats show the close link between conditioned respiratory and arousal responses. The conditioned fear model may be a suitable and largely unexplored model of emotionally induced hyperventilation. The parabrachial nucleus and periacqueducal grey may play a pivotal role in the ventilatory component of conditioned fear. The sensitivity of breathing to conditioning in newborn and adult animals suggests that learning processes may shape breathing pattern throughout life.