Respiratory-Like Rhythmic Activity Can Be Produced by an Excitatory Network of Non-Pacemaker Neuron Models

Robust network oscillations during mammalian respiratory rhythm generation driven by synaptic dynamics

How might synaptic dynamics generate synchronous oscillations in neuronal networks? We address this question in the preBötzinger complex (preBötC), a brainstem neural network that paces robust, yet labile, inspiration in mammals. The preBötC is composed of a few hundred neurons that alternate bursting activity with silent periods, but the mechanism underlying this vital rhythm remains elusive. Using a computational approach to model a randomly connected neuronal network that relies on short-term synaptic facilitation (SF) and depression (SD), we show that synaptic fluctuations can initiate population activities through recurrent excitation. We also show that a two-step SD process allows activity in the network to synchronize (bursts) and generate a population refractory period (silence). The model was validated against an array of experimental conditions, which recapitulate several processes the preBötC may experience. Consistent with the modeling assumptions, we reveal, by electrophysiological recordings, that SF/SD can occur at preBötC synapses on timescales that influence rhythmic population activity. We conclude that nondeterministic neuronal spiking and dynamic synaptic strengths in a randomly connected network are sufficient to give rise to regular respiratory-like rhythmic network activity and lability, which may play an important role in generating the rhythm for breathing and other coordinated motor activities in mammals.

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.

Computational models and emergent properties of respiratory neural networks

Computational models of the neural control system for breathing in mammals provide a theoretical and computational framework bringing together experimental data obtained from different animal preparations under various experimental conditions. Many of these models were developed in parallel and iteratively with experimental studies and provided predictions guiding new experiments. This data-driven modeling approach has advanced our understanding of respiratory network architecture and neural mechanisms underlying generation of the respiratory rhythm and pattern, including their functional reorganization under different physiological conditions. Models reviewed here vary in neurobiological details and computational complexity and span multiple spatiotemporal scales of respiratory control mechanisms. Recent models describe interacting populations of respiratory neurons spatially distributed within the Bötzinger and pre-Bötzinger complexes and rostral ventrolateral medulla that contain core circuits of the respiratory central pattern generator (CPG). Network interactions within these circuits along with intrinsic rhythmogenic properties of neurons form a hierarchy of multiple rhythm generation mechanisms. The functional expression of these mechanisms is controlled by input drives from other brainstem components,including the retrotrapezoid nucleus and pons, which regulate the dynamic behavior of the core circuitry. The emerging view is that the brainstem respiratory network has rhythmogenic capabilities at multiple levels of circuit organization. This allows flexible, state-dependent expression of different neural pattern-generation mechanisms under various physiological conditions,enabling a wide repertoire of respiratory behaviors. Some models consider control of the respiratory CPG by pulmonary feedback and network reconfiguration during defensive behaviors such as cough. Future directions in modeling of the respiratory CPG are considered.

Respiratory circuits: development, function and models

Breathing is a rhythmic motor behavior generated and controlled by hindbrain neuronal networks. Respiratory motor output arises from two distinct, but functionally interacting, rhythmogenic networks: the pre-Bötzinger complex (preBötC) and the retrotrapezoïd nucleus/parafacial respiratory group (RTN/pFRG). This review outlines recent advances in delineating the genetic specification of the neuronal constituents of these two rhythmogenic networks, their respective roles in respiratory function and how they interact to constitute a functional respiratory circuit ensemble. The often lethal consequences of disruption to these networks found in naturally occurring developmental disorders, transgenic animals, and highly specific lesion studies are described. In addition, we discuss how recent computational models enhance our understanding of how respiratory networks generate and regulate respiratory behavior.

Functional anatomical evidence for respiratory rhythmogenic function of endogenous bursters in rat medulla

Endogenous burster neurons (EBs) have been found at the level of the facial nucleus (VIIn), and 500 mum caudally, within the pre-Bötzinger complex (preBötC). They have been proposed as either causal to or playing no role in respiratory rhythmogenesis. Little is known about their broader distribution in ventrolateral medulla. Here, a Ca(2+) indicator was used to record respiratory network activity in ventrolateral medulla, and, following synaptic blockade, to identify EBs active at perfusate K(+) concentrations ([K(+)](o)) of 3, 6, and 9 mm. Recordings were made along the respiratory column, extending 300 mum rostrally, and 1100 mum caudally from the caudal pole of VIIn (VIIc), in the in vitro tilted sagittal slab preparation, isolated from neonate male and female Sprague Dawley rats. Activity under matching [K(+)](o) in the intact respiratory network was subsequently investigated. Respiratory neurons (n = 401) formed statistically significant clusters at the VIIc, within the preBötC, and 100 mum caudal to the preBötC. EBs (n = 693) formed statistically significant clusters that overlapped with respiratory clusters at the VIIc and preBötC. EB activity increased significantly as [K(+)](o) was increased, as did neurons that remained coupled following synaptic blockade. The overlap between respiratory and EB clusters in regions of ventrolateral medulla identified as rhythmogenic supports the hypothesis that EBs are constituents of rhythmogenic networks. In addition, the observation of truncated inspiratory bursts and ectopic bursting in respiratory neurons when [K(+)](o) was elevated in the intact network is consistent with a causal role for EBs in respiratory rhythmogenesis.

Large-conductance calcium-activated potassium channels in the neurons of pre-Bötzinger complex and their participation in the regulation of central respiratory activity in neonatal rats

The present study was conducted to test our hypothesis that the large-conductance calcium-activated potassium channels (BK(Ca) channels) exist in the neurons of the pre-Bötzinger complex (PBC), a brainstem region that may generate respiratory rhythm in mammals, and play roles in central regulation of respiratory activity in neonatal rats. Immunohistochemical technique revealed that BK(Ca) channels expressed in the neurons of PBC region. Whole cell voltage clamp recordings from the neurons in the PBC showed that BK(Ca) channels could be activated by membrane depolarization and blocked by 1mM tetraethylammonium (TEA) or 10microM paxilline in the preparation of thin (about 300microm) medullary slices of neonatal rats. The rhythmic respiratory-like discharge of hypoglossal rootlets could be changed by perfusing the thick (700-900microm) medullary slices with 1mM TEA or 10microM paxilline. Both TEA and paxilline could prolong the inspiratory duration, shorten the expiratory duration and increase the respiratory frequency. The results suggest that BK(Ca) channels exist in the PBC neurons and may be involved in the central control of rhythmic respiration in the neonatal rats.

Role of neurokinin receptors and ionic mechanisms within the respiratory network of the lamprey

We have suggested that in the lamprey, a medullary region called the paratrigeminal respiratory group (pTRG), is essential for respiratory rhythm generation and could correspond to the pre-Bötzinger complex (pre-BötC), the hypothesized kernel of the inspiratory rhythm-generating network in mammals. The present study was performed on in vitro brainstem preparations of adult lampreys to investigate whether some functional characteristics of the respiratory network are retained throughout evolution and to get further insights into the recent debated hypotheses on respiratory rhythmogenesis in mammals, such as for instance the "group-pacemaker" hypothesis. Thus, we tried to ascertain the presence and role of neurokinins (NKs) and burst-generating ion currents, such as the persistent Na(+) current (I(NaP)) and the Ca(2+)-activated non-specific cation current (I(CAN)), described in the pre-Bötzinger complex. Respiratory activity was monitored as vagal motor output. Substance P (SP) as well as NK1, NK2 and NK3 receptor agonists (400-800 nM) applied to the bath induced marked increases in respiratory frequency. Microinjections (0.5-1 nl) of SP as well as the other NK receptor agonists (1 microM) into the pTRG increased the frequency and amplitude of vagal bursts. Riluzole (RIL) and flufenamic acid (FFA) were used to block I(NaP) and I(CAN), respectively. Bath application of either RIL or FFA (20-50 microM) depressed, but did not suppress respiratory activity. Coapplication of RIL and FFA at 50 microM abolished the respiratory rhythm that, however, was restarted by SP microinjected into the pTRG. The results show that NKs may have a modulatory role in the lamprey respiratory network through an action on the pTRG and that I(NaP) and I(CAN) may contribute to vagal burst generation. We suggest that the "group-pacemaker" hypothesis is tenable for the lamprey respiratory rhythm generation since respiratory activity is abolished by blocking both I(NaP) and I(CAN), but is restored by enhancing network excitability.

Mechanism for the universal pattern of activity in developing neuronal networks

Spontaneous episodic activity is a fundamental mode of operation of developing networks. Surprisingly, the duration of an episode of activity correlates with the length of the silent interval that precedes it, but not with the interval that follows. Here we use a modeling approach to explain this characteristic, but thus far unexplained, feature of developing networks. Because the correlation pattern is observed in networks with different structures and components, a satisfactory model needs to generate the right pattern of activity regardless of the details of network architecture or individual cell properties. We thus developed simple models incorporating excitatory coupling between heterogeneous neurons and activity-dependent synaptic depression. These models robustly generated episodic activity with the correct correlation pattern. The correlation pattern resulted from episodes being triggered at random levels of recovery from depression while they terminated around the same level of depression. To explain this fundamental difference between episode onset and termination, we used a mean field model, where only average activity and average level of recovery from synaptic depression are considered. In this model, episode onset is highly sensitive to inputs. Thus noise resulting from random coincidences in the spike times of individual neurons led to the high variability at episode onset and to the observed correlation pattern. This work further shows that networks with widely different architectures, different cell types, and different functions all operate according to the same general mechanism early in their development.

Synaptically activated burst-generating conductances may underlie a group-pacemaker mechanism for respiratory rhythm generation in mammals

Breathing, chewing, and walking are critical life-sustaining behaviors in mammals that consist essentially of simple rhythmic movements. Breathing movements in particular involve the diaphragm, thorax, and airways but emanate from a network in the lower brain stem. This network can be studied in reduced preparations in vitro and using simplified mathematical models that make testable predictions. An iterative approach that employs both in vitro and in silico models argues against canonical mechanisms for respiratory rhythm in neonatal rodents that involve reciprocal inhibition and pacemaker properties. We present an alternative model in which emergent network properties play a rhythmogenic role. Specifically, we show evidence that synaptically activated burst-generating conductances-which are only available in the context of network activity-engender robust periodic bursts in respiratory neurons. Because the cellular burst-generating mechanism is linked to network synaptic drive we dub this type of system a group pacemaker.

Calcium-activated nonspecific cation current and synaptic depression promote network-dependent burst oscillations

Central pattern generators (CPGs) produce neural-motor rhythms that often depend on specialized cellular or synaptic properties such as pacemaker neurons or alternating phases of synaptic inhibition. Motivated by experimental evidence suggesting that activity in the mammalian respiratory CPG, the preBötzinger complex, does not require either of these components, we present and analyze a mathematical model demonstrating an unconventional mechanism of rhythm generation in which glutamatergic synapses and the short-term depression of excitatory transmission play key rhythmogenic roles. Recurrent synaptic excitation triggers postsynaptic Ca(2+)-activated nonspecific cation current (I(CAN)) to initiate a network-wide burst. Robust depolarization due to I(CAN) also causes voltage-dependent spike inactivation, which diminishes recurrent excitation and thus attenuates postsynaptic Ca(2+) accumulation. Consequently, activity-dependent outward currents-produced by Na/K ATPase pumps or other ionic mechanisms-can terminate the burst and cause a transient quiescent state in the network. The recovery of sporadic spiking activity rekindles excitatory interactions and initiates a new cycle. Because synaptic inputs gate postsynaptic burst-generating conductances, this rhythm-generating mechanism represents a new paradigm that can be dubbed a 'group pacemaker' in which the basic rhythmogenic unit encompasses a fully interdependent ensemble of synaptic and intrinsic components. This conceptual framework should be considered as an alternative to traditional models when analyzing CPGs for which mechanistic details have not yet been elucidated.

AMPA and metabotropic glutamate receptors cooperatively generate inspiratory-like depolarization in mouse respiratory neurons in vitro

Excitatory transmission mediated by AMPA receptors is critical for respiratory rhythm generation. However, the role of AMPA receptors has not been fully explored. Here we tested the functional role of AMPA receptors in inspiratory neurons of the neonatal mouse preBötzinger complex (preBötC) using an in vitro slice model that retains active respiratory function. Immediately before and during inspiration, preBötC neurons displayed envelopes of depolarization, dubbed inspiratory drive potentials, that required AMPA receptors but largely depended on the Ca(2+)-activated non-specific cation current (I(CAN)). We showed that AMPA receptor-mediated depolarization opened voltage-gated Ca(2+) channels to directly evoke I(CAN). Inositol 1,4,5-trisphosphate receptor-mediated intracellular Ca(2+) release also evoked I(CAN). Inositol 1,4,5-trisphosphate receptors acted downstream of group I metabotropic glutamate receptor activity but, here too, AMPA receptor-mediated Ca(2+) influx was essential to trigger the metabotropic glutamate receptor contribution to inspiratory drive potential generation. This study helps to elucidate the role of excitatory transmission in respiratory rhythm generation in vitro. AMPA receptors in preBötC neurons initiate convergent signaling pathways that evoke post-synaptic I(CAN), which underlies inspiratory drive potentials. The coupling of AMPA receptors with I(CAN) suggests that latent burst-generating intrinsic conductances are recruited by excitatory synaptic interactions among preBötC neurons in the context of respiratory network activity in vitro, exemplifying a rhythmogenic mechanism based on emergent properties of the network.

Fluctuation-driven rhythmogenesis in an excitatory neuronal network with slow adaptation

We study an excitatory all-to-all coupled network of N spiking neurons with synaptically filtered background noise and slow activity-dependent hyperpolarization currents. Such a system exhibits noise-induced burst oscillations over a range of values of the noise strength (variance) and level of cell excitability. Since both of these quantities depend on the rate of background synaptic inputs, we show how noise can provide a mechanism for increasing the robustness of rhythmic bursting and the range of burst frequencies. By exploiting a separation of time scales we also show how the system dynamics can be reduced to low-dimensional mean field equations in the limit N --> infinity. Analysis of the bifurcation structure of the mean field equations provides insights into the dynamical mechanisms for initiating and terminating the bursts.

4-Aminopyridine-sensitive outward currents in preBötzinger complex neurons influence respiratory rhythm generation in neonatal mice

We measured a low-threshold, inactivating K+ current, i.e. A-current (I(A)), in respiratory neurons of the preBötzinger complex (preBötC) in rhythmically active slice preparations from neonatal C57BL/6 mice. The majority of inspiratory neurons (21/34 = 61.8%), but not expiratory neurons (1/8 = 12.5%), expressed I(A). In whole-cell and somatic outside-out patches I(A) activated at -60 mV (half-activation voltage measured -16.3 mV) and only fully inactivated above -40 mV (half-inactivation voltage measured -85.6 mV), indicating that I(A) can influence membrane trajectory at baseline voltages during respiratory rhythm generation in vitro. 4-Aminopyridine (4-AP, 2 mm) attenuated I(A) in both whole-cell and somatic outside-out patches. In the context of rhythmic network activity, 4-AP caused irregular respiratory-related motor output on XII nerves and disrupted rhythmogenesis as detected with whole-cell and field recordings in the preBötC. Whole-cell current-clamp recordings showed that 4-AP changed the envelope of depolarization underlying inspiratory bursts (i.e. inspiratory drive potentials) from an incrementing pattern to a decrementing pattern during rhythm generation and abolished current pulse-induced delayed excitation. These data suggest that I(A) opposes excitatory synaptic depolarizations at baseline voltages of approximately -60 mV and influences the inspiratory burst pattern. We propose that I(A) promotes orderly recruitment of constituent rhythmogenic neurons by minimizing the activity of these neurons until they receive massive coincident synaptic input, which reduces the periodic fluctuations of inspiratory activity.

What role do pacemakers play in the generation of respiratory rhythm?

The pacemaker hypothesis that specialized neurons with conditional oscillatory- bursting properties are obligatory for respiratory rhythm generation in vitro has gained widespread acceptance, despite lack of direct proof. Here we critique the pacemaker hypothesis and provide an alternative explanation for rhythmogenesis based on emergent network properties. Pacemaker neurons in the preBötC depend on either persistent Na+ current I(NaP) or Ca(2+)-activated nonspecific cationic current (I(CAN)). Activity in slice preparations and synaptically- isolated pacemaker neurons undergo similar frequency modulation by perturbations including hypoxia and changes in external K+. These data have been used to argue that pacemaker cells must be rhythmogenic, but may simply reflect the action of these perturbations on intrinsic membrane properties throughout the preBötC and does not constitute proof that pacemakers necessarily drive the rhythm with synaptic coupling in place. Likewise, bath-applied drugs, such as riluzole (RIL) and flufenamic acid (FFA), attenuate I(NaP) and I(CAN), respectively, throughout the slice. Thus, when these drugs stop the rhythm, a widespread depression of excitability is likely the underlying cause, not selective blockade of bursting-pacemaker activity. We propose that rhythmogenesis is an emergent network property, wherein recurrent synaptic excitation initiates a positive feedback cycle among interneurons and that intrinsic currents like I(CAN) and I(NaP) promote inspiratory burst generation by augmenting synaptic excitation in the context of network activity. In this group-pacemaker framework, individual pacemaker neurons can be embedded but play the same role as every other network constituent.

Reconfiguration of a vertebrate motor network: specific neuron recruitment and context-dependent synaptic plasticity

Motor networks typically generate several related output patterns or gaits where individual neurons may be shared or recruited between patterns. We investigate how a vertebrate locomotor network is reconfigured to produce a second rhythmic motor pattern, defining the detailed pattern of neuronal recruitment and consequent changes in the mechanism for rhythm generation. Hatchling Xenopus tadpoles swim if touched, but when held make slower, stronger, struggling movements. In immobilized tadpoles, a brief current pulse to the skin initiates swimming, whereas 40 Hz pulses produce struggling. The classes of neurons active during struggling are defined using whole-cell patch recordings from hindbrain and spinal cord neurons during 40 Hz stimulation of the skin. Some motoneurons and inhibitory interneurons are active in both swimming and struggling, but more neurons are recruited within these classes during struggling. In addition, and in contrast to a previous study, we describe two new classes of excitatory interneuron specifically recruited during struggling and define their properties and synaptic connections. We then explore mechanisms that generate struggling by building a network model incorporating these new neurons. As well as the recruitment of new neuron classes, we show that reconfiguration of the locomotor network to the struggling central pattern generator (CPG) reveals a context-dependent synaptic depression of reciprocal inhibition: the result of increased inhibitory neuron firing frequency during struggling. This provides one possible mechanism for burst termination not seen in the swimming CPG. The direct demonstration of depression in reciprocal inhibition confirms a key element of Brown's (1911) hypothesis for locomotor rhythmogenesis.

Role of persistent sodium current in mouse preBötzinger Complex neurons and respiratory rhythm generation

Breathing movements in mammals depend on respiratory neurons in the preBötzinger Complex (preBötC), which comprise a rhythmic network and generate robust bursts that form the basis for inspiration. Persistent Na(+) current (I(NaP)) is widespread in the preBötC and is hypothesized to play a critical role in rhythm generation because of its subthreshold activation and slow inactivation properties that putatively promote long-lasting burst depolarizations. In neonatal mouse slice preparations that retain the preBötC and generate a respiratory-related rhythm, we tested the role of I(NaP) with multiple Na(+) channel antagonists: tetrodotoxin (TTX; 20 nM), riluzole (RIL; 10 microM), and the intracellular Na(+) channel antagonist QX-314 (2 mM). Here we show that I(NaP) promotes intraburst spiking in preBötC neurons but surprisingly does not contribute to the depolarization that underlies inspiratory bursts, i.e. the inspiratory drive potential. Local microinjection in the preBötC of 10 microM RIL or 20 nM TTX does not perturb respiratory frequency, even in the presence of bath-applied 100 microM flufenamic acid (FFA), which attenuates a Ca(2+)-activated non-specific cation current (I(CAN)) that may also have burst-generating functionality. These data contradict the hypothesis that I(NaP) in preBötC neurons is obligatory for rhythmogenesis. However, in the presence of FFA, local microinjection of 10 microM RIL in the raphe obscurus causes rhythm cessation, which suggests that I(NaP) regulates the excitability of neurons outside the preBötC, including serotonergic raphe neurons that project to, and help maintain, rhythmic preBötC function.

Insights from models of rhythmic motor systems

Computational models of rhythmic motor systems are valuable tools for the study of motor pattern generation and control. Recent modeling advances, together with experimental results, suggest that rhythmic behaviors, such as breathing or walking, are influenced by complex interactions among motor system components. Such interactions occur at all levels of organization, from the subcellular through to the cellular, synaptic, and network levels to the level of neuromuscular interactions and that of the whole organism. Simultaneously, safety mechanisms at all levels contribute to network stability and the generation of robust motor patterns.

Acetazolamide affects breathing differently in ICR and C57 mice

Acetazolamide (ACZ) administration was compared on ventilation in outbred male ICR Swiss Webster (ICR) and inbred C57BL/6J (C57) mice, used in development of transgenic strains. We hypothesized that in both strains ACZ would affect breathing similarly. Mice received intraperitoneally vehicle and the next week ACZ (40 mg/kg), and were exposed to air for 90 min, followed by 5-min exposure to 10% O(2), air for 15 min, and to 5 min of 5% CO(2) in O(2). Ventilation was evaluated using plethysmography. ACZ stimulated ventilation in both stains exposed to air. C57 mice minimally increased frequency and tidal volume, whereas ICR mice markedly increased frequency. Strain differences in the ventilatory pattern in response to hypoxia and hypercapnia occurred. ACZ-treated ICR mice decreased hypoxic responsiveness to 50% of vehicle values, whereas ACZ had no effect in C57 mice. ACZ decreased hypercapnic ventilatory responsiveness in both strains. Differential effects of ACZ breathing in these two strains suggest that genetic factors modulate its effect on breathing.

GABAergic and glycinergic inhibitory mechanisms in the lamprey respiratory control

The specific role of gamma-aminobutyric acid (GABA) and glycine receptors in respiratory rhythm generation and pattern formation was investigated in in vitro brainstem preparations from adult lampreys by analyzing the changes in respiratory activity induced by bath application of specific antagonists, agonists, and uptake blockers. GABAA receptor blockade by bicuculline or picrotoxin increased both the frequency and amplitude of respiratory bursts. Similar effects were observed after glycine receptor blockade by strychnine. Combined bath application of bicuculline and strychnine markedly increased the frequency and amplitude of respiratory activity. These responses were associated, especially at the higher concentrations of the two drugs, with the appearance of tonic activity and irregular, high-frequency bursts followed by transient depression of respiratory activity. GABAA and glycine receptor agonists suppressed respiratory activity. These effects were prevented by bath application of the corresponding specific antagonists. GABAB receptor blockade by 2-hydroxysaclofen reduced the respiratory frequency but increased the peak amplitude of respiratory bursts. Activation of GABAB receptors suppressed respiratory activity. These responses were prevented by 2-hydroxysaclofen. Neither GABAC receptor agonist nor antagonist had any effects on respiration. Depression of both the frequency and amplitude of respiratory bursts was induced by blockades of GABA and glycine uptake using, respectively, nipecotic acid and sarcosine. The results suggest that GABA- and glycine-mediated inhibition is not essential for respiratory rhythm generation in the adult lamprey, although it appears to exert potent influences on respiratory activity and to have a role in maintaining a stable and regular breathing pattern.

Perinatal maturation of the respiratory rhythm generator in mammals: from experimental results to computational simulation

The survival of neonatal mammals requires a correct function of the respiratory rhythm generator (RRG), and therefore, the processes that control its prenatal maturation are of vital importance. In humans, lambs and rodents, foetal breathing movements (FBMs) occur early during gestation, are episodic, sensitive to bioamines, central hypoxia and inputs from CNS upper structures, and evolve with developmental age. In vitro, the foetal rodent RRG studied in preparations where the upper CNS structures are lacking continuously produces a rhythmic command, which is sensitive to hypoxia and bioaminergic inputs. The rhythm is slow with variable periods 4 days before birth. It becomes faster 2 days before birth, similar to the postnatal rhythm. Compelling evidence suggests that a region of the RRG called the preBötzinger complex (PBC) contains respiratory pacemaker neurones which play a primary role in perinatal rhythmogenesis. Although the RRG functions during early gestation, no pacemakers are found in the putative PBC area and its electrical stimulation and lesion do not affect the early foetal rhythm. To know whether the early foetal and perinatal rhythms originate from either pacemaker neurones or network connection properties, and to know which maturational processes might explain the appearance of PBC pacemakers and the rhythm increase during perinatal development, we computationally modelled maturing RRG. Our model shows that both network noise and persistent sodium conductance are crucial for rhythmogenesis and that a slight increase in the persistent sodium conductance can solve the pacemaker versus network dilemma in a noisy network.