Counterpoint: Medullary pacemaker neurons are essential for gasping, but not eupnea, in mammals

Defining the Rhythmogenic Elements of Mammalian Breathing

Breathing's remarkable ability to adapt to changes in metabolic, environmental, and behavioral demands stems from a complex integration of its rhythm-generating network within the wider nervous system. Yet, this integration complicates identification of its specific rhythmogenic elements. Based on principles learned from smaller rhythmic networks of invertebrates, we define criteria that identify rhythmogenic elements of the mammalian breathing network and discuss how they interact to produce robust, dynamic breathing.

Kinetic properties of persistent Na+ current orchestrate oscillatory bursting in respiratory neurons

The rhythmic pattern of breathing depends on the pre-Bötzinger complex (preBötC) in the brainstem, a vital circuit that contains a population of neurons with intrinsic oscillatory bursting behavior. Here, we investigate the specific kinetic properties that enable voltage-gated sodium channels to establish oscillatory bursting in preBötC inspiratory neurons, which exhibit an unusually large persistent Na⁺ current (I_NaP). We first characterize the kinetics of I_NaP in neonatal rat brainstem slices in vitro, using whole-cell patch-clamp and computational modeling, and then test the contribution of I_NaP to rhythmic bursting in live neurons, using the dynamic clamp technique. We provide evidence that subthreshold activation, persistence at suprathreshold potentials, slow inactivation, and slow recovery from inactivation are kinetic features of I_NaP that regulate all aspects of intrinsic rhythmic bursting in preBötC neurons. The slow and cumulative inactivation of I_NaP during the burst active phase controls burst duration and termination, while the slow recovery from inactivation controls the duration of the interburst interval. To demonstrate this mechanism, we develop a Markov state model of I_NaP that explains a comprehensive set of voltage clamp data. By adding or subtracting a computer-generated I_NaP from a live neuron via dynamic clamp, we are able to convert nonbursters into intrinsic bursters, and vice versa. As a control, we test a model with inactivation features removed. Adding noninactivating I_NaP into nonbursters results in a pattern of random transitions between sustained firing and quiescence. The relative amplitude of I_NaP is the key factor that separates intrinsic bursters from nonbursters and can change the fraction of intrinsic bursters in the preBötC. I_NaP could thus be an important target for regulating network rhythmogenic properties.

[Broncho-pulmonary aspiration of brain and cartilage tissue in a context of gasping]

Evidence of post-mortem breath movements are rarely reported. We present two cases of broncho-pulmonary aspiration of brain and cartilage tissue following two fatal suicidal gunshots to the head. We also discuss the physiopathological implications for the agony.

The cellular building blocks of breathing

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

State-dependent interactions between excitatory neuromodulators in the neuronal control of breathing

All neuronal networks are modulated by multiple neuropeptides and biogenic amines. Yet, few studies investigate how different modulators interact to regulate network activity. Here we explored the state-dependent functional interactions between three excitatory neuromodulators acting on neurokinin1 (NK1), alpha1 noradrenergic (alpha1 NE), and 5-HT2 serotonin receptors within the pre-Bötzinger complex (pre-BötC), an area critical for the generation of breathing. In anesthetized, in vivo mice, the reliance on endogenous NK1 activation depended on spontaneous breathing frequency and the modulatory state of the animal. Endogenous NK1 activation had no significant respiratory effect when stimulating raphe magnus and/or locus ceruleus, but became critical when alpha1 NE and 5-HT2 receptors were pharmacologically blocked. The dependence of the centrally generated respiratory rhythm on NK1 activation was blunted in the presence of alpha1 NE and 5-HT2 agonists as demonstrated in slices containing the pre-BötC. We conclude that a modulator's action is determined by the concurrent modulation and interaction with other neuromodulators. Deficiencies in one neuromodulator are immediately compensated by the action of other neuromodulators. This interplay could play a role in the state dependency of certain breathing disorders.

Degeneracy as a substrate for respiratory regulation

Recent studies in vivo and in vitro suggest that both respiratory rhythmogenesis and its central chemosensory modulation arise from multiple, mechanistically and/or anatomically distinct networks whose outputs are similar. These observations are consistent with degeneracy, defined as the ability of structurally distinct elements to generate similar function. This review argues that degeneracy is an essential feature of respiratory networks, ensuring the survival of the individual organism over the course of development, and accounting for the transformation of respiratory biomechanics over evolutionary time. At faster timescales, respiration must adapt continuously and rapidly to changes in metabolic demand and ambient conditions to maintain blood-gas homeostasis. Control theory, which formalizes homeostasis, states axiomatically that rapid responsiveness can only be achieved with high gain, but high gain comes at the cost of instability. Homeostatic systems displaying highly optimized tolerance (HOT) mitigate the instability accompanying high gain by incorporating regulatory mechanisms that provide protection against expected perturbations, yet these systems remain fragile to catastrophic failure in response to rare events. Because the multiple mechanisms that are conjectured to mediate respiratory rhythmogenesis and chemosensation have distinct ranges of activity and responses to modulatory input, they provide a richer substrate for respiratory regulation than those of any single mechanism. Respiration, though robust, remains fragile to rare perturbations, matching a key feature of HOT. These observations support the conclusion that degeneracy provides the substrate for respiratory regulation, and that the resulting regulatory system conforms to HOT.

Mylohyoid discharge of the in situ rat: a probe of pontile respiratory activities in eupnea and gasping

Our purpose was to characterize respiratory-modulated activity of the mylohyoid nerve. Since its motoneurons are in the trigeminal motor nucleus, mylohyoid discharge could serve as a probe of the role of pontile mechanisms in the generation of respiratory rhythms. Studies were performed in the decerebrate, perfused in situ preparation of the rat. Phrenic discharge was recorded as the index of the respiratory rhythm. In eupnea, the mylohyoid nerve discharged primarily during neural expiration, in the period between phrenic bursts. This expiratory discharge increased greatly in hypoxia and fell in hypercapnia. The hypoxia-induced increase in mylohyoid discharge was due, at least in part, to a direct influence of hypoxia on the brain stem. In ischemia, phrenic discharge increased, and then declined to apnea, which was succeeded by gasping. The mylohyoid nerve discharged tonically during the apneic period, but still declined during each of the phrenic bursts of gasping. This maintenance of a respiratory-modulation of the mylohyoid discharge in gasping supports the concept that a release of medullary mechanisms, rather than a ubiquitous suppression of pontile influences, underlies the neurogenesis of gasping. Results also provide additional support for our conclusion that activity of any single cranial nerve does not provide an accurate index of the type of respiratory rhythm, be it eupnea or gasping.

Noeud vital for breathing in the brainstem: gasping--yes, eupnoea--doubtful

For the past 200 years, various regions of the brainstem have been proposed as a 'noeud vital' for breathing-a critical region which, when destroyed, results in an irreversible cessation of breathing and death. Complicating this search for a noeud vital is the extensive network of neurons that comprises the brainstem respiratory control system of pons and medulla. Does a cessation of breathing following ablation of any region reflect the removal of a critical set of neurons whose activity generates the respiratory rhythm or does it reflect the interruption of one component of the neuronal circuit, such that this circuit cannot function, at least temporarily? An additional complication is that in contemporary neuroscience, a number of in vitro preparations have been introduced for the study of the generation of the respiratory rhythms. However, how are the rhythms that these preparations generate related to normal breathing? Are these rhythms similar to those of gasping, which is recruited when normal, eupnoeic breathing fails, or are these rhythms unique to the in vitro preparation and not related to any breathing pattern in vivo?

Discharge of the hypoglossal nerve cannot distinguish eupnea from gasping, as defined by phrenic discharge, in the in situ mouse

If normal, eupneic breathing fails, gasping is recruited. Serotonin was proposed as essential for gasping, based on findings using an in vitro mouse preparation. This preparation generates rhythmic activities of the hypoglossal nerve that are considered to be akin to both eupnea and gasping. In previous studies, gasping of in situ rat and mouse preparations continued unabated following blockers of receptors for serotonin. However, hypoglossal activity was not recorded in the mouse, and we hypothesized that its discharge during gasping might be dependent on serotonin. In the in situ mouse preparation, hypoglossal discharge had varying and inconsistent patterns during eupnea, discharging concomitant with the phrenic burst, at varying intervals between phrenic bursts, or was silent in some respiratory cycles. In eupnea, phrenic discharge was incrementing, whereas hypoglossal discharge was decrementing in 15 of 20 preparations. During ischemia-induced gasping, peak phrenic height was reached at 205 +/- 17 ms, compared with 282 +/- 27.9 ms after the start of the eupneic burst (P < 0.002). In contrast, rates of rise of hypoglossal discharge in gasping (peak at 233 +/- 25 ms) and eupnea (peak at 199 +/- 19.2 ms) were the same. The uncoupling of hypoglossal from phrenic discharge in eupnea was exacerbated by methysergide, an antagonist of serotonin receptors. These findings demonstrate that hypoglossal discharge alone cannot distinguish eupnea from gasping nor, in eupnea, can hypoglossal activity be used to differentiate neural inspiration from expiration. These findings have significant negative implications for conclusions drawn from the in vitro medullary slice of mouse.

Location and properties of respiratory neurones with putative intrinsic bursting properties in the rat in situ

Using the in situ arterially perfused preparations of both neonatal and juvenile rats, we provide the first description of the location, morphology and transmitter content of a population of respiratory neurones that retains a bursting behaviour after ionotropic receptor blockade. All burster neurones exhibited an inspiratory discharge during eupnoeic respiration. These neurones were predominantly glutamatergic, and were located within a region of the ventral respiratory column that encompasses the pre-Bötzinger complex and the more caudally located ventral respiratory group. Bursting behaviour was both voltage and persistent sodium current dependent and could be stimulated by sodium cyanide to activate this persistent sodium current. The population of burster neurones may overlap with that previously described in the neonatal slice in vitro. Based upon the present and previous findings, we hypothesize that this burster discharge may be released when the brain is subject to severe hypoxia or ischaemia, and that this burster discharge could underlie gasping.

Structure-function analysis of rhythmogenic inspiratory pre-Bötzinger complex networks in "calibrated" newborn rat brainstem slices

Inspiratory pre-Bötzinger complex (preBötC) networks remain active in perinatal rodent brainstem slices. Our analysis of (crescendo-like) inspiratory-related population and cellular bursting in novel histologically identified transversal preBötC slices in physiological (3 mM) superfusate [K(+)] revealed: (i) the preBötC extent sufficient for rhythm in thin slices is at most 175 microm. (ii) In 700 microm thick slices with unilaterally exposed preBötC, a <100 microm kernel generates a eupnea-like inspiratory pattern under predominant influence of caudally adjacent structures or thyrotropin-releasing hormone-like transmitters, but a mixed eupnea-sigh-like pattern when influence of rostral structures or substance-P-like transmitters dominates. (iii) Primarily presynaptic processes may underlie inhibition of rhythm by opioids or raising superfusate [Ca(2+)] from lower to upper physiological limits (1-1.5 mM). (iv) High K(+) reverses depression of rhythm by raised Ca(2+), opioids and anoxia. In summary, distinct activity patterns of spatiochemically organized isolated inspiratory networks depend on both an extracellular "Ca(2+)-K(+) antagonism" and slice dimensions. This explains some discrepant findings between studies and suggests use of "calibrated" slices and more uniform experimental conditions.

Genesis of gasping is independent of levels of serotonin in the Pet-1 knockout mouse

Eupnea is normal breathing. If eupnea fails, as in severe hypoxia or ischemia, gasping is recruited. Gasping can serve as a powerful mechanism for autoresuscitation. A failure of autoresuscitation has been proposed as a basis of the sudden infant death syndrome. In an in vitro preparation, endogenous serotonin is reported to be essential for expression of gasping. Using an in situ preparation of the Pet-1 knockout mouse, we evaluated such a critical role for serotonin. In this mouse, the number of serotonergic neurons is reduced by 85-90% compared with animals without this homozygous genetic defect. Despite this reduction in the number of serotonergic neurons, phrenic discharge in eupnea and gasping of Pet-1 knockout mice was not different from that of wild-type mice. Indeed, gasping continued unabated, even after administration of methysergide, a blocker of many types of receptors for serotonin, to Pet-1 knockout mice. We conclude that serotonin is not critical for expression of gasping. The proposal for such a critical role, on the basis of observations in the in vitro slice preparation, may reflect the minimal functional neuronal tissue and neurotransmitters in this preparation, such that the role of any remaining neurotransmitters is magnified. Also, rhythmic activity of the in vitro slice preparation has been characterized as eupnea or gasping solely on the basis of activity of the hypoglossal nerve or massed neuronal activities of the ventrolateral medulla. The accuracy of this method of classification has not been established.

Brain-derived neurotrophic factor enhances fetal respiratory rhythm frequency in the mouse preBötzinger complex in vitro

Brain-derived neurotrophic factor (BDNF) is required during the prenatal period for normal development of the respiratory central command; however, the underlying mechanisms remain unknown. To approach this issue, the present study examined BDNF regulation of fetal respiratory rhythm generation in the preBötzinger complex (preBötC) of the mouse, using transverse brainstem slices obtained from prenatal day 16.5 animals. BDNF application (100 ng/mL, 15 min) increased the frequency of rhythmic population activity in the preBötC by 43%. This effect was not observed when preparations were exposed to nerve growth factor (100 ng/mL, 30 min) or pretreated with the tyrosine kinase inhibitor K252a (1 h, 200 nm), suggesting that BDNF regulation of preBötC activity requires activation of its cognate tyrosine receptor kinase, TrkB. Consistent with this finding, single-cell reverse transcription-polymerase chain reaction experiments showed that one third of the rhythmically active preBötC neurons analysed expressed TrkB mRNA. Moreover, 20% expressed BDNF mRNA, suggesting that the preBötC is both a target and a source of BDNF. At the network level, BDNF augmented activity of preBötC glutamatergic neurons and potentiated glutamatergic synaptic drives in respiratory neurons by 34%. At the cellular level, BDNF increased the activity frequency of endogenously bursting neurons by 53.3% but had no effect on basal membrane properties of respiratory follower neurons, including the Ih current. Our data indicate that BDNF signalling through TrkB can acutely modulate fetal respiratory rhythm in association with increased glutamatergic drive and bursting activity in the preBötC.

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.

Respiratory control in neonatal rats exposed to prenatal cigarette smoke

RATIONALE

Prenatal cigarette smoke (CS) exposure, increased environmental temperature, and hypoxic episodes have been postulated as major risk factors for sudden infant death syndrome.

OBJECTIVES

To test the hypothesis that maternal CS exposure disrupts eupneic breathing and depresses breathing responses of neonatal rats to thermal and hypoxic challenges.

METHODS

Experiments were performed on 1-week-old rat pups exposed prenatally to CS (n = 39) or room air (sham; n = 30). Breathing patterns were recorded by whole-body plethysmography during thermoneutral or hyperthermic states under normoxic and hypoxic conditions.

MEASUREMENTS AND MAIN RESULTS

Mean pup weight, breaths per minute, and gasping respiratory patterns were measured for both smoke- and sham-exposed groups during thermoneutral and hyperthermic states under normoxic and hypoxic conditions. Under thermoneutral conditions, hypoxia caused gasping in CS-exposed animals but not in sham-exposed animals. Furthermore, under hyperthermic conditions, whereas hypoxia induced gasping in both groups, only CS-exposed animals exhibited a pronounced and longer lasting respiratory depression after the termination of hypoxia.

CONCLUSIONS

We show that prenatal CS exposure increases the likelihood of gasplike respiration and provide the first experimental evidence that the combined effects of prenatal CS exposure and hyperthermia dramatically prolong the time required for neonates to return to eupneic breathing after hypoxia. These observations provide important evidence of how prenatal CS exposure, hypoxic episodes, and hyperthermia might place infants at higher risk for sudden infant death syndrome.

ATP sensitivity of preBötzinger complex neurones in neonatal rat in vitro: mechanism underlying a P2 receptor-mediated increase in inspiratory frequency

P2 receptor (R) signalling plays an important role in the central ventilatory response to hypoxia. The frequency increase that results from activation of P2Y(1)Rs in the preBötzinger complex (preBötC; putative site of inspiratory rhythm generation) may contribute, but neither the cellular nor ionic mechanism(s) underlying these effects are known. We applied whole-cell recording to rhythmically-active medullary slices from neonatal rat to define, in preBötC neurones, the candidate cellular and ionic mechanisms through which ATP influences rhythm, and tested the hypothesis that putative rhythmogenic preBötC neurones are uniquely sensitive to ATP. ATP (1 mm) evoked inward currents in all non-respiratory neurones and the majority of respiratory neurons, which included inspiratory, expiratory and putative rhythmogenic inspiratory neurones identified by sensitivity to substance P (1 microM) and DAMGO (50 microM) or by voltage-dependent pacemaker-like activity. ATP current densities were similar in all classes of preBötC respiratory neurone. Reversal potentials and input resistance changes for ATP currents in respiratory neurones suggested they resulted from either inhibition of a K(+) channel or activation of a mixed cationic conductance. The P2YR agonist 2MeSADP (1 mm) evoked only the latter type of current in inspiratory and pacemaker-like neurones. In summary, putative rhythmogenic preBötC neurones were sensitive to ATP. However, this sensitivity was not unique; ATP evoked similar currents in all types of preBötC respiratory neurone. The P2Y(1)R-mediated frequency increase is therefore more likely to reflect activation of a mixed cationic conductance in multiple types of preBötC neurone than excitation of one, highly sensitive group.

Maintenance of gasping and restoration of eupnea after hypoxia is impaired following blockers of alpha1-adrenergic receptors and serotonin 5-HT2 receptors

In severe hypoxia or ischemia, normal eupneic breathing fails and is replaced by gasping. Gasping serves as part of a process of autoresuscitation by which eupnea is reestablished. Medullary neurons, having a burster, pacemaker discharge, underlie gasping. Conductance through persistent sodium channels is essential for the burster discharge. This conductance is modulated by norepinephrine, acting on alpha 1-adrenergic receptors, and serotonin, acting on 5-HT2 receptors. We hypothesized that blockers of 5-HT2 receptors and alpha 1-adrenergic receptors would alter autoresuscitation. The in situ perfused preparation of the juvenile rat was used. Integrated phrenic discharge was switched from an incrementing pattern, akin to eupnea, to the decrementing pattern comparable to gasping in hypoxic hypercapnia. With a restoration of hyperoxic normocapnia, rhythmic, incrementing phrenic discharge returned within 10 s in most preparations. Following addition of blockers of alpha 1-adrenergic receptors (WB-4101, 0.0625-0.500 microM) and/or blockers of 5-HT2 (ketanserin, 1.25-10 microM) or multiple 5-HT receptors (methysergide, 3.0-10 microM) to the perfusate, incrementing phrenic discharge continued. Fictive gasping was still induced, although it ceased after significantly fewer decrementing bursts than in preparations than received no blockers. Moreover, the time for recovery of rhythmic activity was significantly prolonged. This prolongation was in excess of 100 s in all preparations that received both WB-4101 (above 0.125 microM) and methysergide (above 2.5 microM). We conclude that activation of adrenergic and 5-HT2 receptors is important to sustain gasping and to restore rhythmic respiratory activity after hypoxia-induced depression.

Eupnea of in situ rats persists following blockers of in vitro pacemaker burster activities

Two groups of intrinsically bursting neurons, linked to respiration, have been identified using in vitro medullary slice preparations. One group is dependent upon a calcium-activated nonspecific cationic current that is blocked by flufanemic acid. This group is hypothesized as essential for eupnea, but not gasping. The second group is dependent upon conductance through persistent sodium channels that is blocked by riluzole. This group is proposed to underlie both eupnea and gasping. In the decerebrate in situ preparation of the juvenile rat, flufanemic acid caused an increase in frequency and a decrease in peak level of the phrenic and vagus nerve activities in both eupnea and gasping. Similar changes in eupnea followed the simultaneous blockades by flufanemic acid and riluzole. However, gasping was eliminated. These results do not support the hypothesis that conductances through either persistent sodium channels or calcium-activated nonspecific cationic channels are essential for the neurogenesis of eupnea. However, gasping does depend upon a conductance through persistent sodium channels.