Respiratory Phase Transition Research Articles

Inhibitory control of active expiration by the Bötzinger complex in rats.

Contraction of abdominal muscles at the end of expiration during metabolic challenges (such as hypercapnia and hypoxia) improves pulmonary ventilation. The emergence of this active expiratory pattern requires the recruitment of the expiratory oscillator located on the ventral surface of the medulla oblongata. Here we show that an inhibitory circuitry located in the Bötzinger complex is an important source of inhibitory drive to the expiratory oscillator. This circuitry, mediated by GABAergic and glycinergic synapses, provides expiratory inhibition that restrains the expiratory oscillator under resting condition and regulates the formation of abdominal expiratory activity during active expiration. By combining experimental and modelling approaches, we propose the organization and connections within the respiratory network that control the changes in the breathing pattern associated with elevated metabolic demand. The expiratory neurons of the Bötzinger complex (BötC) provide inhibitory inputs to the respiratory network, which, during eupnoea, are critically important for respiratory phase transition and duration control. Here, we investigated how the BötC neurons interact with the expiratory oscillator located in the parafacial respiratory group (pFRG) and control the abdominal activity during active expiration. Using the decerebrated, arterially perfused in situ preparations of juvenile rats, we recorded the activity of expiratory neurons and performed pharmacological manipulations of the BötC and pFRG during hypercapnia or after the exposure to short-term sustained hypoxia - conditions that generate active expiration. The experimental data were integrated in a mathematical model to gain new insights into the inhibitory connectome within the respiratory central pattern generator. Our results indicate that the BötC neurons may establish mutual connections with the pFRG, providing expiratory inhibition during the first stage of expiration and receiving excitatory inputs during late expiration. Moreover, we found that application of GABAergic and glycinergic antagonists in the BötC caused opposing effects on abdominal expiratory activity, suggesting complex inhibitory circuitry within the BötC. Using mathematical modelling, we propose that the BötC network organization and its interactions with the pFRG restrain abdominal activity under resting conditions and contribute to abdominal expiratory pattern formation during active expiration observed during hypercapnia or after the exposure to short-term sustained hypoxia.

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

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 8-10mm3 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. 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.

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

One of the major goals of modern neuroscience is to understand the relationship between the functional neuroanatomy/connectivity of neural circuits and the behavior of an animal. The key challenge to addressing this goal with respect to control of breathing is the distributed nature of the respiratory network. While in cortical networks, short‐range connections (10–100 μm) out‐number long‐range connections (1+ mm), the opposite may be true of brainstem circuits. Specifically, anatomical tracing studies have identified many long‐range projections between brainstem respiratory nuclei that led to the concept that the brainstem respiratory network is compartmentally organized. Thus, measuring the functional neuroanatomy of the respiratory network—a task required to phenotype the functional role of neuronal populations identified in molecular mapping studies—requires monitoring the activity of respiratory neurons which may be spread over many millimeters.We hypothesized that the spatio‐temporal structure 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 in single preparations. To address our hypothesis, we developed an approach using silicon multi‐electrode arrays to record spontaneous LFPs from hundreds of electrode sites across the ponto‐medullary volume of the respiratory network while monitoring the respiratory motor pattern on phrenic and vagal nerves.Our results revealed the expression of rLFPs across the brainstem respiratory network. rLFPs were expressed selectively 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, rather than being equally distributed across the respiratory cycle. Spatially, the E2‐I (inspiratory on‐switch), and PI‐E2 transitions were localized to the ventral and dorsal respiratory groups, respectively, whereas the I‐PI (inspiratory off‐switch) transition was distributed across the ventral, dorsal and pontine respiratory groups. An independent component analysis (ICA) confirmed this spatio‐temporal organisation of rLFPs and identified a traveling wave of rLFPs that occurred at the I‐PI transition. Finally, a group‐wise ICA demonstrated that all preparations exhibited rLFPs with a similar temporal structure.Overall, under intact network conditions, our results confirm that inspiration is initiated by the pre‐Bötzinger complex, whereas post‐inspiration and late‐expiration depend on activity throughout the brainstem respiratory network. In conclusion, we have developed a general approach to volumetrically map spontaneous‐ or evoked‐respiratory network activity at the brainstem‐wide scale in single preparations to inform our understanding of the network mechanisms underlying the neural control of breathing.Support or Funding InformationThis work was supported by grants from the Australian Research Council (to MD), National Institutes of Health (to TED) and the Hartwell Foundation (to RFG).

Rubral modulation of breathing.

What is the topic of this review? Rubral modulation of pontomedullary respiratory rhythm and pattern generating circuitry powerfully contributes to regulation of breathing. What advances does it highlight? Studies have demonstrated extensive rubromedullary and rubrospinal projections to zones generating and organizing the respiratory rhythm and pattern. Rubral modulation of respiratory output effects inspiratory expiratory phase transitions with stimulation generating inhibitory or excitatory responses of medullary inspiratory and expiratory units. The red nucleus mediates hypoxic ventilatory depression, integrates respiratory output with oromotor and locomotor activity, and modulates respiratory output during noxious stimulation. Although normal triphasic eupnoea can be produced by the pontomedullary respiratory network after pontomesencephalic transection, the midbrain provides important modulation of respiration. Specifically, stimulation of the red nucleus elicits inspiratory inhibition, as manifest in the phrenic neurogram, in addition to excitation and inhibition of individual medullary respiratory-related units, with the majority of premotor units that receive rubral modulation being inhibited. Stimulation of the red nucleus also induces respiratory phase transitions, which appear to be pontine independent. These effects might be mediated by rubrobulbar and/or rubrospinal tracts. Although lesioning of the red nucleus does not alter respiration in normoxic conditions, it eliminates hypoxic ventilatory depression, which is the second phase of the biphasic ventilatory response to low oxygen tension. The finding that the red nucleus also plays a role in anti-nociception suggests that it might coordinate respiratory responses during noxious stimulation and, given that the red nucleus regulates upper limb flexors, it might represent one region in a distributed bulbar network contributing to respiratory-locomotor integration. Modulation of jaw opening by the red nucleus would support a model whereby it coordinates oromotor activity with breathing. Thus, the multiplicity of roles played by the red nucleus aptly position it to coordinate respiration in a variety of behavioural states. In this review, we seek to highlight the different features and regional specializations of the rubral contribution to respiratory control and underscore its vital importance to breathing in the freely behaving mammal.

Abstract MP081: Short-long-short Sequence of RR Intervals is Associated With Increased Risk of Sudden Cardiac Death: the Atherosclerosis Risk in Community Study

Introduction: A Short-long-short sequence of RR’ intervals frequently precedes ventricular tachyarrhythmia. However, an association of RR’ alternans with sudden cardiac death (SCD) in the general population has not been previously studied. Hypothesis: We hypothesized that RR’ alternans is associated with SCD. Methods: We analyzed baseline visit 10-second ECG in 14,250 participants of the Atherosclerosis Risk in Communities (ARIC) cohort (mean age 54.1±5.87y; 45% men; 74% white). Only normal sinus beats were included in the semi-automated analysis. Digital 12-lead ECG was transformed into orthogonal ECG, from which respiratory signal was derived. Joint symbolic analysis was used to assess increase or decrease in successive RR’ intervals and R-instant respiratory phases calculated using the Hilbert transform. Short-long-short or long-short-long RR’ sequence was defined as RR’ alternans. RR’ alternans occurring during respiratory phase transitions were not considered. SCD, non-SCD, and non-cardiac death served as competing outcomes. Three competing risks models were constructed. Model 1 was adjusted for age, sex, race, and study center. Model 2 was in addition adjusted for prevalent CVD. Model 3 was further adjusted for subclinical CVD, its risk factors, including traditional ECG predictors, and medications affecting SA or AV nodes. Results: Over median follow-up of 24.4 years, there were 497 SCDs (incidence 1.66 [95%CI 1.52-1.82], 742 non-sudden cardiac deaths (incidence 2.48 [95%CI 2.31-2.67], and 3,753 non-cardiac deaths (incidence 12.6 [95%CI 12.1-13.0]) per 1,000 person-years. RR alternans was present in 1,679(11.8%) participants. In competing risks analysis (Table) the presence of RR’ alternans, excluding those occurring due to respiratory phase changes, was independently associated with increased risk of SCD. Conclusion: In competing risks analysis the presence of RR’ alternans that are not due to respiratory phase transition is associated with SCD. Further study of RR’ alternans is warranted.

Reproducibility of Heart Rate Variability Characteristics Measured on Random 10-second ECG using Joint Symbolic Dynamics.

In this paper we employed joint symbolic dynamics (JSD) approach to study reproducibility of heart rate variability characteristics measured on 2 randomly selected 10-second segments within 3-minute resting orthogonal ECG in 170 healthy participants. First, the ECG R-peaks were detected using parabolic fitting. Second, the respiratory signal was derived from orthogonal ECG X-lead using QRS slopes. Third, time series of R-R intervals and respiratory phases (calculated using Hilbert transform), were transformed into tertiary symbol vectors based on their successive changes and words of length '3' were formed. Bland-Altman analysis was used to assess the agreement between measured log-transformed JSD characteristics of HRV, and their reproducibility. Traditional HRV measures such as RR' interval changes showed a very high reproducibility. However, agreement between two 10-second JSD indices of HRV was low. Interestingly, a significant decrease in low-high alterations of HRV dynamics measured using JSD was observed when respiratory phase transition intervals were excluded (10s: 4.7±9.4 vs. 24.8±21.0%, p<0.0001; 3min: 9.8±8.1 vs. 24.8±12.3%, p<0.0001).

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

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

Emergence of brain‐derived neurotrophic factor‐induced postsynaptic potentiation of NMDA currents during the postnatal maturation of the Kölliker–Fuse nucleus of rat

The Kölliker-Fuse nucleus (KF) contributes essentially to respiratory pattern formation and adaptation of breathing to afferent information. Systems physiology suggests that these KF functions depend on NMDA receptors (NMDA-R). Recent investigations revealed postnatal changes in the modulation of glutamatergic neurotransmission by brain-derived neurotrophic factor (BDNF) in the KF. Therefore, we investigated postnatal changes in NMDA-R subunit composition and postsynaptic modulation of NMDA-R-mediated currents by BDNF in KF slice preparations derived from three age groups (neonatal: postnatal day (P) 1-5; intermediate: P6-13; juvenile: P14-21). Immunohistochemistry showed a developmental up-regulation of the NR2D subunit. This correlated with a developmental increase in decay time of NMDA currents and a decline of desensitization in response to repetitive exogenous NMDA applications. Thus, developmental up-regulation of the NR2D subunit, which reduces the Mg(2+) block of NMDA-R, causes these specific changes in NMDA current characteristics. This may determine the NMDA-R-dependent function of the mature KF in the control of respiratory phase transition. Subsequent experiments revealed that bath-application of BDNF progressively potentiated these repetitively evoked NMDA currents only in intermediate and juvenile age groups. Pharmacological inhibition of protein kinase C (PKC), as a downstream component of the BDNF-tyrosine kinase B receptor (trkB) signalling, prevented BDNF-induced potentiation of NMDA currents. BDNF-induced potentiation of NMDA currents in later developmental stages might be essential for synaptic plasticity during the adaptation of the breathing pattern in response to peripheral/central commands. The lack of plasticity in neonatal neurones strengthens the hypothesis that the respiratory network becomes permissive for activity-dependent plasticity with ongoing postnatal development.

Nitric oxide and respiratory rhythm in mammals: a new modulator of phase transition?

NO (nitric oxide) modulates several central pattern generators, but its role in respiratory rhythmogenesis and its mode of action on medullary respiratory neurons during normoxia are unknown. We analysed the actions of NO on the mammalian respiratory network at the system and cellular levels. Given systemically, the NO donor diethylamine NONOate increased post-inspiratory duration in vagus, phrenic and hypoglossal nerves, whereas blockade of NO generation with L-NAME (N(G)-nitro-L-arginine methyl ester) produced the opposite response. At the cellular level, we pressure-ejected the NO donor on to respiratory neurons. NO had both inhibitory and excitatory effects on all types of respiratory neurons. Inhibitory effects involved soluble guanylate cyclase, as they were blocked with ODQ (1H-[1,2,4]oxadiazolo[4,3a]quinoxalin-1-one), whereas excitations were antagonized by uric acid and possibly mediated via peroxynitrite. Importantly, NO facilitated both GABA (gamma-aminobutyric acid)- and NMDA (N-methyl-D-aspartate)-induced neuronal responses, but this was restricted to post-inspiratory and pre-inspiratory neurons; other neuron types showed additive effects only. Our results support NO as modulator of centrally generated respiratory activity and specifically of ligand-mediated responses in respiratory neuron types involved in respiratory phase transition.

Modeling the ponto-medullary respiratory network

The generation and shaping of the respiratory motor pattern are performed in the lower brainstem and involve neuronal interactions within the medulla and between the medulla and pons. A computational model of the ponto-medullary respiratory network has been developed by incorporating existing experimental data on the medullary neural circuits and possible interactions between the medulla and pons. The model reproduces a number of experimental findings concerning alterations of the respiratory pattern following various perturbations/stimulations applied to the pons and pulmonary afferents. The results of modeling support the concept that eupneic respiratory rhythm generation requires contribution of the pons whereas a gasping-like rhythm (and the rhythm observed in vitro) may be generated within the medulla and involve pacemaker-driven mechanisms localized within the medullary pre-Botzinger Complex. The model and experimental data described support the concept that during eupnea the respiration-related pontine structures control the medullary network mechanisms for respiratory phase transitions, suppress the intrinsic pacemaker-driven oscillations in the pre-BotC and provide inspiration-inhibitory and expiration-facilitatory reflexes which are independent of the pulmonary Hering-Breuer reflex but operate through the same medullary phase switching circuits.

Models of neuronal bursting behavior: implications for in-vivo versus in-vitro respiratory rhythmogenesis.

Neonatal in-vitro preparations can generate respiratory-like oscillations based on the intrinsic bursting or pacemaker properties in some neurons within the medullary preBötzinger complex (PBC)1-3. This discovery provided a basis for a hybrid pacemaker-network theory that suggests that pacemaker PBC neurons form a kernel of the central pattern generator and hence provide a necessary contribution to the genesis of the respiratory rhythm4. However, it is controversial whether a pacemaker-based mechanism necessarily operates in vivo. Specifically, the pacemaker-based models face principal problems in providing explanations to systems-level phenomena such as an independent regulation of respiratory phases, respiratory reflexes (e.g. the Hering-Breuer reflex), various phase resetting phenomena. On the other hand, the network theories and models based on in-vivo data can explain these phenomena5,6, but demand the respiratory phase transitions to be based on the reciprocal inhibitory interactions between respiratory neurons. Therefore, the network-based models have so far been unable to explain a pacemaker-driven rhythm recorded in vitro which is resistant to blockade of synaptic inhibition7. The current state of knowledge in the field requires a comprehensive computational study of the relationships between in-vivo and in-vitro data. The objective of this study was to explore the concept that the respiratory network can generate a breathing pattern by either a network or a hybrid pacemaker-network mechanism, which is state-dependent.KeywordsRespiratory RhythmRespiratory NeuronTonic FiringPacemaker NeuronExcitatory DriveThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Interactions of breathing with the postural regulation of the fingers

Objectives: The present study was performed to detect the interactions between breathing and the postural motor control of the fingers. A previous study revealed a scattering in the intraindividual motor responses which resembled a grouping. It is hypothesized to result from the influence of breathing. Methods: Torque impulses and torque steps at two intensities were applied to 17 healthy volunteers, to the II–IV fingers of their right hand. The subjects had to compensate for these additional torque loads that were triggered by a breathing-related signal and elicited at 4 different moments within a breath. Results: We demonstrated mutual influences between breathing and the regulation of finger posture. The reaction to a torque load was faster at the beginning of inspiration but more precise when the torque load was applied in mid-expiration. The motor response to torque loads was accompanied by changes in the breathing time course, particularly when the torque load was elicited during inspiration. The effects were most pronounced when torque steps were applied. The intensity of torque loads had no significant influence. Additionally, we observed that the respiratory phase-transitions often coincided with the end of the applied torque steps. Conclusions: The results correspond well with the interactions existing between breathing and single or rhythmical movements. Further investigations of motor functions should consider this interdependence with breathing.

Mutual nervous influences between breathing and precision finger movements.

The precision of short-term finger tracking flexions has been shown to vary with the respiratory cycle. In the present study, we analysed the mutual effects between breathing and short-term finger tracking movements (SFTM)--both flexions and extensions. Moreover, we investigated the preferred phase relationships between breathing and spontaneous single finger flexions and extensions. Two types of experiments were carried out. Fifteen volunteers participated in the finger tracking experiments. In one experimental session, a square-wave function served as a tracking signal that required a rapid finger flexion, and in another session it required a finger extension. In the second type of experiment, 14 volunteers performed spontaneous short-term finger flexions and extensions of a pre-defined amplitude, with the starting point chosen at their convenience. SFTM were associated with modulations in the time course of the respiratory cycle. These were more pronounced for finger flexions than for extensions. Likewise, the precision of finger flexions, but not extensions, showed significant respiratory-phase-dependent differences. The largest tracking errors occurred at the end of expiration in finger flexions and at the end of inspiration in finger extensions. Spontaneous finger flexions tended to start at around the respiratory phase transitions. Spontaneous extensions, however, tended to start at early expiration (12.5-37.5% of expiration time). The results demonstrate that both spontaneous and tracking finger flexions and extensions are influenced by different stages of a breath. However, spontaneous finger movements did not tend to start at the stages of breaths that were associated with the highest movement precision in the tracking tests. Moreover, these results suggest that finger flexions are more closely related to the respiratory rhythm than are finger extensions.

The influence of hypercapnic hyperpnea on the interaction between breathing and finger tracking movements in humans.

Numerous studies on entrainment of breathing to simultaneous limb movements, for instance walking, running, cycling, or finger tapping, exclusively referred to effects of the limb movements on breathing1,2,5,6,10,17. None of them considered effects of breathing on the limb movement which are much less obvious than movement-induced changes in breathing rhythm. In a previous study on finger tracking movements we could show that both influences of finger movements on the respiratory rhythm and breathing influences on the finger movements do exist13. The effect of breathing on the tracking movement consisted in more precise tracking in mid-inspiration and mid-expiration and greater tracking errors at the respiratory phase-transitions. Tracking movements influenced the respiratory rhythm by shortening the coinciding inspiration or expiration, particularly when they were started during inspiration.

Coordination Between Breathing and Finger Tracking in Man.

Arm and leg movements are known to produce temporal pattern changes of breathing. This can be interpreted as coordination, as defined by von Holst (1939). The aim of the present study was to find whether breathing exerts an influence in a reverse direction on a nonrespiratory movement as well. A pursuit tracking test was used, and test individuals (N = 19) were instructed to track a visually presented step function by flexion or extension of their right index finger. Velocity and precision of the step responses proved to be dependent on their relation to the breathing time course; the differences between inspiratory and expiratory responses were smaller than those within each half-cycle. The movements were performed more rapidly and more precisely in about the middle of each half-cycle than immediately after the respiratory phase transition or during the second half of each inspiration or expiration. Discontinuous short-lasting motor actions exerted a coordinative influence on respiration comparablewith that of periodical events: Breaths coinciding with step responses were shortened, preferably when the preset step was given early in the inspiration. It was hypothesized that the reciprocal effect between both motor actions changes periodically. In the first part of each respiratory half-cycle, the respiratory rhythm exerts only a weak influence on additional movements, but it can be altered easily by simultaneous motor processes. Toward the respiratory phase-switching, the respiratory rhythm behaves more stably against coordinative influences and becomes capable of impairing an additional movement.

Involvement of NMDA receptors in the respiratory phase transition is different in the adult guinea pig in vivo and in the isolated brain stem preparation.

1. We investigated the involvement of N-methyl-D-aspartate (NMDA) receptors in the respiratory pattern in an in vitro preparation of adult brain stem compared with in vivo conditions in the guinea pig. 2. In vivo, combining administration of the NMDA channel blocker dizocilpine (MK-801) (3 mg/kg) with a surgical section of the vagus nerves induced an apneustic type of respiration characterized by long inspiratory "holds," as has been shown in other species. The same effect was observed in hypothermic animals (30 degrees C). 3. The isolated in vitro brain stems from these apneustic animals did not present a prolonged inspiratory phase. A second dose of dizocilpine (100 microM perfused vascularly did not induce apneusis, even after increasing brain stem temperature to 35.5 degrees C. 4. In another group of isolated brain stems of adult guinea pigs anesthetized with pentobarbital sodium before decapitation, we perfused dizocilpine and NMDA through the basilar artery. The duration of periodic inspiratory motor activity recorded from the hypoglossal nerve was unaffected by dizocilpine (1-100 microM) or the competitive NMDA antagonist D- or DL-2-amino-5-phosphonopentanoic acid (100 microM and 1 mM), although respiratory frequency decreased. The increase in respiratory activity produced by vascularly perfused NMDA (25-100 microM) was blocked by dizocilpine (100 microM). 5. We conclude that the central mechanism of inspiratory termination in the vagotomized adult guinea pig requires the activation of NMDA receptors in vivo but not in vitro. This difference is not due to the hypothermic environment in vitro. Possible mechanisms for phase switching in vitro are discussed.

Pre-Bötzinger complex in the cat.

1. Patterns of respiratory neuronal activity were examined in pentobarbitone anesthetized adult cats in a circumscribed area of the ventrolateral medulla, which has previously been defined as the pre-Bötzinger complex (pre-BOTC) from electrophysiological and morphological criteria in the brain stem-spinal cord preparation of the neonatal rat. The pre-BOTC has been proposed to play a critical role in respiratory rhythm generation in mammals, but electrophysiological properties of the region have not been thoroughly characterized in the adult brain stem in vivo. 2. From intra- and extracellular recordings, we verified the existence of a well-defined zone with a distinct profile of neuronal activity between the rostral Bötzinger complex containing expiratory neurons and the more caudal medullary pool of inspiratory neurons of the ventral respiratory group (VRG) in the para-ambigual region. This zone corresponds to the pre-BOTC. It was characterized by a concentration of the various types of respiratory neurons, particularly those proposed to be involved in respiratory phase transitions, including neurons discharging immediately before the onset of inspiratory phase activity (pre-inspiratory neurons), early-inspiratory, and postinspiratory neurons. The majority of these neurons were presumed interneurons because they were not antidromically activated by spinal cord or cranial nerve stimulation. 3. The locus of the pre-BOTC corresponded histologically to the rostral part of the nucleus ambiguus and ventrolateral reticular formation. It was located caudal to the retrofacial nucleus and rostral to the lateral reticular nucleus, extending 3.0-3.5 mm rostral to the obex, and 3.2-4.0 mm lateral from the midline. This location was homologous to that established in the neonatal rat. 4. Pre-inspiratory neurons (pre-I neurons) were specifically found in the pre-BOTC. Intracellular recordings from these neurons revealed two types of activity patterns. Type 1 of pre-I neurons exhibited a steady membrane depolarization during expiration and a steep membrane depolarization with a high-frequency burst of action-potential discharge during the phase transition from expiration to inspiration. This was followed by a decline of depolarization and spike discharge during the remainder of the inspiratory phase. A second type of pre-I neurons exhibited a secondary graded membrane depolarization and burst discharge during the late-inspiratory period. 5. Synaptic events were examined in other respiratory neurons during the 40-160 ms preceding the onset of phrenic nerve activity when pre-I neurons exhibited peak spike discharge. Early-inspiratory, throughout-respiratory, and postinspiratory neurons were disinhibited during this period, whereas stage-2 expiratory neurons exhibited a decrease in spike activity and repolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Interaction between central pattern generators for breathing and swallowing in the cat.

1. We examined the interaction between central pattern generators for respiration and deglutition in decerebrate, vagotomized, paralysed and ventilated cats (n = 10), by recording activity from the following nerves: hypoglossal, phrenic, thyroarytenoid and triangularis sterni. Fictive breathing was spontaneous with carbon dioxide above the apnoeic threshold (end-tidal PCO2, 32 +/- 4 mmHg) and fictive swallowing was induced by stimulating the internal branch of the left superior laryngeal nerve (SLN) continuously (0.2 ms pulse duration, 10 Hz). 2. In all ten animals, SLN stimulation evoked short bursts of thyroarytenoid and hypoglossal nerve activity indicative of fictive swallowing. In two of ten animals, respiration was inhibited completely during deglutition. In the other eight animals, fictive breathing and swallowing occurred simultaneously. 3. With SLN stimulation below threshold for eliciting swallowing, the respiratory rhythm decreased, the duration of inspiration did not change but the duration of expiration, especially stage II, increased. Integrated nerve activities indicated that the rate of rise and peak of phrenic nerve activity decreased, stage I expiratory activity of the thyroarytenoid and especially that of the hypoglossal nerve increased and stage II expiratory activity of the triangularis sterni nerve was suppressed completely. However, if inspired carbon dioxide was increased, i.e. hypercapnic ventilation, stage II expiratory activity remained partially during continuous SLN stimulation. 4. Fictive-swallowing bursts occurred only at respiratory phase transitions. At the minimal stimulus intensity that evoked repetitive swallowing bursts, the pattern of interaction between breathing and swallowing central pattern generators was consistent for each animal (n = 7) but was different across animals. In four animals, fictive swallows occurred at the phase transition between stage II expiration and inspiration, at the transition between inspiration and stage I expiration in one animal; and in two other animals, at the transition between stage I and II of expiration. 5. The response to SLN stimulation accommodated during the stimulus train. Accommodation was evident in both the interswallow interval (ISI) which lengthened, and the interaction pattern which had fewer swallows per breath as the stimulus period progressed. In contrast to the ISI, characteristics of the fictive swallow did not accommodate. For example, duration of the swallow was constant, distributed over a narrow range throughout the stimulus train. 6. We conclude that the central pattern generators for swallowing and breathing interact. The pattern of interaction supports the three-phase theory of respiratory pattern generation.(ABSTRACT TRUNCATED AT 400 WORDS)

P-398 - Effect of thiamylal on respiratory unit activity in rabbit brain stem during respiratory phase transition.

P-398 - Effect of thiamylal on respiratory unit activity in rabbit brain stem during respiratory phase transition.

Effect of thiamylal on discharge of brain stem respiratory neurons during the respiratory phase transition

Effect of thiamylal on discharge of brain stem respiratory neurons during the respiratory phase transition