Expiratory neural activities in gasping induced by pharyngeal stimulation and hypoxia

Resuscitation and auto resuscitation by airway reflexes in animals

Various diseases often result in decompensation requiring resuscitation. In infants moderate hypoxia evokes a compensatory augmented breath - sigh and more severe hypoxia results in a solitary gasp. Progressive asphyxia provokes gasping respiration saving the healthy infant - autoresuscitation by gasping. A neonate with sudden infant death syndrome, however, usually will not survive. Our systematic research in animals indicated that airway reflexes have similar resuscitation potential as gasping respiration. Nasopharyngeal stimulation in cats and most mammals evokes the aspiration reflex, characterized by spasmodic inspiration followed by passive expiration. On the contrary, expiration reflex from the larynx, or cough reflex from the pharynx and lower airways manifest by a forced expiration, which in cough is preceded by deep inspiration. These reflexes of distinct character activate the brainstem rhythm generators for inspiration and expiration strongly, but differently. They secondarily modulate the control mechanisms of various vital functions of the organism. During severe asphyxia the progressive respiratory insufficiency may induce a life-threatening cardio-respiratory failure. The sniff- and gasp-like aspiration reflex and similar spasmodic inspirations, accompanied by strong sympatho-adrenergic activation, can interrupt a severe asphyxia and reverse the developing dangerous cardiovascular and vasomotor dysfunctions, threatening with imminent loss of consciousness and death. During progressive asphyxia the reversal of gradually developing bradycardia and excessive hypotension by airway reflexes starts with reflex tachycardia and vasoconstriction, resulting in prompt hypertensive reaction, followed by renewal of cortical activity and gradual normalization of breathing. A combination of the aspiration reflex supporting venous return and the expiration or cough reflex increasing the cerebral perfusion by strong expirations, provides a powerful resuscitation and autoresuscitation potential, proved in animal experiments. They represent a simple but unique model tested in animal experiments.

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.

Phrenic, vagal and hypoglossal activities in rat: pre-inspiratory, inspiratory, expiratory components

During eupnea in an in situ perfused preparation of the rat, inspiratory activities of the hypoglossal and vagal nerves commence before the phrenic; the vagus also discharges in expiration. The hypoglossal discharge has a prominent "pre-inspiratory" component. Power spectral analysis indicated that peak frequencies of oscillations in phrenic, hypoglossal and vagal inspiratory and expiratory activities were the same during eupnea. "Pre-inspiratory" hypoglossal activity had significantly lower peak frequencies. In gasping, "pre-inspiratory" hypoglossal activity ceased and all neural activities became purely inspiratory. High frequency oscillations of phrenic and vagal activities during gasping were shifted upward, compared to those in eupnea, whereas that of the hypoglossal was unaltered. In gasping, the temporal patterns of activities of the phrenic, hypoglossal and vagal nerves, and the level of coherence between these activities implies a restricted and shared set of pre-motor neurons. During eupnea, the activity patterns in the phrenic, hypoglossal and vagal nerves seem to originate from different sets of pre-motor neurons.

Laryngeal muscle activities with cerebral hypoxia-ischemia in newborn lambs

This study tested the hypotheses that (1) acute cerebral hypoxia-ischemia changes laryngeal adductor, laryngeal abductor, and diaphragmatic activities, resulting in central apnea with laryngeal closure; and (2) these laryngeal muscle activities act to maintain absolute lung volume. The respiratory pattern was determined in three asphyxiated, awake preterm lambs after cesarean section birth and in 12 awake, term lambs, with normal lung function, after induction of acute cerebral hypoxia-ischemia by occlusion of the brachiocephalic artery. Electrocorticogram activity, flow, volume, electromyograms of laryngeal abductor and adductor muscles and diaphragm, and, in the term lambs, trans-upper airway pressure and carotid blood flow were recorded. With either preterm birth asphyxia or induced acute cerebral hypoxia-ischemia, minute ventilation initially increased, and then hypopnea occurred. During the hypopnea, laryngeal adductor activity was prominent, accompanied by an increased upper airway pressure and a maintained/raised absolute lung volume. Thus, when acute hypoxia-ischemia limited to the upper body is induced in lambs with normal lung function, expiratory laryngeal adduction with closure of the upper airway occurs and likely functions to aid autoresuscitation.

Trigeminal reflex regulation of the glottis depends on central glycinergic inhibition in the rat

In an unanesthetized decerebrate in situ arterially perfused brain stem preparation of mature rat, strychnine (0.05-0.2 microM) blockade of glycine receptors caused postinspiratory glottal constriction to occur earlier, shifting from early expiration to inspiration. This resulted in a paradoxical inspiratory-related narrowing of the upper airway. Stimulation of the trigeminal ethmoidal nerve (EN5; 20 Hz, 100 micros, 0.5-2 V) evoked a diving response, which included a reflex apnea, glottal constriction, and bradycardia. After strychnine administration, this pattern was converted to a maintained phrenic nerve discharge and a reduced glottal constriction that was interrupted intermittently by transient abductions. The onset of firing of postinspiratory neurons shifted from early expiration into neural inspiration in the presence of strychnine, but neurons maintained their tonic activation during EN5 stimulation, as observed during control. Inspiratory neurons that were hyperpolarized by EN5 stimulation in control conditions were powerfully excited after loss of glycinergic inhibition. Thus the integrity of glycinergic inhibition within the pontomedullary respiratory network is critical for the coordination of cranial and spinal motor outflows during eupnea but also for protective reflex regulation of the upper airway.

Active glottal closure during anoxic gasping in lambs

The present study was aimed at assessing laryngeal dynamics and their consequences during anoxic gasping in ketamine-sedated lambs. We first verified that the glottis was closed between gasps during anoxic gasping in seven chronically instrumented lambs, aged 11-15 days. Recording of glottal constrictor muscle electrical activity, subglottal pressure and lung volume, together with endoscopic observation, confirmed the presence of active glottal closure with maintenance of a high lung volume between gasps. Secondly, we tested whether maintenance of a high lung volume between gasps improved autoresuscitation efficiency. Six sedated lambs aged 8-11 days underwent two anoxic runs, including one with an open tracheostomy to prevent maintenance of a high lung volume. Access back to air was allowed for gasping. No significant difference was found in time to eupnea resumption, hemodynamic parameters or arterial blood gases. We conclude that a high lung volume is actively maintained by glottal closure between anoxic gasps in sedated lambs. Further studies are however needed to define the importance of laryngeal dynamics during gasping.

Intercostal expiratory activity in an in vitro brainstem-spinal cord-rib preparation from the neonatal rat

1. We examined whether expiratory activity can be observed when central chemoreceptors are activated by a decrement in the extracellular pH in an isolated brainstem-spinal cord-rib preparation from 0- to 3-day-old rats. Expiratory activity was defined as the burst activity that occurs in an internal intercostal muscle (IIM) during the silent period between the periodic inspiratory bursts in the C4 ventral root (which contains phrenic motor axons). 2. During perfusion with modified Krebs solution (26 mM HCO3-, 5 % CO2, pH 7.4), there was no consistent activity in IIM, though rhythmic inspiratory motor activity always appeared in the C4 ventral root. 3. When the pH of the perfusate was lowered from about 7.4 to 7.1 by reducing [HCO3-] from 26 to 10 mM, the frequency of the C4 inspiratory rhythm increased, and rhythmic activity appeared in IIM. In most cases, the rhythmic burst in IIM started just after the cessation of the C4 inspiratory burst and coincided with movement of the ribs in a caudal direction. This intercostal expiratory burst was limited to the first half of the expiratory phase. 4. The coordinated reciprocal motor activity between the C4 ventral root and IIM changed to a largely overlapping pattern when strychnine (5-10 microM), a glycine receptor antagonist, was added to the perfusate. 5. These results suggest (i) that the neuronal mechanisms responsible for expiratory motor activity are preserved in this in vitro preparation and (ii) that the glycinergic inhibitory system plays an important role in the coordination between inspiratory and expiratory motor activity during respiration.

Mechanisms and clinicophysiological implications of the sniff- and gasp-like aspiration reflex

Mechanical stimulation of the pharyngeal mucosa in cats and some other mammals evokes the 'aspiration reflex' (AR) characterized by rapid and strong inspiratory efforts not followed by active expirations. It resembles other spasmodic inspiratory acts such as the sniff, the gasp, and the sigh in several aspects, e.g. reflex or semireflex triggering from the upper airways, the sudden onset and termination of such inspirations, the massive recruitment, steep rise and high-peak amplitude of inspiratory unit activity, analogous ventilatory pattern, and contribution to arousal. The similarity of these spasmodic acts is manifested mainly in enhanced speed and volume of inhalation, although of different intensity, which is determined by the varying degree of forced inspiratory activity and a concomitant inhibition of expiratory activity. The extent of the inspiratory dilation of the glottis and the timing and range of late-inspiratory and/or postinspiratory glottal narrowing modulate the depth of aspiration. Thus, the inhalation can be moderate as in sniffing, which provides a transfer of odorants to the olfactory mucosa. In AR the airstream is presumably strong enough to tear off the mechanical particles from the naso- and oropharynx and to convey them into the hypopharynx to allow their subsequent elimination by reflex swallowing or coughing. Prolonged glottal opening allows either the transfer of some additional air to the bronchi by sighing to prevent the development of atelectasis, or redistribution of a larger amount of fresh air into the lungs by gasping to support autoresuscitation. Should aspiration be a common effective component in these spasmodic processes, then the easily elicitable AR could be beneficial as a simple model for studying their properties in health and disease.

NGFI-B, c-fos, and c-jun mRNA expression in mouse brain after acute carbon monoxide intoxication

The expression of immediate early genes (IEG) has been documented in the brain after various kinds of insults such as ischemia and hypoxia. To determine whether acute carbon monoxide intoxication (ACOI) might trigger IEG expression, adult ddY mice were subjected to carbon monoxide exposure at a rate of 30 mL/min for 35 seconds. The levels of NGFI-B, c-fos, and c-jun mRNA were determined by Northern blot analysis. A time-course study in the cerebral cortex indicated that the induction of NGFI-B, c-fos, and c-jun mRNA started as early as 15 minutes, reached a peak at 30 minutes, and returned to the basal level at 1 hour after the ACOI. In addition, the temporal feature of the induction of these IEG mRNA in the hippocampus was very similar to that in the cerebral cortex. Examination of brain regions at 30 minutes after the ACOI revealed a significant induction of NGFI-B mRNA in the cerebellum, thalamus-hypothalamus, brainstem. as well as in the cortex and hippocampus, but not in the striatum or olfactory bulb. Furthermore, the neuroanatomical distribution of c-fos mRNA at 30 minutes after the ACOI was very similar to that of the NGFI-B mRNA. The widespread distribution of these IEG in the brain, especially in the cerebellum and brainstem, indicates that the major cause for the triggering of IEG expression in the brain by the ACOI might be a diffuse hypoxia. These findings show for the first time the temporal and spatial expression of IEG in the brain after ACOI.

Strychnine eliminates reciprocation and augmentation of respiratory bursts of the in vitro frog brainstem

We have recorded rhythmic bursts of efferent action potentials from nerves of respiratory muscles in the frog (Rana pipiens), using a modified in vitro preparation, in which the brainstem lies in situ in the ventral half of the skull. The burst in the sternohyoid branch of the hypoglossal nerve (Hsh) was augmenting, and alternated with a relatively brief augmenting burst in the main branch of the hypoglossal nerve (Hm). The laryngeal branch of the vagus nerve (XI) displayed a biphasic burst, beginning before peak activity of Hsh and spanning the Hm burst. The spatio-temporal patterns of these bursts closely resemble those recorded from the same nerves in intact and in decerebrate frogs, indicating that the bursting rhythm of this in situ preparation constitutes fictive breathing. The nature of neurotransmission responsible for burst reciprocity and augmentation was investigated by applying the glycine receptor blocker, strychnine. Low levels of strychnine (1 and 5 M) increased the frequency of fictive breathing without changing the shape or timing of Hsh, Hm and XI bursts; at higher doses (10 and 20 M) the bursts in all nerves abruptly changed shape and timing to become synchronous and decrementing. The strychnine-induced changes were associated with the appearance of a prominent peak (10-20 Hz) on the spectral analysis of the nerve discharge, possibly indicating a fundamental change in neurogenesis of the respiratory pattern. We conclude that the burst augmentation and reciprocation discharge characteristics of fictive breathing in the frog require strychnine-sensitive inhibitory networks.

The morphology and connections of neurons in the gasping centre of adult rats

Neuronal activities in the intermediate reticular nucleus and adjacent lateral tegmental field are critical for the neurogenesis of the ventilatory pattern of gasping. We report herein the anatomical features of these neurons, their axonal projections and the location of neurons providing afferent inputs. These neuroanatomical evaluations were performed by iontophoretic injection of the tracer Neurobiotin into the region of the intermediate reticular nucleus of the rat. At the site of injection, neurons having soma of 30-50 microns were filled. Labelled axons and terminals were observed in ipsilateral regions which contain neurons having established functions in the control of ventilatory activity. These regions include the nucleus ambiguous and motor nuclei of the hypoglossal and facial nerves. In addition, axonal projections extended to the contralateral region of the intermediate reticular nucleus. From this contralateral region, retrograde tracing revealed projections to the site of injection. Similarly, many ipsilateral regions which received axonal terminals from the region of the intermediate reticular nucleus had reciprocal projections to this region. These anatomical results support the physiological observation that the neurogenesis of gasping involves a synchronized activation of diverse components of the brainstem ventilatory control system.

Medullary regions for neurogenesis of gasping: noeud vital or noeuds vitals?

Gasping is a critical mechanism for survival in that it serves as a mechanism for autoresuscitation when eupnea fails. Eupnea and gasping are separable patterns of automatic ventilatory activity in all mammalian species from the day of birth. The neurogenesis of the gasp is dependent on the discharge of neurons in the rostroventral medulla. This gasping center overlaps a region termed "the pre-Bötzinger complex." Neuronal activities of this complex, characterized in an in vitro brain stem spinal cord preparation of the neonatal rat, have been hypothesized to underlie respiratory rhythm generation. Yet, the rhythmic activity of this in vitro preparation is markedly different from eupnea but identical with gasping in vivo. In eupnea, medullary neuronal activities generating the gasp and the identical rhythm of the in vitro preparation are incorporated into a portion of the pontomedullary circuit defining eupneic ventilatory activity. However, these medullary neuronal activities do not appear critical for the neurogenesis of eupnea, per se.