Ventilatory Rhythms Research Articles

Time-restricted feeding reveals a role for neural respiratory clocks in optimizing daily ventilatory-metabolic coupling in mice.

The master circadian clock, located in the suprachiasmatic nuclei (SCN), organizes the daily rhythm in minute ventilation (V̇e). However, the extent that the daily rhythm in V̇e is secondary to SCN-imposed O2 and CO2 cycles (i.e., metabolic rate) or driven by other clock mechanisms remains unknown. Here, we experimentally shifted metabolic rate using time-restricted feeding (without affecting light-induced synchronization of the SCN) to determine the influence of metabolic rate in orchestrating the daily V̇e rhythm. Mice eating predominantly at night exhibited robust daily rhythms in O2 consumption (V̇o2), CO2 production (V̇co2), and V̇e with similar peak times (approximately ZT18) that were consistent with SCN organization. However, feeding mice exclusively during the day separated the relative timing of metabolic and ventilatory rhythms, resulting in an approximately 8.5-h advance in V̇co2 and a disruption of the V̇e rhythm, suggesting opposing circadian and metabolic influences on V̇e. To determine if the molecular clock of cells involved in the neural control of breathing contributes to the daily V̇e rhythm, we examined V̇e in mice lacking BMAL1 in Phox2b-expressing respiratory cells (i.e., BKOP mice). The ventilatory and metabolic rhythms of predominantly night-fed BKOP mice did not differ from wild-type mice. However, in contrast to wild-type mice, exclusive day feeding of BKOP mice led to an unfettered daily V̇e rhythm with a peak time aligning closely with the daily V̇co2 rhythm. Taken together, these results indicate that both daily V̇co2 changes and intrinsic circadian time-keeping within Phox2b respiratory cells are predominant orchestrators of the daily rhythm in ventilation.NEW & NOTEWORTHY The master circadian clock organizes the daily rhythm in ventilation; however, the extent that this rhythm is driven by SCN regulation of metabolic rate versus other clock mechanisms remains unknown. We report that metabolic rate alone is insufficient to explain the daily oscillation in ventilation and that neural respiratory clocks within Phox2b-expressing cells additionally optimize breathing. Collectively, these findings advance our mechanistic understanding of the circadian rhythm in ventilatory control.

The Molecular Circadian Clock of Phox2b-expressing Cells Drives Daily Variation of the Hypoxic but Not Hypercapnic Ventilatory Response in Mice.

While the suprachiasmatic nucleus (SCN) controls 24-h rhythms in breathing, including minute ventilation (VE), the mechanisms by which the SCN drives these daily changes are not well understood. Moreover, the extent to which the circadian clock regulates hypercapnic and hypoxic ventilatory chemoreflexes is unknown. We hypothesized that the SCN regulates daily breathing and chemoreflex rhythms by synchronizing the molecular circadian clock of cells. We used whole-body plethysmography to assess ventilatory function in transgenic BMAL1 knockout (KO) mice to determine the role of the molecular clock in regulating daily rhythms in ventilation and chemoreflex. Unlike their wild-type littermates, BMAL1 KO mice exhibited a blunted daily rhythm in VE and failed to demonstrate daily variation in the hypoxic ventilatory response (HVR) or hypercapnic ventilatory response (HCVR). To determine if the observed phenotype was mediated by the molecular clock of key respiratory cells, we then assessed ventilatory rhythms in BMAL1fl/fl; Phox2bCre/+ mice, which lack BMAL1 in all Phox2b-expressing chemoreceptor cells (hereafter called BKOP). BKOP mice lacked daily variation in HVR, similar to BMAL1 KO mice. However, unlike BMAL1 KO mice, BKOP mice exhibited circadian variations in VE and HCVR comparable to controls. These data indicate that the SCN regulates daily rhythms in VE, HVR, and HCVR, in part, through the synchronization of the molecular clock. Moreover, the molecular clock of Phox2b-expressing cells is specifically necessary for daily variation in the hypoxic chemoreflex. These findings suggest that disruption of circadian biology may undermine respiratory homeostasis, which, in turn, may have clinical implications for respiratory disease.

Obesity elicits a sex‐specific attenuation of the daily rhythm in ventilation in mice chronically fed a high‐fat diet

To efficiently deal with changes in nutrient availability across the 24‐hr day, animals have evolved daily rhythms in energy metabolism and, consequently, a daily rhythm in ventilation to mediate metabolic gas exchange. Daily ventilatory rhythms have been implicated in a variety of respiratory diseases such as sleep apnea, due to time‐specific occurrence of symptoms. However, it is unknown how obese physiology and intermittent hypoxia (IH), two hallmarks of sleep apnea, influence the daily rhythm in ventilation. Here, we use a dietary mouse model of obesity to determine the extent to which ventilation rhythms are influenced by obese physiology and IH. Using whole‐body plethysmography, we found that obese mice chronically‐fed a high‐fat diet exhibit a dampened minute ventilation rhythm and decreased mean ventilation during the night‐time. The obesity‐associated reduction in minute ventilation was sex‐specific and resulted in part from a failure of female mice to increase minute ventilation in proportion to weight gain. Moreover, we found that 5 days of daytime IH exposure minimally affected the daily rhythm in minute ventilation. We conclude that the daily rhythm in minute ventilation is attenuated by obesity in a sex‐specific manner but largely unaffected by intermittent hypoxia. Taken together, these results have clinical implications for obesity‐associated respiratory diseases such as obstructive sleep apnea.

Photoperiod Manipulation Reveals a Light-Driven Component to Daily Patterns of Ventilation in Male C57Bl/6J Mice.

Obstructive sleep apnea is a common sleep disorder that increases risk for cardiovascular disease and mortality. The severity of sleep-disordered breathing in obstructive sleep apnea patients fluctuates with the seasons, opening the possibility that seasonal changes in light duration, or photoperiod, can influence mechanisms of breathing. Photoperiod can have profound effects on internal timekeeping and can reshape metabolic rhythms in mammals. While the daily rhythm in ventilation is largely shaped by the metabolic rate, less is known about whether ventilatory rhythms are altered in accordance with metabolism under different photoperiods. Here, we investigate the relationship between ventilation and metabolism under different photoperiods using whole-body plethysmography and indirect calorimetry. We find that the daily timing of ventilation is chiefly synchronized to dark onset and that light cues are important for maintaining daily ventilatory rhythms. Moreover, changes in ventilatory patterns are not paralleled by changes in oxygen consumption, energy expenditure, or respiratory exchange rate under different photoperiods. We conclude that ventilatory patterns are not only shaped by the metabolic rate and circadian timing but are also influenced by other light-driven factors. Collectively, these findings have clinical implications for the seasonal variations in sleep-disordered breathing found in individuals with obstructive sleep apnea.

Putative Sympathetic Oscillator in the Thoracic Spinal Cord of the Bullfrog

BACKGROUNDCardiorespiratory control is essential for survival of all vertebrates, yet our understanding of the neuronal circuitry involved is rudimentary ‐‐ in part because there is little data beyond mammals to guide us. Neuronal oscillators are discrete neural circuits located within the central nervous system that are capable of generating rhythmic outputs without phasic input. These circuits are found throughout the animal kingdom and are often critical for regulating overt rhythmic behaviours such breathing. Previously, we found three oscillators in the frog brainstem that control ventilation: one for buccal ventilation, two for lung. Three respiratory oscillators have subsequently been found in mammals. Central neuronal oscillators within the sympathetic nervous system of frogs and mammals have also been suspected, but these oscillators have yet to be precisely located.FINDINGSUsing extracellular recording and transection techniques in the frog superfused, isolated brainstem and spinal cord preparation, we discovered a robust, yet previously unreported buccal‐like rhythm in the third spinal nerve (SNIII). Unlike the ventilatory rhythms we normally study, the SNIII rhythm demonstrates poor bilateral synchrony. Our data also suggest that this rhythm is: (a) not coupled to ventilatory rhythms recorded in cranial nerves; (b) has distinct responses to pharmacological stimulation compared to ventilatory rhythms; (c) does not require the brainstem; and (d) persists in a chunk of spinal cord centered around SNIII. These data suggest an additional neuronal oscillator in the frog spinal cord located at the level of SNIII. However, removing the brainstem alters the frequency, increases variability and degrades left‐right synchrony of the rhythm, suggesting brainstem connections are important for normal function.SIGNIFICANCEThe literature has long debated the existence of central neuronal oscillators that modulate sympathetic nervous system activity in vertebrates. We propose that the bursting activity recorded from the SNIII in frog is the output of such an oscillator, discovery of which could prove critical in shaping our current understanding of the function and evolution of the sympathetic nervous system.Support or Funding InformationNSERC and HBI.

Mechanical ventilatory modes and cardioventilatory phase synchronization in acute respiratory failure patients

Cardioventilatory phase synchronization was studied in ten critically ill patients admitted in intensive care unit (ICU) for acute respiratory failure under two mechanical ventilatory modes: (i) pressure controlled ventilation (PCV); (ii) pressure support ventilation (PSV). The two modalities were administered to the same patient in different times in a random order. Cardioventilatory phase interactions were typified by plotting the relative position of a heartbeat, detected from the electrocardiogram and collected in n groups, within m ventilatory cycles as a function of the progressive cardiac beat number via the synchrogram. n:m phase synchronized patterns were detected by computing the variability of each phase group. The percent duration of the recording featuring phase synchronization was assessed as a measure of the strength of phase synchrony and tested against situations of full phase desynchronization between cardiac and ventilatory rhythms. Indexes quantifying the variability of the cardiac and ventilatory activities were computed as well. Findings proved that: (i) a significant presence of n:m phase synchronized patterns was detected in PCV; (ii) the strength of n:m phase synchronization was stronger during PCV than PSV; (iii) different strengths of cardioventilatory phase synchronization detected during PCV and PSV were found in presence of similar heart and ventilatory rates and alike variability. We conclude that mechanical ventilation can induce a significant presence of cardioventilatory phase synchronized patterns and this amount depends on the mode of mechanical ventilation. Future studies should test the eventual link of the level of phase coordination between heart and mechanical ventilation to a clinical outcome to understand whether featuring a certain degree of cardioventilatory phase synchronization is beneficial for the critical patient in ICU.

Three brainstem areas involved in respiratory rhythm generation in bullfrogs.

Ventilation in mammals consists of at least three distinct phases: inspiration, post-inspiration and late-expiration. While distinct brainstem rhythm generating and pattern forming networks have long been assumed, recent data suggest the mammalian brainstem contains two coupled neuronal oscillators: one for inspiration and the other for active expiration. However, whether additional burst generating ability is required for generating other phases of ventilation in mammals is controversial. To investigate brainstem circuit architectures capable of producing multiphasic ventilatory rhythms, we utilized the isolated frog brainstem. This preparation produces two types of ventilatory motor patterns, buccal and lung bursts. Lung bursts can be divided into two phases, priming and powerstroke. Previously we identified two putative oscillators, the Buccal and Lung Areas. The Lung Area produces the lung powerstroke and the Buccal Area produces buccal bursts and - we assumed - the priming phase of lung bursts. However, here we identify an additional brainstem region that generates the priming phase. This Priming Area extends rostral and caudal of the Lung Area and is distinct from the Buccal Area. Using AMPA microinjections and reversible synaptic blockade, we demonstrate selective excitation and ablation (respectively) of priming phase activity. We also demonstrate that the Priming Area contains neurons active selectively during the priming phase. Thus, we propose that three distinct neuronal components generate the multiphase respiratory motor pattern produced by the frog brainstem: the buccal, priming and powerstroke burst generators. This raises the possibility that a similar multi-burst generator architecture mediates the three distinct phases of ventilation in mammals.

Transmission of the respiratory rhythm to trigeminal and hypoglossal motor neurons in the American Bullfrog (Lithobates catesbeiana)

Spatially distinct, interacting oscillators in the bullfrog medulla generate and coordinated buccal and lung ventilatory rhythms, but how these rhythms are transmitted onto trigeminal and hypoglossal motor neurons is unknown. Using a vertically-mounted isolated brainstem preparation, the Sheep Dip, we identified the regions of the brainstem containing motor nuclei using a solution capable of blocking synaptic release and, following washout, locally exposed these regions to 5μM NBQX and/or 50μM AP5. Local application of NBQX significantly reduced the amplitude of buccal and lung bursts on the trigeminal nerve, and lung bursts on the hypoglossal nerve. Local AP5 caused a significant reduction in lung burst amplitude on both nerves, but for buccal bursts, hypoglossal amplitude increased and trigeminal amplitude was unchanged. Local co-application of NBQX and AP5 eliminated fictive respiratory motor output completely in both nerves. These results are consistent with mammalian data, suggesting a critical role for glutamate in transmission of respiratory activity from oscillators to motor neurons.

Typical ventilatory pattern of the intact locust is produced by the isolated CNS

Ventilatory rhythms of locusts are generated in the central nervous system (CNS). The primary oscillator or central pattern generator (CPG) is located in the metathoracic ganglion. We studied the different patterns of ventilation by recording long-term efferent discharges from the isolated metathoracic ganglion. Two different basic patterns occur: continuous ventilation and discontinuous ventilation. These patterns can be found in the isolated nerve cord as well as in intact animals. In intact animals sensory feedback usually elicits high frequency continuous ventilation as is the case in most physiological experiments. Many studies of ventilation-associated interneurones were performed under what we call stressed conditions i.e. with strong sensory feedback. Under these conditions many interneurones may be recruited which probably do not belong to the basic CPG. In isolated nerve cords of locusts we recognised the two basic types of ventilation. This provides an experimental approach to the origin of rhythmogenesis in ventilation. We can now examine single interneurones under less stressed or even discontinuous ventilatory conditions in the isolated CNS. We suggest the dominance of intrinsic rhythmogenesis of ventilation in the metathoracic ganglion of locusts.

Automatic control of respiration to deal with difference of individual characteristics

A clinical technology generally consistent with a difference of individuals is discussed on the basis of adaptive control theory taking into account the automatic control of alveolar CO2-concentration in both controlled- and assisted-respiration. An experimental system for this purpose is proposed which consists of a newly developed respirator and controlling and measuring devices. Its control system requires safe and accurate operation in the different experimental environments in controlled- and assisted-respiration, which are characterized by pattems of ventilatory rhythms. In order to satisfy both requirements, a control system is designed on the basis of an adaptive pole assignment method, which is capable of compensating characteristic and environmental changes of a controlled object. It yields, thus, a stable control of alveolar CO2-concentration dealing with some disorder of the equipment, mainly with the different characteristics of a respiratory regulation system inclusive of its individual difference such as nonlinearity and chronic change. From clinical experiments on healthy subjects, the performance of the control system is confirmed to follow-up a given desired alveolar CO2-concentration with assurance of its clinical safety. This kind of adaptive control is confirmed to lead to the useful concept and method for the control of an organic function relied upon its less exact recognition because of its time-varying and ambiguous characteristics.

Independent brain stem sites for ventilatory neurogenesis.

The purpose was to determine if independent ventilatory rhythms could be generated in each half of completely separated brain stems. In decerebrate, cerebellectomized, vagotomized, paralyzed, and ventilated cats, activities of phrenic, recurrent laryngeal (RLN), and/or hypoglossal nerves were monitored. Midsaggital brain stem divisions were performed by sections and lesions. Eupnea continued following divisions of mesencephalon and pons. Hypoglossal and phrenic activities were eliminated after sections approximating the obex. In most preparations, RLNs discharged with independent rhythms after completion of midsagittal brain stem section and a C1 transection. Independent rhythms were also obtained from each half of medulla following transections at the pontomedullary junction and at C1, and midsagittal medullary divisions. In other animals with transections between pons and medulla and at C1, synchronized RLN and hypoglossal activities persisted after sagittal medullary divisions, 2.0 mm lateral to midline contralaterally. Data demonstrate that there is more than one potential brain stem site for ventilatory neurogenesis. It is hypothesized that there are many such sites, possibly having pacemaker cells, in pons and medulla.

Phase Co-Ordination in the Cardiac and Ventilatory Rhythms of the Lobster Homarus Americanus

ABSTRACT Relative co-ordination is demonstrated between the rhythms of the right and left scaphognathites (SG) and between the heart and either right or left SG or both simultaneously. The co-ordination between these rhythms results from mutual phasedependent alteration of the period lengths as demonstrated by the phaseresponse curves for the interactions. The standard deviation of the SG period lengths is an inverse linear function of the percentage coupling between the two SGs (r = 0·76; P < 0·01). The right SG motor output may show relative co-ordination with respect to a sinusoidal, dorso-ventral movement applied along the radial axis of that appendage. This is taken to indicate that sensory reafference resulting from the interplay between the applied and intended movements can alter the SG rhythm depending upon the relative phasing of the two movements. The effects of sensory reafference are therefore not merely tonic as has been believed until now. Cautery of the right oval organ, a major phasic mechanoreceptor of the SG, has no significant influence on any of the above relationships. The significance of this and of the widespread occurrence of relative co-ordination between rhythmic activities is discussed.

The use of telemetry to study light synchronization with feeding and gill ventilation rates in Salmo trutta

A technique is described whereby the electromyogram of the m. adductor mandibulae of brown trout is detected by implanted extracellular electrodes and used as the input signal for an ultrasonic transmitter attached externally to the fish. The periodic electrical activity of the muscle during ventilation is relayed by the transmitter using an analogue pulse system.As the ventilatory electromyogram occurs in discrete rhythmic trains, it follows that alterations to this rhythm can be used to telemeter, instantaneously, single feeding events from a free swimming fish. Laboratory tests have shown that the feeding act is unequivocally distinct electromyographically from other manoeuvres such as ‘coughing’.Four adult brown trout have been equipped with this transmitter system and released in Airthrey Loch, Stirling. Using a tracking facility, feeding activity and ventilatory rhythms have been recorded for extended periods.The results indicate the presence of three daily peaks of feeding activity, which is discussed in relation to changes in light levels. The telemetry records indicate that night feeding is a common occurrence in brown trout. In addition, ventilatory rates were found to be at or near resting levels.

The effects of zinc on the cardiac and ventilatory rhythms of rainbow trout ( Salmo Gairdneri, Richardson) and their responses to environmental hypoxia

Recordings were made of the cardiac and ventilatory rhythms of rainbow trout subjected to solutions of 40 and 10 ppm zinc in an experimental circulation. The higher concentration produces clearly-defined increases in ventilatory and coughing frequency and a decrease in heart rate; the percentage coupling between the two rhythms usually increased. During exposure to 10 ppm added zinc the effects are not so readily discernible but are comparable in type. The addition of 10 ppm zinc has a significant effect on the responses of rainbow trout to hypoxia before and after this treatment. It is concluded that zinc interferes with some of the mechanisms involved in the uptake of oxygen at the gills and consequently can significantly affect the ability of fish to respond to the additional stress of oxygen lack in its environment.

Ventilatory muscle activity in restrained and free-swimming dragonfly larvae (Odonata: Anisoptera)

The roles of certain abdominal muscles in ventilatory behaviour are discussed and illustrated with records of muscular activity obtained mainly from free-swimming animals. The following muscles are shown to be active during normal ventilation (V n): respiratory and anterior dorso-ventrals (RDV and ADV), longitudinal tergal (LT), diaphragm, sub-intestinal and adductors of the anal appendages (VADP). The posterior dorso-ventral, lateral primary longitudinal sterno-pleural, and dorso-ventral oblique muscles were found to be inactive duringV n. TheRDV, ADV, LT andVADP are also active during ventilatory movements other thanV n.RDV activity is shown to be more variable in the free-swimming animal than recordings from dissected and restrained preparations previously suggested. Activity in theADV duringV n shows a reciprocal relationship with that in theRDV. Postulated control elements in larval ventilation are found to parallel many described in other ventilatory systems and motor rhythms generally.