Evidence that ventilatory rhythmogenesis in the frog involves two distinct neuronal oscillators

Revisiting the two rhythm generators for respiration in lampreys

In lampreys, respiration consists of a fast and a slow rhythm. This study was aimed at characterizing both anatomically and physiologically the brainstem regions involved in generating the two rhythms. The fast rhythm generator has been located by us and others in the rostral hindbrain, rostro-lateral to the trigeminal motor nucleus. More recently, this was challenged by researchers reporting that the fast rhythm generator was located more rostrally and dorsomedially, in a region corresponding to the mesencephalic locomotor region. These contradictory observations made us re-examine the location of the fast rhythm generator using anatomical lesions and physiological recordings. We now confirm that the fast respiratory rhythm generator is in the rostro-lateral hindbrain as originally described. The slow rhythm generator has received less attention. Previous studies suggested that it was composed of bilateral, interconnected rhythm generating regions located in the caudal hindbrain, with ascending projections to the fast rhythm generator. We used anatomical and physiological approaches to locate neurons that could be part of this slow rhythm generator. Combinations of unilateral injections of anatomical tracers, one in the fast rhythm generator area and another in the lateral tegmentum of the caudal hindbrain, were performed to label candidate neurons on the non-injected side of the lateral tegmentum. We found a population of neurons extending from the facial to the caudal vagal motor nuclei, with no clear clustering in the cell distribution. We examined the effects of stimulating different portions of the labeled population on the respiratory activity. The rostro-caudal extent of the population was arbitrarily divided in three portions that were each stimulated electrically or chemically. Stimulation of either of the three sites triggered bursts of discharge characteristic of the slow rhythm, whereas inactivating any of them stopped the slow rhythm. Substance P injected locally in the lateral tegmentum accelerated the slow respiratory rhythm in a caudal hindbrain preparation. Our results show that the fast respiratory rhythm generator consists mostly of a population of neurons rostro-lateral to the trigeminal motor nucleus, whereas the slow rhythm generator is distributed in the lateral tegmentum of the caudal hindbrain.

Compensatory changes in GABAergic inhibition are differentially expressed in the respiratory network to promote function following hibernation

The respiratory network must produce consistent output throughout an animal's life. Although respiratory motor plasticity is well appreciated, how plasticity mechanisms are organized to give rise to robustness following perturbations that disrupt breathing is less clear. During underwater hibernation, respiratory neurons of bullfrogs remain inactive for months, providing a large disturbance that must be overcome to restart breathing. As a result, motoneurons upregulate excitatory synapses to promote the drive to breathe. Reduced inhibition often occurs in parallel with increased excitation, yet the loss of inhibition can destabilize respiratory motor output. Thus, we hypothesized that GABAergic inhibition would decrease following hibernation, but this decrease would be expressed differentially throughout the network. We confirmed that respiratory frequency was under control of GABAAR signaling, but after hibernation, it became less reliant on inhibition. The loss of inhibition was confined to the respiratory rhythm-generating centers: non-respiratory motor activity and large seizure-like bursts were similarly triggered by GABAA receptor blockade in controls and hibernators. Supporting reduced presynaptic GABA release, firing rate of respiratory motoneurons was constrained by a phasic GABAAR tone, but after hibernation, this tone was decreased despite the same postsynaptic receptor strength as controls. Thus, selectively reducing inhibition in respiratory premotor networks promotes stability of breathing, while wholesale loss of GABAARs causes non-specific hyperexcitability throughout the brainstem. These results suggest that different parts of the respiratory network select distinct strategies involving either excitation (motoneurons) or inhibition (rhythm generator) to minimize pathological network states when engaging plasticity that protects the drive to breathe.

Activation of respiratory-related bursting in an isolated medullary section from adult bullfrogs

Breathing is generated by a rhythmic neural circuit in the brainstem, which contains conserved elements across vertebrate groups. In adult frogs, the 'lung area' located in the reticularis parvocellularis is thought to represent the core rhythm generator for breathing. Although this region is necessary for breathing-related motor output, whether it functions as an endogenous oscillator when isolated from other brainstem centers is not clear. Therefore, we generated thick brainstem sections that encompass the lung area to determine whether it can generate breathing-related motor output in a highly reduced preparation. Brainstem sections did not produce activity. However, subsaturating block of glycine receptors reliably led to the emergence of rhythmic motor output that was further enhanced by blockade of GABAA receptors. Output occurred in singlets and multi-burst episodes resembling the intact network. However, burst frequency was slower and individual bursts had longer durations than those produced by the intact preparation. In addition, burst frequency was reduced by noradrenaline and μ-opioids, and increased by serotonin, as observed in the intact network and in vivo. These results suggest that the lung area can be activated to produce rhythmic respiratory-related motor output in a reduced brainstem section and provide new insights into respiratory rhythm generation in adult amphibians. First, clustering breaths into episodes can occur within the rhythm-generating network without long-range input from structures such as the pons. Second, local inhibition near, or within, the rhythmogenic center may need to be overridden to express the respiratory rhythm.

Synaptic modifications transform neural networks to function without oxygen

BACKGROUND

Neural circuit function is highly sensitive to energetic limitations. Much like mammals, brain activity in American bullfrogs quickly fails in hypoxia. However, after emergence from overwintering, circuits transform to function for approximately 30-fold longer without oxygen using only anaerobic glycolysis for fuel, a unique trait among vertebrates considering the high cost of network activity. Here, we assessed neuronal functions that normally limit network output and identified components that undergo energetic plasticity to increase robustness in hypoxia.

RESULTS

In control animals, oxygen deprivation depressed excitatory synaptic drive within native circuits, which decreased postsynaptic firing to cause network failure within minutes. Assessments of evoked and spontaneous synaptic transmission showed that hypoxia impairs synaptic communication at pre- and postsynaptic loci. However, control neurons maintained membrane potentials and a capacity for firing during hypoxia, indicating that those processes do not limit network activity. After overwintering, synaptic transmission persisted in hypoxia to sustain motor function for at least 2 h.

CONCLUSIONS

Alterations that allow anaerobic metabolism to fuel synapses are critical for transforming a circuit to function without oxygen. Data from many vertebrate species indicate that anaerobic glycolysis cannot fuel active synapses due to the low ATP yield of this pathway. Thus, our results point to a unique strategy whereby synapses switch from oxidative to exclusively anaerobic glycolytic metabolism to preserve circuit function during prolonged energy limitations.

A brainstem preparation allowing simultaneous access to respiratory motor output and cellular properties of motoneurons in American bullfrogs

Breathing is generated by a complex neural circuit, and the ability to monitor the activity of multiple network components simultaneously is required to uncover the cellular basis of breathing. In neonatal rodents, a single brainstem slice can be obtained to record respiratory-related motor nerve discharge along with individual rhythm-generating cells or motoneurons because of the close proximity of these neurons in the brainstem. However, most ex vivo preparations in other vertebrates can only capture respiratory motor outflow or electrophysiological properties of putative respiratory neurons in slices without relevant synaptic inputs. Here, we detail a method to horizontally slice away the dorsal portion of the brainstem to expose fluorescently labeled motoneurons for patch-clamp recordings in American bullfrogs. This 'semi-intact' preparation allows tandem recordings of motor output and single motoneurons during respiratory-related synaptic inputs. The rhythmic motor patterns are comparable to those from intact preparations and operate at physiological temperature and [K+]. Thus, this preparation provides the ability to record network and cellular outputs simultaneously and may lead to new mechanistic insights into breathing control across vertebrates.

Evolution of vertebrate respiratory central rhythm generators

Tracing the evolution of the central rhythm generators associated with ventilation in vertebrates is hindered by a lack of information surrounding key transitions. To begin with, central rhythm generation has been studied in detail in only a few species from four vertebrate groups, lamprey, anuran amphibians, turtles, and mammals (primarily rodents). Secondly, there is a lack of information regarding the transition from water breathing fish to air breathing amniotes (reptiles, birds, and mammals). Specifically, the respiratory rhythm generators of fish appear to be single oscillators capable of generating both phases of the respiratory cycle (expansion and compression) and projecting to motoneurons in cranial nerves innervating bucco-pharyngeal muscles. In the amniotes we find oscillators capable of independently generating separate phases of the respiratory cycle (expiration and inspiration) and projecting to pre-motoneurons in the ventrolateral medulla that in turn project to spinal motoneurons innervating thoracic and abdominal muscles (reptiles, birds, and mammals). Studies of the one group of amphibians that lie at this transition (the anurans), raise intriguing possibilities but, for a variety of reasons that we explore, also raise unanswered questions. In this review we summarize what is known about the rhythm generating circuits associated with breathing that arise from the different rhombomeric segments in each of the different vertebrate classes. Assuming oscillating circuits form in every pair of rhombomeres in every vertebrate during development, we trace what appears to be the evolutionary fate of each and highlight the questions that remain to be answered to properly understand the evolutionary transitions in vertebrate central respiratory rhythm generation.

The lamprey respiratory network: Some evolutionary aspects

Breathing is a complex behaviour that involves rhythm generating networks. In this review, we examine the main characteristics of respiratory rhythm generation in vertebrates and, in particular, we describe the main results of our studies on the role of neural mechanisms involved in the neuromodulation of the lamprey respiration. The lamprey respiratory rhythm generator is located in the paratrigeminal respiratory group (pTRG) and shows similarities with the mammalian preBötzinger complex. In fact, within the pTRG a major role is played by glutamate, but also GABA and glycine display important contributions. In addition, neuromodulatory influences are exerted by opioids, substance P, acetylcholine and serotonin. Both structures respond to exogenous ATP with a biphasic response and astrocytes there located strongly contribute to the modulation of the respiratory pattern. The results emphasize that some important characteristics of the respiratory rhythm generating network are, to a great extent, maintained throughout evolution.

Is there a common drive for buccal movements associated with buccal and lung 'breath' in Lithobates catesbeianus?

In amphibians, there is some evidence that (1) anatomically separate brainstem respiratory oscillators are involved in rhythm generation, one for the buccal rhythm and another for the lung rhythm and (2) they become functionally coupled during metamorphosis. The present analysis, performed on neurograms recorded using brainstem preparations from Lithobates catesbeianus, aims to investigate the temporal organisation of lung and buccal burst types. Continuous Wavelet Transfom applied to the separated buccal and lung signals of a neurogram revealed that both buccal and lung frequency profiles exhibited the same low frequency peak around 1 Hz. This suggests that a common 'clock' organises both rhythms within an animal. A cross-correlation analysis applied to the buccal and lung burst signals revealed their similar intrinsic oscillation features, occurring at approximately 25 Hz. These observations suggest that a coupling between the lung and buccal oscillators emerges at metamorphosis. This coupling may be related to inter-connectivity between the two oscillators, and to a putative common drive.

Development of central respiratory control in anurans: The role of neurochemicals in the emergence of air-breathing and the hypoxic response

Physiological and environmental factors impacting respiratory homeostasis vary throughout the course of an animal's lifespan from embryo to adult and can shape respiratory development. The developmental emergence of complex neural networks for aerial breathing dates back to ancestral vertebrates, and represents the most important process for respiratory development in extant taxa ranging from fish to mammals. While substantial progress has been made towards elucidating the anatomical and physiological underpinnings of functional respiratory control networks for air-breathing, much less is known about the mechanisms establishing these networks during early neurodevelopment. This is especially true of the complex neurochemical ensembles key to the development of air-breathing. One approach to this issue has been to utilize comparative models such as anuran amphibians, which offer a unique perspective into early neurodevelopment. Here, we review the developmental emergence of respiratory behaviours in anuran amphibians with emphasis on contributions of neurochemicals to this process and highlight opportunities for future research.

Buccal rhythmogenesis and CO2 sensitivity in Lithobates catesbeianus tadpole brainstems across metamorphosis

Bullfrog tadpoles ventilate both the buccal cavity and lung. In isolated brainstems, the midbrain/pons influences CO₂ responsiveness and timing of lung ventilatory bursting, depending on larval development. However, little is known about midbrain/pons influences on buccal burst patterns. As such, we investigated how removal of this region affects buccal burst shape and CO₂ responsiveness across development. We measured facial nerve activity in brainstems isolated from tadpoles during early and late developmental stages, under normal and elevated levels of CO₂. Brainstems were either left intact or transected by removing the midbrain/pons. In late stage preparations, buccal burst pattern differed between intact and reduced preparations, and bursts were responsive to elevated CO₂ in these reduced preparations. These results suggest the midbrain/pons affects tadpole buccal burst pattern and CO₂ responsiveness, perhaps similar to its influences on lung ventilation.

Developmental Maturation of Functional Coupling Between Ventilatory Oscillators in the American Bullfrog

Many vital motor behaviors - including locomotion, swallowing, and breathing - appear to be dependent upon the activity of and coordination between multiple endogenously rhythmogenic nuclei, or neural oscillators. Much as the functional development of sensory circuits is shaped during maturation, we hypothesized that coordination of oscillators involved in motor control may likewise be maturation-dependent, i.e., coupling and coordination between oscillators change over development. We tested this hypothesis using the bullfrog isolated brainstem preparation to study the metamorphic transition of ventilatory motor patterns from early rhythmic buccal (water) ventilation in the tadpole to the mature pattern of rhythmic buccal and lung (air) ventilation in the adult. Spatially distinct oscillators drive buccal and lung bursts in the isolated brainstem; we found these oscillators to be active but functionally uncoupled in the tadpole. Over the course of metamorphosis, the rhythms produced by the buccal and lung oscillators become increasingly tightly coordinated. These changes parallel the progression of structural and behavioral changes in the animal, with adult levels of coupling arising by the metamorphic stage (forelimb eruption). These findings suggest that oscillator coupling undergoes a maturation process similar to the refinement of sensory circuits over development.

The rostral medulla of bullfrog tadpoles contains critical lung rhythmogenic and chemosensitive regions across metamorphosis

The development of amphibian breathing provides insight into vertebrate respiratory control mechanisms. Neural oscillators in the rostral and caudal medulla drive ventilation in amphibians, and previous reports describe ventilatory oscillators and CO₂ sensitive regions arise during different stages of amphibian metamorphosis. However, inconsistent findings have been enigmatic, and make comparisons to potential mammalian counterparts challenging. In the current study we assessed amphibian central CO₂ responsiveness and respiratory rhythm generation during two different developmental stages. Whole-nerve recordings of respiratory burst activity in cranial and spinal nerves were made from intact or transected brainstems isolated from tadpoles during early or late stages of metamorphosis. Brainstems were transected at the level of the trigeminal nerve, removing rostral structures including the nucleus isthmi, midbrain, and locus coeruleus, or transected at the level of the glossopharyngeal nerve, removing the putative buccal oscillator and caudal medulla. Removal of caudal structures stimulated the frequency of lung ventilatory bursts and revealed a hypercapnic response in normally unresponsive preparations derived from early stage tadpoles. In preparations derived from late stage tadpoles, removal of rostral or caudal structures reduced lung burst frequency, while CO₂ responsiveness was retained. Our results illustrate that structures within the rostral medulla are capable of sensing CO₂ throughout metamorphic development. Similarly, the region controlling lung ventilation appears to be contained in the rostral medulla throughout metamorphosis. This work offers insight into the consistency of rhythmic respiratory and chemosensitive capacities during metamorphosis.

New insights into sucking, swallowing and breathing central generators: A complexity analysis of rhythmic motor behaviors

Sucking, swallowing and breathing are dynamic motor behaviors. Breathing displays features of chaos-like dynamics, in particular nonlinearity and complexity, which take their source in the automatic command of breathing. In contrast, buccal/gill ventilation in amphibians is one of the rare motor behaviors that do not display nonlinear complexity. This study aimed at assessing whether sucking and swallowing would also follow nonlinear complex dynamics in the newborn lamb. Breathing movements were recorded before, during and after bottle-feeding. Sucking pressure and the integrated EMG of the thyroartenoid muscle, as an index of swallowing, were recorded during bottle-feeding. Nonlinear complexity of the whole signals was assessed through the calculation of the noise limit value (NL). Breathing and swallowing always exhibited chaos-like dynamics. The NL of breathing did not change significantly before, during or after bottle-feeding. On the other hand, sucking inconsistently and significantly less frequently than breathing exhibited a chaos-like dynamics. Therefore, the central pattern generator (CPG) that drives sucking may be functionally different from the breathing CPG. Furthermore, the analogy between buccal/gill ventilation and sucking suggests that the latter may take its phylogenetic origin in the gill ventilation CPG of the common ancestor of extant amphibians and mammals.

The neural control of respiration in lampreys

This review focuses on past and recent findings that have contributed to characterize the neural networks controlling respiration in the lamprey, a basal vertebrate. As in other vertebrates, respiration in lampreys is generated centrally in the brainstem. It is characterized by the presence of a fast and a slow respiratory rhythm. The anatomical and the basic physiological properties of the neural networks underlying the generation of the fast rhythm have been more thoroughly investigated; less is known about the generation of the slow respiratory rhythm. Comparative aspects with respiratory generators in other vertebrates as well as the mechanisms of modulation of respiration in association with locomotion are discussed.

Evolution of lung breathing from a lungless primitive vertebrate

Air breathing was critical to the terrestrial radiation and evolution of tetrapods and arose in fish. The vertebrate lung originated from a progenitor structure present in primitive boney fish. The origin of the neural substrates, which are sensitive to metabolically produced CO2 and which rhythmically activate respiratory muscles to match lung ventilation to metabolic demand, is enigmatic. We have found that a distinct periodic centrally generated rhythm, described as "cough" and occurring in lamprey in vivo and in vitro, is modulated by central sensitivity to CO2. This suggests that elements critical for the evolution of breathing in tetrapods, were present in the most basal vertebrate ancestors prior to the evolution of the lung. We propose that the evolution of breathing in all vertebrates occurred through exaptations derived from these critical basal elements.

Diving into the mammalian swamp of respiratory rhythm generation with the bullfrog

All vertebrates produce some form of respiratory rhythm, whether to pump water over gills or ventilate lungs. Yet despite the critical importance of ventilation for survival, the architecture of the respiratory central pattern generator has not been resolved. In frogs and mammals, there is increasing evidence for multiple burst-generating regions in the ventral respiratory group. These regions work together to produce the respiratory rhythm. However, each region appears to be pivotally important to a different phase of the motor act. Regions also exhibit differing rhythmogenic capabilities when isolated and have different CO2 sensitivity and pharmacological profiles. Interestingly, in both frogs and rats the regions with the most robust rhythmogenic capabilities when isolated are located in rhombomeres 7/8. In addition, rhombomeres 4/5 in both clades are critical for controlling phases of the motor pattern most strongly modulated by CO2 (expiration in mammals, and recruitment of lung bursts in frogs). These key signatures may indicate that these cell clusters arose in a common ancestor at least 400 million years ago.

Three brainstem areas involved in respiratory rhythm generation in bullfrogs

For most multiphasic motor patterns, rhythm and pattern are produced by the same circuit elements. For respiration, however, these functions have long been assumed to occur separately. In frogs, the ventilatory motor pattern produced by the isolated brainstem consists of buccal and biphasic lung bursts. Previously, two discrete necessary and sufficient sites for lung and buccal bursts were identified. Here we identify a third site, the Priming Area, important for and having neuronal activity correlated with the first phase of biphasic lung bursts. As each site is important for burst generation of a separate phase, we suggest each major phase of ventilation is produced by an anatomically distinct part of an extensive brainstem network. Embedding of discrete circuit elements producing major phases of respiration within an extensive rhythmogenic brainstem network may be a shared architectural characteristic of vertebrates.

ABSTRACT

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.

Respiratory neuron characterization reveals intrinsic bursting properties in isolated adult turtle brainstems (Trachemys scripta)

It is not known whether respiratory neurons with intrinsic bursting properties exist within ectothermic vertebrate respiratory control systems. Thus, isolated adult turtle brainstems spontaneously producing respiratory motor output were used to identify and classify respiratory neurons based on their firing pattern relative to hypoglossal (XII) nerve activity. Most respiratory neurons (183/212) had peak activity during the expiratory phase, while inspiratory, post-inspiratory, and novel pre-expiratory neurons were less common. During synaptic blockade conditions, ∼10% of respiratory neurons fired bursts of action potentials, with post-inspiratory cells (6/9) having the highest percentage of intrinsic burst properties. Most intrinsically bursting respiratory neurons were clustered at the level of the vagus (X) nerve root. Synaptic inhibition blockade caused seizure-like activity throughout the turtle brainstem, which shows that the turtle respiratory control system is not transformed into a network driven by intrinsically bursting respiratory neurons. We hypothesize that intrinsically bursting respiratory neurons are evolutionarily conserved and represent a potential rhythmogenic mechanism contributing to respiration in adult turtles.

The phylogeny and ontogeny of autonomic control of the heart and cardiorespiratory interactions in vertebrates

Heart rate in vertebrates is controlled by activity in the autonomic nervous system. In spontaneously active or experimentally prepared animals, inhibitory parasympathetic control is predominant and is responsible for instantaneous changes in heart rate, such as occur at the first air breath following a period of apnoea in discontinuous breathers like inactive reptiles or species that surface to air breathe after a period of submersion. Parasympathetic control, exerted via fast-conducting, myelinated efferent fibres in the vagus nerve, is also responsible for beat-to-beat changes in heart rate such as the high frequency components observed in spectral analysis of heart rate variability. These include respiratory modulation of the heartbeat that can generate cardiorespiratory synchrony in fish and respiratory sinus arrhythmia in mammals. Both may increase the effectiveness of respiratory gas exchange. Although the central interactions generating respiratory modulation of the heartbeat seem to be highly conserved through vertebrate phylogeny, they are different in kind and location, and in most species are as yet little understood. The heart in vertebrate embryos possesses both muscarinic cholinergic and β-adrenergic receptors very early in development. Adrenergic control by circulating catecholamines seems important throughout development. However, innervation of the cardiac receptors is delayed and first evidence of a functional cholinergic tonus on the heart, exerted via the vagus nerve, is often seen shortly before or immediately after hatching or birth, suggesting that it may be coordinated with the onset of central respiratory rhythmicity and subsequent breathing.

Neural mechanisms underlying respiratory rhythm generation in the lamprey

The isolated brainstem of the adult lamprey spontaneously generates respiratory activity. The paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator, has been anatomically and functionally characterized. It is sensitive to opioids, neurokinins and acetylcholine. Excitatory amino acids, but not GABA and glycine, play a crucial role in the respiratory rhythmogenesis. These results are corroborated by immunohistochemical data. While only GABA exerts an important modulatory control on the pTRG, both GABA and glycine markedly influence the respiratory frequency via neurons projecting from the vagal motoneuron region to the pTRG. Noticeably, the removal of GABAergic transmission within the pTRG causes the resumption of rhythmic activity during apnea induced by blockade of glutamatergic transmission. The same result is obtained by microinjections of substance P or nicotine into the pTRG during apnea. The results prompted us to present some considerations on the phylogenesis of respiratory pattern generation. They may also encourage comparative studies on the basic mechanisms underlying respiratory rhythmogenesis of vertebrates.

GABAergic and glycinergic inputs modulate rhythmogenic mechanisms in the lamprey respiratory network

We have previously shown that GABA and glycine modulate respiratory activity in the in vitro brainstem preparations of the lamprey and that blockade of GABAA and glycine receptors restores the respiratory rhythm during apnoea caused by blockade of ionotropic glutamate receptors. However, the neural substrates involved in these effects are unknown. To address this issue, the role of GABAA, GABAB and glycine receptors within the paratrigeminal respiratory group (pTRG), the proposed respiratory central pattern generator, and the vagal motoneuron region was investigated both during apnoea induced by blockade of glutamatergic transmission and under basal conditions through microinjections of specific antagonists. The removal of GABAergic, but not glycinergic transmission within the pTRG, causes the resumption of rhythmic respiratory activity during apnoea, and reveals the presence of a modulatory control of the pTRG under basal conditions. A blockade of GABAA and glycine receptors within the vagal region strongly increases the respiratory frequency through disinhibition of neurons projecting to the pTRG from the vagal region. These neurons were retrogradely labelled (neurobiotin) from the pTRG. Intense GABA immunoreactivity is observed both within the pTRG and the vagal area, which corroborates present findings. The results confirm the pTRG as a primary site of respiratory rhythm generation, and suggest that inhibition modulates the activity of rhythm-generating neurons, without any direct role in burst formation and termination mechanisms.

New insights in gill/buccal rhythm spiking activity and CO(2) sensitivity in pre- and postmetamorphic tadpoles (Pelophylax ridibundus)

Central CO(2) chemosensitivity is crucial for all air-breathing vertebrates and raises the question of its role in ventilatory rhythmogenesis. In this study, neurograms of ventilatory motor outputs recorded in facial nerve of premetamorphic and postmetamorphic tadpole isolated brainstems, under normo- and hypercapnia, are investigated using Continuous Wavelet Transform spectral analysis for buccal activity and computation of number and amplitude of spikes during buccal and lung activities. Buccal bursts exhibit fast oscillations (20-30Hz) that are prominent in premetamorphic tadpoles: they result from the presence in periodic time windows of high amplitude spikes. Hypercapnia systematically decreases the frequency of buccal rhythm in both pre- and postmetamorphic tadpoles, by a lengthening of the interburst duration. In postmetamorphic tadpoles, hypercapnia reduces buccal burst amplitude and unmasks small fast oscillations. Our results suggest a common effect of the hypercapnia on the buccal part of the Central Pattern Generator in all tadpoles and a possible effect at the level of the motoneuron recruitment in postmetamorphic tadpoles.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

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 5 μM NBQX and/or 50 μM 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.

Evidence for rhombomeric organization of multiple respiratory oscillators in the bullfrog brainstem

The anuran brainstem is segmentally organized into rhombomeres (r) and this segmental organization is uniquely preserved throughout development. We hypothesized that rhombomeres associated with cranial nerves (CN) also contain oscillators that are capable of producing rhythmic motor output (bursts) in isolation. We used in vitro brainstem preparations from pre- and post-metamorphic bullfrogs (Lithobates catesbeianus) to determine if rhombomeric organization of oscillators is present throughout development. Brainstems were transected into segments containing one or more rhombomeres and motor output was measured with suction electrodes attached to CN V, X and XII. Rhythmic motor output was observed in 85% of tadpoles and 91% of frogs in an anterior segment (r0-r5), 27% of tadpoles and 18% of frogs in the middle segment (r6-r7), and 77% of tadpoles and 55% of frogs in the caudal segment (r8). There were significant reductions in burst frequency and whole nerve amplitude following transections. These data support the hypothesis that brainstem oscillators associated with specific groups of rhombomeres are present throughout development in anurans.

Persistent lung oscillator response to CO2 after buccal oscillator inhibition in the adult frog

The automatic ventilatory drive in amphibians depends on two oscillators interacting with each other, the gill/buccal and lung oscillators. The lung oscillator would be homologous to the mammalian pre-Bötzinger complex and the gill/buccal oscillator homologous to the mammalian parafacial respiratory group/retrotrapezoid nucleus (pFRG/RTN). Dysfunction of the pFRG/RTN has been involved in the development of respiratory diseases associated to the loss of CO(2) chemosensitivity such as the congenital central hypoventilation syndrome. Here, on adult in vitro isolated frog brainstem, consequences of the buccal oscillator inhibition (by reducing Cl(-)) were evaluated on the respiratory rhythm developed by the lung oscillator under hypercapnic challenges. Our results show that under low Cl(-) concentration (i) the buccal oscillator is strongly inhibited and the lung burst frequency and amplitude decreased and (ii) it persists a powerful CO(2) chemosensitivity. In conclusion, in frog, the CO(2) chemosensitivity depends on cellular contingent(s) whose the functioning is independent of the concentration of Cl(-) and origin remains unknown.

Corticosterone promotes emergence of fictive air breathing in Xenopus laevis Daudin tadpole brainstems

The emergence of air breathing during amphibian metamorphosis requires significant changes to the brainstem circuits that generate and regulate breathing. However, the mechanisms controlling this developmental process are unknown. Because corticosterone plays an important role in the neuroendocrine regulation of amphibian metamorphosis, we tested the hypothesis that corticosterone augments fictive air breathing frequency in Xenopus laevis. To do so, we compared the fictive air breathing frequency produced by in vitro brainstem preparations from pre-metamorphic tadpoles and adult frogs before and after 1 h application of corticosterone (100 nmol l–1). Fictive breathing measurements related to gill and lung ventilation were recorded extracellularly from cranial nerve rootlets V and X. Corticosterone application had no immediate effect on respiratory-related motor output produced by brainstems from either developmental stage. One hour after corticosterone wash out, fictive lung ventilation frequency was increased whereas gill burst frequency was decreased. This effect was stage specific as it was significant only in preparations from tadpoles. GABA application (0.001–0.5 mmol l–1) augmented fictive air breathing in tadpole preparations. However, this effect of GABA was no longer observed following corticosterone treatment. An increase in circulating corticosterone is one of the endocrine processes that orchestrate central nervous system remodelling during metamorphosis. The age-specific effects of corticosterone application indicate that this hormone can act as an important regulator of respiratory control development in Xenopus tadpoles. Concurrent changes in GABAergic neurotransmission probably contribute to this maturation process, leading to the emergence of air breathing in this species.

Effects of maturation and acidosis on the chaos-like complexity of the neural respiratory output in the isolated brainstem of the tadpole, Rana esculenta

Human ventilation at rest exhibits mathematical chaos-like complexity that can be described as long-term unpredictability mediated (in whole or in part) by some low-dimensional nonlinear deterministic process. Although various physiological and pathological situations can affect respiratory complexity, the underlying mechanisms remain incompletely elucidated. If such chaos-like complexity is an intrinsic property of central respiratory generators, it should appear or increase when these structures mature or are stimulated. To test this hypothesis, we employed the isolated tadpole brainstem model [Rana (Pelophylax) esculenta] and recorded the neural respiratory output (buccal and lung rhythms) of pre- (n = 8) and postmetamorphic tadpoles (n = 8), at physiologic (7.8) and acidic pH (7.4). We analyzed the root mean square of the cranial nerve V or VII neurograms. Development and acidosis had no effect on buccal period. Lung frequency increased with development (P < 0.0001). It also increased with acidosis, but in postmetamorphic tadpoles only (P < 0.05). The noise-titration technique evidenced low-dimensional nonlinearities in all the postmetamorphic brainstems, at both pH. Chaos-like complexity, assessed through the noise limit, increased from pH 7.8 to pH 7.4 (P < 0.01). In contrast, linear models best fitted the ventilatory rhythm in all but one of the premetamorphic preparations at pH 7.8 (P < 0.005 vs. postmetamorphic) and in four at pH 7.4 (not significant vs. postmetamorphic). Therefore, in a lower vertebrate model, the brainstem respiratory central rhythm generator accounts for ventilatory chaos-like complexity, especially in the postmetamorphic stage and at low pH. According to the ventilatory generators homology theory, this may also be the case in mammals.

Retrotrapezoid nucleus and parafacial respiratory group

The rat retrotrapezoid nucleus (RTN) contains about 2000 Phox2b-expressing glutamatergic neurons (ccRTN neurons; 800 in mice) with a well-understood developmental lineage. ccRTN neuron development fails in mice carrying a Phox2b mutation commonly present in the congenital central hypoventilation syndrome. In adulthood, ccRTN neurons regulate the breathing rate and intensity, and may regulate active expiration along with other neighboring respiratory neurons. Prenatally, ccRTN neurons form an autonomous oscillator (embryonic parafacial group, e-pF) that activates and possibly paces inspiration. The pacemaker properties of the ccRTN neurons probably vanish after birth to be replaced by synaptic drives. The neonatal parafacial respiratory group (pfRG) may represent a transitional phase during which ccRTN neurons lose their group pacemaker properties. ccRTN neurons are activated by acidification via an intrinsic mechanism or via ATP released by glia. In summary, throughout life, ccRTN neurons seem to be a critical hub for the regulation of CO(2) via breathing.

Adaptive trends in respiratory control: a comparative perspective

In 1941, August Krogh published a monograph entitled The Comparative Physiology of Respiratory Mechanisms (Philadelphia, PA: University of Pennsylvania Press, 1941). Since that time comparative studies have continued to contribute significantly to our understanding of the fundamentals of respiratory physiology and the adaptive trends in these processes that support a broad range of metabolic performance under demanding environmental conditions. This review specifically focuses on recent advances in our understanding of adaptive trends in respiratory control. Respiratory rhythm generators most likely arose from, and must remain integrated with, rhythm generators for chewing, suckling, and swallowing. Within the central nervous system there are multiple “segmental” rhythm generators, and through evolution there is a caudal shift in the predominant respiratory rhythm-generating site. All sites, however, may still be capable of producing or modulating respiratory rhythm under appropriate conditions. Expression of the respiratory rhythm is conditional on (tonic) input. Once the rhythm is expressed, it is often episodic as the basic medullary rhythm is turned on/off subject to a hierarchy of controls. Breathing patterns reflect differences in pulmonary mechanics resulting from differences in body wall and lung architecture and are modulated in different species by various combinations of upper and lower airway mechanoreceptors and arterial chemoreceptors to protect airways, reduce dead space ventilation, enhance gas exchange efficiency, and reduce the cost of breathing.

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.

Serotonergic modulation of respiratory rhythmogenesis and central chemoreception

In addition to evidence supporting serotonergic modulation of respiratory rhythmogenesis, serotonergic mechanisms play a role in central respiratory chemoreception. We examined the role of serotonin 5HT1A receptors in respiratory rhythmicity and central respiratory chemosensitivity in in vitro brainstem preparations of the bullfrog tadpole, Rana catesbeiana. Spontaneous respiratory motor output was recorded from cranial nerve 7 at control bath pH (7.8) and hypercapnic bath pH (7.4) as bath concentrations of a 5HT1A receptor agonist were steadily increased from 0.5 to 25 microM. Activation of the 5HT1A receptor significantly altered the respiratory burst cycle. Significant increases in both gill and lung burst cycle were observed in response to bath application of 8-OH-DPAT; gill burst cycle in response to 8-OH-DPAT was influenced by bath pH, as gill burst cycle at bath pH 7.8 was not significantly increased at 0.5 or 5.0 microM 8-OH-DPAT. However, when the pH was reduced to 7.4 gill burst cycle was significantly increased at these same bath concentrations of 8-OH-DPAT. Gill burst amplitude was not altered in response to bath application of 8-OH-DPAT; however, lung burst amplitude was significantly decreased at 25.0 microM 8-OH-DPAT at bath pH 7.8. These data indicate that 5HT1A receptors are involved in neural respiratory rhythmogenic and chemoreceptive circuits in the bullfrog tadpole, and support the hypothesis that abnormalities in serotonergic systems may be an underlying component of Sudden Infant Death Syndrome.

Neural model of frog ventilatory rhythmogenesis

In the adult frog respiratory system, periods of rhythmic movements of the buccal floor are interspersed by lung ventilation episodes. The ventilatory activity results from the interaction of two hypothesized oscillators in the brainstem. Here, we model these oscillators with two coupled neural networks, whose co-activation results in the emergence of new dynamics. One of the networks is built with "loop chains" of excitatory and inhibitory neurones producing periodic activities. We define two groups of excitatory neurones whose oscillatory antiphasic sums of activities represent output signals as possible motor commands towards antagonist buccal muscles. The other oscillator is a small network with a self-modulated excitatory input to an excitatory neurone whose episodic firings synchronise some neurones of the first network chains. When this oscillator is silent, the output signals exhibit only regular oscillations, and, when active, the synchronisation process reconfigures the output signals whose new features are representative of lung ventilation motor patterns. The biological interest of this formal model is illustrated by the persistence of the relevant dynamical features when perturbations are introduced in the model, i.e. dynamic noises and architecture modifications. The implementation of the networks with clock-driven continuous time neurones provides simulations with physiological time scales.

Lung respiratory rhythm and pattern generation in the bullfrog: role of neurokinin-1 and mu-opioid receptors

Location of the lung respiratory rhythm generator (RRG) in the bullfrog brainstem was investigated by examining neurokinin-1 and mu-opioid receptor (NK1R, muOR) colocalization by immunohistochemistry and characterizing the role of these receptors in lung rhythm and episodic pattern generation. NK1R and muOR occurred in brainstems from all developmental stages. In juvenile bullfrogs a distinct area of colocalization was coincident with high-intensity fluorescent labeling of muOR; high-intensity labeling of muOR was not distinctly and consistently localized in tadpole brainstems. NK1R labeling intensity did not change with development. Similarity in colocalization is consistent with similarity in responses to substance P (SP, NK1R agonist) and DAMGO (muOR agonist) when bath applied to bullfrog brainstems of different developmental stages. In early stage tadpoles and juvenile bullfrogs, SP increased and DAMGO decreased lung burst frequency. In juvenile bullfrogs, SP increased lung burst frequency, episode frequency, but decreased number of lung bursts per episode and lung burst duration. In contrast, DAMGO decreased lung burst frequency and burst cycle frequency, episode frequency, and number of lung bursts per episode but increased all other lung burst parameters. Based on these results, we hypothesize that NK1R and muOR colocalization together with a metamorphosis-related increase in muOR intensity marks the location of the lung RRG but not necessarily the lung episodic pattern generator.

Role of glutamate and substance P in the amphibian respiratory network during development

This study tested the hypothesis that glutamatergic ionotropic (AMPA/kainate) receptors and neurokinin receptors (NKR) are important in the regulation of respiratory motor output during development in the bullfrog. The roles of these receptors were studied with in vitro brainstem preparations from pre-metamorphic tadpoles and post-metamorphic frogs. Brainstems were superfused with an artificial cerebrospinal fluid at 20-22 degrees C containing CNQX, a selective non-NMDA antagonist, or with substance P (SP), an agonist of NKR. Blockade of glutamate receptors with CNQX in both groups caused a reduction of lung burst frequency that was reversibly abolished at 5 microM (P<0.01). CNQX, but not SP, application produced a significant increase (P<0.05) in gill and buccal frequency in tadpoles and frogs, respectively. SP caused a significant increase (P<0.05) in lung burst frequency at 5 microM in both groups. These results suggest that glutamatergic activation of AMPA/kainate receptors is necessary for generation of lung burst activity and that SP is an excitatory neurotransmitter for lung burst frequency generation. Both glutamate and SP provide excitatory input for lung burst generation throughout the aquatic to terrestrial developmental transition in bullfrogs.

Role of pontine neurons in central O(2) chemoreflex during development in bullfrogs (Lithobates catesbeiana)

The present study used an in vitro brainstem preparation from pre-metamorphic tadpoles and adult bullfrogs (Lithobates catesbeiana) to understand the neural mechanisms associated with central O(2) chemosensitivity and its maturation. In this species, brainstem hypoxia increases fictive lung ventilation in tadpoles but decreases in adults. Previous studies have shown that alpha(1)-adrenoceptor inactivation prevents these responses, suggesting that noradrenergic neurons are involved. We first tested the hypothesis that the pons (which includes noradrenergic neurons from the locus coeruleus; LC) plays a role in the lung burst frequency response to central hypoxia by comparing the effects of brainstem transection at the LC level between pre-metamorphic tadpoles and adults. Data show that brainstem transection prevents the lung burst frequency response in both stage groups. During development, the progressive decrease in the Na(+)/K(+)/Cl(-) co-transporter NKCC1 contributes to the maturation of neural networks. Because NKCC1 becomes activated during hypoxia, we then tested the hypothesis that NKCC1 contributes to maturation of the central O(2) chemoreflex. Double labeling experiments showed that the proportion of tyrosine hydroxylase positive neurons expressing NKCC1 in the LC decreases during development. Inactivation of NKCC1 with bumetanide bath application reversed the lung burst response to hypoxia in tadpoles. Bumetanide inhibited the response in adults. These data indicate that a structure within the pons (potentially the LC) is necessary to the central hypoxic chemoreflex and demonstrate that NKCC1 plays a role in central O(2) chemosensitivity and its maturation in this species.

Postnatal developmental changes in activation profiles of the respiratory neuronal network in the rat ventral medulla

Two putative respiratory rhythm generators (RRGs), the para-facial respiratory group (pFRG) and the pre-Bötzinger complex (preBötC), have been identified in the neonatal rodent brainstem. To elucidate their functional roles during the neonatal period, we evaluated developmental changes of these RRGs by optical imaging using a voltage-sensitive dye. Optical signals, recorded from the ventral medulla of brainstem-spinal cord preparations of neonatal (P0-P4) rats (n = 44), were analysed by a cross correlation method. With development during the first few postnatal days, the respiratory-related activity in the pFRG reduced and shifted from a preinspiratory (P0-P1) to an inspiratory (P2-P4) pattern, whereas preBötC activity remained unchanged. The mu-opioid agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO) augmented preinspiratory activity in the pFRG, while the mu-opioid antagonist naloxone induced changes in spatiotemporal activation profiles that closely mimicked the developmental changes. These results are consistent with the recently proposed hypothesis by Janczewski and Feldman that the pFRG is activated to compensate for the depression of the preBötC by perinatal opiate surge. We conclude that significant reorganization of the respiratory neuronal network, characterized by a reduction of preinspiratory activity in the pFRG, occurs at P1-P2 in rats. The changes in spatiotemporal activation profiles of the pFRG neurones may reflect changes in the mode of coupling of the two respiratory rhythm generators.

Opioid-induced depression in the lamprey respiratory network

The role of opioid receptors in modulating respiratory activity was investigated in in vitro brainstem preparations of adult lampreys by bath application of agonists and antagonists. The vagal motor output was used to monitor respiratory activity. Neuronal recordings were also performed to characterize the rostrolateral trigeminal region that has been suggested to be critical for respiratory rhythmogenesis. Microinjections of the micro-opioid receptor agonist [d-Ala(2), N-Me-Phe(4), Gly(5)-ol]-enkephalin (DAMGO) were also made into this region and at different locations within the brainstem. Bath application of DAMGO (0.5-2 microM) caused marked decreases in respiratory frequency up to complete apnea. Bath application of the delta-opioid receptor agonist [d-Pen(2,5)]-enkephalin (DPDPE) at 10-40 microM induced less pronounced depressant respiratory effects, while no changes in respiratory activity were induced by the kappa-opioid receptor agonist trans-(1S,2S)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] benzeneacetamide (U50488) at 10-40 microM. Bath application of the opioid receptor antagonists naloxone and naltrindole did not affect baseline respiratory activity, but prevented agonist-induced effects. DAMGO microinjections (1 mM; 0.5-1 nl) at sites rostrolateral to the trigeminal motor nucleus, where respiration-related neuronal activity was recorded, abolished the respiratory rhythm. The results show that opioids may have an important role in the lamprey respiratory network and that micro-opioid receptor activation is the most effective in causing respiratory depression. They also indicate that endogenous opioids are not required for the generation of baseline respiratory activity. Apneic responses induced by DAMGO microinjections support the hypothesis that a specific opioid-sensitive region rostrolateral to the trigeminal motor nucleus, that we have termed the paratrigeminal respiratory group (pTRG), likely has a pivotal role in respiratory rhythmogenesis. Since the lamprey diverged from the main vertebrate line around 450 million years ago, our results also imply that the inhibitory role of opioids on respiration is present at an early stage of vertebrate evolution.

Respiratory rhythms generated in the lamprey rhombencephalon

Brainstem networks generating the respiratory rhythm in lampreys are still not fully characterized. In this study, we described the patterns of respiratory activities and we identified the general location of underlying neural networks. In a semi-intact preparation including the brain and gills, rhythmic discharges were recorded bilaterally with surface electrodes placed over the vagal motoneurons. The main respiratory output driving rhythmic gill movements consisted of short bursts (40.9+/-15.6 ms) of discharge occurring at a frequency of 1.0+/-0.3 Hz. This fast pattern was interrupted by long bursts (506.3+/-174.6 ms) recurring with an average period of 37.4+/-24.9 s. After isolating the brainstem by cutting all cranial nerves, the frequency of the short respiratory bursts did not change significantly, but the slow pattern was less frequent. Local injections of a glutamate agonist (AMPA) and antagonists (6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or D,L-amino-5-phosphonopentanoic acid (AP5)) were made over different brainstem regions to influence respiratory output. The results were similar in the semi-intact and isolated-brainstem preparations. Unilateral injection of AP5 or CNQX over a rostral rhombencephalic region, lateral to the rostral pole of the trigeminal motor nucleus, decreased the frequency of the fast respiratory rhythm bilaterally or stopped it altogether. Injection of AMPA at the same site increased the rate of the fast respiratory rhythm and decreased the frequency of the slow pattern. The activity recorded in this area was synchronous with that recorded over the vagal motoneurons. After a complete transverse lesion of the brainstem caudal to the trigeminal motor nucleus, the fast rhythm was confined to the rostral area, while only the slow activity persisted in the vagal motoneurons. Our results support the hypothesis that normal breathing depends on the activity of neurons located in the rostral rhombencephalon in lampreys, whereas the caudal rhombencephalon generates the slow pattern.

Chronic hypoxia attenuates central respiratory-related pH/CO2 chemosensitivity in the cane toad

This study examined the effects of chronic hypoxia (CH) and mid-brain transection on central respiratory-related pH/CO(2) chemosensitivity in cane toads (Bufo marinus). Toads were exposed to 10 days of CH (10% O(2)) following which in vitro brainstem-spinal cord preparations, with the mid-brain attached, were used to examine central pH/CO(2) chemosensitivity. A reduction in artificial cerebral spinal fluid (aCSF) pH increased fictive breathing frequency (fR) and total fictive ventilation. CH reduced fictive fR and total fictive ventilation, compared to controls. Mid-brain transection caused an increase in fictive fR, at the lower aCSF pH levels, in both control and CH preparations. In the CH preparations, mid-brain transection restored fictive breathing to control levels. In both groups, mid-brain transection eliminated fictive breath clustering. The data indicate that CH attenuates central pH/CO(2)-sensitive fictive breathing but a mid-brain transection in the middle of the optic lobes abolishes this attenuation. The results suggest that CH induces inhibition of central pH/CO(2) chemoreceptor function via descending inputs from the mid-brain region.

Role of midbrain in the control of breathing in anuran amphibians

The present study was designed to explore systematically the midbrain of unanesthetized, decerebrate anuran amphibians (bullfrogs), using chemical and electrical stimulation and midbrain transections to identify sites capable of exciting and inhibiting breathing. Ventilation was measured as fictive motor output from the mandibular branch of the trigeminal nerve and the laryngeal branch of the vagus nerve. The results of our transection studies suggest that, under resting conditions, the net effect of inputs from sites within the rostral half of the midbrain is to increase fictive breathing frequency, whereas inputs from sites within the caudal half of the midbrain have no net effect on fictive breathing frequency but appear to act on the medullary central rhythm generator to produce episodic breathing. The results of our stimulation experiments indicate that the principal sites in the midbrain that are capable of exciting or inhibiting the fictive frequency of lung ventilation, and potentially clustering breaths into episodes, appear to be those primarily involved in visual and auditory integration, motor functions, and attentional state.

Breathing and calling: neuronal networks in the Xenopus laevis hindbrain

Xenopus laevis is an aquatic anuran with a complex vocal repertoire. Unlike terrestrial frogs, vocalizations are independent of respiration, and a single muscle group--the laryngeal dilators--produces underwater calls. We sought to identify the premotor neural network that underlies vocal behaviors. Vocal patterns generated by premotor networks control laryngeal motor neurons in cranial nucleus (n.) IX-X. Glottal motor neurons, active during respiration, are also present in n.IX-X. We used horseradish peroxidase (HRP), Lucifer yellow, and fluorescently conjugated dextrans to characterize the organization of n.IX-X and to trace premotor neuron projections. Premotor nuclei include the inferior reticular formation (Ri) adjacent to n.IX-X and the pretrigeminal nucleus of the dorsal tegmental area of the medulla (DTAM), the primary descending input to n.IX-X. Intramuscular HRP injections revealed a spatially segregated pattern, with glottal motor neurons in anterior n.IX-X and laryngeal motor neurons in the caudal portion of the nucleus. Dextran injections identified commissural n.IX-X neurons that project to the contralateral motor nucleus and DTAM-projecting n.IX-X neurons. Both neuronal types are clustered in anteromedial n.IX-X, closely associated with glottal motor neurons. Ri neurons project to ipsilateral and contralateral DTAM. Projections from DTAM target n.IX-X bilaterally, and all four identified subtypes receive DTAM input. In contrast, Ri neurons receive little input from DTAM. We hypothesize that connectivity between neurons in n.IX-X, Ri and DTAM may provide mechanisms to generate laryngeal and glottal activity patterns and that DTAM may coordinate vocal and respiratory motor pools, perhaps acting to switch between these two mutually exclusive behaviors.