Rostral ventrolateral medulla and respiratory rhythmogenesis in mice

Machine learning-based data analytic approaches for evaluating post-natal mouse respiratory physiological evolution

Respiratory parameters change during post-natal development, but the nature of their changes have not been well-described. The advent of commercially available plethysmographic instruments provided improved repeatability of measurements and standardization of measured breathing in mice across laboratories. These technologies thus allowed for exploration of more precise respiratory pattern changes during the post-natal developmental epoch. Current methods to analyze respiratory behavior utilize plethysmography to acquire standing values of frequency, volume and flow at specific time points in murine maturation. These metrics have historically been independently analyzed as a function of time with no further analysis examining the interplay these variables have with each other and in the context of postnatal maturation or during blood gas homeostasis. We posit that machine learning workflows can provide deeper physiological understanding into the postnatal development of respiration. In this manuscript, we delineate a machine learning workflow based on the R-statistical programming language to examine how variation and relationships of frequency (f) and tidal volume (TV) change with respect to inspiratory and expiratory parameters. Our analytical workflows could successfully predict age and found that the variation and relationships between respiratory metrics are dynamically shifting with age and during hypercapnic breathing. Thus, our work demonstrates the utility of high dimensional analyses to provide reliable class label predictions using non-invasive respiratory metrics. These approaches may be useful in large-scale phenotyping across development and in disease.

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

KEY POINTS

The brainstem catecholaminergic (CA) modulation on ventilation changes with development. We determined the role of the brainstem CA system in ventilatory control under normocapnic and hypercapnic conditions during different phases of development [postnatal day (P)7-8, P14-15 and P20-21] in male and female Wistar rats. Brainstem CA neurones produce a tonic inhibitory drive that affects breathing frequency in P7-8 rats and provide an inhibitory drive during hypercapnic conditions in both males and females at P7-8 and P14-15. In pre-pubertal rats, brainstem CA neurones become excitatory for the CO₂ ventilatory response in males but remain inhibitory in females. Diseases such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome have been associated with abnormalities in the functioning of CA neurones; therefore, the results of the present study contribute to a better understanding of this system.

ABSTRACT

The respiratory network undergoes significant development during the postnatal phase, including the maturation of the catecholaminergic (CA) system. However, postnatal development of this network and its effect on the control of pulmonary ventilation ( V̇E ) is not fully understood. We investigated the involvement of brainstem CA neurones in respiratory control during postnatal development [postnatal day (P)7-8, P14-15 and P20-21], in male and female rats, through chemical injury with conjugated saporin anti-dopamine β-hydroxylase (DβH-SAP). Thus, DβH-SAP (420 ng μL^-1 ), saporin (SAP) or phosphate buffered solution (PBS) was injected into the fourth ventricle of neonatal Wistar rats of both sexes. V̇E and oxygen consumption were recorded 1 week after the injections in unanaesthetized neonatal and juvenile rats during room air and hypercapnia. The resting ventilation was higher in both male and female P7-8 lesioned rats by 33%, with a decrease in respiratory variability being observed in males. The hypercapnic ventilatory response (HCVR) was altered in male and female lesioned rats at all postnatal ages. At P7-8, the HCVR for males and females was increased by 37% and 30%, respectively. For both sexes at P14-15 rats, the increase in V̇E during hypercapnia was 37% higher for lesioned rats. A sex-specific difference in HCRV was observed at P20-21, with lesioned males showing a 33% decrease, and lesioned females showing an increase of 33%. We conclude that brainstem CA neurones exert a tonic inhibitory effect on V̇E in the early postnatal days of the life of a rat, increase variability in P7-8 males and modulate HCRV during the postnatal phase.

Phasic inhibition as a mechanism for generation of rapid respiratory rhythms

Central neural networks operate continuously throughout life to control respiration, yet mechanisms regulating ventilatory frequency are poorly understood. Inspiration is generated by the pre-Bötzinger complex of the ventrolateral medulla, where it is thought that excitation increases inspiratory frequency and inhibition causes apnea. To test this model, we used an in vitro optogenetic approach to stimulate select populations of hindbrain neurons and characterize how they modulate frequency. Unexpectedly, we found that inhibition was required for increases in frequency caused by stimulation of Phox2b-lineage, putative CO₂-chemosensitive neurons. As a mechanistic explanation for inhibition-dependent increases in frequency, we found that phasic stimulation of inhibitory neurons can increase inspiratory frequency via postinhibitory rebound. We present evidence that Phox2b-mediated increases in frequency are caused by rebound excitation following an inhibitory synaptic volley relayed by expiration. Thus, although it is widely thought that inhibition between inspiration and expiration simply prevents activity in the antagonistic phase, we instead propose a model whereby inhibitory coupling via postinhibitory rebound excitation actually generates fast modes of inspiration.

Mechanistic Studies of Capsaicin-Induced Apnea in Rodents

Inhalation of capsaicin-based sprays can cause central respiratory depression and lethal apneas. There are contradictory reports regarding the sites of capsaicin action. Furthermore, an understanding of the neurochemical mechanisms underlying capsaicin-induced apneas and the development of pharmacological interventions is lacking. The main objectives of this study were to perform a systematic study of the mechanisms of action of capsaicin-induced apneas and to provide insights relevant to pharmacological intervention. In vitro and in vivo rat and transient receptor potential vanilloid superfamily member 1 (TRPV1)-null mouse models were used to measure respiratory parameters and seizure-like activity in the presence of capsaicin and compounds that modulate glutamatergic neurotransmission. Administration of capsaicin to in vitro and in vivo rat and wild-type mouse models induced dose-dependent apneas and the production of seizure-like activity. No significant changes were observed in TRPV1-null mice or rat medullary slice preparations. The capsaicin-induced effects were inhibited by the TRPV1 antagonist capsazepine, amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) receptor antagonists CNQX, NBQX, perampanel, and riluzole, a drug that inhibits glutamate release and increases glutamate uptake. The capsaicin-induced effects on breathing and seizure-like activity were accentuated by positive allosteric modulators of the AMPA receptors, CX717 and cyclothiazide. To summarize, capsaicin-induced apneas and seizure-like behaviors are mediated via TRPV1 activation acting at lung afferents, spinal cord-ascending tracts, and medullary structures (including nucleus tractus solitarius). AMPA receptor-mediated conductances play an important role in capsaicin-induced apneas and seizure-like activity. A pharmaceutical strategy targeted at reducing AMPA receptor-mediated glutamatergic signaling may reduce capsaicin-induced deleterious effects.

Respiratory Rhythm Generation: Plasticity of a Neuronal Network

The exchange of gases between the external environment and the organism is controlled by a neural network of medullary neurons that produces rhythmic activity that ultimately leads to periodic contractions of thoracic, abdominal, and diaphragm muscles. This occurs in three neural phases: inspiration, postinspiration, and expiration. The present article deals with the mechanisms underlying respiratory rhythm generation and the processes of dynamic adjustment of respiratory activity by neuromodulation as it occurs during normoxia and hypoxia. The respiratory rhythm originates from the “pre-Bötzinger complex,” which is a morphologically defined region within the lower brainstem. There is a primary oscillating network consisting of reciprocally connected early-inspiratory and postinspiratory neurons, whereas various other subgroups of respiratory neurons shape the activity pattern. Rhythm generation and pattern formation result from neuronal interactions within the network, that is, from cooperative adjustments of intrinsic membrane properties and synaptic processes in the respiratory neurons. There is evidence that in neonatal mammals, as well as under certain pathological situations in adult mammals, the respiratory rhythm derives from early-inspiratory burster neurons that drive inspiratory output neurons. The respiratory network is influenced by a variety of neuromodulators. Stimulation of appropriate receptors mostly activates signal pathways that converge on cAMP-dependent protein kinase and protein kinase C. Both pathways exert modulatory effects on voltage- and ligand-controlled ion channels. Many neuromodulators are continuously released within the respiratory region or accumulated under pathological conditions such as hypoxia. The functional significance of such ongoing neuromodulation is seen in variations of network excitability. In this review, the authors concentrate on the modulators serotonin, adenosine, and opioids.

Effects of bile acids and the bile acid receptor FXR agonist on the respiratory rhythm in the in vitro brainstem medulla slice of neonatal Sprague-Dawley rats

Intrahepatic cholestasis of pregnancy is always accompanied by adverse fetal outcomes such as malfunctions of respiration. Farnesoid X receptor (FXR) plays a critical role in the homeostasis of bile acids. Thus, we are determined to explore the effects of farnesoid X receptor (FXR) and five bile acids on respiratory rhythm generation and modulation of neonatal rats. Spontaneous periodic respiratory-related rhythmical discharge activity (RRDA) was recorded from hypoglossal nerves during the perfusion of modified Krebs solution. Group 1–6 was each given GW4064 and five bile acids of chenodeoxycholic acid (CDCA), deoxycholic acid (DCA), lithocholic acid (LCA), cholic acid (CA) as well as ursodeoxycholic acid (UDCA) at different concentrations to identify their specific functions on respiratory rhythm modulations. Group 7 was applied to receive FXR blocker Z-guggulsterone and Z-guggulsterone with the above bile acids separately to explore the role of FXR in the respiratory rhythm modulation. Group 8 was given dimethyl sulfoxide (DMSO) as controls. Apart from UDCA, CDCA, DCA LCA and CA all exerted effects on RRDA recorded from hypoglossal nerves in a concentration-dependent manner. Respiratory cycle (RC), Inspiratory time (TI), Expiratory Time (TE) and Integral Amplitude (IA) were influenced and such effects could be reversed by Z-guggulsterone. FXR may contribute to the effects on the modulation of respiratory rhythm exerted by bile acids.

A simple and novel method to monitor breathing and heart rate in awake and urethane-anesthetized newborn rodents

Rodents are most useful models to study physiological and pathophysiological processes in early development, because they are born in a relatively immature state. However, only few techniques are available to monitor non-invasively heart frequency and respiratory rate in neonatal rodents without restraining or hindering access to the animal. Here we describe experimental procedures that allow monitoring of heart frequency by electrocardiography (ECG) and breathing rate with a piezoelectric transducer (PZT) element without hindering access to the animal. These techniques can be easily installed and are used in the present study in unrestrained awake and anesthetized neonatal C57/Bl6 mice and Wistar rats between postnatal day 0 and 7. In line with previous reports from awake rodents we demonstrate that heart rate in rats and mice increases during the first postnatal week. Respiratory frequency did not differ between both species, but heart rate was significantly higher in mice than in rats. Further our data indicate that urethane, an agent that is widely used for anesthesia, induces a hypoventilation in neonates whilst heart rate remains unaffected at a dose of 1 g per kg body weight. Of note, hypoventilation induced by urethane was not detected in rats at postnatal 0/1. To verify the detected hypoventilation we performed blood gas analyses. We detected a respiratory acidosis reflected by a lower pH and elevated level in CO2 tension (pCO2) in both species upon urethane treatment. Furthermore we found that metabolism of urethane is different in P0/1 mice and rats and between P0/1 and P6/7 in both species. Our findings underline the usefulness of monitoring basic cardio-respiratory parameters in neonates during anesthesia. In addition our study gives information on developmental changes in heart and breathing frequency in newborn mice and rats and the effects of urethane in both species during the first postnatal week.

Coordination of NMDA-induced rhythmic activity in the trigeminal and hypoglossal nerves of neonatal mice in vitro

Suckling is a rhythmic jaw movement that is symmetrical on the left and right side and is highly coordinated with tongue movement. Thus, we investigated the neuronal mechanisms of the left/right and jaw/tongue coordinations during N-methyl-d-aspartate (NMDA)-induced fictive suckling using isolated brainstem-spinal cord preparations obtained from neonatal mice. We observed synchronous low-frequency rhythmic activity in the left/right trigeminal motor nerves, which differed from respiration, and high-frequency rhythmic trigeminal activity, which was side-independent. The low-frequency rhythmic trigeminal activity was also synchronized with the hypoglossal nerve activity. After a complete midline separation of the preparation or a partial midline transection of the brainstem from the anterior inferior cerebellar artery to the junction of the vertebral artery, the low-frequency rhythmic trigeminal activity disappeared, whereas the high-frequency rhythmic trigeminal activity and low-frequency rhythmic hypoglossal activity still remained. These results suggest that the neuronal network that generates low-frequency rhythmic activity likely contributes to the synchronized activity of the left/right jaw muscles and to the jaw/tongue muscles, where it sends its command to the trigeminal motoneurons mainly via the commissural pathway that crosses the transected midline region. Such a neuronal network may underlie the coordinated movements of the jaw and tongue during suckling.

Task2 potassium channels set central respiratory CO2 and O2 sensitivity

Task2 K(+) channel expression in the central nervous system is surprisingly restricted to a few brainstem nuclei, including the retrotrapezoid (RTN) region. All Task2-positive RTN neurons were lost in mice bearing a Phox2b mutation that causes the human congenital central hypoventilation syndrome. In plethysmography, Task2(-/-) mice showed disturbed chemosensory function with hypersensitivity to low CO(2) concentrations, leading to hyperventilation. Task2 probably is needed to stabilize the membrane potential of chemoreceptive cells. In addition, Task2(-/-) mice lost the long-term hypoxia-induced respiratory decrease whereas the acute carotid-body-mediated increase was maintained. The lack of anoxia-induced respiratory depression in the isolated brainstem-spinal cord preparation suggested a central origin of the phenotype. Task2 activation by reactive oxygen species generated during hypoxia could silence RTN neurons, thus contributing to respiratory depression. These data identify Task2 as a determinant of central O(2) chemoreception and demonstrate that this phenomenon is due to the activity of a small number of neurons located at the ventral medullary surface.

Hypothalamic orexins/hypocretins as regulators of breathing

It was suggested half a century ago that electrical impulses from the lateral hypothalamic area stimulate breathing. It is now emerging that these effects may be mediated, at least in part, by neurons containing orexin neuropeptides (also known as hypocretins). These cells promote wakefulness and consciousness, and their loss results in narcolepsy. Recent data also show that orexin neurons directly project to respiratory centres in the brainstem, which express orexin receptors, and where injection of orexin stimulates breathing. Because orexin neurons receive inputs that signal metabolic, sleep/wake and emotional states, it is tempting to speculate that they may regulate breathing according to these parameters. Knockout of the orexin gene in mice reduces CO2-induced increases in breathing by approximately 50% and increases the frequency of spontaneous sleep apneas. The relationship between orexins and breathing may be bidirectional: the rate of breathing controls acid and CO2 levels, and these signals alter the electrical activity of orexin neurons in vitro. Overall, these findings suggest that orexins are important for the regulation of breathing and may potentially play a role in the pathophysiology and medical treatment of respiratory disorders.

Endogenous noradrenaline affects the maturation and function of the respiratory network: Possible implication for SIDS

Breathing is a vital, rhythmic motor act that is required for blood oxygenation and oxygen delivery to the whole body. Therefore, the brainstem network responsible for the elaboration of the respiratory rhythm must function from the very first moments of extrauterine life. In this review, it is shown that the brainstem noradrenergic system plays a pivotal role in both the modulation and the maturation of the respiratory rhythm generator. Compelling evidence are reported demonstrating that genetically induced alterations of the noradrenergic system in mice affect the prenatal maturation and the perinatal function of the respiratory rhythm generator and have drastic consequences on postnatal survival. Sudden Infant Death Syndrome (SIDS), the leader cause of infant death in industrialised countries, may result from cardiorespiratory disorders during sleep. As several cases of SIDS have been observed in infants having noradrenergic deficits, a possible link between prenatal alteration of the noradrenergic system, altered maturation and function of the respiratory network and SIDS is suggested.

Adenosine A1 receptors modulate the discharge activities of inspiratory and biphasic expiratory neurons in the medial region of Nucleus Retrofacialis of neonatal rat in vitro

This study investigated whether adenosine A1 receptors could modulate respiratory rhythm in mammals. Experiments were performed in in vitro brainstem slice preparations from neonatal rats. These preparations included the medial region of Nucleus Retrofacialis (mNRF) with the hypoglossal nerve (XII nerve) rootlets retained. The rhythmical discharges of the biphasic expiratory (biphasic E) neurons/inspiratory neurons (I neurons) and activities of the XII nerve rootlets were simultaneously recorded by using extracellular microelectrodes and suction electrodes, respectively. Roles of adenosine A1 receptors in modulation of respiratory rhythm were investigated by administration of the adenosine A1 receptor agonist R-phenylisopropyl-adenosine (R-PIA, 10 microM) and its specific antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX, 2 microM). DPCPX decreased the respiratory period (27.19%) and expiratory duration (28.27%) of biphasic E neurons and at the same time increased the peak discharge frequency (48.13%). By contrast, R-PIA produced opposite effects. On the other hand, the effects of DPCPX and R-PIA on the I neurons were similar to that on the biphasic E neurons except that R-PIA shortened the discharge duration of I neurons (34.12%) and decreased the peak discharge frequency (37.75%) in the middle phase of inspiration, but not in the initial and terminal phases. These results suggest that adenosine A1 receptors are involved in the phase-switching between expiration and inspiration by affecting biphasic E neurons. Activation of adenosine A1 receptors may modulate the inhibitory synaptic inputs from I neurons to biphasic E neurons.

Modulation of the respiratory rhythm generator by the pontine noradrenergic A5 and A6 groups in rodents

The aim of the present review is to summarise available studies dealing with the respiratory control exerted by pontine noradrenergic neurones in neonatal and adult mammals. During the perinatal period, in vitro studies on neonatal rodents have shown that A5 and A6 neurones exert opposite modulations onto the respiratory rhythm generator, inhibitory and facilitatory respectively, that the anatomical support for these modulations already exists at birth, and that genetically induced alterations in the formation of A5 and A6 neurones affect the maturation of the respiratory rhythm generator, leading to lethal respiratory deficits at birth. The A5-A6 modulation of the respiratory rhythm generator is not transient, occurring solely during the perinatal period but it persists throughout life: A5 and A6 neurones display a respiratory-related activity, receive inputs from and send information to the medullary respiratory centres and contribute to the adaptation of adult breathing to physiological needs.

Somatostatin inhibition of fictive respiration is modulated by pH

We studied the respiratory effects of the tetradecapeptide somatostatin (SST) upon fictive respiration using the in vitro brain stem spinal cord preparation from new-born mouse. We found that SST inhibits respiration, an effect that was potentiated when the chemical drive to respiration was increased. SST inhibited fictive respiration decreasing both the frequency and amplitude in a dose-dependent way. SST inhibition was not antagonized by cyclosomatostatin (cyclo [7-aminoheptanoyl-Phe-D-Trp-Lys-Thr(Bzl)]), a putative SST antagonist, which in contrast behaved as a partial agonist. When the chemical drive to respiration was increased, by lowering the pH of the brain stem superfusion medium from 7.4 to 7.3, the inhibitory effect of SST on respiratory frequency was potentiated. These results suggest an interaction between SST and respiratory central chemoreception in new-born mouse.

Effect of hypothermia on respiratory rhythm generation in hamster brainstem–spinal cord preparations

This study examined the effect of hypothermia on respiratory neural output from brainstem-spinal cord preparations of a cold tolerant rodent, the Syrian hamster. Brainstem-spinal cords from neonatal hamsters (0-6 days) were placed in a recording dish and respiratory-like neural activity was recorded from roots of the first cervical nerve. The preparations were cooled and warmed in a continuous or stepwise fashion. Inputs from the pons completely inhibited neural activity under steady state conditions. With the pons removed, fictive breathing was robust. Cooling caused respiratory arrest, followed by spontaneous resumption of activity on re-warming. Preparations from older hamsters (4-6 days old) were more cold tolerant than younger preparations (0-3 days old). Motor discharge was episodic during continuous cooling, and seizure-like discharge was observed during continuous warming. These phenomena were not observed with stepwise temperature changes suggesting that transient temperature effects on membrane properties may be involved. These preparations were not as cold tolerant as hamster pups in vivo but they retained the ability to autoresuscitate at all ages studied.

Phox2a gene, A6 neurons, and noradrenaline are essential for development of normal respiratory rhythm in mice

Although respiration is vital to the survival of all mammals from the moment of birth, little is known about the genetic factors controlling the prenatal maturation of this physiological process. Here we investigated the role of the Phox2a gene that encodes for a homeodomain protein involved in the generation of noradrenergic A6 neurons in the maturation of the respiratory network. First, comparisons of the respiratory activity of fetuses delivered surgically from heterozygous Phox2a pregnant mice on gestational day 18 showed that the mutants had impaired in vivo ventilation, in vitro respiratory-like activity, and in vitro respiratory responses to central hypoxia and noradrenaline. Second, pharmacological studies on wild-type neonates showed that endogenous noradrenaline released from pontine A6 neurons potentiates rhythmic respiratory activity via alpha1 medullary adrenoceptors. Third, transynaptic tracing experiments in which rabies virus was injected into the diaphragm confirmed that A6 neurons were connected to the neonatal respiratory network. Fourth, blocking the alpha1 adrenoceptors in wild-type dams during late gestation with daily injections of the alpha1 adrenoceptor antagonist prazosin induced in vivo and in vitro neonatal respiratory deficits similar to those observed in Phox2a mutants. These results suggest that noradrenaline, A6 neurons, and the Phox2a gene, which is crucial for the generation of A6 neurons, are essential for development of normal respiratory rhythm in neonatal mice. Metabolic noradrenaline disorders occurring during gestation therefore may induce neonatal respiratory deficits, in agreement with the catecholamine anomalies reported in victims of sudden infant death syndrome.

Ventilatory pattern and chemosensitivity in M1 and M3 muscarinic receptor knockout mice

Acetylcholine (ACh) acting through muscarinic receptors is thought to be involved in the control of breathing, notably in central and peripheral chemosensory afferents and in regulations related to sleep-wake states. By using whole-body plethysmography, we compared baseline breathing at rest and ventilatory responses to acute exposure (5 min) to moderate hypoxia (10% O(2)) and hypercapnia (3 and 5% CO(2)) in mice lacking either the M(1) or the M(3) muscarinic receptor, and in wild-type matched controls. M(1) knockout mice showed normal minute ventilation (V(E)) but elevated tidal volume (V(T)) at rest, and normal chemosensory ventilatory responses to hypoxia and hypercapnia. M(3) knockout mice had elevated V(E) and V(T) at rest, a reduced V(T) response slope to hypercapnia, and blunted V(E) and frequency responses to hypoxia. The results suggest that M(1) and M(3) muscarinic receptors play significant roles in the regulation of tidal volume at rest and that the afferent pathway originating from peripheral chemoreceptors involves M(3) receptors.

MafB deficiency causes defective respiratory rhythmogenesis and fatal central apnea at birth

The genetic basis for the development of brainstem neurons that generate respiratory rhythm is unknown. Here we show that mice deficient for the transcription factor MafB die from central apnea at birth and are defective for respiratory rhythmogenesis in vitro. MafB is expressed in a subpopulation of neurons in the preBötzinger complex (preBötC), a putative principal site of rhythmogenesis. Brainstems from Mafb(-/-) mice are insensitive to preBötC electrolytic lesion or stimulation and modulation of rhythmogenesis by hypoxia or peptidergic input. Furthermore, in Mafb(-/-) mice the preBötC, but not major neuromodulatory groups, presents severe anatomical defects with loss of cellularity. Our results show an essential role of MafB in central respiratory control, possibly involving the specification of rhythmogenic preBötC neurons.

Nasal trigeminal inputs release the A5 inhibition received by the respiratory rhythm generator of the mouse neonate

Experiments were performed on neonatal mice to analyze why, in vitro, the respiratory rhythm generator (RRG) was silent and how it could be activated. We demonstrated that in vitro the RRG in intact brain stems is silenced by a powerful inhibition arising from the pontine A5 neurons through medullary alpha(2) adrenoceptors and that in vivo nasal trigeminal inputs facilitate the RRG as nasal continuous positive airway pressure increases the breathing frequency, whereas nasal occlusion and nasal afferent anesthesia depress it. Because nasal trigeminal afferents project to the A5 nuclei, we applied single trains of negative electric shocks to the trigeminal nerve in inactive ponto-medullary preparations. They induced rhythmic phrenic bursts during the stimulation and for 2-3 min afterward, whereas repetitive trains produced on-going rhythmic activity up to the end of the experiments. Electrolytic lesion or pharmacological inactivation of the ipsilateral A5 neurons altered both the phrenic burst frequency and occurrence after the stimulation. Extracellular unitary recordings and trans-neuronal tracing experiments with the rabies virus show that the medullary lateral reticular area contains respiratory-modulated neurons, not necessary for respiratory rhythmogenesis, but that may provide an excitatory pathway from the trigeminal inputs to the RRG as their electrolytic lesion suppresses any phrenic activity induced by the trigeminal nerve stimulation. The results lead to the hypothesis that the trigeminal afferents in the mouse neonate involve at least two pathways to activate the RRG, one that may act through the medullary lateral reticular area and one that releases the A5 inhibition received by the RRG.

Respiratory responses to pH in the absence of pontine and dorsal medullary areas in the newborn mouse in vitro

The contribution of pons and dorsal medulla in establishing the pattern of fictive respiration and in mediating the respiratory response to acidification was studied using the isolated brainstem-spinal cord preparation from neonatal mouse. About 40% of ponto-medullary preparations (retaining pons) showed spontaneous, but irregular respiratory-like rhythm. In the other 60%, the elimination of the pons often was followed by the initiation of a respiratory-like rhythm. Medullary preparations, derived from either inactive or rhythmic ponto-medullary preparations, showed a regular respiratory-like rhythm, which was also of a higher frequency and a bigger amplitude than that observed in ponto-medullary preparations. In contrast, ventral medullary preparations, derived from medullary preparations by eliminating the dorsal medulla, showed an irregular rhythm with a reduced amplitude of the integrated inspiratory burst. In ponto-medullary and ventral medullary preparations, acidification of the superfusion medium increased the respiratory frequency, while in medullary preparations, it increased the frequency and reduced the amplitude of the inspiratory burst. Our results suggest that pontine structures influence negatively the rate and depth of the respiratory-like rhythm, while dorsal medullary structures influence positively the depth of the rhythm. They also suggest that the pattern of response to pH supported by the ventral medulla is modified by the input provided from pons and dorsal medulla.

Noradrenergic receptors and in vitro respiratory rhythm: possible interspecies differences between mouse and rat neonates

Similar in vitro experiments were performed on brainstem-spinal cord preparations from mouse and rat neonates to compare the noradrenergic regulations of the respiratory network. In preparations retaining the pons, rhythmic phrenic bursts occurred in rats but not in mice. Transection of the pons, electrolytic lesions and noradrenaline applications showed that the pontine noradrenergic A5 group inhibited the respiratory rhythm generator in both species but the inhibition was especially potent in mice. After pons elimination, noradrenaline applications to the medulla decreased the respiratory frequency in rats but increased it in mice. Noradrenergic agent applications revealed that the frequency changes implicated medullary alpha 1 and alpha 2 noradrenergic receptors in mice and rats, respectively. Thus, interspecies differences seem to exist in the noradrenergic regulations of the rat and mouse medullary respiratory networks.

Identification of central nervous system neurons innervating the respiratory muscles of the mouse: a transneuronal tracing study

In recent years, the central control of breathing in mammals has been the subject of numerous studies. The aim of the present one was to characterize the neuronal network projecting to the main respiratory motoneurons, in adult mice. To this end, the morphology and location of the respiratory motoneurons and their sequential connections with other neurons were revealed using a transneuronal tracing technique by means of the rabies virus infection. The injections of the rabies virus in the respiratory muscles resulted in labeling the motoneurons and their serially connected interneurons at multiple levels of the mouse central nervous system: spinal cord, pons and medulla, cerebellum, mesencephalon, diencephalon, and telencephalon. Most of these labeled areas have been previously identified in the control of cardiorespiratory regulation, as well as in other autonomic functions. These anatomical data provide support for the integration of respiratory-related activities in complex behavioral responses. Furthermore, these data suggest similarities in the evolution of central respiratory networks in mammals.

Gap junctions and inhibitory synapses modulate inspiratory motoneuron synchronization

Interneuronal electrical coupling via gap junctions and chemical synaptic inhibitory transmission are known to have roles in the generation and synchronization of activity in neuronal networks. Uncertainty exists regarding the roles of these two modes of interneuronal communication in the central respiratory rhythm-generating system. To assess their roles, we performed studies on both the neonatal mouse medullary slice and en bloc brain stem-spinal cord preparations where rhythmic inspiratory motor activity can readily be recorded from both hypoglossal and phrenic nerve roots. The rhythmic inspiratory activity observed had two temporal characteristics: the basic respiratory frequency occurring on a long time scale and the synchronous neuronal discharge within the inspiratory burst occurring on a short time scale. In both preparations, we observed that bath application of gap-junction blockers, including 18 alpha-glycyrrhetinic acid, 18 beta-glycyrrhetinic acid, and carbenoxolone, all caused a reduction in respiratory frequency. In contrast, peak integrated phrenic and hypoglossal inspiratory activity was not significantly changed by gap-junction blockade. On a short-time-scale, gap-junction blockade increased the degree of synchronization within an inspiratory burst observed in both nerves. In contrast, opposite results were observed with blockade of GABA(A) and glycine receptors. We found that respiratory frequency increased with receptor blockade, and simultaneous blockade of both receptors consistently resulted in a reduction in short-time-scale synchronized activity observed in phrenic and hypoglossal inspiratory bursts. These results support the concept that the central respiratory system has two components: a rhythm generator responsible for the production of respiratory cycle timing and an inspiratory pattern generator that is involved in short-time-scale synchronization. In the neonatal rodent, properties of both components can be regulated by interneuronal communication via gap junctions and inhibitory synaptic transmission.

5-Hydroxytryptamine(2A) and 5-hydroxytryptamine(1B) receptors are differently affected by the monoamine oxidase A-deficiency in the Tg8 transgenic mouse

In brainstem-spinal cord preparations of neonatal control C3H and transgenic Tg8 mice where deletion of the gene encoding monoamine oxidase-A results in serotonin (5-hydroxytryptamine (HT)) excess, whole cell recordings of identified phrenic motoneurons (Phr Mns) were performed to study the modulation of their activity by 5-HT. In C3H mice, a dual effect was observed: (i) a facilitation via 5-HT(2A) receptors and (ii) a decrease of the transmission of the central inspiratory drive via 5-HT(1B) receptors. In Tg8 mice, the 5-HT(2A)-mediated facilitation was present but the 5-HT(1B)-mediated decrease was lacking. Therefore, the conservation of the 5-HT(2A) response vs. the loss of the 5-HT(1B) one suggest that the two types of receptors respond differently to 5-HT level changes.

Possible role of retrotrapezoid nucleus and parapyramidal area in the respiratory response to anoxia: an in vitro study in neonatal rat

The brainstem-spinal cord preparation from neonatal rat has been used in several reports to evaluate the central effect of low oxygen level on the respiratory network. We demonstrate that bilateral lesion of retrotrapezoid nucleus and parapyramidal area unmasks an early reinforcement of the respiratory output in response to anoxia. This suggests that neurons in both areas might trigger or relay a central depressive influence of hypoxia on the respiratory network.

Control of respiration in the isolated central nervous system of the neonatal opossum, Monodelphis domestica

Respiration represents an unusual motor activity with respect to its development. As newly born mammals enter the world, their limb movements are not coordinated; time and experience are required for effective performance to be achieved. Yet the rhythm of respiration is of necessity functionally perfected and unfailing at birth. Inspiratory and expiratory motor neurons are already able to fire at appropriate rates, under the command of rhythmically active neurons in the medulla. In this review, we discuss refinements of control present in the newborn opossum, particularly with respect to mechanisms that allow adaptation of respiration to changes in the level of activity or in the outside environment. Our own studies have been aimed at analyzing respiration at the earliest stages, and at establishing the way in which important variables influence inspiration and expiration. To this end, we have used the central nervous system (CNS) of a neonatal opossum, isolated in its entirety and maintained in culture. Although the opossum is unable to walk and highly immature at birth, its respiration is regular and unfailing. The isolated CNS survives, undergoes development, and maintains its neural activity and fine structure in vitro. Moreover, fictive respiration persists for over a day or longer at rates similar to those of the intact pup. The effects of altered pH, of increased temperature, and of drugs known to alter respiratory rhythm in intact animals can be measured directly, by electrical recordings made from medullary neurons or ventral roots. As in a slice, fluids of different composition can be applied focally, through micropipettes to the surface of the ventral medulla, or diffusely to the brainstem, With highly localized application of procaine hydrochloride (2%) to selected areas of the ventral medulla, the respiratory rhythm is reduced or abolished. As in adult mammals, both the rate and the amplitude of respiration simultaneously increase in response to lowered pH (6.5-.7.1) or to topical application of 1.0 microM carbachol. Conversely, as expected, the rate and amplitude decrease in response to increased pH (pH 7.5-7.7), or 100 microM scopolamine. Two characteristic features of the control of respiration in the neonatal opossum are evident from such tests. First, changes in rate are achieved by changes in the duration of the expiratory phase of respiration. This result suggests that the timing of the respiratory cycle in the neonatal opossum is controlled by an expiratory instead of an inspiratory "off-switch". Second, the rate and the amplitude of the respiratory excursions can be controlled independently, depending on the stimulus. For example, an increase in temperature increases the rate of fictive respiration without changing its amplitude, whereas noradrenaline decreases the rate while increasing the amplitude. Thus, changes of timing and amplitude need not go hand in hand. The opossum CNS offers a favorable preparation for the analysis of neural mechanisms that generate and modulate a motor rhythm, as the animal develops from embryonic to adult stages.

Abnormal phrenic motoneuron activity and morphology in neonatal monoamine oxidase A-deficient transgenic mice: possible role of a serotonin excess

In rodent neonates, the neurotransmitter serotonin (5-HT) modulates the activity of both the medullary respiratory rhythm generator and the cervical phrenic motoneurons. To determine whether 5-HT also contributes to the maturation of the respiratory network, experiments were conducted in vitro on the brainstem-spinal cord preparation of neonatal mice originating from the control strain (C3H) and the monoamine oxidase A-deficient strain, which has a brain perinatal 5-HT excess (Tg8). At birth, the Tg8 respiratory network is unable to generate a respiratory pattern as stable as that produced by the C3H network, and the modulation by 5-HT of the network activity present in C3H neonates is lacking in Tg8 neonates. In addition, the morphology of the phrenic motoneurons is altered in Tg8 neonates; the motoneuron dendritic tree loses the C3H bipolar aspect but exhibits an increased number of spines and varicosities. These abnormalities were prevented in Tg8 neonates by treating pregnant Tg8 dams with the 5-HT synthesis inhibitor p-chlorophenylalanine or a 5-HT(2A) receptor antagonist but were induced in wild-type neonates by treating C3H dams with a 5-HT(2A) receptor agonist. We conclude that 5-HT contributes, probably via 5-HT(2A) receptors, to the normal maturation of the respiratory network but alters it when present in excess. Disorders affecting 5-HT metabolism during gestation may therefore have deleterious effects on newborns.

Respiratory network function in the isolated brainstem-spinal cord of newborn rats

The in vitro brainstem-spinal cord preparation of newborn rats is an established model for the analysis of respiratory network functions. Respiratory activity is generated by interneurons, bilaterally distributed in the ventrolateral medulla. In particular non-NMDA type glutamate receptors constitute excitatory synaptic connectivity between respiratory neurons. Respiratory activity is modulated by a diversity of neuroactive substances such as serotonin, adenosine or norepinephrine. Cl(-)-mediated IPSPs provide a characteristic pattern of membrane potential fluctuations and elevation of the interstitial concentration of (endogenous) GABA or glycine leads to hyperpolarisation-related suppression of respiratory activity. Respiratory rhythm is not blocked upon inhibition of IPSPs with bicuculline, strychnine and saclofen. This indicates that GABA- and glycine-mediated mutual synaptic inhibition is not crucial for in vitro respiratory activity. The primary oscillatory activity is generated by neurons of a respiratory rhythm generator. In these cells, a set of intrinsic conductances such as P-type Ca2+ channels, persistent Na+ channels and G(i/o) protein-coupled K+ conductances mediates conditional bursting. The respiratory rhythm generator shapes the activity of an inspiratory pattern generator that provides the motor output recorded from cranial and spinal nerve rootlets in the preparation. Burst activity appears to be maintained by an excitatory drive due to tonic synaptic activity in concert with chemostimulation by H+. Evoked anoxia leads to a sustained decrease of respiratory frequency, related to K+ channel-mediated hyperpolarisation, whereas opiates or prostaglandins cause longlasting apnea due to a fall of cellular cAMP. The latter observations show that this in vitro model is also suited for analysis of clinically relevant disturbances of respiratory network function.

The fictively breathing tadpole brainstem preparation as a model for the development of respiratory pattern generation and central chemoreception

Spontaneous high-frequency, low-amplitude and low-frequency, high-amplitude efferent bursting patterns of cranial and spinal motor nerve activity in the in vitro brainstem preparation of the bullfrog tadpole Rana catesbeiana have been characterized as fictive gill and lung ventilation, respectively (Gdovin MJ, Torgerson CS, Remmers JE). Characterization of gill and lung ventilatory activity in cranial nerves in the spontaneously breathing tadpole Rana catesbeiana, FASEB J 1996;10(3):A642; Gdovin MJ, Torgerson CS, Remmers JE. Neurorespiratory pattern of gill and lung ventilation in the decerebrate spontaneously breathing tadpole, Respir Physiol 1998;113:135 146; Pack AI, Galante RJ, Walker RE, Kubin LK, Fishman AP. Comparative approach to neural control of respiration, In: Speck DF, Dekin MS, Revelette WR, Frazier DT, editors. Respiratory Control Central and Peripheral Mechanisms. Lexington: University of Kentucky Press, 1993:52-57). In addition, the ontogenetic dependence of central respiratory chemoreceptor stimulation on fictive gill and lung ventilation has been previously described (Torgerson CS, Gdovin MJ, Remmers JE. Fictive gill and lung ventilation in the pre- and post-metamorphic tadpole brainstem, J Neurophysiol 1998, in press). To investigate the neural substrates responsible for central respiratory rhythm generation of gill and lung ventilation in the developing tadpole, we recorded efferent activities of cranial nerve (CN) V, VII, and X and spinal nerve (SN) II during changes in superfusate PCO2 before and after multiple transection of the in vitro brainstem. The brainstem was transected between CN VIII and IX and the response to changes in PCO2 was recorded. A second transection was then made between the caudal margin of CN X and rostral to SN II. Preliminary data reveal that robust gill ventilation was recorded consistently only if the segment of brainstem included CN X, whereas the loci capable of eliciting fictive lung bursting patterns appeared to differ depending on developmental stage. These data demonstrate that the neural substrate required for fictive gill and lung ventilation exists in anatomically separate regions such that the gill central pattern generator (CPG) is located in the caudal medulla at the level of CN X throughout development, whereas the location of the lung CPG is located more rostrally at the level of CN VII in the post-metamorphic larva. Both in vivo and in vitro studies revealed two distinct neural bursting patterns associated with gill and lung ventilation. Sequential activation of CN V, VII, X were observed during gill ventilation of in vivo and fictive gill ventilation in vitro, whereas these nerve activities, along with SN II displayed more synchronous bursting patterns of activation during lung ventilation and fictive lung breaths.

Pulmonary stretch receptor discharges and vagal regulation of respiration differ between two mouse strains

1. Experiments were performed on adult pentobarbitone-anaesthetized mice of the OF1 and the C3H/HeJ (C3H) strains, to analyse the regulation of respiration by pulmonary stretch receptors (PSRs). 2. Although the mean respiratory period, inspiratory and expiratory durations, and tidal volume did not differ significantly between the two strains, the inspiratory onset was drastically inhibited in OF1 mice but only slightly inhibited in C3H mice in response to tracheal occlusion performed at the very end of inspiration. 3. Low current electrical stimulation of the vagus nerve induced inspiratory onset inhibition in both strains, suggesting that the weak inspiratory onset inhibition elicited by tracheal occlusion in C3H mice did not originate from a low sensitivity of the respiratory centres to PSRs. 4. During normal respiration, PSR firing rate increased with tidal volume, but reached significantly higher values in OF1 than C3H mice. During tracheal occlusion, PSR firing rate was significantly higher at the end of inspiration and during the first third of the occlusion period in OF1 than C3H mice. 5. The airway pressure resistance was significantly higher in OF1 than C3H mice. After abolishing the tracheo-bronchial muscle tone with atropine in OF1 mice, tracheal occlusions induced weak inspiratory onset inhibitions resembling the C3H mouse responses. 6. The possibility that differences in tracheo-bronchial tone between OF1 and C3H mice may lead to a greater PSR discharge and thus to a powerful inhibition on the OF1 medullary respiratory centres during tracheal occlusion is discussed.

Temporal patterns of trigeminal respiratory activity in rat brainstem-spinal cord in vitro

Respiratory activity in trigeminal (V) motoneurons was studied in rhythmically active en bloc brainstem-spinal cord preparations isolated from neonatal rats (P0-P3). In the majority of preparations (83%), the temporal pattern of V activity consisted of spontaneous inspiratory phasic discharge with onset delayed or coincident with onset of phrenic motoneuron discharge. Blockade of alpha-2 noradrenergic receptor activation shifted onset of V respiratory discharges earlier than phrenic discharges, while elevation of extracellular potassium concentration or blockade of GABAergic and glycinergic inhibitory synaptic transmission had little effect on temporary pattern of V respiratory discharges. We conclude V motoneurons in the in vitro preparation generate respiratory activity during inspiratory phase, and their temporal patterns are modulated by inhibitory noradrenergic synaptic transmission.

Central respiratory effects of substance P in neonatal mice: an in vitro study

Experiments were performed on neonatal mice to know whether substance P (SP) modified the rhythm and the amplitude of the phrenic bursts generated in vitro in brainstem-cervical cord preparations. In OF1 and C3H neonatal preparations, SP or the tachykinin NK1 receptor agonist [Sar9,Met(O2)11] substance P both increased significantly phrenic burst amplitude (10(-7) M) but had no significant effect on respiratory rhythm unless used at concentrations 10 times larger. In neonates from the monoamine oxidase-A deficient transgenic Tg8 line, SP increased phrenic burst amplitude but had no effect on the respiratory rhythm at the tested concentrations. The role of SP in regulating neonatal respiratory activity is discussed on the basis of rat and mouse results.

Characterization of pontine neurons which respond to hypoxia in fetal sheep

Hypoxia causes apnea and postural muscle hypotonia in fetal sheep, a response thought to arise by descending inhibition from a group of lateral pontine neurons that express FOS protein after hypoxia. To determine the neurochemical phenotype, and whether these neurons project to the cervical spinal cord, the retrograde tracer CTB-gold was injected into the C5-C8 ventral horn of four fetal sheep at 110 days gestation. Then, at 135 days each fetus was made hypoxic for 2 h by allowing the mother to breathe 7-8% O2. Immunocytochemistry showed that FOS-positive neurons in the subcoeruleus and Kolliker-Fuse regions of the pons were catecholaminergic, but not cholinergic or GABAergic, and a proportion of them contained CTB-gold particles, indicating direct connection with the cervical spinal cord. We suggest that these pontine neurons inhibit respiratory and postural muscle activities during hypoxia in fetal sheep.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Pontine and medullary control of the respiratory activity in the trigeminal and facial nerves of the newborn mouse: an in vitro study

In vitro, the respiratory activity in rodents is characterized by: (i) the rapidly peaking, slowly decrementing pattern of spontaneous and rhythmic active phases recorded from the motor rootlets, and (ii) the specific location of their rhythmic generator in the ventrolateral medulla. The aim of the present study was to assess whether the trigeminal and facial motor rootlets still exhibit respiratory activity in the absence of peripheral and higher cerebral structures, and to compare the onset of their active phases with that of other respiratory rootlets, using the in vitro isolated brainstem--spinal cord preparation of the newborn mouse and rat. Spontaneous rhythmic activity was recorded from the trigeminal and facial rootlets. It was regular and synchronized bilaterally and ipsilaterally with the hypoglossal or cervical C1-C6 rootlets. Brainstem transection experiments demonstrated that for both the trigeminal and facial rootlets, the spontaneous rhythmic activity originates from the medulla, in a region consistent with the pre-Bötzinger complex and the rostral ventrolateral medulla. The pattern of the respiratory motor activity recorded from the trigeminal and facial rootlets was identical to the pattern recorded from the hypoglossal and cervical C1-C6 rootlets with rapidly peaking, slowly decrementing characteristics. The duration of the ascending part and the total duration of their active phases were similar. The onset of the active phases of the phrenic rootlets was delayed compared with that of the trigeminal, facial and hypoglossal rootlets. However, no difference in the onsets of the active phases of the cranial rootlets could be observed. Removal of the rostral pons suppressed the delay in onset of the active phases of the phrenic rootlets. Our findings show that: (i) rhythmic activities of the trigeminal and facial rootlets are preserved in absence of control by peripheral or high cerebral structures; (ii) the pattern and the location of the rhythmic generator for these activities are of the respiratory type; and (iii) the rostral pons is responsible for a delay in the onset of the active phases of the phrenic rootlets compared with that of the trigeminal, facial and hypoglossal rootlets.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

Blockade of synaptic inhibition within the pre-Bötzinger complex in the cat suppresses respiratory rhythm generation in vivo

1. The role of synaptic inhibition in respiratory rhythm generation was analysed by microinjections of GABAA and glycine receptor antagonists into the bilateral pre-Botzinger complex (PBC) of anaesthetized cats. Central respiratory activity was monitored by phrenic nerve recordings. 2. Bilateral injections of bicuculline (50 or 100 microM) irreversibly slowed respiratory frequency and induced apneustic patterns. 3. Bilateral injections of strychnine (50 or 100 microM) greatly reduced phrenic burst amplitudes leading to increased burst frequency or irreversibly blocked rhythmic phrenic discharges. After unilateral tetrodotoxin (TTX) blockade in the PBC, strychnine injection into the contralateral PBC blocked rhythmic phrenic discharges. 4. Bilateral blockade of both GABAergic and glycinergic inhibition abolished rhythmic burst discharges and only tonic phrenic activity remained. Such tonic activity was blocked only by TTX (1 microM). 5. Potentiation of synaptic inhibition by the serotonin 1A receptor agonist 8-hydroxydipropylaminotetralin (8-OH-DPAT; 50 microM) restored rhythmic activity only when given shortly after strychnine and bicuculline applications. It was, however, ineffective after blockade of synaptic inhibition was complete. 6. The study demonstrates the significance of synaptic inhibition in the process of respiratory generation in the adult cat in vivo.