Modification by chemical stimuli of temporal difference in the onset of inspiratory activity between vagal (superior laryngeal) or hypoglossal and phrenic nerves of the rat

Temporal differences in the onset of inspiratory activities between the efferent vagal (superior laryngeal, Xsl) or hypoglossal (XII) and phrenic (Phr) nerves were measured at various levels of chemical stimuli in the halothane-anesthetized, vagotomized, and artificially ventilated rat. The onset of Xsl (XII) inspiratory activities always preceded the abrupt start of the Phr discharge. Hyperoxic hypocapnia due to hyperventilation delayed the start of inspiratory activity (reduction in respiratory frequency) and shortened the difference in onset time between the cranial (Xsl, XII) and Phr nerve discharges (Td). During respiratory stimulation due to asphyxia (progressive hypoxia and hypercapnia), the start of Xsl (XII) inspiratory activity became progressively earlier than that of Phr discharge, which extremely prolonged the Td. Severe asphyxia, however, retarded the start of inspiratory activities with accompanying long Td and slow respiratory frequency. The early but gradually augmenting inspiratory activity of the Xsl (XII) nerve was always followed by large bursts synchronized with Phr discharges during altered chemical stimuli. The termination of inspiratory activity, which occurred simultaneously in the three respiratory nerves, was not significantly affected by changes in chemical stimuli except for extreme hypocapnia. The results indicate that changes in chemical stimuli not only alter the start of inspiratory activity but also influence the transition from the initial slow onset to the final synchronized inspiratory activity in the Xsl (XII) nerve. The apparent dissociation of the onset time between the Xsl (XII) and Phr nerve discharges shows that the temporal aspect of the brain stem process(es) for starting inspiratory activities may not be determined from the trajectory of Phr discharges only.     

Roles of vagal afferents on discharge patterns and CO2-responsiveness of efferent superior laryngeal, hypoglossal, and phrenic respiratory activities in anesthetized rats

Discharge patterns and CO2-responsiveness of the efferent respiratory activities in the superior laryngeal (Xsl), hypoglossal (XII), and phrenic (Phr) nerves were compared between vagi-intact and -denervated rats. Bilateral cervical vagotomy decreased the respiratory frequency (f), minute Phr activity (peak integrated Phr activity X f) and consequently elevated end-tidal PCO2 (PETCO2). Augmentation of the peak inspiratory activity following vagotomy was much larger in the Xsl and XII nerves than that in the Phr nerve. After vagotomy, the time delay from the onset of inspiratory activities in the Xsl or XII nerve to the onset of the Phr bursts was greatly prolonged. While in the vagi-intact rats the peak inspiratory activities of these cranial nerves were not increased in response to elevated PETCO2, the activities, in particular of the XII nerve, were augmented by high PETCO2 in the absence of vagal afferents. These results suggested that the vagal afferents inhibit not only phasic Phr discharges but also inspiratory output neurons in the Xsl or XII motor nuclei being activated in normo- and hypercapnia and that they facilitate temporally the onset and progress of inspiratory activities in various groups of respiratory output neurons.     

Effects of hypocapnia on respiratory timing and inspiratory activities of the superior laryngeal, hypoglossal, and phrenic nerves in the vagotomized rat

Effects of acute hypocapnia on respiratory timing (inspiratory and expiratory times (TI, TE) ) and on inspiratory activities of the efferent superior laryngeal (Xs1), hypoglossal (XII), and phrenic (Phr) nerves were studied in artificially ventilated vagotomized, and anesthetized rats. Hyperventilation induced a decrease in respiratory frequency exclusively due to prolongation of TE and resulted in expiratory apnea. Inspiratory activities of three nerves decreased with reduction in CO2 concentration of end-tidal gas (FETCO2), and disappeared simultaneously at a threshold FETCO2 for apnea. The decrease in the peak inspiratory activity by hypocapnia was larger in the XII than in the Phr or Xs1 nerve (XII greater than Phr greater than Xs1). The results suggest that the CO2 stimulus (mainly via a central chemosensor) plays an important role in the process of terminating expiration or of expiratory-inspiratory phase switching and that the responses of the XII or Xs1 motoneurons to variation in CO2 stimulus differ from that of the Phr motoneurons (or of the Phr driving medullary neurons). A possible functional significance of these observations is discussed.     

Respiratory neural activity responses to chemical stimuli in newborn rats: reversible transition from normal to 'secondary' rhythm during asphyxia and its implication for 'respiratory like' activity of isolated medullary preparation

To clarify a possible origin of 'respiratory like' rhythmic activities observed in in vitro brainstem preparation, the phrenic (Phr) and cranial nerve (XII or IX) inspiratory activities were analyzed in halothane-anesthetized, vagotomized and artificially ventilated newborn (2--6 days after birth) and young adult rats (30--50 days) during altered chemical stimuli and prolonged asphyxia at 25 degrees C. The newborn rat showed regular rhythmic inspiratory discharges of short duration, and their responses to CO(2) and hypoxia did not differ from those seen in adult rats. In the newborn rat the Phr and cranial nerve inspiratory discharges increased first, then respiratory frequency decreased and finally ceased completely for approximately 1--2 min during asphyxia. Thereafter, 'secondary' rhythmic inspiratory activity emerged at a slower rate with decremental inspiratory discharge profile, which persisted for a period more than 40 min of asphyxia. A normal respiratory activity recovered after resumption of artificial ventilation. Though young adult rats exhibited similar sequential changes in respiratory activity during asphyxia, the 'secondary' rhythmic activity persisted for a period of several min only. The pattern of 'secondary' respiratory activity corresponded well with that of rhythmic activities seen in the isolated medullary block preparation of newborn rat. 'Respiratory like' activity found in isolated medullary preparations of newborn animals may arise from a mechanism that generates 'secondary' (or so called 'gasping' type) rhythmic inspiratory activity during prolonged asphyxia in in vivo preparations.     

Differential sensitivity to hypoxic inhibition of respiratory processes in the anesthetized rat

To estimate the sensitivity to hypoxic inhibition of various regulatory processes for respiration, changes in breathing pattern during hypoxic ventilatory depression (HVD) were analyzed in the halothane-anesthetized spontaneously breathing rat using a "progressive isocapnic hypoxia test." In the carotid sinus nerve (CSN) intact rats, ventilatory augmentation was followed by depression due to reduction in respiratory frequency (f) at end-tidal PO2 (PETO2) levels below 50-60 mmHg despite increased afferent activities from the carotid chemoreceptors. After CSN section, ventilation was progressively depressed at PETO2 lower than normoxic level with simultaneous decreases of f and tidal volume. An increase in CO2 stimulus or the prevention of arterial hypotension during hypoxia by infusing a vasoconstrictor agent (phenylephrine) inhibited the occurrence of ventilatory depression in both the CSN intact and denervated animals. In all cases studied, the reduction in f resulted mainly from the prolongation of expiratory time (TE). The results suggest that in the anesthetized rat the effect of respiratory stimulation from carotid chemoreceptor afferents becomes inadequate to offset the prolongation of TE due to the central hypoxia at lower PETO2, and that the neural process for regulating TE is the major site of deterioration during central hypoxic inhibition. Roles of CO2 stimulus and systemic circulatory conditions in the generation of HVD were also discussed.     

Effects of a midsagittal lesion of the rabbit medulla. I. Respiratory motoneuronal outputs

The effects of a midsagittal lesion of the medulla on integrated efferent activities of the phrenic (Phr) and vagus nerves (Eff. Vag.) and on external intercostal EMG (EI EMG) activity were studied in 23 rabbits, anaesthetized with chloralose and urethane. To support depressed respiratory movements, controlled ventilation was applied after the lesion. No muscle relaxant was administered. Bilateral recordings of respiratory activities showed that the lesion produced a desynchronization of inspiratory volleys recorded from the left and right Phr and EI EMG. Following the lesion, the phasic activities of EI EMG and Eff. Vag. were reduced more than that of Phr. Both EI EMG and Phr activities were more sensitive to changes in lung volume than in intact animals. We suggest that the lesion raises the motoneuronal threshold for inspiratory firing differentially for Phr, EI EMG and Eff. Vag. activities. Midsagittal lesion of the medulla enhances spinal reflexes.     

Differences in respiratory neural activities between vagal (superior laryngeal), hypoglossal, and phrenic nerves in the anesthetized rat

Respiratory neural activities were recorded from the efferent vagal (superior laryngeal Xsl)), hypoglossal (XII), and phrenic nerves in spontaneously breathing rats anesthetized with halothane. The onset of inspiratory discharges in the cranial nerves was slightly earlier (5-60 msec) but more gradual than that of phrenic bursts. When the anesthesia was deepened by increasing the concentration of halothane or by injection of pentobarbital, inspiratory discharges in the three nerves were well maintained although there was a progressive decrease in respiratory frequency and a prolongation of the delay from the start of Xsl or XII inspiratory activities to the onset of phrenic bursts. Inhalation of CO2 increased respiratory frequency and augmented the peak phrenic activity whereas the peak inspiratory activities in the cranial nerves remained unchanged under elevated end-tidal PCO2. Both in deeper anesthesia and in hypercapnia, changes in respiratory frequency were due mainly to alterations in the duration of expiration. The results indicated that the rat, 1) overall inspiratory activities in various nerves innervating the diaphragm and accessory respiratory muscles in the upper airway are quite resistant to depressing actions of halothane or halothane-pentobarbital anesthesia, although the mechanism controlling respiratory frequency is strongly affected, and 2) excitatory signals elicited by an elevated PCO2 via respiratory chemosensors preferentially augment inspiratory activities in the phrenic nerve. Factors influencing the temporal difference in the onset of inspiratory activities in the cranial and phrenic nerves are discussed.     

Role in the inspiratory off-switch of vagal inputs to rostral pontine inspiratory-modulated neurons

Neurons of the pontine respiratory group (PRG) in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus are believed to play an important role in promoting the inspiratory (I) off-switch (IOS). In decerebrate gallamine-paralyzed cats ventilated with a cycle-triggered pump system (lung inflation during the neural I phase), we studied the effects of vagal (V) afferent inputs on firing of I-modulated neurons (the most numerous population in PRG) and on I duration. The predominant V effect on unit activity was inhibitory, as shown by two types of test: (a) withholding of inflation during an I phase, which produced increase of unit firing and of its respiratory modulation (58/66 neurons in 14 cats), indicating disinhibition due to removal of phasic V input; (b) delivery of afferent V stimulus trains during a no-inflation I phase, which produced decrease of unit firing and of its respiratory modulation (20 neurons). In addition, application of V input during the neural expiratory (E) phase, which lengthened E phase duration, produced no effect on the neurons' firing, suggesting that the inhibition during I was presynaptic in origin. The results may be interpreted by the hypothesis that the medullary late-I presumptive IOS neurons receive excitatory inputs from the PRG I-modulated neurons as well as from V afferents.. With relatively strong V input, this pontine excitatory output is reduced by inhibition, whereas with relatively weak V input that excitatory output is increased due to reduction of inhibition. Thus the pontine and the V influences on the IOS can operate in a complementary manner dependent on state.     

Synaptic effects of intercostal tendon organs on membrane potentials of medullary respiratory neurons

1. Stimulation of intercostal muscle tendon organs or their afferent fibers reduces medullary inspiratory neuron activity, decreases motor output to inspiratory muscles, and increases the activity of expiratory laryngeal motoneurons. The present study examines the synaptic mechanisms underlying these changes to obtain information about medullary neurons that participate in the afferent limb of this reflex pathway. 2. Membrane potentials of medullary respiratory neurons were recorded in decerebrate paralyzed cats. Postsynaptic potentials (PSPs) elicited in these neurons by intercostal nerve stimulation (INS) were compared before and after intracellular iontophoresis of chloride ions. After chloride injection, the normal hyperpolarization caused by inhibitory (I) PSPs is "reversed" to depolarization. 3. In inspiratory neurons, reversal of IPSPs by chloride injection also reversed hyperpolarization produced by INS when applied during any portion of the respiratory cycle. This observation suggests that increased chloride conductance of the postsynaptic membrane mediated the inhibition. Further, it is very likely that the last-order interneuron in the afferent pathway must be excited by INS and alter inspiratory neuron activity via an inhibitory synapse. The linear relationship between the amplitude of the INS induced PSP and membrane potential of inspiratory neurons provided evidence that neurons in the afferent pathway are not respiratory modulated. 4. The membranes of expiratory vagal motoneurons and post-inspiratory neurons were depolarized by INS during all portions of the respiratory cycle before IPSP reversal. Reversal of IPSPs affected neither this depolarization of expiratory vagal motoneurons during stage I and II expiration nor that of post-inspiratory neurons during stage I expiration. Thus this depolarization probably resulted from synaptic excitation.(ABSTRACT TRUNCATED AT 250 WORDS)     

Dependence of phrenic motoneurone output on the oscillatory component of arterial blood gas composition.

1. The hypothesis that respiratory oscillations of arterial blood gas composition influence ventilation has been examined. 2. Phrenic motoneurone output recorded in the C5 root of the left phrenic nerve and the respiratory oscillations of arterial pH in the right common carotid artery were measured in vagotomized anaesthetized dogs which had been paralysed and artificially ventilated. 3. The effect of a change in tidal volume for one or two breaths on phrenic motoneurone output was measured with the inspiratory pump set at a constant frequency similar to, and in phase with, the animal's own respiratory frequency. A reduction of tidal volume to zero or an increase by 30% led to a corresponding change of mean carotid artery pH level. The changes of carotid artery pH resulted in a change of phrenic motoneurone output, predominantly of expiratory time (Te) but to a lesser extent of inspiratory time (T1) and also peak amplitude of 'integrated' phrenic motoneurone output (Phr). Denervation of the carotid bifurcation blocked this response. 4. The onset of movement of the inspiratory pump was triggered by the onset of phrenic motoneurone output. When a time delay was interposed between them, the phase relationship between respiratory oscillations of arterial pH and phrenic motoneurone output altered. The dominant effect was to alter Te; smaller and less consistent changes of Phr and T1 were observed. 5. When the inspiratory pump was maintained at a constant frequency but independent of and slightly different from the animal's own respiratory frequency (as judged by phrenic motoneurone output), the phase relationship between phrenic motoneurone output and the respiratory oscillations of pH changed breath by breath over a sequence of 100-200 breaths, without change of the mean level of arterial blood gas composition. Te varied by up to 30% about its mean value depending on the phase relationship. Ti and Phr were also dependent on the phase relationship but varied to a lesser extent. The changes were comparable to the results obtained in paragraph 4. 6. It was concluded that phrenic motoneurone output is dependent in part on its relationship to the respiratory oscillations of arterial blood gas composition. 7. Information concerning a transient ventilatory disturbance is stored in the arterial blood in the form of an altered pattern of the respiratory oscillations of blood gas composition; this in turn can change breathing by an effect on the carotid bodies.     

Developmental changes in the spatio-temporal pattern of respiratory neuron activity in the medulla of late fetal rat

We investigated how the spatio-temporal pattern of respiratory neuron network activity in the ventral medulla changes during the late fetal period of rat. Brainstem-spinal cord preparations isolated from rat fetuses on embryonic days 17–21 (E17–E21) were stained with a voltage-sensitive dye for optical image analysis of neuronal activity of the ventral medulla. The spatio-temporal pattern of respiratory neuron activity in the preparation from E20 to E21 was basically identical to that of neonatal rat; pre-inspiratory activity in a limited region of the rostral ventrolateral medulla, the para-facial region, preceded by several hundred milliseconds the onset of inspiratory activity in the more caudal ventrolateral medulla, the pre-Bötzinger complex level. In contrast, in E17–E18 specimens, pre-inspiratory activity could not be detected in the rostral medulla at the level of the facial nucleus. Neuronal activity appeared to begin at the pre-Bötzinger complex level shortly before onset of the inspiratory burst. Strong activity then developed in the facial nucleus and peaked in the post-inspiratory phase. The transition of these patterns of respiratory activity occurred at E19. We conclude that the changes in the spatio-temporal pattern of neuronal activity reflect developmental changes in the cellular elements underlying rhythm generation in the fetal respiratory neuron network. We suggest that the pre-inspiratory neuron network of the para-facial region in the rostral ventrolateral medulla functions as the rhythm generator after E19/20.     

Role of vagal and spinal sensory pathways in diaphragmatic response to resistive loading

The effects of severe inspiratory (I) or expiratory (E) resistive loads on diaphragmatic activity were studied in two groups of cats anaesthetized with sodium pentobarbital or ethylcarbamate-chloralose. In intact cats, I or E loading never changed the amplitude of integrated diaphragmatic electric myogram (EMG) measured at 1.0 s (Edi 1.0); only I loading, prolonged the duration of diaphragmatic activity (Tdi). After selective procaine block of non-volume related vagal sensory inputs, I or E loading markedly increased Edi 1.0 and changes in Tdi due to I loading persisted. After bivagotomy, which also suppressed volume related vagal feed back, Edi 1.0 increased during I or E loading but change in Tdi disappeared. Initial spinal section at C8 level only reduced changes in Tdi with inspiratory loading. Bivagotomy plus spinal section abolished all load induced changes in diaphragmatic activity. These results suggest that all vagal information from the lungs participate in the mechanism of load compensation but that spinal sensory pathways play a minor role in this response in anaesthetized cats.     

Respiratory muscle injury, fatigue and serum skeletal troponin I in rat

To evaluate injury to respiratory muscles of rats breathing against an inspiratory resistive load, we measured the release into blood of a myofilament protein, skeletal troponin I (sTnI), and related this release to the time course of changes in arterial blood gases, respiratory drive (phrenic activity), and pressure generation. After ∼1.5 h of loading, hypercapnic ventilatory failure occurred, coincident with a decrease in the ratio of transdiaphragmatic pressure to integrated phrenic activity ( P _di /∫Phr) during sighs. This was followed at ∼1.9 h by a decrease in the P _di /∫Phr ratio during normal loaded breaths (diaphragmatic fatigue). Loading was terminated at pump failure (a decline of P _di to half of steady-state loaded values), ∼2.4 h after load onset. During 30 s occlusions post loading, rats generated pressure profiles similar to those during occlusions before loading, with comparable blood gases, but at a higher neural drive. In a second series of rats, we tested for sTnI release using Western blot–direct serum analysis of blood samples taken before and during loading to pump failure. We detected only the fast isoform of sTnI, release beginning midway through loading. Differential detection with various monoclonal antibodies indicated the presence of modified forms of fast sTnI. The release of fast sTnI is consistent with load-induced injury of fast glycolytic fibres of inspiratory muscles, probably the diaphragm. Characterization of released fast sTnI may provide insights into the molecular basis of respiratory muscle dysfunction; fast sTnI may also prove useful as a marker of impending respiratory muscle fatigue.     

Evidence for respiratory interneurones in the C3-C5 cervical spinal cord in the decorticate rabbit

Summary In mammals, it has long been considered that the bulbo-spinal inspiratory drive provided a direct (monosynaptic) excitation of phrenic motoneurones (Phr Mns). Although such connections have been demonstrated, recent indirect data strongly suggested that the main inspiratory drive is polysynaptic. We tried to directly demonstrate relay respiratory interneurones at the C3–C6 spinal cord level where the Phr Mn pool is located. The experiments were performed on decorticate, unanaesthetized, bilaterally vagotomized and curarized rabbits and the firing pattern of spinal interneurones was compared to the phrenic bursting. Dorsally and dorso-medially to the Phr Mn pool, different classes of inspiratory (54%) and expiratory (46%) interneurones could be identified in the ventral horn. Three classes of inspiratory interneurones were characterized and classified as I all (26%), I late (43%) and I tonic (29%) according to the terminology used by other authors for the bulbospinal inspiratory neurones which drive the spinal respiratory motoneurones. The expiratory interneurones could also be divided into 3 classes: E all (48%), E late (10%) and E tonic (41%). This first direct evidence of inspiratory interneurones at the C3–C6 spinal cord levels can account for the major polysynaptic excitation of the Phr Mns while the presence of numerous expiratory interneurones at this level suggests a polysynaptic bulbo-spinal inhibitory action onto the Phr Mns. These classes of inspiratory and expiratory interneurones did not always coincide with the bulbo-spinal classes of neurones described elsewhere. Unless these discrepancies are due to the different experimental conditions, they may indicate that some of these interneurones are not just relay target cells and they suggest that they might behave as integrative operators between the medullary drive and the Phr Mns.     

Functional associations among simultaneously monitored lateral medullary respiratory neurons in the cat. II. Evidence for inhibitory actions of expiratory neurons

Arrays of extracellular electrodes were used to monitor simultaneously several (2-8) respiratory neurons in the lateral medulla of anesthetized, paralyzed, bilaterally vagotomized, artificially ventilated cats. Efferent phrenic nerve activity was also recorded. The average discharge rate as a function of time in the respiratory cycle was determined for each neuron. Most cells were tested for spinal or vagal axonal projections using antidromic stimulation methods. Cross-correlational methods were used to analyze spike trains of 480 cell pairs. Each pair included at least one neuron most active during the expiratory phase. All simultaneously recorded neurons were located in the same side of the brain stem. Twenty-six percent (33/129) of the expiratory (E) neuron pairs exhibited short time scale correlations indicative of paucisynaptic interactions or shared inputs, whereas 8% (27/351) of the pairs consisting of an E neuron and an inspiratory (I) cell were similarly correlated. Evidence for several inhibitory actions of E neurons was found: 1) inhibition of I neurons by E neurons with both decrementing (DEC) and augmenting (AUG) firing patterns; 2) inhibition of E-DEC and E-AUG neurons by E-DEC cells; 3) inhibition of E-DEC and E-AUG neurons by E-AUG neurons; and 4) inhibition of E-DEC neurons by tonic I-E phase-spanning cells. Because several cells were recorded simultaneously, direct evidence for concurrent parallel and serial inhibitory processes was also obtained. The results suggest and support several hypotheses for mechanisms that may help to generate and control the pattern and coordination of respiratory motoneuron activities.     

Blunted response to low oxygen of rat respiratory network after perinatal ethanol exposure: involvement of inhibitory control

Acute ethanol depresses respiration, but little is known about chronic ethanol exposure during gestation and breathing, while the deleterious effects of ethanol on CNS development have been clearly described. In a recent study we demonstrated that pre- and postnatal ethanol exposure induced low minute ventilation in juvenile rats. The present study analysed in juvenile rats the respiratory response to hypoxia in vivo by plethysmography and the phrenic (Phr) nerve response to ischaemia in situ. Glycinergic neurotransmission was assessed in situ with strychnine application and [(3)H]strychnine binding experiments performed in the medulla. After chronic ethanol exposure, hyperventilation during hypoxia was blunted in vivo. In situ Phr nerve response to ischaemia was also impaired, while gasping activity occurred earlier and recovery was delayed. Strychnine applications in situ (0.05-0.5 microM) demonstrated a higher sensitivity of expiratory duration in ethanol-exposed animals compared to control animals. Moreover, [(3)H]strychnine binding density was increased after ethanol and was associated with higher affinity. Furthermore, 0.2 microM strychnine in ethanol-exposed animals restored the low basal Phr nerve frequency, but also the Phr nerve response to ischaemia and the time to recovery, while gasping activity appeared even earlier with a higher frequency. Polycythaemia was present after ethanol exposure whereas lung and heart weights were not altered. We conclude that chronic ethanol exposure during rat brain development (i) induced polycythaemia to compensate for low minute ventilation at rest; (ii) impaired the respiratory network adaptive response to low oxygen because of an increase in central glycinergic tonic inhibitions, and (iii) did not affect gasping mechanisms. We suggest that ethanol exposure during early life can be a risk factor for the newborn respiratory adaptive mechanisms to a low oxygen environment.     

Glycinergic inhibition is essential for co-ordinating cranial and spinal respiratory motor outputs in the neonatal rat

Eupnoeic breathing in mammals is dependent on the co-ordinated activity of cranial and spinal motor outputs to both ventilate the lungs and adjust respiratory airflow, which they do by regulating upper-airway resistance. We investigated the role of central glycinergic inhibition in the co-ordination of cranial and spinal respiratory motor outflows. We developed an arterially perfused neonatal rat preparation (postnatal age 0–4 days) to assess the effects of blocking glycine receptors with systemically administered strychnine (0.5–1 μM). We recorded respiratory neurones located within the ventrolateral medulla, inspiratory phrenic nerve activity (PNA) and recurrent laryngeal nerve activity (RLNA), as well as dynamic changes in laryngeal resistance. Central recordings of postinspiratory neurones revealed an earlier onset in firing relative to the onset of inspiratory PNA after exposure to strychnine (260 ± 38.9 vs. 129 ± 26.8 ms). After glycine receptor blockade, postinspiratory neurones discharged during the inspiratory phase. Strychnine also evoked a decrease in PNA frequency (from 38.6 ± 4.7 to 30.7 ± 2.8 bursts min⁻¹), but amplitude was unaffected. In control conditions, RLNA comprised inspiratory and postinspiratory discharges; the amplitude of the latter exceeded that of the former. However, after administration of strychnine, the amplitude of inspiratory-related discharge increased (+65.2 ± 15.2%) and exceeded postinspiratory activity. Functionally this change in RLNA caused a paradoxical, inspiratory-related glottal constriction during PNA. We conclude that during the first days of life in the rat, glycine receptors are essential for the formation of the eupnoeic-like breathing pattern as defined by the co-ordinated activity of cranial and spinal motor inspiratory and postinspiratory activities.     

Studies on the synaptic interconnection between bulbar respiratory neurones of cats

In cats anaesthetized with pentobarbital, medullary respiratory neurones of both dorsal and ventral populations were recorded intracellularly with 1 mol·l^–1 KCl-electrodes. The neurones were classified according to the projection of their axons to the spinal cord (bulbospinal neurones) or to the vagal nerves (vagal neurones). Those neurones which could not be activated antidromically (NAA-neurones) by either procedure were subdivided into (inspiratory) R-neurones, which were monosynaptically excited by lung stretch receptor afferents, and into inspiratory and expiratory NAA-neurones, which did not receive a direct synaptic input, from these afferents.All types of neurone investigated revealed postsynaptic activity during both inspiration and expiration. The periods when synaptic activity was minimal were the periods of transition between respiratory phases.The input resistance of most respiratory neurones varied in parallel with the respiratory cycle. A drastic fall of the input resistance during expiration was observed in R-neurones and in some inspiratory vagal neurones. This was not seen in inspiratory bulbospinal neurones.In stable intracellular recordings, periodic postsynaptic inhibition was demonstrated in 52 of 53 respiratory neurones by IPSP reversal following chloride injection. Maximal membrane potential then was generally reached during one of the periods of respiratory phase transition. Reasons for the failure of others to demonstrate these IPSPs are presented and discrepancies between other findings and these are discussed. It is concluded that reciprocal inhibition between bulbar respiratory neurones does exist and is a general phenomenon.It is argued that reciprocal inhibition is the fundamental mechanism underlying respiratory gating of afferent inputs.The probable existence of recurrent inhibition is inferred from the changes in the pattern of membrane depolarization during the active period of neurones.Supported by the Deutsche Forschungsgemeinschaft     

Inspiration-Promoting Vagal Reflex in Anaesthetized Rabbits after Rostral Dorsolateral Pons Lesions

The centrally generated respiratory rhythm is under strong modulation by peripheral information, such as that from the slowly adapting pulmonary stretch receptors (SA-PSRs) conveyed via the vagus nerve. We have already demonstrated that vagal afferent stimulation at a low frequency (5–40 Hz), or holding the lung volume at the end-expiratory level (no-inflation test) prevents spontaneous termination of the inspiratory (I) phase or initiates I activity in anaesthetized rabbits in which the NMDA receptors (NMDA-Rs) are pharmacologically blocked. Here we show that this I-promoting vagal reflex also becomes manifest in animals where the pontine respiratory groups are ablated. Following lesions of the rostral dorsolateral pons, including the nucleus parabrachialis medialis and Kölliker-Fuse nucleus, with radio-frequency current or local injection of kainic acid, low-frequency stimulation of the vagus nerve and the no-inflation test significantly prolonged the I phase in a manner highly similar to that observed in rabbits with NMDA-R block. Brief stimuli at low frequency during the mid-expiratory (E) phase evoked I discharge with a latency significantly smaller and less variable than that before the lesions. It is concluded that low-frequency input from the SA-PSR suppresses I-to-E phase transition and promotes central I activity when the medullary respiratory network is released from pontine influence, which involves NMDA-R-mediated signalling.     

Hypoxia recruits a respiratory-related excitatory pathway to brainstem premotor cardiac vagal neurons in animals exposed to prenatal nicotine

The most ubiquitous form of arrhythmia is respiratory sinus arrhythmia in which the heart beat slows during expiration and heart rate increases during inspiration. Whereas respiratory sinus arrhythmia benefits pulmonary gas exchange respiratory dysfunction presents a major challenge to the cardiorespiratory system. Hypoxia evokes a pronounced bradycardia mediated by increases in parasympathetic cardiac activity. It has been hypothesized that the fatal events in sudden infant death syndrome (SIDS) are exaggerated cardiorespiratory responses to hypoxia. This study tests whether premotor cardiac vagal neurons receive rhythmic respiratory-related excitatory synaptic inputs during normoxia and hypoxia, and if animals exposed to nicotine in the prenatal period have exaggerated responses to hypoxia. Premotor cardiac vagal neurons in the nucleus ambiguus were identified in rats by the presence of a fluorescent tracer in medullary slices that generate rhythmic inspiratory-related motor discharge. Respiratory activity was recorded from the hypoglossal nerve and excitatory synaptic events in cardiac vagal neurons were isolated using patch clamp techniques. Adult female rats were implanted with osmotic minipumps that delivered nicotine at a level approximately equivalent to those that occur in moderate to heavy smokers. During normal eupneic respiration, as well as during hypoxia, premotor cardiac vagal neurons from control animals did not receive any rhythmic respiratory-related excitatory inputs. However in animals exposed to nicotine throughout the prenatal period respiratory bursts during hypoxia dramatically increased the frequency of excitatory synaptic events in cardiac vagal neurons. In summary, in animals exposed to nicotine throughout the prenatal period, but not in unexposed animals, respiratory bursts that occur during hypoxia dramatically increase the frequency of excitatory synaptic events in cardiac vagal neurons. This study establishes a likely neurochemical mechanism for the heart rate responses to hypoxia and a link between prenatal nicotine exposure and exaggerated bradycardia responses during hypoxia that may contribute to sudden infant death syndrome.