Interaction of pulmonary afferents and pneumotaxic center in control of respiratory pattern in cats.

The interaction between the pulmonary afferents (PA) and the pneumotaxic center (PC) in control of respiratory pattern was studied in lightly anesthetized paralyzed cats before and after bivagotomy or lesions of the PC using inflations controlled by the onset or cessation of phrenic nerve discharge, i.e., cycle-triggered inflations. This interaction was also studied using electrical stimulation of the central stumps of cut vagi. Introduction of a delay between inspiratory onset and the commencement of an inflation at constant flow and duration resulted in increases of the durations of inspiration (T1) and expiration (TE) and amplitude of the integrated phrenic nerve discharge (A). The lung volume at inspiratory cutoff, i.e., the volume threshold, increased markedly as T1 increased. There were linear relationships between T1 and TE and between T1 and A. At constant alveolar CO2 and tidal volume, the quantitative effects of delay were dependent on the rate of inflation; i.e., when the flow increased, the volume threshold for a given T1 decreased. Bilateral vagotomy abolished the effects of delay and flow. PC lesions, which resulted in apneusis when the cycle-triggered inflations were stopped, produced the following changes compared to the delay effects seen in intact cats: a) the volume threshold for zero delay doubled and its rate of decrease with increased T1 was significantly smaller, and b) the change in TE for a given change in T1 was reduced markedly. Introduction of a delay between inspiratory onset and the start of electrical stimulation of the afferent vagi resulted in effects similar to those seen for delays in cycle-triggered inflations. The T1-TE relationship remained linear when the stimulus trains ended with inspiratory cessation. These results suggest that: a) the inspiratory cutoff mechanism is responsive to the rate, as well as the level, of lung inflation; b) all of the lung volume information affecting inspiratory cutoff in paralyzed cats is carried via the vagi; c) an intact PC is necessary for the generation of a normal time dependence of the volume threshold for inspiratory cutoff; d) the PC plays an important role in matching TE to T1 when the latter changes. For inflations and vagal stimulations applied during expiration, with introduction of a delay between inspiratory cessation and the start of cycle-triggered inflation or vagal stimulation, the results indicated that the expiratory cutoff mechanism has an irrevocable phase of 300-450 ms.     

Medullary respiratory neural activity during hypoxia in NREM and REM sleep in the cat

Intact unanesthetized cats hyperventilate in response to hypocapnic hypoxia in both wakefulness and sleep. This hyperventilation is caused by increases in diaphragmatic activity during inspiration and expiration. In this study, we recorded 120 medullary respiratory neurons during sleep in hypoxia. Our goal was to understand how these neurons change their activity to increase breathing efforts and frequency in response to hypoxia. We found that the response of medullary respiratory neurons to hypoxia was variable. While the activity of a small majority of inspiratory (58%) and expiratory (56%) neurons was increased in response to hypoxia, the activity of a small majority of preinspiratory (57%) neurons was decreased. Cells that were more active in hypoxia had discharge rates that averaged 183% (inspiratory decrementing), 154% (inspiratory augmenting), 155% (inspiratory), 230% (expiratory decrementing), 191% (expiratory augmenting), and 136% (expiratory) of the rates in normoxia. The response to hypoxia was similar in non-rapid-eye-movement (NREM) and REM sleep. Additionally, changes in the profile of activity were observed in all cell types examined. These changes included advanced, prolonged, and abbreviated patterns of activity in response to hypoxia; for example, some inspiratory neurons prolonged their discharge into expiration during the postinspiratory period in hypoxia but not in normoxia. Although changes in activity of the inspiratory neurons could account for the increased breathing efforts and activity of the diaphragm observed during hypoxia, the mechanisms responsible for the change in respiratory rate were not revealed by our data.     

Blockade of pulmonary stretch receptors reinforces diaphragmatic activity during high-frequency oscillatory ventilation

During apneic periods elicited by high-frequency oscillatory ventilation (HFOV) a tonic diaphragmatic activity was observed, contrasting with the absence of diaphragmatic activity during apnea induced by lung inflation. To clarify the mechanism underlying the persistence of the diaphragmatic activity during HFOV-induced arrest of breathing the reflex responses to short periods of HFOV, and to periods of lung inflation with airway pressure (P _aw) equal to the meanP _aw and/or to maximalP _aw during HFOV were examined both before and after the blockade of slowly adapting stretch receptors (SR) by inhalation of sulphur dioxide (SO₂) in anaesthetized rabbits. In animals with intact SR, the HFOV-induced reflex apnea lasted longer than that induced by lung inflation, the associated diaphragmatic activity being in the most cases higher than the diaphragmatic activity during quiet expiration; inflation, however, completely inhibited diaphragmatic activity. After blockade of SR, spontaneous breathing continued during periods of lung inflation, i.e., the Hering-Breuer inflation reflex was abolished, whereas HFOV still led to a cessation of spontaneous breathing, the associated diaphragmatic activity even exceeding the level observed during quiet inspiration. From these results we conclude that only one part of the reflex response to HFOV is due to SR-stimulation and that in addition other vagal pulmonary receptors (irritant-and/or C-fibre-receptors) are involved. The stimulation of the latter counterbalances the concomitant stimulation of SR, giving rise to the tonic activity of the diaphragm.     

Tonic and phasic respiratory drives to human genioglossus motoneurons during breathing

A tongue muscle, the genioglossus (GG), is important in maintaining pharyngeal airway patency. Previous recordings of multiunit electromyogram (EMG) suggest it is activated during inspiration in humans with some tonic activity in expiration. We recorded from populations of single motor units in GG in seven subjects during quiet breathing when awake. Ultrasonography assisted electrode placement. The activity of single units was separated into six classes based on a step-wise analysis of the discharge pattern. Phasic and tonic activities were analyzed statistically with the coefficient of determination (r2) between discharge frequency and lung volume. Of the 110 motor units, 29% discharged tonically without phasic respiratory modulation (firing rate approximately 19 Hz). Further, 16% of units increased their discharge during expiration (expiratory phasic and expiratory tonic units). Only half the units increased their discharge during inspiration (inspiratory phasic and inspiratory tonic units). Units firing tonically with an inspiratory increase had significantly higher discharge rates than those units that only fired phasically (peak rates 25 vs. 16 Hz, respectively). Simultaneous recordings of two or three motor units showed neighboring units with differing respiratory and tonic drives. Our results provide a classification and the first quantitative measures of human GG motor-unit behavior and suggest this activity results from a complex interaction of inspiratory, expiratory, and tonic drives at the hypoglossal motor nucleus. The presence of different drives to GG implies that complex premotor networks can differentially engage human hypoglossal motoneurons during respiration. This is unlike the ordered recruitment of motor units in limb and axial muscles.     

Co-ordination of spontaneous swallowing with respiratory airflow and diaphragmatic and abdominal muscle activity in healthy adult humans

Co-ordination of breathing and swallowing is essential for normal pharyngeal function and to protect the airway. To allow for safe passage of a bolus through the pharynx, respiration is interrupted (swallowing apnoea); however, the control of airflow and diaphragmatic activity during swallowing and swallowing apnoea are not fully understood. Here, we validated a new airflow discriminator for detection of respiratory airflow and used it together with diaphragmatic and abdominal electromyography (EMG), spirometry and pharyngeal and oesophageal manometry. Co-ordination of breathing and spontaneous swallowing was examined in six healthy volunteers at rest, during hypercapnia and when breathing at 30 breaths min^–1. The airflow discriminator proved highly reliable and enabled us to determine timing of respiratory airflow unambiguously in relation to pharyngeal and diaphragmatic activity. During swallowing apnoea, the passive expiration of the diaphragm was interrupted by static activity, i.e. an 'active breath holding', which preserved respiratory volume for expiration after swallowing. Abdominal EMG increased throughout pre- and post-swallowing expiration, more so during hyper- than normocapnia, possibly to assist expiratory airflow. In these six volunteers, swallowing was always preceded by expiration, and 93 and 85% of swallows were also followed by expiration in normo- and hypercapnia, respectively, indicating that, in man, swallowing during the expiratory phase of breathing may be even more predominant than previously believed. This co-ordinated pattern of breathing and swallowing potentially reduces the risk for aspiration. Insights from these measurements in healthy volunteers and the airflow discriminator will be used for future studies on airway protection and effects of disease, drugs and ageing.     

Reflex effects of laryngeal irritation on the pattern of breathing and total lung resistance

1. The reflex responses to chemical and mechanical irritation of the laryngeal mucosa have been studied by applying stimuli to the open larynx of tracheostomized cats while monitoring ventilatory and circulatory variables. The responses were studied before and after vagotomy and before and after denervation of the larynx by transection of the superior and recurrent laryngeal nerves.

2. The immediate response to laryngeal irritation was not consistent. The most frequent responses were coughing, and slowing and deepening of breathing without coughing. Less common were expiratory apnoea and sustained, simultaneous inspiratory and expiratory activity.

3. A consistent late change in the pattern of breathing occurred. Slower, deeper breathing with increased total lung resistance (bronchoconstriction) was seen after the immediate response abated.

4. The slowing of breathing was due to prolongation of both the time for inspiration and the time for expiration. The rate of increase in phrenic nerve activity was also slowed.

5. Vagotomy did not alter qualitatively the reflex changes in the pattern of breathing, although bronchoconstriction no longer occurred.

6. The responses were abolished by denervation of the larynx.

     

Spinal integration of segmental, cortical and breathing inputs to thoracic respiratory motoneurones

1. The spinal integration of cortical, segmental and breathing inputs to thoracic motoneurones was studied in anaesthetized, paralysed cats: the breathing input was intensified by underventilation or abolished by hyperventilation.2. In apnoeic animals, low intensity stimulation of an internal intercostal nerve evoked a brief latency polysynaptic reflex discharge of expiratory motoneurones (direct response) in several adjacent segments with no or little response of the inspiratory motoneurones.3. A similar direct response of expiratory motoneurones occurred with brief tetanic stimulation of the trunk area in the contralateral sensorimotor cortex.4. Conditioning of an intercostal-intercostal test reflex by a prior stimulus to an intercostal nerve or to the cortex gave conditioning curves showing facilitation of transmission to expiratory motoneurones at short intervals (5-25 msec) and inhibition at long intervals (25-200 msec).5. The direct response of expiratory motoneurones to the cortical or segmental inputs was depressed during the inspiratory phase when the animal was underventilated; conversely the spontaneous activity of the inspiratory motoneurones was inhibited for a period that corresponded with the direct response or to the phase of facilitated transmission to expiratory motoneurones. During the expiratory phase, the cortically or segmentally induced direct response was facilitated but the inhibition of inspiratory motoneurone activity was concealed by the absence of spontaneous activity.6. It was possible with discrete lesions of the spinal cord to differentiate between the pathways subserving the responses to cortical stimulation and the spontaneous activity due to the breathing input.7. To account for the results a working hypothesis is proposed utilizing a segmental interneuronal network which transmits mutual reciprocal inhibition between inspiratory and expiratory motoneurones.     

Abdominal expiratory muscle activity in anesthetized vagotomized neonatal rats

The pattern of respiratory activity in abdominal muscles was studied in anesthetized, spontaneously breathing, vagotomized neonatal rats at postnatal days 0–3. Anesthesia (2.0% isoflurane, 50% O₂) depressed breathing and resulted in hypercapnia. Under this condition, abdominal muscles showed discharge late in the expiratory phase (E2 activity) in most rats. As the depth of anesthesia decreased, the amplitude of discharges in the diaphragm and abdominal muscles increased. A small additional burst frequently occurred in abdominal muscles just after the termination of diaphragmatic inspiratory activity (E1 or postinspiratory activity). Since this E1 activity is not often observed in adult rats, the abdominal respiratory pattern likely changes during postnatal development. Anoxia-induced gasping after periodic expiratory activity without inspiratory activity, and in most rats, abdominal expiratory activity disappeared before terminal apnea. These results suggest that a biphasic abdominal motor pattern (a combination of E2 and E1 activity) is a characteristic of vagotomized neonatal rats during normal respiration.     

Inspiratory on-switch evoked by stimulation of the mesencephalon: Activity of phrenic and laryngeal motoneurones

Summary In anaesthetized cats (chloralose-urethan) the effects of brief tetanic electrical stimulation (50 to 100 ms) of the mesencephalic central gray matter and reticular formation on the inspiratory on-switch were studied during the expiratory (E) phase on the gross and unitary activities of phrenic, laryngeal inspiratory and laryngeal expiratory nerves. On the inspiratory laryngeal and phrenic nerves, stimulation elicited a short latency gross response concomitant with the train: the inspiratory Primary Response (Prim.R.) which is followed by an inspiratory Patterned Response (Patt.R.) of longer duration which corresponded to the inspiratory on-switch. The Patt.R. generally appeared from the Prim.R. within a latent period (Silent Phase: Sil.P.) as long as 100 ms. On the expiratory laryngeal nerve, stimulation elicited a brief activation (expiratory Prim.R.) concomitant with the beginning of the inspiratory laryngeal Prim.R. and which rapidly stopped as the latter continued during the stimulus train. The inspiratory Prim.R. corresponded to a simultaneous activation of both early and late (so defined during their spontaneous discharge) inspiratory motoneurones. The laryngeal motoneurones were more strongly activated than the phrenic ones. During the inspiratory Patt.R. all the phrenic motoneurones presented a recruitment delay earlier, compared with the spontaneous one, whereas the recruitment drastically changed from an inspiratory laryngeal motoneurone to another. Thus, the two pools of motoneurones presented different properties of activation. During the inspiratory Sil.P. no concomitant expiratory laryngeal activation was observed when most of the inspiratory motoneurones were inactive. As some inspiratory laryngeal motoneurones did not stop firing, the existence of some central respiratory neurones exhibiting a similar persistent activity and subserving the inspiratory on-switch mechanisms may be hypothesized.Supported by CNRS (LA 205 and ATP n 4188) and Fondation pour la Recherche Médicale     

Somatostatin selectively ablates post-inspiratory activity after injection into the Bötzinger complex

Somatostatin (SST) neurons in the ventral respiratory column (VRC) are essential for the generation of normal breathing. Little is known about the neuromodulatory role of SST on ventral respiratory neurons other than that local administration induces apnoea. Here, we describe the cardiorespiratory effects of microinjecting SST into the preBötzinger and Bötzinger complexes which together elaborate a normal inspiratory augmenting and expiratory respiratory pattern, and on spinally projecting respiratory subnuclei (rostral ventral respiratory group; rVRG). Microinjections (20–50 nl) of SST (0.15, 0.45, 1.5 mM) were made into respiratory subnuclei of urethane-anaesthetized, paralysed, vagotomized and artificially ventilated Sprague–Dawley rats (n=46). Unilateral microinjection of SST into the Bötzinger complex converted the augmenting activity of phrenic nerve discharge into a square-wave apneustic pattern associated with a lengthening of inspiratory period and shortening of expiratory time. Following bilateral microinjection the apneusis became pronounced and was associated with a dramatic variability in inspiratory duration. Microinjection of SST into the Bötzinger complex also abolished the post-inspiratory (post-I) motor activity normally observed in vagal and sympathetic nerves. In the preBötzinger complex SST caused bradypnoea and with increasing dose, apnoea. In the rVRG SST reduced phrenic nerve amplitude, eventually causing apnoea. In conclusion, SST powerfully inhibits respiratory neurons throughout the VRC. Of particular interest is the finding that chemical inhibition of the Bötzinger complex with SST ablates the post-I activity that is normally seen in respiratory activity and leads to apneusis. This loss of post-I activity is a unique feature of inhibition with SST and is not seen following inhibition with other agents such as galanin, GABA and endomorphin. The effect seen on post-I activity is similar to the effect of inhibiting the Kölliker–Fuse nucleus in the pons. The mechanism by which SST exerts this effect on Bötzinger neurons remains to be determined.     

NMDA and non-NMDA receptors may play distinct roles in timing mechanisms and transmission in the feline respiratory network.

1. Activation of N-methyl-D-aspartate (NMDA) glutamate receptors in the brainstem network of respiratory neurones is required to terminate inspiration in the absence of lung afferents, but it is not required in the inspiratory motor act of lung inflation. In the present study we examined the involvement of non-NMDA ionotropic glutamate receptors in these two mechanisms in the adult mammal. 2. Adult cats were either decerebrated or anaesthetized with sodium pentobarbitone, paralysed and ventilated. Inspiratory motor output was recorded from the phrenic nerve and central respiratory activity from neurones in the bulbar ventral respiratory group. 3. In decerebrate vagotomized cats, ionophoretic application of 2,3-dihydroxy-6-nitro-7-sulphamoylbenzo(F)quinoxaline (NBQX) onto single respiratory neurones decreased their spontaneous discharge rate and abolished the excitatory effect of exogenously applied (RS) alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionic acid (AMPA) but not NMDA. 4. In these animals, intravenous infusion (12 mg kg-1) of the non-NMDA receptor blockers GYKI 52466 (1-(4-aminophenyl)-4-methyl-7,8-methylene-dioxy-5-H-2,3-benzodi aze pine) or NBQX: (1) decreased (in 10/15 cats) or abolished (in 5/15 cats) the inspiratory-related discharge of the phrenic nerve; (2) did not prolong the inspiratory phase; (3) reduced or abolished the spontaneous discharge of respiratory neurones; and (4) profoundly decreased the excitatory effects of AMPA but not NMDA ionophoresed onto these neurones. When both the phrenic nerve and the recorded respiratory neurone were silenced, neuronal excitation by ionophoretic application of NMDA first revealed a subthreshold respiratory modulation without lengthening of the inspiratory phase, then respiratory modulation became undetectable. 5. Additional blockade of NMDA receptors by a small dose (0.15 mg kg-1) of dizocilpine (MK-801), abolished the phrenic nerve activity which persisted after NBQX (apnoea), but the discharge or the subthreshold modulation of the bulbar respiratory neurones showed a lengthening of the inspiratory phase (apneusis). 6. Elevation of FA,CO2 increased or re-established phrenic nerve discharges after blockade of non-NMDA receptors or of both NMDA and non-NMDA receptors. 7. Small doses of NBQX or GYKI 52466 induced apnoea in five of five cats anaesthetized with sodium pentobarbitone. 8. In decerebrate animals with intact vagi, GYKI 52466 and NBQX depressed the Hering-Breuer expiratory-lengthening reflex. 9. The results suggest that: (1) there is a specialization of different classes of glutamate receptors participating in timing mechanisms and transmission within the mammalian respiratory network. Neural transmission predominantly involves activation of non-NMDA receptors, acting in synergy with NMDA receptors.(ABSTRACT TRUNCATED AT 400 WORDS)     

Tonic and phasic drive to medullary respiratory neurons during periodic breathing

It is unknown how central neural activity produces the repetitive termination and restart of periodic breathing (PB). We hypothesized that inspiratory and expiratory neural activities would be greatest during the waxing phase and least during the waning phase. We analyzed diaphragmatic and medullary respiratory neural activities during PB in intact unanesthetized adult cats. Diaphragmatic activity was increased and phasic during the waxing phase and was decreased and tonic during the waning phase. Activity of expiratory (n=21) and inspiratory (n=40) neurons was generally increased and phasic during the waxing phase and was decreased and more tonic during the waning phase. During apneas associated with PB, diaphragmatic activity was silent and most, but not all, inspiratory cells were inactive whereas most expiratory cells decreased activity but remained tonically active. We suggest that reduced strength of reciprocal inhibition, secondary to reduced respiratory drive, allows for simultaneous tonic activity of inspiratory and expiratory neurons of the central pattern generator, ultimately resulting in central apnea.     

Modeling of the expiratory flow pattern of spontaneously breathing cats

A mathematical model was developed describing the entire expiratory flow pattern during spontaneous, tidal breathing in the absence of expiratory muscle activity. It provides estimates for the time constants of the respiratory system (tau RS(model)) and of the decay of continuing inspiratory muscle activity in early expiration (tau mus(model)). In ten anesthetized, tracheostomized cats flow, tracheal pressure and diaphragmatic EMG were measured during normal expirations and expirations with four different added resistances. No significant differences were found between tau RS(model) (0.21-0.49 sec) obtained by fitting the model to the flow data and tau RS obtained from the straight part of the expiratory flow-volume curve. tau mus(model) (0.050-0.052 sec) was comparable to similar time constants obtained from the integrated diaphragmatic EMG or from end-inspiratory, tracheal occlusion pressure. Fitted peak flow and time to peak tidal expiratory flow were not significantly different from those measured. In conclusion, for spontaneously breathing, anesthetized cats our model provides a close fit of the expiratory flow and parameter estimates were comparable with independently measured values.     

Reflex control of discharge in motor fibres to the larynx

1. Action potentials have been recorded from single laryngeal motor fibres, with expiratory or inspiratory phases, in cats anaesthetized with pentobarbitone and breathing through a tracheal cannula.2. Pneumothorax increased the discharge of both inspiratory and expiratory units, the inspiratory response being greatly reduced by bilateral vagotomy below the origin of the recurrent laryngeal nerves.3. Addition of a ;viscous' resistance to breathing, or asphyxial rebreathing through an added dead space, increased the activity of inspiratory units and decreased that of expiratory units.4. Induction of pulmonary oedema decreased the discharge of inspiratory units and increased that of expiratory units. After vagotomy the response of inspiratory units was reversed.5. Intravenous injections of potassium cyanide increased the activity of both types of unit.6. Chemical irritation of the laryngeal mucosa decreased the discharge of inspiratory units and increased that of expiratory units, whether the vagi were intact or cut.7. It is concluded that expiratory unit discharge can be correlated with expiratory laryngeal resistance, but that inspiratory unit discharge does not correlate so well with inspiratory laryngeal resistance.8. The relationship between laryngeal motor-fibre activity and the contractions of the inspiratory and expiratory muscles of breathing is discussed.     

Inspiratory on-switch evoked by mesencephalic stimulation: activity of medullary respiratory neurones

Summary The activity of medullary inspiratory and expiratory neurones was studied in urethan-chloralose anaesthetized cats during stimulus — evoked inspiratory phase (inspiratory on-switch). All neurones were characterized according to their axonal destination (i.e. bulbospinal neurones or vagal motoneurones) or the absence of such axonal projections (i.e. propriobulbar neurones), and to their location in the dorsal or ventral respiratory nuclei. 1. The inspiratory on-switch effects were elicited during expiration (E phase) by brief tetanic electrical stimulation (50 to 100 ms duration; 0.5 mA; 300 Hz) delivered to the mesencephalic periaqueductal central gray and the adjacent reticular formation. The evoked inspiratory effects observed on the phrenic nerve discharge consisted of: (i) an immediate response (latency 20 ± 5 ms) of stable duration related to the stimulus (primary response: Prim.R.), (ii) a delayed response (patterned response: Patt.R.) appearing after a latent period (silent phase: Sil.P.) of 100 ms maximal duration. The later the stimulus in the E phase, the longer was the duration of the Patt.R. (300 to 1000 ms). 2. The stimulation evoked an earlier activation of the inspiratory bulbospinal neurones (latency 12 ± 6 ms) than that obtained in the phrenic nerve (Prim.R.). Hence, the Prim.R. originated from the bulbospinal pathway and not from a pathway directly impinging on the motoneurones. Conversely during stimulation very few inspiratory propriobulbar neurones were activated and no expiratory neurone discharged. 3. During the phrenic Sil.P., 46% of the inspiratory bulbospinal neurones continued to discharge with a firing rate lower than that during the stimulus train, while most of the inspiratory propriobulbar and expiratory neurones were not active. 4. During the Patt.R. all inspiratory bulbospinal neurones discharged early and were strongly activated whatever the Patt.R. duration whereas the expiratory neurones were not active. Inspiratory propriobulbar neurones were either not recruited or recruited later, and the number of active neurones increased as the duration of the Patt.R. lengthened. 5. Our results suggest that the eliciting of the stimulus-evoked inspiration (Patt.R.) primarily depends on the activation of the inspiratory bulbospinal neurones. These neurones therefore would not only be the output neurones of the medullary respiratory centres, but they would serve other roles such as building up of the excitation in other respiratory neurones, thus acting as a component of the inspiratory ramp generator.Abbreviations Prim.R Primary response - Patt.R Patterned response - Sil.P Silent phase - I phase Inspiratory phase - E phase Expiratory phase - IBSN Inspiratory bulbospinal neurones - IPBN Inspiratory propriobulbar neurones - EBSN Expiratory bulbospinal neurones - EPBN Expiratory propriobulbar neurones - DRN Dorsal respiratory nucleus - VRN Ventral respiratory nucleus Supported by CNRS (LA 205 and ATP no 4188) and Fondation pour Ia recherche médicale     

The importance of timing on the respiratory effects of intermittent carotid sinus nerve stimulation

1. The respiratory response, measured directly as tidal volume or indirectly by using integrated peak phrenic activity, to intermittent electrical stimulation of the carotid sinus nerve was determined in anaesthetized cats.2. Stimulation at rates of 20-25 Hz for 0.5 sec had a rapid effect, increasing inspiratory airflow and phrenic discharge, but only if applied during inspiration. An increase in tidal volume or peak level of integrated phrenic discharge occurred only if the stimulus was exhibited during the second half of inspiration. Continuous stimulation had no greater effect on size or frequency of breathing than did intermittent inspiratory stimuli alone. Stimulation during expiration had no effect on the form or magnitude of subsequent breaths.3. Stimuli in expiration led to a prolongation of expiration. Stimuli in late inspiration caused a prolongation of both inspiration and expiration. Because of these effects, the respiratory rate could be changed by stimulation; in some instances entrainment of respiration by the intermittent carotid sinus nerve stimuli occurred.4. The findings are attributable to modulation of incoming carotid sinus nerve information by the central respiratory neurones, which use primarily that which arrives during inspiration. They show a possible mechanism by which oscillating signals may have a different effect than their mean level would indicate.     

Direct and synaptic responses of the cat's medullary respiratory neurons evoked by microstimulation

The responses of single medullary neurons were evoked by microstimulation and recorded extracellularly in cats with spontaneous breathing under pentobarbital anaesthesia. Direct and synaptic responses were observed. In the silent phases of the respiratory neurons the latencies of direct responses were always longer (0.25–0.5 ms) than during the spontaneous discharge of these neurons (0.12–0.35 ms). Such phase-related variations of latency persist even when maximal currents are applied.The latencies of synaptic responses of expiratory and inspiratory neurons differed. The expiratory neurons gave two types of synaptic responses with a latency of about 0.8 ms (monosynaptic) or about 1.5 ms (probably disynaptic). The latencies of the evoked monosynaptic responses of the expiratory neurons varied from 0.6 to 1.2 ms depending on the phase of the respiratory cycle. The minimal latency of the synaptic responses of the inspiratory neurons under study was 1.2 ms. The typical latency varied between 1.75 and 2.8 ms. Some neurons had the late synaptic responses with a latency of about 5–9 ms. The synaptic responses of all the inspiratory neurons as well as the disynaptic responses of the expiratory neurons were evoked only during a spontaneous discharge which was followed by the inhibition of the spontaneous activity (lasting 20–100 ms).Thus it can be concluded that the excitatory pathways to expiratory neurons activated by microstimulation are mono- and disynaptic, whereas excitatory pathways to inspiratory neurons are polysynaptic.     

Reorganisation of respiratory network activity after loss of glycinergic inhibition

gamma-Aminobutyric acid (GABA)-ergic and glycinergic inhibition is believed to play a major role in the respiratory network. In the present study we tested whether specific blockade of glycinergic inhibition resulted in changes in respiratory network interaction and function. Using the working heart-brainstem preparation from adult mice, we recorded phrenic nerve activity and the activity of different types of respiratory neurones located in the ventrolateral medulla. Strychnine (0.03-0.3 microM) was given systemically to block glycine receptors (Gly-R). During exposure to strychnine, post-inspiratory (PI) neurones shifted their onset of discharge into the inspiratory phase. As a consequence, the post-inspiratory phase failed and the rhythm changed from a three-phase cycle (inspiration, post-inspiration, expiration, with a frequency of about. 0.24 Hz) to a faster, two-phased cycle (inspiration expiration, frequency about 0.41 Hz). Inspiratory and expiratory neurones altered their augmenting membrane potential pattern to a rapidly peaking pattern. Smaller voltage oscillations at approximately 10 Hz and consisting of excitatory and inhibitory postsynaptic potential sequences occurred during the expiratory interval. Due to their high frequency and low amplitude, such oscillations would be inadequate for lung ventilation. We conclude that, under physiological conditions, glycinergic inhibition does indeed play a major role in the generation of a normal respiratory rhythm in adult mice. After failure of glycinergic inhibition a faster respiratory rhythm seems to operate through reciprocal GABAergic inhibition between inspiratory and expiratory neurones, while phase switching is organised by activation of intrinsic membrane properties.     

Chemical activation of caudal medullary expiratory neurones alters the pattern of breathing in the cat.

1. The purpose of this work was to ascertain whether the activation of caudal expiratory neurones located in the caudal part of the ventral respiratory group (VRG) may affect the pattern of breathing via medullary axon collaterals. 2. We used microinjections of DL-homocysteic acid (DLH) to activate this population of neurones in pentobarbitone-anaesthetized, vagotomized, paralysed and artificially ventilated cats. Both phrenic and abdominal nerve activities were monitored; extracellular recordings from medullary and upper cervical cord respiratory neurones were performed. 3. DLH (160 mM) microinjected (10-30 nl for a total of 1.6-4.8 nmol) into the caudal VRG, into sites where expiratory activity was encountered, provoked an intense and sustained activation of the expiratory motor output associated with a corresponding period of silence in phrenic nerve activity. During the progressive decline of the activation of abdominal motoneurones, rhythmic inspiratory activity resumed, displaying a decrease in frequency and a marked reduction or the complete suppression of postinspiratory activity as its most consistent features. 4. Medullary and upper cervical cord inspiratory neurones exhibited inhibitory responses consistent with those observed in phrenic nerve activity, while expiratory neurones in the caudal VRG on the side contralateral to the injection showed excitation patterns similar to those of abdominal motoneurones. On the other hand, in correspondence to expiratory motor output activation, expiratory neurones of the Bötzinger complex displayed tonic discharges whose intensity was markedly lower than the peak level of control breaths. 5. Bilateral lignocaine blockades of neural transmission at C2-C3 affecting the expiratory and, to a varying extent, the inspiratory bulbospinal pathways as well as spinal cord transections at C2-C3 or C1-C2, did not suppress the inhibitory effect on inspiratory neurones of either the ipsi- or contralateral VRG in response to DLH microinjections into the caudal VRG. 6. The results show that neurones within the column of caudal VRG expiratory neurones promote inhibitory effects on phrenic nerve activity and resetting of the respiratory rhythm. We suggest that these effects are mediated by medullary bulbospinal expiratory neurones, which may, therefore, have a function in the control of breathing through medullary axon collaterals.     

5-HT-1A receptor-mediated modulation of medullary expiratory neurones in the cat.

The involvement of the 5-HT-1A receptor in serotoninergic responses of stage 2 expiratory (E-2) neurones was investigated in pentobarbitone-anaesthetized, mechanically ventilated cats. The specific agonist of the 5-HT-1A receptor, 8-hydroxy-diproplaminotetralin (8-OH-DPAT), administered systemically or by ionophoresis directly on to the neurones, had a clear depressant effect. Administration of 8-OH-DPAT at doses of 10-50 micrograms kg-1 (I.V.) increased the membrane hyperpolarizations of E-2 neurones during the inspiratory and postinspiratory phases, and shortened their duration of activity in association with shortening of phrenic nerve activity. Discharges of E-2 neurones were also less intense. At doses of 50-90 micrograms kg-1, 8-OH-DPAT reduced or abolished inspiratory hyperpolarizations, and reduced expiratory depolarizations of membrane potential and discharge in parallel with inhibition of phrenic nerve discharges. The effects of the larger doses were reversed by I.V. injection of NAN-190, an antagonist at the 5-HT-1A receptor. Dose-dependent effects on the membrane potential and discharge of E-2 neurones, but not on phrenic nerve activity, were also seen by ionophoretic administration of 8-OH-DPAT on to E-2 neurones. At low currents, ejection of 8-OH-DPAT hyperpolarized the neurones without affecting the duration of inspiratory hyperpolarization and expiratory depolarization. This hyperpolarization depressed the intensity and the duration of expiratory discharges. Ejection with larger currents hyperpolarized the E-2 neurones further, and depressed expiratory depolarization leading to blockade of expiratory discharges. The effects on membrane potential were accompanied by decreased neuronal input resistance. This depressed the excitability of E-2 neurones as tested by discharge evoked by intracellular current injection. The amplitudes of action potentials decreased in parallel with the changes in input resistance. The effects were attributed to a postsynaptic effect of 8-OH-DPAT leading to a gradually developing inhibition by activation of 5-HT-1A receptors. Hyperventilatory apnoea depressed on-going synaptic activity and unmasked the effect of ionophoretically applied 8-OH-DPAT. The responses of the E-2 neurone were enhanced, as evidenced by increased membrane hyperpolarization and greater reduction of input resistance. Both responses faded appreciably, indicating receptor desensitization. The degree and rate of apparent desensitization depended on the dose/ejecting current. The greater sensitivity and faster desensitization to 8-OH-DPAT were attributed to the hyperventilatory alkalinization of the extracellular fluid, which might influence agonist binding to 5HT-1A receptors and/or receptor properties.