Bilateral reflex effects on phrenic nerve activity in response to single-shock vagal stimulation

Modeling Long-Term Facilitation of Respiration During Interval Exercise in Humans

Long-term facilitation (LTF) of respiration has been mainly initiated by intermittent hypoxia and resultant chemoreceptor stimulation in humans. Comparable levels of chemoreceptor stimulation can occur in combined exercise and carbon dioxide (CO2) inhalation and lead to LTF. This possibility was supported by data collected during combined interval exercise and 3% inhaled CO2 in seven normal subjects. These data were further analyzed based on the dynamics involved using mathematical models in this study. Previously estimated peripheral chemoreceptor sensitivity during light exercise (40 W) with air or 3% inhaled CO2 approximately doubled resting sensitivity. Ventilation after a delay increased by 17.0 ± 2.48 L/min (p < 0.001) during recovery following 45% maximal oxygen uptake ([Formula: see text] ) exercise consistent with LTF which exceeded what can be achieved with intermittent hypoxia. Model fitting of the dynamic responses was used to separate neural from chemoreceptor-mediated CO2 responses. Exercise of 45% [Formula: see text] was followed by ventilation augmentation following initial recovery. Augmentation of LTF developed slowly according to second-order dynamics in accordance with plasticity involving a balance between self-excitatory and self-inhibitory neuronal pools.

Synaptic mechanisms of inspiratory off-switching evoked by pontine pneumotaxic stimulation in cats

To elucidate synaptic mechanisms and the involvement of N-methyl-D-aspartate (NMDA) receptors in inspiratory off-switching (IOS) evoked by the stimulation of the nucleus parabrachialis medialis (NPBM), excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were recorded from bulbar augmenting inspiratory (aug-I) and postinspiratory (PI) neurons in vagotomized cats. Stimulation of NPBM produced either transient inhibition or premature termination of inspiration (reversible or irreversible IOS), depending on the stimulus intensity. Each neuron displayed four-phasic postsynaptic responses during the reversible IOS, i.e. Phase 1 EPSPs, Phase 2 IPSPs, Phase 3 EPSPs and Phase 4 IPSPs in aug-I neurons, and Phase 1 plus 2 EPSPs, Phase 3 IPSPs and Phase 4 EPSPs in PI neurons. During the irreversible IOS, Phase 4 responses were replaced by sustained hyperpolarization in aug-I neurons and decrementing depolarization in PI neurons. Blockade of NMDA receptors by dizocilpine (0.3 mg kg(-1) i.v.) selectively increased Phase 4 potentials in both types of neurons and decreased the thresholds for evoking the irreversible IOS. The NPBM-induced responses had a pattern and time-course similar to those induced by vagal stimulation. The present results suggest that pneumotaxic and vagal inputs converge on the common IOS circuit, and the effectiveness of both inputs is modulated by NMDA receptors.

Contribution of NMDA receptors to activity of augmenting expiratory neurons in vagotomized cats

To identify the NMDA receptor-mediated mechanism in augmenting expiratory (E2) neurons, the effects of systemic and local application of dizocilpine on spontaneous and evoked postsynaptic potentials (PSPs) were investigated in decerebrate and vagotomized cats. Intravenously applied dizocilpine reduced the inhibitory PSPs during inspiration and stage 1 expiration, but had little effect on the excitatory PSPs during stage 2 expiration. Iontophoresed dizocilpine caused a continuous hyperpolarization throughout the respiratory cycle. Dizocilpine had no effect on vagally evoked PSPs. These results suggest that the NMDA mechanisms are involved presynaptically in periodic postsynaptic inhibitions and postsynaptically in tonic excitation in E2 neurons.

Synaptic interactions between respiratory neurons during inspiratory on-switching evoked by vagal stimulation in decerebrate cats

To elucidate neuronal mechanisms underlying phase-switching from expiration to inspiration, or inspiratory on-switching (IonS), postsynaptic potentials (PSPs) of bulbar respiratory neurons together with phrenic nerve discharges were recorded during IonS evoked by vagal stimulation in decerebrate and vagotomized cats. A single shock stimulation of the vagus nerve applied at late-expiration developed an inspiratory discharge in the phrenic neurogram after a latency of 79+/-11 ms (n = 11). Preceding this evoked inspiratory discharge, a triphasic response was induced, consisting of an early silence (phase 1 silence), a transient burst discharge (phase 2 discharge) and a late pause (phase 3 pause). During phase 1 silence, IPSPs occurred in augmenting inspiratory (aug-I) and expiratory (E2) neurons, and EPSPs in postinspiratory (PI) neurons. During phase 2 discharge, EPSPs arose in aug-I neurons and IPSPs in PI and E2 neurons. These initial biphasic PSPs were comparable with those during inspiratory off-switching evoked by the same stimulation applied at late-inspiration. In both on- and off-switching, phase-transition in respiratory neuronal activities started to arise concomitantly with the phrenic phase 3 pause. These results suggest that vagal inputs initially produce a non-specific, biphasic response in bulbar respiratory neurons, which consecutively activates a more specific process connected to IonS.

GABA(A) receptor-mediated inspiratory termination evoked by vagal stimulation in decerebrate cats

To identify the GABAergic inhibitory mechanisms involved in inspiratory termination or off-switching (IOS), the effects of a specific enhancer of GABA(A) receptors, midazolam, and an antagonist, bicuculline, on vagally evoked inspiratory inhibitions and IOS were investigated in decerebrate cats. Stimulation of vagal afferents at late inspiration provoked either reversible inspiratory inhibition or IOS, depending on the stimulus intensity. Each response occurred at a constant latency (phase 1). The reversible response was triphasic, consisting of an early (phase 2) inhibition, a brief (phase 3) excitation and a late (phase 4) inhibition in the phrenic neurogram, and early (phase 2) IPSPs, brief (phase 3) EPSPs and late (phase 4) IPSPs in bulbar inspiratory (I) neurones. With an increasing stimulus intensity, phase 4 inhibitions were increased in amplitude and duration, leading to IOS. Midazolam (0.1 mg/kg i.v.) increased more selectively phase 4 IPSPs than phase 2 IPSPs in I neurones, and decreased the threshold for evoking IOS by producing an earlier and larger phase 4 IPSPs. Bicuculline (1.0 mg/kg i.v.) had an opposite effect. These results suggest that the late inhibitory response evoked by vagal stimulation in the I neuronal pool organizes an initial phase of IOS which is mediated by GABA(A) receptors.

Effects of intravenous bicuculline and strychnine on inspiratory inhibitory responses in the cat

Single shock stimulation of the superior laryngeal nerve (SLN), intercostal nerve (ICN), phrenic nerve (PN) or within the medullary respiratory groups (DRG-VRG) produces a transient, short-latency attenuation of inspiratory motor activity. Trains of stimuli delivered to SLN and ICN cause premature termination of inspiration. This study examined involvement of glycine and GABAA receptors in these reflex inhibitions. Experiments were conducted in decerebrate, vagotomized, and paralyzed cats. Control responses of left PN activity to threshold single shock stimulation of SLN, PN, ICN and the DRG-VRG were recorded and the thresholds for SLN- and ICN-evoked inspiratory termination were determined. Five min after intravenous injection of bicuculline (1 mg/kg) or strychnine (50 micrograms/kg), the responses to stimulation were again recorded. This procedure was reiterated until the cumulative dose elicited marked convulsions. Neither drug affected the inspiratory terminating reflexes. Systemic bicuculline had no effect on transient inspiratory inhibition. However strychnine prolonged the onset latency and the duration of all four inhibitory responses. Since the degree of transient inhibition was not lessened (only delayed), it appears that these inspiratory inhibitory reflexes do not rely exclusively on actions of glycine or GABAA receptors.

Effects of electrical and chemical stimulation of the Bötzinger complex on respiratory activity in the cat

The effects of electrical and chemical stimulation of the expiratory neuronal population in the region of the retrofacial nucleus, the so called 'Bötzinger complex' (Böt. c.), on respiratory activity were investigated in vagotomized cats under pentobarbitone anaesthesia. Some of the experiments were performed on paralyzed or bilaterally thoracotomized, artificially ventilated animals. Sustained tetanic electrical stimulation (20 to 100-Hz, 0.5-ms current pulses at intensities of 5-60 microA) induced strong depressant effects on the inspiratory motor output which could lead to complete apnoea. The apnoeic response was accompanied by tonic activation of expiratory muscles; the appearance and the strength of tonic expiratory activity were dependent upon the frequency of stimulation. Brief tetani (40 to 100 ms trains of 0.5-ms rectangular pulses at 100-300 Hz) timed either during the inspiratory or the expiratory phase caused depression of inspiratory activity and prolongation of expiratory time, respectively. These effects increased gradually as the onset of stimulation was progressively delayed during each respiratory phase. The effects of sustained tetanic stimulation were mimicked by microinjections (25-100 nl) of 0.5 M L-glutamate or 0.16 M DL-homocysteic acid in the same region, thus indicating that they were the result of the stimulation of cell bodies and not of axons of passage. The present results support the hypothesis that Böt. c. neurons play an important role in the control of the breathing pattern by exerting inhibitory influences on inspiratory activity and, possibly, by contributing to the off-switch mechanisms. Furthermore, they suggest that these neurons are involved in the central control of expiratory activity.

Role of the ventrolateral region of the nucleus of the tractus solitarius in processing respiratory afferent input from vagus and superior laryngeal nerves

The role of respiratory neurons located within and adjacent to the region of the ventrolateral nucleus of the tractus solitarius (vlNTS) in processing respiratory related afferent input from the vagus and superior laryngeal nerves was examined. Responses in phrenic neural discharge to electrical stimulation of the cervical vagus or superior laryngeal nerve afferents were determined before and after lesioning the vlNTS region. Studies were conducted on anesthetized, vagotomized, paralyzed and artificially ventilated cats. Arrays of 2 to 4 tungsten microelectrodes were used to record neuronal activity and for lesioning. Constant current lesions were made in the vlNTS region where respiratory neuronal discharges were recorded. The region of the vlNTS was probed with the microelectrodes and lesions made until no further respiratory related neuronal discharge could be recorded. The size and placement of lesions was determined in subsequent microscopic examination of 50 micron thick sections. Prior to making lesions, electrical stimulation of the superior laryngeal nerve (4-100 microA, 10 Hz, 0.1 ms pulse duration) elicited a short latency increase in discharge of phrenic motoneurons, primarily contralateral to the stimulated nerve. This was followed by a bilateral decrease in phrenic nerve discharge and, at higher currents, a longer latency increase in discharge. Stimulation of the vagus nerve at intensities chosen to selectively activate pulmonary stretch receptor afferent fibers produced a stimulus (current) dependent shortening of inspiratory duration. Responses were compared between measurements made immediately before and immediately after each lesion so that changes in response efficacy due to lesions per se could be distinguished from other factors, such as slight changes in the level of anesthesia over the several hours necessary in some cases to complete the lesions. Neither uni- nor bi-lateral lesions altered the efficacy with which stimulation of the vagus nerve shortened inspiratory duration. The short latency excitation of the phrenic motoneurons due to stimulation of the superior laryngeal nerve was severely attenuated by unilateral lesions of the vlNTS region ipsilateral to the stimulated nerve. Neither the bilateral inhibition nor the longer latency excitation due to superior laryngeal nerve stimulation was reduced by uni- or bi-lateral lesions of the vlNTS region.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of sagittal medullary section on high-frequency oscillation in rabbit phrenic neurogram

Six rabbits were anaesthetized with chloralose and urethane and were subjected to sagittal section of the medulla in the region of obex to a few mm rostral to obex. The sections had an initial rostro-caudal length of 4 mm and were gradually extended until the respiratory rhythms on the two sides of the animal, as seen in phrenic discharges, became dissociated. High-frequency oscillation in the phrenic discharge, assessed either by visual inspection of the neurogram or by cross-correlation between the discharges in the two phrenic roots on one side, was abolished by sections for which the intensity of the phrenic discharge was hardly affected. These sections were smaller than those required to dissociate the respiratory rhythms on the two sides of the animal.

Neural elements subserving pulmonary stretch receptor-mediated facilitation of phrenic motoneurons

The neural elements responsible for facilitation of phrenic nerve activity by lung inflation were investigated in cats by the simultaneous recording of individual pulmonary stretch receptor afferents, respiratory neurons of the ventrolateral nucleus of the tractus solitarius and phrenic nerve activity. Monosynaptic excitation of I beta neurons by slowly adapting pulmonary stretch receptors was demonstrated by cross-correlational analysis. It was also demonstrated that the majority of these same I beta neurons projected to the contralateral C5 phrenic motoneuron pool. Thus, this study has shown that I beta neurons can act as central neural elements to mediate the facilitatory effect of lung inflation upon phrenic nerve activity.

Effects of graded focal cold block in the solitary and para-ambigual regions of the medulla in the cat

Unilateral focal cold blocks in the region of the nucleus tractus solitarius and the dorsal respiratory group of neurons, DRG, of anaesthetized cats consistently caused apneustic-type breathing. There was no concomitant change in the initial rate of rise of inspiratory activity. The apneustic prolongation of inspiratory duration, TI, was most pronounced in, but was not confined to, the DRG. The apneustic effects were more marked after vagotomy. In cats with intact vagus nerves being given artificial ventilation, focal cooling at certain sites of the DRG region could produce 'unlocking' of the respiratory rhythm from that of the respiratory pump. At other sites in this region, focal cooling could selectively block the effects of the inspiration-facilitating reflex induced by deflation without blocking the inspiration-inhibiting Hering-Breuer reflex. Unilateral focal cold blocks in the region of the intermediate part of the ventral respiratory group of neurons, VRG, generally caused depression of the rate of rise of inspiratory activity, but almost never apneustic effects. All effects of unilateral focal cooling both in the DRG and VRG were bilaterally symmetrical. No systematic differences between the effects on phrenic and external intercostal inspiratory activity were found in response to focal cooling either of the DRG or VRG suggesting that differential control of phrenic and external intercostal motoneurons is not exerted mainly at the level of these medullary structures. The results suggest that the DRG and VRG areas exert somewhat different effects on the respiratory pattern: DRG appears to be more concerned with integration of vagal and other inputs contributing to the inspiratory off-switch mechanisms which, however, are not confined only to the DRG. The VRG inspiratory mechanisms, on the other hand, appear to be more involved in the gain control of the inspiratory output intensity.

Effect of synchronous activation of medullary inspiratory bulbo-spinal neurones on phrenic nerve discharge in cat

The effects on phrenic nerve discharge elicited by intraspinal stimulation which produced synchronous activation of bulbo-spinal inspiratory neurones were investigated in chloralose-urethane anaesthetized, paralysed, vagotomized and artificially ventilated cats. Descending respiratory axons were activated in the ventrolateral spinal cord at the second cervical level using either monopolar or bipolar stimulation (25-200 microA, 100 microseconds, 1-300 Hz). Activation of bulbo-spinal axons was confirmed by recording both orthodromic phrenic nerve excitation and antidromic spike invasion of single, inspiratory modulated units in either the dorsal respiratory group (d.r.g.) or ventral respiratory group (v.r.g.). Antidromic activation of inspiratory bulbo-spinal neurones was confirmed by the criteria of high frequency following and collision tests. Spinal cord stimulation at intensities of 100 microA antidromically activated approximately half of the inspiratory bulbo-spinal neurones in the d.r.g. and v.r.g. Stimulation pulses delivered to the spinal cord elicited an orthodromic excitation of the ipsilateral phrenic nerve lasting 2-12 ms during inspiration. The onset latency of excitation was 2-4 ms, decreasing as inspiration progressed. Following the initial excitation there was a 4-30 ms period of reduced phrenic nerve discharge. Continuous trains of stimuli (less than 100 microA, 100 microseconds, 1-300 Hz) or phrenic gated trains delivered during every fourth inspiratory or expiratory cycle had little or no effect on the duration of inspiration or expiration. Brief trains (400 ms, 50 Hz, 100 microA) of bilateral spinal cord stimulation delivered at various delays from the onset of inspiration had only a transient effect on the pattern of phrenic nerve discharge, with no noticeable effect 60 ms after termination of stimulation. Based on the assumption that synchronous activation of a portion of the central pattern generator for respiration would phase shift or reset the rhythm, we conclude that the bulbo-spinal inspiratory neurones are not responsible for generation of respiratory timing signals and play, at most, a limited role in the generation of the augmenting central inspiratory activity.