Respiratory frequency control during hypercapnia in vagotomized, anesthetized cats

Integration-differentiation and gating of carotid afferent traffic that shapes the respiratory pattern

The phase-dependent plasticity of carotid chemoafferent signaling was studied with electrical stimulation of a carotid sinus nerve during either inspiration or expiration in anesthetized, glomectomized, vagotomized, paralyzed, and ventilated rats. Stroboscopic and interferometric analyses of the resulting phase-contrast disturbances of the respiratory rhythm revealed that carotid chemoafferent traffic was dynamically filtered centrally by a parallel bank of leaky integrators and differentiators, each being logically gated to the inspiratory or expiratory phase in a stop-and-go manner as follows: 1) carotid short-term potentiation of inspiratory drive was mediated by dual integrators that both shortened inspiration and augmented phrenic motor output cooperatively in long and short timescales; 2) carotid short-term depression of respiratory frequency was mediated by a (possibly pontine) integrator that lengthened expiration with a relatively long memory; and 3) carotid "chemoreflex" shortening of expiration was mediated by an occult fast integrator, which, together with carotid short-term depression, formed a differentiator. These effects were modulated anteriorly by integrators in the nucleus tractus solitarius that were "auto-gated" to, or recruited by, the carotid sinus nerve input. Such phase-selective and activity-dependent time-frequency filtering of carotid chemoafferent feedback in parallel neurological-neurodynamic central pathways may profoundly affect respiratory stability during hypoxia and sleep and could contribute to the dynamic optimization of the respiratory pattern and maintenance of homeostasis in health and in disease states.

Respiratory neural activity during long-term facilitation

Intermittent hypoxia results in a long-term facilitation (LTF) of respiratory efferent activity. The studies reviewed here presented data from both anesthetized and decerebrate, paralyzed, vagotomized, artificially ventilated adult cats. Multiple arrays of tungsten microelectrodes were used to record the concurrent responses of brain stem neurons that contribute to respiratory motor pattern generation. Spike trains were analyzed with firing rate histograms, peristimulus time histograms, cycle triggered histograms, spike triggered averages with multiunit phrenic efferent activity, cross correlation histograms, joint peristimulus time histograms and the gravity method. These studies addressed several hypotheses. (1) There is parallel processing of input from carotid chemoreceptors to the brain stem. (2) Respiratory related midline neurons are involved in the induction and maintenance of LTF. (3) There is a change in effective connectivity of brain stem neurons with LTF. (4) Neural networks involved in the induction and maintenance of LTF have patterns of synchrony that recur with a frequency greater than expected by chance.

Responses of simultaneously recorded respiratory-related medullary neurons to stimulation of multiple sensory modalities

This study addresses the hypothesis that multiple afferent systems share elements of a distributed brain stem network that modulates the respiratory motor pattern. Data were collected from 18 decerebrate, bilaterally vagotomized, paralyzed, artificially ventilated cats. Up to 28 neurons distributed in the rostral and caudal ventral respiratory group, nucleus tractus solitarius, and raphe obscurus were recorded simultaneously with microelectrode arrays. Phases of the respiratory cycle and inspiratory drive were assessed from integrated efferent phrenic nerve activity. Carotid chemoreceptors were stimulated by injection of CO2-saturated saline solution via the external carotid artery. Baroreceptors were stimulated by increased blood pressure secondary to inflation of an embolectomy catheter in the descending aorta. Cutaneous nociceptors were stimulated by pinching a footpad. Four hundred seventy-four neurons were tested for respiratory modulated firing rates and responses; 403 neurons were tested with stimulation of all 3 modalities. Chemoreceptor stimulation and pinch, perturbations that tend to increase respiratory drive, caused similar responses in 52 neurons; 28 responded oppositely. Chemoreceptor and baroreceptor stimulation resulted in similar primary responses in 45 neurons; 48 responded oppositely. Similar responses to baroreceptor stimulation and pinch were recorded for 38 neurons; opposite effects were measured in 26 neurons. Among simultaneously recorded neurons, distinct combinations of firing rate changes were evoked in response to stimulation of the different modalities. The results show a functional convergence of information from carotid chemoreceptors, baroreceptors, and cutaneous nociceptors on respiratory-modulated neurons distributed in the medulla. The data are consistent with the hypothesis that brain stem neurons have overlapping memberships in multifunctional groups that influence the respiratory motor pattern.

Inspiratory drive and phase duration during carotid chemoreceptor stimulation in the cat: medullary neurone correlations

1. This study addressed the hypothesis that there is a parallel processing of input from carotid chemoreceptors to brainstem neurones involved in inspiratory phase timing and control of inspiratory motor output amplitude. Data were from fifteen anaesthetized, bilaterally vagotomized, paralysed, artificially ventilated cats. Carotid chemoreceptors were stimulated by close arterial injection of 200 microliters of CO2-saturated saline solution. 2. Planar arrays of tungsten microelectrodes were used to monitor simultaneously up to twenty-two neurones in the nucleus tractus solitarii (NTS) and ventral respiratory group (VRG). Spike trains were analysed with two statistical tests of respiratory modulation, cycle-triggered histograms, peristimulus-time histograms, cumulative sum histograms and cross-correlograms. 3. In NTS, 16 of 26 neurones with respiratory and 12 of 27 without respiratory modulation changed firing rate during carotid chemoreceptor stimulation. In the VRG 72 of 112 respiratory and 14 of 48 non-respiratory neurones changed firing rate during stimulation. 4. The spike trains of 85 of 1276 pairs (6.7%) of cells exhibited short time scale correlations indicative of paucisynaptic interactions. Ten pairs of neurones were each composed of a rostral VRG phasic inspiratory neurone that responded to carotid chemoreceptor stimulation with a decline in firing rate and a caudal VRG phasic inspiratory neurone that increased its firing rate. Cross-correlograms from two of the pairs had features consistent with excitation of the caudal neurones by the rostral cells. A decrease in the duration of activity of the rostral VRG neurones was paralleled by the decrease in inspiratory time of phrenic nerve activity. Caudal VRG inspiratory neurones increased their activity as phrenic amplitude increased. Spike-triggered averages of all four neurones indicated post-spike facilitation of phrenic motoneurones. 5. The results support the hypothesis that unilateral stimulation of carotid chemoreceptors results in parallel actions. (a) Inhibition of rostral VRG I-Driver neurones decreases inspiratory duration. (b) Concurrent excitation of premotor VRG and dorsal respiratory group inspiratory neurones increases inspiratory drive to phrenic motoneurones. Other data suggest that responsive ipsilateral neurones act to regulate contralateral neurones.

Verification of a model for the mechanisms controlling expiratory duration in rabbits under various conditions

A mathematical model for the mechanisms controlling expiratory (E) duration was verified through comparison of its prediction with the relationships between TE and waveform parameters of various input patterns obtained by bilateral, simultaneous electrical stimulation of the largest vagal afferent fibers in vagotomized, paralyzed rabbits at normal or elevated chemical drive under light or deep barbiturate anesthesia. The model accurately reproduced experimental results in each animal and condition; it suggests that in the absence of vagal input (v.i.) an exponentially decaying central activity (phi) prevents inspiratory (I) onset until a time independent threshold (phi thr) is reached; the vagal component of phi results from fixed gain, long time constant (tau = 1.07 +/- 0.03 sec; mean +/- SE for N = 37 for N = 37 animals) temporal summation of expiratory v.i., being unaffected by inspiratory v.i.; central and vagal components share a common mechanism, the same value of tau applying to v.i. integration and to phi decay in the absence of v.i. under all conditions; the amplitude of the central component is proportional to tidal changes in central I activity, independently of the condition studied; and phi thr increases at elevated chemical drive, is unaffected by v.i., and decreases under deep anesthesia.

Open-loop comparison of carotid sinus reflex respiratory and circulatory effects in cats

Isolated control of pressure in the carotid sinus was exercised bilaterally by means of pump-driven autoperfusion in anaesthetized heparinized cats breathing 100% O2. In cats with vagus nerves intact, a rise of intrasinus pressure resulted in a non-adapting fall in arterial blood pressure and a short-lasting depression in tidal volume and frequency of breathing that adapted fully in the steady state. Vagotomy increased the steady-state gain of the intrasinus pressure-vasodepressor relationship and resulted sometimes in a measurable sustained inhibition of breathing, following its initial adaptation, at the upper levels of intrasinus pressure tested. Pharmacological stabilization of the blood pressure with the use of guanethidine plus phenylephrine did not detectably affect the adapting character of the sinus reflex respiratory response. Denervation of the carotid sinuses abolished all responses previously evoked by shifts of intrasinus pressure. It is concluded that the reflex effects of carotid sinus autoperfusion are produced by selective activation of the baroreceptors and that the respiratory adaptation results from signal processing in the central nervous system different from the processing involved in the non-adapting vasodepressor effect.

The involvement of expiratory termination in the vagally mediated facilitation of ventilatory CO2 responsiveness during hyperoxia

In anaesthetized rabbits the influence of differential vagal cold blockade on the ventilatory response to inhaled CO2 during hyperoxia was investigated. Following total inactivation, the relationship between ventilation (V) and arterial PCO2 (PaCO2) was shifted to the left and steepened slightly over a range of modest hypercapnia, but was progressively flattened as hypercapnia intensified. The latter effect, suggestive of a vagally mediated facilitation of ventilatory CO2 responsiveness, was studied further. Differential vagal cold blockade to a temperature (5-11 degrees C) which abolished the Breuer-Hering inflation reflex (end-inspiratory tracheal occlusion no longer eliciting a prolongation of expiratory duration, TE) had no effect on V either during normocapnia or at a substantial level of hypercapnia. Only with further vagal cooling to 0 degrees C did the ventilatory depression during hypercapnia emerge, largely because TE failed to shorten in response to the hypercapnic stimulus. It is concluded that the integrity of expiratory-terminating mechanisms is crucial for the manifestation of the vagally mediated facilitation of V and its CO2 responsiveness which is evident during hyperoxic hypercapnia. A possible role is suggested for lung epithelial irritant receptors or for the tonic late-expiratory activity from pulmonary stretch receptors.

Time course of intercostal afferent termination of the inspiratory process

The influence exerted by several somatic nerves on the inspiratory off-switch mechanism has been assessed in decerebrate cats. These animals were paralyzed, artificially ventilated, and bilaterally vagotomized. Inspiratory activity was monitored by a phrenic neurogram. Brief stimulation of either the superficial radical nerve or the sciatic nerve had an inconsistent effect on both the depth of inspiration and the timing of the respiratory cycle. However, stimulation of the T6 intercostal nerve during inspiration elicited a premature phase switch to expiration. Distinct, repeatable thresholds were determined for 10 delays in 100 msec increments after the onset of inspiration. As the delay increased, the threshold current was observed to decrease in all 30 decerebrate cats studied. An increase in the end-expiratory %CO2 caused an elevation of the stimulus threshold. These results correspond to the known characteristics of the inspiratory off-switch. Also, since the intercostal afferents are not normally a major determinant of respiratory rhythmicity in eupnea, this work establishes intercostal nerve stimulation as a very useful technique in the study of inspiratory to expiratory phase switching mechanisms.

Interaction between vagal and chemoreceptors afferents in ventilatory response to transient hypercapnia (awake rabbits)

The ventilatory response to a transient hypercapnia was studied in four awake rabbits maintained in a volume displacement plethysmograph : the increase in inspiratory volume (VI) was associated or not with an increase in inspiratory and expiratory durations (TI and TE). These ventilatory variations were consistent with the activation of the peripheral chemoreceptors by carbon dioxide (short latency of the initial response). After vagal blockade by local anaesthesia, relative ventilatory variations were not significantly different from those previously measured. Central activity seems an important factor reducing inhibitory vagal input and favouring peripheral chemoreceptor afferents.