Phrenic Neurogram Research Articles

Age related alterations in the complexity of respiratory patterns.

The objective of this study is to investigate the relative contributions of maturation to the dynamic behavior of respiration during ontogeny in the neonate. The phrenic neurogram, an output of the respiratory network, was recorded during eupnea at three postnatal ages (3-7, 12-19, and 26-31 days) in decerebrate piglets. The inter-breath interval (IBI) time series were reconstructed from phrenic neurograms for each piglet and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy were high during the first seven days of the postnatal age [1.02 +/- 0.02 (standard error)] and decreased for the 12-19 days (0.67 +/- 0.008) and increased during subsequent maturation [the 26-31 days age group (0.92 +/- 0.015)]. The mean approximate entropy values for the 3-7 days age group was significantly different from those of the 12-19 days age groups (p < 0.05) using the nonparametric statistical test. The complexity values for the 3-7 days age group were slightly higher than those of the 26-31 days age group, but these differences were not statistically significant (p > 0.05). Therefore, these findings suggest that a decrease in the complexity values is unique to the 12-19 days age groups. This could be due to a reduction in the number of dendritic terminals per cell for the 12-19 days age groups. The results of these preliminary experiments also indicate that the behavior of the respiratory pattern generator in the neonate fluctuates during the early maturation period.

Episodic phrenic-inhibitory vagus nerve stimulation paradoxically induces phrenic long-term facilitation in rats.

All respiratory long-term facilitation (LTF) is induced by inspiratory-excitatory stimulation, suggesting that LTF needs inspiratory augmentation and is the result of a Hebbian mechanism (coincident pre- and post-synaptic activity strengthens synapses). The present study examined the long-term effects of episodic inspiratory-inhibitory vagus nerve stimulation (VNS) on phrenic nerve activity. We hypothesized that episodic VNS would induce phrenic long-term depression. The results are compared with those obtained following serotonin receptor antagonism or episodic carotid sinus nerve stimulation (CSNS). Integrated phrenic neurograms were measured before, during and after three episodes of 5 min VNS (50 Hz, 0.1 ms), each separated by a 5 min interval, at a low (approximately 50 microA), medium (approximately 200 microA) or high (approximately 500 microA) stimulus intensity in anaesthetized, vagotomized, neuromuscularly blocked and artificially ventilated rats. Medium- and high-intensity VNS eliminated rhythmic phrenic activity during VNS, while low-intensity VNS only reduced phrenic burst frequency. At 60 min post-VNS, phrenic amplitude was higher than baseline (35 +/- 5% above baseline, mean +/- S.E.M., P < 0.05) in the high-intensity group but not in the low- (-4 +/- 4%) or medium-intensity groups (-10 +/- 15%), or in the high-intensity with methysergide group (4 mg kg(-1), i.p.) (-11 +/- 5%). These data, which are inconsistent with our hypothesis, indicate that phrenic-inhibitory VNS induces a serotonin-dependent phrenic LTF similar to that induced by phrenic-excitatory CSNS (33 +/- 7%) and may require activation of high-threshold afferent fibres. These data also suggest that the synapses on phrenic motoneurons do not use the Hebbian mechanism in this LTF, as these motoneurons were suppressed during VNS.

Effects of hypoxia on the complexity of respiratory patterns during maturation.

During hypoxic gasping, the hypoxic neurogram has a steeper rate of rise, an augmented amplitude, and a shorter duration than is seen during eupnea. Because hypoxia reduces neural activity, we hypothesized that gasping would be characterized by low complexity (irregularity) values compared with eupnea in piglets. In this study, we define and quantify changes in the complexity of the phrenic neurogram, the output of the respiratory neural network in piglets using the approximate entropy (ApEn) method which provides a model independent measure of the complexity of the phrenic neurogram. The phrenic neurogram in vagotomized, peripherally chemodenervated, decerebrated piglets was recorded from the C5 phrenic nerve during eupnea and gasping at four postnatal ages; 3-6 days of age (n=8), 7-13 days of age (n=3), 15-21 days of age (n=4), 29-35 days of age (n=10). Nonlinear dynamical analysis of the phrenic neurogram was performed using the approximate entropy method. The mean approximate entropy values for a recording of 5 consecutive breaths during eupnea and 6-29 consecutive breaths during gasping for each piglet in each group during eupnea was calculated. Our results suggested that the mean approximate entropy values for the 3-6 days age group were 1.46+/-0.003 during eupnea and 0.85+/-0.001 during hypoxic gasping. For the 7-13 days age group, the mean approximate entropy values were 1.35+/-0.009 during eupnea and 1.00+/-0.001 during hypoxic gasping. For the 15-21 days age group, they were 1.33+/-0.005 during eupnea and 0.94+/-0.001 during hypoxic gasping. Finally, for the 29-35 days age group, they were 1.38+/-0.002 during eupnea and 0.93+/-0.001 during hypoxic gasping. The shift from eupnea to gasping caused a drastic drop in the mean values of the approximate entropy values at each of these four age groups. These differences in the complexity values of the phrenic neurogram between eupnea and gasping are statistically different at each age group (p<0.001). These findings suggest that during hypoxic gasping, regardless of degree of development, the output of the central pattern generator becomes less complex probably because hypoxia reduces the neural activity.

Altered Respiratory Motor Drive after Spinal Cord Injury: Supraspinal and Bilateral Effects of a Unilateral Lesion

Because some bulbospinal respiratory premotor neurons have bilateral projections to the phrenic nuclei, we investigated whether changes in contralateral phrenic motoneuron function would occur after unilateral axotomy via C(2) hemisection. Phrenic neurograms were recorded under baseline conditions and during hypercapnic and hypoxic challenge in C(2) hemisected, normal, and sham-operated rats at 1 and 2 months after injury. The rats were anesthetized, vagotomized, and mechanically ventilated. No group differences were seen in contralateral neurograms at 1 month after injury. At 2 months, however, there was a statistically significant decrease in respiratory rate (RR) at normocapnia, an elevated RR during hypoxia, and an attenuated increase in phrenic neurogram amplitude during hypercapnia in the C(2)-hemisected animals. To test whether C(2) hemisection had induced a supraspinal change in respiratory motor drive, we recorded ipsilateral and contralateral hypoglossal neurograms during hypercapnia. As with the phrenic motor function data, no change in hypoglossal output was evident until 2 months had elapsed when hypoglossal amplitudes were significantly decreased bilaterally. Last, the influence of serotonin-containing neurons on the injury-induced change in phrenic motoneuron function was examined in rats treated with the serotonin neurotoxin, 5,7-dihydroxytryptamine. Pretreatment with 5,7-dihydroxytryptamine prevented the effects of C(2) hemisection on contralateral phrenic neurogram amplitude and normalized the change in RR during hypoxia. The results of this study show novel neuroplastic changes in segmental and brainstem respiratory motor output after C(2) hemisection that coincided with the spontaneous recovery of some ipsilateral phrenic function. Some of these effects may be modulated by serotonin-containing neurons.

Chronic Intermittent Hypoxia Elicits Serotonin-Dependent Plasticity in the Central Neural Control of Breathing

We tested the hypothesis that chronic intermittent hypoxia (CIH) elicits plasticity in the central neural control of breathing via serotonin-dependent effects on the integration of carotid chemoafferent inputs. Adult rats were exposed to 1 week of nocturnal CIH (11-12% O(2)/air at 5 min intervals; 12 hr/night). CIH and untreated rats were then anesthetized, paralyzed, vagotomized, and artificially ventilated. Time-dependent hypoxic responses were assessed in the phrenic neurogram during and after three 5 min episodes of isocapnic hypoxia. Integrated phrenic amplitude (integralPhr) responses during hypoxia were greater after CIH at arterial oxygen pressures (PaO(2)) between 25 and 45 mmHg (p < 0.05), but not at higher PaO(2) levels. CIH did not affect hypoxic phrenic burst frequency responses, although the post-hypoxia frequency decline that is typical in rats was abolished. integralPhr and frequency responses to electrical stimulation of the carotid sinus nerve were enhanced by CIH (p < 0.05). Serotonin-dependent long-term facilitation (LTF) of integralPhr was enhanced after CIH at 15, 30, and 60 min after episodic hypoxia (p < 0.05). Pretreatment with the serotonin receptor antagonists methysergide (4 mg/kg, i.v.) and ketanserin (2 mg/kg, i.v.) reversed CIH-induced augmentation of the short-term hypoxic phrenic response and restored the post-hypoxia frequency decline in CIH rats. Whereas methysergide abolished CIH-enhanced phrenic LTF, the selective 5-HT(2) antagonist ketanserin only partially reversed this effect. The results suggest that CIH elicits unique forms of serotonin-dependent plasticity in the central neural control of breathing. Enhanced LTF after CIH may involve an upregulation of a non-5-HT(2) serotonin receptor subtype or subtypes.

Firing patterns of pre-Bötzinger and Bötzinger neurons during hypocapnia in the adult rat

Controversy exists about how a coordinated respiratory rhythm is generated in the brainstem. Some authors suggest that neurons in the pre-Bötzinger complex are key to initiation of all types of breathing. While, on the other hand, it has been reported that some pre-Bötzinger neurons fail to maintain a rhythmic discharge in phase with phrenic nerve discharge during mechanical hyperventilation. Extracellular recordings were made from respiratory units in the pre-Bötzinger and Bötzinger complexes of 13 anaesthetised, paralysed and vagotomised rats. Central respiratory activity was monitored from the C5 phrenic nerve. During mechanical hyperventilation, several changes were observed in the phrenic neurogram. Firstly, the frequency and amplitude of integrated phrenic nerve discharge were reduced and reversibly stopped. Secondly, the patterned discharges changed from an augmenting to a variety of non-augmenting patterns in 53 of 60 cases. In some cases ( n=9) we observed that the pattern appeared to have two components, an early short duration discharge followed by a longer duration discharge. Respiratory units also started to show different firing patterns during mechanical hyperventilation. In general, they were divided into those units that fired tonically ( n=28) and units that became silent ( n=32), before phrenic nerve discharge ceased coincidently with complete apnoea. Of particular interest were those expiratory–inspiratory units in the pre-Bötzinger complex ( n=8) that narrowed their firing period towards late expiration and early inspiration during mechanical hyperventilation. Given their firing features, it is possible that these expiratory–inspiratory units may participate in generation of the early inspiratory component of phrenic nerve discharge.

Pre-Bötzinger complex functions as a central hypoxia chemosensor for respiration in vivo.

Recently, we identified a region located in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) in which activation of ionotropic excitatory amino acid receptors using DL-homocysteic acid (DLH) elicits a variety of excitatory responses in the phrenic neurogram, ranging from tonic firing to a rapid series of high-amplitude, rapid rate of rise, short-duration inspiratory bursts that are indistinguishable from gasps produced by severe systemic hypoxia. Therefore we hypothesized that this unique region is chemosensitive to hypoxia. To test this hypothesis, we examined the response to unilateral microinjection of sodium cyanide (NaCN) into the pre-BötC in chloralose- or chloralose/urethan-anesthetized vagotomized, paralyzed, mechanically ventilated cats. In all experiments, sites in the pre-BötC were functionally identified using DLH (10 mM, 21 nl) as we have previously described. All sites were histologically confirmed to be in the pre-BötC after completion of the experiment. Unilateral microinjection of NaCN (1 mM, 21 nl) into the pre-BötC produced excitation of phrenic nerve discharge in 49 of the 81 sites examined. This augmentation of inspiratory output exhibited one of the following changes in cycle timing and/or pattern: 1) a series of high-amplitude, short-duration bursts in the phrenic neurogram (a discharge similar to a gasp), 2) a tonic excitation of phrenic neurogram output, 3) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a gasplike burst), or 4) an increase in frequency of phrenic bursts accompanied by small increases or decreases in the amplitude of integrated phrenic nerve discharge. Our findings identify a locus in the brain stem in which focal hypoxia augments respiratory output. We propose that the respiratory rhythm generator in the pre-BötC has intrinsic hypoxic chemosensitivity that may play a role in hypoxia-induced gasping.

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.

Role of the Bötzinger complex in fastigial nucleus–mediated respiratory responses

We have reported that the phrenic neurogram (PN) is modulated by stimulation of the fastigial nucleus (FN) of the cerebellum. The present study was undertaken to search for brainstem site(s) involved in the FN efferent pathway to modulate phrenic nerve activities. Experiments were performed on 35 anesthetized, paralyzed, and ventilated cats, using the PN as the index of the respiratory motor output. Results showed that bilateral electrolytic lesions of the red nucleus (RN), the paramedian reticular nucleus (PRN), or the pontine respiratory group (PRG) had little effect on the ability of FN stimulation to modulate the respiratory output. However, the modulation was abolished by bilateral electrolytic lesions of the Bötzinger complex (BötC). Further studies showed that bilateral chemical inactivation of BötC neurons produced by topical microinjection of kainic acid or cobalt chloride failed to abolish the modulation. We concluded that fibers of passage, not synapses or cell bodies in the BötC, were involved in the modulatory effect of FN stimulation on the PN. The RN, PRN, and PRG appear not to be important in the neural circuitry responsible for the FN modulation of the phrenic activity. Anat Rec 254:542–548, 1999. © 1999 Wiley-Liss, Inc.

Role of the Bötzinger complex in fastigial nucleus-mediated respiratory responses.

We have reported that the phrenic neurogram (PN) is modulated by stimulation of the fastigial nucleus (FN) of the cerebellum. The present study was undertaken to search for brainstem site(s) involved in the FN efferent pathway to modulate phrenic nerve activities. Experiments were performed on 35 anesthetized, paralyzed, and ventilated cats, using the PN as the index of the respiratory motor output. Results showed that bilateral electrolytic lesions of the red nucleus (RN), the paramedian reticular nucleus (PRN), or the pontine respiratory group (PRG) had little effect on the ability of FN stimulation to modulate the respiratory output. However, the modulation was abolished by bilateral electrolytic lesions of the Bötzinger complex (BötC). Further studies showed that bilateral chemical inactivation of BötC neurons produced by topical microinjection of kainic acid or cobalt chloride failed to abolish the modulation. We concluded that fibers of passage, not synapses or cell bodies in the BötC, were involved in the modulatory effect of FN stimulation on the PN. The RN, PRN, and PRG appear not to be important in the neural circuitry responsible for the FN modulation of the phrenic activity.

Patterns of phrenic motor output evoked by chemical stimulation of neurons located in the pre-Bötzinger complex in vivo.

The pre-Bötzinger complex (pre-BötC) has been proposed to be essential for respiratory rhythm generation from work in vitro. Much less, however, is known about its role in the generation and modulation of respiratory rhythm in vivo. Therefore we examined whether chemical stimulation of the in vivo pre-BötC manifests respiratory modulation consistent with a respiratory rhythm generator. In chloralose- or chloralose/urethan-anesthetized, vagotomized cats, we recorded phrenic nerve discharge and arterial blood pressure in response to chemical stimulation of neurons located in the pre-BötC with DL-homocysteic acid (DLH; 10 mM; 21 nl). In 115 of the 122 sites examined in the pre-BötC, unilateral microinjection of DLH produced an increase in phrenic nerve discharge that was characterized by one of the following changes in cycle timing and pattern: 1) a rapid series of high-amplitude, rapid rate of rise, short-duration bursts, 2) tonic excitation (with or without respiratory oscillations), 3) an integration of the first two types of responses (i.e., tonic excitation with high-amplitude, short-duration bursts superimposed), or 4) augmented bursts in the phrenic neurogram (i.e., eupneic breath ending with a high-amplitude, short-duration burst). In 107 of these sites, the phrenic neurogram response was accompanied by an increase or decrease (>/=10 mmHg) in arterial blood pressure. Thus increases in respiratory burst frequency and production of tonic discharge of inspiratory output, both of which have been seen in vitro, as well as modulation of burst pattern can be produced by local perturbations of excitatory amino acid neurotransmission in the pre-BötC in vivo. These findings are consistent with the proposed role of this region as the locus for respiratory rhythm generation.

NMDA receptor-mediated inspiratory off-switching in pneumotaxic-disconnected cats

The pneumotaxic center is thought to govern inspiratory off-switching (IOS), and blockade of N-methyl- d-aspartate (NMDA) receptors by dizocilpine impairs IOS causing apneusis. The present study is to examine whether the NMDA receptor-mediated IOS mechanism functions in the medullary respiratory network after disconnecting the pneumotaxic center. In decerebrate and vagotomized cats, the nucleus parabrachialis medialis (NPBM) and vagus nerves were stimulated to evoke IOS and a dorsal pontine transection was performed while the central respiratory activity was recorded in phrenic neurogram. The transection eliminated the NPBM-stimulated IOS but not the vagally evoked IOS, and developed two types of respiration; eupnea in 12 and apneusis in six out of 18 cats. Apneustic respiration was not changed into eupneic one by changing the end-tidal CO 2 level. In animals displaying eupnea after the transection, dizocilpine (0.3 mg/kg i.v.) produced apneusis, characterized by a prolonged inspiration, a shortened stage 1 expiration and an unchanged stage 2 expiration. Dizocilpine caused no further change in the apneustic pattern induced by the transection. The present results suggest that the medullary respiratory network is able to generate a eupneic respiration after disconnecting the pontine pneumotaxic center, and the NMDA mechanism plays an important role in the medullary respiratory network.

Transient respiratory augmentation elicited by acute head-down tilt in the anesthetized cat.

Acute head-down tilt (AHDT, -30 degrees) in humans induces a transient ventilatory augmentation for 1-2 min accompanied by a high venous return. However, the mechanisms underlying this respiratory response remain obscure because of limitations of experiments carried out in human subjects. The present study was undertaken to determine whether AHDT-induced respiratory augmentation exists in the anesthetized, paralyzed, and ventilated cat and, if so, whether this response depends on 1) the cerebellum, 2) the carotid sinus (CS) and/or vagal afferents, and 3) elevation of central venous return. The integrated phrenic neurogram, arterial blood pressure, central venous pressure (CVP), and end-tidal PCO2 were recorded before, during, and after AHDT. The results showed that AHDT produced a transient ( approximately 2 min) enhancement of minute phrenic activity (approximately 30%) primarily via an increase in peak integrated phrenic neurogram amplitude associated with a remarkable elevation of CVP (approximately 3 min). Cerebellectomy, CS denervation, bilateral vagotomy, or clamping CVP did not affect the presence of the AHDT-induced minute phrenic activity response. These findings demonstrate that the anesthetized cat is a suitable model for investigating the mechanisms involved in AHDT-induced respiratory augmentation. Preliminary studies suggest that this response does not require the cerebellum, CS/vagal afferents, or an associated rise in central venous return.

Respiratory responses to selective blockade of carotid sinus baroreceptors in the dog.

Activation of carotid sinus (CS) baroreceptors has been shown to increase inspiratory time (TI) and expiratory time (TE) and to have a varied effect on tidal volume. The contribution of two functionally different types of baroreceptors to changes in respiratory function were examined in the current study. The techniques of DC anodal block and bupivacaine anesthetic block were used to selectively block fibers, from largest (type I) to smallest (type II) and smallest to largest, respectively, in the CS nerve (CSN) from an isolated CS in an anesthetized, paralyzed, vagotomized, artificially ventilated dog. Anodal blocking currents from 25 to 60 microA, which blocked primarily large A fibers, produced significant decreases in TI and TE and increased the slope of the average phrenic neurogram [PNG(t)], with no change in peak PNG(t). Further increases in blocking current to levels that also blocked small C fibers did not result in additional changes. Bupivacaine blockade using concentrations that blocked primarily C fibers did not block changes in TI and TE to step CS pressure changes. Increasing bupivacaine concentration to 20 mg/100 ml blocked all CSN conduction, and respiratory responses were eliminated. Therefore respiratory responses arising from CS baroreceptors appear to originate from the larger type I baroreceptors.

Integration of bronchomotor and ventilatory responses to chemoreceptor stimulation in developing sheep

We analyzed the changes induced by central chemoreceptor stimulation on the lung resistances and phrenic neurogram of anesthetized newborn (3–6 days, n=9) and 9 week old lambs ( n=3). Starting from hypocapneic apnea, 5% CO 2 inhalation evoked a reversible increase in total lung resistance in both newborn and 9 week old lambs (median=112%). The resistance increase preceded phrenic breathing and was greater for the peripheral (233%) than for the central airways (57%), independent of age. Increases in lung and airway resistance caused by CO 2 were reversed totally by atropine and only partially by apnea-producing doses of fentanyl. Our results demonstrate that parasympathetic outflow to the sheep airways is already driven by central chemoreceptor inputs during the newborn period. Even at this early age, bronchomotor responses to central chemoreceptor stimulation are more prominent in the peripheral than in the central airways and exhibit a lower threshold for activation and less sensitivity to opioid inhibition than phrenic responses.

Effects of halothane on the phrenic nerve responses to carbon dioxide mediated by carotid body chemoreceptors in vagotomized dogs.

Previous studies in dogs showed that the phrenic nerve response to an acute hypoxic stimulus was dose dependently depressed by 0.5-2.0 minimum alveolar concentration (MAC) of halothane but not abolished. Because a carbon dioxide stimulus is transduced by a different mechanism in the carotid body chemoreceptors (CBCRs) than is a hypoxic stimulus, inhalational anesthetics may preferentially depress one of these transduction processes, the central neuronal processing, or both, of the integrated responses to these two types of inputs. Carotid body chemoreceptor stimulation was produced by short (1-1.5 s), bilateral, 100% carbon dioxide in saline infusions into the carotid arteries during neural inspiration in unpremedicated, halothane-anesthetized, paralyzed, vagotomized dogs during constant mechanical ventilation. The phrenic neurogram quantified the neural inspiratory response. Four protocols were performed in the study: (1) the dose-dependent effects of halothane anesthesia (0.5-2.0 MAC) during hyperoxic hypercapnia on phrenic nerve activity, (2) the effects of three background levels of the partial pressure of carbon dioxide (PaCO2) on the magnitude of the carbon dioxide infusion responses at 1 MAC halothane, (3) the effects of anesthetic type on the magnitude of the carbon dioxide infusion response, and (4) the effects of CBCR denervation. Peak phrenic nerve activity (PPA) increased significantly during the carbon dioxide-stimulated phrenic burst in protocols 1-3; after denervation there was no response (protocol 4). Halothane produced a dose-dependent reduction in the PPA of control and carbon dioxide infusion-stimulated phrenic bursts and in the net carbon dioxide response. The net PPA responses for the different PaCO2 background levels were not different but were somewhat larger for sodium thiopental anesthesia than for 1.0 MAC halothane. The phrenic nerve response to an acute, severe carbon dioxide stimulus was dose dependently depressed by surgical doses of halothane. The observed responses to carbon dioxide infusion were mediated by the CBCRs because they were eliminated by CBCR denervation. These results suggest that the CBCR transduction and central transmission of the carbon dioxide signal in terms of inspiratory excitatory drive are not abolished at surgical levels of halothane anesthesia.

Involvement of the fastigial nuclei in vagally mediated respiratory responses.

Previous studies have demonstrated that the cerebellum, especially the fastigial nucleus (FN), is capable of modulating respiratory responses to chemical and mechanical stimuli. Because there is evidence to show projections from vagal afferents to the FN, the goal of this study was to determine the role of the FN in the respiratory reflexes elicited by activation of vagal afferents. Experiments were performed in anesthetized (chloralose), paralyzed, and artificially ventilated cats with an occipital exposure of the cerebellum. Administration of capsaicin (Cap; 5-10 micrograms/kg) via the right external jugular vein at the end of inspiration and application of lung inflation (LI; 10 cmH2O) during inspiration were carried out to stimulate nonmyelinated and myelinated vagal afferents, respectively. The phrenic neurogram was recorded as an index of the respiratory motor output. Control cardiorespiratory variables [expiratory duration (TE), arterial blood pressure] and their immediate responses to stimuli were compared before and after bilateral lesions of the FN. The results showed the following. 1) Cap injection and LI resulted in a dramatic increase in TE (apnea). 2) FN lesions did not significantly alter the control TE; however, the apneic duration induced by Cap injection was prolonged. 3) Neither FN lesions nor cerebellectomy affected the apneic duration that resulted from application of LI. 4) Cold blockade of the vagi (6-8 degrees C) eliminated the respiratory responses elicited by LI but not Cap injection; vagotomy abolished the responses to both stimuli. 5) FN lesions did not change the control ABP or its responses to either LI or Cap injection. It is concluded that the FN is involved in vagally mediated respiratory reflexes elicited by activation of nonmyelinated (C-fiber) vagal afferents.

Respiratory-related neurons of the fastigial nucleus in response to chemical and mechanical challenges.

Responses of cerebellar respiratory-related neurons (CRRNs) within the rostral fastigial nucleus and the phrenic neurogram to activation of respiratory mechano- and chemoreceptors were recorded in anesthetized, paralyzed, and ventilated cats. Respiratory challenges included the following: 1 ) cessation of the ventilator for a single breath at the end of inspiration (lung inflation) or at functional residual capacity, 2) cessation of the ventilator for multiple breaths, and 3) exposure to hypercapnia. Nineteen CRRNs having spontaneous activity during control conditions were characterized as either independent (basic, n = 14) or dependent (pump, n = 5) on the ventilator movement. Thirteen recruited CRRNs showed no respiratory-related activity until breathing was stressed. Burst durations of expiratory CRRNs were prolonged by sustained lung inflation but were inhibited when the volume was sustained at functional residual capacity; it was vice versa for inspiratory CRRNs. Multiple-breath cessation of the ventilator and hypercapnia significantly increased the firing rate and/or burst duration concomitant with changes noted in the phrenic neurogram. We conclude that CRRNs respond to respiratory inputs from CO2 chemo- and pulmonary mechanoreceptors in the absence of skeletal muscle contraction.

Involvement of the motor trigeminal nucleus in respiratory phase-switching in the cat.

We investigated the hypothesis that the motor trigeminal nucleus, consisting of expiratory motoneurons, might be influential in termination of inspiration. We addressed the issue by comparing the effects on neural respiration of a reversible, unilateral, pharmacologic blockade of the motor trigeminal nucleus (5M), the medial parabrachial nucleus (PB), and of other nearby structures that are neutral for respiration in anesthetized, vagotomized, paralyzed, and ventilated cats. The blockade was achieved by microinjections of 2% xylocaine, laced with Pontamine Sky Blue to identify sites of injections, from the tip of a penetrating microelectrode. Integrated phrenic neurograms were recorded to quantify the time of neural inspiration (TI), expiration (TE), and the peak phrenic amplitude. We found that blockade of the 5M caused a pattern of apneustic respiration, consisting of a selective prolongation of inspiratory phases that were interrupted by short expiratory pauses. In contrast, blockade of the PB resulted in a prolongation of both TI and TE, which corresponds to a mere slowing of respiration. The results confirmed important functions of the rostral pons in ventilatory control but pointed to the 5M rather than PB as a structure underlying the inspiratory off-switch. We conclude that the motor trigeminal nucleus may have a part in the pontine pneumotaxic mechanism.