Subtype composition and responses of respiratory neurons in the pre-botzinger region to pulmonary afferent inputs in dogs.

The brain stem pre-Botzinger complex (pre-BC) plays an important role in respiratory rhythm generation. However, it is not clear what function each subpopulation of neurons in the pre-BC serves. The purpose of the present studies was to identify neuronal subpopulations of the canine pre-BC and to characterize the neuronal responses of subpopulations to experimentally imposed changes in inspiratory (I) and expiratory (E) phase durations. Lung inflations and electrical stimulation of the cervical vagus nerve were used to produce changes in respiratory phase timing via the Hering-Breuer reflex. Multibarrel micropipettes were used to record neuronal activity and for pressure microejection in decerebrate, paralyzed, ventilated dogs. The pre-BC region was functionally identified by eliciting tachypneic phrenic neural responses to localized microejections of DL-homocysteic acid. Antidromic stimulation and spike-triggered averaging techniques were used to identify bulbospinal and cranial motoneurons, respectively. The results indicate that the canine pre-BC region consists of a heterogeneous mixture of propriobulbar I and E neuron subpopulations. The neuronal responses to ipsi-, contra-, and bilateral pulmonary afferent inputs indicated that I and E neurons with decrementing patterns were the only neurons with responses consistently related to phase duration. Late-I neurons were excited, but most other types of I neurons were inhibited or unresponsive. E neurons with augmenting or parabolic discharge patters were inhibited by ipsilateral inputs but excited by contra- and bilateral inputs. Late-E neurons were more frequently encountered and were inhibited by ipsi- and bilateral inputs, but excited by contralateral inputs. The results suggest that only a limited number of neuron subpopulations may be involved in rhythmogenesis, whereas many neuron types may be involved in motor pattern generation.

Differential processing of excitation by GABAergic gain modulation in canine caudal ventral respiratory group neurons.

The discharge frequency (F(n)) patterns of medullary respiratory premotor neurons are subject to potent tonic GABAergic gain modulation. Studies in other neuron types suggest that the synaptic input for tonic inhibition is located on the soma where it can affect total neuronal output. However, our preliminary data suggested that excitatory responses elicited by highly local application of glutamate receptor agonists are not gain modulated. In addition, modulation of the amplitude of spike afterhyperpolarizations can gain modulate neuronal output, and this mechanism is located near the spike initiation zone and/or soma. The purpose of this study was to determine if these two gain-modulating mechanisms have different functional locations on the somatodendritic membrane of bulbospinal inspiratory and expiratory neurons. Four-barrel micropipettes were used for extracellular single-neuron recording and pressure ejection of drugs in decerebrate, paralyzed, ventilated dogs. The net increases in F(n) due to repeated short-duration picoejections of the glutamate receptor agonist, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid (AMPA), was quantified before and during locally induced antagonism of GABA(A) receptors by bicuculline or small-conductance, calcium-activated potassium channels by apamin. The AMPA-induced net increases in F(n) were not significantly altered by BIC, although it produced large increases in the respiratory-related activity. However, the AMPA-induced net responses were amplified in accordance with the gain increase of the respiratory-related activity by apamin. These findings suggest that GABAergic gain modulation may be functionally isolated from the soma/spike initiation zone, e.g., located on a dendritic shaft. This could allow other behavioral signals requiring strong neuronal activation (e.g., coughing, sneezing, vomiting) to utilize the same neuron without being attenuated by the GABAergic modulation.

Differential modulation of respiratory neuronal discharge patterns by GABA(A) receptor and apamin-sensitive K(+) channel antagonism.

The discharge patterns of respiratory neurons of the caudal ventral respiratory group (cVRG) appear to be subject to potent GABAergic gain modulation. Local application of the GABA(A) receptor antagonist bicuculline methochloride amplifies the underlying discharge frequency (F(n)) patterns mediated by endogenous excitatory and inhibitory synaptic inputs. Gain modulation can also be produced by alterations in the amplitude of spike afterhyperpolarizations (AHPs) mediated by apamin-sensitive small-conductance Ca(2+)-activated K(+) (SK) channels. Since methyl derivatives of bicuculline (BICm) also have been shown to reduce the amplitude of AHPs, in vitro, it is possible that the BICm-induced gain modulation is due to a block of SK channels. The purpose of these studies was to determine the mechanisms by which BICm produces gain modulation and to characterize the influence of SK channels in the control of respiratory neuron discharge. Six protocols were used in this in vivo study of cVRG inspiratory (I) and expiratory (E) neurons in decerebrate, paralyzed, ventilated dogs. The protocols included characterizations of the neuronal responses to 1) BICm and apamin on the same neuron, 2) BICm during maximum apamin-induced block of AHPs, 3) apamin during maximum BICm-induced gain modulatory responses, 4) the specific GABA(A) receptor antagonist, (+)beta-hydrastine, 5) the specific GABA(A) receptor agonist, muscimol, and 6) the GABA uptake inhibitor, nipecotic acid. For protocols 3, 5, and 6, only E neurons were studied. Four-barrel micropipettes were used for extracellular single neuron recording and pressure ejection of drugs. Cycle-triggered histograms were used to quantify the F(n) patterns and to determine the drug-induced changes in the gain (slope) and offset of the F(n) patterns. Compared to apamin at maximum effective dose rates, BICm produced a 2.1-fold greater increase in peak F(n) and a 3.1-fold greater increase in average F(n). BICm and apamin produced similar increases in gain, but the offsets due to apamin were more negative. The responses to hydrastine were similar to BICm. During maximum apamin block, BICm produced an additional 112 +/- 22% increase in peak F(n). Conversely, apamin produced an additional 176 +/- 74% increase in peak F(n) during the maximum BICm-induced response. Muscimol and nipecotic acid both decreased the gain and offset of the discharge patterns. Taken together, these results suggest that the gain modulatory effect of BICm is due to a reduction of GABA(A)-ergic shunting inhibition rather than a reduction in AHPs by block of SK channels in canine cVRG neurons.

Effects of sevoflurane on excitatory neurotransmission to medullary expiratory neurons and on phrenic nerve activity in a decerebrate dog model.

BACKGROUND: Sevoflurane is a new volatile anesthetic with a pronounced respiratory depressant effect. Synaptic neurotransmission in canine expiratory bulbospinal neurons is mainly mediated by excitatory N-methyl-D-aspartatic acid (NMDA) receptor input and modulated by inhibitory gamma-aminobutyric acid type A (GABA(A)) receptors. The authors investigated the effect of sevoflurane on these mechanisms in decerebrate dogs. METHODS: Studies were performed in decerebrate, vagotomized, paralyzed and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of 1 minimum alveolar concentration (MAC; 2.4%) sevoflurane on extracellularly recorded neuronal activity was measured during localized picoejection of the glutamate agonist NMDA and the GABA(A) receptor blocker bicuculline in a two-part protocol. First, complete blockade of the GABA(A)ergic mechanism by bicuculline allowed differentiation between the effects of sevoflurane on overall GABA(A)ergic inhibition and on overall glutamatergic excitation. In a second step, the neuronal response to exogenous NMDA was used to estimate sevoflurane's effect on postsynaptic glutamatergic neurotransmission. RESULTS: One minimum alveolar concentration sevoflurane depressed the spontaneous activity of 16 expiratory neurons by 36.7+/-22.4% (mean +/- SD). Overall glutamatergic excitation was depressed 19.5+/-16.2%, and GABA(A)ergic inhibition was enhanced 18.7+/-20.6%. However, the postsynaptic response to exogenous NMDA was not significantly altered. In addition, 1 MAC sevoflurane depressed peak phrenic nerve activity by 61.8+/-17.7%. CONCLUSIONS: In the authors' in vivo expiratory neuronal model, the depressive effect of sevoflurane on synaptic neurotransmission was caused by a reduction of presynaptic glutamatergic excitation and an enhancement of GABA(A)ergic inhibition. The effects on expiratory neuronal activity were similar to halothane, but sevoflurane caused a stronger depression of phrenic nerve activity than halothane.

Effect of central CO(2) drive on lung inflation responses of expiratory bulbospinal neurons in dogs.

The purpose of these studies is to better understand the nature of the reflex interactions that control the discharge patterns of caudal medullary, expiratory (E) bulbospinal neurons. We examined the effect of central chemodrive inputs measured as arterial CO(2) tension (Pa(CO(2))) during hyperoxia on the excitatory and inhibitory components of the lung inflation responses of these neurons in thiopental sodium-anesthetized, paralyzed dogs. Data from slow ramp inflation and deflation test patterns, which were separated by several control inflation cycles, were used to produce plots of neuronal discharge frequency (F(n)) versus transpulmonary pressure (P(t)). P(t) was used as an index of the activity arising from the slowly adapting pulmonary stretch receptors (PSRs). Changes in inspired CO(2) concentrations were used to produce Pa(CO(2)) levels that ranged from 20 to 80 mmHg. The data obtained from 41 E neurons were used to derive an empirical model that quantifies the average relationship for F(n) versus both P(t) and Pa(CO(2)). This model can be used to predict the time course and magnitude of E neuronal responses to these inputs. These data suggest that the interaction between Pa(CO(2)) and PSR-mediated excitation and inhibition of F(n) is mainly additive, but synergism between Pa(CO(2)) and excitatory inputs is also present. The implications of these findings are discussed.

Relative magnitude of tonic and phasic synaptic excitation of medullary inspiratory neurons in dogs.

The relative contribution of phasic and tonic excitatory synaptic drives to the augmenting discharge patterns of inspiratory (I) neurons within the ventral respiratory group (VRG) was studied in anesthetized, ventilated, paralyzed, and vagotomized dogs. Multibarrel micropipettes were used to record simultaneously single-unit neuronal activity and pressure microejected antagonists of GABAergic, glycinergic, N-methyl-D-aspartate (NMDA) and non-NMDA glutamatergic, and cholinergic receptors. The discharge patterns were quantified via cycle-trigger histograms. The findings suggest that two-thirds of the excitatory drive to caudal VRG I neurons is tonic and mediated by NMDA receptors and the other third is ramp-like phasic and mediated by non-NMDA receptors. Cholinergic receptors do not appear to be involved. The silent expiratory phase is produced by phasic inhibition of the tonic activity, and approximately 80% of this inhibition is mediated by gamma-aminobutyric acid receptors (GABA(A)) and approximately 20% by glycine receptors. Phasic I inhibition by the I decrementing neurons does not appear to contribute to the predominantly step-ramp patterns of these I neurons. However, this decrementing inhibition may be very prominent in controlling the rate of augmentation in late-onset I neurons and those with ramp patterns lacking the step component.

Pattern formation and rhythm generation in the ventral respiratory group.

1. There is increasing evidence that the kernel of the rhythm-generating circuitry for breathing is located within a discrete subregion of a column of respiratory neurons within the ventrolateral medulla referred to as the ventral respiratory group (VRG). It is less clear how this rhythm is transformed into the precise patterns appearing on the varied motor outflows. 2. Two different approaches were used to test whether subregions of the VRG have distinct roles in rhythm or pattern generation. In one, clusters of VRG neurons were activated or inactivated by pressure injection of small volumes of neuroactive agents to activate or inactivate groups of respiratory neurons and the resulting effects on respiratory rhythm and pattern were determined. The underlying assumption was that if rhythm and pattern are generated by neurons in different VRG subregions, then we should be able to identify regions where activation of neurons predominantly alters rhythm with little effect on pattern and other regions where pattern is altered with little effect on rhythm. 3. Based on the pattern of phrenic nerve responses to injection of an excitatory amino acid (DL-homocysteate), the VRG was divided into four subdivisions arranged along the rostrocaudal axis. Injections into the three rostral regions elicited changes in both respiratory rhythm and pattern. From rostral to caudal the regions included: (i) a rostral bradypnoea region, roughly associated with the Bötzinger complex; (ii) a dysrhythmia/tachypnoea area, roughly associated with the pre-Bötzinger complex (PBC); (iii) a second caudal bradypnoea area; and, most caudally, (iv) a region from which no detectable change in respiratory motor output was elicited. 4. In a second approach, the effect of unilateral lesions of one subregion, the PBC, on the Breuer-Hering reflex changes in rhythm were determined. Activation of this reflex by lung inflation shortens inspiration and lengthens expiration (TE). 5. Unilateral lesions in the PBC attenuated the reflex lengthening of TE, but did not change baseline respiratory rhythm. 6. These findings are consistent with the concept that the VRG is not functionally homogenous, but consists of rostrocaudally arranged subregions. Neurons within the so-called PBC appear to have a dominant role in rhythm generation. Nevertheless, neurons within other subregions contribute to both rhythm and pattern generation. Thus, at least at an anatomical level resolvable by pressure injection, there appears to be a significant overlap in the circuitry generating respiratory rhythm and pattern.

Unraveling the mechanism for respiratory rhythm generation.

Breathing is generated by a neuronal network located within the caudal brainstem. One area of particular significance for respiratory rhythm generation is the pre-Bötzinger (preBotC) complex in the ventrolateral medulla. An important step towards understanding the cellular and network basis by which neurons within this region generate the respiratory rhythm was made in a recent study by Koshiya and Smith.(1) Using simultaneous image analysis and electrophysiological techniques these authors identified a discrete population of synaptically-coupled pacemaker neurons within the preBotC. They postulated that these neurons constitute the minimal essential network component (kernel) for generating the respiratory rhythm. BioEssays 22:6-9, 2000.

Effects of halothane on excitatory neurotransmission to medullary expiratory neurons in a decerebrate dog model.

BACKGROUND: The activity of canine expiratory (E) neurons in the caudal ventral respiratory group is primarily dependent on N-methyl-D-aspartic acid (NMDA) receptor-mediated excitatory chemodrive inputs and modulated by an inhibitory mechanism mediated via gamma-aminobutyric acidA (GABA(A)) receptors. In an intact canine preparation, halothane depressed the activity of these neurons mainly by reduction in overall glutamatergic excitation. A new decerebrate preparation allows comparison of the effects of halothane on these synaptic mechanisms with an anesthetic-free baseline state. METHODS: Two separate studies were performed in decerebrate, vagotomized, paralyzed, mechanically ventilated dogs during hypercapnic hyperoxia. In study 1, the effect of 1 minimum alveolar concentration (MAC) halothane on extracellularly recorded E neuronal activity was studied before and during complete GABA(A) receptor blockade by localized pressure ejection of bicuculline. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall GABA(A)-mediated inhibition and on overall NMDA receptor-mediated excitation. In study 2, the effect of 1 MAC halothane on the dose response of neurons to localized picoejection of the glutamate agonist NMDA was used to estimate halothane effect on postsynaptic glutamatergic excitatory neurotransmission. RESULTS: In study 1, the spontaneous activity of 14 E neurons was depressed 38.6 +/- 20.6% (mean +/- SD) by 1 MAC halothane. Overall excitation was depressed 31.5 +/- 15.5%. The GABAergic inhibition showed a 11.7 +/- 18.3% enhancement during halothane. In study 2, the spontaneous activity of 13 E neurons was again significantly depressed by 1 MAC halothane (27.9 +/- 10.6%), but the postsynaptic response of the neurons to exogenous NMDA was not significantly depressed by halothane (3.3 +/- 38.4%). CONCLUSIONS: Together these results suggest that in our E neuron paradigm, halothane exerted its depressive effect mainly via reduction of glutamatergic presynaptic mechanisms.

Effects of halothane on synaptic neurotransmission to medullary expiratory neurons in the ventral respiratory group of dogs.

BACKGROUND: The activity of canine expiratory neurons is primarily dependent on N-methyl-D-aspartic acid (NMDA)-receptor mediated excitatory chemodrive inputs and a powerful inhibitory gain modulatory mechanism mediated via gamma-aminobutyric acidA (GABA(A)) receptors. We examined whether the depressant effect of halothane on expiratory neuronal activity is primarily caused by a reduction in glutamatergic excitation or a potentiation of the inhibitory mechanism. METHODS: Experiments were performed in halothane-anesthetized, vagotomized, paralyzed, and mechanically ventilated dogs during hypercapnic hyperoxia. The effect of a halothane dose increase from one minimum alveolar concentration (MAC) to 2 MAC on extracellularly recorded expiratory neuronal activity was studied before and during complete GABA(A) receptor blockade by localized picoejection of bicuculline close to the neuron. Complete blockade of the inhibitory mechanism allowed differentiation between the effects of halothane on overall NMDA-mediated excitation and on GABA(A)-mediated inhibition. RESULTS: The spontaneous activity of 12 expiratory neurons was significantly depressed (18.1%) by the 1-MAC halothane dose increase. Overall glutamatergic excitation was depressed 38.3+/-12.3% (mean +/- SD) by the 1-MAC halothane increase. The prevailing GABA(A)ergic attenuation of neuronal output decreased significantly from 49.5+/-10 to 32.0+/-10.4%. Thus overall inhibition was reduced by halothane by 33.5+/-17.2%. CONCLUSIONS: These results suggest that the depressive effect of a 1-MAC halothane dose increase on expiratory neuronal activity in our in vivo preparation with an intact neural network was mainly caused by a reduction of synaptic excitatory mechanisms and not an enhancement of synaptic inhibitory mechanisms.

Differential roles of ionotropic glutamate receptors in canine medullary inspiratory neurons of the ventral respiratory group.

The relative roles of ionotropic N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors in supplying excitatory drive to inspiratory (I) augmenting pattern neurons of the ventral respiratory group were studied in anesthetized, ventilated, paralyzed, and vagotomized dogs. Multibarrel micropipettes were used to record simultaneously single-unit neuronal activity and pressure microeject the NMDA antagonist, 2-amino-5-phosphonovalerate (AP5; 2 mM), the non-NMDA antagonist 2, 3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX; 0.25 mM), and an artificial cerebrospinal fluid vehicle. Ejected volume-rates were measured directly via meniscus level changes. The moving time average of phrenic nerve activity was used to determine respiratory phase durations and to synchronize cycle-triggered histograms of the discharge patterns. Both AP5 and NBQX produced dose-dependent reductions in peak spontaneous I neuronal discharge frequency (Fn). The average (+/- SE) maximum reduction in peak Fn produced by AP5 was 69.1 +/- 4.2% and by NBQX was 47.1 +/- 3.3%. Blockade of both glutamate receptor subtypes nearly silenced these neurons, suggesting that their activity is highly dependent on excitatory synaptic drive mediated by ionotropic glutamate receptors. Differential effects were found for the two glutamatergic antagonists. AP5 produced downward, parallel shifts in the augmenting pattern of discharge, whereas NBQX reduced the slope of the augmenting discharge pattern. These results suggest that time-varying excitatory input patterns to the canine I bulbospinal neurons are mediated by non-NMDA glutamate receptors and that constant or tonic input patterns to these neurons are mediated by NMDA receptors.

Differential effects of GABAA receptor antagonists in the control of respiratory neuronal discharge patterns.

To ascertain the role of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) in shaping and controlling the phasic discharge patterns of medullary respiratory premotor neurons, localized pressure applications of the competitive GABAA receptor antagonist bicuculline (BIC) and the noncompetitive GABAA receptor antagonist picrotoxin (PIC) were studied. Multibarrel micropipettes were used in halothane anesthetized, paralyzed, ventilated, vagotomized dogs to record single unit activity from inspiratory and expiratory neurons in the caudal ventral respiratory group and to picoeject GABAA receptor antagonists. The moving time average of phrenic nerve activity was used to determine respiratory phase durations and to synchronize cycle-triggered histograms of discharge patterns. Picoejection of BIC and PIC had qualitatively different effects on the discharge patterns of respiratory neurons. BIC caused an increase in the discharge rate during the neuron's active phase without inducing activity during the neuron's normally silent phase. The resulting discharge patterns were amplified replicas (x2-3) of the underlying preejection phasic patterns. In contrast, picoejection of PIC did not increase the peak discharge rate during the neuron's active phase but induced a tonic level of activity during the neuron's normally silent phase. The maximum effective BIC dose (15 +/- 1.8 pmol/min) was considerably smaller than that for PIC (280 +/- 53 pmol/min). These findings suggest that GABAA receptors with differential pharmacology mediate distinct functions within the same neuron, 1) gain modulation that is BIC sensitive but PIC insensitive and 2) silent-phase inhibition blocked by PIC. These studies also suggest that the choice of an antagonist is an important consideration in the determination of GABA receptor function within the respiratory motor control system.

Improved method of canine decerebration.

We describe an improved decerebration method for dogs that is suitable for studies of brain stem neurons in the absence of anesthesia. Previously reported techniques of canine decerebration often lead to respiratory and hemodynamic instability and lack of typical decerebrate rigidity. We have developed a precise, visually controlled, midcollicular brain stem transection technique that overcomes these problems. Our method results in only moderate blood loss while preserving carotid and basilar artery circulations. Consistent levels of brain stem transection routinely lead to stable postdecerebration hemodynamic parameters, allowing prolonged brain stem neuronal recordings. The same model should also be useful for a variety of studies involving other physiological systems in dogs in the absence of anesthesia and for studies of anesthetic effects.

Modulation of the synaptic drive to respiratory premotor and motor neurons.

The characteristics of GABAergic inhibitory modulation of respiratory bulbospinal neuronal activity and short-term potentiation (STP) of phrenic motoneuronal activity were studied. Extracellular unit recording and picoejection techniques in anesthetized dogs showed that both the spontaneous rhythmic and reflexly induced discharge patterns of inspiratory (I) and expiratory (E) premotor neurons were proportionately amplified by the localized application of picomole amounts of bicuculline (Bic), a competitive GABAA antagonist. Intracellular recording and paired-pulse stimulation techniques in anesthetized rats demonstrated an STP of phrenic motor output that appears to be mediated by NMDA receptors and is associated with facilitation of EPSPs and prolonged depolarization of individual phrenic motoneurons. We speculate that both GABAergic gain modulation of premotor neuronal activity and NMDA-mediated STP of phrenic activity may be neural substrates which are involved with the optimization of respiratory and non-respiratory behaviors, via adaptive and/or differential control of breathing.

Dose-dependent effects of halothane on the phrenic nerve responses to acute hypoxia in vagotomized dogs.

BACKGROUND: Previous studies in dogs and humans suggest that the carotid body chemoreceptor response to hypoxia is selectively impaired by halothane. The present studies in an open-loop canine preparation were performed to better delineate the effects of anesthetic concentrations of halothane on the carotid body chemoreceptor-mediated phrenic nerve response to an acute hypoxic stimulus. METHODS: Three protocols were performed to study the effects of halothane anesthesia on the phrenic nerve response to 1 min of isocapnic hypoxia (partial pressure of oxygen [PaO2] at peak hypoxia, 35-38 mmHg) in unpremedicated, anesthetized, paralyzed, vagotomized dogs during constant mechanical ventilation. In protocol 1, the dose-dependent effects of halothane from 0.5-2.0 minimum alveolar concentration (MAC) on the hypoxic response during moderate hypercapnia (partial pressure of carbon dioxide [PaCO2], 60-65 mmHg) were studied in 10 animals. In protocol 2, the hypoxic responses at 1 MAC halothane near normocapnia (PaCO2, 40-45 mmHg) and during moderate hypercapnia were compared in an additional four animals. In protocol 3, the hypoxic response of 4 of 10 dogs from protocol 1 was also studied under sodium thiopental (STP) anesthesia after they completed protocol 1. RESULTS: Protocol 1: Peak phrenic nerve activity (PPA) increased significantly during the hypoxic runs compared with the isocapnic hyperoxic controls at all halothane doses. The phrenic nerve response to the hypoxic stimulus was present even at the 2 MAC dose. Protocol 2: The net hypoxic responses for the two carbon dioxide background levels at 1 MAC were not significantly different. Protocol 3: The net hypoxic response of PPA for the STP anesthetic was not significantly different from the 1 MAC halothane dose. Bilateral carotid sinus denervation abolished the PPA response to hypoxia. CONCLUSIONS: The phrenic nerve response to an acute, moderately severe isocapnic hypoxic stimulus is dose-dependently depressed but not abolished by surgical doses of halothane. This analysis does not suggest a selective depression of the carotid body chemoreceptor response by halothane. The observed hypoxic phrenic response was mediated by the carotid body chemoreceptors in vagotomized dogs because bilateral carotid sinus denervation abolished all increases in PPA.

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

BACKGROUND: 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. METHODS: 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. RESULTS: 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. CONCLUSIONS: 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.

Reflex effects of stimulation of sympathetic afferents on the triangularis sterni.

The purpose of this study was to determine whether contralateral inhibition of the triangularis sterni is produced by stimulation of intrathoracic sympathetic afferents. Dogs were anesthetized with sodium pentobarbital and placed on positive pressure ventilation. The chest was opened through a mid-sternal incision. Diaphragm and left and right triangularis sterni EMGs were recorded, post-vagotomy, before and during electrical stimulation of the left ventral ansa subclavia (VA), vagosympathetic trunk, ventrolateral and ventromedial cardiac nerves and, when present, the stellate cardiac nerve. Peak of the phasic diaphragm EMG and expiratory time were not significantly affected by stimulation of the VA. A significant decrease in inspiratory time was observed. Ipsilateral excitation and contralateral inhibition of the left and right triangularis sterni EMGs, respectively, were produced by stimulation of the VA. Stimulation of the other intrathoracic nerves produced a similar pattern of results. Conduction velocity determinations suggested that the afferents which produced the reflex responses are, at least in part, small A fibers.

NMDA receptor-mediated transmission of carotid body chemoreceptor input to expiratory bulbospinal neurones in dogs.

1. This study tested the hypothesis that excitatory amino acid receptors mediate the excitatory response of expiratory bulbospinal neurones to carotid body chemoreceptor inputs. 2. Studies were carried out in thiopental sodium anaesthetized, paralysed, ventilated, vagotomized dogs. 3. Brisk, short-duration chemoreceptor activation was produced by bilateral bolus injections of CO2-saturated saline (PCO2 > 700 mmHg) into the autoperfused carotid arteries. A pressurized-reservoir-solenoid valve system was used to deliver the CO2 bolus injections just prior to the onset of the neural expiratory phase, as determined from the phrenic neurogram, about once per minute. 4. Multibarrelled micropipettes were used to record neuronal unit activity and deliver neurotransmitter agents. Net responses of expiratory bulbospinal neurones to peripheral chemoreceptor activation were determined by subtracting the mean discharge frequencies (Fn) during three control expiratory cycles from the Fn during administration of a CO2 test bolus. The role of excitatory amino acid receptors in mediating this response was determined by comparing the baseline and bolus expiratory neuronal Fn before, during and after the pressure microejection of the NMDA receptor antagonist 2-amino-5-phosphonovalerate (AP5) or the non-NMDA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulphamoyl- benzo(f)quinoxaline (NBQX). Ejection rates of AP5 and NBQX were measured by monitoring the movement of the pipette meniscus. 5. AP5 reduced Fn during both the control and bolus cycles, as well as reducing the change in Fn between control and bolus cycles. NBQX had no effect on either baseline or bolus responses. 6. AP5 did not prevent excitation of expiratory bulbospinal neurones by AMPA. Coadministration of AMPA with AP5 prevented the AP5-mediated decrease in Fn but not the dose-dependent reduction in the CO2 bolus response. 7. Taken together, these data strongly suggest that the carotid chemoreceptor-mediated excitation of expiratory bulbospinal neurones is dependent on NMDA but not non-NMDA glutamate receptors.

Dose-dependent effects of halothane on the carbon dioxide responses of expiratory and inspiratory bulbospinal neurons and the phrenic nerve activities in dogs.

BACKGROUND: Expiratory bulbospinal and inspiratory bulbospinal neurons in the ventral respiratory group provide drive for thoracoabdominal expiratory and phrenic and thoracic inspiratory motor neurons. Potent inhalational agents such as halothane may have differential effects on inspiratory and expiratory neurons, but detailed studies comparing neurons at a homologous level are lacking. METHODS: The dose-dependent effects of anesthesia with 1.0-2.5 minimum alveolar concentration halothane on the CO2 responses of single expiratory and inspiratory bulbospinal neurons of the ventral respiratory group and on phrenic neural activities were studied in nonpremedicated, anesthetized, paralyzed, vagotomized dogs. Hyperventilation with O2 and the addition of CO2-O2 mixtures were used to produce low, medium, and high steady-state levels of central CO2 drive. RESULTS: Peak neuron discharge frequency decreased progressively with increasing halothane dose at all levels of CO2 drive for both types of neurons. The sensitivities of inspiratory and expiratory bulbospinal neuronal activities to halothane were not significantly different from one another, whereas the sensitivity to halothane of the peak phrenic activity was markedly greater than those of the neurons. Increasing halothane dose caused a downward, predominantly parallel shift of the CO2 response curves. Phrenic nerve activity also showed a decrease in slope of the CO2 response. CONCLUSIONS: The activities of respiratory premotor neurons are less depressed by increasing doses of halothane than is phrenic nerve activity. The greater depression of phrenic activity may result from additional anesthetic actions on the efferent motor pathways, resulting in decreased descending synaptic inputs to phrenic motor neurons.

Interaction between chemoreceptor and stretch receptor inputs at medullary respiratory neurons.

The interaction between afferent inputs from carotid body chemoreceptors (CCRs) and from slowly adapting pulmonary stretch receptors (PSRs) on the discharge patterns of medullary inspiratory (I) and expiratory (E) neurons was characterized in thiopental sodium-anesthetized, paralyzed, ventilated dogs. A cycle-triggered ventilator was used to produce control and test pulmonary afferent input patterns. The CCRs were stimulated by phase-synchronized bolus injections of CO2-saturated saline into the common carotid arteries. Only those neurons whose discharge time course was altered by both inflation and CCR activation were studied. The dorsal respiratory group (DRG) I inflation-insensitive neurons were also included. Cycle-triggered histograms of unit activity were obtained for the neuronal responses to inflation, CO2 bolus, and their combination, as well as for the spontaneous control condition. Linearity of the interaction was tested by comparing the sum of the net individual responses to the net response of the combined afferent inputs. The results suggest that a linear (additive) interaction between CCR and PSR inputs exists for the DRG I inflation-sensitive neurons, the ventral respiratory group (VRG) I decrementing, and caudal VRG E augmenting neurons, while a nonadditive interaction exists for caudal VRG E decrementing bulbospinal neurons. The implications of these findings are discussed.