Respiratory effects of pneumotaxic center lesions and subsequent vagotomy in chronic cats

Changes in pontine and preBötzinger/Bötzinger complex neuronal activity during remifentanil-induced respiratory depression in decerebrate dogs

Introduction: In vivo studies using selective, localized opioid antagonist injections or localized opioid receptor deletion have identified that systemic opioids dose-dependently depress respiratory output through effects in multiple respiratory-related brainstem areas. Methods: With approval of the subcommittee on animal studies of the Zablocki VA Medical Center, experiments were performed in 53 decerebrate, vagotomized, mechanically ventilated dogs of either sex during isocapnic hyperoxia. We performed single neuron recordings in the Pontine Respiratory Group (PRG, n = 432) and preBötzinger/Bötzinger complex region (preBötC/BötC, n = 213) before and during intravenous remifentanil infusion (0.1-1 mcg/kg/min) and then until complete recovery of phrenic nerve activity. A generalized linear mixed model was used to determine changes in Fn with remifentanil and the statistical association between remifentanil-induced changes in Fn and changes in inspiratory and expiratory duration and peak phrenic activity. Analysis was controlled via random effects for animal, run, and neuron type. Results: Remifentanil decreased Fn in most neuron subtypes in the preBötC/BötC as well as in inspiratory (I), inspiratory-expiratory, expiratory (E) decrementing and non-respiratory modulated neurons in the PRG. The decrease in PRG inspiratory and non-respiratory modulated neuronal activity was associated with an increase in inspiratory duration. In the preBötC, the decrease in I-decrementing neuron activity was associated with an increase in expiratory and of E-decrementing activity with an increase in inspiratory duration. In contrast, decreased activity of I-augmenting neurons was associated with a decrease in inspiratory duration. Discussion: While statistical associations do not necessarily imply a causal relationship, our data suggest mechanisms for the opioid-induced increase in expiratory duration in the PRG and preBötC/BötC and how inspiratory failure at high opioid doses may result from a decrease in activity and decrease in slope of the pre-inspiratory ramp-like activity in preBötC/BötC pre-inspiratory neurons combined with a depression of preBötC/BötC I-augmenting neurons. Additional studies must clarify whether the observed changes in neuronal activity are due to direct neuronal inhibition or decreased excitatory inputs.

Neurochemistry of the Kölliker-Fuse nucleus from a respiratory perspective

The Kölliker-Fuse nucleus (KF) is a functionally distinct component of the parabrachial complex, located in the dorsolateral pons of mammals. The KF has a major role in respiration and upper airway control. A comprehensive understanding of the KF and its contributions to respiratory function and dysfunction requires an appreciation for its neurochemical characteristics. The goal of this review is to summarize the diverse neurochemical composition of the KF, focusing on the neurotransmitters, neuromodulators, and neuropeptides present. We also include a description of the receptors expressed on KF neurons and transporters involved in each system, as well as their putative roles in respiratory physiology. Finally, we provide a short section reviewing the literature regarding neurochemical changes in the KF in the context of respiratory dysfunction observed in SIDS and Rett syndrome. By over-viewing the current literature on the neurochemical composition of the KF, this review will serve to aid a wide range of topics in the future research into the neural control of respiration in health and disease.

The Use of Neuromodulation for Symptom Management

Pain and other symptoms of autonomic dysregulation such as hypertension, dyspnoea and bladder instability can lead to intractable suffering. Incorporation of neuromodulation into symptom management, including palliative care treatment protocols, is becoming a viable option scientifically, ethically, and economically in order to relieve suffering. It provides further opportunity for symptom control that cannot otherwise be provided by pharmacology and other conventional methods.

CO2 retention: The key to stopping hiccups

BACKGROUND

While investigating the mechanisms behind hiccups, our team discovered what could be the sufficient physiological conditions for terminating even persistent cases.

METHODS

To investigate the role of CO₂ retention, a healthy male volunteer was asked to perform three kinds of rebreathing experiments using different materials: (I) a 20 L air-filled plastic bag, (II) a 20 L air-filled plastic bag with a 1.5 × 1.5 cm hole and (III) a 20 L oxygen-filled plastic bag. During each experiment, CO₂ level upon expiration (EtCO₂ ) and inspiration (InspCO₂ ) were measured until the volunteer gave up. Once the safety of this manoeuvre was demonstrated with the volunteer, we performed the technique using the materials from experiment (I) on two actual patients with persistent hiccups.

RESULTS

In experiments (I) and (III), InspCO₂ increased from the beginning and reached almost the same level as EtCO₂ after 90 seconds. Both levels continued simultaneously increasing, finally reaching 56 mm Hg in (I) and 79 mm Hg in (III), respectively. In (II), both increased; however, after 120 seconds, EtCO₂ plateaued at 47 mm Hg and InspCO₂ at 37 mm Hg. In the actual patients, both CO₂ levels reached the same value of 35.9 mm Hg at 60 seconds and 37.0 mm Hg at 90 seconds, and hiccups stopped at 195 seconds and at 359 seconds when EtCO₂ reached 50 mm Hg and 53 mm Hg, respectively.

CONCLUSION

The study determined that to successfully obstruct the mechanisms causing hiccups, it is necessary that the level of InspCO₂ not only increases at the same level as EtCO₂ , but also reaches approximately 50 mm Hg.

Deficiency of GABAergic synaptic inhibition in the Kölliker-Fuse area underlies respiratory dysrhythmia in a mouse model of Rett syndrome

KEY POINTS

Life threatening breathing irregularity and central apnoeas are highly prevalent in children suffering from Rett syndrome. Abnormalities in inhibitory synaptic transmission have been associated with the physiopathology of this syndrome, and may underlie the respiratory disorder. In a mouse model of Rett syndrome, GABAergic terminal projections are markedly reduced in the Kölliker-Fuse nucleus (KF) in the dorsolateral pons, an important centre for control of respiratory rhythm regularity. Administration of a drug that augments endogenous GABA localized to this region of the pons reduced the incidence of apnoea and the respiratory irregularity of Rett female mice. Conversely, the respiratory disorder was recapitulated by blocking GABAergic transmission in the KF area of healthy rats. This study helps us understand the mechanism for generation of respiratory abnormality in Rett syndrome, pinpoints a brain site responsible and provides a clear anatomical target for the development of a translatable drug treatment. Central apnoeas and respiratory irregularity are a common feature in Rett syndrome (RTT), a neurodevelopmental disorder most often caused by mutations in the methyl-CpG-binding protein 2 gene (MECP2). We used a MECP2 deficient mouse model of RTT as a strategy to obtain insights into the neurobiology of the disease and into mechanisms essential for respiratory rhythmicity during normal breathing. Previously, we showed that, systemic administration of a GABA reuptake blocker in MECP2 deficient mice markedly reduced the occurrence of central apnoeas. Further, we found that, during central apnoeas, post-inspiratory drive (adductor motor) to the upper airways was enhanced in amplitude and duration in Mecp2 heterozygous female mice. Since the pontine Kölliker-Fuse area (KF) drives post-inspiration, suppresses inspiration, and can reset the respiratory oscillator phase, we hypothesized that synaptic inhibition in this area is essential for respiratory rhythm regularity. In this study, we found that: (i) Mecp2 heterozygous mice showed deficiency of GABA perisomatic bouton-like puncta and processes in the KF nucleus; (ii) blockade of GABA reuptake in the KF of RTT mice reduced breathing irregularity; (iii) conversely, blockade of GABAA receptors in the KF of healthy rats mimicked the RTT respiratory phenotype of recurrent central apnoeas and prolonged post-inspiratory activity. Our results show that reductions in synaptic inhibition within the KF induce rhythm irregularity whereas boosting GABA transmission reduces respiratory arrhythmia in a murine model of RTT. Our data suggest that manipulation of synaptic inhibition in KF may be a clinically important strategy for alleviating the life threatening respiratory disorders in RTT.

Pontine mechanisms of respiratory control

Pontine respiratory nuclei provide synaptic input to medullary rhythmogenic circuits to shape and adapt the breathing pattern. An understanding of this statement depends on appreciating breathing as a behavior, rather than a stereotypic rhythm. In this review, we focus on the pontine-mediated inspiratory off-switch (IOS) associated with postinspiratory glottal constriction. Further, IOS is examined in the context of pontine regulation of glottal resistance in response to multimodal sensory inputs and higher commands, which in turn rules timing, duration, and patterning of respiratory airflow. In addition, network plasticity in respiratory control emerges during the development of the pons. Synaptic plasticity is required for dynamic and efficient modulation of the expiratory breathing pattern to cope with rapid changes from eupneic to adaptive breathing linked to exploratory (foraging and sniffing) and expulsive (vocalizing, coughing, sneezing, and retching) behaviors, as well as conveyance of basic emotions. The speed and complexity of changes in the breathing pattern of behaving animals implies that "learning to breathe" is necessary to adjust to changing internal and external states to maintain homeostasis and survival.

The emerging role of the parabrachial complex in the generation of wakefulness drive and its implication for respiratory control

The parabrachial complex is classically seen as a major neural knot that transmits viscero- and somatosensory information toward the limbic and thalamic forebrain. In the present review we summarize recent findings that imply an emerging role of the parabrachial complex as an integral part of the ascending reticular arousal system, which promotes wakefulness and cortical activation. The ascending parabrachial projections that target wake-promoting hypothalamic areas and the basal forebrain are largely glutamatergic. Such fast synaptic transmission could be even more significant in promoting wakefulness and its characteristic pattern of cortical activation than the cholinergic or mono-aminergic ascending pathways that have been emphasized extensively in the past. A similar role of the parabrachial complex could also apply for its more established function in control of breathing. Here the parabrachial respiratory neurons may modulate and adapt breathing via the control of respiratory phase transition and upper airway patency, particularly during respiratory and non-respiratory behavior associated with wakefulness.

Sleep-disordered breathing: effects on brain structure and function

Sleep-disordered breathing is accompanied by neural injury that affects a wide range of physiological systems which include processes for sensing chemoreception and airflow, driving respiratory musculature, timing circuitry for coordination of breathing patterning, and integration of blood pressure mechanisms with respiration. The damage also occurs in regions mediating emotion and mood, as well as areas regulating memory and cognitive functioning, and appears in structures that serve significant glycemic control processes. The injured structures include brain areas involved in hormone release and action of major neurotransmitters, including those playing a role in depression. The injury is reflected in a range of structural magnetic resonance procedures, and also appears as functional distortions of evoked activity in brain areas mediating vital autonomic and breathing functions. The damage is preferentially unilateral, and includes axonal projections; the asymmetry of the injury poses unique concerns for sympathetic discharge and potential consequences for arrhythmia. Sleep-disordered breathing should be viewed as a condition that includes central nervous system injury and impaired function; the processes underlying injury remain unclear.

Controlling the Lungs Via the Brain: A Novel Neurosurgical Method to Improve Lung Function in Humans

BACKGROUND:

Deep brain stimulation (DBS) of subcortical brain areas such as the periaqueductal grey and subthalamic nucleus has been shown to alter cardiovascular autonomic performance. The supramedullary circuitry controlling respiratory airways is not well defined and has not been tested in humans.

OBJECTIVE:

To use direct electric stimulation via DBS macroelectrodes to test whether airway resistance could be manipulated by these areas in awake humans.

METHODS:

Thirty-seven patients with in-dwelling deep brain electrodes for movement disorders or chronic pain underwent spirometry according to the European Respiratory Society guidelines. Testing was performed randomly 3 times on stimulation and 3 times off stimulation; patients were blinded to the test. Thoracic diameter changes were measured by a circumferential pressure-sensitive thoracic band. Ten periaqueductal grey and 10 subthalamic nucleus patients were tested. To control for confounding pain and movement disorder relief, the sensory thalamus in 7 patients and globus pallidus interna in 10 patients, respectively, were also tested.

RESULTS:

Peak expiratory flow rate (PEFR) increased significantly with periaqueductal grey and subthalamic nucleus stimulation by up to 14% (P = .02 and .005, respectively, paired-samples Student t tests). Stimulation of control nuclei produced no significant PEFR change. Similarly, changes in thoracic diameter reflecting skeletal activity rather than airway caliber did not correlate with the improvement in PEFR. Forced expiratory volume in 1 second was unchanged by stimulation.

CONCLUSION:

DBS can improve PEFR in chronic pain and movement disorder patients. This finding provides insights into the neural modulation of respiratory performance and may explain some of the subjective benefits of DBS.

The pontine respiratory group, particularly the Kölliker-Fuse nucleus, mediates phases of the hypoxic ventilatory response in unanesthetized goats

The objective of the present study was to test the hypothesis that, in the in vivo awake goat model, perturbation/lesion in the pontine respiratory group (PRG) would decrease the sensitivity to hypercapnia and hypoxia. The study reported herein was part of two larger studies in which cholinergic modulation in the PRG was attenuated by microdialysis of atropine and subsequently ibotenic acid injections neurotoxically lesioned the PRG. In 14 goats, cannula were bilaterally implanted into either the lateral (n=4) or medial (n=4) parabrachial nuclei or the Kölliker-Fuse nucleus (KFN, n=6). Before and after cannula implantation, microdialysis of atropine, and injection of ibotenic acid, hypercapnic and hypoxic ventilatory sensitivities were assessed. Hypercapnic sensitivity was assessed by three 5-min periods at 3, 5, and 7% inspired CO2. In all groups of goats, CO2 sensitivity was unaffected (P>0.05) by any PRG perturbations/lesions. Hypoxic sensitivity was assessed with a 30-min period at 10.8% inspired O2. The response to hypoxia was typically triphasic, with a phase 1 increase in pulmonary ventilation, a phase 2 roll-off, and a phase 3 prolonged increase associated with shivering and increased metabolic rate and body temperature. In all groups of goats, the phase 1 of the hypoxic ventilatory responses was unaffected by any PRG perturbations/lesions, and there were no consistent effects on the phase 2 responses. However, in the KFN group of goats, the phase 3 ventilatory, shivering, metabolic rate, and temperature responses were markedly attenuated after the atropine dialysis studies, and the attenuation persisted after the ibotenic acid studies. These findings support an integrative or modulatory role for the KFN in the phase 3 responses to hypoxia.

Noeud vital for breathing in the brainstem: gasping--yes, eupnoea--doubtful

For the past 200 years, various regions of the brainstem have been proposed as a 'noeud vital' for breathing-a critical region which, when destroyed, results in an irreversible cessation of breathing and death. Complicating this search for a noeud vital is the extensive network of neurons that comprises the brainstem respiratory control system of pons and medulla. Does a cessation of breathing following ablation of any region reflect the removal of a critical set of neurons whose activity generates the respiratory rhythm or does it reflect the interruption of one component of the neuronal circuit, such that this circuit cannot function, at least temporarily? An additional complication is that in contemporary neuroscience, a number of in vitro preparations have been introduced for the study of the generation of the respiratory rhythms. However, how are the rhythms that these preparations generate related to normal breathing? Are these rhythms similar to those of gasping, which is recruited when normal, eupnoeic breathing fails, or are these rhythms unique to the in vitro preparation and not related to any breathing pattern in vivo?

Learning to breathe: control of the inspiratory-expiratory phase transition shifts from sensory- to central-dominated during postnatal development in rats

The hallmark of the dynamic regulation of the transitions between inspiration and expiration is the timing of the inspiratory off-switch (IOS) mechanisms. IOS is mediated by pulmonary vagal afferent feedback (Breuer-Hering reflex) and by central interactions involving the Kölliker-Fuse nuclei (KFn). We hypothesized that the balance between these two mechanisms controlling IOS may change during postnatal development. We tested this hypothesis by comparing neural responses to repetitive rhythmic vagal stimulation, at a stimulation frequency that paces baseline breathing, using in situ perfused brainstem preparations of rats at different postnatal ages. At ages < P15 (P, postnatal days), phrenic nerve activity (PNA) was immediately paced and entrained to the afferent input and this pattern remained unchanged by repetitive stimulations, indicating that vagal input stereotypically dominated the control of IOS. In contrast, PNA entrainment at > P15 was initially insignificant, but increased after repetitive vagal stimulation or lung inflation. This progressive adaption of PNA to the pattern of the sensory input was accompanied by the emergence of anticipatory centrally mediated IOS preceding the stimulus trains. The anticipatory IOS was blocked by bilateral microinjections of NMDA receptor antagonists into the KFn and PNA was immediately paced and entrained, as it was seen at ages < P15. We conclude that as postnatal maturation advances, synaptic mechanisms involving NMDA receptors in the KFn can override the vagally evoked IOS after 'training' using repetitive stimulation trials. The anticipatory IOS may imply a hitherto undescribed form of pattern learning and recall in convergent sensory and central synaptic pathways that mediate IOS.

Normal breathing pattern and arterial blood gases in awake and sleeping goats after near total destruction of the presumed pre-Botzinger complex and the surrounding region

Abrupt neurotoxic destruction of >70% of the pre-Bötzinger complex (preBötzC) in awake goats results in respiratory and cardiac failure (Wenninger JM, Pan LG, Klum L, Leekley T, Bastastic J, Hodges MR, Feroah TR, Davis S, Forster HV. J Appl Physiol 97: 1629-1636, 2004). However, in reduced preparations, rhythmic respiratory activity has been found in other areas of the brain stem (Huang Q, St. John WM. J Appl Physiol 64: 1405-1411, 1988; Janczewski WA, Feldman JL. J Physiol 570: 407-420, 2006; Lieske SP, Thoby-Brisson M, Telgkamo P, Ramierz JM. Nature Neurosci 3: 600-607, 2000; St. John WM, Bledsoe TA. J Appl Physiol 59: 684-690, 1985); thus we hypothesized that, when the preBötzC is destroyed incrementally over weeks, time-dependent plasticity within the respiratory network will result in a respiratory rhythm capable of maintaining normal blood gases. Microtubules were bilaterally implanted into the presumed preBötzC of seven goats. After recovery from surgery, studies were completed to establish baseline values for respiratory parameters. At weekly intervals, increasing volumes (in order 0.5, 1, 5, and 10 microl) of ibotenic acid (IA; 50 mM) were then injected into the preBötzC. All IA injections resulted in an acute tachypnea and dysrhythmia featuring augmented breaths, apneas, and increased breath-to-breath variation in breathing. In studies at night, apneas were nearly all central and occurred in the awake state. Breath-to-breath variation in breathing was greater (P < 0.05) during wakefulness than during non-rapid eye movement sleep. However, one week after the final IA injection, the breathing pattern, breath-to-breath variation, and arterial blood gases and pH were unchanged from baseline, but there was a 20% decrease in respiratory frequency (f) and CO(2) sensitivity (P < 0.05), as well as a 40% decrease in the ventilatory response to hypoxia (P < 0.001). In subsequent histological analysis of the presumed preBötzC region of lesioned goats, it was determined that there was a 90 and 92% reduction from control goats in total and neurokinin-1 receptor neurons, respectively. Therefore, it was concluded that 1) the dysrhythmic effects on breathing are state dependent; and 2) after incremental, near total destruction of the presumed preBötzC region, time-dependent plasticity within the respiratory network provides a rhythm capable of sustaining normal arterial blood gases.

Effects of intertrigeminal region NMDA and non-NMDA receptors on respiratory responses in rats

Respiratory disturbance, including apnea, can be induced by microinjection of glutamate into the intertrigeminal region (ITR) of the lateral pons, a region that is anatomically coupled to both the dorsal and ventral respiratory groups of the medulla. We showed that the ITR plays a functional role in regulating both vagal reflex apnea and spontaneous sleep-related apnea in rats, but the mechanisms have not been determined. This study shows that functional NMDA receptors are expressed in the ITR since the blockade of these receptors by AP5, a specific NMDA receptor antagonist, was fully effective in blocking apnea induced by glutamate injection within this region. Selective blockade of ITR NMDA receptors had no effect on the immediate apnea evoked by an intravenous 5-HT bolus, whereas the nonspecific glutamate receptor antagonist kynurenic acid significantly increased the duration of this vagal reflex apnea. These findings are of interest because pontine NMDA receptors participate in inspiratory off-switch mechanisms and have been implicated in various short- and long-term potentiation and depression phenomena. These data support the involvement of ITR non-NMDA receptors in modulation of reflex apnea per se, whereas NMDA receptors play a role in damping respiratory responses to transient disturbances.

Role in the inspiratory off-switch of vagal inputs to rostral pontine inspiratory-modulated neurons

Neurons of the pontine respiratory group (PRG) in the region of the nucleus parabrachialis medialis and the Kolliker-Fuse nucleus are believed to play an important role in promoting the inspiratory (I) off-switch (IOS). In decerebrate gallamine-paralyzed cats ventilated with a cycle-triggered pump system (lung inflation during the neural I phase), we studied the effects of vagal (V) afferent inputs on firing of I-modulated neurons (the most numerous population in PRG) and on I duration. The predominant V effect on unit activity was inhibitory, as shown by two types of test: (a) withholding of inflation during an I phase, which produced increase of unit firing and of its respiratory modulation (58/66 neurons in 14 cats), indicating disinhibition due to removal of phasic V input; (b) delivery of afferent V stimulus trains during a no-inflation I phase, which produced decrease of unit firing and of its respiratory modulation (20 neurons). In addition, application of V input during the neural expiratory (E) phase, which lengthened E phase duration, produced no effect on the neurons' firing, suggesting that the inhibition during I was presynaptic in origin. The results may be interpreted by the hypothesis that the medullary late-I presumptive IOS neurons receive excitatory inputs from the PRG I-modulated neurons as well as from V afferents.. With relatively strong V input, this pontine excitatory output is reduced by inhibition, whereas with relatively weak V input that excitatory output is increased due to reduction of inhibition. Thus the pontine and the V influences on the IOS can operate in a complementary manner dependent on state.

Functional and structural models of pontine modulation of mechanoreceptor and chemoreceptor reflexes

The dorsolateral and ventrolateral pons (dl-pons, vl-pons) are critical brainstem structures mediating the plasticity of the Hering-Breuer mechanoreflex (HBR) and carotid chemoreflex (CCR). Review of anatomical evidence indicates that dl-pons and vl-pons are connected reciprocally with one another and with medullary nucleus tractus solitarius (NTS) and ventral respiratory group (VRG). With this structural map, functional models of HBR and CCR are proposed in which the respiratory rhythm is modulated by short-term depression (STD) or potentiation (STP) of corresponding primary NTS-VRG and auxiliary pons-VRG excitatory or inhibitory pathways. Behaviorally, STD and STP of respiratory reflexes are akin to non-associative learning such as habituation, sensitization or desensitization to afferent inputs. Computationally, the STD and STP effects amount to signal differentiation and integration in the time domain, or high-pass and low-pass filtering in the frequency domain, respectively. These functional and structural models of pontomedullary signal processing provide a novel conceptual framework that unifies a wealth of experimental observations regarding mechanoreceptor and chemoreceptor reflex control of breathing.

Role of pontile mechanisms in the neurogenesis of eupnea

We have proposed a "switching concept" for the neurogenesis of ventilatory activity. Eupnea reflects the output of a pontomedullary neuronal circuit, whereas gasping is generated by medullary pacemaker mechanisms. Pontile mechanisms, then, are hypothesized to play a fundamental role in the neurogenesis of eupnea. If pontile mechanisms do play such a critical role, several criteria must be fulfilled. First, perturbations of pontile regions must alter eupnea under all experimental conditions. Second, neuronal activities that are consistent with generating the eupneic rhythm must be recorded in pons. Finally, medullary mechanisms alone cannot fully explain the neurogenesis of eupnea. Evidence from previous studies that support the validity of these criteria is presented herein. We conclude that pontile mechanisms play a critical role in the neurogenesis of eupnea.

Intertrigeminal region attenuates reflex apnea and stabilizes respiratory pattern in rats

The goal of the present study was to determine whether the newly described anatomical pathway involving the pontine intertrigeminal region (ITR), (Chamberlin and Saper, 1998), has a physiological role in mediating or modulating vagally-induced reflex apnea. We explored the ITR impact on vagal reflex apnea elicited by intravenuously injected 5-HT in ten anesthetized rats. The animals had a catheter inserted into the femoral vein for administration of 5-HT (0.00375 mg) and respiration was recorded by piezo-electric crystal. Multibarrel pipettes were used to pressure inject glutamate (5-10 nl, 10 mM), kynurenic acid (10 nl, 50 mM, a glutamate receptor antagonist), and red dye into the ITR, unilaterally and bilaterally. Intravenous administration of 5-HT produced an immediate 3-s apnea. Microinjections of glutamate into the ITR produced apneas, while microinjections of kynurenic acid blocked the glutamate effect. Following glutamate antagonism, subsequent administration of 5-HT produced apneas of much longer duration (8 s). Acute ponto-medullary transection in two animals yielded even greater prolongation of 5-HT-induced apnea. We conclude that a physiological role for the ITR in respiration is to attenuate vagally-induced reflex apneas. This finding is in agreement with the general modulatory role of pontine structures in autonomic activities including respiration, heart rate and regulation of blood pressure. In addition, our data indicate that medullary circuits, independent of pontine structures, are sufficient to produce 5-HT induced reflex apnea.

Plasticity in the control of breathing following sensory denervation

The purpose of this manuscript is to review the results of studies on the recovery or plasticity following a denervation- or lesion-induced change in breathing. Carotid body denervation (CBD), lung denervation (LD), cervical (CDR) and thoracic (TDR) dorsal rhizotomy, dorsal spinal column lesions, and lesions at pontine, medullary, and spinal sites all chronically alter breathing. The plasticity after these is highly variable, ranging from near complete recovery of the peripheral chemoreflex in rats after CBD to minimal recovery of the Hering-Breuer inflation reflex in ponies after LD. The degree of plasticity varies among the different functions of each pathway, and plasticity varies with the age of the animal when the lesion was made. In addition, plasticity after some lesions varies between species, and plasticity is greater in the awake than in the anesthetized state. Reinnervation is not a common mechanism of plasticity. There is evidence supporting two mechanisms of plasticity. One is through upregulation of an alternate sensory pathway, such as serotonin-mediated aortic chemoreception after CBD. The second is through upregulation on the efferent limb of a reflex, such as serotonin-mediated increased responsiveness of phrenic motoneurons after CDR, TDR, and spinal cord injury. Accordingly, numerous components of the ventilatory control system exhibit plasticity after denervation or lesion-induced changes in breathing; this plasticity is uniform neither in magnitude nor in underlying mechanisms. A major need in future research is to determine whether "reorganization" within the central nervous system contributes to plasticity following lesion-induced changes in breathing.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.

Role of the parabrachial nuclei in the airway dilation evoked by the Hering-Breuer reflex

We tested the hypothesis that blockade of glutamatergic receptors in the parabrachial nucleus (PBN) of chloralose-anesthetized cats attenuated the reflex airway dilation evoked by activation of pulmonary stretch receptors. Unilateral microinjection of kynurenic acid (50 nl, 100 mM) into the PBN reversibly attenuated the reflex relaxation of the trachealis muscle in 7 cats. These findings suggest that the PBN is part of the central pathway mediating the airway dilation component of the Hering-Breuer reflex.

Involvement of N-methyl-D-aspartate (NMDA) receptors in respiratory rhythmogenesis

The involvement of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in the control of inspiratory termination was studied in paralyzed decerebrated cats. Cats were either vagotomized, or had intact vagus nerves and were ventilated with a ventilator driven by the discharge of the phrenic nerve. The systemic administration of NMDA antagonists acting non-competitively (MK-801, ketamine, phencyclidine) or competitively (2-amino-7-phosphonoheptanoic acid: AP7), produced an apneusis in vagotomized animals or in animals transiently deprived of vagal pulmonary feedback by the 'no inflation test'. After NMDA receptor blockade, the inspiratory phase could be terminated by lung inflation or sensory stimulation. Thus pharmacologically distinct mechanisms control the termination of inspiration: vagal afferents which are NMDA-independent, and a central mechanism acting through the activation of NMDA receptors. The apneustic pattern induced by NMDA receptor blockade was characterized by a decrease of the amplitude of integrated phrenic nerve activity, the persistence of CO2 sensitivity and an enhancement of apneusis by anaesthesia. After injection of NMDA antagonists there was a decrease of the duration of expiration which thereafter remained constant and dissociated from inspiratory duration. The possible mechanisms by which NMDA receptors may contribute to respiratory rhythmogenesis are discussed.

Central respiratory depression induced by acetylcholinesterase inhibition: involvement of anaesthesia

We have studied the effects of anaesthesia on the changes in central respiratory activity following the inhibition of acetylcholinesterase in chronically implanted cats. The organophosphate paraoxon was administered to the brainstem respiratory centres by intracerebroventricular (i.c.v.) injection (3 mg) into the IVth ventricle, thus avoiding peripheral effects such as paralysis of respiratory muscles. Paraoxon had opposite effects on respiratory activity depending on whether the cats were anaesthetized or not: it induced respiratory depression and sometimes respiratory arrest during pentobarbital (30 mg/kg i.v.) or halothane anaesthesia, but in the same animals in the waking state, the same dose of paraoxon always stimulated respiration. These results show a strong interaction between anaesthetics and the effects of acetylcholine (ACh) accumulation on central respiratory activity. This study extends previous results showing an interaction between ACh and pentobarbital on single respiratory neurons and stresses the importance of a 'wakefulness stimulus' for sustaining respiratory activity after organophosphate poisoning.

Changes in cerebrospinal fluid homovanillic acid in children with Ondine's curse

The cerebrospinal fluid (CSF) concentrations of three acid monoamine metabolites, two purines, and a group of amino acids were determined in two children with chronic central alveolar hypoventilation (Ondine's curse). The levels of all assayed neuroactive substances, metabolites, and amino acids, with one exception, were normal compared to an age-matched group of neurologically healthy children. The levels of the dopamine metabolite homovanillic acid in the children with Ondine's curse were approximately 2.4 times higher than expected for age range. The present findings may indicate a link between central nervous system dopamine activity and chronic central alveolar hypoventilation. Among other possible explanations, the changes seen might represent a primary alteration in dopamine activity or may reflect a change in dopamine turnover resulting from the chronic hypoventilation.

Lesions in the upper lateral pons abolish the hypoxic depression of breathing in unanaesthetized fetal lambs in utero

1. The effects of stereotaxically placed lesions made throughout the mid-brain and upper pons were studied in chronically instrumented fetal lambs from 120 days gestation, after recovery in utero. 2. Isocapnic hypoxia caused an increase in the rate and depth of breathing movements in fetuses in which bilateral lesions encompassed the upper lateral pons in the region of and slightly rostral to the principal sensory and motor nuclei of the trigeminal nerve. 3. Fetal lambs with lesions which did not bilaterally encompass the upper lateral pons showed the normal fetal depressive response to hypoxia. 4. None of the lesions induced permanent continuous breathing as previously described in mid-brain transected fetuses, although periods of continuous breathing lasting several hours were seen at times in some fetuses with lesions in the upper lateral pons. 5. It is concluded that an area in the lateral pons close to areas with well known involvement in respiratory control is involved in the hypoxic depression of breathing in the fetal lamb.

Maintenance of respiratory modulation by pneumotaxic mechanisms in deep anesthesia

My purpose was to evaluate the hypothesis that the pneumotaxic center should continue to regulate ventilatory activity in deep anesthesia. In decerebrate cats, the respiratory pattern was reversibly altered from eupnea to apneusis by cooling the rostral pons. Such cooling markedly changed phrenic activity even after doses of halothane, pentobarbital, or chloralose-urethane had almost entirely eliminated this activity in eupnea. It is concluded that both pontile and medullary mechanisms control eupneic ventilatory activity in unanesthetized as well as deeply anesthetized preparations.

An electrophysiological study of medullary neurons projecting to nucleus parabrachialis of the cat

Neuronal activity was recorded from a total of 868 cells in the medulla of cats in regions known to project to nucleus parabrachialis (NPB) of the pons and thought to be involved in respiration. Of 196 neurons antidromically (85) or synaptically (111) activated from NPB, 178 were silent in the decerebrate or barbiturate-anesthetized preparation, and could not be induced to activity by increasing CO2, activating Breuer-Hering reflexes, stimulation of the vagus or vagotomy. We conclude that the direct pathway from the lateral tegmental field of the medulla to NPB is not significantly involved in respiratory function in these preparations.

Sleep-related apneic and apneustic breathing following pneumotaxic lesion and vagotomy

Sleep-wakefulness state was found to be a crucial determinant of respiratory pattern in chronic cats with bilateral lesions of the rostral pontine pneumotaxic complex (PC). Lesions resulted in increased TE, TI, and VT in all sleep and waking states. Several state-specific respiratory effects were also observed: (1) comparatively eupneic breathing during alert wakefulness (WI); (2) greatly increased TE in slow wave sleep (SWS); (3) decreased TE during rapid eye movement sleep (REM), relative to SWS; (4) increased tendency for prolonged TI (brief apneusis) during REM. Bilateral vagotomy at 2-5 weeks after PC lesion exaggerated these effects and caused distinct apneusis during REM. The results confirm that the PC is not essential for the occurrence of either rhythmic breathing or for expression of state changes in respiration, although the effects of the PC on breathing in the intact cat may vary as a function of sleep-wakefulness state. It is suggested that other regulatory systems that influence the central respiratory rhythm generator (RRG) are similarly modulated by state, and that variations in respiratory pattern observed following PC lesion and vagotomy are the result of state-dependent changes in the balance between multiple inputs to the RRG.

Apneusis and apnea after parabrachial or Kölliker-Fuse N. lesion; influence of wakefulness

Comparisons were made between the effects of bilateral lesions of either the nucleus parabrachialis medialis (NPBM) or the Kölliker-Fuse (KF) nuclei in bivagotomized, spinalized and immobilized cats. In a first group animals were electrolytically decerebrated; in a second group animals were locally anaesthetized and atraumatically restrained (semi-chronic encéphale isolé preparation). Lesions resulted in: (1) a marked increase in TI (apneusis); (2) 50--80% decrease in amplitude of the integrated phrenic discharge (IPD); (3) variable lengthening, of TE. Following KF lesion, effects were significantly larger on TE in the decerebrate group, and on TI in the encéphale isolé group. In the encéphale isolé group awakening reduced TE and TI and brought them close to their prelesion values following both NPBM and KF lesion; on the other hand light sleep induced by pentobarbital led to expiratory apnea after KF lesion and reduced IPD amplitude to zero after NPBM lesion. It is proposed that the onset, tonic drive and cut off of the I discharge are normally controlled by three differently weighted influences originating from NPBM, KF and reticular formation respectively.

The role of the pneumotaxic mechanism in the tachypnea of pulmonary vagal origin

The tachypnea due to stimulation of pulmonary vagal irritant and J.receptors with histamine (5%, aerosol) and phenyldiguanide (150 mu g, i.v.) was studied in 11 pentobarbitone anesthetized cats before and after uni (UL) and bilateral (BL) lesion of sites of the pneumotaxic mechanism. BL prolonged the timing parameters of spontaneous and occluded breaths (no phasic input from pulmonary stretch receptors) more under drug stimulation than in control: thus the shortening effect of drug stimulation increased going from intact to BL. Drug stimulation caused a decrease in volume threshold for inspiratory cut-off and this effect was reduced by about 50% by BL. Under hyperoxic iso-PCO2 conditions drug stimulation reduced the respiratory output (indexed by the rate of change of the pressure developed in the airways during occluded breaths) and this effect was left unaltered by BL. The results suggest that the pneumotaxic mechanism plays a role in the reflex respiratory excitatory frequency effect resulting from activation of irritant and J receptors while it seems to be excluded from a control on respiratory output.

Apneustic respiration normalized by an atypical cholinergic drug action in cats

Cats were anesthetized with a mixture of pentobarbital and barbital given intraperitoneally and vagotomized. All animals were pretreated with atropine i.v. which blocked conventional muscarinic receptors and at the same time revealed atropine-insensitive sites whose stimulation by the cholinergic drug, bethanechol, was earlier found to produce respiratory analepsis. Respiration was recorded by pneumotachograph and intrapleural cannula. Apneustic breathing was produced by placing electrolytic lesions in the pneumotaxic area or by injecting a local anesthetic agent into the cerebrospinal fluid. Experiments were performed under isocapnic conditions. Bethanechol, administered intracerebro-ventricularly, restored regular breathing and increased the tidal volume in a dose-related manner within minutes of its injection. In cats with lesions, the expiratory pause generally decreased first and was followed by shortening of the inspiratory plateau. In cats made apneustic by intracerebro-ventricular injection of procaine, bethanechol acted competitively to normalize the distorted waveshape. The central excitant drug pentylenetetrazol was given intravenously for comparison with bethanechol. Apneustic breathing was partially corrected by pentylenetetrazol through a facilitatory influence on the brain stem interpreted to be independent of its convulsant action. These experiments demonstrate correction of pathological breathing by two chemically unrelated agents which share the ability to increase central excitatory state.

Topology of ascending brainstem projections to nucleus parabrachialis in the cat

The afferent projections to nucleus parabrachialis (NPB) and nearby pontine areas from the lower brainstem were studied in cats using retrograde horseradish peoxidase (HRP) and anterograde autoradiographic tracing techniques. Two groups of medullary neurons send major projections to NPB and the Kölliker-Fuse nucleus (KF): 1) the solitary complex, especially the medial nucleus of the solitary tract (SM), nearby smaller cells of the dorsal motor nucleus of the vagus (DMV) and the commissural nucleus; and 2) the lateral tegmental field (FTL), or parvocellular reticular formation. Autoradiographic tracing from these areas demonstrated terminal fields in NPB/KF and emphasized a ventrolateral route to NPB from both sources, with axons ascending between the facial nerve and superior olive and passing rostral to the trigeminal nuclei. Minor projections to NBPB/KF originate in the ventrolateral nucleus of the solitary tract, area subpostrema, the alaminar spinal trigeminal nucleus, the gigantocellular and magnocellular tegmental fields, and an area dorsal to the ipsilateral inferior olive. Topographical features of the major projections were studied by correlating the locus of overlap of injection sites with the locations of HRP-positive cells. Medial areas of SM/DMV project mostly to medial parts of NPB, while lateral areas near the solitary tract project to lateral parts of NPB and KF. Crossing projections from SM/DMV favor dorsolateral NPB and KF. FTL neurons in dorsomedial areas project more to medial NPB, and ventrolateral areas project to lateral NPB/KF. Using a new coordinate system to locate and normalize the positions of FTL neurons, data from many brains were collated. FTL cells projecting to NPB/KF were found to be on discrete longitudinal sheets, running radially with respect to the fourth ventricle. This substructure and related evidence suggest a preferred pattern for neuroanatomical connections and information processing in the lateral reticular areas of the brainstem, and help in understanding the topography of the projections to NPB/KF.

Pentobarbitone effects on respiration related units; selective depression of bulbopontine reticular neurones

Respiration related units (RRUs) were recorded in cats, locally anesthetized, vagotomized and immobilized, under two different experimental conditions: (1) receiving pentobarbitone intravenously at a dose inducing slight but highly significant changes in phrenic discharge; (2) unanesthetized, with a spinal section at C6. RRU characteristics were analysed in unitary volumes constituted by cubes of a matrix including the brain stem. Four parameters of RRU discharges were compared in cubes = RRU density (RRUD), RRU proportion and two indices of respiratory modulation. Cubes were sampled in: (1) dorsal and ventral bulbar respiratory nuclei, (2) pneumotaxic centre (PC), and (3) seven fields of the bulbopontine and mesencephalic reticular formation. Using the paired sample method for comparing data cube by cube in the two conditions it was shown that under pentobarbitone, RRU activity was profoundly depressed in the reticular formation surrounding dorsal and ventral bulbar respiratory nuclei, in the region bridging the gap between these and the pneumotaxic centre and extending from the pneumotaxic level to the decussation of the brachium conjunctivum. In contrast RRU activity was unchanged at the level of dorsal and ventral bulbar respiratory nuclei and in the nucleus parabrachiallis medialis (NPBM) and was increased in the Kölliker-Fuse nucleus (KF). In the mesencephalic reticular formation, increased activity was observed for non-modulated units and to a larger extent for units driven by the bulbopontine respiratory system.

Alteration from apneusis to more regular rhythmic respiration in decerebrate cats

In spontaneously breathing or paralyzed, ventilated, decerebrate cats. bilateral pneumotaxic center lesions and bilateral vagotomy resulted in apneusis characterized by a marked prolongation of inspiratory duration (TI) and, concomitantly, an elevation of tidal volume (VT) and reduction of respiratory frequency (f). Subsequent placement of medial or lateral lesions in the caudal pons or medulla reduced TI and VT, increased f, and restored a more regular rhythmic respiratory pattern. Placement of these pontile or medullary lesions in animals with pneumotaxic center lesions prevented the development of a typical apneustic pattern upon vagotomy. Large rostral pontile lesions did not change apneusis in spontaneous breathing animals whereas, in paralyzed, ventilated cats, these lesions resulted in some decrease of TI. It is concluded that apneusis generation is not equatable simply with a summation of caudal pontile respiratory unit activities. Rather, activity inherent to intrapontile and/or pontomedullary feedback circuits is considered as a necessary requisite for apneusis development. Interaction of these circuits with an inspiratory off-switch mechanism is considered probable.

Three dimensional representation of bulbo-pontine respiratory networks architecture from unit density maps

Results of the preceding paper32 suggest that bulbo-pontine respiration related units (RRU) could be classed into 4 populations according to the relation of their peak firing frequency to the respiratory cycle: one inspiratory (I) and 3 expiratory (E) (early, mid and late). A further study of their spatial distribution is reported. Recorded units totalling 26,520 were distributed through a matrix of unitary cubes of which about 2,000 were found within the limits of the medulla and pons. Both percentage and density of respiratory and non-respiratory units were estimated in each cube. Statistical analysis of unit distribution showed a high probability of finding RRU in certain regions. The probability of finding I and E units in separate cubes was high, whereas there was no probability better than chance for recording one of the 3 E types separately from the 2 others. Therefore, only two populations, I and E, could be considered for spatial analysis. A thresholding technique applied to I and E unit density figures in unitary cubes demonstrated a definite limit for I and E populations. A computerized, three-dimensional (3-D) reconstruction of the outer surfaces of the I and E networks showed that half a medullary system consisted of 2 contiguous and uninterrupted columns extending from the cervical junction up to the pneumotaxic system. The main body of the I population was found in the caudal third and dorsolateral part of the bulbopontine region, while the main body of the E population was in the middle third in a ventromedial position. Both have caudal and rostral extensions that run parallel to the main axis of the brainstem and give rise to several lateral branches; these lie in close contact and interdigitate extensively, especially in the rostral third of the bulbopontine region. From the outer limits of I and E networks, a progressive increase of unit density levels was observed towards the core; it showed that inside both I and E populations, RRU aggregate into high density foci (HDF). Large HDF were found in the main body of each population. Smaller HDF were observed scattered throughout the medulla; I, and to a lesser extent E, HDF invade and surround sensory and motor nuclei and roots of the branchial nerves.

Effects of lesions in the parabrachial nucleus on the mechanisms for central and reflex termination of inspiration in the cat

The effects of PCO2 and body temperature on the time course and peak amplitude of the central inspiratory activity (CIA) and the inspiratory "off-switch" threshold was studied in apneustic and non-apneustic cats. Apneusis resulted from lesions of the inspiratory inhibiting structures of the medial parabrachial nucleus (NPBM) and by interrupting vagal volume feedback. The cats were paralyzed and ventilated either proportionally to their phrenic output or at predetermined rate and volume. The dependence of the rate of rise and maximal amplitude of phrenic activity on PCO2 and body temperature were comparable in apneustic and non-lesioned animals. The Hering-Breuer volume threshold for inspiratory termination was increased following the rostral pontine lesions. Both hyperthermia and hypercapnia caused augmentation of the absolute rate of rise of inspiratory activity but hypercapnia, in contrast to hyperthermia, caused virtually no change in the fractional increment per unit time. With hypercapnia the inspiratory "off-switch" threshold was raised in the apneustic animals in intact ones, whereas hyperthermia did not seem to influence this threshold. In apneustic conditions expiratory duration remained constant, independent of the large variations in the inspiratory durations. Our results suggest that the NPBM merely provides an excitatory, threshold-lowering input to the inspiratory "off-switch" mechanism.