Blockade of glutamate receptors in CVLM and NTS attenuates airway dilation evoked from parabrachial region

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.

Microinjection of kynurenic acid in the rostral nucleus of the tractus solitarius disrupts spatiotemporal aspects of mechanically induced tracheobronchial cough

The importance of neurons in the nucleus of the solitary tract (NTS) in the production of coughing was tested by microinjections of the nonspecific glutamate receptor antagonist kynurenic acid (kyn; 100 mM in artificial cerebrospinal fluid) in 15 adult spontaneously breathing anesthetized cats. Repetitive coughing was elicited by mechanical stimulation of the intrathoracic airway. Electromyograms (EMG) were recorded from inspiratory parasternal and expiratory transversus abdominis (ABD) muscles. Bilateral microinjections of kyn into the NTS rostral to obex [55 ± 4 nl total in 2 locations (n = 6) or 110 ± 4 nl total in 4 locations (n = 5)], primarily the ventrolateral subnucleus, reduced cough number and expiratory cough efforts (amplitudes of ABD EMG and maxima of esophageal pressure) compared with control. These microinjections also markedly prolonged the inspiratory phase, all cough-related EMG activation, and the total cough cycle duration as well as some other cough-related time intervals. In response to microinjections of kyn into the NTS rostral to the obex respiratory rate decreased, and there were increases in the durations of the inspiratory and postinspiratory phases and mean blood pressure. However, bilateral microinjections of kyn into the NTS caudal to obex as well as control vehicle microinjections in the NTS location rostral to obex had no effect on coughing or cardiorespiratory variables. These results are consistent with the existence of a critical component of the cough rhythmogenic circuit located in the rostral ventral and lateral NTS. Neuronal structures of the rostral NTS are significantly involved specifically in the regulation of cough magnitude and phase timing.NEW & NOTEWORTHY The nucleus of the solitary tract contains significant neuronal structures responsible for control of 1) cough excitability, 2) motor drive during cough, 3) cough phase timing, and 4) cough rhythmicity. Significant elimination of neurons in the solitary tract nucleus results in cough apraxia (incomplete and/or disordered cough pattern). The mechanism of the cough impairment is different from that for the concomitant changes in breathing.

Control of the lungs via the human brain using neurosurgery

Neurosurgery can alter cardiorespiratory performance via central networks and includes deep brain stimulation (DBS), a routinely employed therapy for movement disorders and chronic pain syndromes. We review the established cardiovascular effects of DBS and the presumed mechanism by which they are produced via the central autonomic network. We then review the respiratory effects of DBS, including modulation of respiratory rate and lung function indices, and the mechanisms via which these may occur. We conclude by highlighting the potential future therapeutic applications of DBS for intractable airway diseases.

Airway responsiveness to psychological processes in asthma and health

Psychosocial factors have been found to impact airway pathophysiology in respiratory disease with considerable consistency. Influences on airway mechanics have been studied particularly well. The goal of this article is to review the literature on airway responses to psychological stimulation, discuss potential pathways of influence, and present a well-established emotion-induction paradigm to study airway obstruction elicited by unpleasant stimuli. Observational studies have found systematic associations between lung function and daily mood changes. The laboratory-based paradigm of bronchoconstrictive suggestion has been used successfully to elicit airway obstruction in a substantial proportion of asthmatic individuals. Other studies have demonstrated modulation of airway responses to standard airway challenges with exercise, allergens, or pharmacological agents by psychological factors. Standardized emotion-induction techniques have consistently shown airway constriction during unpleasant stimulation, with surgery, blood, and injury stimuli being particularly powerful. Findings with various forms of stress induction have been more mixed. A number of methodological factors may account for variability across studies, such as choice of measurement technique, temporal association between stimulation and measurement, and the specific quality and intensity of the stimulus material, in particular the extent of implied action-orientation. Research has also begun to elucidate physiological processes associated with psychologically induced airway responses, with vagal excitation and ventilatory influences being the most likely candidate pathways, whereas the role of specific central nervous system pathways and inflammatory processes has been less studied. The technique of emotion-induction using films has the potential to become a standardized challenge paradigm for the further exploration of airway hyperresponsiveness mediated by central nervous system processes.

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.

Role of central neurotransmission and chemoreception on airway control

This review summarizes work on central neurotransmission, chemoreception and CNS control of cholinergic outflow to the airways. First, we describe the neural transmission of bronchoconstrictive signals from airway afferents to the airway-related vagal preganglionic neurons (AVPNs) via the nucleus of the solitary tract (nTS) and, second, we characterize evidence for a modulatory effect of excitatory glutamatergic, and inhibitory GABAergic, noradrenergic and serotonergic pathways on AVPN output. Excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second order neurons within the nTS, via a glutamate-AMPA (alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) receptor signaling pathway. These nTS neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command through descending fibers and airway intramural ganglia to airway smooth muscle, submucosal secretory glands, and the vasculature. The strength and duration of this reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs. In addition, central noradrenergic and serotonergic inhibitory pathways appear to participate in the regulation of cholinergic drive to the tracheobronchial system. Down-regulation of these inhibitory influences results in a shift from inhibitory to excitatory drive, which may lead to increased excitability of AVPNs, heightened airway responsiveness, greater cholinergic outflow to the airways and consequently bronchoconstriction. In summary, centrally coordinated control of airway tone and respiratory drive serve to optimize gas exchange and work of breathing under normal homeostatic conditions. Greater understanding of this process should enhance our understanding of its disruption under pathophysiologic states.

Brain stem excitatory and inhibitory signaling pathways regulating bronchoconstrictive responses

This review summarizes recent work on two basic processes of central nervous system (CNS) control of cholinergic outflow to the airways: 1) transmission of bronchoconstrictive signals from the airways to the airway-related vagal preganglionic neurons (AVPNs) and 2) regulation of AVPN responses to excitatory inputs by central GABAergic inhibitory pathways. In addition, the autocrine-paracrine modulation of AVPNs is briefly discussed. CNS influences on the tracheobronchopulmonary system are transmitted via AVPNs, whose discharge depends on the balance between excitatory and inhibitory impulses that they receive. Alterations in this equilibrium may lead to dramatic functional changes. Recent findings indicate that excitatory signals arising from bronchopulmonary afferents and/or the peripheral chemosensory system activate second-order neurons within the nucleus of the solitary tract (NTS), via a glutamate-AMPA signaling pathway. These neurons, using the same neurotransmitter-receptor unit, transmit information to the AVPNs, which in turn convey the central command to airway effector organs: smooth muscle, submucosal secretory glands, and the vasculature, through intramural ganglionic neurons. The strength and duration of reflex-induced bronchoconstriction is modulated by GABAergic-inhibitory inputs and autocrine-paracrine controlling mechanisms. Downregulation of GABAergic inhibitory influences may result in a shift from inhibitory to excitatory drive that may lead to increased excitability of AVPNs, heightened airway responsiveness, and sustained narrowing of the airways. Hence a better understanding of these normal and altered central neural circuits and mechanisms could potentially improve the design of therapeutic interventions and the treatment of airway obstructive diseases.

Involvement of glutamate in gastrointestinal vago-vagal reflexes initiated by gastrointestinal distention in the rat

Vago-vagal reflexes play an integral role in the regulation of gastrointestinal function. Although there have been a number of reports describing the effects of various stimuli on the firing rates of vagal afferent fibers and vagal motor neurons, little is known regarding the neurotransmitters that mediate the vago-vagal reflexes. In the present work, we investigated the role of glutamate in the vago-vagal reflex induced by gastrointestinal distention. Using single-cell recording techniques, we determined the effects of gastric and duodenal distention on the firing rates of gut-related neurons in the dorsal vagal complex, in the absence and presence of glutamate antagonists. Kynurenic acid, a competitive glutamate receptor antagonist, injected into the dorsal vagal complex, blocked the neuronal response of neurons in the dorsal motor nucleus of the vagus and the nucleus of the solitary tract to gastrointestinal distention. Injection of glutamate into the nucleus of the solitary tract produced inhibition of dorsal motor nucleus of the vagus neurons that were also inhibited by gastric and/or duodenal distention. Thus, the distention-induced inhibition of dorsal motor nucleus of the vagus neurons may be mediated by glutamate-induced excitation of gut-related nucleus of the solitary tract neurons. To investigate the role of the various glutamate receptor subtypes in the distention-induced events, we studied the effects of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a selective non-NMDA receptor antagonist, and DL-2-amino-5-phosphonopentanoic acid (DL-AP5), a selective NMDA receptor antagonist. CNQX injected into the dorsal vagal complex either blocked or attenuated the inhibitory response of the neurons in the dorsal motor nucleus of the vagus and nucleus of the solitary tract neurons to gastric and duodenal distention. In contrast, DL-AP5 had less effect, especially in the vago-vagal reflex elicited by gastric distention. The results suggest (1) distention activates vagal afferents in the gastrointestinal tract; (2) the central branches of the vagal afferents from the gut terminate in the nucleus of the solitary tract and release glutamate that mainly act on non-NMDA receptors; (3) glutamate activates the inhibitory neurons in the nucleus of the solitary tract that project to the dorsal motor nucleus of the vagus; and (4) the inhibitory neurotransmitter suppresses the activity of the dorsal motor nucleus of the vagus neurons. For the excitatory neuronal responses of the dorsal motor nucleus of the vagus neurons to gastrointestinal distention, the possible circuit is that the vagal afferents containing glutamate directly activate the receptors on the dendrites of the dorsal motor nucleus of the vagus.

Glutamate mediates an excitatory influence of the paraventricular hypothalamic nucleus on the dorsal motor nucleus of the vagus

Data have shown that the paraventricular nucleus of the hypothalamus (PVN) and the dorsal motor nucleus of the vagus (DMNV) play important roles in the regulation of gastrointestinal function and eating behavior. Anatomical studies have demonstrated direct projections from the PVN to the DMNV and physiological studies showed that the DMNV mediates many of the effects of PVN stimulation and electrical current stimulation of the PVN excites a subset of DMNV neurons. The aim of this study was to characterize the role of glutamate receptors in the excitatory influence of the PVN on gut-related DMNV neurons. Using single-cell recording techniques, we determined the effects of kynurenic acid, 6-cyano-7-nitroquinoxalene-2,3-dione (CNQX), and DL-2-amino-5-phosphonopentanoic acid (DL-AP5) on the increase in firing rate due to electrical current stimulation of the PVN. In initial experiments, we studied 24 DMNV neurons excited by electrical current stimulation of the PVN. Kynurenic acid, a broad-spectrum glutamate receptor antagonist, prevented the PVN effect in 22 neurons and significantly attenuated the effect in the other cells. Nine of these neurons demonstrated an inhibition in firing rate with PVN stimulation after pretreatment with kynurenic acid. In a separate group of 12 neurons, we determined the effects of CNQX (1.2 nmol) injected into the DMNV. This AMPA receptor antagonist completely blocked the excitatory response to PVN stimulation of six DMNV neurons and significantly attenuated the response of the other six DMNV neurons. The addition of 1.2 nmol DL-AP5, a N-methyl-D-aspartate (NMDA) receptor antagonist, further attenuated the response to PVN stimulation in four of the five DMNV neurons that were still excited after CNQX treatment. The fifth neuron demonstrated PVN- induced inhibition of firing rate after treatment with CNQX and DL-AP5. In a separate group of 11 DMNV neurons excited by electrical stimulation of the PVN, DL-AP5 partially attenuated the excitatory responses of only four DMNV neurons and did not block the excitation of any cells. The mean latency (14 neurons tested) from the PVN to the DMNV was 37.71 +/- 2.40 (SE) ms. Monosynaptic action potentials and excitatory postsynaptic potentials were demonstrated in three DMNV neurons by intracellular recording. Our results indicate that glutamate released from PVN neurons projecting to the DMNV excite the gut-related vagal motor neurons by acting predominantly on the AMPA receptor. The NMDA receptor plays only a minor role in the excitatory effect.

Laryngeal effects of stimulation of rostral and ventral pons in the anaesthetized rat

In order to study the importance of two pontine regions modulating laryngeal resistance, electrical current or microinjections of glutamate (10-30 nl, 1-3 nmol) were made into the pontine parabrachial complex and the A5 region in spontaneously breathing anaesthetized rats. Two distinct patterns of laryngeal and respiratory responses were elicited. An increase of subglottal pressure was accompanied with an expiratory facilitatory response consisted of a decrease in both respiratory rate and phrenic nerve activity. A decrease of subglottal pressure was accompanied with an inspiratory facilitatory response consisted of an increase in both respiratory rate and phrenic nerve activity. The modification of laryngeal calibre occurred during both respiratory phases in most cases. The concomitant cardiovascular changes of these responses were also analyzed. Controls using guanethidine to block autonomic responses which might interact with respiratory control were also made. Histological analysis of stimulation sites showed a topographical organization of these responses: laryngeal constriction was evoked from Kölliker-Fuse, medial parabrachial nuclei and A5 region, whilst the laryngeal dilation was evoked from the lateral parabrachial nucleus.

Caudal midline medulla mediates behaviourally-coupled but not baroreceptor-mediated vasodepression

Within the caudal medulla there are two regions whose activation leads to vasodepression and bradycardia, the caudal ventrolateral medulla and a discrete region of the caudal midline medulla. This study investigated, in the halothane anaesthetized rat, the contribution of these two vasodepressor regions to "homeostatic" and "behaviourally-coupled" cardiovascular regulation. In an initial set of experiments the contribution of each of these two regions to the hypotension and bradycardia evoked by acute hypovolaemia (15% haemorrhage) was investigated. It was found that inactivation of the caudal midline medulla significantly attenuated (cobalt chloride) or completely blunted (lignocaine) the hypotension and bradycardia evoked by acute hypovolaemia. In contrast, inactivation of the caudal ventrolateral medulla using cobalt chloride, although attenuating the magnitude of the hypotension and completely blocking the bradycardia, did not delay the onset of the hypotension evoked by acute hypovolaemia. The caudal ventrolateral medulla is known to be critical in homeostatic cardiovascular control through the expression of the "baroreceptor reflex" and the hypotension and bradycardia evoked by activation of cardiopulmonary afferents. In a second series of experiments we found inactivation of the caudal midline medulla played no role in baroreflex-evoked bradycardia (i.v. phenylephrine) or the hypotension and bradycardia evoked by cardiopulmonary afferent activation (i.v. 5-hydroxytryptamine). These data suggest that the caudal midline medulla and caudal ventrolateral medulla play different roles in cardiovascular control. The caudal ventrolateral medulla is involved in mediating cardiovascular changes associated with a variety of stimuli including "homeostatic" and "behaviourally-coupled" cardiovascular changes, whereas the caudal midline medulla is critical for mediating "behaviourally-coupled" changes in arterial pressure and heart rate.