Dynamic changes in glottal resistance during exposure to severe hypoxia in neonatal rats in situ

Comparison of the effects of fentanyls and other μ opioid receptor agonists on the electrical activity of respiratory muscles in the rat

Introduction: Deaths due to overdose of fentanyls result primarily from depression of respiration. These potent opioids can also produce muscle rigidity in the diaphragm and the chest muscles, a phenomenon known as Wooden Chest Syndrome, which further limits ventilation. Methods: We have compared the depression of ventilation by fentanyl and morphine by directly measuring their ability to induce muscle rigidity using EMG recording from diaphragm and external and internal intercostal muscles, in the rat working heart-brainstem preparation. Results: At equipotent bradypnea-inducing concentrations fentanyl produced a greater increase in expiratory EMG amplitude than morphine in all three muscles examined. In order to understand whether this effect of fentanyl was a unique property of the phenylpiperidine chemical structure, or due to fentanyl's high agonist intrinsic efficacy or its lipophilicity, we compared a variety of agonists with different properties at concentrations that were equipotent at producing bradypnea. We compared carfentanil and alfentanil (phenylpiperidines with relatively high efficacy and high to medium lipophilicity, respectively), norbuprenorphine (orvinolmorphinan with high efficacy and lipophilicity) and levorphanol (morphinan with relatively low efficacy and high lipophilicity). Discussion: We observed that, agonists with higher intrinsic efficacy were more likely to increase expiratory EMG amplitude (i.e., produce chest rigidity) than agonists with lower efficacy. Whereas lipophilicity and chemical structure did not appear to correlate with the ability to induce chest rigidity.

Neural Control of Breathing and CO2 Homeostasis

Recent advances have clarified how the brain detects CO2 to regulate breathing (central respiratory chemoreception). These mechanisms are reviewed and their significance is presented in the general context of CO2/pH homeostasis through breathing. At rest, respiratory chemoreflexes initiated at peripheral and central sites mediate rapid stabilization of arterial PCO2 and pH. Specific brainstem neurons (e.g., retrotrapezoid nucleus, RTN; serotonergic) are activated by PCO2 and stimulate breathing. RTN neurons detect CO2 via intrinsic proton receptors (TASK-2, GPR4), synaptic input from peripheral chemoreceptors and signals from astrocytes. Respiratory chemoreflexes are arousal state dependent whereas chemoreceptor stimulation produces arousal. When abnormal, these interactions lead to sleep-disordered breathing. During exercise, central command and reflexes from exercising muscles produce the breathing stimulation required to maintain arterial PCO2 and pH despite elevated metabolic activity. The neural circuits underlying central command and muscle afferent control of breathing remain elusive and represent a fertile area for future investigation.

Recurrent laryngeal nerve activity exhibits a 5-HT-mediated long-term facilitation and enhanced response to hypoxia following acute intermittent hypoxia in rat

A progressive and sustained increase in inspiratory-related motor output (“long-term facilitation”) and an augmented ventilatory response to hypoxia occur following acute intermittent hypoxia (AIH). To date, acute plasticity in respiratory motor outputs active in the postinspiratory and expiratory phases has not been studied. The recurrent laryngeal nerve (RLN) innervates laryngeal abductor muscles that widen the glottic aperture during inspiration. Other efferent fibers in the RLN innervate adductor muscles that partially narrow the glottic aperture during postinspiration. The aim of this study was to investigate whether or not AIH elicits a serotonin-mediated long-term facilitation of laryngeal abductor muscles, and if recruitment of adductor muscle activity occurs following AIH. Urethane anesthetized, paralyzed, unilaterally vagotomized, and artificially ventilated adult male Sprague-Dawley rats were subjected to 10 exposures of hypoxia (10% O2 in N2, 45 s, separated by 5 min, n = 7). At 60 min post-AIH, phrenic nerve activity and inspiratory RLN activity were elevated (39 ± 11 and 23 ± 6% above baseline, respectively). These responses were abolished by pretreatment with the serotonin-receptor antagonist, methysergide ( n = 4). No increase occurred in time control animals ( n = 7). Animals that did not exhibit postinspiratory RLN activity at baseline did not show recruitment of this activity post-AIH ( n = 6). A repeat hypoxia 60 min after AIH produced a significantly greater peak response in both phrenic and RLN activity, accompanied by a prolonged recovery time that was also prevented by pretreatment with methysergide. We conclude that AIH induces neural plasticity in laryngeal motoneurons, via serotonin-mediated mechanisms similar to that observed in phrenic motoneurons: the so-called “Q-pathway”. We also provide evidence that the augmented responsiveness to repeat hypoxia following AIH also involves a serotonergic mechanism.

Upper airway dysfunction of Tau-P301L mice correlates with tauopathy in midbrain and ponto-medullary brainstem nuclei

Tauopathy comprises hyperphosphorylation of the microtubule-associated protein tau, causing intracellular aggregation and accumulation as neurofibrillary tangles and neuropil treads. Some primary tauopathies are linked to mutations in the MAPT gene coding for protein tau, but most are sporadic with unknown causes. Also, in Alzheimer's disease, the most frequent secondary tauopathy, neither the cause nor the pathological mechanisms and repercussions are understood. Transgenic mice expressing mutant Tau-P301L suffer cognitive and motor defects and die prematurely from unknown causes. Here, in situ electrophysiology in symptomatic Tau-P301L mice (7-8 months of age) revealed reduced postinspiratory discharges of laryngeal motor outputs that control laryngeal constrictor muscles. Under high chemical drive (hypercapnia), postinspiratory discharge was nearly abolished, whereas laryngeal inspiratory discharge was increased disproportionally. The latter may suggest a shift of postinspiratory laryngeal constrictor activity into inspiration. In vivo double-chamber plethysmography of Tau-P301L mice showed significantly reduced respiratory airflow but significantly increased chest movements during baseline breathing, but particularly in hypercapnia, confirming a significant increase in inspiratory resistive load. Histological analysis demonstrated hyperphosphorylated tau in brainstem nuclei, directly or indirectly involved in upper airway motor control (i.e., the Kölliker-Fuse, periaqueductal gray, and intermediate reticular nuclei). In contrast, young Tau-P301L mice did not show breathing disorders or brainstem tauopathy. Consequently, in aging Tau-P301L mice, progressive upper airway dysfunction is linked to progressive tauopathy in identified neural circuits. Because patients with tauopathy suffer from upper airway dysfunction, the Tau-P301L mice can serve as an experimental model to study disease-specific synaptic dysfunction in well defined functional neural circuits.

Paradoxical vocal cord motion: A review focused on multiple system atrophy

OBJECTIVE

Paradoxical vocal cord motion (PVCM) is a well recognized respiratory condition in which active adduction of the vocal cords during inspiration causes functional airway obstruction. It is considered that laryngeal reflex acceleration underlies the generation of nonorganic PVCM. In various situations producing PVCM, multiple system atrophy (MSA) is a representative neurological disease causing nocturnal laryngeal stridor attributed to PVCM. The purpose of this review is to identify the underlying mechanisms associated with nonorganic and MSA-related PVCM. The following issues are addressed in this review: (1) the pathophysiology of nonorganic and MSA-related PVCM, (2) the relationships between PVCM and airway reflexes, and (3) the treatment for MSA-related PVCM.

METHODS

Review.

RESULTS AND CONCLUSIONS

An abnormality of the laryngeal output-feedback control underlies nonorganic PVCM, which is usually triggered by an excessive response to external and internal airway stimuli. Similarly, several clinical and experimental evidence suggest that MSA-related PVCM is attributed to the airway reflex as well as to paradoxical central outputs resulting from the MSA-induced damage to the pontomedullary respiratory center. Application of continuous positive airway pressure (CPAP), which suppresses the reflexive inspiratory activation of adductors, is recommended as the treatment for MSA-related PVCM.

Optical imaging of medullary ventral respiratory network during eupnea and gaspingIn situ

In severe hypoxia, respiratory rhythm is shifted from an eupneic, ramp-like motor pattern to gasping characterized by a decrementing pattern of phrenic motor activity. However, it is not known whether hypoxia reconfigures the spatiotemporal organization of the central respiratory rhythm generator. Using the in situ arterially perfused juvenile rat preparation, we investigated whether the shift from eupnea to gasping was associated with a reconfiguration of the spatiotemporal pattern of respiratory neuronal activity in the ventral medullary respiratory network. Optical images of medullary respiratory network activity were obtained from male rats (4-6 weeks of age). Part of the medullary network was stained with a voltage-sensitive dye (di-2 ANEPEQ) centred both within, and adjacent to, the pre-Bötzinger complex (Pre-BötC). During eupnea, optical signals initially increased prior to the onset of phrenic activity and progressively intensified during the inspiratory phase peaking at the end of inspiration. During early expiration, fluorescence was also detected and slowly declined throughout this phase. In contrast, hypoxia shifted the respiratory motor pattern from eupnea to gasping and optical signals were restricted to inspiration only. Areas active during gasping showed fluorescence that was more intensive and covered a larger region of the rostral ventrolateral medulla compared to eupnea. Regions exhibiting peak inspiratory fluorescence did not coincide spatially during eupnea and gasping. Moreover, there was a recruitment of additional medullary regions during gasping that were not active during eupnea. These results provide novel evidence that the shift in respiratory motor pattern from eupnea to gasping appears to be associated with a reconfiguration of the central respiratory rhythm generator characterized by changes in its spatiotemporal organization.

A decerebrate, artificially-perfused in situ preparation of rat: utility for the study of autonomic and nociceptive processing

By extending established technology, we have developed a relatively simple in situ preparation from juvenile rat that overcomes some technical restrictions present in vivo (e.g. need for anaesthesia, mechanical instability for intracellular recording and control over the extracellular milieu). The in situ preparation is decerebrate and artificially perfused via the left ventricle with a colloid containing solution. It exhibits an eupneic pattern of respiratory motor activity and demonstrates numerous somatic and visceral reflexes including those evoked by stimulation of the tail, hindlimbs, bladder, baroreceptors and peripheral chemoreceptors. We have employed this preparation to allow recordings from multiple sympathetic motor outflows such as thoracic and lumbar chain, inferior cardiac, splanchnic, renal and adrenal nerves. We show that the sympathetic motor discharge shows strong respiratory modulation and exhibits pronounced reflex modulation indicating intact communication between the periphery, the brainstem and the spinal cord. Further, we have made extracellular and whole cell recordings from neurones in the spinal cord, demonstrating good mechanical stability. The decerebrate, artificially-perfused rat (DAPR) provides a powerful methodology with which to study peripheral and central control of the autonomic nervous system with many of the benefits of an in vitro environment.