Excitatory mechanism of deflationary slowly adapting pulmonary stretch receptors in the rat lung

Baroreceptor Modulation of the Cardiovascular System, Pain, Consciousness, and Cognition

Baroreceptors are mechanosensitive elements of the peripheral nervous system that maintain cardiovascular homeostasis by coordinating the responses to external and internal environmental stressors. While it is well known that carotid and cardiopulmonary baroreceptors modulate sympathetic vasomotor and parasympathetic cardiac neural autonomic drive, to avoid excessive fluctuations in vascular tone and maintain intravascular volume, there is increasing recognition that baroreceptors also modulate a wide range of non-cardiovascular physiological responses via projections from the nucleus of the solitary tract to regions of the central nervous system, including the spinal cord. These projections regulate pain perception, sleep, consciousness, and cognition. In this article, we summarize the physiology of baroreceptor pathways and responses to baroreceptor activation with an emphasis on the mechanisms influencing cardiovascular function, pain perception, consciousness, and cognition. Understanding baroreceptor-mediated effects on cardiac and extra-cardiac autonomic activities will further our understanding of the pathophysiology of multiple common clinical conditions, such as chronic pain, disorders of consciousness (e.g., abnormalities in sleep-wake), and cognitive impairment, which may result in the identification and implementation of novel treatment modalities. © 2021 American Physiological Society. Compr Physiol 11:1373-1423, 2021.

Deflation-activated receptors, not classical inflation-activated receptors, mediate the Hering-Breuer deflation reflex

Many airway sensory units respond to both lung inflation and deflation. Whether those responses to opposite stimuli come from one sensor (one-sensor theory) or more than one sensor (multiple-sensor theory) is debatable. One-sensor theory is commonly presumed in the literature. This article proposes a multiple-sensor theory in which a sensory unit contains different sensors for sensing different forces. Two major types of mechanical sensors operate in the lung: inflation- and deflation-activated receptors (DARs). Inflation-activated sensors can be further divided into slowly adapting receptors (SARs) and rapidly adapting receptors (RARs). Many SAR and RAR units also respond to lung deflation because they contain DARs. Pure DARs, which respond to lung deflation only, are rare in large animals but are easily identified in small animals. Lung deflation-induced reflex effects previously attributed to RARs should be assigned to DARs (including pure DARs and DARs associated with SARs and RARs) if the multiple-sensor theory is accepted. Thus, based on the information, it is proposed that activation of DARs can attenuate lung deflation, shorten expiratory time, increase respiratory rate, evoke inspiration, and cause airway secretion and dyspnea.

Sensory nerves in lung and airways

Sensory nerves innervating the lung and airways play an important role in regulating various cardiopulmonary functions and maintaining homeostasis under both healthy and disease conditions. Their activities conducted by both vagal and sympathetic afferents are also responsible for eliciting important defense reflexes that protect the lung and body from potential health-hazardous effects of airborne particulates and chemical irritants. This article reviews the morphology, transduction properties, reflex functions, and respiratory sensations of these receptors, focusing primarily on recent findings derived from using new technologies such as neural immunochemistry, isolated airway-nerve preparation, cultured airway neurons, patch-clamp electrophysiology, transgenic mice, and other cellular and molecular approaches. Studies of the signal transduction of mechanosensitive afferents have revealed a new concept of sensory unit and cellular mechanism of activation, and identified additional types of sensory receptors in the lung. Chemosensitive properties of these lung afferents are further characterized by the expression of specific ligand-gated ion channels on nerve terminals, ganglion origin, and responses to the action of various inflammatory cells, mediators, and cytokines during acute and chronic airway inflammation and injuries. Increasing interest and extensive investigations have been focused on uncovering the mechanisms underlying hypersensitivity of these airway afferents, and their role in the manifestation of various symptoms under pathophysiological conditions. Several important and challenging questions regarding these sensory nerves are discussed. Searching for these answers will be a critical step in developing the translational research and effective treatments of airway diseases.

Spectrum of myelinated pulmonary afferents (II)

Recently, it has been recognized that a single airway sensory unit may contain multiple receptive fields and that each field houses at least one encoder. Since some units respond to both lung inflation and deflation, we hypothesized that these units contain heterogeneous encoders for sensing inflation and deflation, respectively. Single unit activities were recorded from the cervical vagus nerve in anesthetized, open chest, and mechanically ventilated rabbits. Fifty-two airway sensory units with multiple receptive fields that responded to both lung inflation and deflation were identified. Among them, 13 units had separate receptive fields for inflation and deflation, where one of the fields could be blocked by local injection of 2% lidocaine (10 μl). In 8 of the 13 units, the deflation response was blocked without affecting the unit's response to inflation, whereas in the remaining five units, the inflation response was blocked without affecting the deflation response. Our results support the hypothesis that a single mechanosensory unit may contain heterogeneous encoders that can respond to either inflation or deflation.

A joint computational respiratory neural network-biomechanical model for breathing and airway defensive behaviors

Data-driven computational neural network models have been used to study mechanisms for generating the motor patterns for breathing and breathing related behaviors such as coughing. These models have commonly been evaluated in open loop conditions or with feedback of lung volume simply represented as a filtered version of phrenic motor output. Limitations of these approaches preclude assessment of the influence of mechanical properties of the musculoskeletal system and motivated development of a biomechanical model of the respiratory muscles, airway, and lungs using published measures from human subjects. Here we describe the model and some aspects of its behavior when linked to a computational brainstem respiratory network model for breathing and airway defensive behavior composed of discrete "integrate and fire" populations. The network incorporated multiple circuit paths and operations for tuning inspiratory drive suggested by prior work. Results from neuromechanical system simulations included generation of a eupneic-like breathing pattern and the observation that increased respiratory drive and operating volume result in higher peak flow rates during cough, even when the expiratory drive is unchanged, or when the expiratory abdominal pressure is unchanged. Sequential elimination of the model's sources of inspiratory drive during cough also suggested a role for disinhibitory regulation via tonic expiratory neurons, a result that was subsequently supported by an analysis of in vivo data. Comparisons with antecedent models, discrepancies with experimental results, and some model limitations are noted.

Flecainide inhibits the stimulatory effect of veratridine on the response of airway mechanoreceptors to maintained inflations in rats

AIMS

the purpose of the present study was to investigate (a) whether maintained inflations result in the inhibition of slowly adapting pulmonary stretch receptor (SAR) discharge to elicit an abrupt change in receptor activity and (b) whether pretreatment with veratridine, a Na(+) channel opener, and/or flecainide, a Na(+) channel blocker, alters the responses of SAR properties to maintained inflations.

MAIN METHODS

we measured the properties of SAR activity during maintained inflations at different pressures in 31 anesthetized, artificially ventilated rats with unilateral vagotomy.

KEY FINDINGS

During maintained inflations (approximately 5, 10 and 15 cmH(2)O) for about 5s, the procedures did not produce the induction of inhibition of either 16 low-threshold SARs (firing during both inflation and deflation) or 15 high-threshold SARs (firing during inflation only). In these preparations, the excitatory responses of SARs to maintained inflations at three different pressures were markedly enhanced after administration of veratridine (50 μg/kg), but under such conditions, the inhibition of SAR discharges was not observed. In the same SAR preparations, after flecainide treatment (9 mg/kg) sufficient for the blockade of veratridine (50 μg/kg)-induced SAR stimulation, maintained inflations at higher pressures (10 and 15 cmH(2)O) greatly inhibited SAR discharges. Under these conditions, the inhibition of SAR discharges was not observed during maintained inflations at 5 cmH(2)O.

SIGNIFICANCE

These results suggest that neither low-threshold SARs nor high-threshold SARs in the rat lung are deactivated during maintained inflations at higher pressures.

Effect of ouabain on the afterhyperpolarization of slowly adapting pulmonary stretch receptors in the rat lung

In anesthetized, artificially ventilated rats with one vagus nerve section, the purposes of the present study were to investigate whether release from phasic consecutive hyperinflations (inflation volume=3 tidal volumes) results in the afterhyperpolarization (AHP) of the slowly adapting pulmonary stretch receptor (SAR) activity and whether the effect of ouabain, a Na+-K+ ATPase inhibitor, alters AHP of the SAR activity seen after release from maintained inflations. Release from 10 consecutive phasic hyperinflations did not cause any significant inhibition of SAR activity. Release from maintained inflations (for approximately 10 and 15 cmH2O) for 5 s produced the induction of disappearance of SAR activity, corresponding with the AHP. Intravenous administration of ouabain (20 and 40 microg/kg) had no significant effects on the responses of SAR activity and SAR adaptation index (AI) to maintained inflations, but ouabain treatment with at 40 microg/kg resulted in a significant increase in the SAR activity after stopping the respirator and significantly attenuated the AHP of the SAR activity. In the immunohistochemical study, we found Na+-K+ ATPase alpha3-subunit-isoforms-like immunoreactivity in SAR terminals, forming leaflike extensions in the intrapulmonary bronchioles at different diameters, and those terminals were buried in the smooth muscle. In the same sections, the alpha1 subunit immunoreactivity of SAR terminals was not found. These results suggest that the mechanism of generating the AHP of SARs is mainly mediated by the activation of Na+-K+ ATPase alpha3 subunit isoform.

Effects of acetazolamide and 4-aminoprydine on the responses of deflationary slowly adapting pulmonary stretch receptors to CO2 inhalation in the rat

The inhibitory effect of CO(2) on deflationary slowly adapting pulmonary stretch receptors (deflationary SARs) was investigated before and after administration of acetazolamide, a carbonic anhydrase (CA) inhibitor, or 4-aminopyridine (4-AP), a K(+) channel blocker, in anesthetized, artificially ventilated rats after unilateral vagotomy. CO(2) inhalation (maximum tracheal CO(2) concentration ranging from 9 to 12%) for approximately 60 s decreased the impulse activity of deflationary SARs but had no significant effect on tracheal pressure (P(T)) as an index of bronchomotor tone. Acetazolamide treatment (20 mg/kg) diminished the inhibitory response of deflationary SARs to CO(2) inhalation. 4-AP (0.7 and 2.0 mg/kg) dose-dependently attenuated the decrease in deflationary SAR activity induced by CO(2) inhalation. When comparing the maximum attenuation due to 4-AP (2.0 mg/kg) and acetazolamide (20 mg/kg) in CO(2)-induced deflationary SAR inhibition, blockade of K(+) channels had a more pronounced effect. These results suggest that inhibition of deflationary SARs by CO(2) inhalation may be largely mediated by the stimulating action of 4-AP-sensitive K(+) currents in the nerve terminals of the receptors.