Synaptic interactions between respiratory neurons during inspiratory on-switching evoked by vagal stimulation in decerebrate cats

The Modulation by Anesthetics and Analgesics of Respiratory Rhythm in the Nervous System

Rhythmic eupneic breathing in mammals depends on the coordinated activities of the neural system that sends cranial and spinal motor outputs to respiratory muscles. These outputs modulate lung ventilation and adjust respiratory airflow, which depends on the upper airway patency and ventilatory musculature. Anesthetics are widely used in clinical practice worldwide. In addition to clinically necessary pharmacological effects, respiratory depression is a critical side effect induced by most general anesthetics. Therefore, understanding how general anesthetics modulate the respiratory system is important for the development of safer general anesthetics. Currently used volatile anesthetics and most intravenous anesthetics induce inhibitory effects on respiratory outputs. Various general anesthetics produce differential effects on respiratory characteristics, including the respiratory rate, tidal volume, airway resistance, and ventilatory response. At the cellular and molecular levels, the mechanisms underlying anesthetic-induced breathing depression mainly include modulation of synaptic transmission of ligand-gated ionotropic receptors (e.g., γ-aminobutyric acid, N-methyl-D-aspartate, and nicotinic acetylcholine receptors) and ion channels (e.g., voltage-gated sodium, calcium, and potassium channels, two-pore domain potassium channels, and sodium leak channels), which affect neuronal firing in brainstem respiratory and peripheral chemoreceptor areas. The present review comprehensively summarizes the modulation of the respiratory system by clinically used general anesthetics, including the effects at the molecular, cellular, anatomic, and behavioral levels. Specifically, analgesics, such as opioids, which cause respiratory depression and the "opioid crisis", are discussed. Finally, underlying strategies of respiratory stimulation that target general anesthetics and/or analgesics are summarized.

Computational models and emergent properties of respiratory neural networks

Computational models of the neural control system for breathing in mammals provide a theoretical and computational framework bringing together experimental data obtained from different animal preparations under various experimental conditions. Many of these models were developed in parallel and iteratively with experimental studies and provided predictions guiding new experiments. This data-driven modeling approach has advanced our understanding of respiratory network architecture and neural mechanisms underlying generation of the respiratory rhythm and pattern, including their functional reorganization under different physiological conditions. Models reviewed here vary in neurobiological details and computational complexity and span multiple spatiotemporal scales of respiratory control mechanisms. Recent models describe interacting populations of respiratory neurons spatially distributed within the Bötzinger and pre-Bötzinger complexes and rostral ventrolateral medulla that contain core circuits of the respiratory central pattern generator (CPG). Network interactions within these circuits along with intrinsic rhythmogenic properties of neurons form a hierarchy of multiple rhythm generation mechanisms. The functional expression of these mechanisms is controlled by input drives from other brainstem components,including the retrotrapezoid nucleus and pons, which regulate the dynamic behavior of the core circuitry. The emerging view is that the brainstem respiratory network has rhythmogenic capabilities at multiple levels of circuit organization. This allows flexible, state-dependent expression of different neural pattern-generation mechanisms under various physiological conditions,enabling a wide repertoire of respiratory behaviors. Some models consider control of the respiratory CPG by pulmonary feedback and network reconfiguration during defensive behaviors such as cough. Future directions in modeling of the respiratory CPG are considered.

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.

Control of breathing by interacting pontine and pulmonary feedback loops

The medullary respiratory network generates respiratory rhythm via sequential phase switching, which in turn is controlled by multiple feedbacks including those from the pons and nucleus tractus solitarii; the latter mediates pulmonary afferent feedback to the medullary circuits. It is hypothesized that both pontine and pulmonary feedback pathways operate via activation of medullary respiratory neurons that are critically involved in phase switching. Moreover, the pontine and pulmonary control loops interact, so that pulmonary afferents control the gain of pontine influence of the respiratory pattern. We used an established computational model of the respiratory network (Smith et al., 2007) and extended it by incorporating pontine circuits and pulmonary feedback. In the extended model, the pontine neurons receive phasic excitatory activation from, and provide feedback to, medullary respiratory neurons responsible for the onset and termination of inspiration. The model was used to study the effects of: (1) "vagotomy" (removal of pulmonary feedback), (2) suppression of pontine activity attenuating pontine feedback, and (3) these perturbations applied together on the respiratory pattern and durations of inspiration (T(I)) and expiration (T(E)). In our model: (a) the simulated vagotomy resulted in increases of both T(I) and T(E), (b) the suppression of pontine-medullary interactions led to the prolongation of T(I) at relatively constant, but variable T(E), and (c) these perturbations applied together resulted in "apneusis," characterized by a significantly prolonged T(I). The results of modeling were compared with, and provided a reasonable explanation for, multiple experimental data. The characteristic changes in T(I) and T(E) demonstrated with the model may represent characteristic changes in the balance between the pontine and pulmonary feedback control mechanisms that may reflect specific cardio-respiratory disorders and diseases.

Synaptic mechanisms of inspiratory off-switching evoked by pontine pneumotaxic stimulation in cats

To elucidate synaptic mechanisms and the involvement of N-methyl-D-aspartate (NMDA) receptors in inspiratory off-switching (IOS) evoked by the stimulation of the nucleus parabrachialis medialis (NPBM), excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were recorded from bulbar augmenting inspiratory (aug-I) and postinspiratory (PI) neurons in vagotomized cats. Stimulation of NPBM produced either transient inhibition or premature termination of inspiration (reversible or irreversible IOS), depending on the stimulus intensity. Each neuron displayed four-phasic postsynaptic responses during the reversible IOS, i.e. Phase 1 EPSPs, Phase 2 IPSPs, Phase 3 EPSPs and Phase 4 IPSPs in aug-I neurons, and Phase 1 plus 2 EPSPs, Phase 3 IPSPs and Phase 4 EPSPs in PI neurons. During the irreversible IOS, Phase 4 responses were replaced by sustained hyperpolarization in aug-I neurons and decrementing depolarization in PI neurons. Blockade of NMDA receptors by dizocilpine (0.3 mg kg(-1) i.v.) selectively increased Phase 4 potentials in both types of neurons and decreased the thresholds for evoking the irreversible IOS. The NPBM-induced responses had a pattern and time-course similar to those induced by vagal stimulation. The present results suggest that pneumotaxic and vagal inputs converge on the common IOS circuit, and the effectiveness of both inputs is modulated by NMDA receptors.

Neuropharmacology of control of respiratory rhythm and pattern in mature mammals

This review summarizes the current understanding of the neurotransmitters and neuromodulators that are involved, firstly, in respiratory rhythm and pattern generation, where glutamate plays an essential role in the excitatory mechanisms and glycine and gamma-aminobutyric acid mediate inhibitory postsynaptic effects, and secondly, in the transmission of input signals from the central and peripheral chemoreceptors and of motor outputs to respiratory motor neurons. Finally, neuronal mechanisms underlying respiratory modulations caused by respiratory depressants and excitants, such as general anesthetics, benzodiazepines, opioids, and cholinergic agents, are described.