Exploring the role of the Kölliker-Fuse nucleus in breathing variability by mathematical modelling.

The Kölliker-Fuse nucleus (KF), which is part of the parabrachial complex, participates in the generation of eupnoea under resting conditions and the control of active abdominal expiration when increased ventilation is required. Moreover, dysfunctions in KF neuronal activity are believed to play a role in the emergence of respiratory abnormalities seen in Rett syndrome (RTT), a progressive neurodevelopmental disorder associated with an irregular breathing pattern and frequent apnoeas. Relatively little is known, however, about the intrinsic dynamics of neurons within the KF and how their synaptic connections affect breathing pattern control and contribute to breathing irregularities. In this study, we use a reduced computational model to consider several dynamical regimes of KF activity paired with different input sources to determine which combinations are compatible with known experimental observations. We further build on these findings to identify possible interactions between the KF and other components of the respiratory neural circuitry. Specifically, we present two models that both simulate eupnoeic as well as RTT-like breathing phenotypes. Using nullcline analysis, we identify the types of inhibitory inputs to the KF leading to RTT-like respiratory patterns and suggest possible KF local circuit organizations. When the identified properties are present, the two models also exhibit quantal acceleration of late-expiratory activity, a hallmark of active expiration featuring forced exhalation, with increasing inhibition to KF, as reported experimentally. Hence, these models instantiate plausible hypotheses about possible KF dynamics and forms of local network interactions, thus providing a general framework as well as specific predictions for future experimental testing. KEY POINTS: The Kölliker-Fuse nucleus (KF), a part of the parabrachial complex, is involved in regulating normal breathing and controlling active abdominal expiration during increased ventilation. Dysfunction in KF neuronal activity is thought to contribute to respiratory abnormalities seen in Rett syndrome (RTT). This study utilizes computational modelling to explore different dynamical regimes of KF activity and their compatibility with experimental observations. By analysing different model configurations, the study identifies inhibitory inputs to the KF that lead to RTT-like respiratory patterns and proposes potential KF local circuit organizations. Two models are presented that simulate both normal breathing and RTT-like breathing patterns. These models provide testable hypotheses and specific predictions for future experimental investigations, offering a general framework for understanding KF dynamics and potential network interactions.

Abdominal expiratory activity in the rat brainstem-spinal cord in situ: patterns, origins and implications for respiratory rhythm generation.

We studied respiratory neural activity generated during expiration. Motoneuronal activity was recorded simultaneously from abdominal (AbN), phrenic (PN), hypoglossal (HN) and central vagus nerves from neonatal and juvenile rats in situ. During eupnoeic activity, low-amplitude post-inspiratory (post-I) discharge was only present in AbN motor outflow. Expression of AbN late-expiratory (late-E) activity, preceding PN bursts, occurred during hypercapnia. Biphasic expiratory (biphasic-E) activity with pre-inspiratory (pre-I) and post-I discharges occurred only during eucapnic anoxia or hypercapnic anoxia. Late-E activity generated during hypercapnia (7-10% CO(2)) was abolished with pontine transections or chemical suppression of retrotrapezoid nucleus/ventrolateral parafacial (RTN/vlPF). AbN late-E activity during hypercapnia is coupled with augmented pre-I discharge in HN, truncated PN burst, and was quiescent during inspiration. Our data suggest that the pons provides a necessary excitatory drive to an additional neural oscillatory mechanism that is only activated under conditions of high respiratory drive to generate late-E activity destined for AbN motoneurones. This mechanism may arise from neurons located in the RTN/vlPF or the latter may relay late-E activity generated elsewhere. We hypothesize that this oscillatory mechanism is not a necessary component of the respiratory central pattern generator but constitutes a defensive mechanism activated under critical metabolic conditions to provide forced expiration and reduced upper airway resistance simultaneously. Possible interactions of this oscillator with components of the brainstem respiratory network are discussed.

Multiple pontomedullary mechanisms of respiratory rhythmogenesis.

Mammalian central pattern generators producing rhythmic movements exhibit robust but flexible behavior. However, brainstem network architectures that enable these features are not well understood. Using precise sequential transections through the pons to medulla, it was observed that there was compartmentalization of distinct rhythmogenic mechanisms in the ponto-medullary respiratory network, which has rostro-caudal organization. The eupneic 3-phase respiratory pattern was transformed to a 2-phase and then to a 1-phase pattern as the network was physically reduced. The pons, the retrotrapezoid nucleus and glycine mediated inhibition are all essential for expression of the 3-phase rhythm. The 2-phase rhythm depends on inhibitory interactions (reciprocal) between Bötzinger and pre-Bötzinger complexes, whereas the 1-phase-pattern is generated within the pre-Bötzinger complex and is reliant on the persistent sodium current. In conditions of forced expiration, the RTN region was found to be essential for the expression of abdominal late expiratory activity. However, it is unknown whether the RTN generates or simply relays this activity. Entrained with the central respiratory network is the sympathetic nervous system, which exhibits patterns of discharge coupled with the respiratory cycle (in terms of both gain and phase of coupling) and dysfunctions in this coupling appear to underpin pathological conditions. In conclusion, the respiratory network has rhythmogenic capabilities at multiple levels of network organization, allowing expression of motor patterns specific for various physiological and pathophysiological respiratory behaviors.

Spatial and functional architecture of the mammalian brain stem respiratory network: a hierarchy of three oscillatory mechanisms.

Mammalian central pattern generators (CPGs) producing rhythmic movements exhibit extremely robust and flexible behavior. Network architectures that enable these features are not well understood. Here we studied organization of the brain stem respiratory CPG. By sequential rostral to caudal transections through the pontine-medullary respiratory network within an in situ perfused rat brain stem-spinal cord preparation, we showed that network dynamics reorganized and new rhythmogenic mechanisms emerged. The normal three-phase respiratory rhythm transformed to a two-phase and then to a one-phase rhythm as the network was reduced. Expression of the three-phase rhythm required the presence of the pons, generation of the two-phase rhythm depended on the integrity of Bötzinger and pre-Bötzinger complexes and interactions between them, and the one-phase rhythm was generated within the pre-Bötzinger complex. Transformation from the three-phase to a two-phase pattern also occurred in intact preparations when chloride-mediated synaptic inhibition was reduced. In contrast to the three-phase and two-phase rhythms, the one-phase rhythm was abolished by blockade of persistent sodium current (I(NaP)). A model of the respiratory network was developed to reproduce and explain these observations. The model incorporated interacting populations of respiratory neurons within spatially organized brain stem compartments. Our simulations reproduced the respiratory patterns recorded from intact and sequentially reduced preparations. Our results suggest that the three-phase and two-phase rhythms involve inhibitory network interactions, whereas the one-phase rhythm depends on I(NaP). We conclude that the respiratory network has rhythmogenic capabilities at multiple levels of network organization, allowing expression of motor patterns specific for various physiological and pathophysiological respiratory behaviors.

Nitric oxide and respiratory rhythm in mammals: a new modulator of phase transition?

NO (nitric oxide) modulates several central pattern generators, but its role in respiratory rhythmogenesis and its mode of action on medullary respiratory neurons during normoxia are unknown. We analysed the actions of NO on the mammalian respiratory network at the system and cellular levels. Given systemically, the NO donor diethylamine NONOate increased post-inspiratory duration in vagus, phrenic and hypoglossal nerves, whereas blockade of NO generation with L-NAME (N(G)-nitro-L-arginine methyl ester) produced the opposite response. At the cellular level, we pressure-ejected the NO donor on to respiratory neurons. NO had both inhibitory and excitatory effects on all types of respiratory neurons. Inhibitory effects involved soluble guanylate cyclase, as they were blocked with ODQ (1H-[1,2,4]oxadiazolo[4,3a]quinoxalin-1-one), whereas excitations were antagonized by uric acid and possibly mediated via peroxynitrite. Importantly, NO facilitated both GABA (gamma-aminobutyric acid)- and NMDA (N-methyl-D-aspartate)-induced neuronal responses, but this was restricted to post-inspiratory and pre-inspiratory neurons; other neuron types showed additive effects only. Our results support NO as modulator of centrally generated respiratory activity and specifically of ligand-mediated responses in respiratory neuron types involved in respiratory phase transition.