Tonic and phasic drive to medullary respiratory neurons during periodic breathing

Carotid chemoreceptors tune breathing via multipath routing: reticular chain and loop operations supported by parallel spike train correlations

We tested the hypothesis that carotid chemoreceptors tune breathing through parallel circuit paths that target distinct elements of an inspiratory neuron chain in the ventral respiratory column (VRC). Microelectrode arrays were used to monitor neuronal spike trains simultaneously in the VRC, peri-nucleus tractus solitarius (p-NTS)-medial medulla, the dorsal parafacial region of the lateral tegmental field (FTL-pF), and medullary raphe nuclei together with phrenic nerve activity during selective stimulation of carotid chemoreceptors or transient hypoxia in 19 decerebrate, neuromuscularly blocked, and artificially ventilated cats. Of 994 neurons tested, 56% had a significant change in firing rate. A total of 33,422 cell pairs were evaluated for signs of functional interaction; 63% of chemoresponsive neurons were elements of at least one pair with correlational signatures indicative of paucisynaptic relationships. We detected evidence for postinspiratory neuron inhibition of rostral VRC I-Driver (pre-Bötzinger) neurons, an interaction predicted to modulate breathing frequency, and for reciprocal excitation between chemoresponsive p-NTS neurons and more downstream VRC inspiratory neurons for control of breathing depth. Chemoresponsive pericolumnar tonic expiratory neurons, proposed to amplify inspiratory drive by disinhibition, were correlationally linked to afferent and efferent "chains" of chemoresponsive neurons extending to all monitored regions. The chains included coordinated clusters of chemoresponsive FTL-pF neurons with functional links to widespread medullary sites involved in the control of breathing. The results support long-standing concepts on brain stem network architecture and a circuit model for peripheral chemoreceptor modulation of breathing with multiple circuit loops and chains tuned by tegmental field neurons with quasi-periodic discharge patterns. NEW & NOTEWORTHY We tested the long-standing hypothesis that carotid chemoreceptors tune the frequency and depth of breathing through parallel circuit operations targeting the ventral respiratory column. Responses to stimulation of the chemoreceptors and identified functional connectivity support differential tuning of inspiratory neuron burst duration and firing rate and a model of brain stem network architecture incorporating tonic expiratory "hub" neurons regulated by convergent neuronal chains and loops through rostral lateral tegmental field neurons with quasi-periodic discharge patterns.

Functional connectivity in raphé-pontomedullary circuits supports active suppression of breathing during hypocapnic apnea

Hyperventilation is a common feature of disordered breathing. Apnea ensues if CO2 drive is sufficiently reduced. We tested the hypothesis that medullary raphé, ventral respiratory column (VRC), and pontine neurons have functional connectivity and persistent or evoked activities appropriate for roles in the suppression of drive and rhythm during hyperventilation and apnea. Phrenic nerve activity, arterial blood pressure, end-tidal CO2, and other parameters were monitored in 10 decerebrate, vagotomized, neuromuscularly-blocked, and artificially ventilated cats. Multielectrode arrays recorded spiking activity of 649 neurons. Loss and return of rhythmic activity during passive hyperventilation to apnea were identified with the S-transform. Diverse fluctuating activity patterns were recorded in the raphé-pontomedullary respiratory network during the transition to hypocapnic apnea. The firing rates of 160 neurons increased during apnea; the rates of 241 others decreased or stopped. VRC inspiratory neurons were usually the last to cease firing or lose rhythmic activity during the transition to apnea. Mayer wave-related oscillations (0.04-0.1 Hz) in firing rate were also disrupted during apnea. Four-hundred neurons (62%) were elements of pairs with at least one hyperventilation-responsive neuron and a correlational signature of interaction identified by cross-correlation or gravitational clustering. Our results support a model with distinct groups of chemoresponsive raphé neurons contributing to hypocapnic apnea through parallel processes that incorporate disfacilitation and active inhibition of inspiratory motor drive by expiratory neurons. During apnea, carotid chemoreceptors can evoke rhythm reemergence and an inspiratory shift in the balance of reciprocal inhibition via suppression of ongoing tonic expiratory neuron activity.

State-dependent control of breathing by the retrotrapezoid nucleus

KEY POINTS

This study explores the state dependence of the hypercapnic ventilatory reflex (HCVR). We simulated an instantaneous increase or decrease of central chemoreceptor activity by activating or inhibiting the retrotrapezoid nucleus (RTN) by optogenetics in conscious rats. During quiet wake or non-REM sleep, hypercapnia increased both breathing frequency (fR ) and tidal volume (VT ) whereas, in REM sleep, hypercapnia increased VT exclusively. Optogenetic inhibition of RTN reduced VT in all sleep-wake states, but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased VT but raised fR only in quiet wake and non-REM sleep. Phasic RTN stimulation produced active expiration and reduced early expiratory airflow (i.e. increased upper airway resistance) only during wake. We conclude that the HCVR is highly state-dependent. The HCVR is reduced during REM sleep because fR is no longer under chemoreceptor control and thus could explain why central sleep apnoea is less frequent in REM sleep.

ABSTRACT

Breathing has different characteristics during quiet wake, non-REM or REM sleep, including variable dependence on PCO2. We investigated whether the retrotrapezoid nucleus (RTN), a proton-sensitive structure that mediates a large portion of the hypercapnic ventilatory reflex, regulates breathing differently during sleep vs. wake. Electroencephalogram, neck electromyogram, blood pressure, respiratory frequency (fR ) and tidal volume (VT ) were recorded in 28 conscious adult male Sprague-Dawley rats. Optogenetic stimulation of RTN with channelrhodopsin-2, or inhibition with archaerhodopsin, simulated an instantaneous increase or decrease of central chemoreceptor activity. Both opsins were delivered with PRSX8-promoter-containing lentiviral vectors. RTN and catecholaminergic neurons were transduced. During quiet wake or non-REM sleep, hypercapnia (3 or 6% FI,CO2 ) increased both fR and VT whereas, in REM sleep, hypercapnia increased VT exclusively. RTN inhibition always reduced VT but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased VT but raised fR only in quiet wake and non-REM sleep. Blood pressure was unaffected by either stimulation or inhibition. Except in REM sleep, phasic RTN stimulation entrained and shortened the breathing cycle by selectively shortening the post-inspiratory phase. Phasic stimulation also produced active expiration and reduced early expiratory airflow but only during wake. VT is always regulated by RTN and CO2 but fR is regulated by CO2 and RTN only when the brainstem pattern generator is in autorhythmic mode (anaesthesia, non-REM sleep, quiet wake). The reduced contribution of RTN to breathing during REM sleep could explain why certain central apnoeas are less frequent during this sleep stage.

Common mechanisms of compensatory respiratory plasticity in spinal neurological disorders

In many neurological disorders that disrupt spinal function and compromise breathing (e.g. ALS, cervical spinal injury, MS), patients often maintain ventilatory capacity well after the onset of severe CNS pathology. In progressive neurodegenerative diseases, patients ultimately reach a point where compensation is no longer possible, leading to catastrophic ventilatory failure. In this brief review, we consider evidence that common mechanisms of compensatory respiratory plasticity preserve breathing capacity in diverse clinical disorders, despite the onset of severe pathology (e.g. respiratory motor neuron denervation and/or death). We propose that a suite of mechanisms, operating at distinct sites in the respiratory control system, underlies compensatory respiratory plasticity, including: (1) increased (descending) central respiratory drive, (2) motor neuron plasticity, (3) plasticity at the neuromuscular junction or spared respiratory motor neurons, and (4) shifts in the balance from more to less severely compromised respiratory muscles. To establish this framework, we contrast three rodent models of neural dysfunction, each posing unique problems for the generation of adequate inspiratory motor output: (1) respiratory motor neuron death, (2) de- or dysmyelination of cervical spinal pathways, and (3) cervical spinal cord injury, a neuropathology with components of demyelination and motor neuron death. Through this contrast, we hope to understand the multilayered strategies used to "fight" for adequate breathing in the face of mounting pathology.