State‐dependent control of breathing by the retrotrapezoid nucleus

Abstract

Key pointsr 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.r During quiet wake or non-REM sleep, hypercapnia increased both breathing frequency (f R ) and tidal volume (V T ) whereas, in REM sleep, hypercapnia increased V T exclusively.r Optogenetic inhibition of RTN reduced V T in all sleep-wake states, but reduced fR only during quiet wake and non-REM sleep. RTN stimulation always increased V T but raised f R only in quiet wake and non-REM sleep.r Phasic RTN stimulation produced active expiration and reduced early expiratory airflow (i.e. increased upper airway resistance) only during wake.r We conclude that the HCVR is highly state-dependent. The HCVR is reduced during REM sleep because f R 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 P CO 2 . 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 (f R ) and tidal volume (V T ) 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% F I,CO 2 ) increased both f R and V T whereas, in REM sleep, hypercapnia increased V T exclusively. RTN inhibition always reduced V T but reduced f R only during quiet wake and non-REM sleep. RTN stimulation always increased V T but raised f R 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. V T is always regulated by RTN and CO 2 but f R is regulated by CO 2 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.

Selective optogenetic stimulation of the retrotrapezoid nucleus in sleeping rats activates breathing without changing blood pressure or causing arousal or sighs

“…Indeed, the average V E increase elicited by RTN stimulation (83%; ϩ20 ml·min Ϫ1 ·100 g Ϫ1 ) was equivalent to the breathing stimulation elicited by ϳ4% FI CO 2 at steady state (23,43,50). This result suggests that the degree of hyperventilation may not be critical to CO 2 -induced arousal.…”

“…78) increased V E by an average of 83% (ϩ 20 ml·min Ϫ1 ·100 g Ϫ1 ; see Fig. 5, F and I), the equivalent of exposing rats to ϳ4% FI CO 2 (23,50).…”

“…We add here that combined stimulation of RTN (rostral part) plus A5 neurons does not produce arousal from REM sleep either. All these observations could be explained by a generalized reduction in the ability of any stimulus (intero-or exteroceptive) to produce arousal during REM sleep or to regulate breathing (23,28,74).…”

Neural Control of Respiration by the Retrotrapezoid Nucleus.

“…Trials were conducted during nonREM sleep or quiet waking only. No trial was conducted during REM sleep, a state in which breathing is labile and the contribution of RTN to breathing is drastically reduced (Burke et al, 2015b). Trials during which the animals moved, as indicated by EMG activity, were also ignored.…”

“…Optogenetic stimulation of RTN neurons increases mainly F R in quietly awake and non-REM states in rats and mice including conditions where CBs are silent (hyperoxia) (Abbott et al, 2011;Burke et al, 2015b;Holloway et al, 2015;Kumar et al, 2015). Immediate and reversible inhibition of RTN neurons through optogenetics after CBD results in an immediate decrease in respiration.…”