State-dependent control of breathing by the retrotrapezoid nucleus

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 (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% FI,CO2 ) 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 CO2 but f(R) 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.