State-dependent regulation of breathing by the retrotrapezoid nucleus

In this study we stimulated the retrotrapezoidal neurons (RTN) by optogenetics in behaving animals to determine ventilatory responses across sleep and wake states. We targeted the rostral RTN region (n=12) with a PRSX8 promoter lentivirus and limited ChannelRhodopsin expression to RTN neurons and noradrenergic A5 neurons. A subset of rats (n=6) also received intraspinal anti-DBH-saporin, a catecholaminergic-specific neurotoxin, to eliminate A5 neurons. We observed no significant differences for the resting or evoked ventilatory behavior after effective lesion. In quiet awake and non-rapid eye movement (nREM; quiet asleep) states, RTN stimulation increased respiratory frequency and tidal volume, and could entrain respiratory rate between 1.2Hz - 2Hz. We also observed active expiration in the wake state only. The increase in breathing by RTN stimulation was occluded by hypercapnia (6% CO2), but not hypoxia (12% O2). Surprisingly, RTN stimulation in REM (active sleep) failed to increase or pace respiratory rate and only tidal volume was significantly increased. This REM phenomenon draws an intriguing parallel to voluntary breathing (active wake), where homeostatic mechanisms are limited to adjusting the depth of each cortically-triggered breath.