Behavioural state‐specific neurons in the mouse medulla involved in sleep‐wake switching

The medullary reticular formation (RF) is involved in the maintenance of several vital physiological functions and level of vigilance. In this study, in nonanesthetised, head-fixed mice, I examined the role of medullary RF neurons in the control of sleep-wake states, that is, wakefulness (W), slow-wave sleep (SWS) and paradoxical (or rapid eye movement) sleep (PS). I showed, for the first time, that the mouse medullary RF contains presumed SWS-promoting, SWS-on neurons that remain silent during W, display a sharp increase in discharge rate at sleep onset, and discharge tonically and selectively during SWS. In addition, I showed the presence in the medullary RF of both PS-on and PS-off neurons, which, respectively, commence discharging or cease firing selectively just prior to, and during, PS. PS-off neurons were located in the raphe nuclei and ventral medulla, while PS-on neurons were found in both the lateral part of the ventral gigantocellular reticular nucleus and the raphe nuclei, as were SWS-on neurons. PS-off and SWS-on neurons appear to play an important role in both the W-SWS and SWS-PS switches, while PS-on and PS-off neurons play an important role in the PS-W switch. The present findings on the trends in spike activity at the transitions from SWS to PS and from PS to W are in line with the reciprocal interaction hypothesis according to which PS occurs as a result of the cessation of discharge of PS-off neurons, while PS ends as a result of the start of discharge of PS-off neurons.