Role of the dorsal paragigantocellular reticular nucleus in paradoxical (rapid eye movement) sleep generation: a combined electrophysiological and anatomical study in the rat.

It is well known that noradrenergic locus coeruleus neurons decrease their activity during slow wave sleep and are quiescent during paradoxical sleep. It was recently proposed that their inactivation during paradoxical sleep is due to a tonic GABAergic inhibition arising from neurons located into the dorsal paragigantocellular reticular nucleus (DPGi). However, the discharge profile of DPGi neurons across the sleep-waking cycle as well as their connections with brain areas involved in paradoxical sleep regulation remain to be described. Here we show, for the first time in the unanesthetized rat that the DPGi contained a subtype of neurons with a tonic and sustained firing activation specifically during paradoxical sleep (PS-on neurons). Noteworthy, their firing rate increase anticipated for few seconds the beginning of the paradoxical sleep bout. By using anterograde tract-tracing, we further showed that the DPGi, in addition to locus coeruleus, directly projected to other areas containing wake-promoting neurons such as the serotonergic neurons of the dorsal raphe nucleus and hypocretinergic neurons of the posterior hypothalamus. Finally, the DPGi sent efferents to the ventrolateral part of the periaqueductal gray matter known to contain paradoxical sleep-suppressing neurons. Taken together, our original results suggest that the PS-on neurons of the DPGi may have their major role in simultaneous inhibitory control over the wake-promoting neurons and the permissive ventrolateral part of the periaqueductal gray matter as a means of influencing vigilance states and especially PS generation.