Is REM sleep a paradoxical state showing muscle atonia and a cortical activity similar to waking?

Abstract

Michel Jouvet renamed REM sleep paradoxical sleep (PS) after his discovery that it displays an EEG similar to waking but with a complete muscle paralysis. To determine whether the cortical activation is similar during PS and waking, we recently identified at cellular level the populations of cortical neurons activated and displaying plasticity during waking and PS hypersomnia by means of functional neuroanatomy [3] . Our mapping clearly shows for the first time that only a small number of limbic structures are activated during PS in contrast to waking. These structures are the cortical amygdaloid nucleus, the anterior cingulate, retrosplenial and medial entorhinal cortices, the claustrum and the dentate gyrus (DG) [3] . Further, combining retrograde tracing, neurotoxic lesion and FOS immunostaining, we showed that neurons of the claustrum and from the lateral part of the supramammillary nucleus (SuML) are responsible for the activation of the cortical structures and the DG during PS [3] . We further recently showed using stimulation of the SuML/DG pathway using optogenetic induces an increase in theta power and frequency indicating that this pathway plays a role in theta. These surprising results pointed out for the first time that the claustrum and the SuML activate a subset of limbic cortical neurons specifically during PS in contrast to waking during which the aminergic, cholinergic and the hypocretin systems activate the cortex. We propose that the limbic cortical activation revealed in our study might play a key role in the previously reported beneficial effect of PS on learning and memory. Indeed, many studies clearly indicate that PS is instrumental for memory consolidation [1] . Further, it has recently been shown that PS deprivation in rats impairs consolidation of contextual fear conditioning [2] .