Optogenetic control of the neuronal network responsible for paradoxical (REM) sleep atonia – OPTOREM

The recent development of opto- and pharmacogenetics, two technologies that allow in mice the manipulation of the firing activity of genetically-targeted neurons with unprecedented temporal and spatial resolutions, offers an exceptional opportunity to achieve this goal. In the present project, we therefore propose a series of experiments combining the use of these molecular tools with intracellular and polysomnographic recordings in freely moving or head-restrained mice. By this means, we will directly assess the respective contribution of the glutamate SLD neurons and GABA/glycine neurons of the GiV-LPGi in the muscle atonia of natural PS. Further, we will establish whether the motor cortex induce, through phasic motoneuron excitation, the PS muscle twitches and the violent movements observed during PS in RBD patients when the muscle atonia is pathologically disabled. We will finally test the hypothesis that cataplexy during waking is due to an inappropriate activation of the same SLD-Giv-LPGi pathway, usually recruited only during PS.

We found out that the genetic inactivation of glutamatergic neurons within the pontine sublaterodorsal tegmental nucleus or GABA/glycinergic neurons of the GiV induces REM sleep behavior disorder (RBD) in rats, similar to that described in humans. These original data highlight the functional role of this subset of brainstem neurons in natural PS regulation and their potential contribution to the RBD pathogenesis.

We pursue our effort to demonstrate the role of the glutamatergic and GABAergic neurons identified in the muscle atonia of paradoxical sleep using optogenetic. We will also determine whether the phasic movements occurring during REM sleep behavior disorder are generated by the motor cortex.

Two articles will be submitted this year on our first results

Sleep encompasses two distinct entities, slow wave sleep and paradoxical sleep (also coined Rapid Eye Movement, REM sleep). Paradoxical sleep (PS) is characterized by a cortical activation and REMs associated with an atonia of the whole somatic musculature sporadically interrupted by muscle twitches. The identification of the mechanisms controlling the motor activity during PS is clinically relevant since an abnormal regulation of the muscle tone occurs in three of the major disabling sleep disorders, i.e. REM sleep behavior disorder (RBD), narcolepsy with cataplexy or obstructive sleep apnea (OSA). Although the symptoms of these pathologies are well described, the underlying neurobiological mechanisms remain unknown. In that context, the objective of the basic research project is to understand the anatomical and functional organization of the neuronal networks generating muscle atonia and phasic twitches during PS in mice.
Based on a large body of functional, neuroanatomical, electrophysiological and local pharmacological data collected in rats over the last decade, our team, internationally recognized in the field of PS basic research, recently proposed a new functional model assigning to brainstem glutamate and GABA/glycine neurons a central interactive role in the mechanisms generating the muscle atonia of PS. We indeed identified a population of glutamate neurons specifically active during PS (PS-on) located in a dorsal pontine nucleus (sublaterodorsal tegmental nucleus, SLD). These SLD neurons may generate muscle atonia via descending projections to GABA/glycine inhibitory neurons located in the ventro-medial medullary reticular formation (ventral gigantocellular, GiV and lateral paragigantocellular reticular nuclei, LPGi). These later PS-on inhibitory neurons may induce during PS a sustained hyperpolarization of brainstem-spinal somatic motoneurons and by this means the block, although incomplete, of the phasic excitatory drives leading to muscle twitches, likely resulting from the motor cortex activation. The present functional model is still a matter of debate and needs to be experimentally validated. The objective of the present project is thus to fill this gap.
The recent development of opto- and pharmacogenetics, two technologies that allow in mice the manipulation of the firing activity of genetically-targeted neurons with unprecedented temporal and spatial resolutions, offers an exceptional opportunity to achieve this goal. In the present project, we therefore propose a series of experiments combining the use of these molecular tools with intracellular and polysomnographic recordings in freely moving or head-restrained mice. By this means, we will directly assess the respective contribution of the glutamate SLD neurons and GABA/glycine neurons of the GiV-LPGi in the muscle atonia of natural PS. Further, we will establish whether the motor cortex induce, through phasic motoneuron excitation, the PS muscle twitches and the violent movements observed during PS in RBD patients when the muscle atonia is pathologically disabled. We will finally test the hypothesis that cataplexy during waking is due to an inappropriate activation of the same SLD-Giv-LPGi pathway, usually recruited only during PS.
We are convinced that the achievement of this project will constitute a critical step in the identification of the neuronal networks responsible for the muscle control during PS. Furthermore, the identification of the neurobiological mechanisms underlying cataplexy and RBD by using elegant opto- and pharmacogenetic tools will help to open up new paths for the development of targeted pharmacological treatments or the design of original therapies of these two pathologies.