Mechanistic Studies Of Sleep Transition Based On Electrophysiological Default Mode Network

As a fundamental phenomenon of life,sleep is divided into two modes: non-rapid eye movement sleep(NREMS)and rapid eye movement sleep(REMS).The rapid transitions between NREMS and REMS can help brain achieve different physiological functions in sleep,suggesting the importance for the mechanistic studies of these transitions during sleep.The default mode network(DMN)is one of fundamental networks in the brain which is closely related to the levels of consciousness.Accumulating evidence indicates that there exists distinct functional connectivity in the DMN in different types of sleep.However,it is still unclear how these differences of DMN connectivity are formed during the rapid transitions in sleep.To explore how DMN realize the functional transformation during the sleep transition period,we investigated how the activity of electrophysiological DMN changes dynamically during the transitions from NREMS to REMS in rats.Our main results are as follows:1.DMN had two functional modules in sleep.The module called frontal module contained prelimbic cortex,orbitofrontal cortex and cingulate cortex.The other module called posterior module contained posterior parietal cortex,medial secondary visual cortex and retrosplenial cortex.The electroencephalogram(EEG)characteristics of the brain regions in the same module changed in a similar way during the sleep transition period.It indicated that the stability of DMN functional modules can be maintained while the mechanism of sleep transition promoted the changes of electrophysiological characteristics.In addition,the temporal lobe and hippocampus enhanced the functional connectivity with the two modules during the sleep transition period.This maybe the foundation for achieving more complex brain functions in REMS.2.Before attaining REMS,the changes of EEG characteristics were mainly characterized by the decrease of energy in Delta band(0.5-6 Hz)and the increase of energy in Theta band(6-10 Hz).After attaining REMS,the changes of EEG characteristics were mainly characterized by the enhancement of functional connectivity and the increase of complexity.It meant that DMN focused on changing the energy characteristics of EEG before attaining REMS and achieving the REMS functions after attaining REMS.This sequential process can ensure the function of DMN changes smoothly.3.From the research of energy distribution characteristic,we found that the change of energy characteristics in Theta band started from hippocampus and frontal module,then this phenomenon occurred on other DMN regions.After attaining REMS,the energy characteristics still needed some time to become stable.It indicated that DMN needed a relative long time to finish a complete sleep transition process.In addition,the sleep transition process of several DMN regions(includes hippocampus,retrosplenial cortex,temporal lobe,prelimbic cortex and orbitofrontal cortex)related to memory function were affected by the circadian rhythm which may lead to a weaker memory function of DMN at night in REMS.The above results indicated that DMN not only maintained the stability of the functional module,but also completed the changes of energy characteristics and functional connectivity orderly during the sleep transition period in rats.These results were important for understanding how the DMN function changes in sleep.