A research group led by Assistant Professor Yoko Ikoma, Professor Ko Matsui and graduate students Yusuke Takahashi and Daichi Sasaki of the Graduate School of Life Sciences at Tohoku University announced that they had discovered that glial cell astrocytes in the hypothalamus become acidified during REM sleep. They found that acidification and other environmental changes in the brain occur during REM sleep by modifying a proprietary fiber photometry method and examining astrocytes in the hypothalamus of various genetically modified mice. They also confirmed that brain waves such as REM sleep can be measured by artificially manipulating acidification. They also found acidification to be greatly enhanced in mice with exacerbated epileptiform seizure symptoms. The results are expected to lead to the development of astrocyte‐targeted therapies for epilepsy. Their research was published in the international journal 'Brain.'
It is thought that during sleep, organisms shift from non‐REM to REM sleep, and that during REM sleep, active neural activity similar to that of being awake occurs and dreams occur in order to organize memories. REM sleep can be identified by electroencephalogram (EEG) measurements and electrocardiography, which occur in neural activity. The transition to REM sleep is instantaneous, with synchronous activity of many nerve cells measured, which is different from non‐REM sleep.
Up until now, research groups have reported that astrocytes in the hypothalamus amplify neural oscillations initiated in the hippocampus and may also induce exacerbation and plasticity of epilepsy. The hypothalamus is located in the diencephalon, the central region of the autonomic nervous system responsible for vital phenomena and is closely related to sleep, wakefulness and metabolic energy in the brain and the whole body.
The research group posited that the synchronous neural activity that occurs in REM sleep requires a great deal of energy and that there are changes in the brain environment that support such information processing. They therefore set about studying the brain environment using an improved fiber photometry method, focusing on astrocytes in the hypothalamus. The method measures optical signals by inserting an optical fiber into the brain that has been modified to screen out the effects of fluctuations in local blood flow and enable accurate measurements.
They began by using this method to measure autofluorescence during sleep, and an increase in blood flow during REM sleep was recorded. Fluorescent sensor‐expressing mice were then generated that express intracellular pH and calcium ion (Ca2+) sensors in an astrocyte‐specific manner, respectively. They measured brain environmental fluctuations in these two types of mice. The pH in hypothalamic astrocyte cells was then found to become more acidic and calcium concentrations decreased with REM sleep.
Analysis of the timing of these changes in the brain environment and the onset of REM sleep as measured by EEG showed that the changes in the brain environment precede the onset of REM sleep by approximately 20 seconds.
To test whether acidification regulates REM sleep, mice were generated in which channelrhodopsin 2, which can artificially cause acidification by light stimulation, was introduced into astrocytes, and light stimulation was given to the mice, then, REM sleep was induced. This revealed a close relation between REM sleep and glial cells.
Finally, focusing on the relation between REM sleep and diseases, in particular, epilepsy, among which is the subject of the laboratory's research, the laboratory examined whether the environmental changes in the hypothalamus differ from those in healthy mice.
The lab created epilepsy model mice by electrically stimulating the hippocampus of mice. A comparison of environmental changes induced by REM sleep in the hypothalamus with healthy mice showed that acidification was greatly enhanced in a mouse model of epileptic pathology. There were no differences in local blood flow or calcium concentration.
Unlike neurons, glial cells were thought not to be involved in information processing in the brain as they do not have action potentials, but this study showed that they may regulate the brain and state of mind. In addition, in epilepsy, acidification is increased during REM sleep, and this response could be used as an indicator to diagnose the level of development.
"We believe that glial cells may create conditions in which neuronal plasticity related to learning and memory is more or less likely to occur," said Matsui. "From this perspective, we would like to investigate how the state of glial cells changes in various areas of the brain, not only in sleep but also in learning and diseases."
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