While we know that rapid eye movement – or REM – sleep is an essential part of restful sleep, we do not know much about what controls it. Now, researchers – using a new technology called optogenetics – have discovered they can trigger REM episodes in mice by shining a light directly onto selected brain cells or neurons.
The team – including members from the Massachusetts Institute of Technology (MIT) and Harvard Medical School, both in Massachusetts – report their work in the Proceedings of the National Academy of Sciences.
In their study report, the researchers explain that we already know the brainstem contains cholinergic cells or neurons that are involved in controlling REM. But – because that brain region also contains a lot of other cell types – it is not easy to tease out the unique role of cholinergic neurons.
Lead author Dr. Christa Van Dort, of the Department of Brain and Cognitive Sciences at MIT, explains that previous studies have suggested brainstem cholinergic neurons are active when we are awake and also during REM sleep, but, “nobody could actually say whether the firing of these specific cells was responsible for the transition to REM sleep.”
So, in their study, the team set out to investigate whether cholinergic neurons could induce REM sleep. They used a radical new technology called optogenetics that is helping scientists understand the brain’s wiring by controlling brain cell activity with light.
In optogenetics, neurons are made to react to light with the insertion of a protein found in algae. In nature, the protein responds to certain wavelengths of light, allowing the algae to move around.
In 2005, researchers at Stanford University discovered that if you insert the protein into certain types of brain cell, you could then shine a light on them and activate them – essentially controlling brain activity at the level of individual cells.
For their study, Dr. Van Dort and colleagues used mice where the light-sensitive protein had been inserted in their cholinergic neurons. The neurons could be activated via a fiber-optic device mounted on the heads of the mice.
They found if they activated the light-sensitized cholinergic neurons during non-REM sleep, it increased the number – but not the duration – of REM sleep episodes the mice had. Further analysis showed the induced REM episodes matched natural REM episodes very closely.
The team is now exploring how the cholinergic brain system links up to brain systems that are already known to be important to REM sleep. And they are also developing and testing ways to produce better non-REM sleep.
The study is important because it gives new clues about how REM sleep is controlled – a step toward understanding how to design natural sleep in humans.
Getting the right kind of sleep and enough of it is important for helping the brain recuperate and restore itself. It also helps us process memories, recharge the immune system and maintain other body functions.
Different stages of sleep are good for different things, says study senior author Emery Brown, the Edward Hood Taplin Professor of Medical Engineering at MIT.
Animal studies show that learning occurs during REM sleep, while slow wave sleep – known as non-REM stage three – is important for making us feel rested and refreshed.
So far, drugs have not been able to replicate the benefits of natural sleep – where REM and non-REM states alternate every 90 minutes, as Prof. Brown explains: “What they do is create sedation. If you are lucky, the sedation allows your natural sleep mechanisms to take over.”
The team’s ultimate goal is to find better ways to create natural sleep. In order to do this, they intend to create and study the different stages of sleep separately and then together.
Medical News Today recently reported on another optogenetics study where researchers found a way to read and write brain signals using flashes of light.
In that study, researchers from University College London in the UK showed they could use light to trigger activity in selected brain cells and get individual cells to emit a unique color when active. Thus, effectively, they could select brain cells in different patterns and measure how the chosen circuit responded.