Researchers at Brandeis University have found that the link between sleep and memory is stronger than we thought. It is well known that sleep is important for learning and memory, and many people can attest to having a hard time focusing and remembering things after a bad night’s sleep. Students often receive advice about getting a good night’s sleep instead of late-night cramming before a test. Simply put, scientists have learned that the brain takes advantage of the quiet hours during sleep to transfer newly-learned memories into long-term storage.
But how exactly are these complex behaviors connected in the brain? Does sleep simply permit memory storage to take place, such that the part of the brain involved in memory just takes advantage of sleep whenever it can? Or are sleep and memory physically connected, and the same mechanism in the brain is involved in both? In a recent study published in eLife, researchers in the Griffith lab may have finally uncovered the answer. They found that a single pair of neurons, known as the DPM neurons, are actively involved in both sleep and memory storage in fruit flies.
Why the fly? Fruit flies may be less complex than humans, but they have similar behaviors such as sleep and memory, and their brains have a similar organization. You may have heard of the hippocampus: the seahorse-shaped brain region in mammals that is responsible for learning and memory. The hippocampus receives a lot of information from other parts of the brain, and it has been very difficult for researchers to sort it all out. Fortunately, fruit flies have a similar region called the mushroom bodies (MBs), which are also important for learning and memory. Even better, fruit fly researchers have identified many of the neurons that send information to the MBs. One such example is the DPM neurons, which are critical for long-term memory storage. If the DPM neurons (there’s just two of them!) are “turned off” so that they can’t communicate with the MBs, flies can’t form long-term memories. This gave the researchers a great place to start for studying how sleep and memory are linked in the brain.
To find out if the DPM neurons are also involved in sleep, the group manipulated the activity levels of the DPM neurons and observed whether the flies showed any changes in their sleep patterns (Click here if you want to learn more about exactly how we study sleep in flies). They found that the DPM neurons had a dramatic effect: hyper-activating them increased the amount of time the flies slept, while silencing them decreased sleep (remember that silencing them also shut down long-term memory storage). Thus, sleep doesn’t just permit memory storage. These behaviors are actually tied to the same mechanism—the same neurons!—in the fruit fly brain.
As the researchers delved further, they found that the DPM neurons were dampening part of the MBs’ activity using GABA and serotonin (both are chemical messengers that neurons use for communicating with each other). That part of the MBs was important for learning and, as it turns out, also signaling wakefulness. It’s almost as if that section of the MBs were saying “Hey, stay awake and learn this”. After a while, however, the DPM neurons may start signaling to suppress the MBs, as if to say “You’re going to need sleep if you want to remember this later”.
Finally, there was another interesting insight uncovered by this study. It is widely believed that long-term memory is stored when groups of neurons signal back and forth in an excitatory manner, progressively strengthening their connections with one another (you may have heard the adage “neurons that fire together, wire together”). Yet, the authors of this study found that the DPM neurons, which are critical for memory storage, are not actually excitatory. To the contrary, they inhibit a section of the MBs necessary for learning. What role does inhibition play in memory? This finding doesn’t answer that question, but it does demonstrate just how much work is left to be done.
- Haynes P.R., Christmann, B.L. & Leslie C. Griffith (2015). A single pair of neurons links sleep to memory consolidation in Drosophila melanogaster , eLife, 4 DOI: http://dx.doi.org/10.7554/elife.03868