A new mouse study shows how sleep solidifies memories by encouraging new nerve connections in the brain to grow.
We’ve known for a while that sleep helps you remember things you’ve learned and experienced, but exactly how this works is still a mystery. Previous work has shown that teaching mice a new motor skill (like balancing on a rod) causes new dendritic spines to form in their brains. These tiny protrusions connect brain cells to each other and facilitate the passage of information across synapses, or the junctions where brain cells meet. Since the number of spines correlates to the number of nerve synapses, it appears that synaptic change in the brains of mammals underlies learning.
Now, to see if sleep influences those same synaptic changes, a team led by Wen-Biao Gan of New York University School of Medicine found a way to visualize dendritic spines in the brains of 15 live mice who were genetically engineered to express a fluorescent protein in the neurons of their motor cortex. Using a laser-scanning microscope to illuminate the glowing proteins, the team tracked and imaged the growth of spines along individual branches of dendrites as the mice were trained to run on a spinning rod. After the training, some mice got to sleep immediately for seven hours, while other mice had to stay awake for another eight hours.
The researchers found that sleep caused the number of new spines to increase. In the rested mice, new spines formed on different sets of dendritic branches in response to different learning tasks (like running forwards versus running backwards). And these are protected from being eliminated when multiple tasks are learned. About 5 percent of spines in the motor cortex were formed anew in the 8 to 24 hour period after the mice woke up, lead coauthor Guang Yang of NYU tells New Scientist.
“Imagine a tree that grows leaves (spines) on one branch but not another branch. When we learn something new, it’s like we’re sprouting leaves on a specific branch,” Gans explains in a news release.
On the other hand, not only did sleep deprivation do the opposite, its effects were so strong, it overwhelmed extra training. When sleep-deprived mice were trained harder on a skill, they still formed fewer new spines than well-rested mice. That’s because neuron activity associated with newly learned skills were replayed (or reactivated) during subsequent non–rapid eye movement (non-REM) sleep. This reactivation is linked to spine growth, strengthening synapses forged during recent experiences as we sleep.
So, while sleep enhances memory and improves the execution of learned behaviors, disrupting this neuronal reactivation prevents spine formation. “Here we’ve shown how sleep helps neurons form very specific connections on dendritic branches that may facilitate long-term memory,” Gan says. “If you don’t sleep well you won’t learn well.”
In this video, dendrites and dendritic spines in the mouse cortex are imaged through a thinned-skull window using two-photon microscopy. Dendrites are labeled by Yellow Fluorescent Protein. The first 14 frames are images of the skull. Frames 54-125 are images of dendrites and dendritic spines.
The work was published in Science last week.