Research has found that parents are to be believed when it comes to their advice on adequate sleep before an exam - because boffins say that sleep boosts memory.
And now, they have understood how.
Marcos Frank, PhD, Assistant Professor of Neuroscience, at the University of Pennsylvania School of Medicine, postdoctoral researcher Sara Aton, PhD, and colleagues have found for the first time how cellular changes in the sleeping brain promote the formation of memories.
For the study, the researchers used an experimental model of cortical plasticity - the rearrangement of neural connections in response to life experiences.
"That's fundamentally what we think the machinery of memory is, the actual making and breaking of connections between neurons," Frank said.
In this case, the experience Frank and his team used was visual stimulation. Animals that were young enough to still be establishing neural networks in response to visual cues were deprived of stimulation through one eye by covering that eye with a patch.
The researchers then compared the electrophysiological and molecular changes that resulted with control animals whose eyes were not covered. Some animals were studied immediately following the visual block, while others were allowed to sleep first.
From previous work, Frank's team already knew that sleep induced a stronger reorganization of the visual cortex in animals that had an eye patch versus those that were not allowed to sleep. Now they know why.
A molecular explanation is emerging. The key cellular player in this process is a molecule called N-methyl D-aspartate receptor (NMDAR), which acts like a combination listening post and gate-keeper. It both receives extracellular signals in the form of glutamate and regulates the flow of calcium ions into cells.
Essentially, once the brain is triggered to reorganize its neural networks in wakefulness, intra- and intercellular communication pathways engage, setting a series of enzymes into action within the reorganizing neurons during sleep.
To start the process, NMDAR is primed to open its ion channel after the neuron has been excited. The ion channel then opens when glutamate binds to the receptor, allowing calcium into the cell. In turn, calcium, an intracellular signaling molecule, turns other downstream enzymes on and off.
Some neural connections are strengthened as a result of this process, and the result is a reorganized visual cortex. And, this only happens during sleep.
"To our amazement, we found that these enzymes never really turned on until the animal had a chance to sleep. As soon as the animal had a chance to sleep, we saw all the machinery of memory start to engage," Frank said.
Equally important was the demonstration that inhibition of these enzymes in the sleeping brain completely prevented the normal reorganization of the cortex.
The new study could pave the way to understanding, on a molecular level, why humans need sleep, and why they are so affected by the lack of it. It could also lead to novel therapeutics that could compensate for the lack of sleep, by mimicking the molecular events that occur during sleep.
The study is published this week in Neuron.