Studying mice, scientists at Johns Hopkins have fortified evidence that a key purpose of sleep is to recalibrate the brain cells responsible for learning and memory so the animals can “solidify” lessons learned and use them when they awaken — in the case of nocturnal mice, the next evening.

The researchers, all of the Johns Hopkins University School of Medicine, also report they have discovered several important molecules that govern the recalibration process, as well as evidence that sleep deprivation, sleep disorders and sleeping pills can interfere with the process.

“Our findings solidly advance the idea that the mouse and presumably the human brain can only store so much information before it needs to recalibrate,” says Dr. Graham Diering, the postdoctoral fellow who led the study. “Without sleep and the recalibration that goes on during sleep, memories are in danger of being lost.”

According to the study published in the journal Science, current scientific understanding of learning suggests that information is “contained” in synapses, the connections among neurons through which they communicate.

Scientists believe memories are encoded through synaptic changes. But there’s a hitch in this thinking, because while mice and other mammals are awake, the synapses throughout its brain tend to be strengthened, not weakened, pushing the system toward its maximum load. When neurons are “maxed out” and constantly firing, they lose their capacity to convey information, stymieing learning and memory.

One possible reason that neurons don’t usually max out is a process that has been well-studied in lab-grown neurons but not in living animals, asleep or awake. Known as homeostatic scaling down, it is a process that uniformly weakens synapses in a neural network by a small percentage, leaving their relative strengths intact and allowing learning and memory formation to continue.

To find out if the process does occur in sleeping mammals, Diering focused on the areas of the mouse brain responsible for learning and memory: the hippocampus and the cortex. He purified proteins from receiving synapses in sleeping and awake mice, looking for the same changes seen in lab-grown cells during scaling down.

Results showed a 20 percent drop in receptor protein levels in sleeping mice, indicating an overall weakening of their synapses, compared to mice that were awake.

As the final test of their hypothesis that scaling down during sleep is crucial for learning and memory, the researchers tested the mice’s ability to learn without scaling down. Individual mice were placed in an unfamiliar arena and given a mild electrical shock, either as they woke up or right before they went to sleep. Some mice then received a drug known to prevent scaling down.

When an undrugged mouse received a shock just before sleep, its brain went through the scaling-down process and formed an association between that arena and the shock. When placed in that same arena, those mice spent about 25 percent of their time motionless, in fear of another shock. When placed in a different unfamiliar arena, they froze sometimes, but only about 9 percent of their time there, probably because they were relatively good a telling the difference between the two unfamiliar arenas.

The bottom line is that sleep is not really downtime for the brain. It has important work to do then, and we in the developed world are short-changing ourselves by skimping on it.

Sleep is still a big mystery. The study only examined what goes on in two areas of the brain during sleep. There are probably equally important processes happening in other areas, and throughout the body, for that matter.

 

 

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