Wednesday, November 13, 2019
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Good sleep recalibrates your brain cells

Between tweeting and other smartphone activity, it has been reported that President Donald Trump gets about four hours of sleep at night. President Obama reportedly got between five and six and George W Bush about six, although it was reported that W, like Ronald Reagan and John F Kennedy before him, compensated for the impact the presidency has on good sleep by taking afternoon naps. How do you think President Trump compensates?

In a new study, summarized February 3 in the journal Science, scientists at Johns Hopkins have fortified evidence, using mice, 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 Graham Diering, PhD, the postdoctoral fellow who led the study. “Without sleep and the recalibration that goes on during sleep, memories are in danger of being lost.”

Diering explains that current scientific understanding of learning suggests that information is “contained” in synapses, the connections among neurons through which those neurons communicate with other neurons.

On the “sending side” of a synapse, signaling molecules called neurotransmitters are released by a brain cell as it “fires”; on the “receiving side,” those molecules are captured by receptor proteins, which pass the “message” along. If a cell receives enough input through its synapses, it fires off its own neurotransmitters.

More specifically, experiments in animals have shown that the synapses on the receiving neuron can be toggled by adding or removing receptor proteins, thereby strengthening or weakening them and allowing the receiving neuron to receive more or less input from nearby signaling neurons.

Scientists believe memories are encoded through these synaptic changes. But there’s a hitch in this thinking, Diering says, 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, cutting off our capacity for learning and memory.

One possible reason that neurons don’t usually max out is a process that has been 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.

“That was the first evidence of homeostatic scaling down in live animals,” says Richard Huganir, PhD, professor of neuroscience, director of the Department of Neuroscience and lead author of the study. “It suggests that synapses are restructured throughout the mouse brain every 12 hours or so, which is quite remarkable.”

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

Huganir says that sleep is still a big mystery. “In this study, we 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,” he adds.

Paul Katula is the executive editor of the Voxitatis Research Foundation, which publishes this blog. For more information, see the About page.

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