Sleep is known to play a key role in learning and memory formation, but what happens to these important brain functions when we fail to get enough sleep? Researchers from the University of Michigan provide some answers with their new study.

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Researchers suggest that sleep deprivation disrupts the rhythm of neuronal firing in a specific area of the hippocampus, which might interfere with memory formation.

The study found that sleep deprivation interferes with the rhythm of neuronal firing in a region of the hippocampus called CA1. The hippocampus is the brain structure responsible for the formation of long-term memories.

Study leader Nicolette Ognjanovski, of the Department of Molecular, Cellular, and Developmental Biology at Michigan, and colleagues found that disrupting the rhythm of neuronal firing, or oscillations, in the CA1 of mice interfered with memory formation.

Ognjanovski and colleagues say that their findings - recently reported in the journal Nature Communications - shed further light on the role that sleep plays in memory.

Current guidelines recommend that adults should aim to get around 7 to 9 hours of sleep each night for optimal health. According to the American Sleep Association, however, around 35 percent of adults in the United States report getting under 7 hours of sleep per night.

Studies have shown that sleep is important for memory consolidation - that is, the brain's ability to convert short-term memories into long-term memories. With this in mind, it is no surprise that a lack of sleep has been found to hinder memory formation.

Previous research has associated sleep deprivation with impaired hippocampal function. However, Ognjanovski and team note that precisely which area of the hippocampus is affected by lack of sleep has been unclear.

To find out, the team conducted a series of experiments in mice, beginning with a learning task.

Stronger rhythmic brain activity in well-rested mice after learning

First, the researchers removed mice from their home and placed them in a new environment. After the rodents had explored their new surroundings for a while, the researchers gave them a mild foot shock. The mice were then returned to their original environment to rest.

On assessing the hippocampal activity of the mice, the researchers found that rodents that had sufficient sleep after the learning task demonstrated stronger sleep-related oscillations in the CA1 region of the hippocampus, compared with sleep-deprived mice.

Next, the researchers set out to determine whether disrupting these sleep-related oscillations might impact memory. To do so, the team gave a new group of mice a drug that prevented a small group of neurons in the CA1 from expressing the protein parvalbumin. This is a process that studies have shown occurs with sleep deprivation.

The rodents then completed the same learning task as the previous group of mice, and hippocampal activity was measured before and after the task.

The researchers found that blocking the activity of parvalbumin-expressing neurons in the CA1 of mice interfered with the rhythmic firing of surrounding neurons.

Furthermore, when these mice were returned to the environment in which they received the foot shock, all memories of this environment appeared to have been eliminated.

"If you return the mouse to that same structure a day or even a couple [of] months later, they will have this very stereotyped fear response, which is that they freeze," explains senior study author Sara Aton, an assistant professor in the Department of Molecular, Cellular, and Developmental Biology at Michigan.

"But if you sleep-deprive an animal for a few hours after that context-shock pairing, the mouse won't remember it the next day," she adds.

'Memories aren't stored in single cells'

According to the researchers, their findings challenge the current understanding of how memories are formed.

"The dominant oscillatory activity, which is so critical for learning, is controlled by a very small number of the total cell population in the hippocampus," says Ognjanovski.

"This changes the narrative of what we understand about how networks work. The oscillations that parvalbumin cells control are linked to global network changes, or stability. Memories aren't stored in single cells, but distributed through the network."

What is more, the team believes that the results highlight the importance of sleep for learning and memory formation, and they shed light on how sleep deprivation might hamper this critical process.

"It seems like this population of neurons that is generating rhythms in the brain during sleep is providing some informational content for reinforcing memories. The rhythm itself seems to be the most critical part, and possibly why you need to have sleep in order to form these memories."

Sara Aton

The team now plans to investigate whether simulating the effects of sleep in the CA1 region by restoring oscillations aids memory formation in sleep-deprived mice.

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