Researchers from the University of Michigan provided some answers with their new study showing 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, along with colleagues found that disrupting the rhythm of neuronal firing, or oscillations, in the CA1 of mice interfered with memory formation. 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 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. “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. Sara Aton said that, “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.”