As we sleep, our bodies cycle through several stages. One is the REM or rapid eye movement stage, during which most dreams occur. Reporting in Science, an international team of researchers has now suggested that the brain is intentionally forgetting what we experience during this stage. Neurons that are known to create a hormone that triggers appetite and lie deep within the brain are thought to be involved in the process.
“Ever wonder why we forget many of our dreams?” asked the senior author of the study, Thomas Kilduff, Ph.D., director of the Center for Neuroscience at SRI International in Menlo Park, California. “Our results suggest that the firing of a particular group of neurons during REM sleep controls whether the brain remembers new information after a good night’s sleep.”
The REM stage starts around 90 minutes after we fall asleep. Our eyes dart, heart rate increases, limbs become paralyzed, brain waves are similar to those we have during wakefulness, and we dream. Scientists have long thought that sleep has a role in memory. Recent studies using mouse models have suggested that the brain prunes synapses (connections between neurons) that are related to some kinds of learning during REM sleep.
“Understanding the role of sleep in forgetting may help researchers better understand a wide range of memory-related diseases like post-traumatic stress disorder and Alzheimer’s,” noted Janet He, Ph.D., program director, at NINDS. “This study provides the most direct evidence that REM sleep may play a role in how the brain decides which memories to store.”
In this work, Kilduff's team collaborated with the lab of Akihiro Yamanaka, Ph.D., of Nagoya University and the lab of Akira Terao, D.V.M., Ph.D., at Hokkaido University. They wanted to know more about MCH neurons, so-named because they generate a molecule called melanin concentrating hormone, which helps regulate sleep and appetite. In mice experiencing REM sleep, 52.8 percent of MCH neurons in the hypothalamus were active and firing. When the mice were awake, around 35 percent were firing, and around twelve percent of the neurons were active during sleep and wakefulness.
The researchers were also able to show that many of these long hypothalamic MCH neurons were sending inhibitory signals all the way to the memory center of the brain - the hippocampus. This suggested that the MCH neurons are involved in memory and learning.
Learn more about the stages of sleep from the video.
“From previous studies done in other labs, we already knew that MCH cells were active during REM sleep. After discovering this new circuit, we thought these cells might help the brain store memories,” said Kilduff.
In their mouse model, the scientists were able to activate and deactivate MCH neurons while memory tests were bring conducted. They used several tests that focused on the retention of new information just after it's learned, but before it ends up as a long-term memory. They found that the MCH neurons were exerting their influence exclusively during REM sleep. During that time, if the MCH neurons were turned on, the mice had a harder time retaining new information, and when the neurons were turned off, memory was improved. The activity of MCH neurons during other sleep stages had no impact on memory.
“These results suggest that MCH neurons help the brain actively forget new, possibly, unimportant information,” noted Kilduff. “Since dreams are thought to primarily occur during REM sleep, the sleep stage when the MCH cells turn on, activation of these cells may prevent the content of a dream from being stored in the hippocampus – consequently, the dream is quickly forgotten.”
The researchers now want to know more about how these neurons are related to sleep and memory disorders.