Forgetting the Stuff That Dreams Are Made On

Why do we forget some dreams and remember others? Researchers in a study funded by the National Institute of Neurological Disorders and Stroke say our dream memory may be controlled by a group of neurons commonly associated with appetite. Their findings could shed light on a wide range of memory-related conditions, including posttraumatic stress disorder (PTSD) and Alzheimer disease.

Studies have already shown that sleep helps the brain store new memories and eliminate excess information. Recent mouse studies have found that during sleep the brain prunes synaptic connections made between neurons involved in some types of learning.

But this study shows how that might happen.

The researchers have previously demonstrated that narcolepsy might be linked to the loss of hypocretin/orexin-making neurons in the hypothalamus. In this study, they looked at neighboring cells that produce melanin-concentrating hormone (MCH), which is involved in the control of both sleep and appetite. A majority (53%) of hypothalamic MCH cells fired in mice during REM sleep; 35% fired when they were awake, and 12% fired at both times.

Those cells also may play a role in learning and memory, the researchers suggest. To test their theory that MCH cells might help the brain store memories, they used “genetic tools” to turn MCH neurons on and off during memory tests.

Surprisingly, the researchers say, pharmacogenetic activation—turning on the MCH cells—worsened memory; genetic ablation—turning them off—improved memory. Further experiments suggested MCH neurons exclusively played this role during REM sleep.

The results suggest that MCH neurons help the brain actively forget new, possibly unimportant information. “Since dreams are thought to primarily occur during REM sleep,” says Thomas Kilduff, PhD, senior author of the study, “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.”