Time of the day may affect memory, says Japanese study

Washington D.C. (USA): A study has identified a specific protein that controls the expression of the genes that determine forgetfulness during the course of a day.

The experiment was carried out on mice to find out how the presence or absence of this protein influences their scores in a memory test. The findings of this study, as the researchers hope, can be extended to the human model to better understand the disorders that adversely affect memory.

The results were published in the journal Nature Communications. Prof Satoshi Kida from the University of Tokyo's Department of Applied Biological Chemistry, said: "We may have identified the first gene in mice specific to memory retrieval."

Every time you forget something, it could be because you didn't truly learn it -- like the name of the person you were just introduced to a minute ago; or it could be because you are not able to recall the information from where it is stored in your brain -- like the lyrics of your favourite song slipping your mind.

"We designed a memory test that can differentiate between not learning versus knowing but not being able to remember," said Prof Kida. Researchers tested the memories of young adult male and female mice. In the "learning," or training, phase of the memory tests, researchers allowed mice to explore a new object for a few minutes.

Later, in the "recall" phase of the test, researchers observed how long the mice touched the object when it was reintroduced. Mice spend less time touching objects that they remember seeing previously.

Researchers tested the mice's recall by reintroducing the same object at different times of the day. They did the same experiments with healthy mice and mice without BMAL1, a protein that regulates the expression of many other genes. BMAL1 normally fluctuates between low levels just before waking up and high levels before going to sleep.

Healthy mice and mice without BMAL1 had the same pattern of results, but the mice without BMAL1 were even more forgetful just before they normally woke up. Researchers saw the same results when they tested mice on recognizing an object or recognizing another mouse.

According to Prof Kida, the memory research community has previously suspected that the body's internal, or circadian, clock that is responsible for regulating sleep-wake cycles also affects learning and memory formation.

Researchers have traced the role of BMAL1 in memory retrieval to a specific area of the brain called the hippocampus. Additionally, researchers connected normal BMAL1 to the activation of dopamine receptors and the modification of other small signalling molecules in the brain.

"If we can identify ways to boost memory retrieval through this BMAL1 pathway, then we can think about applications to human diseases of memory deficit, like dementia and Alzheimer's disease," added Prof Kida.

However, the purpose of having memory recall abilities that naturally fluctuate depending on the time of day remains a mystery. "We really want to know what is the evolutionary benefit of having naturally impaired memory recall at certain times of day," said Prof Kida.