'Lost' memories in Alzheimer's may be retrieved: study

Memories lost in Alzheimer's disease may be retrieved by stimulating nerve cells to grow new connections, according to a new study conducted in mice by researchers including one of Indian-origin.
In the early stages of Alzheimer's disease patients are often unable to remember recent experiences, however scientists from Massachusetts Institute of Technology (MIT) in US have shown that those memories are still stored in the brain - they just cannot be easily accessed.
Mice in the early stages of Alzheimer's can form new memories just as well as normal mice but cannot recall them a few days later, researchers said.

Scientists were able to artificially stimulate those memories using a technique known as optogenetics, suggesting that those memories can still be retrieved with a little help.
Although optogenetics cannot currently be used in humans, the findings raise the possibility of developing future treatments that might reverse some of the memory loss seen in early-stage Alzheimer's, researchers said.
"The important point is, this a proof of concept. That is, even if a memory seems to be gone, it is still there. It is a matter of how to retrieve it," said Susumu Tonegawa from MIT.

Researchers studied two different strains of mice genetically engineered to develop Alzheimer's symptoms, plus a group of healthy mice.

All of these mice, when exposed to a chamber where they received a foot shock, showed fear when placed in the same chamber an hour later.
However, when placed in the chamber again several days later, only the normal mice still showed fear. The Alzheimer's mice did not appear to remember the foot shock.
"Short-term memory seems to be normal, on the order of hours. But for long-term memory, these early Alzheimer's mice seem to be impaired," said Dheeraj Roy from MIT.
Researchers then tagged the engram cells associated with the fearful experience with a light-sensitive protein called channelrhodopsin, using a technique they developed in 2012.

Whenever tagged engram cells are activated by light, normal mice recall the memory encoded by that group of cells.
"Directly activating the cells that we believe are holding the memory gets them to retrieve it. This suggests that it is indeed an access problem to the information, not that they are unable to learn or store this memory," said Roy.
The researchers were also able to induce a longer-term reactivation of the "lost" memories by stimulating new connections between the entorhinal cortex and the hippocampus.
To achieve this, they used light to optogenetically stimulate entorhinal cortex cells that feed into the hippocampal engram cells encoding the fearful memory.

After three hours of this treatment, the researchers waited a week and tested the mice again.
This time, the mice could retrieve the memory on their own when placed in the original chamber, and they had many more dendritic spines on their engram cells.
The findings were published in the journal Nature.