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For the study, the researchers destroyed cells in the hippocampus, an area of the brain vital to memory formation and where neurons often die. They then gave place and object recognition tests to healthy mice and mice with brain injuries to test memory.

Memories of place depend upon the hippocampus, and memories of objects depend more upon the cortex.

In the place test, healthy mice retained the information about their surroundings about 70 percent of the time, but mice with brain injuries remembered it just 40 percent of the time.

In the object test, healthy mice remembered objects about 80 percent of the time, while injured mice remembered as poorly as about 65 percent of the time.

Later, the scientists set out to learn whether neural stem cells from a mouse could improve memory in mice with brain injuries.

For testing this, each mouse was injected with about 200,000 neural stem cells that were engineered to appear green under ultraviolet light. The colour allowed the scientists to track the stem cells inside the mouse brain after transplantation.

Three months later, after implanting the stem cells the mice were tested on place recognition and the researchers found that mice with brain injuries that received stem cells remembered their surroundings about 70 percent of the time, which was the same level as the healthy mice. In contrast, control mice that didn’t receive stem cells still had memory impairments.

Subsequently, the scientists took a closer look at how the green-coloured stem cells behaved in the mouse brain and it was found that only about 4 percent of them turned into neurons, indicating the stem cells were not improving memory simply by replacing the dead brain cells.

In the healthy mice, the stem cells migrated throughout the brain, but in the mice with neuronal loss, the cells congregated in the hippocampus, the area of the injury.

“We know that very few of the cells are becoming neurons, so we think that the stem cells are instead enhancing the local brain microenvironment. We have evidence suggesting that the stem cells provide support to vulnerable and injured neurons, keeping them alive and functional by making beneficial proteins called neurotrophins,” Blurton-Jones said.

The authors believe that the stem cells secreted proteins called neurotrophins that protected vulnerable cells from death and rescued memory.

The finding creates a hope that a drug to boost production of the proteins could be developed to restore the ability to remember in patients with neuronal loss, according to Blurton-Jones.

“Our research provides clear evidence that stem cells can reverse memory loss. This gives us hope that stem cells someday could help restore brain function in humans suffering from a wide range of diseases and injuries that impair memory formation,” LaFerla said.

The study is published in the Journal of Neuroscience. (ANI)