The reduced capacity of older people to learn and remember could be caused by their neural stem cells dividing less often, according to U.S. researchers.

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SYDNEY: The reduced capacity of older people to learn and remember may be caused by stem cells in the brain dividing less frequently, rather than a shortage of neural stem cells as was previously thought.

The findings suggest it may be possible to treat degenerative disorders, including Alzheimer's disease, dementia and depression, by stimulating the stem cells' ability to divide and produce new nerve cells.

Previous studies have demonstrated that as the brain ages, fewer new nerve cells, or neurons, are born in the hippocampus - the brain's learning and memory centre. But the cause of this decline had remained a mystery, until now.

The common assumption had been that the 'brain drain' was due to a decreasing supply of neural stem cells in the ageing hippocampus. Neural stem cells are immature cells that have the ability to give rise to all types of nerve cells in the brain.

However in this new study the researchers found that stem cells in ageing brains are not reduced in number - instead they divide less frequently, resulting in dramatic reductions in the number of new neurons added to the hippocampus. Their findings are reported online in the journal Neurobiology of Aging.

To conduct their census, the researchers attached fluorescent tags to the neural stem cells in the hippocampus of young, middle-aged and older rats.

They found that in young rats, the hippocampus contained 50,000 stem cells. Significantly, this number did not diminish with age. According to the researchers, this finding suggests that the decreased production of new neurons in an aged brain is not due to a lack of starting material.

The researchers then used another fluorescent marker to tag all stem cells undergoing division - potentially to become mature nerve cells. They found that in young rats, approximately 25 per cent of the neural stem cells were actively dividing, while only 8 per cent of the cells in middle-aged rats and 4 per cent in old rats were dividing.

According to the team, this finding suggests that decreased division of stem cells is what causes the decreased neurogenesis - or birth of nerve cells - seen with ageing.

"This discovery provides a new avenue to pursue in trying to combat the cognitive decline associated with conditions such as Alzheimer's disease and with ageing in general," said co-author Bharathi Hattiangady, of Duke University.

The team is now searching for ways to stimulate the brain to replace its own cells in order to improve learning and memory function in the elderly.

One approach being explored is to treat older rats with drugs designed to mimic the action of compounds called neurogenic factors, which encourage stem cells in the brain to divide, said co-author Ashok Shetty, also of Duke University.

The researchers are also grafting neural stem cells grown in culture dishes into the hippocampus, to stimulate those already present.

Additional approaches include using behavioural modification techniques - such as physical exercise and exposure to an enriching environment - that are known to stimulate proliferation of stem cells.