Young mind on old shoulders

A way to restore the intellectual vigour of youth, and even make blind mice see, has been uncovered by US researchers.

It's often said that you cannot teach an old dog new tricks. And with good reason, because as we age we tend to become less intellectually agile and more set in our ways.

From an evolutionary standpoint this makes sense, because a young individual has a lot to learn, and very quickly, while an older person needs to hang onto their accrued wisdom in the longterm. But the mechanism of this cognitive rigidity previously hadn't been well understood.

Now Stanford neuroscientist Carla Shatz and her colleagues, writing in Science Translational Medicine, have uncovered one way that might make it possible to revert the aged brain back to a youthful state capable of 'sponge-like' learning.

"We've found a molecule called PirB," explains Shatz. "If we turn this off we can block the pruning of nerve connections, so nerve cells make, net, more connections to each other, like a young brain does."

Intriguingly, the PirB moecules, which are present on nerve cells, are also used elsewhere in the body to enable immune cells to communicate with each other. "It's intriguing that two structures - the brain and the immune system - which both have to learn in their own way - use the same system."

To demonstrate its effects, the team blocked the action of PirB in a group of mice using both a genetic technique to knock our the gene and also by introducing a decoy molecule to prevent the PirB signal from being picked up by neurones.

In both cases, animals with the mouse equivalent of a squint that had led to blindness in one eye, recovered their vision in that eye through the formation of new nerve connections that re-established the correct visual pathways from the formerly blind eye.

"This suggests that it is possible to put the nervous system back into the same plastic state that exists when we are younger, although there might be a downside," says Shatz. "You might have to go back to med-school! When I first discovered this I thought 'Cool!' Then I realised that, actually, at the same time as learning something new, I might also be forgetting something else."

Nevertheless, the discovery does open a window into the possibility of boosting brain plasticity which, if done temporarily, might hold promise for helping to improve the recovery and prospects of victims of strokes, head injuries or dementias.