Stanford University School of Medicine scientists have found substances in the blood of old mice that make young brains act older.

The researchers connected the circulatory systems of pairs of old and young mice via a surgical procedure, which produced brain changes in  areas critical to memory and learning (like the hippocampus) in both type of mice: the older mouse of the pair produced more new nerve cells, while the young one produced fewer.

So what in the blood was producing the effect? When they analyzed different immune-signaling proteins, they found high concentrations of six of them in the blood of both unpaired old mice and young mice that had been paired with older ones.

At the top of the list was eotaxin, a small protein that impaired performance on spatial-memory tests, and in humans is associated with allergic responses and asthma. The negative effects of eotaxin could be countered by injections of another substance that blocks its action.

The researchers speculated that it might be possible to shield the brain from aging by eliminating or mitigating the effects of these proteins, or perhaps by identifying other blood-borne substances that exert rejuvenating effects on the brain, but whose levels decline with age.

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Ref.: Saul A. Villeda, et al., The ageing systemic milieu negatively regulates neurogenesis and cognitive function, Nature, 2011; [DOI:10.1038/nature1035]