The old brain has more tricks up its sleeve than researchers thought. In addition to two previously identified regions, a third spot in the mammalian adult brain can generate neurons, according to a new study. This particular patch degenerates in Parkinson's and other diseases. But whether the new-growth phenomenon plays a role in age-related illness has yet to be determined.
Researchers have known for several years that adult mammals can grow new neurons, but the only parts of the brain that have been found to do so are the hippocampus, where new memories are made, and the olfactory bulb. Scientists have been hoping to find signs of new growth in the substantia nigra, a midbrain region mangled by Parkinson's disease, which trashes neurons that produce the neurotransmitter dopamine. But last year, researchers looking for such growth in adult rats found none.
In the new study, Ann Marie Janson of the Karolinska Institute in Stockholm, Sweden, and colleagues counted the number of dopamine-producing neurons in the substantia nigra of mice between 2 and 20 months of age. The team found that, over time, the number of neurons remained the same even though some neurons died. This suggested that the brain replaced the dead ones with new growth. The researchers then injected a dye that colors actively growing neurons into the brains of living mice. After continuously staining brains for 3 weeks, the team removed the brains and found about 20 new cells in each.
To determine the source of the new neurons, the group tracked chemically labeled neural stem cells. They turned up as dopamine neurons in the substantia nigra. This method also allowed them to calculate that the mouse brain made about 20 new dopamine neurons every day. Additional tests suggested that the new neurons formed the proper connections to other cells in the midbrain, they report online this week in the Proceedings of the National Academy of Sciences. "We speculate that Parkinson's disease may be due to too little formation of new nerve cells," says Janson, who suggests the process could be manipulated to fight the disorder.
Not everyone is so optimistic. Neuroscientist John Trojanowski of the University of Pennsylvania Medical School in Philadelphia says the "technically superb" work "clarifies the phenomenon of neurogenesis in the substantia nigra." He adds, however, that Parkinson's disease affects many more parts of the brain than the substantia nigra, and loss of new neurons probably does not explain the ailment.