New neurons are constantly being produced in the adult brain, but their ability to fine tune connections may be short lived, report Shaoyu Ge, Hongjun Song (JHMI, Baltimore, MD), and colleagues. This short window of plasticity could explain why learning continues throughout life, yet the stability of neuronal circuits is maintained.
Learning and memory require a balance of neuronal plasticity and stability. Too much plasticity and we'd constantly have to learn the same things over and over. Too much stability and we'd have little chance of learning at all. Plasticity peaks in the juvenile brain. By adulthood, however, plasticity is limited.
One area that maintains plasticity is the hippocampus, a region of the brain involved in learning and memory. By labeling adult-born mouse hippocampal neurons and then measuring their electrical activity at regular age increments, Ge et al. found that new neurons are not permanently highly plastic. Instead, plasticity peaks when neurons are one to one-and-a-half months old.
This peak in plasticity was coupled with synaptic expression of a type of glutamate receptor called NR2B, which is involved in developmental plasticity (such as that seen in the juvenile brain). Inhibition of NR2B almost completely abolished the telltale electrical patterns of plasticity in 1-month-old neurons but had no effect on mature neurons.
Limiting the period of NR2B-dependent plasticity could enable adult-born neurons to establish and then stabilize new connections in response to experience. The team now plans to inhibit this critical period of plasticity in adult animals to see whether learning is indeed impaired. 
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