It is now generally accepted that the vertebrate brain continues to produce new neurons throughout life, but it has not been clear whether new neurons are essential for memory formation. In the 15 March Nature, Tracey Shors of Rutgers University, New Jersey, and colleagues present evidence that newly generated neurons in the hippocampus of adult rats are essential for at least one form of memory — the kind concerning the timing of learned responses and temporal relationships between events (Nature 2001, 410:372-376).

Shors et al injected rats with methylazoxymethanol acetate (MAM), a DNA methylating agent that is toxic to proliferating cells. Daily injections over 14 days resulted in a reduction in the number of newly generated cells in the hippocampus by 84%. This treatment impaired the ability of rats to perform a task, called hippocampal-dependent trace conditioning, in which they had to associate stimuli separated in time. When the rats were allowed to recover over a period of 21 days from MAM treatment, during which time the number of proliferating cells in the hippocampus increased, the ability to perform the task was restored.

"The results support the idea that it might, one day, be possible to add new, fully functional neurons into existing brain circuitry to treat diseases of the nervous system," says Jeffrey Macklis of Harvard Medical School, Boston, in an accompanying News and Views article.