New Neurons Not So Important?

By Kelli WhitlockBurton
ScienceNOW Daily News
3 May 2006

Give a mouse more room and a few toys, and good things happen. New neurons sprout in the hippocampus, while spatial memory improves and anxiety eases. As tempting as it might be to tie the new neurons to the behavioral changes, a new study finds no link between them. The results contradict a popular assumption among scientists that new neurons in the hippocampus contribute to the cognitive boost that comes with a more stimulating environment.

The notion that two parts of the brain--the hippocampus and olfactory bulb--continue to produce new neurons into adulthood has been widely accepted since the late 1990s. But just what role those new cells play in cognitive function remains a mystery. Recent studies have found that animals housed in larger cages with opportunities for exercise and social interaction generate more new neurons in the hippocampus than do animals in more cramped quarters with no playmates. Scientists in the lab of Columbia University neurobiologist René Hen hoped to find the link between hippocampal neurogenesis and certain behaviors such as learning and memory that involve the hippocampus.

Hen's team zapped mice with a focused dose of radiation to halt neurogenesis in a portion of the animals' hippocampuses. They then placed half the animals in regular cages and half in enhanced environments for 6 weeks before testing their anxiety and spatial memory. To the researchers' surprise, the animals with better accommodations had improved spatial memory skills and were less anxious than mice in smaller confines, despite not having any new neurons in their hippocampuses. "We thought we would see a dependence on neurogenesis in some of the behaviors we saw in the enriched environment, but that's not what we found," says Hen, whose team published the findings online 30 April in Nature Neuroscience.

"This doesn't say that adult neurogenesis plays no role in functional changes [in the brain] associated with enriched environments, but it does highlight that neurogenesis is definitely not the whole story," says Elizabeth Gould, a neurobiologist at Princeton University. Gould points out that Hen's study examines only two specific behaviors, a limitation Hen plans to address in future work. "The challenge is to figure out which of the many hippocampal-dependent tasks are affected and which are not," Hen says.

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