Learning to make, keep adult neurons.

Certain types of learning, and even regular exercise, appear to enhance the formation and survival of new brain cells in adult rodents, two teams of neuroscientists find.

Their investigations, both published in the March NATURE NEUROSCIENCE, focused on cell births in the mature hippocampus. A growing body of research indicates that this small neural structure, which contributes to learning and memory, produces fresh nerve cells throughout the lives of humans, monkeys, and other animals (SN: 10/31/98, p. 276).

A group led by Elizabeth Gould of Princeton University injected a group of rats with a chemical that labels newborn cells. A week later, some of these rats received training in one of four tasks.

The number of newly generated cells doubled in rats after training on one of the tasks that are known to require an intact hippocampus, the scientists report. One consisted of learning to expect delivery of an electric shock to the eyelids (as evidenced by eye blinks) just after hearing a distinctive blast of noise; the other required animals to find and remember the location of a submerged platform in a water-filled maze.

Two tasks that did not depend on the hippocampus--learning to associate noise blasts with concurrent electric shocks and to swim to a visible platform placed at random locations in a water maze--failed to generate increases in labeled neurons. Untrained mice also produced no extra neurons.

Hippocampus-dependent learning aids the survival of cells created before training, rather than sparking more cell production during training, Gould's team proposes. Labeled cells did not surge in number in rats that were injected immediately after starting any of the training sessions.

A second study, directed by Henriette van Praag of the Salk Institute for Biological Studies in La Jolla, Calif., finds marked increases in the surviving number of new hippocampal cells in adult mice that were put in cages with running wheels immediately after receiving injections of the chemical label for neural newcomers. Enriched housing, which featured opportunities for frequent social interaction and varied types of play, also yielded substantially greater numbers of surviving newborn cells.

Unlike the adult brain's outer layer, or cortex, the hippocampus may cultivate fresh neurons to deal with novel information as it discards others that have become obsolete, theorize William T. Greenough of the University of Illinois at Urbana-Champaign and his colleagues.