Grown-up monkey brains get growing.

During adulthood, according to traditional views of primate brain development, neurons check out, but they don't check in. In the densely connected mass of mature brain tissue, cells die and leave behind no fresh replacements.

Overturning tradition, researchers have for the first time documented the creation of new neurons in the adult primate brain -- in an area linked with learning and memory. What's more, a single, highly stressful event can interfere with the production of neurons in monkeys for at least 3 weeks, reports a research group headed by neuroscientist Elizabeth Gould of Princeton University.

"In the classical scientific view of the adult brain, our findings seem ridiculous," Gould says. "But the production of new neurons during adulthood and its inhibition by stressful experience may be common to many species, including humans."

Previous research conducted separately by Gould's team and a group in La Jolla, Calif., documented neural generation throughout adulthood in the hippocampus and adjacent regions of the inner brain of rats and tree shrews. This so-called hippocampal formation helps to regulate memory formation and the learning of information.

In the new research, described in the March 16 Proceedings of the National Academy of Sciences, Gould and her colleagues injected into the brains of six adult male marmoset monkeys a substance that marks neurons in early phases of development. Either 2 hours or 3 weeks later, the researchers added another chemical marker to slices of the animals' brain tissue to tag fully developed neurons previously identified in their formative stages.

Over 3 weeks, sizable neural production occurred in a part of the hippocampal formation called the dentate gyrus, Gould holds.

The scientists also transferred four adult monkeys individually to the cage of another adult monkey for 1 hour, a stressful situation in which aggressive displays by the resident animal elicited submissive behavior from the intruder. The four stressed monkeys subsequently produced fewer new dentate gyrus neurons than the nonstressed monkeys.

This finding adds to prior indications from several animal species, including humans (SN: 6/3/95, p. 340), that stress and trauma trigger the release of hormones that can damage the hippocampal formation.

"The discovery that new cells are made in the adult primate dentate gyrus is novel and very exciting," says neuroscientist William T. Greenough of the University of Illinois at Urbana-Champaign.

Further investigations are needed to examine whether the fresh neurons are incorporated into the web of preexisting cellular connections, and, if so, how they function, Greenough says.

Gould suspects that dentate gyrus cells generated during adulthood rapidly form neuronal connections and become involved in learning and memory.

She and her coworkers have also reported that neurons are generated in the adult rat brain in a section of the outer layer, or cortex, that processes smells. This raises the possibility, also open to future research, that parts of the primate cortex produce new cells during adulthood, Gould notes.