In both rats and humans, chronic stress early in life can hobble memory during adulthood. Now researchers have identified a substance that might be to blame. Immature rats injected with high levels of a hormone that is produced in response to stress suffer later memory deficits, and parts of their brains are shrunken.
Adult rat survivors of stressful puphoods typically are missing neurons in a part of the brain called the hippocampus that's necessary for recording memories. But researchers haven't been sure what depletes these neurons. Stress hormones called glucocorticoids are an obvious possibility, except for the fact that they don't normally interact with the hippocampus. So in the new study, neuroscientists tested another stress-related suspect, called corticotropin-releasing hormone (CRH).
To test the effects of CRH on memory, a team of researchers led by Tallie Z. Baram of the University of California, Irvine, injected CRH into 10-day-old rats, while keeping their levels of glucocorticoids steady. When the rats were 3 months old, the researchers tested the rats' memory. CRH-treated rats were stymied by a water maze and an object-recognition test that healthy rats breezed through, and their impairments got even worse at 6 and 10 months of age. When the researchers examined the rats' brains, they saw a possible reason for the poor performance: Their hippocampi were missing crucial neurons, they report in the 10 July issue of the Proceedings of the National Academy of Sciences.
"The conventional wisdom has been that damage [to hippocampal neurons] is mediated by the glucocorticoids," says neurobiologist Paul M. Plotsky of Emory University in Atlanta. But if CRH is to blame for memory problems, keeping its levels down may be a treatment option in childhood cases of chronic stress or brain injury, which causes some of the same memory deficits later in life.