It is known that people with stress-related illnesses, such as post-traumatic stress disorder (PTSD), have abnormalities in the brain, including differences in the amount of gray matter versus white matter.
Gray matter consists mostly of cells - neurons, which store and process information, and support cells called glia - while white matter is composed of axons, which create a network of fibres that interconnect neurons.
White matter gets its name from the white, fatty myelin sheath that surrounds the axons and speeds the flow of electrical signals from cell to cell.
Daniela Kaufer, from the University of California Berkeley, and her colleagues, including graduate students Sundari Chetty and Aaron Freidman, discovered that chronic stress generates more myelin-producing cells and fewer neurons than normal.
This results in an excess of myelin - and thus, white matter - in some areas of the brain, which disrupts the delicate balance and timing of communication within the brain.
"We studied only one part of the brain, the hippocampus, but our findings could provide insight into how white matter is changing in conditions such as schizophrenia, autism, depression, suicide, ADHD and PTSD," she said.
Kaufer's lab, which conducts research on the molecular and cellular effects of acute and chronic stress, focused in this study on neural stem cells in the hippocampus of the brains of adult rats.
These stem cells were previously thought to mature only into neurons or a type of glial cell called an astrocyte. The researchers found, however, that chronic stress also made stem cells in the hippocampus mature into another type of glial cell called an oligodendrocyte, which produces the myelin that sheaths nerve cells.
The finding, which they demonstrated in rats and cultured rat brain cells, suggests a key role for oligodendrocytes in long-term and perhaps permanent changes in the brain that could set the stage for later mental problems.
Oligodendrocytes also help form synapses - sites where one cell talks to another - and help control the growth pathway of axons, which make those synapse connections.
The fact that chronic stress also decreases the number of stem cells that mature into neurons could provide an explanation for how chronic stress also affects learning and memory, Kaufer said. The study was published in the journal Molecular Psychiatry.