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Q. I’ve heard that stress, including prolonged adaption, can temporarily suspend or interfere with some frontal-lobe functions. What else might stress do to the brain?
A. Stress can make the blood-brain barrier (BBB)--the selective anatomical and physiological barrier designed to protect the brain from large molecules--permeable to substances not intended to reach the brain. A breakdown of the BBB could permit the passage of substances into brain tissue that normally are kept out. Diseases such as Alzheimer’s are believed to render the BBB more permeable. Recent studies indicate stress can do so, too.
Research with mice showed that tiny doses of a large molecule (AChE), previously not thought to be capable of penetrating the BBB, breached this barrier when the mice were stressed by being forced to swim (an accepted protocol).
The potential implications of this are mind-boggling. Effective stress-management techniques may take on a new complexion. According to Al Komaroff in Journal Watch (January 15, 1997), for example, it is plausible that the stress of being in a war zone may have allowed some toxic molecules (and small viral agents) to reach the brain, conceivably producing the symptoms of "Gulf War" syndrome.
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Stress and the Brain 
