Washington - Stress activates an enzyme in the brain which interferes with short-term memory and other functions of the prefrontal cortex, located just above the eyes, in rats and monkeys, according to a US study published on Friday.

Known as "protein kinase C" (PKC), the enzyme is also implicated in bipolar disorder and schizophrenia, said Amy Arnsten of Yale Medical School in New Haven, Connecticut, who led the study, published in the journal Science.

She said the first sign of psychological problems becomes evident during major, stressful changes in lifestyle, such as when a young person leaves home for university.

By acting on the prefrontal cortex, PKC could be a factor in distractibility, impaired judgment, impulsivity and thought disorder, all of which have been tied to this brain region.

Arnsten said it gives scientists "some rational understanding of what is causing these irrational disorders".

"I think it's an important breakthrough in understanding the causes of some kinds of mental illnesses," since it could lead to new PKC-inhibiting drugs, she added.

Arnsten also said other recent research has also found high PKC activity among people with mental disorders.

She said her team used chemical substances to provoke stress in laboratory rats and monkeys in a controlled environment.

Similar stress levels are found in humans when confronted with a loud noise or before an exam, she added. - Sapa-AFP