Chronic exposure to estrogen can impair memory, U.S. researchers report.

The brain area affected is the prefrontal cortex, which controls working memory and allows a person to remember information pertaining to a certain task.

Also affected are the ability to plan, respond to changing conditions and moderate or control one's behaviour.

The pre-frontal cortex has estrogen receptors, meaning it binds to estrogen.

Researchers conducted tests on rats, administering them to animals that had been given estrogen and to those who hadn't.

Those who were exposed to estrogen performed poorly compared to those who hadn't been exposed, in both memory and waiting tests that involved pressing levers and responding to a stimulus.

"That's the test where we really saw the most striking effects with estradiol," said Susan Schantz, lead author of the study. The estradiol-treated rats "were not as good at waiting," she said.

The rats that had received estrogen were a lot more active that the rats that didn't receive estrogen, the researchers found.

The authors point out that previous studies have shown that estrogen can enhance certain cognitive functions while impairing others. A recent study showed that estrogen enhances spatial learning.

The findings further complicate women's decisions to take estrogen supplements during menopause to reduce hot flashes and protect their hearts.

The study is published Friday in the journal Behavioural Neuroscience.