Chronic exposure to oestrogen may impair some cognitive functions, according to new research from the University of Illinois.
While experimenting with rats, the university researchers observed that the animals exposed to a steady dose of oestradiol, the main oestrogen in the body, were impaired on tasks involving working memory and response inhibition.
A research article in the journal Behavioral Neuroscience reveals that the researchers made the discovery while they were studying the effects of oestradiol on activities mediated by the prefrontal cortex - a brain region that is vital to working memory, and to the ability to plan, respond to changing conditions and moderate or control one's behaviour.
Susan Schantz, the bioscientist who led the study, says that working memory is the ability to briefly remember information needed for a particular task, such as a phone number which is forgotten soon after the number is dialled.
"With working memory you're just keeping it active until you use it," she said.
During the study, rats were trained to press one of two levers to obtain a food reward.
The animals that alternated between the levers received a reward, while those that hit the same lever twice in a row got nothing.
The researchers said that the rats exposed to oestradiol performed worse than their counterparts on this task, earning significantly fewer rewards.
In another experiment, the researchers measured the rats' ability to wait before responding to a stimulus: the animals had to wait 15 seconds before pushing a lever to get a reward.
The researchers said that the rats exposed to oestradiol performed worse on than those that were not exposed.
"That's the test where we really saw the most striking effects with oestradiol," Schantz said.
She said that the oestradiol-treated rats "were not as good at waiting".
Lead author on the study Victor Wang, a neuroscience graduate student, said: "Rats treated with oestradiol are definitely a lot more active and make a lot more lever presses. That's not conducive toward being rewarded."
Schantz revealed that the study had been designed to get baseline information for a separate inquiry into the effects of soybean oestrogens on cognitive function.
She said that the research team planned to compare the effects of chronic oestradiol exposure to the effects of chronic exposure to genistein, a phytoestrogen found in soybeans.
According to her, genistein is believed to have similar effects in the body as natural or synthetic estrogens, though no study has definitively proven that it does.
Schantz said that her team focused on the prefrontal cortex because it is rich in oestrogen receptor beta (ER-beta), a protein that spurs gene expression and other activities in the cell when it binds to oestradiol. Genistein also activates ER-beta.
She pointed out that some women took genistein supplements or eat soy-based foods to reduce hot flashes or other symptoms of menopause.
"Women take them thinking they'll be a safe alternative to hormone-replacement therapy and they might help hot flashes," she said.
The researcher said that some even ate soy or took genistein supplements hoping that it would improve cardiac or brain function in the same manner as hormone replacement does.
Schantz said that multiple factors influence the effects of oestradiol on the brain.
She believes that the timing of the exposure, the types of brain functions or structures studied, and the age of the test subjects can all generate different results.