According to a study of rats published in the June issue of Behavioral Neuroscience, activation of different parts of the brain may depend on the presence or absence of estrogen. Rats treated with the hormone learned a place-oriented task faster than rats not getting it, but those not on estrogen were faster completing a response-driven task. These tasks are believed to be controlled by different neural or memory systems.
"What we found is that given these analogous tasks that require different cognitive strategies, estrogen biased the rats to use a place, or spatial, strategy," said Donna L. Korol, a psychology professor at the University of Illinois at Urbana-Champaign. "Those not given estrogen are better using a response, or non-spatial, strategy. This suggests that estrogen isn't just good for all kinds of memory. Rather, it is very specific in dictating what strategy one takes. Estrogen may enhance some and impair other forms of learning."
In the National Science Foundation-funded study, Korol and Lacy L. Kolo (St. Louis University School of Medicine) used young rats whose ovaries had been removed to decrease circulating estrogen levels. Three weeks later, some of them received injections of estrogen, while others got a placebo, before learning to find food in two similar four-arm mazes.
In the place-training test, food always was at the same place, but the required turn changed, depending on the rats' starting point. Rats on estrogen learned the task faster than the untreated group. For the response-training test, the rats always found food by turning right (or left) at the first opportunity regardless of where they had started. Rats without estrogen learned this task quicker than the estrogen-treated ones.
"If estrogen was simply enhancing learning, the results should have been the same for each task, but that was not the case," Korol said. "It appears that estrogen is enhancing place learning at the expense of response learning. Both these task scenarios are important, because they reveal that estrogen isn't just up- or down-regulating something. It is shifting what individuals are good at solving - without estrogen, they still are good at something."
A postmenopausal woman not on HRT may believe that her ability to solve a problem as she always had is slipping. In reality, the brain may be shifting gears into a different strategy that the woman is not used to harnessing, Korol said.
In a chapter for a book published last year, "Animal Research and Human Health," Korol and Carol A. Manning of the University of Virginia noted that when an aging woman's hormone level declines, her brain might actually shift into a problem-solving mode more common to men. "Women may actually get better at performing a task from a different approach, but they are not used to doing it that way, so they view the change as an impairment," Korol said. "Theoretically, this may be true, but we don't know this for sure yet. The question is, Can you tap into the other strength?"
Korol is among a growing body of researchers studying the cognitive impacts of estrogen. Researchers so far have found that estrogen increases nerve-cell communications and brain excitability, in general, but findings related to memory and learning have often conflicted as to whether cognition was impaired or enhanced.
Many previous studies involved water-escape tests, in which rats are stressed as they begin to learn new tasks. The stress, Korol said, "seems to impair cognition in the presence of estrogen, but when there is no stress estrogen helps the capacity to learn."
The positive-reward, food-based tests used in Korol's lab remove stress from the equation. "Now we are going in and looking at the specific brain structures," she said. "Having estrogen at high physiological levels will shift the strategy that you use to solve a task. It might be doing so by changing the functioning or activity of certain brain areas."