Scientists are now examining these variables, in order to shed light on how and why HT may cause deterioration of cognition.
Animal models are proving useful in the study of HT because better controls are possible than with human studies. For example, like women, female mice experience natural cyclical fluctuations in estrogen levels. In addition, during middle age (17 months for mice) hormone cycling becomes irregular and hormone levels begin to decline, similar to women in menopause. Mice are also known to experience memory loss as they get older.
Jodi Gresack, MS, and Karyn Frick, PhD, of Yale University 's Department of Psychology capitalized on the similarities between female mice and women in a study designed to examine the effects of continuous versus cyclical estrogen on cognition.
Researchers removed the ovaries of 32 middle-aged mice before starting them on various courses of HT lasting three months. A continuous group received estrogen injections daily, a cyclical group was administered estrogen every four days, and a control group received daily injections with no estrogen.
After three months, the mice underwent a variety of cognitive tests. In the spatial water-escape radial arm maze test, mice had to swim in a wheel-shaped maze until they were able to locate a submerged platform escape route. Mice were tested every day for two weeks for both spatial reference memory (long-term memory for information that did not change during the test session) and working memory (short-term memory for information that changed in each trial).