IN the wake of recent and widely publicized studies hinting that estrogen replacement therapy may help prevent Alzheimer's disease, neuroscientists have expressed relief that at last the world may begin to appreciate the importance of the hormone to the health of the human brain. They are also exploring the mechanisms through which estrogen may protect against Alzheimer's and other types of dementia.

New research suggests that estrogen operates in brain cells throughout life to assure the production of critical enzymes and to maintain the densest possible mesh of fibers connecting one nerve cell to the next, the kind of synaptic complexity that characterizes a thinking, remembering, robust brain.

The work implies that when estrogen levels drop sharply upon menopause, the complexity of connections could slowly diminish, perhaps making neurons -- brain cells -- more likely to degenerate and die. Alzheimer's disease is, above all, a disease of cell death.

And in yet another example of what some may see as the unfair advantages that aging men have over their female counterparts -- beyond the social perks -- men's supply of testosterone could help safeguard them against Alzheimer's disease. Women are known to suffer from the disorder more frequently than men, although this discrepancy has commonly been attributed to women's longer life span. The new work hints at yet another reason for the difference. It turns out that much of a man's circulating testosterone is converted to estrogen in the brain; and because men's testosterone levels do not dip sharply at any point and often stay reasonably high well into the 80's, men could be getting the protective influence of estrogen in the brain for years after women's natural estrogens have dropped off. A New Dimension

The work highlights neurobiologists' mounting conviction that estrogen participates in neural development and performance throughout life, helping to establish brain architecture in the fetus and then to maintain that intricate structure throughout adulthood. Such revelations add an impressive new dimension to a hormone that many people, including doctors, have regarded largely and mistakenly as a female hormone, of interest for its role in a woman's reproductive cycle and for its less benign participation in some types of breast cancer.

"Most people, including doctors, are not aware that the brain is a major target of estrogen," said Dr. C. Dominique Toran-Allerand, a developmental neuroscientist at Columbia University College of Physicians and Surgeons in New York. "People worry about bone density and coronary artery disease in the postmenopausal state, but they should also be worrying about what's happening to the neurons."

And while exercise and a low-fat, high-calcium diet may go far in counteracting the dangers of an estrogen dropoff to bone and heart, a hormone-free mechanism for keeping neurons in shape is much less clear.

Dr. Toran-Allerand emphasized that she was not a clinician, nor was she making a formal recommendation that all women over 50 start taking estrogen replacement therapy. Women who come from families with a history of breast cancer, for example, should discuss with their doctors whether or not to take hormone replacement. But Dr. Toran-Allerand does believe that the role of estrogen in the brain, in men and women alike, has been woefully ignored. Suggestions of Link

The first widely reported indications that estrogen replacement therapy could benefit the brain came last fall, when Dr. Victor Henderson of the University of Southern California announced his results with a study of 2,418 women, some of whom had taken estrogen pills and some who had not. He found that those on hormone replacement were 40 percent less likely to develop Alzheimer's disease than the women who were not on the hormone. Dr. Henderson emphasized that it was still too early to recommend estrogen as a protection against dementia, but he and others have since started new and much larger studies investigating the link.

Researchers have seen similar mental benefits of estrogen treatment in studies of premenopausal women who have had their ovaries, and thus their primary source of estrogen, surgically removed. In experiments conducted over the past 10 years, Dr. Barbara B. Sherwin, a professor of psychology at McGill University in Montreal, looked at two groups of ovariectomized women, one of them given estrogen tablets, the other receiving dummy pills. Subjecting the women to a battery of neuro psychological tests, Dr. Sherwin found that the women taking estrogen performed comparatively better in tests of their verbal memory, for example in their ability to remember paragraphs they had read.

"The effect of estrogen is not global, it's specific," said Dr. Sherwin. Nor can she say that the differences measured in a clinical setting would make much of a difference in the real world. The women who did not receive estrogen pills still managed to live their lives and do their jobs quite reasonably, she said, although all the women decided to take hormone replacement once the trials were through.