The brains of people with the memory-robbing form of dementia are cluttered with a plaque made up of beta-amyloid, a sticky protein.

But there long has been a question of whether that is a cause of the disease or a side effect. Also involved are tangles of a protein called tau; some scientists suspect this is the cause.

Now, researchers have caused Alzheimer's symptoms in rats by injecting them with one particular form of beta-amyloid. Injections with other forms of the protein did not cause illness, which may explain why some people have beta-amyloid plaque in their brains but do not show disease symptoms.

The findings, by a team led by Ganesh M. Shankar and Dennis J. Selkoe of Harvard Medical School, were reported yesterday in the journal Nature Medicine.

The researchers used extracts from the brains of people who donated their bodies to medicine. Forms of soluble beta-amyloid containing different numbers of molecules, as well as insoluble cores of the brain plaque, were injected into the brains of mice. There was no detectable effect from the insoluble plaque or the soluble one-molecule or three-molecule forms, the researchers found.

But the two-molecule form of soluble beta-amyloid produced characteristics of Alzheimer's in the rats, they reported.

Those rats had impaired memory function, especially for newly learned behaviors. When the rat brains were inspected, the density of brain cells was reduced by 47 percent, with the beta-amyloid seeming to affect synapses, the connections between cells that are essential for communication between them.

The research, for the first time, showed the effect of a particular type of beta-amyloid in the brain, said Marcelle Morrison-Bogorad, director of the division of neuroscience at the National Institute on Aging, which helped fund the research.

She said the findings may help explain the discovery of plaque in the brains of people who do not develop dementia.

Now, the question is why one has the damaging effect and not others.

Richard J. Hodes, director of the National Institute on Aging, said that "while more research is needed to replicate and extend these findings, this study has put yet one more piece into place in the puzzle that is Alzheimer's."