Washington ? Researchers have uncovered a new clue to the cause of Alzheimer’s disease.

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 whether this 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 beta-amyloid 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 Dr. Ganesh M. Shankar and Dr. Dennis J. Selkoe of Harvard Medical School were reported in Sunday’s online edition of 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 rats. 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. Studies were also done on mice and when their brains were inspected, the density brain cells were reduced by 47 percent. The beta-amyloid seemed to affect synapses, the connections between cells that are essential for communication between them.