Monkeys may provide AIDS dementia model.

Physiologist Elisabeth Murray remembers hearing tales circulated within research circles that some monkeys turned clumsy after becoming infected with simian immunodeficiency virus (SIV). At the time, she didn't know whether the stories were true, let alone whether they might have implications for humans. Now she does.

In a year-long study conducted at the National Institute of Mental Health in Bethesda, Md., Murray and four colleagues have established for the first time that rhesus monkeys infected with SIV develop cognitive and motor-skill problems resembling those seen in many people infected with human immunodeficiency virus (HIV), which causes AIDS. Because of the similarity, the researchers think SIV-infected monkeys may provide a useful model for studying this impairment--sometimes called AIDS dementia--in humans. They present their findings in the March 6 Science.

Distinguished by loss of memory and muscle coordination, AIDS dementia affects both children and adults infected with HIV, although it occurs far more often in children, says Philip A. Pizzo, head of pediatrics at the National Cancer Institute in Bethesda, Md. "In children [with AIDS], the figures go anywhere from about 50 to 90 percent," he says.

A simian model for AIDS dementia would allow researchers to test not only new drug therapies but also new hypotheses about what actually causes these neurological symptoms. For example, one theory implicates quinolinic acid, a substance produced in the brain. HIV infection may lead to overproduction of this substance, which acts as a neurotoxin at high levels.

Right now, the researchers don't know what triggered the dementia-like symptoms in the monkeys. "We're really in the dark here," Murray told Science News. "We don't understand what's happening between SIV infection and this behavioral impairment."

For the study, Murray and her colleagues trained a group of rhesus monkeys to perform a series of tasks designed to test their memory, reasoning ability and motor skills. The researchers then infected eight of the monkeys with SIV; five uninfected monkeys served as controls. Within six months, four of the infected monkeys showed impaired reactions in one or more behavioral tasks. By 10 months, all but one of the infected animals showed signs of neurological impairment, while all of the control monkeys continued to perform normally.

"It's another piece of the puzzle for understanding HIV and SIV," Murray says.