Scientists at the U.S. Department of Energy’s Brookhaven National Laboratory have discovered a key mechanism in the brains of people with human immunodeficiency virus (HIV) dementia.
The study is the first to document decreases in the neurotransmitter dopamine in those with the condition, and may lead to new, more effective therapies. HIV dementia is a type of cognitive decline that is more common in the later stages of HIV infection.
“Our results offer the first evidence of dopamine terminal injury — specifically injury to dopamine transporters — in HIV dementia patients,” says Brookhaven physician Gene-Jack Wang, the study’s lead author. “This suggests that a decrease in transporters may contribute to the disease process. We believe our findings also indicate a new direction for treatment.” The study appears in the September 2004 issue of the British scientific journal Brain. Physician Linda Chang, formerly a researcher at Brookhaven Lab and now with the University of Hawaii, initiated this research.
“This study clearly demonstrates that HIV infection damages dopamine-associated brain cells, and provides a pathway for developing more effective treatments,” says National Institute on Drug Abuse (NIDA) Director Nora Volkow, co-investigator, who worked on the study while at Brookhaven Lab.
Using positron emission tomography (PET), a brain-scanning technique, Wang and colleagues looked at the brains of 15 HIV-positive study volunteers, some of whom had symptoms of dementia, and 13 HIV-negative volunteers. The purpose of the scans was to determine the presence of dopamine transporters and dopamine receptors.
Dopamine transporters, found on the terminals of some brain cells (neurons), recycle dopamine, a brain chemical that sends signals between the cells. Dopamine receptors are “docking ports” on the receiving neurons. Dopamine is associated with pleasure and reward and is also essential for movement; low levels are found in conditions such as Parkinson’s disease. Decreased dopamine transporters represent neuron damage and lower levels of functional dopamine neurons, as well as decreases in the release of dopamine.
Prior to brain scanning, each volunteer was given an injection containing a radiotracer, a radioactive chemical “tag” designed to “light up” activated areas of the brain. The PET camera picks up the radioactive signal and measures the level and location of the tracer. Two hours after the first scan, each volunteer was given a second radiotracer, and his/her brain was scanned again. The first scan looked specifically for the presence of dopamine receptors, and the second searched for dopamine transporters.
The researchers found that HIV-positive patients with, but not those without dementia, had significantly lower dopamine transporter levels in the putamen and ventral striatum, the brain regions containing the most transporters, compared with HIV-negative volunteers. Higher levels of HIV virus correlated with lower transporter levels in the caudate — part of the striatum — and in the putamen. Dopamine receptor levels showed only mild decreases in HIV-positive volunteers.
“These findings suggest that HIV patients with dementia may benefit from dopamine-enhancing treatments, especially those patients with more severe cognitive motor deficits,” says Wang. “Studies have shown, however, that some of these agents may actually exacerbate HIV-induced neurotoxicity, but antioxidants may block this mechanism. Further studies should focus on adjunctive approaches combining drugs in both classes.”
“Viral suppression seems to correlate with less injury to the dopamine system,” Wang continues. “Because of this correlation, future studies should look at whether viral suppression after antiretroviral treatment can lead to recovery of deficits in the dopamine system.”