Brain Scans Prove Dopamine's Involvement In Cocaine Abuse
BALTIMORE, MD -- November 17, 1997 -- Scientists at Johns Hopkins have used
brain scans to show that intravenous doses of cocaine increase the
availability of dopamine, the brain's feel-good chemical. Dopamine's
activity appears to increase two to three times over baseline levels in the
brain area studied, the putamen, compared to a control area, the cerebellum.
Although the increase cannot yet be directly linked to a cocaine
user's high, investigators report this is the first time anyone has directly
demonstrated cocaine makes more dopamine available in the human brain.
Improvements in scanning technology eventually may track cocaine's
effects on the dopamine-generating nucleus accumbens, a smaller area nearby
in the brain that is known to play a role in addictive behavior in animals,
added Godfrey Pearlson, M.D., professor of psychiatry and a lead author on
the paper.
"The new finding should advance efforts to understand addiction and
treat it by blocking the euphoric effects of drugs," Pearlson said.
Brain cells use dopamine by binding the chemical to specific
openings on their surfaces. Pearlson used these openings to measure dopamine
activity. First, he injected cocaine users with the compound raclopride,
which binds to these same receptors. The raclopride was equipped with a
mildly radioactive tag visible on positron emission tomography (PET) brain
scans.
Soon after, scientists gave the subjects an injection of a placebo
and scanned their brains. Several hours later, the same subjects received a
second dose of raclopride followed by a street-equivalent dose of cocaine.
Then they scanned the patients again.
"Because the raclopride and dopamine compete for the right to bind
to the same receptors, we could compare the two sets of scans and be
virtually certain that the differences in the second group were caused by
extra dopamine produced by cocaine exposure," said Thomas Schlaepfer, M.D.,
now at the University of Bern in Switzerland.
"It's likely cocaine affects other neurotransmitters besides
dopamine and these may also be helping create the immediate rush or feeling
of euphoria caused by cocaine," Pearlson explained. "But dopamine is still
obviously a very important part of drug addiction. Marijuana, alcohol and
heroin all initially act on different brain systems, but the common bond
between them is that they all also increase dopamine availability."
The study was published with an accompanying commentary in the
September issue of the American Journal of Psychiatry.
