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mrz
09-04-03, 20:21
Cocaine addicts get a high before the hit

19:00 09 April 03

NewScientist.com news service

Cocaine addicts may get spikes in a brain chemical associated with a high at the mere anticipation of a "hit", suggests a new study in rats.

US researchers found that dopamine, a neurotransmitter associated with pleasure, surged before rats pressed a lever to release a dose of cocaine and even while they were waiting for the hard drug to be delivered.

Dopamine is known to peak during addictive behaviours like drug taking, eating and sex, but this is the first study to demonstrate an increase before the actual event. The finding could help understand why recovering drug addicts relapse, say the researchers.

"It may be relevant with respect to human addiction because drug related cues and paraphernalia often result in intense carvings," says Regina Carelli, a psychologist at the University of North Carolina in Chapel Hill, and one of the team. "This craving is one of the leading causes of relapse and it is very important we understand the biology of this."

Dopamine might represent "both the chicken and the egg" in the events underlying the behaviour of addicts, says David Self, at the University of Texas in Dallas.

"As a rat chases its tail, drug addicts may suffer a similar vicious circle of priming and reward controlled by these dopamine signals," he writes in an commentary accompanying the Nature paper. "Therapies aimed at preventing one or both of these dopamine signals could be effective treatments for addiction."


Sub-second timescale


Scientists have suspected dopamine plays a role in drug-seeking behaviour. But this had not been demonstrated because until now scientists could only measure changes in dopamine over minutes.

"What makes our study unique is that we were actually able to measure dopamine on a sub-second timescale - in real time," Carelli told New Scientist.

This was possible thanks to a carbon electrode system, developed by Carelli's colleague Mark Wightman, that measures dopamine levels in the brain of the rats 10 times a second.

A strong dopamine surge was seen when the addicted rats approached a lever they knew would deliver a shot of cocaine. A second surge occurred when the rat pressed the lever prompting several audiovisual cues while it waited for the injection.

Carelli said the rats trained to receive cocaine in this way showed this increase in dopamine when exposed to the audiovisual cues alone, unlike rats unfamiliar with this association.

"Ideally, one approach to treating cocaine addiction in humans would be a drug targeted at reducing 'cue-evoked craving' for the drug," said Carelli. This could be used together with counseling.

Journal reference Nature (vol 422, p 615)


Shaoni Bhattacharya

http://www.newscientist.com/news/news.jsp?id=ns99993612