Addiction clue: Just say dopamine
Abused drugs appear to share a biochemical event in the sequence of actions they cause in the brain, conclude scientists studying drug addiction. The results of their studies, performed in rats, support the controversial theory that drugs as diverse as alcohol and cocaine feed into the brain's "reward system," setting off a mechanism thought to underlie all forms of drug addiction.
In past experiments, scientists have identified which drugs rats find rewarding by allowing them to self-administer drugs by pushing levers. They found that rats learn quicly to push a lever when it leads to a dose of cocaine, but take longer when it gives alcohol. In recent years, they also have discovered that lesions in a brain area involved in emotion seem to stop rats from pushing the cocaine lever. These experiments and others have convinced many scientists that this area, the nucleus accumbens, is the "hotspot" for the activity of cocaine and amphetamines, and that a chemical called dopamine transmits the rewarding effects. Several researchers have attempted to show that dopamine is critical to all addictive drug pathways, but their experiments have yielded conflicting results. One source of ambiguity may have been that they examined anesthetized animals.
In the recent study, Gaetano Di Chiara and Assunta Imperato of the University of Cagliari, Italy, used a relatively new technique called brain dialysis to measure brain chemicals directly in live, freely moving rats. In the July PROCEEDING OF THE NATIONAL ACADEMY OF SCIENCES (Vol.85, No.14), they describe how they implanted small tubes in the nucleus accumbens and in the caudate nucleus, an area involved in movememt, in rats briefly anesthetized for the procedure. They then administered various doses of addictive and nonaddictive drugs and allowed the rats to move freely. …