Researchers in the UK and America now show one way to break this vicious circle - at least in rats. Addiction to opiates such as heroin depends not only on their positive, pleasurable effects, but also on escaping the grim symptoms of withdrawal. So just as Pavlov's dog learned to drool to a dinnertime bell, addict's brains quickly associate withdrawal symptoms with certain situations. So when the same stimuli crop up in future, the brain demands drugs. Destroying certain brain cells in rats addicted to another opiate, morphine, ends this 'conditioned withdrawal response' B. J. Everitt of Cambridge University, UK, and colleagues now report in Nature 1 .
Addiction is the body's way of coping with repeated doses of harmful substances.
Biochemical changes allow cells to endure more of the drug, so the addict must take greater amounts to experience the same high. But the body is sneaky. Instead of waiting for the drug to arrive before triggering its biochemical defences, it watches for early warning signs in the surroundings. Conditioned withdrawal responses have been studied widely in people. Experiments with heroin addicts for example, showed they quickly learn to associate peppermint smells with withdrawal.