US researchers have linked the behaviour of seeking drugs after a period of abstinence to specific nerve cells in a part of the brain called the nucleus accumbens. Previous studies have shown that drug addiction depends on the mesocorticolimbic dopamine system innervating the nucleus accumbens. Dr Udi Ghitza and colleagues from Rutgers University tested whether the accumbens neurons exhibit responses to external stimuli previously associated with self-administration of cocaine by rats. The rats learnt that on hearing a tone they could press a lever and self-administer cocaine. No cocaine was available if the animal pressed the lever in the absence of the tone.
Microelectrodes were attached to the brains of the animals and these recorded the activity of single neurons in the nucleus accumbens.

After two weeks of self-administration of cocaine, the lever was removed and no tone sounds were made. The animals abstained from the drug for almost a month. When the lever was returned to the cage, the animals ignored it when no cocaine and no tone were provided. However, when the original tone was made, the animals began to press the lever at a high rate even though no cocaine was available. During this relapse into drug seeking, the neurons – in an area of nucleus accumbens known at the shell – were activated by the tone. The rats eventually stopped pressing the lever when the tone was made. However, nucleus accumbens neurons still responded to the tone. Dr Mark West, one of the co-authors, says, This activity may reflect the processing of memories that persist even after a long abstinence and may partially explain why environmental cues can provoke a relapse. This  suggests that the existence of a neuroadaptation that may make individuals more vulnerable to assuming drug-taking behaviour.”