Rats yield active clues to drug addiction.

Rats yield active clues to drug addiction 

Researchers have begun to develop an animal model of drug addiction that takes into account individual differences in the propensity to ingest drugs. The new approach may lead to a better understanding of the biology of animal and human vulnerability to addiction, according to neurobiologist Pier Vincenzo Piazza and his co-workers at the University of Bordeaux, France.

Like humans, some rats avidly self-administer amphetamine when given the chance, while others abstain. Piazza and his colleagues found they could predict responses to amphetamine among 30 rats by first placing each animal in a novel environment -- a circular corridor -- and tracking its movement for two hours. Photoelectric cells embedded in the corridor detected the animals' activity.

Half the animals were classed as having a low activity response to the novel situation; the rest responded with high activity levels.

The next day, researchers again placed the rats in the corridor and injected each with a low dose of amphetamine. Activity levels increased significantly more among the high-response rats, indicating they were more sensitive to the drug than were the low-response animals.

The researchers then divided a second group of 40 rats according to high or low activity in the unfamiliar corridor. Half the high-activity rats and half the low-activity rats received four injections of amphetamine to accustom them to the drug's effects. The remaining animals received saline injections.

Acatheter connected to a pump-driven syringe was then inserted in each rat. Brief amphetamine injections were triggered when a rat poked its nose through a hole in the side of its cage.

High-activity rats that received saline injections developed a strong taste for self-administered amphetamine, where-as saline-treated low responders did not, the scientists report in the Sept. 29 SCIENCE. However, both low and high responders who first received amphetamine injections acquired a strong preference for self-administered doses of the drug, suggesting previous contact with amphetamine critically enhances the drug's addictive powers.

The researchers say the rats' varying activity levels in response to novelty may relate to differences in their response to stress and may also reflect neurobiological mechanisms that foster drug use.