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Drug‐induced reinstatement of heroin‐ and cocaine‐seeking behaviour following long‐term extinction is associated with expression of behavioural sensitization

The present study was designed to evaluate the relationship between reinstatement of drug‐seeking behaviour following long‐term extinction of intravenous (i.v.) drug self‐administration (an animal model for craving) and long‐term behavioural sensitization. Rats were allowed to self‐administer heroin (50 μg/kg per inj., 14 daily sessions), cocaine (500 μg/kg per inj., 10 daily sessions) or saline. Following a 3‐week extinction period, reinstatement tests were performed to evaluate priming effects of amphetamine, cocaine and heroin on non‐reinforced drug‐seeking behaviour. In addition, the occurrence of long‐term behavioural sensitization in rats with a history of heroin or cocaine self‐administration was determined. Heroin‐seeking behaviour was reinstated by heroin (0.25 mg/kg), amphetamine (1.0 mg/kg) and cocaine (10 mg/kg). In addition, animals with a history of heroin self‐administration displayed locomotor sensitization to both heroin and amphetamine. Cocaine‐seeking behaviour was reinstated by cocaine and amphetamine, but not by heroin. Interestingly, locomotor sensitization to amphetamine, but not heroin, was observed in animals with a history of cocaine self‐administration. In other words, the induction of drug‐seeking behaviour following a prolonged drug‐free period was found to be associated with the expression of long‐term behavioural sensitization. These data provide experimental evidence for a role of behavioural sensitization in the incentive motivation underlying drug‐seeking behaviour. If drug hyperresponsiveness would indeed be a crucial factor in drug‐induced craving in human addicts, pharmacological readjustment of the neuroadaptations underlying drug sensitization may prevent relapse to drug use long after detoxification.