The effects of excitotoxic lesions of the basolateral amygdala on the acquisition of heroin-seeking behaviour in rats

Rationale: Second-order schedules of drug-self-administration provide a method of examining drug-seeking behaviour, which is maintained in part by the presentation of a discrete, drug-associated light CS. Previous results have found that lesions of the basolateral amygdala (BLA) impair the acquisition of IV cocaine self-administration under this type of schedule. Objectives: The present experiments examined the effects of excitotoxic lesions of the BLA on the acquisition of IV heroin self-administration under both continuous reinforcement and second-order schedules, in order to investigate possible commonalties in the neural basis of heroin- and cocaine-seeking behaviour. Methods: Rats received quinolinic acid or sham vehicle lesions of the BLA prior to IV self-administration training. Initially, heroin self-administration under a continuous reinforcement schedule was acquired. Each active lever-press resulted in a 0.04 mg IV heroin infusion, paired with pre sentation of a 20-s light conditioned stimulus. Following acquisition of responding under this schedule, the response requirement was gradually increased to a second-order schedule of FI15(FR5:S). Results: There was no effect of lesions of the BLA on the acquisition of heroin self-administration under a continuous reinforcement schedule. The acquisition of heroin-seeking behaviour under a second-order schedule of self-administration was not affected by lesions of the BLA, but lesioned rats showed a significantly higher baseline level of responding. Conclusions: These results indicate that the rewarding effects of heroin do not depend on the integrity of the BLA. The BLA is also not critically involved in mediating heroin-seeking behaviour under a second-order schedule of reinforcement, and this stands in marked contrast to the effects of BLA lesions on the acquisition of cocaine-seeking behaviour. These findings suggest that discrete heroin cues were not critical in maintaining heroin-seeking behaviour under the second-order schedule used here and that other learning systems are engaged in the control of this behaviour.