Baclofen enhances extinction of opiate conditioned place preference

Conditioned opiate reward (COR) is rapidly acquired and slowly extinguished. The slow rate of extinction of the salience of drug-related cues contributes to drug craving and relapse. The γ-aminobutyric acid receptor type B (GABAB) agonist, baclofen, attenuates the unconditioned rewarding actions of several drugs of abuse and was investigated for effects on the extinction of COR. C57BL/6 mice were utilized in an unbiased conditioned place preference (CPP) protocol using morphine (10 mg/kg, s.c.) and saline. CPP was measured by increases in time spent in the morphine-associated (CS+) vs. the saline-associated (CS−) chamber in a 15-min test after four morphine and four saline alternated conditioning sessions. CPP and locomotor sensitization were produced to the CS+ chamber. Subsequently, sixteen daily extinction sessions were conducted with either vehicle or baclofen (1 or 2.5 mg/kg, s.c.) treatment given either before or after the sessions. This design was used to create the baclofen drug state before or after the activation of the CPP memory trace in the extinction protocol. After morphine CPP development, its extinction was significantly facilitated in a dose-dependent manner by post-session, but not by pre-session, baclofen treatments. No significant sedative effects of baclofen were detected during any extinction training or testing phase. Baclofen treatment facilitated the extinction of COR and reduced conditioned sensitization during extinction when given after, but not before, the activation of the CPP memory trace. Baclofen appears to have disrupted reconsolidation of conditioned reward memory during extinction training and might similarly facilitate extinction learning in human opiate addiction.