Heroin-induced locomotor activity

Before heroin administration, each animal was placed into a locomotor detection chamber (Accuscan Instruments) for 3 days (4 h per day) for environmental habituation. WT and mGluR2-KO rats were then treated with saline (0.5 mg, i.p.) for 3 days followed by heroin (1.0 mg/kg i.p.) for 7 days. Each day, rats were placed in the chamber for 1 h of habituation, then removed and given either saline or heroin, and locomotor activity was recorded for two additional hours. Seven days after the last heroin injection rats were challenged again with the same heroin dose prior to being placed back into the locomotor chambers for 2 h. Data were collected in 10-min intervals each day using the VersaMax data analysis system (Accuscan Instruments, Inc., Columbus, OH, USA). Distance traveled (cm) was used to compare basal and heroin-enhanced locomotion in WT and mGluR2-KO rats.