Repeated morphine injections induce behavioural sensitization in Roman high-, but not in Roman low-avoidance rats

The selective breeding of Roman high- (RHA) and low-avoidance (RLA) rats for, respectively, rapid vs poor active avoidance acquisition has resulted in two phenotypes that differ in their behavioural and neurochemical responses to addictive drugs, including morphine. To compare the ability of these lines to develop behavioural sensitization to morphine, female RHA and RLA rats were treated twice daily with either saline or escalating doses of morphine (5, 10, and 20 mg/kg, s.c. on the 1st, 2nd, and 3rd day of treatment, respectively), and were challenged with morphine (0.5 or 2 mg/kg, s.c) 1 day before and 3 weeks after repeated morphine administration. The locomotor activation produced by either challenge dose of morphine was more pronounced in RHA rats repeatedly treated with morphine vs the respective saline-treated controls, whereas no significant change in locomotor activity was observed in RLA rats. The results show that behavioral sensitization to morphine was induced in RHA but not in RLA rats.