Differential effects of morphine on conditioned place preference and locomotor activity in adult and adolescent rats

Abstract

Objective To test whether adolescent rats are more sensitive than adult rats to morphine' s rewarding actions and locomotor activating effects. Methods Conditioned place preference(CPP) procedure was used to examine drug-induced reward and locomotor activity. Adolescent ( Postnatal day 35 ) and adult ( P67 ) male rats received either saline or morphine (3 mg/kg) for 6 days and the locomotor response in the drug paired chamber was recorded during conditioned training. CPP effects were tested 24h after the last training. Results In adolescent rats, morphine CPP effect was not present [ saline group (20. 89 ±31. 14) s, morphine group (90. 75 ± 27.91)s, P = 0. 15], however, morphine CPP effect occurred in adult rats[saline group (31.5 ± 41. 24) s, morphine group(266. 13 ± 32. 26) s, P < 0. 001 ]. Adult rats presented higher CPP score than adolescent rats(P < 0.001). The locomotor response was significantly higher in day 3 ～ 6 than day 1 in the adolescent treated with morphine (P<0.05 or <0. 01). The locomotor response was significantly higher in day 2-6 than day 1 in the adolescent treated with morphine(P<0. 05 or < 0. 01 ) .which suggested that both adolescent and adult rats exhibited similar behavioral sensitization to morphine administration. Two-way ANOVA indicated that the main effect of age was not significant(F_(1,14) =0. 33, P = 0. 57) ,the interaction effect of age x treat was not significant(F_(5,70) = 0.85, P = 0.52) .there was no age difference between adolescent and adult rats in behavioral sensitization effects. Conclusion Compared with adult rats, adolescent rats don' t present morphine CPP, but show the similar morphine behavioral sensitization. Key words: Drug addiction; Morphine; Conditioned place preference; Locomotor sensitization