Abstract: The long-term effects of repeated exposure to drugs of abuse are one of the central themes in drug addiction research. This study investigates the relationship between the behavioral locomotor sensitization induced by morphine and neurotransmitter acetylcholine. Rats were randomly assigned to two experimental groups and one control group receiving pretreatment, which were given four 10.0-mg/kg injections (i.p.) of morphine alone or with 0.2-mg/kg physostigmine or of saline alone over a 36 h period. At week 1 after pretreatment, all rats were challenged with 4.0-mg/kg morphine. Saline-pretreated rats received injection of physostigmine (0.2mg/kg) at week 2. Rats pretreated with morphine alone were challenged with 4.0-mg/kg morphine and 0.2-mg/kg physostigmine at week 2, and were challenged with 4.0-mg/kg morphine again at week 3. The results of this study showed that physostigmine could inhibit both development and expression of behavioral locomotor sensitization induced by morphine. In conclusion, these findings suggest that behavioral sensitization induced by morphine is related to inhibition of the central neurotransmitter acetylcholine release.

Key words: morphine, physostigmine, acetylcholine, behavioral locomotor sensitization