Abstract: Behavioral sensitization, which has some common properties with learning, memory and long-term potentiation (LTP), is thought to play a key role in certain aspects of drug addiction such as compulsive drug-seeking behavior. It has been demonstrated that behavioral sensitization to amphetamine is blocked by glutamatergic N-methyl-D-aspartate (NMDA) receptor antagonist MK801，which has been proved to block learning, memory and LTP. Scopolamine, an antagonist of muscarinic cholinergic receptor, has also been known to inhibit learning, memory and LTP. However, whereas several studies have showed that scopolamine blocks behavioral sensitization to drugs of abuse, others have suggested that scopolamine plays the role of potentiation to it. This conflict led us to examine further the effects of scopolamine on the development as well as the transfer of behavioral sensitization induced by morphine.
Method
Experiment 1: Rats were given four saline +saline (control group) or 10-mg/kg morphine +saline (morphine group) or 10-mg/kg morphine + 3-mg/kg scopolamine (morphine-scopolamine group) injections (i.p.) over a 36-h period (days1-2), followed by a 7-days withdrawal period (days 3-9). On day 10, all animals were challenged with 4-mg/kg morphine (i.p.) and their locomotor activity was measured for two hours. On day 24, morphine group and morphine-scopolamine group were challenged with 4-mg/kg morphine again. Experiment 2: Rats were given four saline (control group) or 10-mg/kg morphine (morphine group) or 10-mg/kg morphine (morphine-scopolamine group) injections (i.p.) over a 36-h period (days 1-3). After a 12-hour’s interval, control group and morphine group were administered with four saline and morphine-scopolamine group with four 3-mg/kg scopolamine over a 36-h period (day 3-5), followed by a 4-days withdrawal period (days 6-9). On day 10 and 17, all animals were challenged with 4-mg/kg morphine and their locomotor activity was measured for two hours.
Results
In Experiment 1, rats in morphine group showed significantly greater locomotor activity than other two groups (control group and morphine-scopolamine group) on day 10 and than morphine-scopolamine group on day 24. In Experiment 2, there was significant difference between the locomotor activity of morphine group and that of control group on day 10, but morphine-scopolamine group had no significant difference compared with control group or with morphine group. On day 17, the locomotor activity of morphine-scopolamine group significantly increased compared with control group and had no significant difference compared with morphine group.
Conclusions
The results of this study showed that scopolamine could inhibit the development of sensitization and delay to a certain extent but could not inhibit transfer of sensitization induced by morphine

Key words: morphine, scopolamine, development of behavioral sensitization, transfer of behavioral sensitization