Abstract: Conditioned placed preference (CPP), which reflects the rewarding and the strengthening effects of addictive drug, is a classical experimental model to evaluate drug addiction. Behavioral sensitization is defined as an increased (behavioral) response to a given dose of drug or a response of a similar magnitude upon treatment with a lower dose of drug after repeated treatment. It reflects the behavioral activation effect of addictive drug. Many studies have demonstrated the high relevance between abuse-like effects of some drugs (such as nicotine, cocaine and heroin) and impulsivity. However, little is known of the relationship between impulsivity and CPP induced by morphine. Therefore, this experiment attempted to investigate the effects of impulsivity on CPP and behavioral sensitization induced by morphine, using delayed reward model (T maze) and CPP model.
Adult male Wistar rats were stratified into three groups (high impulsivity, HI; middle impulsivity, MI; low impulsivity, LI) according to their choices in a delayed reward model, and then received morphine administration in morphine-paired chamber or saline administration in morphine-unpaired chamber. The following behavior of rats was recorded: the time of stay in morphine–paired chamber before and after morphine administration, the effect of morphine on the development of behavioral sensitization (the locomotor activity of 45 minutes in morphine–paired chamber) and the locomotor activities in the expression period (the locomotor activity of 120 minutes after conversion of behavioral sensitization).
The result of experiment suggested that the magnitude of the place preference of the three groups of rats was: LI > MI > HI. LI and MI animals developed CPP while HI group did not develop CPP. These results demonstrated that in CPP, the higher the innate impulse in rats, the less the rewarding and the strengthening effects of morphine (HI﹥MI﹥LI).
In the test of behavioral sensitization, morphine induced significant behavioral sensitization in HI and MI rats but not in LI rats.
The present data suggested that the impulsivity level in rats is a negative correlation with the CPP and a positive correlation with the behavioral sensitization, which induced by morphine. It also indicated that the effects of the innate differences of impulsivity on CPP and behavioral sensitization are inconsistent.

Key words: rats, impulsivity, conditioned placed preference, behavioral sensitization