Preference for cocaine by the weight pulling method in rats.

The purpose of the present study is to show the efficiency of the weight pulling method in evaluating quantitatively the positive reinforcing effect of cocaine. Rats were trained to pull the weight in order to eat the drug-admixed food (DAF). The experiments began with the preexposure of the drug through the repetition of CFF schedule. The schedule consisted of one choice trial (C) between the intake of normal food and DAF followed by two consecutive forced trials (F), in which the rats were forced to take the DAF only. The study consisted of Experiment I, where cocaine concentration in DAF was varied while the period of cocaine preexposure was kept constant and Experiment II, where the period of preexposure was varied while the cocaine concentration was kept constant. Results show that the reinforcing effect of cocaine was dependent on cocaine intake. On the other hand, the reinforcing effect of cocaine was independent of cocaine preexposure period. The effect of cocaine on the drug-seeking behavior was evident on the first day of cocaine exposure. It is concluded that the weight pulling method is sufficient to evaluate quantitatively the reinforcing effects of cocaine in rats, and this method may be useful for the prediction of dependence potential in man.