Abstract: Behavioral sensitization has been defined as progressive increase of the rewarding and reinforcing effect of drugs of abuse, which has been considered to underlie the etiology of drug craving and relapse. Aim: To explore morphine behavioral sensitization and its conditioned effect, with further investigation into potential individual difference manifestation. Method: Rats were characterized as high and low responders (HR vs. LR) via initial motor activity developed in an inescapable chamber. By using computer-interfaced monitoring system, the locomotor activities of rats were observed under 5-day successive drug treatment to test the development of morphine sensitization. Then a saline challenge was given on day 6 to test its conditioned effect. Results: 1: LR rats expressed increased locomotor activities while HR rats were without this effect upon repeated morphine exposure. 2: Rats successively with drug treatment in the test chamber (paired group) expressed heightened motor activity compared with unpaired and control group upon saline challenge. This effect existed in both HR and LR group. 3: Locomotor activity from toward drug to saline treatment significantly decreased in LR but not in HR rats. Conclusion: 1: Under successive drug treatment, LR, other than HR rats developed morphine behavior sensitization. 2: Both HR and LR rats possessed the capability to develop conditioned behavioral sensitization toward morphine. 3: LR rats responded more strongly than HR rats to drug treatment.

Key words: morphine, behavioral sensitization, conditioned sensitization, individual difference