The expression of locomotor sensitization to apomorphine is dependent on time interval between injection and testing

This study examined the onset of locomotor sensitization induced by apomorphine as a function of the temporal delay between drug injection and testing. In experiment 1, rats received three daily administrations of 2.0 mg/kg apomorphine or vehicle either immediately (0 min) or 20 min before being placed into the test environment for 20 min test sessions. Apomorphine given immediately before testing induced a stimulant effect during the first session and sensitization by the second session. However, when testing was delayed 20 min, apomorphine induced stimulant effects only after the third injection. In experiment 2, separate groups received a single 2.0 (mg/kg) apomorphine/vehicle injection immediately before being placed into the test environment for 60 min. In this experiment, apomorphine induced a stimulant effect at 0-20 and 20-40 min. However, the 20-40 interval increase in locomotion was relative to the low level of activity in the vehicle group and was not greater than the 0-20 min locomotion of the vehicle group. Thus, sensitization depends both on peak drug concentration and habituation state of the control group. The variable post-injection delays could be a useful method to study sensitization because it can avoid ceiling effects and changing baselines in the control groups.