Apomorphine locomotor sensitization can be potentiated by environmental change: evidence for a non-Pavlovian...

We investigated environmental contrast effects on the expression of apomorphine locomotor sensitization by using two arena formats (round and square) that elicited differential levels of spontaneous locomotion. Eight groups of rats received 3 daily injections of apomorphine (APO) (2.0mg/kg) or vehicle (VEH) to induce locomotor sensitization: four groups in the square arena (2 APO and 2 VEH) and four in the round arena (2 APO and 2 VEH). Subsequently, the groups were retested with the same APO/VEH regimen for 3 days. One APO group and one VEH group originally tested in the square arena were re-tested in the square arena while the other square arena induction pair was re-tested in the round arena. A similar re-test protocol (same/different arena) was implemented for the round arena groups. The change from a square induction arena to a round test arena potentiated sensitization as compared to the sensitization observed in the round test arena following induction in the same round arena. In contrast, the change from a round induction arena to a square test arena attenuated sensitization compared to sensitization in the square test arena following induction in the square arena. This asymmetric sensitization profile is inconsistent with a Pavlovian response conditioning mediation of sensitization. We suggest that psycho-stimulant experience in low or high activity environments can evoke positive and negative behavioral contrast effects when the environments are switched. This behavioral plasticity in response to environmental change linked to prior drug environment experience implicates contrast effects in the expression of psychostimulant sensitization.