Circling from unilateral VTA morphine: direction is controlled by environmental stimuli

Crystalline morphine was applied unilaterally to the ventral tegmental area of rats. Contralateral circling was observed in traditional test chambers and inside the walls of an elevated square enclosure. When tested outside the walls of the same enclosure, the animals turned in the opposite direction, maintaining the same flank in the position nearest the wall. This dopamine-dependent circling thus reflects a more complex interaction between the side of activation and the direction of circling than is implied in current views of dopamine function.