Lesions in the mesencephalic part of pedunculopontine nuclei modify goal-directed behaviour.

Eleven Long-Evans hooded rats were preoperatively tested in an open field (OF), then trained in a Y-maze and thereafter symmetrically lesioned in the mesencephalic part of the pedunculopontine nuclei (MPPN). The OF test on the 11th postoperative day revealed no significant changes in exploratory behaviour, but a significant increase of ambulatory activity during all periods of 10 min exposure. This points to the role of the lesioned structure in response inhibition and habituation. High postoperative retention of preoperatively learnt conditioned avoidance responses (CAR) and of brightness discrimination showed no memory deficit. Reaction times were significantly decreased in the MPPN rats without symptoms of enhanced emotional activation. The escape speed from start to goal box was significantly slowed down after lesion. The formation of a new CAR stereotype in the jump test was impossible after MPPN lesions, because the rats did not find the escape possibility. The results differ from that of other mesencephalic reticular lesions.