Reduced behavioural inhibition after lesions in the dorsomedial pontine reticular formation of rats.

Male hooded rats of the Long-Evans strain were investigated in an open field (OF), in an Y-maze and in a pole-climbing (jump) test before and after small bilateral symmetric lesions of the dorsomedial pontine reticular formation, medial from the locus coeruleus including lateral parts of the central gray. Postoperative values of ambulatory activity were increased above 300%. This hyperactivity in the OF was characterized by weakened habituation in the OF, increased locomotor speed, increased climbing rate, whereas horizontal exploratory activity was significantly decreased. The preoperatively learnt Y-maze avoidance tasks were correctly performed after lesions without any retention deficit and without errors in brightness-discrimination. The postoperative acquisition of a jump test avoidance task was impossible because the lesioned rats did not find the escape possibility in spite of a great number of variant jumps. They never showed tendencies of arrest behaviour ("learned helplessness"). The syndrome is characterized by weakened behavioural inhibition of incorrect or redundant responses.