Catecholamines and conditioned blocking: effects of ventral tegmental, septal and frontal 6-hydroxydopamine lesions in rats.

The performance of rats on the conditioned blocking test of learned inattention was measured in a two-way shuttle avoidance task after sham and dopamine (DA)-depleting lesions of the frontal cortex, septum and ventral tegmental area (VTA). Animals were trained on two sessions with tone and/or light as conditioned stimuli. One group was trained with both stimuli on both sessions. A second group was trained on the first session with one stimulus and on the second with both stimuli. The blocking of conditioning to the added stimulus (b) was tested by presenting the stimuli (a and b) separately and measuring the blocking ratio (avoidance to b/a + b) and response latencies. No deficits were recorded on tests of sensory and motor ability. The VTA group alone showed a hyperlocomotor response to apomorphine treatment and did not acquire the avoidance response. The appearance of blocking in the septal group was delayed until the end of the test session. Blocking was mildly attenuated in the frontal group. DA levels were depleted by about 80% and noradrenaline (NA) levels by, respectively, 20 and 50% in frontal and septal areas. This suggests that the level of DA activity or the balance between the activity of DA and NA in frontal and limbic regions can contribute to efficient associative conditioning and/or the normal ability of rats not to attend to a redundant stimulus.