Substantia nigra pars reticulata modulates spontaneous and goal-directed behaviour of rat

Spontaneous and active avoidance behaviour was compared pre- and postoperatively on 11 six month old male hooded rats of the Long-Evans strain. Seven of them with small bilateral symmetric lesions only in the ventromedial part of substantia nigra pars reticulata (SNR) were characterized by a strong decrease of exploratory parameters except rearings, without differences of ambulatory activity in the open field (OF) test. The SNR group showed a significant retention loss, increased reaction times and run durations in three variants of preoperatively consolidated Y-maze performance and weakened brightness discrimination. They were unable to relearn the tasks and to reduce errors to the preoperative level which was zero. Postoperative acquisition of a new active avoidance stereotype in the jump test box was impossible. They ignored the hanging rod in this box and did not find the escape possibility. Prevailing flexor tonus of trunk and forelegs after SNR lesions was no sufficient reason for these changes, because inborn and automated programs were far less concerned than learnt or operative programs and the accuracy of goal-directed behaviour.