Disruption of one-trial appetitive learning and passive avoidance following stimulation of the substantia nigra pars compacta

Rats had bipolar electrodes chronically implanted either in the substantia nigra pars compacta (SNC) or in control placements dorsal or ventral to the SNC, prior to training in one-trial avoidance and appetitive tasks. Experimental subjects received low-intensity electrical stimulation to either the SNC or control placements for at least 2 min during acquisition of both tasks. When retested 24 hr later all animals receiving stimulation of the SNC showed impaired retention of both the step-down passive avoidance response and the appetitive tasks, regardless of the order in which the two tasks were tested. Animals stimulated in areas adjacent to the SNC were not significantly different from implanted and unoperated controls when retested on either task. These data confirm previous reports that stimulation of the dopaminergic SNC impairs retention of shock-motivated avoidance responses and show that the disruptive effects are not restricted to avoidance learning.