EFFECTS OF DOPAMINE DEPLETIONS IN THE MEDIAL PREFRONTAL CORTEX ON ACTIVE-AVOIDANCE AND ESCAPE IN THE RAT

Dopamine systems have been implicated in the performance of avoidance behavior, and the dopaminergic innervation of medial prefrontal cortex is known to be responsive to stressful stimuli. In the present invest igation, injections of 6-hydroxydopamine were used to produce moderate depletions of dopamine in the medial prefrontal cortex of rats traine d to perform an active avoidance/escape task. In this task, 0.5 mA sho ck was presented for 5 s every 30 s, and the rat could escape shock pr esentation, or avoid the shock for 30 s, by pressing a lever. Depletio n of dopamine in the medial prefrontal cortex did not affect total num ber of responses, and did not impair avoidance responding (i.e. respon ding when the shock was off), and in fact dopamine-depleted animals te nded to make slightly more avoidance responses than control animals. P refrontal dopamine depletions did result in a significant decrease in the number of escape responses (i.e. responding to terminate shock whe n the shock was on). Moreover, dopamine depletions significantly decre ased response efficiency, which is an index of the reduction of shock time produced per lever pressing response. Previous work has indicated that dopamine antagonists and accumbens dopamine depletions have dram atic effects on avoidance behavior; thus, the present results indicate that prefrontal cortex dopamine depletions do not mimic the effects o f interference with subcortical dopamine systems. The selective effect s of dopamine depletions on escape behavior in the present study sugge st that rats with medial prefrontal dopamine depletions have an impair ment in the ability to respond appropriately to the direct presentatio n of footshock. These findings may have implications for the functiona l significance of stress-induced increases in prefrontal cortex dopami ne activity.