6-Hydroxydopamine and avoidance: Possible role of response suppression.

The intraventricular administration of 6-HD to rats pretreated with pargyline resulted in severe, long-lasting decreases in avoidance responding with little or no effect on escape responding. Despite the fact that the rats failed to avoid, they appeared to be able to discriminate the CS, as was evident from freezing behavior and other symptoms of an apparent fear reaction during the CS. The increase in freezing, a response that was incompatible with avoidance was seen during the first few test sessions after 6-HD treatment and seemed to be largely responsible for a gradual decline in avoidance responding during this same period. The role of CA depletion in the animal's response to aversive stimuli thus appears to be a significant aspect of the avoidance decrement that follows 6-HD administration.