Successive negative contrast in one-way avoidance: effect of thiopental sodium and chlorpromazine.

The successive negative contrast effect on one-way avoidance was induced by shifting rats from a large reward (30 s spent in the safe compartment after completion of the avoidance response, pre-shift phase) to a small reward (1 s, post-shift phase). Under these conditions, the previously learned avoidance response deteriorated (negative contrast) when compared to a control group for which 'safe time' remained constant throughout the experimental situation (1 s). Thiopental sodium at a dose of 5 or 10 mg/kg, but not at 1, 2, 15 or 20 mg/kg i.p., abolished the negative contrast effect, and did not affect performance of the one-way avoidance task. Similar results were found when rats were treated with diazepam (1 mg/kg i.p.). Chlorpromazine at a dose of 0.5 or 1 mg/kg i.p. did not affect negative contrast, although at higher doses (2 or 3 mg/kg) there was an increase in the number of trials needed to reach the criterion for learning the avoidance response. This increase was evident in both pre-shift and post-shift phases, although only in the experimental situations involving a low level of reinforcement (1 s in the safe compartment). On the basis of these results, we tentatively suggest that the successive negative contrast effect in one-way avoidance in rats can be considered a useful pharmacological animal model for research into anxiety.