SUCCESSIVE NEGATIVE CONTRAST IN ONE-WAY AVOIDANCE - EFFECT OF THIOPENTAL SODIUM AND CHLORPROMAZINE

The successive negative contrast effect on one-way avoidance was induc ed by shifting rats from a large reward (30 s spent in the safe compar tment after completion of the avoidance response, pre-shift phase) to a small reward (1 s, post-shift phase). Under these conditions, the pr eviously learned avoidance response deteriorated (negative contrast) w hen compared to a control group for which 'safe time' remained constan t 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 o ne-way avoidance task. Similar results were found when rats were treat ed 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 increas e was evident in both pre-shift and post-shift phases, although only i n the experimental situations involving a low level of reinforcement ( 1 s in the safe compartment). On the basis of these results, we tentat ively suggest that the successive negative contrast effect in one-way avoidance in rats can be considered a useful pharmacological animal mo del for research into anxiety.