Rats avoided a distinctive environment in which they had previously received inescapable electric shocks; the amounts of passive avoidance were taken as indices of the levels of conditioned fear on repeated unpunished tests. Chlordiazepoxide, 7.5 and 15.0 mg/kg tended to reduce fear, but did not accelerate its extinction; 30.0 mg/kg however, retarded the extinction of fear by making the rats inactive and thus reducing the number of unpunished entries into the fear-evoking environment. The effects of chlordiazepoxide on locomotor activity were complex; entries were increased by all three doses of chlordiazepoxide on the first trial only, and following this, activity was markedly depressed by 30.0 mg/kg. Chlorpromazine on the other hand, consistently reduced locomotor activity and at the same time it increased avoidance, possibly by augmenting fear. As a result of this increased avoidance, the extinction of fear tended to be retarded by 1.5 mg/kg of chlorpromazine. The slowing of the “free” extinction of fear by both of these drugs was critically dependent on the doses used, but in no case was there a “beneficial” effect, indicating hastened extinction of fear.
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Kumar, R. Extinction of fear II: Effects of chlordiazepoxide and chlorpromazine on fear and exploratory behaviour in rats. Psychopharmacologia 19, 297–312 (1971). https://doi.org/10.1007/BF00401946