The effects of chlorpromazine on the expression of an acquired passive avoidance response in mice

Abstract

Mice given one-trial passive avoidance training were examined 24 hours later for retention of the acquired response. Testing was carried out with subjects given either chlorpromazine or saline injections before the test session. Three chlorpromazine doses (0.5, 2.0, and 3.5 mg/kg) were used, and three injection times (10, 90, or 180 min before testing). Chlorpromazine was found to impair the expression of the acquired response, both by depressing its initial elicitation and also by apparently facilitating extinction. A second experiment confirmed that extinction rate was indeed increased. A clear dosage effect was observed but injection time was not important in determining the drug's effect. Further experiments were undertaken to clarify the interpretation of the drug's action; in particular, the possibility that the effects might have been caused by a dissociation of learning between the training and test situations was examined. It is suggested that the elevated extinction rates observed during testing when subjects were given chlorpromazine represents a temporary effect resulting from the reduced stimulus control of behaviour. Permanent effects of pre-test drug administration were noted on the initial expression of the learned response.