Conditioned freezing behaviour as observed during the fear extinction session conducted 24 hours after fear conditioning.

<p>Freezing behaviour was impaired in 5-HTT^−/− (n = 10) compared to 5-HTT^+/+ (n = 10) rats housed in open cages (A); The replication experiment also showed impaired fear extinction in 5-HTT^−/− (n = 5) <i>versus</i> 5-HTT^+/+ (n = 5) rats housed in open cages (B); Lack of differences in freezing behaviour across trials between 5-HTT^−/− (n = 7) and 5-HTT^+/+ (n = 8) rats housed in IVC cages (C); This lack of differences in freezing behaviour was confirmed in an independent replication experiment consisting of five 5-HTT^−/− and five 5-HTT^+/+ rats housed in IVC cages (D). Data (trial blocks averaged over 3 trials) represent mean percentage of freezing time (± S.E.M.) during CS presentation. * P<0.05.