Towards chronic contextual conditioning in rats: The effects of different numbers of unpaired tone-shock presentations on freezing time and startle

Contextual conditioning in rats is typically quantified using freezing time or startle amplitude. In this study, we combined both anxiety measures in one procedure and systematically examined the effect of training with 0, 5, 10 or 15 unpaired tone-shock (0.8 mA - 250 ms) presentations on the expression of contextual conditioning in a chronic protocol with two training and testing days. Such a chronic procedure may be valuable as a chronic anxiety model. Training with 5, 10 or 15 explicitly unpaired shocks resulted in significant contextual freezing. There was no significant increase in freezing time from post-test 1 to post-test 2 and there were no differences between the three shocked groups, implying that the different numbers of shocks did not affect the degree of contextual freezing, probably because the ceiling freezing value had already been reached. Surprisingly, we observed no potentiated startle in the conditioned context. To summarize, our protocol produced consistent contextual freezing over two testing days.