Abstract
We investigated whether juvenile rats do not express renewal following extinction of conditioned fear due to their inability to form a long-term contextual fear memory. In experiment 1, postnatal day (P) 18 and 25 rats received 3 white-noise and footshock pairings, followed by 60 white-noise alone presentations the next day. When tested in a different context to extinction, P25 rats displayed renewal whereas P18 rats did not. Experiments 2A and 2B surprisingly showed that P18 and P25 rats do not show differences in contextual and cued fear, regardless of the conditioning-test intervals and the number of white-noise-footshock pairings received. Finally, we observed age differences in contextual fear when P25 rats were weaned at P21 in experiment 3. These results indicate that the developmental dissociation observed in renewal of extinguished fear is not related to the widely believed late emergence of contextual fear learning.
Highlights
This is because previous studies examining renewal or context conditioned fear have notable differences between each other, such as weaning, and the number of CS-US pairings during conditioning
There was no effect of age, and all rats showed higher contextual freezing when they were tested immediately after fear conditioning compared to 24 hours later, in both experiment 2A and 2B (Figure 2B & 2E)
Our findings strongly suggest that the lack of renewal in P18 rats observed in our study and previous studies is not due to impaired contextual fear learning, but is because extinction may cause ‘unlearning’ of the conditioned fear at this age (Gogolla et al, 2009; Kim & Richardson, 2008; Kim & Richardson, 2007a)
Summary
This is because previous studies examining renewal or context conditioned fear have notable differences between each other, such as weaning, and the number of CS-US pairings during conditioning. Experiment 2A tested the putative age differences in the consolidation of context fear learning using identical conditioning parameters as experiment 1 in P18 and P25 rats. The physical characteristics of Context A chambers included the white house-light, round stickers t on the back wall and wood chip bedding beneath the grid floor, with a eucalyptus-scented rip cleaning agent used before each session. On day 1, after a 2Au min baseline period, rats received 3 CS-US pairings in Context A. On day 2, after a 2-min baseline period, rats received 60 trials of 10s CS with 10s ITIs in Context B. The design and behavioural procedures were identical to experiment 2, except rats received a single CS-US pairing during fear conditioning
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