| Abstract No.: | B-G2197 |
| Country: | Canada |
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| Title: | EVIDENCE FOR THE PERSISTENCE OF CONTEXTUAL FEAR MEMORIES THAT HAVE UNDERGONE IMMEDIATE EXTINCTION. |
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| Authors/Affiliations: | 1 Georgina Archbold*; 2 Mark Bouton; 1 Karim Nader;
1 McGill University, Montreal, QC, Canada; 2 University of Vermont, Burlington, USA.
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| Content: | Extinction is one form of memory inhibition commonly studied in Pavlovian fear conditioning. In extinction, CS-US pairings are followed by presentations of the CS alone, which suppress responding learned in the first phase. Most evidence suggests that extinction is a consequence of new inhibitory learning, rather than "erasure" of the original trace. Recent findings, however, suggest that extinction given immediately after training reflects memory loss rather than inhibition (Myers, Ressler and Davis, 2006).
Objectives: Using contextual fear conditioning, we examine the nature of extinction further using a novel behavioral paradigm that probes for the absence or presence of a memory. First, we studied whether training strength moderates the likelihood of spontaneous recovery, a return of performance commonly taken as indicating the attenuation of memory inhibition (Experiment 1). However, recovery alone may not be sufficient to indicate the return of the original memory.
In order to further test the status of the original memory trace, in our second study we exploited the effect that only the first, but not second learning of contextual fear requires NMDA receptors (NMDAr) in the dorsal hippocampus (Sanders and Fanselow, 2003). Our lab has shown previously that after delayed extinction of contextual fear, the second learning of contextual fear does not require NMDAr. This independence of NMDAr suggests that delayed extinction leads to memory inhibition, but not to memory erasure, which should have rendered the second learning of contextual fear NMDAr-dependent. Here we use this property of second learning to determine if memory of an immediately extinguished fear also persists (Experiment 2).
Materials and Methods: In Experiment 1 rats received a context paired with one of three different shock intensities (either 0.8mA, 1.2mA, or 1.6mA), and received extinction either immediately (15 mins) or after a delay (24 hrs). In Experiment 2 rats received low-intensity training (0.8mA shocks) in one context and then extinguished either immediately or after a delay. Following a delay of five days animals received bilateral infusions of the NMDAr antagonist AP5 (2-amino-5-phosphonopentanoic acid) into the dorsal hippocampus and were fear conditioned to a novel context.
Results: In Experiment 1 spontaneous recovery was roughly equivalent in the immediate and delayed extinction groups when they were tested 24 hr after extinction. Results from experiment 2 show that memory for the second learning is not affected by NMDAr blockade in either group, suggesting that memory was not lost, but inhibited.
Conclusion: Overall, the results provide little evidence that extinction conducted immediately after conditioning destroys or erases the original memory trace.
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