title: Traces of memory: reacquisition of fear following forgetting is NMDAr-independent.
abstract: Recent research shows that while initial learning is dependent on N-methyl-D-aspartate receptors (NMDArs), relearning can be NMDAr-independent. In the present study we examined whether this switch also occurs following forgetting. The developing animal exhibits much more rapid rates of forgetting than adults, so infant rats were used. It was found that infant rats required NMDArs to learn fear (Experiment 1), and that this memory was forgotten after 14 d (Experiment 2). Despite forgetting, relearning fear did not require NMDAr activation (Experiment 3), even if it occurred in adulthood (Experiment 5). Importantly, animals only showed NMDAr-independent reacquisition if they had received paired (white noise-shock) training during conditioning and not if they received unpaired presentations of the white noise and shock (Experiment 4). In addition, this transition following forgetting was not stimulus specific as learning about a novel stimulus (i.e., light, Experiment 6) was also NMDAr-independent. However, reacquisition to a novel stimulus was NMDAr-dependent if the original fear memory was retained at the time of retraining (Experiment 7). Taken together, these results demonstrate how fear memories acquired early in life can have a long-lasting impact on later learning, even when they have been apparently forgotten (i.e., they are not expressed in the animal's overt behavior). Further, they support the idea that while memories may be forgotten, they are not gone.
authors.affiliation: School of Psychology, The University of New South Wales, Sydney, New South Wales 2052, Australia. ssli@psy.unsw.edu.au
authors.forename: Stella
authors.initials: S
authors.lastname: Li
authors.forename: Rick
authors.initials: R
authors.lastname: Richardson
mesh_terms: D000818:Animals; D003214:Conditioning, Classical / Q000187:drug effects / Q000502:physiology; D003213:Conditioning, Psychological / Q000187:drug effects / Q000502:physiology*; D016291:Dizocilpine Maleate / Q000494:pharmacology; D018691:Excitatory Amino Acid Antagonists / Q000494:pharmacology; D005108:Extinction, Psychological / Q000187:drug effects / Q000502:physiology*; D005239:Fear / Q000187:drug effects; D005260:Female; D046352:Freezing Reaction, Cataleptic / Q000187:drug effects; D008297:Male; D051381:Rats; D017207:Rats, Sprague-Dawley; D016194:Receptors, N-Methyl-D-Aspartate / Q000378:metabolism*; D012153:Retention, Psychology / Q000187:drug effects / Q000502:physiology; D013997:Time Factors
chemical_list: D018691:Excitatory Amino Acid Antagonists; D016194:Receptors, N-Methyl-D-Aspartate; D016291:Dizocilpine Maleate