Schema and memory consolidation
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Date
27/06/2011Author
Tse, Dorothy
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Abstract
The traditional view of systems memory consolidation is that it is a gradual process
that takes place over days or weeks. Within this approach, the hippocampus (HPC) is
thought to be involved in the rapid encoding of specific events, whilst neocortex is
thought to be involved in slow learning. An idea posited recently is that systems
consolidation can occur rapidly if an appropriate “schema” into which the new
information can be incorporated has been previously created. Using a hippocampaldependent
paradigm, rats were trained to learn a schema involving 6 flavour-place
paired-associates (PAs). Once the schema was acquired, relevant new information
then became assimilated into extra-hippocampal regions and rapidly became
hippocampal-independent. Building upon this foundation and the PAs schema
paradigm, this thesis has explored several aspects of the neurobiology of schemas in
animals. The first part of the thesis examined the importance of a relevant schema in
new information processing. Rats were trained in both a consistent and inconsistent
schema. In the consistent schema, rats could learn new PAs in a single trial; however,
in the inconsistent schema, rats failed to learn the new PAs as they had not established
an appropriate schema that could facilitate rapid learning. The second part of the
thesis investigated the role of hippocampal NMDA receptors and dopamine receptors
during encoding of new PAs. Bilateral hippocampal infusion of either the NMDA
receptor antagonist D-AP5 or the D1/D5 dopamine receptor antagonist SCH23390
before encoding of new PAs resulted in impaired memory tested at 24 hr. This result
suggests that the encoding of new PAs is dependent upon NMDA receptors in the
HPC and also that dopamine is involved in the modulation of encoding new PAs. The
final chapters of the thesis attempted to identify the extrahippocampal regions in
which these new PAs are integrated with the schema during encoding. To identify the
regions that may be involved, immediate early genes (Zif268 and Arc) were used. In a
group of cortical structures, including the prelimbic cortex, there was significantly
higher Zif268 and Arc expression when encoding 2 new PAs compared to the
reactivation of previously learned (original) PAs or the encoding of 6 new PAs. These
findings indicate that the prelimbic cortex may be critical for rapid assimilation of
new information into a pre-existing schema. Finally, the last experiment in the thesis
investigated this finding using bilateral microinfusions of either the AMPA receptor
antagonist CNQX or the NMDA receptor antagonist D-AP5 into the prelimbic cortex. Infusions of CNQX and D-AP5 resulted in poor learning of the new PAs in the schema
task. This indicates that parallel encoding of new PAs occurred in the prelimbic cortex
and the HPC. The experimental results presented in this thesis suggest that the
prelimbic cortex, in particular, plays a crucial role along with the HPC during
encoding of new information in rapid memory formation.