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Abstract:
According to Tolman, rats cognitively map an environment in
the absence of reinforcement, a view supported by the "latent
learning" experiments published in the 1930's. The current study
extends those findings to contextual fear conditioning and examines
the importance of NMDA receptors in this type of learning. In
contextual fear conditioning, a rat is placed in a novel context
and after a specified interval, it receives an electric footshock.
When tested at a later time, the animal shows a freezing response
to the contextual cues. NMDA-receptor mediated processes have been
implicated in contextual fear conditioning because the
NMDA-receptor antagonist AP-5 (DL-2-amino-5-phosphenovaleric acid)
blocks acquisition of contextual fear. Preexposure to the
conditioning context in the absence of shock enhances contextual
fear. Since rats are learning about the context during preexposure
but do not express it until after conditioning, this represents a
form of aversive latent learning. In Phase 1 of this study, rats
were preexposed to a novel chamber where they explored for 10 min.
In Phase 2, they were placed in the chamber again and after a short
interval, received a shock. Animals that were administered AP-5 (5
ug, ICV) during the preexposure phase did not show the facilitatory
effects of context preexposure. These results suggest that NMDA
receptors are important for the latent learning that occurs during
the context preexposure period.
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