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J. E. LEDOUX, N. HARANHALLI, L. W. MASSIE, J. P. LITTLE, *L. R.
JOHNSON; How memory is organized within neural networks is a fundamental question
in neuroscience. We used Pavlovian fear conditioning to study the discrete
organization patterns of neurons activated in an associative memory paradigm.
In Pavlovian fear conditioning a neutral stimulus, such as an auditory
tone, is temporally paired with an aversive unconditioned stimulus (US),
such as a foot shock. The previously neutral stimulus becomes a conditioned
stimulus (CS) capable of eliciting defensive responses. Given that the
lateral amygdala (LA) is a locus for the storage of memories created through
Pavlovian fear conditioning and that individual neurons within LA store
aspects of the associative fear memory, we asked whether these neurons
are organized in a specific manner and whether a patterned organization
maybe present across different animals storing the same memory? Rats were
habituated and subject to either 5 paired or unpaired presentations of
the CS (20sec, 5kHz, 75db tone) and US (0.5sec, 1.0mA foot shock). Animals
were then studied behaviorally or anatomically. In the behavioral study
rats that received paired training exhibited significantly higher levels
of conditioned fear (freezing) to the CS in a novel context. In the anatomical
study, exactly 60mins following training (Schafe et al, 2000), brains
were prepared for immunocytochemical detection of phosphorylated mitogen
activated kinase (pMAPK). pMAPK activated neurons were mapped and quantified
using Neurolucida (Microbrightfield, VT). Specific regions of the LA showed
greater pMAPK activation. This suggests associatively activated neurons
are discretely localized to regions of the nucleus. We next asked if matching
constellations of activated neurons could be found across paired animals.
We found examples of common constellations in paired animals and in contrast
control animals shared fewer matching neurons. We hypothesize that those
neurons that are activated in both paired and unpaired animals represent
common aspects of the training experience, such as the context and the
US, while the neurons unique to the paired animals are representative
of the associative aspects of the stored memory. We conclude that associative
memory formation in LA involves systematic constellations of neurons.
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