*N. HARANHALLI, L. W. MASSIE, J. P. LITTLE, J. E. LEDOUX, L. R. JOHNSON;
New York Univ., New York, NY

How memories are organized within neural networks is a fundamental question in neuroscience. This question includes how are ‘stronger’ memories differentially encoded from a ‘weaker’ memories. We used Pavlovian fear conditioning to study the discrete number 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. Under these conditions we modeled a ‘weaker’ and ‘stronger’ memory as a response to the same CS that could elicit a different amount of time freezing. The amygdala and especially the lateral amygdala (LA) is a key nucleus for the formation of associative emotional memories. In Pavlovian fear conditioning a previously neutral stimulus such as a tone is paired with an unconditioned stimulus such as a mild shock. As a result of associative memory formation the previously neutral stimulus becomes a conditioned stimulus (CS) capable of eliciting defensive responses. Evidence indicates plastic changes in lateral amygdala neurons, which includes activation of the mitogen activated protein kinase (MAPK) signally cascade leads to regulation of protein synthesis, underlies memory formation (Schafe, et al 2000). Neurons were counted from rats subject to either one or five paired (P1 or P5) or unpaired (UP1 or UP5) presentations of the CS (20sec, 5kHz, 75db tone) and US (0.5sec, 1.0mA foot shock). Animals were then studied anatomically and a parallel group was studied behaviorally for verification of acquisition of conditioned emotional memories. We used Neurolucida (Microbrightfield, VT) to map and quantify pMAPK neuron numbers and ImageJ (NIH, MD, http://rsb.info.nih.gov/ij/) to quantify grayscale density of the immunoreaction product. In the behavioral tests we found rats receiving P5 as compared to P1 showed greater freezing to the same 20sec tone. We next compared P5 and P1 rats for number of neurons and for grayscale density of immunoreaction product. Both measures were higher in the P5 group indicating a relationship between neuron activation measures, neuron number and immunoreactive reaction product, and the conditioned behavioral memory. These data begin to show aspects of the network organization of emotional memories in the amygdala.

Support Contributed By: R01 MH46516, R37 MH38774, K05 MH067048, P50 MH58911

Program No. 307.9/BBB22
Poster presentation:Sunday, Nov 04, 2007, 1:00 PM - 2:00 PM
Location: San Diego Convention Center: Halls B-H