Differential activation of anatomically defined neuronal subpopulations in the amygdala during fear conditioning and extinction

9 1 Abstract The amygdala is a key structure of the neuronal circuitry mediating expression and extinction of conditioned fear. However, fear memories are thought to be encoded in a larger network comprising the medial prefrontal cortex (mPFC) and the hippocampus (HC). Thus, amygdala projections to the mPFC and to the HC are likely to be involved in fear learning, yet the cellular substrates of that learning remain unknown. To examine the role of identified amygdala projection neurons in the expression and extinction of conditioned fear responses, I used a combination of in vivo retrograde tracing techniques and analysis of expression of the activity-dependent immediate early genes (IEGs) cFos and Zif268. I show that amygdala neurons projecting to the mPFC or to the HC exhibit differential cFos and Zif268 expression in fear conditioned animals compared with nonconditioned control animals and with animals subjected to extinction. In particular, extinction resulted in a selective induction of cFos in mPFC projecting neurons. A more detailed analysis revealed that neurons projecting to the infralimbic subdivision of the mPFC (IL), but not those projecting to the prelimbic subdivision (PL), account for the specific cFos expression in mPFC-projecting neurons following extinction. To investigate the physiological correlates of fear extinction in anatomically defined subpopulations of amygdala projection neurons I used an electrophysiological ex vivo approach. In these experiments, I recorded from identified BA neurons projecting to PL or IL in slices obtained from mice subjected to extinction. Extinction differentially affects intrinsic properties of PLand ILprojecting cells. While there was no change in PL-projecting neurons, ILprojecting BA cells showed a learning-related increase in spike half-width and a concomitant decrease in the fast after-hyperpolarization (AHPfast). In control animals, spike half-width and AHPfast were controlled by the activation of voltagedependent potassium channels (VDPCs) and large-conductance Ca dependent potassium channels (BK-channels). After extinction training only VDPCs contribute to the AHPfast in IL-projecitng cells. This indicates a specific modulation of BK-channels in IL-projecting neurons following extinction learning. Our findings suggest that a change in the balance of activity between ILand PL-projecting BA neurons may be involved in the extinction of conditioned fear.