Ultrastructural analysis revealed that the great majority of asymmetric (presumably excitatory) synapses in the LA occur on spines. In order to determine whether fear conditioning altered the proportion of axo-spinous synapses immunoreactive for GluR1, we captured electron microscopic images from the LA until more than 200 synapses were encountered per animal. Data were collected from three treatment groups: paired, naïve control, and unpaired. The unpaired group served as a second control, in that these animals were exposed to the same stimuli of tone and foot shock, but the tone followed the shock. This sequence of stimuli does not yield the conditioned response of freezing (Cain and LeDoux, 2007).
