The present EM immunocytochemical study revealed two contrasting types of cellular responses, distinguished by the location of asymmetric synapses, e.g., dendritic spine versus shaft. Following the paired presentation of CS and US, GluR1 immunoreactivity undergoes augmentation at both the spine and the shaft. In contrast, the unpaired presentation of CS and US elicits a decrease in GluR1 immunoreactivity, only at the asymmetric synapses on shafts, leaving the asymmetric synapses on spines relatively unchanged. This difference in the cellular response may be critical for eliciting the fear response to the CS following the paired presentation of stimuli, but not after the unpaired presentation of stimuli. The identification of such diversity points to the importance of conducting anatomical studies that have the resolving power of differentiating synaptic from nonsynaptic domains, and of distinguishing synapses that form on spines from those that form directly on dendritic shafts. Moreover, only by performing analysis upon intact tissue were we able to identify synaptic changes in the LA that link sensory experience to fear learning within the first hour post conditioning.
