Although the proportion of synapses immunoreactive to the GluR1 subunit was small, our findings were still able to demonstrate that the increase in the number of GluR1-containing AMPARs at the synaptic junction occurs rapidly enough to support short-term fear memory. Earlier studies showed that STM for fear conditioning is measurable 1 hour after training, even in the presence of protein synthesis inhibitors (Schafe and LeDoux, 2000). Because the time point that we analyzed overlaps with the reported phase that is resistant to protein synthesis inhibitors, synapse strengthening via GluR1-containing AMPAR accumulation at the LA synapses is likely to proceed mainly, although by no means exclusively, by phosphorylation and trafficking of reserve pools of GluR1-containing AMPARs from nonsynaptic sites to the postsynaptic sites.
