The frequency of asymmetric synapses occurring along dendritic shafts in the LA was much less than at spines, ranging from 7 to 20% of all asymmetric synapses encountered. Although less frequent, these asymmetric synapses on shafts would still be expected to affect neuronal excitability, because the absence of the spine neck makes these shaft synapses electrotonically and biochemically “closer” to the axon initial segment and soma. In order to determine whether axo-shaft excitatory synapses also undergo changes evoked by the paired presentation of stimuli, we analyzed this population of synapses separately from the axo-spinous asymmetric synapses. Results of one-way ANOVA revealed a robust increase in the proportion of axo-shafts synapses immunolabeled for GluR1 at the synaptic junction (F(2,7) = 23.013; P < 0.001), reflecting a 167% change in the proportion of synapses from the LA of the paired group that were immuno-labeled, relative to those of the Naives (P < 0.005) (Fig. 7A). PEG counts at axo-shaft synapses were also strongly altered by the fear conditioning treatment (F(2,7) = 102.81; P < 0.00001), due to the 167% elevation among synapses of the paired group, relative to those of the naïves (P < 0.00005) (Fig. 7B).
