Large and Small Dendritic Spines Serve Different Interacting Functions in Hippocampal Synaptic Plasticity and Homeostasis

Figure 4

Comparison of small (blue) and large (red) spine head distributions in control versus TEA and control versus sorbitol experiments. ((a), (b)) Distribution of diameters estimated from computed volume measurement of spines in CA1 pyramidal cell apical dendrites (a) 10 and (b) 60 min after application of control (unbroken lines) versus upper panel, TEA (dashed lines), or lower panel, sorbitol (dotted lines), at the same time points. By 60 min, large spines (red) have returned to their original size being no different from control spines, whereas the small spines have grown, indicated by the shift of the distribution (blue) to the right. (c) Dentate granule cell distribution of spine diameters at 60 min. Upper panel: the distribution of small spines (blue) is not greatly affected by TEA (dashed lines) whereas the distribution of large spines (red) shifts to the left showing the persistent decrease in spine head diameter compared to controls (unbroken lines). Lower panel: sorbitol (dotted lines) shows no significant change compared to control (unbroken lines). The blue and red backgrounds represent the diameters defined as small or large, respectively, in the initial category definition at −10 min according to mean diameters (vertical dashed lines). (d) Percentages of spines belonging to each size category (as defined at −10 min) that cross the mean into another category at specified time points after application of TEA or sorbitol. Starting category indicated by font colour (blue text, small; red text, large); final category indicated by background colour (blue, small; red, large). Hence, blue writing on a red background indicates a spine that was initially in the small category but that moved across the threshold to the large category by the time point indicated.