Evidence For Learninginduced Remodeling Of Existing Synapses

Among various organelles of excitatory glutamatergic synapses involving dendritic spines, the PSD is most likely to undergo learning-related structural alterations. Although the PSD consists of many different components (reviewed in 34), the addition of AMPARs to the PSD emerges as a leading mechanism for enhancing the efficacy of synaptic transmission and enlarging PSD size. AMPARs mediate most of fast excitatory synaptic transmission, and their number is a major determinant of synaptic strength35. Being highly dynamic PSD components, AMPARs move into and out of the PSD on a timescale of minutes in an activity-dependent manner (reviewed in 36). There is a strong positive correlation between PSD dimensions and AMPAR content in axospinous synapses11, 37-39. This raises the question of whether PSD size is increased in existing axospinous synapses as a consequence of behavioral learning.

The results of earlier studies based on analysis of single ultrathin sections indicate that the length of PSD profiles is increased in relevant regions of the vertebrate brain following acquisition of new behaviors (reviewed in 8). These results, however, are biased by the size, shape, and orientation of synapses, as well as by section thickness and overprojection (i.e., apparent enlargement of opaque profile images because of the finite thickness of transparent sections). Recently, advanced stereological techniques have been employed to assess the size of PSD area. In our studies, the length of PSD profiles was measured in all consecutive serial sections through each sampled synapse, and the PSD area was estimated using appropriate corrections for variations in section thickness and overprojection25. The other serial section approach was to make three-dimensional reconstructtion of dendritic segments together with spines emanating from them and to measure PSD surface area on reconstructed images40.

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