Ultrastructural analyses were performed on a Jeol-1010 electron microscope from sections at a magnification of 40,000X. Spine/synapse density was estimated by using the unbiased physical dissector method. In brief, data were collected from pairs of serial sections (dissectors). Synapses were identified on the reference section by the presence of: 1) a postsynaptic density (PSD), 2) synaptic vesicles at the presynaptic terminal, and 3) opposing membranes between the pre- and the postsynaptic terminals. Only PSDs found on dendritic spines and only one PSD per spine were analyzed. An unbiased counting frame was superimposed over each of the two micrographs, and the PSD was used as a counting unit. Synapses were labeled on the reference section micrograph if their PSD profiles were located either entirely or partly within the frame and did not intersect the forbidden edges of the frame and its extensions. Finally, only synapses that had a PSD profile in the reference, but not in the look-up section, were counted. At least seven neuropil fields (each 163.65 μm2) were photographed from each animal, corresponding to a total section area of
