[A quantitative study on the synaptic ultrastructural alterations in visual cortex in the maintenance of LTP]

The ultrastructure of synapses three hours after formation of long-term potentiation (LTP) was examined in the local microslices of visual cortical brain slices of 18-20 d rats. Slices without potentiating stimulation which were similarly incubated served as controls. The following structural changes were examined using a graph analyzer: (1) synaptic cleft width; (2) thickness of the postsynaptic densities(PSD); (3) length of the active zones; and (4) curvature of the synaptic interface. The number of synapses of different types in layer II/III of visual cortex was quantified by double-blind scoring procedures. The various counts were converted to the number of synapses per unit volume using stereological quantitation method. Analysis of variance was used for statistical evaluation. Our results suggest that field potentials reached their peak values at about one and a half hours after tetanus and could be maintained as long as three hours without decay. In comparison with the control groups, synaptic cleft width, thickness of PSD, surface density per unit volume(Sv) of the active zones, curvature of the synaptic interface, numeric density per unit volume (Nv) of all synapses, spine synapses and the Nv of perforated synapses were all increased significantly. These data suggest that the increase of Sv of active zones and the increase of the curvature of synaptic interface may be the morphological feature characterizing the maintenance of LTP, in addition to the formation of perforated synapses.