ULTRASTRUCTURAL SYNAPTIC CORRELATES OF SPATIAL-LEARNING IN RAT HIPPOCAMPUS

Memory formation is believed to alter neural circuitry at the synaptic level. Although the hippocampus is known to play an important role in spatial learning, no experimental data exist on the synaptic correlat es of this process at the ultrastructural level. Here, we have employe d quantitative electron microscopy in order to compare the density, si ze and spatial arrangement of synapses in the dentate gyrus, and in ar ea CA1, of spatially trained (water maze, invisible platform) versus c ontrol(visible platform) rats. No training-associated changes of hippo campal volume were found using a stereological estimation (disector) o f the volume density of dentate granule, or CA1 pyramidal cells. Nor w ere changes found in either density, or sizes of synapses (spinous or dendritic), in CA1 or dentate gyrus. However, analysis of synaptic spa tial distribution showed a training-associated increase in the frequen cy of shorter distances (i.e. clustering) between synaptic active zone s in CA1, but not dentate, thus indicating alterations in local neural circuitry. This finding indicates subtle changes in synaptic organiza tion in area CA1 of the hippocampus following a learning experience, s uggesting that spatial memory formation in mammalian hippocampus may i nvolve topographical changes in local circuitry without synapse format ion de novo. (C) 1997 IBRO. Published by Elsevier Science Ltd.