Re-structuring of synapses 24 hours after induction of long-term potentiation in the dentate gyrus of the rat hippocampus in vivo

In male rats, long-term potentiation was induced unilaterally in the dentate gyrus, either by high frequency (200Hz) or theta rhythm stimulation. Structural synaptic changes were examined 24h after induction using quantitative electron microscopy. A disector technique was employed in order to estimate the density of synapses (using 70-80-nm sections) and of granule cell nuclei (using 2-μm sections) in the middle, and inner molecular layer in both hemispheres. Synaptic height and total lateral areas of synaptic active zones per unit tissue volume were assessed via assumption-free stereological techniques coupled with image analysis. The results obtained indicated that both synaptic density and number (corrected per neuron) of axo-spinous, but not axo-dendritic, synapses were ~40% higher in the middle, but not inner molecular layer of the potentiated hemisphere compared to the contralateral (control hemisphere). No significant inter-hemispheric difference was found in the volume densities of lateral areas of active zones.These data suggest that 24h after long-term potentiation induction, active zones of existing axo-spinous synapses either split forming separate contacts, or decrease in size while new synapses are formed. Copyright (C) 2000 IBRO.