Brief bursts of high-frequency stimulation produce two types of structural change in rat hippocampus.

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Abstract
Electrophysiological and EM techniques were used to investigate possible structural modifications associated with the induction of long-term synaptic potentiation in the hippocampal formation. Stimulation and recording were carried out using the Schaffer collateral-commissural projections from field CA3 to field CA1 of the rostral hippocampus of anesthetized rats. In 1 group of animals repetitive stimulation was administered at a frequency of 100 s-1 for 1 s (potentiated); another group was stimulated at 0.2 s-1 for 3 min (control). The 1st produced a marked increase in the strength of the postsynaptic potentials, which persisted without decrement for the 15 min periods of control testing; the repetitive low-frequency activation had no detectable effects on synaptic transmission. The rats were then perfused and prepared for EM. The dendritic zone adjacent to the recording microelectrode tip was found and measurements made of the numbers of synapses as well as of the area and length of various constituents of the neuropil. The number of synaptic contacts on dendritic spines was not different between the 2 groups, but the incidence of synapses onto dendritic shafts was 33% higher in animals that received high-frequency stimulation. No statistically significant changes were observed in the mean size of any of the following: area of synaptic boutons contacting dendritic spines, area of dendritic spines, width of dendritic spine stalks, length of postsynaptic densities (PSD) on dendritic spines, area of synaptic boutons contacting dendritic shafts and length of PSD on dendritic shafts. There were distinct changes in the within-animal variance and distribution of the dendritic spine measures. In the potentiated group there was a reduction in the coefficient of variation in the area of dendritic spines, the length of PSD on dendritic spines and the width of spine stalks. The extent to which the within-animal distributions of each of these measures were positively skewed was reduced in the potentiated group. Brief bursts of high-frequency stimulation seem to produce 2 very different types of structural change: an apparent increase in the number of shaft synapses; and a decrease in the variability of the dendritic spines. The possible relationship of these morphological effects to long-term potentiation of synaptic responses is discussed.