We previously reported in the stable organotypic slice cultures of murine hippocampus that the repeated inductions of LTP and LTD lead to long-lasting synaptic enhancement and suppression, respectively, coupled with synapse formation and elimination. Assuming these long-lasting plasticity phenomena as in vitro models for the cell-biological analyses of memory consolidation, we have examined various aspects of these structural changes. On the synaptic enhancement after repetitive LTP, Oe et al. examined the dynamics of dendritic spines by time-intermittent confocal microscopy using the slice culture in which a limited number of CA1 pyramidal neurons express YFP. They found that the spines, being perpetually generated and retracted, increase in number following two phases: 1) transient increment of both rates of generation and retraction, and 2) recovery of the retraction rate to its basal level earlier than the generation rate (Sci. Rep. 3:1857, 2013).
In this study we examined the dynamics of dendritic spines on the synaptic suppression after repetitive LTD. After the third LTD, but not after the first LTD, the rate of spine retraction was elevated leaving the rate of spine generation unaffected. The lifetime of individual spines was shortened. When the shape of spines was classified into thin-, stubby- and mushroom-shaped, the population of mushroom-shaped spines was significantly lowered, meaning that the mushroom-shaped spines are not exceptionally stable. Oe et al. reported that the spine formation after repeated LTP occurs preferentially in the dendritic segments having low pre-existing spine density. However, the spine elimination after repeated LTD occurred irrespectively to the pre-existing spine density. Go6983, a blocker for protein kinase C, inhibited the spine elimination when applied 3-6hr after the third LTD induction.