Theta stimulation polymerizes actin in dendritic spines of hippocampus.

It has been proposed that the endurance of long-term potentiation (LTP) depends on structural changes entailing reorganization of the spine actin cytoskeleton. The present study used a new technique involving intracellular and extracellular application of rhodamine-phalloidin to conventional hippocampal slices to test whether induction of LTP by naturalistic patterns of afferent activity selectively increases actin polymerization in juvenile to young adult spines. Rhodamine-phalloidin, which selectively binds to polymerized actin, was detected in perikarya and proximal dendrites of CA1 pyramidal cells that received low-frequency afferent activity but was essentially absent in spines and fine dendritic processes. Theta pattern stimulation induced LTP and caused a large (threefold), reliable increase in labeled spines and spine-like puncta in the proximal dendritic zone containing potentiated synapses. The spines frequently occurred in the absence of labeling to other structures but were also found in association with fluorescent dendritic processes. These effects were replicated (>10-fold increase in labeled spines) using extracellular applications of rhodamine-phalloidin. Increases in labeling appeared within 2 min, were completely blocked by treatments that prevent LTP induction, and occurred in slices prepared from young adult rats. These results indicate that near-threshold conditions for inducing stable potentiation cause the rapid polymerization of actin in mature spines and suggest that the effect is both sufficiently discrete to satisfy the synapse-specificity rule of LTP as well as rapid enough to participate in the initial stages of LTP consolidation.