Concentration of F-actin in synaptic formations of the hippocampus as visualized by staining with fluorescent phalloidin.

Polymerized actin (F-actin) was localized in the guinea pig hippocampus by fluorescent staining with rhodamine-labeled phalloidin. Phalloidin displayed a preferential affinity for the main synaptic areas where the fluorescence was confined to 0.5-1 micron particles, roughly the size of synaptic structures (dendritic spines and presynaptic terminals). These particles appeared to stud the surface of dendrites. Particularly large and strongly fluorescent particles mainly 1-3 micron in diameter were found in the termination zone of mossy fibres which form the largest synapses of the hippocampus. The apparent high concentration of F-actin in synaptic formations suggests a role for actin in synaptic function.