Selective localization of high concentrations of F-Actin in subpopulationsof dendritic spines in rat central nervous system: A three-dimensional electron microscopic study

Dendritic spines differ considerably in their size, shape, and internal org anization between brain regions. We examined the actin cytoskeleton in dend ritic spines in hippocampus (areas CA1, CA3, and dentate gyrus), neostriatu m, and cerebellum at both light and electron microscopic levels by using a novel high-resolution photoconversion method based in the high affinity of phalloidin for filamentous (F)-actin. In all brain regions, labeling was st rongest in the heads of dendritic spines, diminishing in the spine neck. Th e number of labeled spines varied by region. Compared with the cerebellar m olecular layer and area CA3, where nearly every dendritic spine was labeled , less than half the spines were labeled in CA1, dentate gyrus, and neostri atum. Serial section reconstructions of spines in these areas indicated tha t phalloidin labeling was restricted to the largest and most morphologicall y diverse dendritic spines. The resolution of the photoconversion technique allowed us to examine the localization and organization of actin filaments in the spine. The most intense staining for actin was found in the postsyn aptic density and associated with the spines internal membrane system. In m ushroom-shaped spines, F-actin staining was particularly strong between the lamellae of the spine apparatus. Three-dimensional reconstruction of label ed spines by using electron tomography showed that the labeled dense materi al was in continuity with the postsynaptic density. These results highlight differences in the actin cytoskeleton between different spine populations and provide novel information on the organization of the actin cytoskeleton in vivo. (C) 2001 Wiley-Liss, Inc.