Activity-Dependent Morphological Plasticity of Dendritic Spines in Cultured Hippocampal Neurons Menahem Segal The Weizmann Institute, Rehovot, Israel and NINDS, NIH, Bethesda, MD D. D. Murphy NINDS, NIH, Bethesda, MD E. Korkotian The Weizmann Institute, Rehovot, Israel Dendritic spines are assumed to be the morphological loci of long-term plasticity. Numerous studies in the past century searched for changes in dendritic spine shape, density, and connectivity, which may underlie the role of the spines in information storage (Segal, 1995a). Studies conducted in the intact brain are tedious and time consuming, and little consensus has been reached as to the nature of the stimuli causing the change, as well as the nature of the change. We have developed an in vi- tro, tissue culture system that will allow us to examine systematically the role of dendritic spines in plasticity (Papa, Bundman, Greenberger, & Segal, 1995; Segal, 1995b). We focus on factors that regulate formation of new dendritic spines in ma- ture cultured neurons, and on the role of the shape of dendritic spine in their interac- tion with the parent dendrite. We found that exposure to estradiol doubles the number of dendritic spines in cul- tured hippocampal neurons, as it does in vivo. We obtained evidence for the involve- ment of cAMP-associated genomic function in formation of new spines (Murphy & Segal, 1997). More recently, we found that estradiol reduces GABAergic inhibition, leading to an increase in spontaneous activity of the cultured neurons (Murphy, Cole, Greenberger, & Segal, 1998). The enhanced action potential discharge causes a rise of intracellular calcium concentration, and a subsequent large increase in phosphorylated CREB. Blockade of cAMP-regulated protein kinase A eliminates estradiol-evoked spine formation, as well as the CREB response. A specific DEVELOPMENTAL NEUROPSYCHOLOGY, 16(3), 329–330 Copyright © 1999, Lawrence Erlbaum Associates, Inc.