Release of calcium from stores alters the morphology of dendritic spines in cultured hippocampal neurons

The ability to monitor ongoing changes in the shape of dendritic spines has important implications for the understanding of the functional correlates of the great variety of shapes and sizes of dendritic spines in central neu rons. We have monitored and three-dimensionally reconstructed dendritic spi nes in cultured hippocampal neurons over several hours of observation in a confocal laser scanning microscope. In the absence of extrinsic stimulation , the dimensions of dendritic spines of 3-week-old cultured neurons did not change to any significant degree over 3-4 hr in the culture dish, unlike t he case with younger cultures. Releasing calcium from stores with pulse app lication of caffeine causes a transient rise of [Ca2+](i) in dendrites and spines, monitored with the calcium dye Oregon-green. Application of caffein e to a dendrite imaged with calcein caused a fast and significant increase in the size of existing dendritic spines and could lead to formation of new ones. This effect is mediated by calcium released from the ryanodine-sensi tive stores, as application of caffeine in the presence of ryanodine blocke d this effect on the morphology of dendritic spines. Thus, release of calci um from stores is sufficient to produce significant changes in the shape of dendritic spines of cultured hippocampal neurons.