Formation of CaMKII clusters upon exposure of hippocampal cultures to energy-depleting conditions. Hippocampal cultures were exposed to glucose-free medium containing either 2 μm (C) or 10 μm(B, D) CCCP for 30 and 15 min, respectively. Control samples (A) were incubated for 30 min in corresponding glucose-containing medium with carrier only (0.1% DMSO). In thin sections of control samples, diffuse labeling for CaMKII (each spot corresponds to an individual enhanced gold complex) is observed throughout the cytoplasm of neurons. In treated samples, diffuse labeling is greatly diminished, but intense labeling of electron-dense round structures is observed (B, arrows). C andD show that these round structures are virtually identical to the CaMKII clusters observed in subcellular fractions (Figure 3C). Scale bars, 100 nm.