ACTIVITY-DEPENDENT CALCIUM SEQUESTRATION IN DENDRITES OF HIPPOCAMPAL-NEURONS IN BRAIN-SLICES

Synaptic activity-dependent changes in the spatio-temporal distributio n of calcium ions regulate important neuronal functions such as dendri tic integration and synaptic plasticity but the processes that termina te the free Ca2+ transients associated with these changes remain uncle ar. We have characterized at the electron microscopic level the intrac ellular compartments involved in buffering free Ca2+ transients in den dritic cytoplasm of CA3 neurons by measuring the larger changes in the concentrations of total Ca that persist for several minutes after neu ronal activity. Quantitative energy-dispersive x-ray microanalysis of cryosections from hippocampal slice cultures rapidly frozen 3 min afte r afferent synaptic activity identified a subset of dendritic endoplas mic reticulum (ER) as a high-capacity Ca2+ buffer. Calcium sequestrati on by cisterns of this subset of ER was graded, reversible, and depend ent on a thapsigargin-sensitive Ca2+-ATPase. Sequestration was so robu st that after repetitive high-frequency stimulation the Ca content of responsive ER cisterns increased as much as 20-fold. These results dem onstrate that a subpopulation of ER is the major dendritic Ca sequestr ation compartment in the minutes after neuronal activity.