The Journal of Neurophysiology Vol. 78 No. 1 July 1997, pp. 10-18
Copyright ©1997 The American Physiological Society

Calcium Dynamics in Thorny Excrescences of CA3 Pyramidal Neurons

David B. Jaffe and Thomas H. Brown

Departments of Psychology and Cellular and Molecular Physiology, Yale University, New Haven, Connecticut 06520

Jaffe, David B. and Thomas H. Brown. Calcium dynamics in thorny excrescences of CA3 pyramidal neurons. J. Neurophysiol. 78: 10-18, 1997. Confocal laser scanning microscopy was used to visualize Ca²⁺ transients in a particular type of dendritic spine, known as a thorny excrescence, in hippocampal CA3 pyramidal neurons. These large excrescences or thorns, which serve as the postsynaptic target for the mossy-fiber synaptic inputs, were identified on the basis of their location, frequency, and size. Whole cell recordings were made from superficial CA3 pyramidal neurons in thick hippocampal slices with the use of infrared video microscopy; cells with proximal apical dendrites close to the surface of the slice were selected. Changes in intracellular Ca²⁺ levels were monitored by imaging changes in fluorescence of the dyes Calcium Green-1 and Fluo-3. Dual-emission fluorescence imaging was also employed with the use of a combination of Fluo-3 and the Ca²⁺insensitive dye seminaphthorhodafluor-1. This method was used todecrease the potential influence of background fluorescence on the calculated changes in intracellular Ca²⁺ concentration ([Ca²⁺]_i). Somatic depolarization produced increases in [Ca²⁺]_i in both the thorn and the immediately adjacent dendrite. Changes in [Ca²⁺]_i were time locked with the onset of depolarization and the decay began immediately after the termination of depolarization. The peak increase in the Ca²⁺ signal was significantly greater in the thorns than in the adjacent dendritic shafts. With the use of high-temporal-resolution methods (line scans), differences were also seen in the time course of Ca²⁺ signals in these two regions. The decay time constants of the Ca²⁺ signal were faster in thorns than in the adjacent dendritic shafts. These observations suggest that voltage-gated Ca²⁺ channels are localized directly on the dendritic spines receiving mossy-fiber input. Furthermore, Ca²⁺ homeostasis within thorny excrescences is distinct from Ca²⁺ regulation in the dendritic shaft, at least over brief time periods, a finding that could have important implications for synaptic plasticity and signaling.

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