Termination Of Depolarization Research Articles

Dissimilar Roles of the Mitochondria in the Modulation of Intracellular Calcium Signals in Primary and Secondary Nociceptive Neurons

The role of different Ca2+-regulated mechanisms in the generation of cytosolic Ca2+ transients during neuronal excitation was compared in isolated primary and secondary nociceptive neurons of the rat. Application of carbonyl cyanide m-chlorophenylhydrazone (CCCP) significantly increased the peak amplitude of depolarization-induced transients in dorsal root ganglion (DRG) neurons in contrast to what was observed in spinal dorsal horn (DH) neurons. Application of CCCP immediately after termination of depolarization induced in DRG neurons massive Ca2+ release from the mitochondria into the cytosol. Application of CCCP immediately after termination of depolarization elicited a small Ca2+ release in DH neurons, which became more intense when application of the agent was delayed.

Mitochondrial participation in modulation of calcium transients in DRG neurons.

The effects of the mitochondrial Na+/Ca2+ exchange blocker tetraphenylphosphonium (TPP+) and the permeability transition blocker cyclosporinA (CysA) on the ability of mitochondria to participate in the regulation of intracellular calcium were investigated on freshly isolated mice sensory DRG neurons. The free intracellular calcium level ([Ca2+]in) was measured using indo-1 based microfluorimetry. The characteristics of depolarization-induced [Ca2+]in transients were changed in the presence of 25 microM TPP+. The amplitude of [Ca2+]in transients became decreased and the restoration of resting [Ca2+]in level speeded up in the presence of TPP+. Application of 5 microM cyclosporinA induced substantial residual elevation of [Ca2+]in after termination of depolarization. We conclude that the mitochondrial Na+/Ca2+ exchanger mechanism plays an important role in the regulation of calcium signals during neuronal activity, prolonging them by releasing Ca2+ stored during transient peak. Activation of permeability transition pores does not participate in these processes.

Calcium dynamics in thorny excrescences of CA3 pyramidal neurons.

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 to decrease 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.