Abstract
Commonly used inbred murine strains differ substantially in their vulnerability to excitotoxic insults. We investigated whether differences in dendritic Ca(2+) signaling could underlie the differential vulnerability of C57BL/6 (resistant to kainate excitotoxicity) and C57BL/10 strains (vulnerable). A striking difference was found in fine dendrite Ca(2+) responses after kainate exposure. Ca(2+) signals in distal dendrites were large in C57BL/10 neurons, and, if a threshold concentration of approximately 1.5 microm was reached, a region of sustained high Ca(2+) was established in the distal dendritic tree. This region then served as an initiation site for a degenerative cascade, producing high Ca(2+) levels that slowly spread to involve the entire neuron and led to cell death. Dendritic Ca(2+) signals in C57BL/6 neurons were much smaller and did not trigger these propagating secondary responses. Strain differences in dendritic Ca(2+) signaling were also evident after tetanic stimulation of Schaffer collaterals. Ca(2+) responses were much larger and peaked earlier in distal dendrites of C57BL/10 compared with those in C57BL/6. Neurons from both strains had similar membrane properties and responded to kainate with intense action potential firing. Degenerative Ca(2+) responses were seen in both strains if soma Ca(2+) could be sustained above 1.5 microm. The early phases of secondary Ca(2+) responses were attributable to Ca(2+) influx and were abolished rapidly by buffered zero Ca(2+) saline. Taken together, these data indicate that the substantial difference in Ca(2+) signals in fine distal dendrites and in the initiation of spreading secondary responses may underlie the selective vulnerability of these neurons to excitotoxic insults.
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