Enhanced Calcium Influx in Hippocampal CA3 Neurons of Spontaneously Epileptic Rats

Summary: Purpose: The spontaneously epileptic rat (SER: tm/tm, zi/zi) shows both absence-like seizures and tonic convulsions. Our previous electrophysiologic studies have demonstrated that SER has abnormal excitability of hippocampal CA3 neurons, which shows a long-lasting depolarization shift by a single stimulation of mossy fibers, probably resulting from the Ca²⁺ channel abnormalities. The present study was performed to determine whether Ca²⁺ influx is actually enhanced in the CA3 area of SER.

Methods: Hippocampal slices were prepared from normal Wistar rats and SER aged 11–16 weeks old, when the epileptic seizures had been observed, and loaded with fura-2AM. Intracellular Ca²⁺ concentration ([Ca²⁺]_i) was monitored as the ratio of fluorescence intensities excited at wavelengths of 340 and 380 nm (RF340/F380) with photometric devices.

Results: High K⁺ (10∼60 mM) applied to the bath for 2 min increased [Ca²⁺]_i in hippocampal CA1, CA3, and dentate gyrus (DG) areas of both the normal rats and SER in a concentration-dependent manner. However, the high K⁺–induced increase in [Ca²⁺]_i was significantly more pronounced in the CA3 area of the SER than in that of the normal animals, whereas there were no significant differences in high K⁺–induced increases of [Ca²⁺]_i in CA1 or DG between the SER and controls. The high K⁺–induced increases in [Ca²⁺]_i of CA1, CA3, and DG were inhibited by nifedipine (1∼10 nM), a Ca²⁺ channel antagonist in both SER and controls. However, the inhibition of the high K⁺–induced increase in [Ca²⁺]_i by nifedipine (1 nM) was significantly greater in the CA3 area of SER than that of controls.

Conclusions: These findings suggest that Ca²⁺ influx through the L-type Ca²⁺ channels is much greater in the CA3 area of SER than in that of normal animals and is involved in the epileptic seizures of the SER.