Abnormality of Ca2+ Channels in CA3 Pyramidal Neurons of Noda Epileptic Rats (NERs)

Abstract

Purpose: The Noda epileptic rat (NER), which was found in a colony of Crj‐Wistar rats, shows spontaneous tonic‐clonic convulsions. After 8–16 weeks of age, a long‐lasting depolarization shift accompanied by repetitive firing is observed in the CA3 pyramidal neurons of the hippocampus in NERs with seizures. This electrophysiologic study using hippocampal slice preparations from NERs was performed to determine whether this abnormal firing in CA3 neurons developed with age. and its probable relationship with the abnormality of Ca2+ channels was investigated pharmacologically. Methods: Hippocampal slices (400 μm) were prepared from NERs and normal Wistar rats (age, 4–24 weeks). A single rectangular pulse stimulus of 0.1‐ms duration was delivered to the mossy fibers every 5 s through a bipolar electrode placed in the granule cell layer of the dentate gyrus. A microelectrode was introduced into the pyramidal cell layer of CA3 for intracellular recording. A Ca2+ spike was elicited by applying a depolarizing pulse (1 nA, 120 ms) in the cell through the recording electrode filled with 3 M KCI under a blockade of Na+ and K+ channels by using 1 μM tetrodotoxin and 10 mM tetraethylammo‐nium added to the artificial cerebrospinal fluid (CSF), respectively. Nicardipine (1–100 nM), a Ca2+ channel blocker, was applied to the bath. Repeated weekly acoustic stimuli applied to NERs beginning at 3 weeks of age induced tonicxlonic convulsions from 6 weeks of age. Results: Twenty‐four slices from 12 NERs and 12 slices from 10 normal Wistar rats were used. The resting membrane potentials of CA3 neurons did not show any significant differences between the NER and Wistar rats tested. When a single stimulus was delivered to the mossy fibers of NERs >6 weeks of age, the CA3 neurons showed a long‐lasting depolarization shift (60.3 ± 5.8 ms; n = 3) accompanied by repetitive firing and an afterhyperpolarization after the abnormal firing. In addition, when higher stimulus intensities (>I2 V) were given, the abnormal firings were also evoked in the CA3 neurons of 4‐ to 5‐week‐old NERs when they did not show any seizures, although the depolarization shift in younger NERs (28.1 ± 1.9 ms; n = 3) was shorter than that in NERs older than 6 weeks. These abnormal firings were evoked in ∼30% of all the CA3 neurons of NERs tested in both groups. The age‐matched Wistar rats showed only a single action potential after a single mossy‐fiber stimulation. The input impedances of the CA3 neurons with abnormal firing (before 6 weeks of age, 24.9 ± 6.4 MR; after 6 weeks of age, 28.2 ± 1.6 MΩ) were lower than those without abnormal firing (37.8 ± 2.8 MΩ) and those of Wistar rats (40.1 ± 3.4 Ma). Bath application of nicardipine (10 nM) inhibited this long‐lasting depolarization shift accompanied by repetitive firing and an afterhyperpolarization without affecting the first spikes induced by mossy‐fiber stimulation. Furthermore, nicardipine (10 nM) inhibited the Ca2+ spikes elicited by applying a depolarizing pulse to neurons of NERs with seizures, although a higher dose (100 nM) did not affect those in Wistar rats. Conclusions: These findings indicate that the abnormal excitability of the CA3 pyramidal neurons in NERs is due to an abnormality in their Ca2+ channels. The abnormal excitability of NERs at an age when they did not display tonic‐clonic convulsions suggests that the hippocampus probably serves as an epileptogenic focus in NERs.