Occurrence of two Types of Granule Cells with Different Excitability in Rat Dentate Gyrus Granule Cell Layer Following Pilocarpine-Induced Status Epilepticus

Aims: The evaluation of the neuronal excitability of dentate gyrus granule cells during the development of epilepsy in pilocarpine model of TLE in rats.
Place and Duration of Study: Neuroscience Research Center, Shahid Beheshti University of Medical Sciences, Tehran, Iran, between October 2012 and January 2013.
Methodology: Status epilepticus (SE) was induced by pilocarpine injection (350mg/kg; i.p) to male rats. Twenty minutes before pilocarpine injection, N-methyl scopolamine (1mg/kg; s. c) was injected to reduce peripheral effects of pilocarpine and after 3h, diazepam (4 mg/kg; i. p) was injected to stop seizures. Twenty four hour (acute phase) and 30 days (chronic phase) after inducing SE, the excitability of granule cells was evaluated using whole cell patch clamp recording.
Results: Fifty seven percent of granule cells in the acute phase were hyperexcitable and another 43% of cells were less excitable. In chronic phase, the majority of cells (71%) were hyperexcitable, while 29% of the cells had a low excitability. In both phases of epilepsy, membrane input resistance (Rin) in hyperexcitable cells was similar to that of control group, while Rin was significantly lower in less excitable cells compared to control group.
Conclusion: Results showed the presence of two groups of granule cells in pilocarpine-induced SE in rats: one group of cells with high excitability and another group of cells with less excitability. Although less excitability might, at least partly, protect granule cells from seizure-induced neurotoxicity, but hyperexcitability in majority of granule cells and the occurrence of spontaneous recurrent seizures during chronic phase suggest the existence of insufficient compensatory mechanisms, possibly facilitating the propagation of seizure activity.