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J Neurophysiol (April 14, 2004). doi:10.1152/jn.00131.2004
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Submitted on February 9, 2004
Accepted on April 8, 2004

Increased excitatory synaptic activity and local connectivity of hippocampal CA1 pyramidal cells in rats with kainate-induced epilepsy

Li-Rong Shao and F. Edward Dudek*

* To whom correspondence should be addressed. E-mail: ed.dudek{at}colostate.edu.

Formation of local excitatory circuits may contribute to epileptogenesis. We tested the hypothesis that epileptogenesis is associated with increased recurrent excitation in the hippocampal CA1 area of rats with kainate-induced epilepsy. Whole-cell recordings were obtained during focal flash photolysis of caged glutamate, which served as a focal excitant to activate local pyramidal cells and study possible connections between neurons. Kainate-treated rats with spontaneous seizures were studied months after status epilepticus, and were compared to saline-injected control rats. Experiments were done in isolated CA1 minislices and in bicuculline to block GABAA receptors. Spontaneous excitatory postsynaptic currents (sEPSCs) were present in 42% of the CA1 pyramidal cells from controls and 62% from kainate-treated rats. The frequency of sEPSCs in the kainate group was significantly higher than that in the control group, but mean amplitude was not different. Flash photolysis of caged glutamate on the somatodendritic area of CA1 pyramidal neurons caused a burst of action potentials. Local excitatory connections between CA1 pyramidal cells were found in 4 of 48 neurons (8%) in slices from control animals, but significantly more neurons (12 of 37, 32%) from rats with kainate-induced epilepsy exhibited interconnections (p<0.001). Photoactivation of glutamate on recorded CA1 pyramidal cells in the kainate group sometimes caused afterdischarges, but not in controls. The kainate-treated rats with pyramidal cells that responded to photostimulaltion with repetitive EPSCs appeared to have experienced more severe seizures. These data provide new electrophysiological evidence for the formation of recurrent excitatory circuits in the CA1 area of rats with kainate-induced epilepsy.




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