Electrophysiological characterization of CA2 pyramidal cells from epileptic humans

The CA2 region of the hippocampus is more resistant to the principal cell loss seen in CA1 and CA3 in both animal models of temporal lobe epilepsy and in medial temporal lobe sclerosis (MTS), a common neuropathological finding in human temporal lobe epilepsy. There is extensive synaptic reorganization in the MTS hippocampi that is not seen in the hippocampi of patients with tumor-associated temporal lobe epilepsy (TTLE). The authors examined the electrophysiological properties of CA2 pyramidal cells from these two types of human hippocampi. The two main findings are that most MTS cells do not have clear evidence for inhibition yet do not fire synaptically evoked bursts; and that mossy fiber stimulation could evoke excitatory postsynaptic potentials (EPSPs) in the MTS tissue, but not the TTLE cells. These data suggest that in MTS, CA2 cells are resistant to firing epileptiform bursts which may account for their survival. Moreover, the granule cell-CA2 cell connection represents a novel form of synaptic plasticity in this disease.