The loss of Ivy cells and the hippocampal input modulatory O-LM cells contribute to the emergence of hyperexcitability in the hippocampus.

Epilepsy is a frequent neurological disorder that affects directly 0.5-1.5% of the world's population. Despite advances regarding therapy, about 30% of patients cannot be relieved of seizures, mainly because the pathophysiological mechanisms are still not elucidated completely. Basket, axo-axonic, bistratified, oriens-lacunosum moleculare (O-LM) and Ivy cells exert spatially and temporary different inhibition on principal neurons. Our aim was to evaluate the alterations of these interneuron populations during epileptogenesis. We induced status epilepticus in male Wistar rats using intraperitoneal pilocarpine injection, which was followed, after a latency period, by spontaneous recurrent seizures (SRS). Nissl staining was used for the analysis of gross morphological changes, whereas triple immunofluorescent-labeled sections (parvalbumin, somatostatin, neuropeptide-Y) were used for differentiation of the selected interneuron types. Putative interneurons identified by their neurochemical contents were quantified, and the cell density was calculated. Although animals developing SRS showed similar behavior, the degree of hippocampal sclerosis was different. In animals with hippocampal sclerotic cell death pattern the density of perisomatic inhibitory neurons was higher, but not significantly. The dendritic inhibitory bistratified cells were preserved, whereas the number of O-LM cells showed a significant decrease. A substantial loss was observed in the number and density of Ivy cells. We suggest that the loss of hippocampal input modulatory O-LM cells, and overall excitation controlling Ivy cells, has a role in the emergence of hyperexcitability. In the same time, alterations of output controlling interneurons might contribute to the propagation of the pathological synchronization to the cortex.