Seizure, Neuron Loss, and Mossy Fiber Sprouting in Herpes Simplex Virus Type 1–Infected Organotypic Hippocampal Cultures

Purpose: Epileptic seizures are frequently seen after viral encephalitis. Herpes simplex virus type 1 (HSV‐1) encephalitis is the most common cause of acquired epilepsy in humans. However, little information is available about the neuropathogenesis of HSV‐1–associated seizures. We have developed an in vitro HSV‐1–infected organotypic hippocampal slice culture to elucidate the underlying mechanisms of HSV‐1–associated acute seizure activity. Methods: Hippocampal slice cultures were prepared from postnatal day 10 to 12 rat pups. Wild‐type HSV‐1 strain RE (1 × 105 PFU) was applied to cultures at 14 days in vitro. The excitability of CA3 pyramidal cells and hippocampal network properties were measured with electrophysiological recordings. Hematoxylin–eosin (H&E) and Timm stains were used. Results: HSV‐1 infection induces epileptiform activity, neuron loss, and subsequently a dramatic increase of mossy fiber sprouting in the supragranular area. With intracellular recordings, surviving CA3 pyramidal cells exhibited a more depolarizing resting membrane potential concomitant with an increase in membrane input resistance and had a lower threshold to generate synchronized bursts and a decrease in the amplitude of afterhyperpolarization than did controls. When the antiherpes agent acyclovir was applied with a delay of 1 or 24 h after HSV‐1 infection, a dramatic inhibition of HSV‐1 replication and protection of the neuron loss were observed. Conclusions: These results suggest that a direct change in the excitability of the hippocampal CA3 neuronal network and HSV‐1–induced neuron loss resulting in subsequent mossy fiber reorganization may play an important role in the generation of epileptiform activity.