Current concepts in neurology: the contribution of virologic research to clinical neurology

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 x 10(5) 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.

Nonfulminant herpes simplex encephalitis as a cause for mesial temporal sclerosis

Although mesial temporal sclerosis has been recognized for more than 100 years, its etiology remains unknown. It is proposed that a common infectious agent, herpes simplex virus type-1, may cause this disorder by means of a nonfulminant infection of mesial temporal lobe structures, which is resolved by the immune system and becomes gliotic in the course of healing by the central nervous system. Brain sections from a long-term experiment in a model of herpes simplex encephalitis reveal such a scar, which shows a high concentration of glial fibrillary acidic protein, without any evidence of residual herpes antigen, by immunocytochemistry.

Neurovirulence in an experimental focal herpes encephalitis: relationship to observed seizures

An animal model of focal herpes simplex encephalitis was used to study several strains of type-1 herpes simplex virus. Rabbits were inoculated in the olfactory bulb by a standardized technique. Virus strains resulting in mortality of greater than 70% produced seizures of 3 types, and all animals that seized became moribund or died. In contrast, a virus strain resulting in a 20% mortality produced no seizures. Administration of 60 mg phenobarbital intramuscularly daily reduced mortality significantly in animals given the epileptogenic viruses. Cultures from temporal and frontal lobes showed viral growth more frequently than did cultures of other brain areas. Microscopic examination of routine and immunoperoxidase-stained brain sections confirmed the focal nature of the infection. Clinical syndromes such as seizures arising from viral brain disease may influence mortality in animal model systems.

Zoster in the elderly: clinical, immunologic and therapeutic considerations

Herpes zoster is common in the elderly. Persons over the age of 50 years have an attack rate double that of patients less than 50 years old. In the very aged, this rate nearly doubles again. The loss of cell-mediated immunity for the VZ virus appears to be the primary factor in the disruption of the dynamic containment process responsible for VZ virus latency within the sensory ganglion. Humoral immunity may play a role in the maintenance of latency, but the degree is unproven, except in the case of dissemination where loss of detectable antibodies seems to correlate with extradermatomal dissemination. Severe forms of the disease and its complications, e.g., postherpetic neuralgia, are likely among the elderly. Direct immunofluorescent staining or cytologic examination is useful for early diagnosis, and serologic changes can document the confusing clinical entity of zoster sine herpete.