Human isolates of dengue (DEN) type 1 viruses FGA/89 and BR/90 differ in their membrane fusion properties in mosquito cell lines (P. Desprès et al., Virology 196:209-216, 1993). FGA/89 and BR/90 were assayed for their neurovirulence in newborn mice, and neurons were the major target cells for both DEN-1 virus strains within the central nervous system. To study the susceptibility of neurons to DEN virus infection, DEN virus replication was analyzed in the murine neuroblastoma cell line Neuro 2a. Infection of Neuro 2a cells with FGA/89 or BR/90 induced apoptotic DNA degradation after 25 h of infection. Studies of DEN protein synthesis revealed that accumulation of viral proteins leads to apoptotic cell death. The apoptotic process progressed more rapidly following BR/90 infection than it did after FGA/89 infection. The higher cytotoxicity of BR/90 for Neuro 2a cells was linked to an incomplete maturation of the envelope proteins, resulting in abortive virus assembly. Accumulation of viral proteins in the endoplasmic reticulum may induce stress and thereby activate the apoptotic pathway in mouse neuroblastoma cells.

SummaryThis report describes the effect of the dissociative anaesthetic ketamine, a non-competitive antagonist of the N-methyl-D-aspartate (NMDA) receptor, on the infectivity of the neurotropic rabies virus, in neuronal culture systems and in vivo. produced an approximate 10Q-1000-fold decrease in the production of rabies virus in neuronal cell cultures. Rabies virus infection was also inhibited in non-neuronal cell lines (Baby Hamster Kidney fibroblasts) but the inhibitory effects appeared better expressed in cells of neuronal origin (neuroblastoma and primary neuronal cultures). The kalnate/quisqualate competitive antagonist CNQX did not modify the course of rabies virus infection and modulators of known NMDA regulatory sites (Mg 2 + , Zn 2 + , HA-966) failed to antagonize the ketamine-rnediated inhibition of rabies virus production in neuroblastoma cells. Furthermore, Ca 2 +-mobilization does not appear to be involved because Ca 2 +-depleted and EGTA-treated medium did not affect the normal production of rabies virus. The action of ketamine on rabies virus infection in vivo was also investigated; peripheral treatment ofrats with this drug inhibited rabies virus infection in the thalamus, cortex and hippocampal formation (in particular the pyramidal layer of the CA1 region). Taken together, these data suggest that the ketamine-mediated inhibition of rabies virus production in vitro, although highly selective, is not acting via classic NMDA receptor-mediated mechanisms. However, the ketaminemediated retardation of rabies virus infection in vivo may offer new prospects for post-exposure rabies antiviral therapy.

We investigated the effect of rabies virus infection on the actin cytoskeleton using various techniques. Confocal microscopic examination of rabies virusinfected neuroblastoma cells at late stages of infection revealed a dramatic decrease in F-actin staining. The results of a fluorimetric assay with pyrenylated actin indicated that purified rabies virus nucleocapsid has no direct action on the kinetics of actin polymerization and only a weak effect on the final extent of polymerization. Videomicroscopy experiments with purified components showed that rabies virus nucleocapsid inhibits the actin-bundling effect induced by dephospho-synapsin I, a neuron-specific protein which is known to exert a control on the actin-based cytoskeleton. Thus, the observed decrease in F-actin staining in infected cells might be ascribed to an indirect action of rabies nucleocapsid on the effects of actinbinding proteins such as synapsin I.

Ionizing radiation has been shown to affect a broad range of viral diseases including neurotropic infections through an immunosuppression mechanism. In the present study we have investigated the effect of ionizing radiation on the characteristics of neurotropic infection by rabies virus, which has the unusual feature of infecting almost exclusively neurons. In order to analyze better the effect produced, the study concerned the spread of an apathogenic rabies virus variant in mouse brain. Irradiation was shown to increase both the intensity and duration of the infection in a reversible and dose-dependent manner and was effective in whole-body irradiation and in head-protected body irradiation, whereas cephalic irradiation had no effect. These results underline the role played by the immune system in the regulation of neurotropic virus infections in the brain and show that phenomena such as viral clearance and time-course of a neurotropic viral infection may be significantly modified by ionizing radiation, even for viruses whose infection involves only neurons.