PSEUDORABIES VIRUS-INFECTION OF THE RAT CENTRAL-NERVOUS-SYSTEM - ULTRASTRUCTURAL CHARACTERIZATION OF VIRAL REPLICATION, TRANSPORT, AND PATHOGENESIS

Pseudorabies virus (PRV) has been used extensively to map synaptic cir cuits in the CNS and PNS. A fundamental assumption of these studies is that the virus replicates within synaptically linked populations of n eurons and does not spread through the extracellular space or by cell- to-cell fusion. In the present analysis we have used electron microsco py to characterize pathways of viral replication and egress that lead to transneuronal infection of neurons, and to document the non-neurona l response to neuronal infection. Three strains of PRV that differ in virulence were used to infect preganglionic motor neurons in the dorsa l motor nucleus of the vagus (DMV). The data demonstrate that viral re plication and transneuronal passage occur in a stepwise fashion that u tilizes existing cellular processes, and that the non-neuronal respons e to infection serves to restrict nonspecific spread of virus by isola ting severely infected neurons. Specifically, capsids containing viral DNA replicate in the cell nucleus, traverse the endoplasmic reticulum to gain access to the cytoplasm, and acquire a bilaminar membrane env elope from the trans cisternae of the Golgi. The outer leaf of this en velope fuses with the neuron membrane to release virus adjacent to axo n terminals that synapse upon the infected cell. A second fusion event involving the viral envelope and the afferent terminal releases the n aked capsid into the bouton. Systematic analysis of serial sections de monstrated that release of virus from infected neurons occurs preferen tially at sites of afferent contact. Nonspecific diffusion of virus fr om even the most severely infected cells is restricted by astrocytes a nd other non-neuronal elements that are mobilized to the site of viral infectivity. The ability of glia and macrophages to restrict spread o f virus from necrotic neurons is the product of (1) temporal differenc es in the mobilization of these cells to the site of infection, (2) di fferential susceptibility of these cells to PRV infection, and (3) abo rtive viral replication in cells that are permissive for infection. Th e findings provide further insight into the intracellular routes of vi ral assembly and egress and support the contention that transneuronal spread of virus in the brain results from specific passage of virions through synaptically linked neurons rather than through cell fusion or release of virus into the extracellular space.