Neuroinvasiveness of pseudorabies virus injected intracerebrally is dependent on viral concentration and terminal field density

Pseudorabies virus (PRV), a neurotropic swine a herpesvirus, has been used extensively for transneuronal analysis of multisynaptic circuitry after per ipheral injection. In the present analysis, we examined the influence of vi ral concentration and neuronal architecture on the invasiveness, replicatio n, and transynaptic passage of an attenuated strain of PRV (PRV-Bartha) inj ected into rat striatum. Different concentrations of PRV-Bartha were inject ed into the striatum at a constant rate of infusion (10 nl/minute), and ani mals were killed 50 hours later. Viral concentration was manipulated by eit her altering the volume of the inoculum (100, 50, 20 nl) or by diluting the inoculum within a constant volume of 100 nl. Immunohistochemical localizat ion of infected neurons revealed dramatic differences in the progression of infection that were dependent directly on the concentration of injected vi rus. In every case, the pattern of infection was consistent with preferenti al uptake of virions by axon terminals and retrograde transynaptic passage of virus from the injection site. The known topographically organized corti costriatal projections permitted a precise definition of the zone of viral uptake. This analysis demonstrated that the "effective zone of viral uptake " (i.e., the zone within which viral uptake led to productive replication o f virus) varied in relation to the concentration of injected virus, with th e highest concentration of PRV invading terminals within a 500 mu m radius of the canula. Concentration-dependent changes in the progression of retrog rade transynaptic infection also were observed. The highest concentration o f virus produced the most extensive infection. The distribution of infected neurons in these cases included those with known afferent projections to s triatum as well as those that became infected by retrograde transynaptic in fection. Lesser concentrations of PRV-Bartha produced an increasingly restr icted infection of the same circuitry within the same postinoculation inter val. It is noteworthy that neurons known to elaborate dense striatal termin al fields were less sensitive to reduction in viral concentration than thos e giving rise to terminal fields of lesser density. Collectively, the data indicate that the onset of viral replication after intracerebral injection of PRV is directly dependent on virus concentration and terminal field dens ity at the site of virus injection. J. Comp. Neurol. 1999;407:438-452. (C) 1999 Wiley-Liss, Inc.