Pseudorabies virus (PRV) has been shown to be an effective
transneuronal tracer within both the peripheral and the central
nervous system. The only investigations of this virus in the
visual system have examined anterograde transport of PRV from
injection sites in the retina. In the present study, we injected
attenuated forms of PRV into the primary visual cortex of both
rats and cats to determine whether transneuronal retrograde
infection would occur back to the retina. In rats, we made small
injections into visual cortex of a strain of PRV (Bartha Blu)
that contained a β-galactosidase promoter insert. In cats,
we injected PRV-M201 into area V1 of visual cortex. After a
2- to 4-day incubation period, we examined tissue from these
animals for the presence of the β-galactosidase marker (rats)
or the virus itself (cats). Cortical PRV injections resulted
in transneuronal retrograde infection of the lateral geniculate
nucleus (LGN), thalamic reticular nucleus (TRN), and retina.
PRV was retinotopically distributed in the pathway. In addition,
double-labeling experiments in cats using an antibody against
gamma-aminobutyric acid (GABA) were conducted to reveal PRV-labeled
interneurons within the LGN and TRN. All TRN neurons were GABA+,
as was a subset of LGN neurons. Only the subset of TRN neurons
adjacent to the PRV-labeled sector of LGN was labeled with PRV.
In addition, a subset of GABA+ interneurons in LGN was also
labeled with PRV. We processed some tissue for electron microscopy
to examine the morphology of the virus at various replication
stages. No mature virions were detected in terminals from efferent
pathways, although forms consistent with retrograde infection
were encountered. We conclude that the PRV strains we have used
produce a local infection that progresses primarily in the
retrograde direction in the central visual pathways. The infection
is transneuronal and viral replication maintains the intensity
of the label throughout the chain of connected neurons, providing
a means of examining detailed circuitry within the visual pathway.