Golgi-like, transneuronal retrograde labelling with CNS injections of herpes simplex virus type 1

Transneuronal labeling of neurons in rabbit brain after injection of herpes simplex virus type 1 into the aortic depressor nerve

Herpes simplex virus type 1 (HSV1) was injected into either the aortic depressor nerve or the vagus nerve in the rabbit. Four or 5 days after injection of virus, the rabbit brain was processed immunohistochemically to demonstrate viral antigen. After injection into the aortic nerve HSV1 positive cells were found principally ipsilaterally within the nucleus tractus solitarius, area postrema, caudal and rostral ventrolateral medulla oblongata, the spinal trigeminal complex, raphe nuclei, A5 area, locus coeruleus, parabrachial nucleus, periaqueductal gray, ventrolateral hypothalamic area, paraventricular nucleus, amygdala, bed nucleus of the stria terminalis and insular cortex. Double labeling studies indicated that approximately 85% of the virus-containing neurons in the ventrolateral medulla, and virtually all the HSV-positive neurons in the A5 area and locus coeruleus also contained tyrosine hydroxylase. In the raphe nuclei and parapyramidal region approximately 33% of virus-containing cells reacted positively with PH8 antibody, a marker for serotonin synthesis. After injection of HSV1 into the vagus nerve labeled cells were found in similar brain areas, with a more bilateral distribution. The HSV-positive neurons may be involved in the processing of baroreceptor-derived information.

beta-Galactosidase expressing recombinant pseudorabies virus for light and electron microscopic study of transneuronally labeled CNS neurons

A beta-galactosidase expression pseudorabies virus (Bartha strain) was constructed, injected into the adrenal gland of rats, and subsequently shown to transneuronally label the CNS autonomic neurons that project to the sympathoadrenal preganglionic neurons. Virally infected neurons were visualized with a one-step histochemical reaction using the Bluo-Gal substrate (halogenated indolyl-beta-D-galactoside) for the localization of beta-galactosidase activity. In some infected neurons, a Golgi-like staining of the primary and sometimes secondary dendrites could be obtained. For electron microscopic studies, the Bluo-Gal substrate produces an electron-dense reaction product that is easily identified at both low and high magnification. This virus may be useful for the study of the cell architecture and synaptic organization of transneuronally labeled neurons of functionally defined neural circuits. These results also demonstrate that it is possible to deliver foreign genes into specific chains of neurons in the mammalian CNS by means of the retrograde transneuronal vial labeling method.

Spread of the CVS strain of rabies virus and of the avirulent mutant AvO1 along the olfactory pathways of the mouse after intranasal inoculation

After intranasal instillation in the mouse, rabies virus (CVS strain) selectively infected olfactory receptor cells. In the main olfactory bulb (MOB), infection was observed in periglomerular, tufted, and mitral cells and in interneurons located in the internal plexiform layer. Beyond the MOB, CVS spread into the brain along the olfactory pathways. This infection is specific to chains of functionally related neurons but at the death of the animal some nuclei remain uninfected. CVS also penetrated the trigeminal system. The avirulent mutant AvO1, carrying a mutation in position 333 of the glycoprotein, infected the olfactory epithelium and the trigeminal nerve as efficiently as CVS. During the second cycle of infection, the mutant was able to infect efficiently periglomerular cells in the MOB and neurons of the horizontal limb of the diagonal band, which indicates that maturation of infective particles is not affected in primarily infected neuronal cells. On the other hand, other neuronal cells permissive for CVS, such as mitral cells or the anterior olfactory nucleus, are completely free of infection with the mutant, indicating that restriction is related to the ability of AvO1 to penetrate several categories of neurons. From these observations, we concluded that CVS should be able to bind several different receptors to penetrate neurons, while the mutant would be unable to recognize some of them.

Two alpha-herpesvirus strains are transported differentially in the rodent visual system

Uptake and transneuronal passage of wild-type and attenuated strains of a swine alpha-herpesvirus (pseudorabies [PRV]) were examined in rat visual projections. Both strains of virus infected subpopulations of retinal ganglion cells and passed transneuronally to infect retino-recipient neurons in the forebrain. However, the location of infected forebrain neurons varied with the strain of virus. Intravitreal injection of wild-type virus produced two temporally separated waves of infection that eventually reached all known retino-recipient regions of the central neuraxis. By contrast, the attenuated strain of PRV selectively infected a functionally distinct subset of retinal ganglion cells with restricted central projections. The data indicate that projection-specific groups of ganglion cells are differentially susceptible to the two strains of virus and suggest that this sensitivity may be receptor mediated.

Transneuronal transport of herpes simplex virus from the cervical vagus to brain neurons with axonal inputs to central vagal sensory nuclei in the rat

The recent introduction of live viruses as intra-axonal tracing agents has raised questions concerning which central neurons are transneuronally labelled after application of the virus to peripheral organs or peripheral nerves. Since the central connections of the vagus nerve have been well described using conventional neuronal tracing agents, we chose to inject Herpes Simplex Virus Type 1 into the cervical vagus of the rat. After survival times of up to 3 days the rat brains were processed immunohistochemically using a polyclonal antiserum against herpes simplex virus. Two days after injection of the virus we observed viral antigen in the area postrema and in the nucleus tractus solitarius and the dorsal motor nucleus of the vagus (dorsal vagal complex), principally ipsilaterally. At this survival time the viral antigen in the dorsal vagal complex was largely confined to glial cells. After 3 days the viral antigen was localized both in glia and in nerve cells within the dorsal vagal complex and in brain regions previously demonstrated, using conventional tracing procedures, to contain neurons with axonal projections to the dorsal vagal complex. This was true for medullary, pontine, midbrain and hypothalamic regions and for telencephalic regions including the amygdala, the bed nucleus of the stria terminalis, and the insular and medial frontal cortices. Many of the nerve cells containing viral antigen were displayed in a Golgi-like manner, with excellent visualization of the dendritic tree. Axonal processes, in contrast, were not visualized. We used co-localization studies to confirm previous findings concerning monoamine neurotransmitter-related antigens present in medullary and pontine neurons projecting to the dorsal vagal complex. After 3 days there were many Herpes Simplex Virus Type 1-containing glial cells along the intra-medullary course of the vagal rootlets. However, no viral antigen was found in brain regions containing neurons whose axons pass through the region of glial cell-labelled rootlets. Glial cells containing viral antigen were particularly numerous in brain regions known to receive an input from neurons in the area postrema and the dorsal vagal complex. Taken together with our observation concerning the early appearance of viral antigen within glial cells in the dorsal vagal complex, this suggests that when the virus reaches the axon terminal portion it is transferred to nearby glial cells and possibly enters central neurons by way of these structures.

Viruses as transneuronal tracers

Tracing chains of neurones requires the use of transneuronal tracers, which are transferred between connected neurones. The conventional transneuronal tracers used so far produce weak labelling of recipient neurones, probably because only a small amount of tracer is transferred. Live neurotropic viruses are beginning to be used as transneuronal tracers. The viruses are replicated in recipient neurones after transneuronal transfer. This replication, which is a unique characteristic of viruses, produces strong transneuronal labelling. The findings indicate that herpes-viruses in particular represent powerful tools for demonstrating neuronal connections across synapses, for example between peripheral nerves and neurones in the brain.