The CVS strain of rabies virus as transneuronal tracer in the olfactory system of mice.

The sequential distribution of transneuronally infected neurons was studied in the olfactory pathway of mice after unilateral inoculation of the challenge virus standard (CVS) strain in the nasal cavity. A first cycle of viral multiplication was observed in a subpopulation of receptor cells scattered in the main olfactory epithelium and in the septal organ. No viral spread from cell body to cell body was reported even in later stages of infection. The second round of viral replication which took place in the ipsilateral main olfactory bulb at 2 and 2.5 days post-inoculation (p.i.), involved second order neurons and periglomerular cells, known to be directly connected with the axon terminals of receptor cells. Also reported as a result of a second cycle of viral replication, was surprisingly the spread of CVS at 2 and 2.5 days p.i. in bulbar interneurons located in the internal plexiform layer and in the superficial granule cell layer, as well as that of 2 ipsilateral cerebral nuclei, the anterior olfactory nucleus and the horizontal limb of the diagonal band. From day 3, a rapid spread of CVS was suggested by detection of virus in all ipsilateral direct terminal regions of the second order neurons and in most tertiary olfactory projections. The locus coeruleus, a noradrenergic nucleus which sends direct afferents to the olfactory bulb, never appeared immunoreactive. In spite of a certain inability of CVS to infect some neuron types, the virus appears relevant to provide new information regarding the complex network of olfactory-related neurons into the CNS.

Rabies RNA synthesis, detected with cDNA probes, as a marker for virus transport in the rat nervous system.

The kinetics of viral RNA synthesis in different parts of the rat brain, infected with fixed or street rabies virus strains, is correlated with their anatomical neuronal connections with the masseter muscles, using hybridization with rabies cDNA probes. Viral RNA synthesis is first detected in the brain-stem and in the pons where the direct anatomical projection of the masseter muscle nervous arborization into the sensory and motor nuclei is located, through the trigeminus nerve. Rabies RNA detection is delayed in the other regions of the rat brain depending on the time course of virus transport from the trigeminal nuclei through multiple nervous connections.

Retrograde transneuronal transfer of herpes simplex virus type 1 (HSV 1) from motoneurones.

The use of Herpes simplex virus (HSV) as a retrograde transneuronal tracer would have the unique advantage that the virus would be replicated in the second order neurones, resulting in strong labelling. HSV was injected in the XII nerve (mice). The virus was detected immunohistochemically. Four stages in the brainstem distribution of HSV-positive neurones were distinguished. These stages were correlated with injected amounts/survival time. In stage 1, positive neurones were restricted to the XII nucleus; glial cells were present around the intramedullary XII rootlets. In stages 2-4, positive neurones and glial cells were also present outside the XII nucleus: (a) in the lateral reticular formation, Kölliker-Fuse nucleus, raphe and nucleus coeruleus; and (b) in the area around the XII rootlets, including parts of the inferior olive. In view of their distribution, many of the neurones in (a) must have received the virus by retrograde transneuronal transfer from XII motoneurones. The neurones in (b) were probably infected through a different route, i.e. local transfer of virus from XII axons via glial cells. This local transfer does not lead to extensive spread of the infection, yet, when using HSV for retrograde transneuronal tracing it may represent a source of error.