Neuronal and transneuronal tracing in the trigeminal system of the rat using the herpes virus suis.

The herpes virus suis has been used as a tracer for the pathways in the central nervous system of the rat. The viruses have been inoculated in various peripheral structures innervated by the trigeminal nerve, namely in the cornea by instillation or scarification, in the anterior chamber of the eye by injection and also by subconjunctival injection, nasal instillation and injection in the masseter muscle. The herpes virus suis is easy to detect by immunofluorescence or electron microscopy, the tracing is precise because it does not diffuse, as some other tracers. The virus is replicated at the site of inoculation and at each neuronal relay, thus 'fresh' tracer is continuously brought into the system. The herpes virus suis is transported by retrograde axonal flow. It has been observed in the motor sensory, sympathetic and parasympathetic pathways up to the central nuclei, which demonstrates transneuronal transport. The selectivity of this tracer, applied to the trigeminal pathways, has allowed us to understand the function of the 3 types of neurons present in the trigeminal ganglion, namely to confirm their somatotopy and establish their central projections in the trigeminal system.