THE CVS STRAIN OF RABIES VIRUS AS TRANSNEURONAL TRACER IN THE OLFACTORY SYSTEM OF MICE

The sequential distribution of transneuronally infected neurons was st udied in the olfactory pathway of mice after unilateral inoculation of the challenge virus standard (CVS) strain in the nasal cavity. A firs t cycle of viral multiplication was observed in a subpopulation of rec eptor cells scattered in the main olfactory epithelium and in the sept al organ. No viral spread from cell body to cell body was reported eve n 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 d ays post-inoculation (p.i.), involved second order neurons and periglo merular cells, known to be directly connected with the axon terminals of receptor cells. Also reported as a result of a second cycle of vira l 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 ipsilatera l cerebral nuclei, the anterior olfactory nucleus and the horizontal l imb of the diagonal band. From day 3, a rapid spread of CVS was sugges ted by detection of virus in all ipsilateral direct terminal regions o f the second order neurons and in most tertiary olfactory projections. The locus coeruleus, a noradrenergic nucleus which sends direct affer ents to the olfactory bulb, never appeared immunoreactive. In spite of a certain inability of CVS to infect some neuron types, the virus app ears relevant to provide new information regarding the complex network of olfactory-related neurons into the CNS.