Rabies virus glycoprotein (RVG) is known to be the only factor that mediates rabies infection. The neurotrophin receptor (p75 NTR ), through its cysteine-rich domain 1, is a specific receptor for RVG and neutralizes virus infectivity, but its role in virus infection has remained obscure. We used adult mouse dorsal root ganglion (DRG) neurons as a model to study the role of p75 NTR in RV infection of primary neurons. We show that RV infects around 20% of DRG neurons, of which more than 80% are p75 NTR positive, have large diameters, and are capsaicin insensitive. Surprisingly, RV binding and infection are absent in about half of the p75 NTRexpressing DRG neurons which have small diameters and are often capsaicin sensitive. This indicates that p75 NTR is not sufficient to mediate RV interaction in sensory neurons. The rate and specificity of neural infection are unchanged in RV-infected p75 NTRExonIV؊/؊ mice that lack all extracellular receptor domains and in wild-type mice infected with two independent RV mutants that lack p75 NTR binding. Accordingly, the mortality rate is unchanged in the absence of RV-p75 NTR interaction. We conclude that although p75 NTR is a receptor for soluble RVG in transfected cells of heterologous expression systems, an RVG-p75 NTR interaction is not necessary for RV infection of primary neurons. This means that other receptors are required to mediate RV infection in vivo and in vitro.Rabies causes severe progressive encephalitis, myelitis, and paralysis and affects more than 50,000 people worldwide each year (59). After onset of symptoms, the infection progresses relentlessly, and there is only one documented case of a nonvaccinated human patient who survived the disease (56). Rabies virus (RV) belongs to the genus Lyssavirus in the family Rhabdoviridae and is a simple, enveloped, nonsegmented, negative-strand RNA virus that encodes only five proteins (44). The glycoprotein (G) is known to be the only protein component of the viral envelope that mediates viral entry into host cells (44).RV is highly neurotropic, and after inoculation it is retrogradely transported by peripheral neurons before being passed on to second and higher-order neurons without being taken up by glia. It is this unique property of the virus that makes it a useful transsynaptic neuronal tracer over many synapses (28,52), which is dependent on RVG (17).Although several receptors have been suggested to be responsible for RV infection, none has been conclusively identified. In previous studies we adopted an expression cloning strategy using a soluble form of RVG as a ligand. That work identified the neurotrophin receptor p75 NTR as a unique interacting receptor (50). We have also shown that the glycoprotein from another lyssavirus genotype (EBL2) is also able to interact with p75 NTR (51). p75 NTR belongs to the tumor necrosis factor receptor family (11,14) and is the common receptor for all known mature neurotrophins as well immature proneurotrophins (35). In addition, several other ligands implicated in the patholo...