Bovine herpesvirus 5 induces an overproduction of nitric oxide in the brain of rabbits that correlates with virus dissemination and precedes the development of neurological signs.

We herein report an investigation of nitric oxide (NO) levels, a candidate molecule for neuronal toxicity and dysfunction, in the brain of rabbits during experimental neurological infection by bovine herpesvirus 5 (BoHV-5). Spectrophotometry for NO products (NO(2) and NO(3)) revealed that NO levels were significantly increased (F(4, 40) = 3.33; P <.02) in several regions of the brain of rabbits with neurological disease, correlating with moderate to high BoHV-5 titers. Immunohistochemistry of brain regions revealed a group of cells with neuronal and astrocyte morphology expressing the enzyme inducible NO synthase (iNOS) close to virus antigen-positive neurons. In addition, the investigation of nitric oxide levels between 2 and 6 days post infection (d.p.i.) revealed an initial increase in NO levels in the olfactory bulb and cortex (OB/OC) and anterior cortex (AC) at day 3 p.i., correlating with the initial detection of virus. As the infection proceeded, increased NO levels-and infectivity-were progressively being detected in the OB/CO and AC at day 4 p.i. (F(12, 128) = 2.82; P <.003); at day 5 p.i. in several brain regions (P <.003 in the OB/OC); and at day 6 p.i. in all regions (P <.003) but the thalamus. These results show that BoHV-5 replication in the brain of rabbits induces an overproduction of NO. The increase in NO levels in early infection correlated spatially and temporally with virus dissemination within the brain and preceded the development of neurological signs. Thus, the overproduction of NO in the brain of BoHV-5-infected rabbits may be a component of the pathogenesis of BoHV-5-induced neurological disease.