The effect of herpes simplex virus-1 on nitric oxide synthase in the rat brain: the role of glucocorticoids

Objective: Herpes simplex virus-1 (HSV-1) is a common cause of viral encephalitis manifested by activation of the adrenocortical axis, fever and behavioral changes. We investigated the early effects of HSV-1 on constitutive (c) and inducible (i) nitric oxide synthase (NOS) activity in rat brain and in mixed glial cell culture. The effect of glucocorticoids (GCs) on NOS responses to HSV-1 was also determined. Methods: NOS activity was evaluated by the conversion of 3H-arginine to 3H-citrulline. Nitrites were measured in supernatants of activated glial cells. Results: Under basal conditions, the highest cNOS activity was found in the cerebellum, while activity was much lower in the pons and negligible in the hypothalamus and hippocampus. Forty-eight hours after intracerebral injection of HSV-1, serum corticosterone was increased and NOS activity in the cerebellum and pons was inhibited. Adrenalectomy had no effect on the basal NOS activity but completely abrogated the inhibitory effect of HSV-1. Administration of the iNOS inhibitor aminoguanidine did not significantly change NOS activity, suggesting that the activity found in the cerebellum and pons can be attributed to the cNOS isoform. In mixed glial cell culture infected with HSV-1 and then activated with lipopolysaccharide, NOS activity and nitrite production were inhibited by 77 and 53%, respectively. Conclusions: These results suggest that brain NOS activity is inhibited in the early stages of HSV-1 infection and requires the presence of circulating GCs. HSV-1-induced brain NOS inhibition may play a role in neuronal viral invasion and in the activation of the adrenocortical axis.