Cortical neurons of mouse embryo were cultured for 7 days.The infection group was co cultured with HTNV(A9 strain) and the control group with devitalized HTNV(A9 strain) for 4h.Both groups were divided randomly into 9 subgroups acccording to different time points.The expression of fos gene was stained with immunohistochemistry.The resulfs showed that cell nuclei of neurons in the infection group displaying a purple blue color.At o,1,2,3,and 4h after infection,the expression rates of fos positive cells were 50 0%( P