“…Viral vectors, especially which permit e cient gene transfer to central nervous system (CNS) from axonal terminals or across the blood-brain barrier, are useful for analyzing structure and function of speci c neuronal circuits around the injection site [1][2][3][4][5][6][7][8][9][10][11], and have become one of the most potential and promising therapy tools by delivery therapeutic genes to a distant target area [5,12]. Compared with traditional retrograde tracers, viral vectors can express genes in speci c neuron groups [9,13], and have been widely used to monitor and manipulate neuronal activities by expressing optogenetic [14,15], chemogenetic [16,17] and calcium-sensitive functional probes [18][19][20]. Some natural and engineered neurotropic viruses exhibit retrograde infection capabilities, including pseudorabies virus (PRV) [21], herpes simplex virus (HSV) [22], rabies virus (RABV) [13,23], lentivirus (LV) [24][25][26][27], canine adenovirus (CAV) [6,28], and adeno-associated virus (AAV) [5,[29][30][31], etc.…”