Abstract The use of alpha herpesviruses for defining functionally interconnected populations of neurones has generated considerable interest in recent years (1). Two of the most attractive aspects of using viruses for circuit definition are the fact that they pass transneuronally and replicate in synaptically linked populations of neurones. Thus, the virus is transported through a multi-synaptic pathway and the signal, (i.e. intracellular concentration of virus) progressively increases with time (Figures 1 and 2). These two attributes are clearly superior to those offered by other neuronal tracers which have been shown to pass transneuronally. Wheatgerm agglutinin horseradish peroxidase (WGA-HRP) and the C fragment of tetanus toxin (CTT) are both known to cross synapses, but they do so in such limited amounts that it is at times difficult to identify synaptic transfer with confidence. For these reasons, there has been a dramatic increase in the number of studies which have utilized herpesviruses to characterize functionally distinct populations of neurones.

Strick, P. L., & Card, J. P. (1992). Transneuronal mapping of neural circuits with alpha herpesviruses. Experimental Neuroanatomy, 81–102.