A dual infection pseudorabies virus conditional reporter approach to identify projections to collateralized neurons in complex neural circuits.

Card JP, Kobiler O, Ludmir EB, Desai V, Sved AF, Enquist LW.  A dual infection pseudorabies virus conditional reporter approach to identify projections to collateralized neurons in complex neural circuits.  PLoS One  2011;6(6):e21141. Epub 2011 Jun 16.

http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3116869/pdf/pone.0021141.pdf 

This is a second article from the group at Pittsburgh that is using viral tract tracing in order to understand more about the pathways of sympathetic innervation of various organs involved in blood pressure regulation.  In this latest work, they continue the use of viral tracers that can cross synapses and retrogradely label sympathetic post-ganglionic; pre-ganglionic; and pre-motor neurons including those in the RVLM.  What's unique about this study is they inject two different viruses, one into the left and one into the right kidney. Although both viruses express the mtomato label which expresses a red color, each virus has its own characteristic labeling pattern.  One fills the cytoplasm and dendrites whereas the other provides only punctate (i.e. dotted) staining of the neurons.  Also by use of the Brainbow cassette and Cre recombinase, when a cell is infected by both viruses (i.e. a cell presumably innervates both kidneys) the ability of the cell to express the red label is excised and the cell now expresses blue or yellow.  Furthermore once the first cell in this pathway is dually-infected, not only does it turn yellow or blue, one of the virus loses it's ability to replicate in any subsequent cell that is transynaptically labelled.  That means that a cell with punctate staining in yellow or blue is a neuron that was infected transynaptically by the cell that had the original dual labelling by both viruses.  In this way the authors can examine what are called 1st and 2nd order dually infected neurons in succession. 
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