Conditional Transneuronal Tracing with PRV Figure 5. Fluorphor Expression in Caudal Brainstem. The distribution of infected neurons throughout the caudal brainstem is illustrated. The red lines described in figure A illustrate the location of the coronal planes illustrated in figures B through E. Figures B through E map the distribution of infected neurons detected by immunoperoxidase localization of viral antigens in case 1. Each red dot indicates the position of an infected neuron and the position of each section with respect to Bregma (b) is indicated below each map. Figures b through e illustrate the fluorescent profiles of neurons in the RVLM (b & d), VMM (c) and raphe pallidus (Rpal; e) in sections adjacent to those mapped for immunoperoxidase localization of viral antigens. The relative position of each field illustrated in figures b through e is illustrated in the boxed area of figures B through E. The fluorescence in figures b through e is a composite of that revealed by the filters specific for dTomato, mCerulean and EYFP. The fluorescence signal from individual color channels in the boxed areas of figures b through e is shown at higher magnification in adjacent photomicrographs and insets (lower case letters marked apostrophes). White arrows mark dual infected neurons expressing VP-mRFP labeled capsids and EYFP and/or mCerulean reporters of the recombined Brainbow cassette. Cells marked by the asterisks are expressing reporters of the recombined Brainbow cassette but do not contain VP26- mRFP labeled capsids. The absence of labeled capsids in these cells indicates that they were infected by transneuronal passage of virus from a dual infected neuron. The photomicrographs illustrated in b–d are from case 1 and that shown in figure e (and at higher magnification in figure 2H) is from PLoS ONE | www.plosone.org 8 June 2011 | Volume 6 | Issue 6 | e21141