FIGURE 1.

TrAC (Tracing Axon Collaterals) permits fluorescent labeling of genetically defined neuron populations based on axonal projections. (Top) Neurons with a history of En1 and Dbh expression constitute the locus coeruleus and an adjacent portion of dorsal subcoeruleus. (Center) In mice heterozygous for En1^Dre, Dbh^Flpo, and the RC:RFLTG indicator allele, locus coeruleus (LC)-norepinephrine (NE) neurons are labeled constitutively with tdTomato (red fluorescence) and switch to EGFP (green fluorescence) after Cre recombination. (Bottom) After injection of a retrograde CAV2-Cre virus (left schematic), EGFP labels LC-NE neurons projecting to the injection site (right image) as well as all their axon collaterals in other brain regions (middle image), scale bars = 100 μm. Adapted from Plummer et al. (2020).