The expression of fluorescent proteins in defined subsets of neurons has become a powerful tool for neurobiology. One limitation of this technology is that the majority of BAC transgenic mice harnessing this technology use enhanced green fluorescent protein (EGFP) as the fluorophore. As a result, the ability to simultaneously visualize two cell populations in the same preparation is limited by the lack of lines using contrasting fluorophores. Here we report the successful application of tdTomato protein for this purpose. Historically, the creation of transgenic lines expressing non-EGFP fluorophores in defined neuronal populations has been met with only limited success, possibly because of toxicity issues, particularly for red spectral variants (Vintersten et al., 2004; Long et al., 2005). Using the regulatory promoter elements of the type 1a dopamine receptor gene (Drd1a), we created a BAC transgenic line that identifies the direct pathway of the basal ganglia (Drd1a-tdTomato). In combination with the existing GENSAT BAC transgenic line identifying the indirect pathway (Drd2-EGFP), this new transgenic line creates a novel platform for the study of basal ganglia physiology. The choice of tdTomato as a reporter may also prove useful in other circuits in which the simultaneous visualization of two defined cell populations is desired.

The basal ganglia are important for a wide range of behaviors from decision making to movement (Graybiel, 2005). Within the basal ganglia circuitry, striatal medium spiny neurons (MSNs) project either directly or indirectly to the substantia nigra pars reticulata. The so-called “direct”(or striatonigral) and “indirect”(or striatopallidal) pathways of the basal ganglia are functionally important because they exhibit opposing influences both on the activity of the output thalamic nuclei and on behavior (Albin et al., 1995; Wichmann and DeLong, 1998). Despite the well described anatomical and functional differences between the direct and indirect pathways, the ability to study the synaptic physiology of MSNs in these two pathways has been limited until recently. BAC transgenic mice created by the