Syn-mCherry adenovirus

Neurotensin (Nts) promotes activation of dopamine (DA) neurons in the ventral tegmental area (VTA) through mechanisms that are not fully understood. Nts can signal through G protein-coupled Nts receptors 1 and 2 (NtsR1 and NtsR2), but understanding of the mechanism of action of Nts is limited by the lack of methods to detect NtsR1- and NtsR2-expressing cells. To overcome this challenge, researchers generated dual-recombinase mice that express FlpO-dependent Cre recombinase in either NtsR1 or NtsR2 cells. This strategy allowed for temporal control of recombination so that researchers could identify cells expressing NtsR1 or NtsR2 and determine whether their distribution differs between the developing and adult brains. Using this system, they found that NtsR1 is transiently expressed in nearly all DA neurons during development and in many non-DA neurons in the VTA. However, NtsR1 expression is more restricted in the adult brain, with only two-thirds of ventral tegmental area (VTA) DA neurons expressing NtsR1. In contrast, NtsR2 expression remains constant throughout life, but is expressed primarily in glial cells. Anterior tract tracing revealed that NtsR1 is expressed by mesolimbic but not mesocortical DA neurons, suggesting that VTA NtsR1 neurons may represent a functionally distinct subset of VTA DA neurons. In summary, this study reveals a cellular mechanism by which Nts can directly interact with NtsR1-expressing DA neurons to modify DA signaling.

Here, researchers injected the Cre-mediated tracer Ad-syn-mCherry into the ventral tegmental area (VTA) of adult NtsR^1ΔNEO-Cre;GFP mice and examined the brains 7-10 days later (Figure 1A). Visualization of the VTA revealed that nearly all mCherry+ neurons co-expressed GFP; however, many GFP+ neurons within the injection site did not co-express mCherry (Figure 1B, white arrows). These results are consistent with previous detection of nearly twice the number of NtsR1-GFP+ neurons in NtsR1Dev;GFP mice compared to NtsR1^Adult;GFP mice, and confirm that NtsR1 expression (and induction by Cre) is restricted to a limited set of VTA neurons in the adult brain. Of the five injected mice, three had mCherry labeling restricted to the VTA and were used for analysis (Figure 1C). The researchers also injected Ad-syn-mCherry into the ventral tegmental area (VTA) of NtsR2^ΔNEO-Cre;GFP mice to validate a small population of NtsR2 neurons in the adult brain and define their projections. Examination of NtsR2^ΔNEO-Cre;GFP mice revealed only very few mCherry+ cells in the VTA, compared to the large number of mCherry-labeled neurons observed in NtsR1^ΔNEO-Cre;GFP mice. This small population of VTA NtsR2 neurons formed fewer than 10 single terminal neurons throughout the brain, including small projections to the nucleus cerebralis (NAc) and the interlimbus anterior commissure (IPAC; Figure 1E, white arrows). These data confirm the lack of NtsR2-expressing neurons in the adult VTA and support a major role for NtsR1 in directly modifying VTA DA signaling.

Figure 1. Ad-Syn-mCherry reveals projections of VTA NtsR1 neurons. (Woodworth H L, et al., 2018)