UC Berkeley

Midbrain dopamine (DA) neurons in the substantia nigra pars compacta (SNc) and ventral tegmental area (VTA) project throughout the brain to modulate a diverse set of behaviors and brain states. Although their effects are widespread, they represent a very small fraction of neurons. Dopamine neurons rely on their diverse inputs, outputs, physiology, and gene expression in order to maintain their broad behavioral influence. To better understand the different roles dopaminergic projections play in the brain, the circuits need to be studied in isolation as adjacent neurons and circuits often mediate distinct behaviors. Because of their close anatomical proximity, novel tools and methods must be devised in order to precisely dissect dopaminergic sub-circuits. New tool development will depend on identifying the underlying genetic signature of dopaminergic subpopulations to allow consistent, reproducible labeling of distinct populations. Gene expression analysis will also reveal unique genes expressed in DA neurons that could lend functional importance to already established dopaminergic circuits.

Here we used single-cell RNA-sequencing to determine the unique gene expression signature of DA neurons and identify genes that define distinct dopaminergic subpopulations. We used retrograde tracing, electrophysiology, and disease models to show that genetically defined populations are functionally distinct with defined projections, unique physiological characteristics, and selective sparing in a Parkinson’s disease model. Through this work, we defined two circuits that arise from populations in the ventromedial VTA marked by their expression of Neurod6 and Grp and determined that single genes are not sufficient to label dopaminergic subtypes. Because of this, we developed a Flp-recombinase dopaminergic reporter mouse (DAT-Flp). Using this mouse, we were able to use intersectional genetic tools to selectively label Neurod6-DA neurons to enable future investigations into the role that these neurons play in dopaminergic functions.