Neurons in the ventral tegmental area (VTA) and substantia nigra pars compacta (SNc) are central to motivation and reward processing. These nuclei contain three core neuronal cell types: dopamine, GABA and glutamate (as well as combinatorial neurons). Dopamine neurons - the largest population - have been intensely studied, but little is known about the GABA neurons- the second largest population. They provide local inhibitory control of dopamine neurons with behavioural consequences, as well as sending long-range projections. It is unknown whether these are distinct populations of interneurons and projection neurons, or axon collaterals from the same population. Given what is known about interneurons and their control over principal cells in other networks, we hypothesised that diverse populations of interneurons are likely to be present in the VTA/SNc. However, there is no way of distinguishing them from projecting GABA populations, because there are no known molecular markers of GABA subpopulations in the VTA/SNc. My aim was to uncover novel molecular markers of subpopulations of GABA neurons in the VTA/SNc to provide new genetic entry points for future investigation of their function.
First, I investigated the expression of two cardinal interneuron markers, neuronal nitric oxide synthase (nNOS) and parvalbumin (PV), in the VTA/SNc using immunostaining and viral tracing. nNOS+ neurons are present throughout the VTA/SNc and neuronal phenotype is sub-region specific. PV+ neurons are also present and they are likely to be GABA interneurons.
Next, I isolated and sequenced mRNA from GABA and dopamine neurons in the midbrain using tagged polyribosomes. A comparison of transcriptomes revealed genes that are enriched in GABA neurons, offering novel candidates for subpopulation markers. These candidates were then validated through immunostaining. In conclusion, I have uncovered molecular markers of subpopulations of GABA neurons in the VTA/SNc as well as a novel population of nNOS+ glutamate neurons.