The Basal Ganglia consist of a number of different nuclei that form a diverse circuitry of GABAergic, dopaminergic and glutamatergic neurons. This complex network is further organized in subcircuits that govern limbic and motor functions in humans and other vertebrates. Due to the interconnection of the individual structures, dysfunction in one area or cell population can affect the entire network, leading to synaptic and molecular alterations in the circuitry as a whole. The studies in this doctoral thesis aimed at characterizing restricted subpopulations of neurons in the Basal Ganglia circuitry and their importance in the wider function of the network. To this end, we identified subpopulations of neurons in the subthalamic nucleus (STN), substantia nigra (SN) and ventral tegmental area (VTA), characterized their molecular profile and investigated their physiological role in the circuitry.

Within the mouse STN, reduction of glutamatergic neurotransmission in a subpopulation expressing Paired-like homeodomain transcription factor 2 (Pitx2) led to structural alterations in the nucleus as well as biochemical alterations of the dopaminergic system in the Nucleus accumbens (NAc) and changes in reward-related behavior. In the ventral midbrain, we identified and characterized novel marker genes selective to the VTA or SN. Of these, transient receptor potential cation channel subfamily V member 1 (TrpV1) marks a population of mainly glutamatergic neurons in the VTA which project to the NAc, while gastrin releasing peptide (Grp) is expressed in a population of dopaminergic neurons neuroprotected in Parkinson's disease. Furthermore, we discovered that disruption of glutamatergic co-release of dopaminergic neurons expressing dopamine transporter (DAT), diminishes fast EPSCs and glutamate release but does not affect the acquisition of reward-related behavioral tasks. To selectively quantify glutamate release from specific subpopulations, we devised a technique combining glutamate-amperometry and optogenetics. This was used to measure glutamate released from Pitx2-expressing synaptic terminals in the Globus pallidus as well as DAT- or TrpV1-expressing terminals in the NAc.

In summary, this doctoral thesis has furthered understanding of the function and importance of specific subpopulations within the Basal Ganglia circuitry and provides a novel means to investigate glutamate in the intact rodent brain within clearly defined, restricted cell populations.