The subthalamic nucleus in motor and affective functions : An optogenetic in vivo-investigation

Sammanfattning: The basal ganglia form a group of subcortical interconnected nuclei involved in motor, limbic and cognitive functions. According to the classical model of the basal ganglia, two main pathways exert opposing control over movement, one facilitating movement and the other suppressing movement. The subthalamic nucleus (STN) plays a critical role in this function, and has also been implicated in reward processing. Despite ample knowledge of the role of the STN in motor dysfunctions in relation to Parkinson’s disease, less is known about STN’s natural role in healthy subjects.The studies described in this thesis aimed to address the functional role of the STN in its natural neurocircuitry by using a transgenic mouse line which expresses Cre recombinase under the Pitx2 promoter. The Pitx2 gene is restricted to the STN and the use of Pitx2-Cre mice thereby allows selective manipulation of STN neurons by using optogenetics. By expressing Channelrhodopsin (ChR2) or Archaerhodopsin (Arch) in Pitx2-Cre neurons, we could optogenetically excite or inhibit STN Pitx2-Cre neurons and investigate the role of the STN in motor and affective functions. We showed that optogenetic inhibition and excitation of the STN induce opposite effects on motor activity. STN excitation reduced locomotion while STN inhibition enhanced locomotion, thereby providing experimental evidence to classical motor models postulating this role. We also showed that optogenetic excitation of the STN induces potent place avoidance, a behaviour relevant to aversion. Projections from the STN to the ventral pallidum (VP) exist that when excited induced the same behaviour. The VP projects to the lateral habenula (LHb), a structure known for its role in aversion. A glutamatergic multi-synaptic connection between the STN and the LHb was confirmed.Aversive behaviour is also mediated by the hypothalamic-mesencephalic area. The Trpv1 gene is expressed within the posterior hypothalamus. By applying optogenetics in a Trpv1-Cre mouse line, projection patterns to limbic brain areas were identified, and optogenetic excitation of Trpv1-Cre neurons was found to induce place avoidance.The STN and posterior hypothalamus are thereby demonstrated as new players in the aversion neurocircuitry, while the long-assumed role of the STN in motor behaviour is confirmed. To enable future analyses of how STN manipulation might rescue motor and affective deficiency relevant to human disorders, a neuronal degeneration mouse model was generated.To conclude, the results presented in this thesis contribute to enhanced neurobiological understanding of the role played by the STN in motor and affective functions.