Functional properties of the intact and compromised midbrain dopamine system
Keywords: In vivo electrophysiology
Substantia nigra pars compacta
Dopamine
Dopamine neurons
Foxa
LRRK2
Parkinson's Disease
Juxtacellular labelling
Ventral tegmental area

The midbrain dopamine system is involved in many aspects of purposeful behaviour and, when compromised, can have devastating effects on movement and cognition as seen in diseases like Parkinson’s. In the healthy brain, dopamine neurons are thought to play particularly important roles in learning by signalling errors in reward prediction.
The objective of this thesis was to investigate the diversity in the functional properties of the midbrain dopamine system, and how this is altered through genetic variation of relevance to Parkinson's and development of cell phenotype. This objective was addressed with a combination of behavioural experiments, in vivo single-cell recording and labelling (both in anaesthetised as well as awake rodents), immunofluorescence labelling, retrograde tracing and stereology.
In a first set of experiments, it was demonstrated that chronic as well as acute genetic challenges can alter the firing patterns of midbrain dopamine neurons. Using a novel bacterial artificial chromosome-transgenic rat model, it was shown that the R1441C mutation in human leucine-rich repeat kinase 2, which is linked to Parkinson's, leads to motor deficits and an age-dependent reduction in the in vivo firing variability and burst firing of substantia nigra pars compacta (SNc) dopamine neurons. These findings help reveal processes of early, pre-degenerative dysfunction in dopamine neurons in Parkinson's. Similar effects on firing variability and burst firing of SNc dopamine neurons were found in a mouse model with conditional knock- out of the transcription factors Forkhead box A1 and A2 (FoxA1/2) in midbrain dopamine neurons. These findings indicate that FoxA1/2 are not only crucial for the early development of dopamine neurons, but also their function in the mature brain.
In a second set of experiments in wildtype mice, it was demonstrated that midbrain dopamine neurons (located in SNc and ventral tegmental area) show diverse expression of the molecular markers Calbindin, Calretinin, Aldh1a1, Sox6, Girk2, SatB1 and Otx2. It was found that selective expression of these markers is of use for discriminating between midbrain dopamine neurons that project to dorsal striatum or nucleus accumbens. To elucidate whether the diverse molecular marker expression would map onto dopamine neurons whose firing correlates with distinct behavioural events, midbrain dopamine neurons were recorded and labelled in head-fixed awake mice either exposed to neutral sensory stimuli or performing a classical conditioning paradigm. The population activity of midbrain dopamine neurons was not modulated by neutral sensory stimuli. Interestingly, fewer than 50% of identified dopamine neurons showed phasic firing increases following reward- predicting cue and/or reward delivery, despite the common assumption that most (if not all) midbrain dopamine neurons signal reward prediction errors. Instead, firing was modulated by other explanatory factors, such as licking, or showed no modulation during the task. Response types of midbrain dopamine neurons were not correlated with their anatomical location nor the selective or combinatorial expression of the markers Aldh1a1, Calbindin and Sox6.
In conclusion, the first set of experiments identified how different genetic burdens can alter the in vivo firing of midbrain dopamine neurons, and provide new insights into how circuits can change in pathological or compensatory ways at early disease stages in Parkinson's. The second set of experiments revealed striking heterogeneity of midbrain dopamine neurons in the intact system, and established further a functional diversity in the response types of identified midbrain dopamine neurons that is only partially consistent with canonical reward prediction error signalling.