Prämotorisches Modell der Parkinson-Krankheit: Elektrophysiologische Charakterisierung von Locus coeruleus Neuronen nach α-Synuklein-Überexpression im Mausmodell

The Parkinson`s disease is the second most common neurodegenerative disorder after Alzheimer´s disease with an increasing prevalence. The characteristic of the Parkinson´s disease are the Lewy bodies, which mainly contain the aggregated α-synuclein. The Lewy pathology is located in distinct areas of the human nerve system and spreads continuously beginning in the dorsal motor nucleus. In the course of time the intraneuronal inclusion bodies reach the medullary raphe nuclei and the locus coeruleus. After the pathology has spread over the substantia nigra and amygdala, it finally reaches the cerebral cortex. The degeneration of the dopaminergic substantia nigra pars compacta neurons causes the typical motor symptoms of Parkinson´s disease including hypokinesia, tremor, rigor and postural instability. Some years in advance there are often symptoms of a prodromal Parkinson´s disease. These premotor symptoms include hyposmia, constipation, orthostatic hypotension and sleep disorders. The locus coeruleus has a probably underestimated role in the pathogenesis of the Parkinson´s disease. First it provides the motor symptoms of the disease and second it is likely to cause some of the non-motor symptoms like sleep disorders, depression and fear. The locus coeruleus is located next to the fourth ventricle at the level of the pons and it is the major noradrenergic source in the central nervous system. To guarantee a continuous release of norepinephrine the neurons of the locus coeruleus possess an intrinsic pacemaker activity. In my thesis work I focused on a premotors mouse model of the Parkinson´s disease, in which human mutant A53T-α-synuclein was overexpressed in the locus coeruleus through an adeno-associated virus. In the control group luciferase was overexpressed to exclude general effects of the protein overexpression and to examine specially the effects of α-synuclein. Using the patch-clamp-technique I examined electrophysiological changes in the locus coeruleus neurons in acute brain slices. All of the noradrenergic neurons were spontaneously active and showed the characteristic intrinsic pacemaker activity with a frequency of 1-3 Hz. After overexpressing A53T-α-synuclein the firing frequency time-dependently increased significantly, while the firing rate after overexpressing luciferase remained constant. As a next step the action potentials were analysed separately and they showed a significant decrease of the afterhyperpolarisation amplitude post α-synuclein overexpression compared to the luciferase group. There was no difference in the action potential width and the threshold between these two groups. Searching for mechanism behind this increased firing frequency und reduced afterhyperpolarization several potassium currents were examined. Potassium selective ion channels play a central role in maintaining the resting membrane potential, the repolarisation and afterhyperpolarisation of the action potentials. The potassium channels, which were examined in this here present work, include the transient outward potassium channels, the delayed rectifier potassium channels and the calcium-actiated potassium channels. The first two potassium channels are activated by depolarisation of the membrane potential, whereas the calcium-activated potassium channels are activated by an intracellular calcium increase. The current responses to depolarizing voltage steps showed unchanged transient outward potassium channels and delayed rectifier potassium channels post α-synuclein and luciferase overexpression. However the calcium-activated potassium channels significantly decreased after A53T-α-synuclein overexpression, whereas these currents stayed constant after luciferase overexpression. Therefore the increased intrinsic pacemaker activity and the reduced afterhyperpolarization could be explained by reduced calcium-activated potassium currents in this here present work. As a consequence the activation of calcium-activated potassium channels is postulated as a promising approach protecting the neurons of the locus coeruleus during the early stages of Parkinson´s disease.