Dopamine Signaling in Parkinson's Disease
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Amyloid Plaque and Neurofibrillary Tangle Formation in Alzheimer's Disease
Parkinson’s disease is the second most prevalent neurodegenerative disorder. Clinically, this disease is characterized by bradykinesia, resting tremors, and rigidity due to loss of dopaminergic neurons within the substania nigra section of the ventral midbrain. In the normal state, release of the neurotransmitter dopamine in the presynaptic neuron results in signaling in the postsynaptic neuron through D1- and D2-type dopamine receptors. D1 receptors signal through G proteins to activate adenylate cyclase, causing cAMP formation and activation of PKA. D2-type receptors block this signaling by inhibiting adenylate cyclase. Parkinson’s disease can occur through both genetic mutation (familial) and exposure to environmental and neurotoxins (sporadic). Recessively inherited loss-of-function mutations in parkin, DJ-1, and PINK1 cause mitochondrial dysfunction and accumulation of reactive oxidative species (ROS), whereas dominantly inherited missense mutations in α-synuclein and LRRK2 may affect protein degradation pathways, leading to protein aggregation and accumulation of Lewy bodies. Mitochondrial dysfunction and protein aggregation in dopaminergic neurons may be responsible for their premature degeneration. Another common feature of the mutations in α-synuclein, parkin, DJ-1, PINK1, and LRRK2 is the impairment in dopamine release and dopaminergic neurotransmission, which may be an early pathogenic precursor prior to death of dopaminergic neurons. Exposure to environmental and neurotoxins can also cause mitochondrial functional impairment and release of ROS, leading to a number of cellular responses including apoptosis and disruption of protein degradation pathways. There is also an inflammatory component to this disease, resulting from activation of microglia that cause the release of inflammatory cytokines and cell stress. This microglia activation causes apoptosis via the JNK pathway and by blocking the Akt signaling pathway via REDD1.
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We would like to thank Prof. Jie Shen, Harvard Medical School, Boston, MA, for contributing to this diagram.
created November 2009
revised September 2012