, 2010; for similar conclusions on mitochondrial dysfunction in AD, see Cho et al., 2009;

for a critical review, see Fukui and Moraes, 2008). The upshot of these Bioactive Compound Library datasheet studies is that elevated ROS levels and mitochondrial dysfunction in DA SNc neurons are clearly associated with PD. Mitochondrial vulnerability might be a first hit target, predisposing DA SNc neurons for vulnerability to PD, but the mitochondrial respiratory chain dysfunctions may be an aggravating consequence rather than a cause of disease. Further supporting the notion that a selective vulnerability of DA SNc neurons to PD is linked to mitochondrial dysfunction, genes whose mutations are causally related to PD code Epigenetics inhibitor for proteins that either accumulate at mitochondria (Pink1, DJ-1, Htra2, LRRK2), are implicated in mitochondrial functionality (Parkin), or influence each other’s role in disease (Parkin and α-synuclein, Parkin and Pink1; e.g., Bogaerts et al., 2008 and Banerjee et al., 2009). However, like for the pharmacological evidence, it is not clear whether the genes cause PD by specifically impairing mitochondrial functions. For example, studies of α-synuclein function have provided evidence that this protein

has a critical role as a chaperone for SNARE assembly, and in regulating synaptic vesicle cycling (e.g., Nemani et al., 2010 and Burré et al., 2010). The mechanistic relationship between α-synuclein mutations and PD may thus involve synaptic transmission and excitability. Likewise, DJ-1 has a role as oxidative stress sensor, and Parkin also has a role in stress protection (e.g., Banerjee et al., 2009 and Guzman

et al., 2010), suggesting that the relationship between these genes and PD may involve cellular stress pathways. The etiology of PD may thus involve overburdening of stress pathways involving mitochondria, which are particularly sensitive in DA SNc neurons. How might DA SNc neurons be more vulnerable to mitochondrial dysfunction than other neurons? Studies addressing the excitability properties of PD-vulnerable neurons have provided exciting evidence as to how this vulnerability may come about. The studies show that adult DA SNc neurons are Ca-dependent next pacemakers whose intrinsic activity is driven by Cav1.3 low voltage-dependent L-type Ca-channels (Chan et al., 2007). These particular channels open at relatively hyperpolarized membrane potentials, leading to high Ca flux loads in DA SNc neurons. Notably, reducing Ca load with L-type Ca channel antagonists reduced the susceptibility of the SNc neurons to parkinsonism-inducing drugs (Chan et al., 2007). A recent study provided evidence that the pacemaking produces oxydative stress selectively in SNc dopaminergic neurons (Guzman et al., 2010). The oxydative stress is compensated by partial uncoupling of mitochondria, which is impaired in the absence of DJ-1 (Guzman et al., 2010).