These findings suggest the dysregulation of monoamine neurotransmitter systems in the development of depression in PD, and α-Syn has been shown to regulate dopamine, serotonin and norepinephrine transporter synaptic availability through subcellular binding, reducing the frequency of intracellular trafficking and resulting in reduced neurotransmitter levels at the cell surface [25]. This evidence concerns the gene SLC6A2 and major depressive disorder.