There is a selective loss of high-metabolic-rate cells in PD and T2D, i.e., insulin-releasing pancreatic islets’ β-cells in patients with diabetes and SNpc dopaminergic neurons with well-branched unmyelinated axons with tonic action potential activity, that releases neurotransmitters across a large number of brain synapses in PD patients. This evidence concerns the gene INS and type 2 diabetes mellitus.