INS and diabetes mellitus: Currently, the association between the functional of neuronal insulin signaling pathway and PD has to be clearly confirmed because diabetes may be one of the few modifiable risk factors for PD, and a few Food and Drug Administration-approved antiglycemic agents have demonstrated neuroprotective effects on PD experimental models (Dehmer et al., 2004; Quinn et al., 2008; Ismaiel et al., 2016; Katila et al., 2017).