INS and Parkinson disease: The neuroprotective effect of insulin was described in several in vitro models, e.g., PD-model in retinoic acid-differentiated human neuroblastoma SH-SY5Y cells with MPP(+)-induced neurotoxicity [29] and H2O2 toxicity [49], and models of ischemia (oxygen–glucose deprivation) in the rat hippocampal neurons and primary cortical neurons [49,50].